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MANUFACTURING TECHNOLOGY RESEARCH

ADDITIVE MANUFACTURING MATERIALS

STANDARDS, TESTING AND APPLICABILITY

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ADDITIVE MANUFACTURING MATERIALS

STANDARDS, TESTING AND APPLICABILITY

LILLIAN WHITE EDITOR

New York


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Published by Nova Science Publishers, Inc. † New York

ISBN: 978-1-63483-303-5 (eBook)


CONTENTS

Preface vii

Chapter 1 Mechanical Properties Testing for Metal Parts Made via Additive Manufacturing: A Review of the State of the Art of Mechanical Property Testing 1 John Slotwinski, April Cooke and Shawn Moylan

Chapter 2 Properties of Metal Powders for Additive Manufacturing: A Review of the State of the Art of Metal Powder Property Testing 21 April Cooke and John Slotwinski

Chapter 3 Applicability of Existing Materials Testing Standards for Additive Manufacturing Materials 49 John Slotwinski and Shawn Moylan

Chapter 4 Materials Testing Standards for Additive Manufacturing of Polymer Materials: State of the Art and Standards Applicability 67 Aaron M. Forster

Index 125



PREFACE This book is the first step in a process to develop standards appropriate for

the testing of the mechanical properties of metal parts made via additive manufacturing. This book investigates the current state-of-the-art methods for determining the properties of metal powders. This assessment evaluates both existing standards that provide guidance for powder property measurement methods, as well as technical publications describing methods that are not covered as completely by standards. Furthermore, the analysis shows that additive manufacturing-specific materials standards for characterizing the properties of metal powders and metal parts do not have to be developed from scratch. Decades of powder property testing (born out of powder metallurgy processes) and mechanical property testing has resulted in a suite of existing standards that can form the basis needed for some additive manufacturing materials.



In: Additive Manufacturing Materials ISBN: 978-1-63483-302-8 Editor: Lillian White © 2015 Nova Science Publishers, Inc.

Chapter 1

MECHANICAL PROPERTIES TESTING FOR METAL PARTS MADE VIA ADDITIVE

MANUFACTURING: A REVIEW OF THE STATE OF THE ART OF MECHANICAL

PROPERTY TESTING∗ John Slotwinski, April Cooke and Shawn Moylan

ABSTRACT

This report is the first step in a process to develop standards appropriate for the testing of the mechanical properties of metal parts made via additive manufacturing.

INTRODUCTION

Background This National Institute of Standards and Technology Internal Report

(NISTIR) is the first in a series of planned reports from the National Institute

∗ This is an edited, reformatted and augmented version of a report, NISTIR 7847, issued by the

National Institute of Standards and Technology, March 2012.


John Slotwinski, April Cooke and Shawn Moylan 2

of Standards and Technology’s (NIST) Engineering Laboratory project titled Materials Standards for Additive Manufacturing.1 This project provides the measurement science for the additive manufacturing industry to measure material properties in a standardized way. Currently there are no consensus-based standards in this area, except for those pertaining to terminology and data file formats. This project, in conjunction with NIST’s Fundamental Measurement Science for Additive Processes project,2 will provide the technical foundation and documentary standards development necessary to develop new consensus-based standards. This will be done via ASTM-International’s (hereafter referred to as ‘ASTM’) Committee F42 on Additive Manufacturing Technologies and the newly formed International Organization for Standardization (ISO) TC261 committee on Additive Manufacturing.

Determining the properties of the powder used for metal-based additive manufacturing, as well as the properties of the resulting bulk metal material, is a necessary condition for industry to be able to confidently select powder and produce consistent parts with known and predictable properties. By 2014, the project team will develop and deliver enhanced measurement techniques that support new, standardized methods for quantifying the material properties of both the powders used for additive manufacturing and the resulting manufactured products.

The project’s research plan includes assessments of the current state-of-the-art testing methods for determining properties of both bulk metal materials, which is the focus of this report, and raw metal powder, which will be reported on in mid-2012. These methods will then be evaluated for applicability and enhanced for use on additively manufactured parts and raw additive powder. NIST’s new Direct Metal Laser Sintering (DMLS) machine will be used to make parts, and these new methods will be rigorously implemented. Using these enhanced methods, the sensitivity of part material properties to variations in initial powder properties will be determined. This is a critical step necessary for determination of scopes of relevant material standards for additive manufacturing and for the production of additive manufacturing parts with consistent properties.

Scope In order to perform this state-of-the-art assessment of bulk metal material

property measurements in a rational and reasonable way, the focus was on existing consensus-based standards. Careful scoping of existing standards was


Mechanical Properties Testing for Metal Parts … 3

first performed to ensure that the assessment was both representative of thestate-of-the-art, and at the same time not unwieldy. To do this, three criteria were applied to determine which standards should be included in this assessment:

• Metals – Only standardized methods for measuring the mechanical

properties of metal parts were included. Mechanical property measurements for non-metals such as polymers or ceramics were excluded.

• Bulk Mechanical Properties – Only standardized methods for measuring bulk properties were included. Mechanical property measurements of extremely localized properties, such as those obtained by micro-indentation methods, were excluded. This criterion does not preclude most standardized hardness tests, because the results of these tests are reflective of the bulk properties of the specimen.

• Focus on International Standards – This was done in order to make the assessment practical. A cursory review of standards from the major Standards Development Organizations showed that the ASTM-International and the International Organization for Standardization mechanical standards are representative of all the pertinent standardized mechanical testing methods.

In addition, care was taken to ensure that both all of the mechanical testing

characteristics required by the MMPDS3 and the characteristics typically reported by the additive manufacturing original equipment suppliers were included in the assessment. The MMPDS includes both required tests (tensile, compression, shear, bearing, moduli) and recommended tests (tests at elevated temperatures, fatigue, fracture toughness, crack growth [1].) EOS – a German producer of additive manufacturing machines with a significant market presence in the U.S. – reports tensile strength, yield point, hardness, and fatigue strength for their materials [2]. The criteria above were applied to a total of 86 standards – 58 ASTM and 29 ISO – covering the measurement of material properties.

The following is organized into two sections: (1) deformation properties (where the tests attempt to quantify how a material will yield or deform) and (2) failure properties (where the tests attempt to quantify the potential for the component to rupture or fail.) Deformation property tests include tension, compression, bearing, modulus, and hardness tests. Failure property tests



include fatigue, fracture toughness, and crack growth tests. Within each of these general test classifications the most general test is first described, followed by modified tests that have testing features or parameters that make them distinct from the most general test.

DEFORMATION PROPERTIES

General Description One criterion for the selection of engineering materials for particular

applications is how those materials behave when subjected to forces, since parts made from those materials will typically be subjected to forces while in service. This is particularly true for parts and structures used in the aerospace and biomedical industry.

If a material is subjected to a static or very slowly changing stress and that stress is applied uniformly over a cross section of that material, then the resulting deformation behavior can be characterized by a simple stress-strain test. The stress can be applied in several different ways relative to the sample including tension (stretching), compression (squeezing), shear (sliding), and torsion (twisting.) The applied stress causes a certain amount of displacement, or strain, in the test specimen. The graphical representation of the amount of strain (on the abscissa) resulting from the applied stress (on the ordinate) is referred to as a stress-strain diagram.

For most metals that are stressed in tension at low levels, the stress and strain are proportional to each other and the constant of proportionality is Young’s Modulus [3]. This type of low-level stress where this proportionality is maintained is called elastic deformation. Under elastic deformation the material returns to its original configuration after the stress is removed. For some materials the initial elastic portion of the stress-strain relationship is not linear; in these cases the tangent (the slope of the stress-strain curve at a specified value of stress) and/or secant modulus (the slope of a line drawn between the origin and a given point on the stress-strain curve) is used.

Above a certain level of strain most metal materials no longer strain elastically, and non-recoverable (i.e., the material does not return to its original configuration when the stress is released) plastic deformation occurs. A subset of these exhibit yield-point phenomena, where the stress-strain curve drops precipitously from the end-point of the elastic deformation (called the upper yield point), and then remains nearly constant for further increasing strain



(called the lower yield point) before again gradually increasing for further increases in strain [3]. If the strain is continued to even higher levels most materials eventually fail via rupturing.

Indentation tests, which measure the resistance of a material to plastic deformation, are also included here. These types of measures use a specified load and loading condition to force a small indenter into the material surface. The size of the indent is then measured, from which the material hardness can be computed. Since different hardness tests employ different indenters with different sizes and geometries, the resulting hardness value applies only to the particular test being used. Hence hardness measurements are mostly relative in nature [3, 4].

Properties Measured Deformation property tests, which include tension, compression, modulus,

and hardness tests, provide the measured material properties listed below. Definitions for many of these terms can be found in Appendix A.

Stress-Strain Diagram Torque-Twist Diagram Yield Strength Yield Point Tensile Strength Rupture Strength Upper Yield Strength Lower Yield Strength Compressive Strength Bearing Strength Ductility Young’s Modulus Shear Modulus

Poisson’s Ratio Tangent Modulus Secant Modulus Chord Modulus Brinell Hardness Number Rockwell Hardness Number Knoop Hardness Number Vickers Hardness Number Scleroscope Hardness Number Webster Hardness Indentation Hardness Indentation Modulus Elasto-plastic Hardness

Specific Tests

Tension Tests ASTM E8 [5] is the basic method for uniaxial tension testing of metals at

room temperatures (10°C – 38°C). ISO 6892-1 [6] is the equivalent ISO test and includes additional test sample geometries such as sheet and wire. Both



methods give the material’s measured yield and tensile strengths. ASTM E21 [7] and ISO 6892-2 [8] provide methods for tension test of metals at elevated temperatures (above 38°C.) ISO 15579 is similar to ASTM E8 and ISO 6892-1 but provides guidance for testing at low temperatures, between 10°C and -196°C [9]. Guidance for tension testing of metals at cryogenic temperatures (less than -196°C) is covered in ASTM E1450 [10] and ISO 19819 [11].

ASTM also has two metal material tension tests, ASTM E292 [12], and ASTM E740 [13], that are similar to E8, except that the samples are first prepared with a notch or surface-crack before subjecting them to tension. ASTM E292 is explicitly performed at elevated temperatures. These tests provide the test material’s rupture strength (E292) or metal plate yield strength (E740.)

Finally, there are two ISO standards, ISO 26203 [14] and ISO 26203 [15], that describe testing of metal sheet material at high-strain rates (10-2 s-1 to 103

s-1 and higher), such as the testing of sheet metal for automotive bodies. There do not appear to be any equivalent standards in ASTM.

Compression Tests

ASTM E9 [16] is the basic method for uniaxial compression testing of metallic samples at room temperatures. ASTM standard practice E209 [17] is the same test performed at elevated and uniform temperatures, up to and beyond 538°C. There do not appear to be equivalent test standards in ISO.

Bearing Tests

ASTM E238 appears to be the only ASTM or ISO method for pin-type bearing tests [18], which determines the bearing yield strength and bearing strength for a rectangular metal specimen containing a hole for a bearing pin. The load on the bearing pin is increased at a rate of 0.05 bearing-strain per minute and a plot of the bearing pin load versus bearing deformation is made. From this data the bearing yield strength and bearing strength are determined.

Modulus Tests

ASTM E111 is the basic method for conducting modulus tests [19]; it builds on the test methods specified in ASTM E8 for tension and E9 for compression by providing additional guidance on the number of required trials, specimen preparation, and test temperatures. It also defines how to determine the Young’s, Tangent, and Chord modulus values from the tension and compression test data. ASTM E111 includes guidance for modulus measurements at both high and low temperatures. ASTM E143 provides the



basic method for measuring the shear modulus at room temperature [20]. Both of the E143 and E111 methods involve subjecting the test specimens to macroscopic tension, compression, or twisting.

Alternatively, dynamic methods that employ more microscopic deformations may be employed. ASTM E1875 describes a vibrational method that induces a sonic resonance throughout the entire sample, using a variable-frequency audio oscillator to generate a sinusoidal signal and a power-amplified transducer to convert that signal into a mechanical driving vibration [21]. A suspension-coupling system supports the test specimen and another transducer detects the mechanical vibration in the sample. If both the flexural mode and torsional mechanical resonances of the specimen are measureable, and if the geometry and mass are known, then the Young’s and Shear modulus and Poisson’s Ratio can be determined. This method includes guidance for making measurements made at room, elevated, and very low temperatures, across the range -195°C to 1200°C. ASTM E1876 is similar, and is also a micro-scale deformation method; however this test uses a more localized elastic excitation, typically generated by an impulse tool [22]. Both E1875 and E1876 methods give the dynamic Young’s and Shear Moduli and Poisson’s Ratio. With careful experiments, ultrasonic nondestructive testing techniques, which are similar conceptually to E1876, can measure the various dynamic moduli and Poisson’s Ratio of a suitably-sized metal material with a measurement uncertainty of better than 1% (k=2) [23].

Hardness Tests

Hardness tests give a general indication of the strength of a material and its resistance to deformation [3]. Hardness is not a fundamental material property, however, due to the wide variety of indenters and a material’s resistance to indentation being dependant on the size and geometry of the indenter and the applied load [3, 4]. Hence it is difficult to make quantified comparisons between material tests made with different indenters. ASTM E140 [24] does provide approximate conversions of the hardness values measured with different types of indenters.

ASTM E10 [25] and ISO 6506-1 [26] provide guidance for hardness tests of metal samples using Brinell indenters, which are spherical in shape, at room temperatures (10°C to 35°C). ASTM E18 [27] and ISO 6508 [28] provide the same for Rockwell (either tungsten carbide balls (Rockwell B) or diamond spheroconical indenters (Rockwell C) hardness tests, also at room temperatures. Knoop and Vickers indentation tests are covered in ASTM E384 [29] and ISO 4545-1 [30] (for Knoop) and ISO 6507-1 [31] for Knoop and



Vickers, respectively. Both Knoop and Vickers indenters are pyramidal in shape, but with different face angles.

The above methods all involve the application of a static (or quasi-static) load. ASTM E448 [32] describes a Scleroscope Hardness measurement, which is a dynamic measurement. For this test an indenter is dropped from a height above the metal test sample, and the magnitude of the rebound height is used to determine the hardness. ISO 14577-1 is also an elastic method, and measures both the plastic and elastic deformation of the metal material during application of the indenter, in order to compute the indentation modulus and the elasto-plastic hardness [33].

Four ASTM standards also provide guidance on three hand-held instrument methods which are useful in production environments for quality control purposes. These include the Webster [34] and Barcol Indentation Hardness [35] (both of which are not as sensitive to material properties as Rockwell or Brinell), and the Rockwell B-Scale Hardness, which is measured using a Newage Instrument [36]. All three of these portable methods are only standardized in ASTM for use on aluminum materials. ASTM E110 also includes portable hardness testers [37].

Finally, ISO Technical Report 29381 [38] summarizes the state of the art in deriving bulk material tensile properties from the indentation response of the material. It describes three techniques: representative stress-strain, inverse finite element analysis methods, and the use of neural networks. However, all of these techniques assume a test piece that is free of residual stresses.

FAILURE PROPERTIES

General Description Part failure is an undesirable aspect of in-service parts that is difficult to

predict. A number of mostly qualitative tests have been devised to determine a part’s or material’s resistance to failure under cyclical stress or impacts. These include impact tests and cyclical stress tests, which are often made on specimens that include pre-made cracks. These types of tests are also useful for materials that don’t strain well and rupture quickly under applied stress, thus making them problematic for deformation testing.



Properties Measured Failure property tests, which include fatigue, fracture toughness, and crack

growth tests, provide the measured material properties listed below. Definitions for many of these terms can be found in Appendix A.

Number of Cycles to Failure Stress/Strain Ranges Strain Ratio Fatigue Crack Growth Rates as a Function of Stress-Intensity Factor Range

(∆K) Fatigue Life Tensile and Compressive Stresses as a Function of Number of Fatigue Cycles Representative Cycles of Mechanical Strain Versus Stress/Temperature Plane-Strain Fracture Toughness Fracture Toughness Plain-Strain Crack-Arrest Fracture Toughness Crack-Tip Opening displacement Absorbed Impact Energy Specimen Residual Strength Creep Crack Growth Rate Threshold Stress Intensity Factor Crack Resistance Curve

Specific Tests

Fatigue In basic fatigue tests a test specimen is subjected to cyclical stresses (e.g.,

tension and compression) until the specimen fails. ASTM E466 is the basic ASTM method for room-temperature fatigue testing of metals [39] and ISO 1099 is the equivalent ISO standard test method [40], although 1099 includes guidance on performing the tests at higher temperatures. ISO 1099 provides a quantitative measure of the fatigue life, while E466 just determines whether a part will fail for a given set of material and testing conditions, such as material composition, geometry, and surface condition. The basic test subjects either un-notched or pre-notched specimens to a constant amplitude periodic axial force. ASTM E606 [41] is similar to E466, but is strain-controlled instead of force-controlled fatigue, and provides guidance on determining the fatigue life,



similar to ISO 1099. In addition, E606 determines the cyclic stresses and strains at any time during the tests.

ASTM E647 is a fatigue test that measures the rate of crack growth in a specimen [42] and determines the rate as a function of the stress-intensity factor range (∆K). The method uses cyclic loading of notched specimens that have been pre-cracked. The crack size is measured as a function of the number of fatigue cycles, and the crack growth rates are expressed in terms of ∆K, which is calculated from linear stress analysis. ISO 12108 is the equivalent ISO test [43].

ASTM E2368 is a practice for strain-controlled thermomechanical fatigue testing, which occurs when a uniform temperature and strain field over the specimen are simultaneously varied and independently controlled [44]. ISO 12111 is the equivalent ISO test method [45]. ASTM E2714 is the ASTM test method for creep-fatigue testing [46]. This test determines deformation and crack formation or crack nucleation as a result of constant-amplitude strain-controlled tests or constant-amplitude force-controlled tests (see ASTM E606, ASTM 466, ISO 12106, and ISO 1099.) These tests are typically done at elevated temperatures and involve sequential or simultaneous application of the loading conditions necessary to generate cyclical deformation or damage enhanced by creep deformation or damage. The distinction between E2714 and E466 is that E2714 involves much longer hold times. ISO 12111 is the equivalent ISO test method for strain-controlled thermomechanical fatigue testing [45].

ASTM E2760 is the ASTM test method for creep-fatigue crack growth testing [47]. E2760 covers fatigue cycling with long loading/unloading rates and/or hold times. This causes creep deformation in the pre-cracked crack tip and the creep deformation is then responsible for enhanced crack growth during each loading cycle.

ASTM E2789 is the ASTM guide for fretting fatigue testing [48]. E466 is still the basic method, but E2789 provides guidance for fatigue testing of small amplitude oscillatory tangential motion between two solid surfaces in contact. Fretting fatigue is generally characterized by a sharp decrease in fatigue life at the same stress level of a standard specimen. This decrease is due to the shortened time required to form a crack and the acceleration of the crack growth under the coupling of the fretting and bulk cyclical stresses and strains.

ISO 1143 measures the fatigue life of rotating bar bending fatigue testing [49]. This method uses metal samples with a circular cross-section, which are rotated and subjected to a bending moment. ISO 1352 is similar to ISO 1143 but is for torque-controlled fatigue testing [50]. ISO 12106 covers fatigue testing of metal samples where the axial-strain is controlled [51]. This is similar to ISO



1099 but is a low-cycle fatigue test that is performed until specimen failure. The standard has guidance on performing the test at both low and high temperatures.

Fracture Toughness

The basic fracture toughness test subjects a pre-cracked specimen to strain, in an attempt to initiate crack growth and fracture the material. The ability of a material to resist this fracture is a measure of its fracture toughness. ASTM E399 [52] and E1820 [53] are the basic ASTM test methods for determining the linear-elastic plain-strain fracture toughness of metals. The E399 test is performed on metals under linear-elastic, plane-strain conditions using fatigue pre-cracked specimens that are subjected to a slowly increasing crack-displacement force. ISO 12737 [54] is the equivalent ISO test method of E399. In ASTM E1820 a precracked fatigue test specimen is loaded to induce either stable or unstable crack extensions. ISO 12135 [55] is the ISO equivalent of E1820, and ISO 22889 is similar, but provides guidance for specimens that are very small, and hence have size sensitivities [56]. ASTM B646 [57] is the standard practice for fracture toughness of aluminum alloys, and provides information supplementary to E399 on specimen size, analysis, and interpretation of results, especially for parts of varying thickness, when aluminum alloys are tested. ASTM B645 [58] is supplementary to both E399 and B646, and is the basic test method for plane-strain fracture toughness measurements of aluminum specimens. ASTM B909 [59] provides additional guidance for fracture toughness tests of aluminum where complete stress relief of the aluminum samples is not possible and ASTM E1304 covers plane-strain fracture toughness of metal materials that have a Chevron-shaped-notch [60].

ASTM E23 contains the standard methods for absorbed impact energy measurements of notched metal bars using both Charpy and Izod test methods [61]. These two methods have differences in the shape of the notches, the bar holding mechanisms, the impact locations, and the sample dimensions. ASTM E23 has detailed information about testing at different temperatures. ISO 148-1 is the equivalent ISO Charpy test [62] and in addition provides guidance on performing the test at elevated or decreased temperatures using liquid or gaseous mediums. ISO 14556 is similar to 148-1 but is for steel materials [63].

ASTM E1221 is the ASTM test method for determining the plane-strain crack-arrest fracture toughness of ferritic steels [64]. There does not appear to be an equivalent ISO test. ASTM E1290 [65] is the ASTM test method for crack-tip opening displacement (CTOD) fracture toughness measurements. This test determines the critical CTOD values, which are used to measured cleavage crack



initiation toughness for materials that exhibit a change from ductile to brittle behavior with decreasing temperatures. Finally, ISO 27306 is a standard method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel components [66]. Specifically, this method converts fracture toughness from laboratory specimens to the equivalent value for structural components. There does not appear to be an equivalent ASTM standard.

Crack Growth

In basic crack growth testing a pre-cracked specimen is subjected to stress, and the rate of growth of the crack(s) is measured. ASTM has four standards that cover crack growth testing. ASTM E740 [13] is the practice for fracture testing with surface-crack tension specimens. This practice covers the design, preparation, and testing of surface-crack specimens, and the test is performed with a continuously increasing force, and excludes cyclical and sustained loadings. This test determines the residual strength of specimens with semi-elliptical or circular-segment fatigue cracks. ASTM E1457 [67] is a test method for measurement of creep crack growth times in metal specimens. It determines the creep crack growth in metals at elevated temperatures using pre-cracked specimens that are subjected to static or quasi-static loading conditions. ASTM E1681 [68] is a test method for determining the threshold stress intensity factor for environmentally-assisted cracking of metallic materials, and requires an environmental chamber. Finally, ASTM E2472 [69] is a test method for determination of resistance to stable crack extension under low-constraint conditions, which occurs when the crack-length-tothickness and uncracked-ligament-to-thickness ratios are greater than or equal to 4. This test is performed only under a slowly increasing remote applied displacement.

CONCLUSIONS AND NEXT STEPS As mentioned in the introduction, this report is the first step of a longer

process to develop standards appropriate for the testing of the mechanical properties of metal parts made via additive manufacturing. A future report will assess the practical applicability of the existing test methods and standards summarized in this report for use on additively-made metal parts. Nevertheless, some initial conclusions can be made.

First, this assessment shows that a large number of international, consensus-based standards covering a wide range of material mechanical properties already exists. This is fortunate because it means that the baseline technical development



for these standard tests has already been done. These tests include the typical mechanical properties specified in the introduction for both the MMPDS and a typical additive parts manufacturer. The hope is that these existing tests can be used, in either their current or slightly modified forms, on metal additive parts.

However, given the way in which these parts are manufactured, supplementary guidance will have to accompany these tests. This will be necessary to account, for example, for the anisotropy and potential porosity that may be present in these parts. Users should not assume that specimens made via additive manufacturing processes are isotropic, and the influence of anisotropy and porosity on the results obtained with these tests will need to be determined. In addition, not all of the test specimens described in these standards are easily built with additive manufacturing; very thin test specimens for example will be problematic because the resulting residual stresses may warp the specimens. Finally, there is a strong need for a careful study of the sensitivity of the material properties to both the additive manufacturing build parameters and the properties of the initial powder.

The next step is to assess the state-of-the-art measurements for the properties of metal powders. The properties of interest include measurements of particulate size, powder chemical composition, powder viscosity, and powder particulate morphology. This will be reported on in mid-2012. Following that, the suitability of these existing measurements - both mechanical properties of metal parts and properties of metal powders - will be assessed for use in additive manufacturing.

ACKNOWLEDGMENTS The authors would like to thank William Luecke (NIST/Metallurgy

Division) and David McColskey (NIST/Materials Reliability Division) for their helpful comments, suggestions, and discussions.

APPENDIX A: DEFINITIONS OF MATERIAL PROPERTY TERMS

Absorbed Impact Energy – In an Izod or Charpy test, the amount of energy

required to fracture a material. Bearing Strength – The maximum bearing stress which a material is capable of

sustaining. [70]



Brinell Hardness Number – Result from indentation hardness test in which a number proportional to the quotient obtained by dividing the test force by the curved surface area of the indentation which is assumed to be spherical and of the diameter of the ball. [70]

Chord Modulus – The slope of the chord drawn between any two specified points on the stress-strain curve. [19]

Compressive Strength – The maximum compressive stress that a material is capable of sustaining. [70]

Crack-Tip Opening Displacement – The crack displacement resulting from the total deformation (elastic plus plastic) at variously defined locations near the original (prior to force application) crack tip. [71]

Creep Crack Growth Rate – The rate of crack extension caused by creep damage and expressed in terms of average crack extension per unit time. [71]

Ductility – The ability of a material to deform plastically before fracturing. [70]

Elasto-plastic Hardness – Hardness measured from the recorded time record of the force and displacement data of an indentation during plastic and elastic deformation.

Fatigue Cycle – One complete sequence of values of force (strain) that is repeated under constant amplitude loading (straining). [71]

Fatigue Life – The number of cycles of a specified character that a given specimen sustains before failure of a specified nature occurs. [71]

Fracture Toughness – A generic term for measures of resistance to extension of a crack. [71]

Indentation Hardness – The hardness as evaluated from measurements of area or depth of the indentation made by pressing a specific indenter into the surface of a material under specified static loading conditions. [70]

Indentation Modulus – Modulus measured from the recorded time record of the force and displacement data of an indentation during plastic and elastic deformation.

Knoop Hardness Number – A number related to the applied force and to the projected area of the permanent impression made by a rhombic-based pyramidal diamond indenter having included edge angles of 172° 30’ and 130° 0’. [70]

Lower Yield Strength – The minimum stress recorded during discontinuous yielding, ignoring transient effects. [70]

Plane-Strain Fracture Toughness – The crack-extension resistance under conditions of crack-tip plane strain in Mode I for slow rates of loading



under predominately linear-elastic conditions and negligible plastic-zone adjustment. [71]

Poisson’s Ratio – The negative of the ratio of transverse strain to the corresponding axial strain resulting from an axial stress below the proportional limit of the material. [70]

Rockwell Hardness Number – A number derived from the net increase in the depth of indentation as the force on an indenter is increased from a specified preliminary test force to a specified total test force and then returned to the preliminary test force. [70]

Rupture Strength – The stress at which rupture (a failure that is accompanied by significant plastic deformation, often associated with creep failure) occurs. [3]

Scleroscope Hardness Number – A number related to the height of rebound of a diamond-tipped hammer dropped on a material being tested. [70]

Secant Modulus – On a stress-strain curve, the slope of a line between the origin and any specified stress. [3]

Shear Modulus (aka torsional modulus) – The ratio of shear stress to corresponding shear strain below the proportional limit. [70]

Strain Ratio – The ratio of width to thickness strain determined in the uniform elongation portion of a tension test. [72]

Stress-Intensity Factor – The magnitude of the mathematically ideal, crack-tip stress field for a particular mode in a homogeneous, linear-elastic body. [71]

Stress-Intensity Factor Range – In fatigue, the variation in the stress-intensity factor in a cycle. [71]

Stress-Strain Diagram (aka Stress-Strain Curve) – A diagram in which corresponding values of stress and strain are plotted against each other. Values of stress are usually plotted as ordinates and values of strain as abscissas. [70]

Tangent Modulus – The slope of the stress-strain curve at any specified stress or strain. [19]

Tensile Strength (aka Ultimate Tensile Strength) – The maximum tensile stress which a material is capable of sustaining. Tensile strength is calculated from the maximum force during a tension test carried to rupture and the original cross-sectional area of the specimen. [70]

Torque-Twist Diagram (aka Torque-Twist Curve) – In shear testing, a diagram in which corresponding values of torque and twist are plotted against each other. Values of torque are usually plotted as ordinates and values of twist as abscissas.



Upper Yield Strength – See Yield Point. Vickers Hardness Number – A number related to the applied force and the

surface area of the permanent impression made by a square-based pyramidal diamond indenter having included face angles of 136°. [70]

Webster Hardness – A hardness number measured by a Webster Hardness gauge. Webster Hardness gauges can measure a range of hardness that corresponds to 5 HRE to 110 HRE on the Rockwell hardness scale. [34]

Yield Point (aka Upper Yield Strength) – In a uniaxial test, the first stress maximum associated with discontinuous yielding at or near the onset of plastic deformation. [70]

Yield Strength – The engineering stress at which it is considered that plastic elongation of the material has commenced. [70]

Young’s Modulus – The ratio of tensile or compressive stress to corresponding strain below the proportional limit of the material. [70]

REFERENCES

[1] J. Jackson, Definition of Design Allowables for Aerospace Metallic Materials, AeroMat Presentation. (2007).

[2] EOS, Aluminum AlSi10MG Material Data Sheet (2009). [3] J. William D. Callister, Materials Science and Engineering: An

Introduction. (1994). [4] S. Kalpakjian, Manufacturing Engineering and Technology. (1995). [5] ASTM, E8: Standard Test Methods for Tension Testing of Metallic

Materials (2009). [6] ISO, 6892-1: Metallic materials -- Tensile testing -- Part 1: Method of

test at room temperature. (2009). [7] ASTM, E21: Test Methods for Elevated Temperature Tension Tests of

Metallic Materials. (2009). [8] ISO, 6892-2: Metallic materials -- Tensile testing -- Part 2: Method of

test at elevated temperature. (2011). [9] ISO, 15579: Metallic materials -- Tensile testing at low temperature.

(2000). [10] ASTM, E1450: Test Method for Tension Testing of Structural Alloys in

Liquid Helium. (2009). [11] ISO, 19819: Metallic materials -- Tensile testing in liquid helium.

(2004).



[12] ASTM, E292: Test Methods for Conducting Time-for-Rupture Notch Tension Tests of Materials. (2009).

[13] ASTM, E740: Practice for Fracture Testing with Surface-Crack Tension Specimens. (2010).

[14] ISO, 26203-1: Metallic materials -- Tensile testing at high strain rates -- Part 1: Elastic-bar-type systems. (2010).

[15] ISO, 26203-2: Metallic materials -- Tensile testing at high strain rates -- Part 2: Servo-hydraulic and other test systems. (2011).

[16] ASTM, E9: Test Methods of Compression Testing of Metallic Materials at Room Temperature. (2009).

[17] ASTM, E209: Practice for Compression Tests of Metallic Materials at Elevated Temperatures with Conventional or Rapid Heating Rates and Strain Rates. (2010).

[18] ASTM, E238: Test Method for Pin-Type Bearing Test of Metallic Materials. (2008).

[19] ASTM, E111: Test Method for Young's Modulus, Tangent Modulus, and Chord Modulus. (2010).

[20] ASTM, E143: Test Method for Shear Modulus at Room Temperature. (2008).

[21] ASTM, E1875: Test Method for Dynamic Young's Modulus, Shear Modulus, and Poisson's Ratio by Sonic Resonance. (2008).

[22] ASTM, E1876: Test Method for Dynamic Young's Modulus, Shear Modulus, and Poisson's Ratio by Impulse Excitation of Vibration. (2009).

[23] J. A. Slotwinski and G. V. Blessing, Ultrasonic Measurement of the Dynamic Elastic Moduli of Small Metal Samples, Journal of Testing and Evaluation. 27 (2), 164-166 (1999).

[24] ASTM, E140: Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, and Scleroscope Hardness. (2007).

[25] ASTM, E10: Test Method for Brinell Hardness of Metallic Materials. (2010).

[26] ISO, 6506: Metallic materials -- Brinell hardness test -- Part 1: Test method. (2005).

[27] ASTM, E18: Test Methods for Rockwell Hardness of Metallic Materials. (2008).

[28] ISO, 6508: Metallic materials -- Rockwell hardness test -- Part 1: Test method (scales A, B, C, D, E, F, G, H, K, N, T). (2005).



[29] ASTM, E384: Test Method for Knoop and Vickers Hardness of Materials. (2011).

[30] ISO, 4545: Metallic materials -- Knoop hardness test -- Part 1: Test method. (2005).

[31] ISO, 6507-1: Metallic materials -- Vickers hardness test -- Part 1: Test method. (2005).

[32] ASTM, E448: Practice for Scleroscope Hardness Testing of Metallic Materials. (2008).

[33] ISO, 14577-1: Metallic materials -- Instrumented indentation test for hardness and materials parameters -- Part 1: Test method. (2002).

[34] ASTM, B647: Standard Test Method for Indentation Hardness of Aluminum Alloys by Means of a Webster Hardness Gage. (2010).

[35] ASTM, B648: Standard Test Method for Indentation Hardness of Aluminum Alloys by Means of a Barcol Impressor. (2010).

[36] ASTM, B724: Standard Test Method for Indentation Hardness of Aluminum Alloys by Means of a Newage, Portable, Non-Caliper-Type Instrument. (2006).

[37] ASTM, E110: Standard Test Method for Indentation Hardness of Metallic Materials by Portable Hardness Testers. (2010).

[38] ISO, 29381: Metallic materials -- Measurement of mechanical properties by an instrumented indentation test -- Indentation tensile properties. (2008).

[39] ASTM, E466: Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials. (2007).

[40] ISO, 1099: Metallic materials -- Fatigue testing -- Axial force-controlled method. (2006).

[41] ASTM, E606: Practice for Strain-Controlled Fatigue Testing. (2004). [42] ASTM, E647: Test Method for Measurement of Fatigue Crack Growth

Rates. (2011). [43] ISO, 12108: Metallic materials -- Fatigue testing -- Fatigue crack growth

method. (2002). [44] ASTM, E2368: Practice for Strain Controlled Thermomechanical

Fatigue Testing. (2010). [45] ISO, 12111: Metallic materials -- Fatigue testing -- Strain-controlled

thermomechanical fatigue testing method. (2011). [46] ASTM, E2714: Test Method for Creep-Fatigue Testing. (2009). [47] ASTM, E2760: Test Method for Creep-Fatigue Crack Growth Testing.

(2010). [48] ASTM, E2789: Guide for Fretting Fatigue Testing. (2010).



[49] ISO, 1143: Metallic materials -- Rotating bar bending fatigue testing. (2010).

[50] ISO, 1352: Metallic materials -- Torque-controlled fatigue testing. (2011).

[51] ISO, 12106: Metallic materials -- Fatigue testing -- Axial-strain-controlled method. (2003).

[52] ASTM, E399: Test Method for Linear-Elastic Plane-Strain Fracture Toughness Ic of Metallic Materials. (2009).

[53] ASTM, E1820: Test Method for Measurement of Fracture Toughness. (2011).

[54] ISO, 12737: Metallic materials -- Determination of plane-strain fracture toughness. (2010).

[55] ISO, 12135: Metallic materials -- Unified method of test for the determination of quasistatic fracture toughness. (2002).

[56] ISO, 22889: Metallic materials -- Method of test for the determination of resistance to stable crack extension using specimens of low constraint. (2007).

[57] ASTM, B646: Standard Practice for Fracture Toughness Testing of Aluminum Alloys. (2006).

[58] ASTM, B645: Standard Practice for Linear-Elastic Plane-Strain Fracture Toughness Testing of Aluminum Alloys. (2010).

[59] ASTM, B909: Standard Guide for Plane Strain Fracture Toughness Testing of Non-Stress Relieved Aluminum Products. (2006).

[60] ASTM, E1304: Test Method for Plane-Strain (Chevron-Notch) Fracture Toughness of Metallic Materials. (2008).

[61] ASTM, E23: Test Methods for Notched Bar Impact Testing of Metallic Materials. (2007).

[62] ISO, 148-1: Metallic materials -- Charpy pendulum impact test -- Part 1: Test method. (2009).

[63] ISO, 14556: Steel -- Charpy V-notch pendulum impact test -- Instrumented test method. (2000).

[64] ASTM, E1221: Test Method for Determining Plane-Strain Crack-Arrest Fracture Toughness, Ia, of Ferritic Steels. (2010).

[65] ASTM, E1290: Test Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurement. (2008).

[66] ISO, 27306: Metallic materials -- Method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel components. (2009).



[67] ASTM, E1457: Test Method for Measurement of Creep Crack Growth Times in Metals. (2007).

[68] ASTM, E1681: Test Method for Determining Threshold Stress Intensity Factor for Environment-Assisted Cracking of Metallic Materials. (2008).

[69] ASTM, E2472: Test Method for Determination of Resistance to Stable Crack Extension under Low-Constraint Conditions. (2006).

[70] ASTM, E6: Terminology Relating to Methods of Mechanical Testing. (2009).

[71] ASTM, E1823: Terminology Relating to Fatigue and Fracture Testing. (2010).

[72] online: http://www.worthingtonsteel.com. Accessed

End Notes

1 http://www.nist.gov/el/isd/sbm/matstandaddmanu.cfm 2 http://www.nist.gov/el/isd/sbm/fundmeasursci.cfm 3 The MMPDS is the Metallic Materials Properties Database and Standardization. This database

replaced MIL-HDBK-5 and is the preeminent source for aerospace component design allowables. See http://http://projects.battelle.org/mmpds/ for further information.



Chapter 2

PROPERTIES OF METAL POWDERS FOR ADDITIVE MANUFACTURING: A REVIEW OF THE STATE OF THE ART OF METAL POWDER

PROPERTY TESTING∗

April Cooke and John Slotwinski

ABSTRACT

This chapter investigates the current state-of-the-art methods for determining the properties of metal powders. This assessment evaluates both existing standards that provide guidance for powder property measurement methods, as well as technical publications describing methods that are not covered as completely by standards.

INTRODUCTION This report is the second in a series of reports from the National Institute

of Standards and Technology’s (NIST’s) Engineering Laboratory project titled Materials Standards for Additive Manufacturing.1 This project provides the measurement science for the additive manufacturing (AM) industry to measure ∗ This is an edited, reformatted and augmented version of a report, NISTIR 7873, issued by the

National Institute of Standards and Technology, July 2012.


April Cooke and John Slotwinski 22

material properties in a standardized way. Currently there are few consensus-based standards in this area. This project, in conjunction with NIST’s Fundamental Measurement Science for Additive Processes project,2 will provide the technical foundation necessary to develop new consensus-based standards. Development of standards will be done via ASTM-International’s (hereafter referred to as ‘ASTM’) Committee F42 on Additive Manufacturing Technologies and the newly formed International Organization for Standardization (ISO) TC261 committee on Additive Manufacturing.

Determining the properties of the powder used for metal-based additive manufacturing, as well as the properties of the resulting bulk metal material, is a necessary condition for industry to be able to confidently select powder and produce consistent parts with known and predictable properties. By 2014, the project team will develop and deliver enhanced measurement techniques that support new, standardized methods for quantifying the material properties of both the powders used for additive manufacturing and the resulting manufactured products.

The project’s research plan includes assessments of the current state-of-the-art testing methods for determining properties of both bulk metal materials, as reported in [1], and raw metal powder, which is the focus of this report. These methods will then be evaluated for applicability and enhanced for use on additively manufactured parts and powder used as raw material for powder-bed fusion additive manufacturing processes. NIST’s Direct Metal Laser Sintering (DMLS) machine will be used to make parts, and these new testing methods will be rigorously implemented. Using these enhanced methods, the sensitivity of part material properties to variations in initial powder properties will be determined. This is a critical step necessary for determining the scopes of relevant material standards for additive manufacturing and for the production of additive manufacturing parts with consistent properties.

Metal powders for additive processes are typically made through one of two processes, gas atomization and plasma rotating electrode process (PREP). Gas atomized powders are the most commonly used raw materials for the additive manufacturing of metal parts [2]. They are made by disrupting a molten metal stream with one or more gas jets. In PREP, a rotating bar of feedstock is arced with gas plasma, and the molten metal is centrifugally atomized as it is flung off the bar. The molten metal droplets then cool into spherical powder particles [3].

In order to perform this assessment of the existing test methods for metal powder properties, 21 relevant consensus-based ASTM standards and 31


Properties of Metal Powders for Additive Manufacturing 23

technical publications were surveyed. The report is organized into the most important properties of metal powders, which are size, morphology, chemical composition, flow, thermal properties, and density. Test methods for these properties described in various ASTM standards and other literature are summarized. Sampling methods, which are important to ensure that measurements of small powder samples are representative of the entire batch, are also covered.

TESTING OF METAL POWDER PROPERTIES

Powder Sampling Due to large lot sizes it is not practical to measure an entire batch of

powder. Instead, smaller representative samples are taken from the batch, and these are characterized. Sampling must be done judiciously to ensure that the measured properties of these smaller samples are representative of the entire batch of powder. A detailed synopsis of powder sampling techniques is provided in another NIST report, which addresses the determination of the particle size distributions of ceramic powders [4]. When a sample is taken from the batch of powder, it is preferable that the powder be in motion (i.e., being poured from one container to another) as the sample is taken from it. However, if motion is not possible, samples can be taken through a static sampling procedure using a device called a “sample thief,” which is shown in Figure 1. This is a long rod with a sampling chamber along its length and a device that can open and close the sampling chamber at the operator’s end. This tool should be placed in multiple representative locations within an amount of powder to extract powder representative of the bulk lot. For example, if the powder is located in a bag, representative locations include the bottom, center, top, front, and rear of the bag.

Figure 1. Sample Thief.



After bulk sampling, the material needs to be further sub-divided in order to make the sample size manageable. The methods described in [4] include Cone and Quartering, Scoop Sampling, Table Sampling, Chute Splitting, and Spin Riffling. Cone and Quartering involves mounding a pile of powder into a cone-shaped heap, flattening it with a spatula, dividing it into four sections, and repeating the process on one of the sections so that the final sample is 1/16th the size of the original sample. Scoop Sampling simply involves using a scoop to select a portion of the bulk sample. Table Sampling involves pouring the bulk sample of powder down an inclined plane that has a series of structures and holes used to divide it. Chute Splitting is a process in which samples are divided into two lots via dispersion through a series of chutes. Spin Riffling involves pouring the bulk sample into a hopper that empties onto a vibratory chute that leads to a series of sample containers contained in a rotating ring. Figure 2 shows illustrations of a table sampler, a chute splitter, and a spin riffler. Allen [5] reported that the statistically significant results provided by the spin riffler are so superior to other methods that it should be used whenever possible.

(a ) (b) (c)

Figure 2. Illustrations of (a) a table sampler, (b) a chute splitter, and (c) a spin riffler.

ASTM B215-10 [6] also describes procedures for sampling metal powders. This standard focuses on metal powders transferred from blenders or storage tanks, as well as metal powders already packaged in containers such as bags. Specific instructions for the method of collecting metal powder from a moving stream are also included in this standard. If the lot of powder to be sampled is flowing into one container, then the guidance is to pass a rectangular vessel at a constant speed through the stream of flowing powder



when the container to which the powder is flowing is 1/4, 1/2, and 3/4 full. If the lot of powder is to fill several containers, then the first portion of the sample should be taken when the first container is 1/2 full, the second portion of the sample should be taken in the middle of the run, and the third portion of the sample should be taken near the end of the run.

Size Determination Methods In additive manufacturing, powder particle size determines the minimum

part layer thickness, as well as the minimum buildable feature sizes on a part. Several methods were described in the literature [4] for particle size determination, including sieving, gravitational sedimentation, microscopy-based techniques, and laser light diffraction. Sieving is the technique of shaking powder through a stacked series of sieves with decreasing mesh sizes. Gravitation sedimentation uses X-ray absorption or light scattering to determine the particle concentration at different heights within a monitored container of liquid at different times. Microscopy-based techniques use optical light microscopes, scanning electron microscopes (SEM), or transmission electron microscopes (TEM) to visually discern size information. Laser light diffraction involves deconvolving and inverting the summation of the scattered light pattern produced by each sampled particle.

Several ASTM standards address particle size determination. More detailed specifications for the sieving process than what is discussed in NIST SP 960-1 are provided in ASTM B214-07(2011) [7]. This standard suggests arranging a group of sieves in consecutive order by the size of their openings, with the coarsest sieve on top and a solid collecting pan below the bottom sieve. The powder is placed in the top sieve, covered, and the entire apparatus is fastened into a sieve shaker where it is agitated for 15 min. The sieve mesh sizes can range from 5 µm to 1 mm, and the sieve mesh, or wire cloth, is commonly made of bronze, brass, or stainless steel. ASTM B214-07(2011) also states that the entire sieve apparatus should be 200 mm in diameter and either 25.4 mm or 50.8 mm deep. However, ASTM E161-00(2010) [8] states that the sieve can be as small as 76.2 mm in diameter with the same depth range.

A method of using a horizontally collimated beam of X-rays of constant intensity to measure the relative mass concentration of particles in a liquid medium is described in ASTM B761-06(2011) [9]. First the intensity of the X-ray beam that is projected through a clear liquid medium in the central region (vertically) of



a cell is measured to establish a reference. Then the cell is filled with the homogenous suspension of powder that is continuously circulated, and the intensity of the X-ray is measured after it passes through. The powder particles absorb some of the X-ray energy, and this establishes a value for full scale attenuation. Then the circulation of the mixture is ceased, which allows the particles to settle to the bottom of the cell. While this is happening, the X-ray intensity is monitored. During sedimentation, the larger particles fall below the X-ray beam at a faster rate than the smaller particles. Each mass measurement represents the cumulative mass fraction of the remaining fine particles. Eventually, all of the particles settle, which allows the X-ray to pass through the cell unattenuated, and the intensity value reaches the value of the reference. The particle size can be determined from velocity measurements by applying Stokes’ law, since the conditions of liquid density and viscosity and particle density are known. Stokes’ law demonstrates that the terminal settling velocity of a spherical particle in a fluid medium is a function of the diameter of the particle [10]. This process is applicable to particles of uniform density and composition having a particle size distribution of 0.1 µm to 100 µm. Additionally, the relationship between size and sedimentation velocity assumes that the particles settle within the laminar flow regime. Therefore, analysis for particles that settle with a Reynold’s number greater than 0.3 may be incorrect due to turbulent flow [9].

The use of light scattering to measure the particle size distribution is described in ASTM B822-10 [11]. In this technique, which is applicable for powders whose particle diameters are in the range of 0.4 µm to 2 mm, a sample of the material is dispersed in liquid and circulated through the path of a light beam. Alternatively, a dry sample can be aspirated through light in a carrier gas. The light beam is scattered by the passing particles. Photodetector arrays collect the scattered light and convert it into electrical signals, which are then analyzed. The analyzed signal is converted to size distribution through the theories of Fraunhofer Diffraction, Mie Scattering, or a combination of both [11]. Fraunhofer Diffraction approximates the laws of light scattering, while Mie Scattering represents the laws of scattering more precisely with a solution to Maxwell’s Equations, which are the set of four partial differential equations that govern the behavior of electric and magnetic fields [12].

In a study by Biancaniello [13], it was determined that using the ASTM B214 method with a modified sieve measurement, incorporating electroformed micromesh sieves with openings of 5 µm and larger provided particle size distribution data that was more reliable than data produced through sedimentation and light scattering techniques.



Morphology Characterization Methods The morphology of powder particles determines how well the particles lay

or pack together and thus, in additive manufacturing, is an important factor in realizing the minimum part layer thickness and density. Hawkins [14] offers a comprehensive review of many of the principles for characterizing the morphology of powder particles. The first principle is the importance of ensuring that every particle of a powder lot has an equal chance of being selected for observation. This is done through the previously discussed sampling techniques.

The next principle discussed in Hawkins’ book indicates that silhouettes of the individual powder particles must be made visible so that their geometric shape can be characterized. Observation of the silhouettes can be performed by a method described by Cox [15], during which “the grains may be sprinkled over a lantern slide that has been previously covered with glue or a suitable cement, the slide inserted in a projection camera, and the image obtained on the screen.” This method, archaic in its description, has been modernized through the use of current microscopy techniques. The first and simplest means of morphology characterization is qualitative. Over the years, researchers have assigned adjectives to certain shapes, and most recently, ASTM B243-11 [16] established definitions for many powder shapes. The relevant terms are listed in Table 1. The fact that some of these definitions are similar suggests that the terms are not defined through scientific rigor.

Table 1. Terms describing powder shapes in ASTM B243-11

Term Definition acicular needle-shaped

flake flat or scale-like, whose thickness is small compared with other dimensions

granular approximately equidimensional, nonspherical shapes irregular lacking symmetry needles elongated and rod-like nodular irregular, having knotted, rounded, or similar shaped platelet composed of flat particles having considerable thickness plates flat particles of metal powder having considerable thickness spherical globular-shaped



Figure 3. Shapes useful in illustrating ambiguity of single-number classification system (e.g., projected area).

Other means of powder morphology characterization are quantitative. One method is single-number classification, in which a shape is defined by using only one number associated with a feature of a particle [14]. The single-number classification has two disadvantages. First, it is ambiguous in that more than one outline shape can have the same number. An example of this situation is presented in Luerkens [17], and the shapes displayed in Figure 3 are useful in its visualization.

All of the objects in Figure 3 have the same projected area, because they are all constructed from the same number of squares. If the number used to characterize these shapes is the projected area, then this is an example of how more than one shape can be defined by the same number.

The second disadvantage of the single-number classification system is that a number alone is not descriptive enough to enable the reproduction of the shape. However, they are mathematically convenient and can be associated with the shape characteristic of interest such as the projected area. The single-number classification system is divided into four groups: dimensional, sphericity, roundness, and perimeter.

In the dimensional group the form of a large particle can be expressed by a parameter that is based on the longest, shortest, and intermediate orthogonal axes. This notion is discussed in Barrett [18], which is a review paper on the subject that tabulates 12 of the dimensional parameters that have been used since 1920. Table 2 provides five of them.



Table 2. Dimensional parameters used to characterize large particles, where L = Long Axis, S = Short Axis, and I = Intermediate Axis [18]

Formula Name/Description Range

Flatness Index 1 to ∞

Ordinate and Abscissa for a Plot to Characterize Shape 0 to 1

Elongation 0 to 100

Flatness 0 to 100

Flatness 0 to 1

However, it is generally accepted that small particles can be evaluated in

two dimensions [14], thus allowing for the use of measurements taken of particle silhouettes. The terms of the single-number classification groups mentioned and described from this point forward are two-dimensional. One dimensional characteristic is the elongation of a grain, or the degree of anisometry (having unsymmetrical parts or unequal measurements). This is obtained by the measurement of the greatest length (L) and the greatest width at right-angles (B), and it is expressed as:

Elongation = L/B or B/L. (1) Since small particle outlines may not be perfect, or even representative of

geometric figures, statistical diameters have sometimes been used as dimensional characterizations. Two statistical diameters, “Feret’s Diameter” and “Martin’s Diameter” are illustrated in Figure 4.

“Feret’s Diameter” is the distance between two tangents on opposite sides of the particle being evaluated. “Martin’s Diameter” is the distance between opposite sides of the particle measured crosswise of the particle and on a line bisecting the projected area. As illustrated in Figure 4, the two diameters are different for the same outline.



Another dimensional characterization is called the breadth of a particle. This was defined by Heywood [19] as the minimum distance between two parallel lines tangential to the projected outline of the particle when placed in the most stable position. Figure 5 illustrates this definition, as well as the length of the particle, which is the distance between two lines that are perpendicular to the tangents defining breadth and also tangent to the projected outline of the particle.

Figure 4. Illustration of (a) “Feret’s Diameter” and (b) “Martin’s Diameter.”

Figure 5. Tangential lines defining the distances of breadth (B) and length (L).



Using the breadth (B) and length (L), the two following ratios can be formulated:

Length Ratio = L/B, and (2)

(3) The next group in the single-number classification system is the sphericity

group. Using the term “sphericity” as a two-dimensional description of a particle

silhouette, although confusing, has been practiced for many years, and the best method of attaining the sphericity of a particle has long been debated. There are many different equations used to define it. Wadell [20] defines it as follows:

(4) Another definition is given by Riley [21]:

(5) The third group in the single-number classification system is the

roundness group, which defines roundness as the sharpness or smoothness of the perimeter of the silhouette of the particle.

Analogous to sphericity, there are several methods of obtaining a roundness value. One method was introduced by Wadell [22] and is shown in Figure 6.



Figure 6. Illustration of technique used by Wadell to attain roundness values.

The illustration in Figure 6 depicts that the radius of curvature (r) of each of the N projections (which contain the solid, red arcs) and the radius (R) of the maximum inscribed circle (denoted by the green, dashed circle) are measured. The method of attaining the roundness is to divide the average of the radii (r) of the N corners of the silhouette by the radius of the maximum inscribed circle (R), as shown in Equation 6.

(6) Two other equations used to determine a roundness value were introduced

by Wentworth [14] and Cailleux [14]. They use the two distances, illustrated in Figure 7, of the greatest length (L) of the silhouette outline and the greatest breadth (B), which is the perpendicular to the line indicating the greatest length.

Figure 7. The greatest length (L) and greatest breadth (B) of a particle silhouette.



Using the distances depicted in Figure 7, Wentworth’s equation for roundness [14] is

(7)

and Cailleux’s equation for roundness [14] is

(8) Finally, the perimeter group describes the circularity shape factor, widely

accepted as the ratio of the square of the perimeter of the particle outline to 4ð

times the cross-sectional or projection area of the particle outline [14]. The equation form of this expression is as follows:

(9) Representation by series involves approximating the outlines of powder

particles with complicated mathematical formulas. For example, an article by Schwarcz and Shane [23] poses that a Fourier series expression for the boundary of a particle silhouette can be expressed as follows:

(10)

where the constants ai, bi, and a0 are defined as:

(11)

(12)



(13) As the order of the Fourier series increases, the accuracy of the

approximation increases. This phenomenon can be simplified for better visualization. Figure 8 shows a series of six points, fit with curves of equations with increasing degrees. These curves can be used to approximate a surface created by those six points. The solid, red line represents a linear function fit to the data. The green, dashed line represents parabolic function fit to the data. A sixth-order polynomial is fit to the data, and it is represented by the purple, dotted line. Figure 8 clearly indicates that as the complexity of the equations used for curve-fitting increases, the accuracy of the approximation increases as well. By using Fourier series, a surface as complex as a human skull can be represented, which was accomplished by Lu [24].

Figure 8. Series of six points with curves fit to them.

Chemical Composition Test Methods

In practice raw metal powders used in additive manufacturing are not 100

percent pure, and contain other materials. These added materials consist of



different elements that when combined and used in AM produce material properties specific to the bulk material type. Other materials might be introduced to ensure powder fidelity. The overall chemical composition of the metal powder may be a factor in the final part’s mechanical properties. Powder properties may also change over time, or in the case of recycled powder, due to repeated exposures in an AM build chamber environment. The techniques that can be used to determine the chemical composition of powders include micro analysis methods, surface analysis methods, and bulk analysis methods. The descriptions of these methods are as they are applied to bulk material, so the term “specimen” refers to bulk material, such as a block of metal, unless otherwise stated.

Micro Analysis Methods Common micro analysis methods are energy dispersive spectroscopy and

the use of an electron microprobe. Energy dispersive spectroscopy takes advantage of the fact that each element has a unique atomic structure. A high-energy beam of charged particles or a beam of X-rays is focused onto the sample. The incident beam may excite an electron, which would cause it to jump into a higher energy level, creating a hole where the electron was. An electron from a higher-energy level fills the hole and the difference in energy between the two levels is released in the form of an X-ray. The intensity and wavelength of the X-rays are measured with an energy dispersive spectrometer. The energy of the X-rays is characteristic of the difference in energy between the two energy levels and of the atomic structure from which they were emitted, thus allowing the elemental composition of the specimen to be determined [25].

An electron microprobe focuses an electron beam onto the surface of a specimen. An X-ray is emitted as a result, and its intensity is recorded. The intensity is then divided by the intensity of the X-rays emitted by the pure substance. The result is equal to the mass concentration of the element at that location on the specimen. This method is used to determine the composition of a volume on the order of (10 to 30) µm3 or less. Also, the elements making up the specimen have to be known beforehand [26], so it is not applicable for samples that have unknown constituents.



Surface Analysis Methods Surface analysis methods are atomic emission spectroscopy, X-ray

photoelectron spectroscopy, and secondary ion mass spectrometry. Atomic emission spectroscopy uses the intensity of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample. The wavelength of the atomic spectral line identifies the element while the intensity of the emitted light is proportional to the number of atoms of the element [27].

X-ray photoelectron spectroscopy, also called electron spectroscopy for chemical analysis (ESCA), is another method of determining the chemical composition of a specimen. Spectra are obtained by flooding X-rays onto the surface of the specimen while simultaneously measuring the kinetic energy and number of electrons that escape from the top 1 nm to 10 nm of the material. This procedure requires ultra-high vacuum conditions, and it detects all elements with an atomic number of 3 or higher. The detection of Hydrogen and Helium (atomic numbers of 1 and 2, respectively) is not possible because the diameters of these atoms are so small [28].

Secondary ion mass spectrometry sputters the surface of the specimen with a focused primary ion beam and collects and analyzes ejected secondary ions. Secondary ions are measured with a mass spectrometer to determine elemental, isotopic, or molecular composition of the surface. This process requires a high vacuum, and is a very sensitive surface analysis technique that can determine quantities to parts per billion [29].

Bulk Analysis Methods Relevant bulk analysis methods include atomic absorption spectroscopy,

inductively coupled plasma optical emission spectroscopy, flame emission spectroscopy, X-ray fluorescence, X-ray powder diffraction, and inert gas fusion. Atomic absorption spectroscopy requires the sample to be atomized and reference samples with a known chemical composition to establish the relation between the measured absorbance and the elemental concentration. Energy is passed through the atomized sample, and each element absorbs a particular wavelength of light, since each element requires a unique amount of energy to promote its electrons into higher energy levels. The radiation flux with the sample in the atomizer is measured by a detector, and the radiation flux without the sample in the atomizer is measured by the same detector. The



ratio between the two values (the absorbance) is converted to component concentration or mass using the Beer-Lambert Law [30].

Inductively coupled plasma optical emission spectroscopy uses liquid samples that are injected into plasma using a nebulizer or other method of sample introduction. The mist is quickly dried, vaporized, and energized through collisional excitation at a high temperature (10 000 K), which promotes the atoms to excited states. The excited state atoms/ions may relax to a ground state through the emission of a photon. This atomic emission is then analyzed. The photons have characteristic energies, so the wavelengths can be used to identify the elements from which they originated. The total number of photons is directly proportional to the concentration of the originating element in the sample [31].

Flame emission spectroscopy is the process of introducing a specimen in the form of a gas or sprayed solution into a flame. The heat creates free atoms by evaporating the solvent and breaking chemical bonds. The atoms are excited by the addition of the thermal energy, so they emit light as they return to relaxed states. Characteristic wavelengths are emitted by each element, which are detected by a spectrometer [32].

In X-ray fluorescence, a specimen is excited due to exposure to X-rays or gamma rays. This causes the ejection of an electron from an atom and the emission of a photon when an electron from a higher energy level falls into the ejected electron’s original energy level. The emitted radiation has an energy that is characteristic of the atoms present [33].

X-ray powder diffraction exploits the crystalline structure of a material. X-rays are aimed at a sample, and some of the photons in the incident beam are deflected upon collision with electrons, much like billiard balls bouncing off of each other. If the atoms are arranged periodically, the diffracted waves will form interference peaks with a symmetry that matches that of the distribution of atoms. This allows the determination of the distribution of atoms in the sample, thus enabling its identification [34].

The inert gas fusion technique, which is used to determine the oxygen, nitrogen, and hydrogen content in metals, is described by ASTM E1409-08 [35], E1447-09 [36], E1569-09 [37], and E2792-11 [38]. In this process a sample is held in a chamber directly above a graphite crucible. The crucible is brought to an extremely high temperature (roughly 3000°C). At the same time, the inert gas flows over it to remove any contaminants in what is called an “out-gassing” procedure. After “out-gassing,” the crucible temperature is lowered and the sample is added to it by lowering it from above. As the sample melts, the oxygen present reacts with the carbon in the graphite crucible to form Carbon Monoxide (CO) and Carbon Dioxide (CO2). The Nitrogen present is released as molecular



Nitrogen (N2), and the Hydrogen present is released as Hydrogen gas (H2). The gases are carried out of the chamber onto a detector by the inert gas flow.

Flow Characteristics Test Methods How well a powder flows is an important factor for many additive

manufacturing processes. Methods of determining the flow rate of powders using two types of flowmeters, the Hall flowmeter funnel and the Carney funnel, are described in ASTM B213-11 [39] and ASTM B964-09 [40], respectively. There are two types of methods for determining the flow rate: a static flow method and a dynamic flow method. The static flow method requires the user to cover the funnel opening with a dry finger and pour in a predetermined mass of powder. Then, when all the powder is loaded, the process is to start timing the flow of powder through the opening upon removal of the finger and stop timing as soon as the last of the powder has egressed. The dynamic flow method doesn’t involve covering the orifice with a dry finger. The timing begins as soon as the powder begins flowing out of the funnel and stops when all powder has finished flowing through the funnel. The flow rate value, reported in units of seconds per gram, is determined by dividing the measured time taken for all of the powder to exit the funnel by the mass of the powder sample. The difference between the Hall flowmeter funnel and the Carney funnel is the size of their orifice, with the Hall flowmeter having a 2.54 mm opening, and the Carney flowmeter having a 5.08 mm opening. The Carney flowmeter is used only for powder that will not flow through the Hall flowmeter.

The process of timing powder as it flows through a Hall flowmeter is described in ASTM B855-11 [41]. However, this specifies measuring the flow by volume instead of by mass, thereby eliminating the variable of powder density. The ability to flow and pack is a function of interparticle friction, so as the surface area increases, the friction increases, which gives less efficient flow and packing. The acquisition of the correct volume of powder to use requires the use of an Arnold meter, a process that will be described later in the Density section of this report. Once the volume to be tested is attained, the process follows that of ASTM B213-11. This flow rate value is determined by dividing the measured time taken for all of the powder to exit the funnel by the volume of the powder sample, and it is reported in units of seconds per cm3.



Thermal Characterization Methods Melting of powder by a high energy source is the essence of several AM

processes. As such, thermal properties of powders are extremely important to AM. Sih and Barlow [42] provide a good summary of techniques used to evaluate the thermal conductivities of materials, dividing the techniques into two categories: steady-state and transient. While Sih and Barlow claim that the methods are applicable to powders, the descriptions that follow assume the specimen to be solid, bulk material in the required shape for the measurement technique. The first steady-state method is the Plate Method. An example of this method is the Guarded-Hot-Plate Method described in ASTM C177-10 [43]. Figure 9 is a diagram of a guarded-hot-plate apparatus.

Figure 9. Diagram of a Guarded-Hot-Plate apparatus.

The components used in this method are two isothermal cold plates, a guarded-hot-plate, and two samples of the test specimen. The two samples of the test specimen sandwich the guarded-hot-plate, and the two isothermal cold plates then sandwich the test specimen/guarded-hot-plate combination. The necessary values for the calculation of the thermal conductivity of the specimen are the power supplied to the guarded-hot-plate, various surface temperature measurements, and the area and thickness of the specimen



samples. Using these quantities, the thermal conductivity of the specimen can be determined through heat transfer principles.

Another steady-state method of determining the thermal conductivity of a material is the Cylindrical Method. In this method, a heater resides along the axis of a cylinder of material with an unknown thermal conductivity. Guard rings are located at each end of the cylinder to prevent heat loss out of the ends. Power is supplied to the heater so that heat is radiated outward from the center of the cylinder. The temperatures at two different radii are measured and heat transfer principles are used to determine the thermal conductivity of the sample [42].

Finally, a third steady-state method consists of Spherical and Ellipsoidal Methods. In these methods, a spherical heater is placed in the center of a spherical or ellipsoidal sample. Similar to the Cylindrical Method, power is supplied to the heater so that heat radiates outward from the center of the sample. Temperatures are measured at different radii and heat transfer principles allow the calculation of the thermal conductivity [42].

In all of the steady-state methods, the calculation of the thermal conductivity involves simple heat transfer equations, and the heat losses by radiative heat transfer are assumed to be negligible. However, there is often difficulty in obtaining the required specimen shape. On the other hand, with transient methods, obtaining the thermal conductivity is much quicker although the calculations are more involved.

One transient technique for measuring thermal conductivity is the Transient Hot Wire Method. For this method a long, thin heating wire is placed into a large specimen of material that is initially at a uniform temperature. The heater emits a constant heat when power is supplied to it. While heating up, the temperature at a specific point within the sample it monitored as a function of time. Heat transfer principles allow the calculation of the thermal conductivity by using two of the temperatures recorded at two discrete times. The Thermal Probe Method is quite similar to the Transient Hot Wire Method, but the heat source is enclosed inside a probe. This protects the wire and allows for easier insertion into the sample [42].

The Transient Hot Strip Method is another transient technique used to determine the thermal conductivity of a sample. In this method, a thin metal strip is placed within the specimen. The strip acts as a heat source and indicates its own temperature increase as well. A temperature increase causes an increase in the electrical resistance of the strip, so by monitoring the output voltage of the strip, one can obtain information about the thermal conductivity of the surrounding material [44]. A constant current is supplied to the metal strip, which provides a



constant output of power. The voltage is monitored in the strip, and a change in voltage is indicative of a temperature increase within the strip.

A fourth transient method used to determine the thermal conductivity of a sample is the Flash Method. In this method a brief, high-intensity light pulse is focused onto the surface of a specimen (a bed of powder). The temperature of the surface on the opposite side of the specimen is then measured. The thermal diffusivity is determined by the shape of the curve of temperature versus time at the opposing surface. The heat capacity is determined by the maximum temperature reached by the opposing surface. Finally, the thermal conductivity is determined by multiplying the heat capacity, thermal diffusivity, and the density [45, 46].

Density Determination Methods Several methods for determining the apparent density of metal powder are

specified by ASTM standards. The apparent density is the ratio of the mass to a given volume of powder. The method of determining apparent density using the Hall flowmeter funnel is described in ASTM B212-09 [47]. The basic process is to let powder flow through the Hall flowmeter into a cup of definite volume. The powder should mound over the cup and a nonmagnetic spatula should be used to level off the top surface of the powder flush with the sides of the cup. Then, the cup of powder should be placed on a balance to determine the mass. The apparent density is the measured mass divided by the volume. The process of determining the apparent density of metal powder through the use of a Carney funnel is outlined in ASTM B417-11 [48]. This process mirrors that of the apparent density determination process using the Hall flowmeter funnel, only the Carney flowmeter funnel is used instead.

The process of determining apparent density using the Scott volumeter is described in ASTM B329-06 [49]. Figure 10 shows an illustration of this apparatus.

In this process, powder that is poured into a series of funnels at the top of the apparatus travels through the funnels, then through a series of baffles, into another funnel under the baffle box, and into a collection cup upon exiting the final funnel. The collection cup is located within an overflow tray, so after it is filled, the entire tray can be removed. The powder should form a heap over the collection cup so that a nonmagnetic spatula can scrape the excess off into the overflow tray, making the powder flush with the sides of the container. Then



the collection cup is placed on a balance to determine the mass, which is divided by the volume to determine the density.

Figure 10. Illustration of a Scott volumeter.

The technique of determining the apparent density using the Arnold meter is described in ASTM B703-10 [50]. An Arnold meter is a steel block with a cavity in the middle and powder delivery sleeve. Figure 11 illustrates an Arnold meter. In this process, the powder delivery sleeve is placed on either side of the die cavity. Powder is poured into the sleeve, and it is slid across the cavity to allow powder to fall through and fill the die cavity. Then the sleeve is slid back across the cavity to level the amount of powder flush with the steel block. The amount of powder in the die cavity is then placed on a balance to acquire the mass. The apparent density is the mass divided by the volume of the die cavity.



Figure 11. Illustration of an Arnold meter.

The method of determining the tap density of metallic powders and compounds is described in ASTM B527-06 [51]. The tap density is the density of a powder that has been tapped to settle the contents in a container. The basic process is to pour a known mass of a powder specimen into a graduated cylinder. The cylinder must be loaded into a tapping apparatus that permits tapping against a firm base at a rate between 100 taps per minute and 300 taps per minute. When there is no further decrease in volume due to tapping, that volume is used in the calculation of the tap density, which is the mass divided by the volume.

The method of obtaining the skeletal density of metal powders by Helium or Nitrogen pycnometry is described in ASTM B923-10 [52]. This is a process during which a known mass of powder is placed into a chamber of known volume, the chamber is purged to create a vacuum, and then Helium or Nitrogen is added so that it occupies the entire chamber except what is occupied by the sample. The gas molecules are small enough to fill the nooks and crannies present in non-spherical powder particles. The volume of powder is the volume of the chamber less the volume of the added gas. To determine the skeletal density, the mass of the powder is divided by the calculated volume.

CONCLUSIONS AND NEXT STEPS This assessment of the current methods used in evaluating the properties

of metal powder demonstrates that there are several international, consensus-



based standards already in existence that cover some of the properties pertinent for metal powder used for additive manufacturing processes. They cover powder sampling techniques and methods used in determining size, flow, and density characteristics. However, other property measurements, such as those regarding the morphology, chemical composition, and thermal properties of metal powders are described by a very limited number of standards, although there are some literature references addressing those topics.

The applicability of the principles discussed in this report to powders used in additive manufacturing will be addressed in a future report. At first glance though, it appears that powder sampling techniques and the methods of determining size, flow, and density characteristics are readily applicable. However, standardizing the methods of measuring the chemical composition, thermal properties, and morphology of metal powders might prove to be more challenging.

ACKNOWLEDGMENTS The authors would like to thank Shawn Moylan (NIST/Intelligent Systems

Division), Stephen Ridder (NIST/Surface and Microanalysis Science Division), Alexis Seven (Rowan-Cabarrus Community College/Science, Biotechnology, Mathematics and Information Technologies Department), and Elizabeth Harvey (Philadelphia School of the Future/Science Department) for their helpful comments, suggestions, and discussions.

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[5] T. Allen, Particle Size Measurement. England, Chapman and Hall Ltd. (1981).

[6] ASTM, B215-10: Standard Practices for Sampling Metal Powders. (2011). [7] ASTM, B214-07 (Reapproved 2011): Standard Test Method for Sieve

Analysis of Metal Powders. (2011). [8] ASTM, E161-00 (Reapproved 2010): Standard Specification for

Precision Electroformed Sieves. (2010). [9] ASTM, B761-06 (Reapproved 2011): Standard Test Method for Particle

Size Distribution of Metal Powders and Related Compounds by X-Ray Monitoring of Gravity Sedimentation. (2011).

[10] B. R. Muson, D. F. Young and T. H. Okiishi, Fundamentals of Fluid Mechanics. New York, John Wiley & Sons, Inc. (1998).

[11] ASTM, B822-10: Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering. (2010).

[12] J. H. Simmons and K. S. Potter, Optical Properties. Academic Press. (2000).

[13] F. S. Biancaniello, J. J. Conway, P. I. Espina, G. E. Mattingly and S. D. Ridder, Particle Size Measurement of Inert-gas-atomized Powder, Materials Science and Engineering: A. 124 (1), 9-14 (1990).

[14] E. Hawkins, The Shape of Powder-Particle Outlines. England, Research Studies Press Ltd. (1993).

[15] E. P. Cox, A Method of Assigning Numerical and Percentage Values to the Degree of Roundness of Sand Grains, Journal of Paleontology. 1 (3), 179-183 (1927).

[16] ASTM, B243-11: Standard Terminology of Powder Metallurgy. (2011). [17] D. W. Luerkens, Theory and Application of Morphological Analysis:

Fine Particles and Surfaces. CRC Press. (1991). [18] P. J. Barret, The shape of rock particles, a critical review, Sedimentology.

27 (3), 291-303 (1980). [19] H. Heywood, Numerical definitions of particle size and shape, Journal

of Society of Chemical Industry. 56 (7), 149-154 (1937). [20] H. Wadell, Volume, Shape, and Roundness of Rock Particles, The

Journal of Geology. 40 (5), 443-451 (1932).



[21] N. A. Riley, Projection Sphericity, Journal of Sedimentary Petrology. 11 (2), 94-97 (1941).

[22] H. Wadell, Volume, Shape, and Roundness of Quartz Particles, The Journal of Geology. 43 (3), 250- 280 (1935).

[23] H. P. Schwarcz and K. C. Shane, Measurement of Particle Shape by Fourier Analysis, Sedimentology. 13 (3/4), 213-231 (1969).

[24] K. H. Lu, Harmonic Analysis of the Human Face, Biometrics. 21 (2), 491-505 (1965).

[25] J. Goldstein, D. Newbury, D. Joy, C. Lyman, P. Echlin, E. Lifshin, L. Sawyer and J. Michael, Scanning Electron Microscopy and X-ray Microanalysis. New York, Kluwer Academic/Plenum Publishers. (2002).

[26] D. B. Wittry, Electron Probe Microanalyzer, U.S. Patent 2,916,621. (1959).

[27] P. C. Uden, Element-specific chromatographic detection by atomic emission spectroscopy. Columbus, OH, American Chemical Society. (1992).

[28] J. F. Moulder, W. F. Stickle, P. E. Sobol and K. D. Bomben, Handbook of X-ray Photoelectron Spectroscopy. Eden Prairie, MN, USA, Perkin-Elmer Corp. (1992).

[29] Benninghoven, F. G. Rüdenauer and H. W. Werner, Secondary Ion Mass Spectrometry: Basic Concepts, Instrumental Aspects, Applications, and Trends. New York, Wiley. (1987).

[30] Walsh, The application of atomic absorption spectra to chemical analysis, Spectrochimica Acta. 7 108-117 (1955).

[31] X. Hou and B. T. Jones, Inductively Coupled Plasma/Optical Emission Spectroscopy, Encyclopedia of Analytical Chemistry. 9468-9485 (2000).

[32] R. J. Reynolds and K. C. Thompson, Atomic absorption, fluorescence, and flame emission spectroscopy: a practical approach. New York, Wiley. (1978).

[33] Beckhoff, B. Kanngießer, N. Langhoff, R. Wedell and H. Wolff, Handbook of Practical X-Ray Fluorescence Analysis. Springer. (2006).

[34] L. Brügemann and E. K. E. Gerndt, Detectors for X-ray diffraction and scattering: a user's overview, Nuclear Instruments and Methods in Physics Research A. 531 (1-2), 292-301 (2004).

[35] ASTM, E1409-08: Standard Test Method for Determination of Oxygen and Nitrogen in Titanium and Titanium Alloys by the Inert Gas Fusion Technique. (2008).



[36] ASTM, E1447-09: Standard Test Method for Determination of Hydrogen in Titanium and Titanium Alloys by Inert Gas Fusion Thermal Conductivity/Infrared Detection Method. (2009).

[37] ASTM, E1569-09: Standard Test Method for Determination of Oxygen in Tantalum Powder by Inert Gas Fusion Technique. (2009).

[38] ASTM, E2792-11: Standard Test Method for Determination of Hydrogen in Aluminum and Aluminum Alloys by Inert Gas Fusion. (2011).

[39] ASTM, B213-11: Standard Test Methods for Flow Rate of Metal Powders Using the Hall Flowmeter Funnel. (2011).

[40] ASTM, B964-09: Standard Test Methods for Flow Rate of Metal Powders Using the Carney Funnel. (2009).

[41] ASTM, B855-11: Standard Test Method for Volumetric Flow Rate of Metal Powders Using the Arnold Meter and Hall Flowmeter Funnel. (2011).

[42] S. S. Sih and J. W. Barlow, The Measurement of the Thermal Properties and Absorptances of Powders Near Their Melting Temperatures, Proceedings of the Solid Freeform Fabrication Symposium. 131-140 (1992).

[43] ASTM, C177-10: Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus. (2010).

[44] S. E. Gustafsson, E. Karawacki and M. N. Khan, Transient hot-strip method for simultaneously measuring thermal conductivity and thermal diffusivity of solids and fluids, Journal of Physics D: Applied Physics. 12 (9), 1411-1421 (1979).

[45] W. J. Parker, R. J. Jenkins, C. P. Butler and G. L. Abbott, Flash Method of Determining Thermal Diffusivity, Heat Capacity, and Thermal Conductivity, Journal of Applied Physics. 32 (9), 1679-1684 (1961).

[46] ASTM, E1461-07: Standad Test Method for Thermal Diffusivity by the Flash Method. (2008).

[47] ASTM, B212-09: Standard Test Method for Apparent Density of Free-Flowing Metal Powders Using the Hall Flowmeter Funnel. (2010).

[48] ASTM, B417-11: Standard Test Method for Apparent Density of Non-Free-Flowing Metal Powders Using the Carney Funnel. (2011).

[49] ASTM, B329-06: Standard Test Method for Apparent Density of Metal Powders and Compounds Using the Scott Volumeter. (2006).

[50] ASTM, B703-10: Standard Test Method for Apparent Density of Metal Powders and Related Compounds Using the Arnold Meter. (2010).



[51] ASTM, B527-06: Standard Test Method for Determination of Tap Density of Metallic Powders and Compounds. (2006).

[52] ASTM, B923-10: Standard Test Method for Metal Powder Skeletal Density by Helium or Nitrogen Pycnometry. (2010).

End Notes

1 http://www.nist.gov/el/isd/sbm/matstandaddmanu.cfm 2 http://www.nist.gov/el/isd/sbm/fundmeasursci.cfm



Chapter 3

APPLICABILITY OF EXISTING MATERIALS TESTING STANDARDS FOR ADDITIVE

MANUFACTURING MATERIALS∗

John Slotwinski and Shawn Moylan

ABSTRACT

The analysis in this report shows that additive manufacturing-specific materials standards for characterizing the properties of metal powders and metal parts do not have to be developed from scratch. Decades of powder property testing (born out of powder metallurgy processes) and mechanical property testing has resulted in a suite of existing standards that can form the basis needed for some additive manufacturing materials.

INTRODUCTION This NIST Internal Report (NISTIR) is the third in a series of reports from

the National Institute of Standards and Technology (NIST) Engineering Laboratory’s project on Additive Manufacturing Materials. This project provides the measurement science for the additive manufacturing (AM) ∗ This is an edited, reformatted and augmented version of a report, NISTIR 8005, issued by the

National Institute of Standards and Technology, June 2014.


John Slotwinski and Shawn Moylan 50

industry to measure material properties of additive manufacturing materials in a standardized way. Currently there are no additive manufacturing-specific consensus-based standards in this area. This project, in conjunction with the NIST Engineering Laboratory’s other additive manufacturing projects, will provide the technical foundation and documentary standards development necessary to develop new consensus-based standards for AM materials, AM processes, and AM part qualification. This will be done via ASTM-International’s (hereafter referred to as ‘ASTM-I’) Committee F42 on Additive Manufacturing Technologies and the newly formed International Organization for Standardization (ISO) TC261 committee on Additive Manufacturing.

Determining the properties of the powder used for metal-based additive manufacturing, as well as the properties of the resulting bulk metal material, is a necessary condition for AM users to be able to confidently select powder and produce consistent parts with known and predictable properties. Standardized methods for characterizing the properties of metal powders and metal parts already exist. Summaries of these existing methods were published in NISTIR 7847 [1], for bulk metal materials properties, and NISTIR 7873 [2], for metal powder properties. While many of these existing standards are applicable for AM powders and parts, others are not, while still others are applicable with some additional modifications or guidance to the existing standards. This report summarizes an assessment of the applicability of the standards described in NISTIR 7847 and NISTIR 7873 for use on metal AM powders and parts.

The applicability assessment presented here is based on our knowledge and experiences of the various test methods, as well as our experiences and knowledge on the capabilities of metal powder-bed AM systems. For each of the ASTM and ISO standards presented here, a qualitative judgment on the degree of applicability of the standard for AM metal powders and parts is presented. This is shown in tabular form in the next section. A description of each applicability classification is described at the beginning of the table.

APPLICABILITY ASSESSMENT In this chart, each standard is given one of the following classifications in

determining if it is applicable for additive manufacturing. Some standards have additional pertinent text in the NOTES column.


Applicability of Existing Materials … 51

• YES – The standard should be applicable for additive manufacturing without any modifications.

• YES WITH GUIDANCE – The standard should be generally applicable for additive manufacturing, but there may be limits on its applicability, or additional considerations. These include:

o Limits on some ranges of test specimens, especially thin sheets and wires that may not be easily realized via metals-based commercial additive manufacturing systems; however some specimens can be readily built via AM.

o Required post-processing such that specimens built via additive manufacturing meets the requirements of the standard; this typically includes surface finish or dimensional requirements.

o Material isotropy requirements. AM specimens usually have some inherent anisotropy. The measurement methods that specify applicability for isotropic materials may still work, but the measured results may have larger uncertainties.

o Application specific considerations, such as elevated testing temperatures.

• NOT A TEST METHOD – These standards may have useful auxiliary information, such as terminology, but they are not by themselves a test method.

• NO – The standard either requires specimens that certainly cannot be built via AM, or the measurement simply is not applicable.

This analysis does not address any measurement-specific safety issues,

such as those that may be encountered while manipulating raw AM powder, which may require additional modifications and/or considerations.

Mechanical Testing of Metal Parts

Standard Designation

Standard Name Applicable for AM Testing?

Notes

ASTM E0006 Terminology Relating to Methods of Mechanical Testing

Not a test method This is not a testing standard, it is a terminology document

ASTM A370 Standard Test Methods and Definitions for Mechanical Testing of Steel Products

Yes For steel, refers to several other testing standards for basics and adds additional requirements/guidance, includes hardness, tension and impact testing



(Continued)



Notes

ASTM A1058 Standard Test Methods for MechanicalTesting of Steel Products—Metric

Yes Same as ASTM A370 but in SI units



Notes

ASTM B557 Standard Test Methods for Tension Testing Wrought and Cast Aluminum and Magnesium-Alloy Products

Yes ASTM B557 is for Al and Magnesium, ASTM E8 is the basic method

ASTM E8 Standard Test Methods for Tension Testing of Metallic Materials

Yes with Guidance

Basic method for tension testing at room temperature (10°C – 38°C, 50°F – 100°F). Not all specimen types can be made additively (e.g., wire and sheet). Includes requirements for powder metallurgy materials that should cover AM specimens.

ASTM E0021 Test Methods for Elevated Temperature Tension Tests of Metallic Materials

Yes with Guidance

Like ASTM E8 but at elevated temperatures (assume > 38°C/100°F). Yes with guidance that depends on application.

ASTM E0292 Test Methods for Conducting Time-for-Rupture Notch Tension Tests of Materials

Yes with Guidance

Similar to ASTM E8 but uses notched specimens. This test is done at elevated temperatures and with constant loads. Pre-test specimens need post-processing (after AM building) to achieve dimensional requirements.

ASTM E0740 Practice for Fracture Testing With Surface-Crack TensionSpecimens

Yes with Guidance

Similar to ASTM E8 but for plate with an existing flaw. Possible specimen thickness limitations for AM specimens.

ASTM E1450 Test Method for Tension Testing of Structural Alloys in Liquid Helium

Yes with Guidance

Similar to ASTM E8 but done at cryogenic temperatures. May require possible application-specific guidance.

ISO 6892-1:2009

Metallic materials--Tensile testing -- Part 1: Method of test at room temperature

Yes with Guidance

Basic ISO method similar to ASTM E8, includes additional test sample geometries (sheet, wire, etc.) that may not be appropriate for AM.

ISO 6892-2:2011

Metallic materials --Tensile testing -- Part 2: Method of test at elevated temperature

Yes with Guidance

Similar to ISO 6892-1 and ASTM E8 but at elevated temperatures.Sheet and wire specimens may not be appropriate for AM.

ISO 15579:2000

Metallic materials-- Tensile testing at low temperature

Yes with Guidance

Similar to ISO 6892-1 and ASTM E8 but at low temperatures (between





Notes

10°C and -196°C.) Sheet and wire specimens may not be appropriate for AM, also potentially application specific.

ISO 19819:2004

Metallicmaterials--Tensile testing in liquid helium

Yes with Guidance

Similar to ISO 6892-1 and ASTM E8 but at a very low temperature (-269°C or 4.2K, liquid He temperature.) Also can be done at cryogenic temperatures (less that -196°C or 77K.) Sheet and wire specimens may not be appropriate for AM, also potentially application specific.

ISO 26203-1:2010

Metallic materials --Tensile testing at high strain rates – Part Elastic-bar-type systems

No Similar to ISO 6892-1 but for sheet materials such as those for car bodies, and at high strain rates (> 102 s-1).These parts cannot currently be built on commercial AM systems due to their large size.

ISO 26203-2:2011

Metallic materials—Tensile testing at high strain rates – Part Servo-hydraulic and other test systems

No Similar to ISO 6892-1 but for sheet materials such as those for car bodies, and at high strain rates (10-2 s-1 to 103 s-1). These parts cannot currently be built on commercial AM systems due to their large size.

ASTM E9 Test Methods of Compression Testing of Metallic Materials at Room Temperature

Yes with Guidance

Basic test for axial-load compression testing of metals at room temperature.Not all of the sample types (e.g., thin sheets) can be made additively.

ASTM E0209 Practice for Compression Tests of Metallic Materials at Elevated Temperatures with Conventional Or Rapid Heating Rates and Strain Rates

Yes with Guidance

Similar to ASTM E9 but for specimens that are heated at a uniform temperature (up to and beyond 1000°F/538°C). Yes, depending on application.

ASTM E0238 Test Method for Pin-Type Bearing Test of Metallic Materials

Yes with Guidance

Basic and only method for pin-type bearing. The surface finish requirements and some thickness requirements are problematic for some metal-based commercial AM systems.

ASTM E0111 Test Method for Young's Modulus, Tangent Modulus, and Chord Modulus

Yes Limited to materials, temperatures, and stresses where creep is negligible. ASTM E8 and ASTM E9 are the basic tension and compression methods; this provides additional guidance (number of trials, specimens, temperature etc.)



(Continued)



Notes

and defines the three moduli. Provides guidance for both high and low temperatures.

ASTM E0143 Test Method for Shear Modulus at Room Temperature

Yes Basic method for shear modulus at room temperature only.

ASTM E1875 Test Method for Dynamic Young's Modulus, Shear Modulus, and Poisson's Ratio by Sonic Resonance

Yes with Guidance

Instead of macro deformation, uses sonic resonance (which may be considered micro deformation.) This standard covers room, elevated, and very low temperatures in the range -195°C to 1200°C. Strict requirements for isotropic materials may be a problem for some AM parts/processes.

ASTM E1876 Test Method for Dynamic Young's Modulus, Shear Modulus, and Poisson's Ratio by Impulse Excitation of Vibration

Yes with Guidance

Similar to ASTM E1875 but with an impulse instead of a body resonance. This might be considered micro-scale deformation. States that can be performedat non-room temperatures. Strict requirements for isotropic materials may be a problem for some AM parts/processes.

ASTM E0466 Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials

Yes with Guidance

Basic method; fatigue testing of axial un-notched and notched specimens subjected to constant amplitude periodic forcing function in air at room temp. Used to test effect of variations in material, geometry, surface condition, etc. Test is finished when it fails or a certain number of cycles are reached. Samples will require post-processing to achieve recommended surface roughness and notch geometry.

ASTM E0467 Practice for Verification of Constant Amplitude Dynamic Forces in an Axial Fatigue Testing System

Not a test method

A “method for testing the tests.” This is not a test method.

ASTM E0468 Practice for Presentation of Constant Amplitude Fatigue Test Results for Metallic Materials

Not a test method

Not a method, it’s about what to report.

ASTM E0606 Practice for Strain-ControlledFatigue Testing

Yes Similar to ISO 1099; strain-controlled instead of force-controlled fatigue. Uniqueness is





Notes

that it yields the determination of cyclic stresses and strains at any time during the tests.

ASTM E0647 Test Method for Measurement of Fatigue Crack Growth Rates

Yes with Guidance

Determines fatigue crack growth rates from near-threshold to Kmax controlled instability. Possible anisotropy issues.

ASTM E1049 Practices for Cycle Counting in Fatigue Analysis

Not a test method

Not a test method but still potentially useful for fatigue analysis of AM specimens.

ASTM E1823 Terminology Relating to Fatigue and Fracture Testing

Not a test method

ASTM E1942 Guide for Evaluating Data Acquisition Systems Used in Cyclic Fatigue and Fracture Mechanics Testing

Not a test method

ASTM E2368 Practice for Strain ControlledThermomechanical Fatigue Testing

Yes with Guidance

Uniform temperature and strain fields over the specimen are simultaneously varied and independently controlled. AM samples would likely need post-processing.

ASTM E2714 Test Method for Creep-Fatigue Testing

Yes with Guidance

Determines deformation and crack formation or nucleation as a consequence of constant-amplitude strain-controlled tests or constant-amplitude force-controlled tests (ASTM E0606 and ASTM E0466, ISO 12106 and ISO 1099.) Typically done at elevated temperatures an involves sequential or simultaneous application of loading conditions necessary to generate cyclic deformation/damage enhanced by creep deformation/damage or vice-versa. The difference with the basic method is the long hold time.

ASTM E2760 Test Method for Creep-Fatigue Crack Growth Testing

Yes with Guidance

ASTM E2760 is to ASTM E0647 as ASTM E2709 is to ASTM E0606 or ASTM E0466. Concerns fatigue cycling with long loading/unloading rates and/or hold times to cause creep deformation at the crack tip and the creep deformation be responsible for enhancing the crack growth per loading cycle.

ASTM E2789 Guide for Fretting Fatigue Testing

Yes ASTM E0466 is the basic method. Small amplitude motion, usually



(Continued)



Notes

tangential, between two solid surfaces in contact.

ISO 1099:2006 Metallic materials--Fatigue testing -- Axial force-controlled method

Yes with Guidance

Samples are similar to ASTM E0466; stress vs. cycles to failure. Can be done at high temperatures. Basic method. AM samples will require post-processing to achieve recommended surface roughness.

ISO 1143:2010 Metallic materials -- Rotating bar bending fatigue testing

Yes with Guidance

Circular cross-section samples, rotated and subjected to bending moment. Application specific, post-processing may be required.

ISO 1352:2011 Metallic materials --Torque- controlled fatigue testing

Yes Similar to ISO 1352 but for torque.

ISO 12106:2003

Metallic materials -- Fatigue testing -- Axial-strain-controlled method

Yes with Guidance

Similar to ISO 1099 but for low-cycle fatigue tests. Imposed constant strain rate; test starts by testing elastic region to measure modulus (checked vs nominal), test stops are failure. High and low temps mentioned. Samples will require post-processing to achieve recommended surface roughness.

ISO 12108:2002

Metallic materials--Fatigue testing -- Fatigue crack growth method

Yes with Guidance

Primarily intended for isotropic materials; variety of samples and tests.

ISO 12111:2011

Metallic materials --Fatigue testing --Strain-controlled thermomechanical fatigue testing method

Yes with Guidance

Similar to ISO 12106 but with the addition of temperature cycling. Samples will require post-processing to achieve recommended surface roughness.

ASTM B645 Standard Practice for Linear-Elastic Plane-Strain Fracture Toughness Testing of Aluminum Alloys

Yes with Guidance

Fracture toughness is resistance to crack extension; Basic test method for plane-strain fracture toughness of aluminum. Supplements E399 and B646. Samples will require post-processing to achieve recommended surface roughness and dimensional tolerances.

ASTM B909 Standard Guide for Plane Strain Fracture Toughness Testing of Non-Stress Relieved Aluminum Products

Yes with Guidance Not a Test Method

ASTM E399 or ISO 12737 is basic method. ASTM B909 provides supplemental information for plane-strain toughness testing of Al where complete stress relief is not possible. Additive manufacturing samples will require post-processing to achieve





Notes

recommended surface roughness and dimensional tolerances.

ASTM B646 Standard Practice for Fracture Toughness Testing of Aluminum Alloys

Provides guidelines for test selection for fracture toughness properties of Al, particularly for quality assurance and material release purposes. Provides supplemental information on specimen size, analysis, and interpretation of results, particularly for varying thicknesses.

ASTM E0023 Test Methods for Notched Bar Impact Testing of Metallic Materials

Yes with Guidance

Notched-bar impact testing of metals by Charpy (simple-beam) and Izod (cantilever- beam) tests. Describes 4 differences between two tests (sample notches, holding mechanism, impact location, sample dimension). Additive manufacturing samples will require post-processing to achieve recommended dimensional tolerances.

ASTM E0399 Test Method for Linear-Elastic Plane-Strain Fracture Toughness KIc of Metallic Materials

Yes with Guidance

Basic method. See also ISO 12737. For metals under linear-elastic, plane-strain conditions using fatigue pre-cracked specimens subjected to a slowly increasing crackdisplacement force. May require post-process machining for notch.

ASTM E1221 Test Method for Determining Plane-Strain Crack-Arrest Fracture Toughness, KIa, of Ferritic Steels

Yes with Guidance

Samples may require post-processing.

ASTM E1290 Test Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurement

Yes with Guidance

Determines critical crack-tip opening displacement (CTOD) values, used to measure cleavage crack initiation toughness for materials that exhibit a change from ductile to brittle behavior with decreasing temp. Notches may require post-processing (additional machining).

ASTM E1304 Test Method for Plane-Strain (Chevron-Notch) Fracture Toughness of Metallic Materials

Yes with Guidance

Chevron-shaped-notch, notches may require post-processing (additional machining).

ASTM E1820

Test Method for Measurement of Fracture Toughness

Yes with Guidance

This also appears to be the basic ASTM method for fracture toughness. Load a fatigue pre-



(Continued)



Notes

cracked test specimen to induce either unstable crack extension and/or stable crack extension. Need to measure force versus load-line displacement or crack mouth opening displacement or both. ASTM E1921 recommended for testing ferric steels that undergo cleavage fracture in the ductile-to-brittle transition. Notches may require post-processing (additional machining).

ISO 148-1:2009

Metallic materials -- Charpy pendulum impact test -- Part 1: Test method

Yes with Guidance

Break notched test piece with a single blow from a swinging pendulum. Mention of heated or cooled tests in a liquid or gaseous medium. Notches may require post-processing (additional machining).

ISO 148-3:2008

Metallic materials -- Charpy pendulum impact test -- Part 3: Preparation and characterization of Charpy V-notch test pieces for indirect verification of pendulum impact machines

Not a Test Method.

This is about sample prep.

ISO 12135:2002

Metallic materials -- Unified method of test for the determination of quasistatic fracture toughness

Yes with Guidance

Fracture toughness (resistance to crack extension) in terms of stress intensity factor (K), crack-tip displacement (δ), loading parameter (J) and resistance curves for homogenous metallic materials. Continually increasing force applied to sample by uniaxial tension or 3-point bending. Notches may require post-processing (additional machining).

ISO 12737:2010

Metallic materials -- Determination of plane-strain fracture toughness

Yes with Guidance

Similar to ASTM E399. Determines plane-strain fracture toughness (Klc) of homogeneous metallic materials using a specimen that is notched and pre-cracked by fatigue, and slowly increasing crack displacement force. Notches may require post-processing (additional machining).

ISO 14556:2000

Steel – Charpy V-notch Pendulum impact test -- Instrumented test method

Yes with Guidance

Similar to ISO 148 but for steels. Notches may require post-processing (additional machining).

ISO Metallic materials -- Method Yes with ISO 12135 is the basic method. ISO





Notes

22889:2007 of test for the determination of resistance to stable crack Extension using specimens of low constraint

Guidance 22889 applies to samples that are very small (i.e., have size sensitivity). May require post-processing.

ISO 27306:2009

Metallic materials -- Method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel components

Not a test method

Converts fracture toughness from lab specimens to equivalent toughness for structural components. This is not a test method; it is a description for how to use method.

ASTM E0740 Practice for Fracture Testing with Surface-Crack Tension Specimens

Yes with Guidance

Covers design, prep, and testing of surface-crack specimens. Test is performed with continuously increasing force and excludes cyclic and sustained loadings. Determines residual strength of a specimen with a semi-elliptical or circular-segment fatigue crack. Requires post-processing to make initial pre-crack.

ASTM E1457 Test Method for Measurement of Creep Crack Growth Times in Metals

Yes with Guidance

Determines creep crack growth in metals at elevated temps using pre-cracked specimens subjected to static or quasi-static loading conditions. Requires post-processing to make initial pre-crack.

ASTM E1681 Test Method for DeterminingThreshold Stress Intensity Factor For Environment-Assisted Cracking of Metallic Materials

Yes with Guidance

Appears to be basic method, requires environmental chamber. Requires post-processing to make initial pre-crack since most metal-based AM cannot produce a test specimen with a small-enough crack.

ASTM E2472 Test Method for Determination of Resistance to Stable Crack Extension under Low-Constraint Conditions

Yes with Guidance

For low-constraint conditions (crack-length-to-thickness and un-cracked ligament-to-thickness ratios are greater than or equal to 4) and that are tested under slowly increasing remote applied displacement. Requires post-processing to make initial pre-crack.

ASTM B769 Standard Test Method for Shear Testing of Aluminum Alloys

Yes with Guidance

Double-shear loading using a tension or compression testing machine. Requires post- processing in order to meet surface finish specification.

ASTM B565 Standard Test Method for Shear Testing of Aluminum and Aluminum-Alloy Rivets and Cold-Heading Wire and

No Metal wire, rivets, and rods are difficult to make via additive manufacturing



(Continued)



Notes

Rods ASTM E0010 Test Method for Brinell

Hardness of Metallic MaterialsYes with Guidance

Basic method for Brinell; tests at temperatures outside of nominal (10°C to 35°C) permitted. Requires post-processing in order to meet surface finish specification.

ASTM E0018 Test Methods for Rockwell Hardness of Metallic Materials

Yes with Guidance

Basic method for Rockwell; tests at temperatures outside of nominal (10°C to 35°C) permitted. Requires post-processing in order to meet surface finish specification.

ASTM E0140 Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, and Scleroscope Hardness

Not a test method.

Tables that convert hardness values of one type of tests to other types.

ASTM E0384 Test Method for Knoop and Vickers Hardness of Materials

Yes with Guidance

Basic methods for Knoop and Vickers; tests at temperatures outside of nominal (10°C to 35°C) permitted. Post-processing likely necessary for proper specimen surface roughness.

ASTM E0448 Practice for Scleroscope Hardness Testing of Metallic Materials

Yes with Guidance

Dynamic indentation hardness (drop and bounce). This is the basic method. Requires post-processing in order to meet surface finish specification.

ASTM B647 Standard Test Method for Indentation Hardness of Aluminum Alloys by Means of a Webster Hardness Gage

Yes with Guidance

Webster gage for Al only. This is a portable handheld device useful for in-situ measurements; good for production/quality control purposes. Not as sensitive as Rockwell or Brinell. Requires post-processing in order to meet surface finish specification.

ASTM B648 Standard Test Method for Indentation Hardness of Aluminum Alloys by Means of a Barcol Impressor

Yes with Guidance

Barcol gage for Al only. This is a portable handheld device useful for in-situ measurements; good for production/quality control purposes. Not as sensitive as Rockwell or Brinell. Requires post-processing in order to meet surface finish specification.





Notes

ASTM B724 Standard Test Method for Indentation Hardness of Aluminum Alloys by Means of a Newage, Portable, Non-CaliperType Instrument

Yes with Guidance

Newage gage for Al only.Good for large pieces that can’t be measured with a caliper-type instrument. Requires post-processing in order to meet surface finish specification.

ISO 4545-1:2005

Metallic materials -- Knoop hardness test—Part 1: Test method

Yes with Guidance

Basic Knoop test like ASTM E0384. Requires post-processing in order to meet surface finish specification.

ISO 6506-1:2005

Metallic materials – Brinell hardness test – Part 1: Test method

Yes with Guidance

Basic Brinell test like ASTM E0010. Requires post-processing in order to meet surface finish specification.

ISO 6507-1:2005

Metallic materials -- Vickers hardness test—Part 1: Test method

Yes with Guidance

Basic Vickers test like ASTM E0384. Requires post-processing in order to meet surface finish specification.

ISO 6508 Metallic materials -- Rockwell hardness test—Part 1: Test method (scales A, B, C, D, E, F, G, H, K, N, T)

Yes with Guidance

Basic Rockwell test like ASTM E0018. Requires post-processing in order to meet surface finish specification.

ISO 14577 Metallic materials -- Instrumented indentation test for hardness and materials parameters – Part 1: Test method

Yes with Guidance

This test considers the force and displacement of the indentation during plastic and elastic deformation; monitoring the complete cycle of increasing and removal of test force. Requires post-processing in order to meet surface finish specification.

ISO/TR 29381:2008

Metallic materials -- Measurement of mechanical properties by an instrumented indentation test -- Indentation tensile properties

Not a Test Method

Can derive tensile properties from indentation measurements via one of three methods (representative stress-strain, inverse FEA methods, or neural networks). Not a test method.

ASTM E0132 Test Method for Poisson's Ratio at Room Temperature

Yes with Guidance

Basic method, tension tests, requires extensometers; room temp only. Requires post- processing; specimens should be stress-relieved, isotropic and homogeneous.

ASTM E0290 Test Methods for Bend Testing of Material for Ductility

Yes Yes with Guidance

Bend testing for ductility of materials

ASTM E0837 Test Method for DeterminingResidual Stresses by the Hole- Drilling Strain-Gage Method

Isotropic and linearly-elastic materials only; drill hole at center of strain rosette, and measure strains. If AM materials are anisotropic then this method may not be applicable.

ASTM E0915 Test Method for Verifying the Alignment of X-Ray Diffraction Instrumentation

No This is not a materials test method, it is a method for verifying the alignment of an X-ray machine.



(Continued)



Notes

for Residual Stress Measurement

ISO 7438:2005 Metallic materials -- Bend test Yes Like ASTM E0290.

ISO 11531:1994

Metallic materials -- Earing test No

Metal AM systems cannot easily make samples with thicknesses of 100 µm.

ISO/TR 14936:1998

Metallic materials -- Strain analysis report

Yes with Guidance

The standard applies to sheet but the thickness requirements are not specified. AM cannot make a very thin sheet.

ASTM E0003 Guide for Preparation of Metallographic Specimens

Not a test method

Not a mechanical test method, but important for micro-structural analysis.

ASTM E0007 Terminology Relating to Metallography

Not a test method

Terminology.

ASTM B348 Standard Specification for Titanium and Titanium Alloy Bars and Billets

Not a test method Not a mechanical test method, but does contain useful specs for various materials that could be compared to those that are made additively.

ASTM B211 Standard Specification for Aluminum and Aluminum-Alloy Bar, Rod, and Wire

Not a test method

Not a mechanical test method, but does contain useful specs for various materials that could be compared to those that are made additively.

Metal Powder Testing


Standard Name Applicable for AM?

Notes

ASTM B212 Standard Test Method for Apparent Density of Free- Flowing Metal Powders Using the Hall Flowmeter Funnel

Yes

ASTM B213 Standard Test Methods for Flow Rate of Metal Powders Using the Hall Flowmeter Funnel

Yes

ASTM B214 Standard Test Method for Sieve Analysis of Metal Powders

Yes with Guidance

Applicable for sieves with openings from 45 µm to 1000 µm. Not suitable for powders smaller than 45 µm.

ASTM B215 Standard Practices for Sampling Metal Powders

Yes

ASTM B243 Standard Terminology of Not a test Terminology, not a test method.





Notes

Powder Metallurgy method ASTM B329 Standard Test Method for

Apparent Density of Metal Powders and Compounds Using the Scott Volumeter

Yes

ASTM B417 Standard Test Method for Apparent Density of Non-Free-Flowing Metal Powders Using the Carney Funnel

No AM powders are typically free flowing, in fact they flow exceptionally well.

ASTM B527 Standard Test Method for Determination of Tap Density of Metallic Powders and Compounds

Yes

ASTM B703 Standard Test Method for Apparent Density of Metal Powders and Related Compounds Using the Arnold Meter

Yes

ASTM B761 Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by X-Ray Monitoring of Gravity Sedimentation

Yes with Guidance

Test method works best “for the analysis of elemental tungsten, tungsten carbide, molybdenum, and tantalum. Other metal powders may be analyzed using this method with caution as to significance until actual satisfactory experience is developed.” It appears that this only works for particles with sizes > 25 µm (and minimum size depends on species).

ASTM B822 Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering

Yes

ASTM B855 Standard Test Method for Volumetric Flow Rate of Metal Powders Using the Arnold Meter and Hall Flowmeter Funnel

Yes

ASTM B923 Standard Test Method for Metal Powder Skeletal Density by Helium or Nitrogen Pycnometry

Yes

ASTM B964 Standard Test Methods for Flow Rate of Metal Powders Using the Carney Funnel

Yes



(Continued)



Notes

ASTM C177 Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus

Yes with Guidance

Method uses solid specimens, applicability to powder specimens is uncertain.

ASTM E161 Standard Specification for Precision Electroformed Sieves

Not a Test Method

This standard presents the specifications for precision electroformed sieves, and is not a test method.

ASTM E1409 Standard Test Method for Determination of Oxygen andNitrogen in Titanium and Titanium Alloys by the Inert Gas Fusion Technique

No This method will likely not work with metal powders since as part of the method the surface of the material must first be removed, either chemically or mechanically. Given the small diameters of AM powders, this is very impractical, and may completely consume the powder under test.

ASTM E1447 Standard Test Method for Determination of Hydrogen in Titanium and Titanium Alloys by Inert Gas Fusion Thermal Conductivity - Infrared Detection Method

No This method requires solid form specimens.

ASTM E1461 Standard Test Method for Thermal Diffusivity by the Flash Method

No This method requires homogeneous isotropic solid disc specimens.

ASTM E1569 Standard Test Method for Determination of Oxygen in Tantalum Powder by Inert Gas Fusion Technique

No This method is only for tantalum powders.

ASTM E2792 Standard Test Method for Determination of Hydrogen in Aluminum and Aluminum Alloys by Inert Gas Fusion

No This method does not seem to be applicable to powder samples.

DISCUSSION AND NEXT STEPS The analysis in this report shows that AM-specific materials standards for

characterizing the properties of metal powders and metal parts do not have to be developed from scratch. Decades of powder property testing (born out of



powder metallurgy processes) and mechanical property testing has resulted in a suite of existing standards that can form the basis for AM-specific materials standards. This report shows that while some of these existing standards are not appropriate for AM materials, many are, either in the current form or with some additional guidance or enhancements to adequately account for the characteristics of AM materials. Standards development organizations such as ASTM-I F42 can now use this information in the development of new AM-specific materials standards, especially new standards that are largely based on the existing standards contained in this report.

REFERENCES

[1] Slotwinski, J.A., Cooke, A.L., Moylan, S.P., “Mechanical Properties Testing for Metal Parts Made via Additive Manufacturing: A Review of the State of the Art of Mechanical Property Testing,” NISTIR 7847, March, 2012.

[2] Cooke, A.L., Slotwinski, J.A., “Properties of Metal Powders for Additive Manufacturing: A Review of the State of the Art of Metal Powder Property Testing,” NIST IR 7873, July, 2012.




Chapter 4

MATERIALS TESTING STANDARDS FOR ADDITIVE MANUFACTURING OF POLYMER

MATERIALS: STATE OF THE ART AND STANDARDS APPLICABILITY*

Aaron M. Forster

ABSTRACT

Additive manufacturing (AM) continues to grow as an advanced manufacturing technique. The most recent industry report from Wohlers and Associates indicates AM represented $1.6B in revenue from parts, systems, and other supporting industries in 2012 and is expected to grow to more than $3.5B by 2017 and to $10B by 2022. The measurement challenges for AM, whether the deposited material is metal or polymer, are similar. Parallels between the additive manufacturing for metals and polymers include: characterization of raw materials, development of material properties for design, in-situ process and feedback control, workflow optimization, and modeling final properties. The scope of this report is to analyze the current trends in polymer additive manufacturing and determine the applicability of current American Society for Testing Materials International (ASTM) and the International Standards Organization (ISO) standard test methods for mechanical properties and

* This is an edited, reformatted and augmented version of a report, NISTIR 8059, issued by the

National Institute of Standards and Technology, May 2015.


Aaron M. Forster 68

failure of polymers and polymer composites generated from the additive manufacturing processes. The current approach to mechanical testing standards utilizes existing guidelines for testing materials, but this analysis highlights the need to develop specific guidelines for testing AM materials. The current AM efforts at NIST towards polymers are supported through the Material Measurement Laboratory (MML) AM program. While this program is addressing critical measurement science to validate polymer physics within AM, it is not directly translating this knowledge into the standards required for engineering design. The emerging engineering and standards challenges for high performance polymers and polymer composites are not currently addressed by the Engineering Laboratory (EL) Additive Manufacturing program, which is focused on metal applications. The development of a program to bridge the measurement gap between molecular architecture of AM materials (MML) and generating engineering properties for design represents an opportunity for the EL effort.

Keywords: additive manufacturing, polymer, mechanical properties, standards, testing

LIST OF ACRONYMS AMC, Additive Manufacturing Consortium – A national consortium of

industry, government, academic, and non-profit research organizations with the mission of accelerating and advancing the manufacturing readiness of metal additive manufacturing technology.

AM, additive manufacturing – a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies [1].

AmericaMakes – is the National Additive Manufacturing Innovation Institute. AmericaMakes is focused on helping the United States grow capabilities and strength in 3D printing. It is based in Youngstown, Ohio and is an extensive network of more than 100 companies, non-profit organizations, academic institutions and government agencies [2].

ARL - Army Research Laboratory. ASTM – American Society of Testing Materials International. Battelle – Battelle Memorial Institute is the world’s largest nonprofit

research and development organization. CAD/CAM – Computer Aided Design/Computer Aided Machining.


Materials Testing Standards for Additive Manufacturing … 69

CAMM, Consortium for Additive Manufacturing Materials – The organizational component of CIMP 3D. It was created from a NIST Advanced Manufacturing Technology Consortia (AMTech) Program grant to foster innovation in additive manufacturing.

CIMP 3D, Center for Innovative Materials Processing Through Direct Digital Deposition – A consortium administered by Pennsylvania State University, Batelle, and Sciaky Corporation. It is a resource for AM technology for critical applications [3].

CNC, computer numerical control – indicative of computer-controlled machinery for cutting various hard materials. These are synonymous with subtractive manufacturing methods.

DARPA – Defense Advanced Research Projects Agency. FDM – a material extrusion process used to make thermoplastic parts

through heated extrusion and deposition of materials layer by layer; term denotes machines built by Stratasys, Inc [1].

ISO – International Standards Organization. LS, laser sintering – a powder bed fusion process used to produce objects

from powdered materials using one or more layers to selectively fuse or melt the particles at the surface, layer by layer, in an enclosed chamber [1].

NAMII, National Additive Manufacturing Innovation Institute - The pilot institute from NNMI established in 2012. This consortium includes 40 companies, nine research universities, five community colleges, and 11 nonprofit organizations. This consortium is also called AmericaMakes [4].

NCDMM, National Center for Defense Manufacturing and Machining – the driver of AmericaMakes. The mission is to deliver optimized manufacturing solutions that enhance the quality, affordability, maintainability, and rapid deployment of existing and yet-to-be developed defense systems [5].

NNMI, National Network for Manufacturing Innovation – A federally backed network established by the Revitalize American Manufacturing Act. The institutes developed under NNMI are intended to create a competitive, effective, and sustainable manufacturing research-tomanufacturing infrastructure for U.S. industry and academia to solve industry-relevant problems.

ME, material extrusion – an additive manufacturing process in which a material is selectively dispensed through a nozzle or orifice [1].

MSAM, Measurement Science for Advanced Manufacturing – A cooperative agreement program that solicits proposals for grants to advanced manufacturing. The program is administered by the National Institutes of


Aaron M. Forster 70

Standards and Technology. This is the program developed to administer grants for the NNMI.

ONR – Office of Naval Research ORNL – Oak Ridge National Laboratory

1. INTRODUCTION† Additive manufacturing is a process of joining materials to make objects

from 3D model data, usually in a layer by layer process [1, 6]. The objects are created in CAD/CAM software, transformed to machining instructions similar to the process for CNC machining, and each layer is directly fabricated on top of previous layers to create a replica of the object. In general, this manufacturing process is not subtractive. Multiple processes exist to create polymeric materials and composites, but they generally fall into three classes: material extrusion, powder bed fusion, and material jetting.

Material Extrusion (ME) is a process that selectively dispenses a thermoplastic polymer through a nozzle. Stratasys has trademarked the term fused deposition modeling (FDM®) to identify their systems that utilize this technique [1]. The extrusion head melts the plastic filament, extrudes material through a nozzle, and places the resin bead onto the substrate. This is a less aggressive process compared to injection molding where the plastic is melted and uniformly blended using a screw extruder to inject the material at high pressure into a mold. There are specific challenges for additive manufacturing that are unique to polymeric materials. In ME, heat is used to melt a polymer filament and the material is directed to a specific location via a nozzle. This places a polymer beads or filament of a specific size and length onto the substrate. Successive beads are layered to create the final 3D structure. The strength of the part is generated from the deposited material properties and the interface between beads. The interface is important because the neighboring beads are at a lower temperature than the molten bead leaving the nozzle. The thermal gradient between the two materials will melt the existing bead and cause polymers molecule to diffuse across the interface. The strength of this fusion is dependent on many factors such as temperature gradient, polymer structure (molecular weight, branching, heat of fusion, glass transition † Certain commercial equipment, instruments, or materials are identified in this paper to foster

understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.



temperature, etc.), and bead geometry. It is possible for this fusion to exist under stress prior to any mechanical loading. As the beads cool, the polymer contracts which creates a localized residual stress. As the performance of AM materials is increased by using semi-crystalline and more rigid polymers, the physics to model interfacial strength and stress are different than amorphous materials. There are additional challenges such as ME surface roughness, void space between beads, and defects (excess material) that can initiate failure modes within the part under loading. The nature of the printing process and the aligned structure of the beads make AM parts highly anisotropic and this anisotropy may exhibit a non-linear dependence on processing parameters.

Laser sintering (LS), or powder bed fusion, is a process that utilizes directed energy to melt a thermoplastic powder similar to the process used to generate parts from metal powders. This process starts with a powder bed of polymer powder of a specific layer height and temperature. A high powered laser is rastered across the surface to locally heat the polymer pellets. At this point, two thermal processes could occur depending on the machine design. The laser could fully melt the polymer pellets and allow the molten material to flow and diffuse into adjacent material. The second process locally heats the pellets to allow diffusion of polymer between adjacent pellets and layers. This is equivalent to a sintering process. In either case, the locally heated polymer diffuses to create a single layer within the part. A fresh layer of powder material is placed on top of the previous layer and the laser again melts local regions of polymer. The strength of parts is dependent on the ability of the process to manage thermal gradients. The molten material must flow into cracks adjacent to previous layers and the thermal gradient must be high enough to allow entanglement between neighboring polymer layers [7]. These parts tend to exhibit anisotropy characteristic of the laser scanning direction or build (Z) direction and can exhibit location dependent voids.

The material jetting process, which is similar to early stereolithography methods used for rapid prototyping [8], utilizes an inkjet print head to deposit a thin layer of photopolymerizable polymer and initiator. Ultraviolet lamps, or similar energy source, at the print head initiate cure of the photopolymer layer as it is deposited [9]. Direct printing processes have challenges related to the chemistry of the photocurable polymers used to build parts. Localized curing may be inhomogeneous which leads to a range of mechanical properties throughout the part. Uncrosslinked material trapped within holes may plasticize or age the part causing premature failure [10].

Additive manufacturing (AM) continues to grow as an advanced manufacturing technique. The most recent industry report from Wohlers and


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Associates indicates AM represented $1.6B in revenue from parts, systems, and other supporting industries in 2012 [11] and is expected to grow to more than $3.5B by 2017 [12] and to $10B by 2022 [13]. The AM material market is expected to grow from $470M in 2013 to over $1.09B in 2022 [13]. Industry, government and academia in the U.S. have been working to support the rate of growth by expanding machine capabilities and developing new high strength and biomedical materials. A report by IDTechEx as reported in The Guardian indicates the dental and medical market is expected to expand to $867M by 2025, the inclusion of additively manufactured organs and tissues would mean a potential of $6B within 10 years [14, 15].

Currently, there are few standards specifically addressing mechanical properties of AM parts. ASTM F42.01 has a number of standards and work items focused on metals AM [16]. There is currently one ASTM standard test method applicable to powder bed fusion of plastic materials; F3091/F3019M-14 Standard Specification for Powder Bed Fusion of Plastic Materials [17]. The subcommittee has one work item that covers evaluation of manufacturing systems: WK 40419 New Test Methods for Performance evaluation of additive manufacturing systems through measurement of a manufactured test piece [18]. The International Standards Organization (ISO) has one active standard: ISO 17296-3:2014 Additive Manufacturing – General Principles—Part 3: Main characteristics and corresponding test methods [19] to address quality characteristics of parts produced by AM. This standard references other ISO standards for mechanical property testing of polymers and metals, but there are no AM specific considerations in testing. The lack of AM specific mechanical standards creates challenges for stakeholders to provide equal comparisons between machines, materials, and models that predict final part properties in order to generate design allowables. Stahl has identified the inferior mechanical performance compared to that for traditionally manufactured parts as a risk for AM [20].

2. SCOPE The measurement challenges for AM, whether it is metal or polymer, are

similar. Parallels between the material systems include: characterization of raw materials, development of design allowables, in-situ process and feedback control, workflow optimization, and modeling final properties [21]. The emerging engineering and standards challenges for high performance polymers and polymer composites are not directly addressed by the Engineering



Laboratory Additive Manufacturing program, which is focused on metal applications. The scope of this report is to analyze the current trends in polymer additive manufacturing and determine the applicability of current ASTM and the ISO standard test methods for mechanical properties and failure of polymers and polymer composites generated from the additive manufacturing processes.

This report follows the style of previous reports, NISTIR 8005 [22] and NISTIR 7847 [23], from EL to document the standards needs in metal AM. This report will provide:

a. State of the art for additive manufacturing of polymers and polymer

composites b. Analysis of the technical hurdles that are preventing these materials

from high performance manufacturing applications. c. Analysis of the current ASTM and ISO standards for measuring

mechanical properties and failure of polymers and composites to include: i. Standard designation, ii. Standard Name, iii. Application to AM testing, iv. Notes concerning each standard relevant to the AM, d. Summary of recommended potential directions for NIST standards

research in polymer additive manufacturing. This report will also describe the emerging effort in the Material

Measurement Laboratory (MML) to support polymer additive manufacturing efforts and the potential opportunities for leveraging collaborative efforts between MML and the Engineering Laboratory. In order to limit the scope of this effort, concessions were made to focus on materials, measurement length scales, and types of standards as described below:

• Polymers – Standardized methods for measuring the mechanical

properties polymeric materials utilized in ME, laser sintering, and direct printing were addressed. Mechanical property measurement standards for polymer matrix fiber reinforced composites were also included for review. Fiber reinforced composites use a polymer matrix to support woven or aligned high strength fibers. Manufacturing composites requires stacking multiple layers of fibers and infusing the interstitial spacing with a polymer resin. The fibers


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are strong in the axial direction and weaker in the radial direction; the fibers provide anisotropy to the strength of the composite.

Therefore, standardized measurements developed for mechanical

properties in fiber reinforced composites may be applicable to additively manufactured materials.

• Bulk Mechanical Properties – Standardized methods for bulk property

measurements were addressed. Mechanical property measurements of localized properties, like those obtained by micro-indentation, hardness, and atomic force microscopy were excluded.

• Focus on International Standards – This was done in order to make the assessment practical. A cursory review of standards from the major Standards Development Organizations showed that ASTM and ISO mechanical standards are representative of all the pertinent standardized mechanical testing methods. A number of industry groups have developed protocols for reference tests. These tests are useful and some eventually become standards. Such tests are not covered here for three reasons: there are many such tests, often they are specific to an industry sector or product, and many are consensus methods that may lack the rigorous scientific basis and round-robin verification that is required for a standard.

3. GOVERNMENT AND ACADEMIC SUPPORT In 2009 the National Science Foundation (NSF) developed a Roadmap for

Additive Manufacturing [24]. This roadmap addressed various challenges faced by the industry in the areas of design, materials, biomaterials, and energy and sustainability. The main recommendations from this roadmap were to:

• expand the capabilities of solid modeling to support additive

manufacturing, • develop better closed loop and feedback control, • develop predictive process-structure-property relationships integrated

into CAD/CAM tools,



• improve physical models of AM processes to maximize the properties of AM parts,develop and adopt internationally recognized standards which are useful to product, process, and material certification

Chapter 6 of this report highlights the types of material properties that are

measured to determine the engineering properties used to design structures. In addition, understanding the anisotropy of AM parts allows specific functionality to be built into the manufacturing process and this allows unique capabilities not achievable through other manufacturing methods. NSF funded additive manufacturing to a total of $200M from 1986 to 2012 [24].

In 2012 the creation of the National Network for Manufacturing Innovation (NNMI) was started by the Executive Office of the President to develop regional advanced manufacturing hubs [4]. One of the first hubs was AmericaMakes. AmericaMakes is a partnership among the Departments of Commerce, Defense, Energy, NASA, and NSF. This effort led to the funding of the National Center for Defense Manufacturing and Machining (NCDMM) in Youngstown Ohio, which brings together a regional network of 14 research universities, community colleges, 40 industry partners, and 10 non-profit organizations spanning Western Pennsylvania, Eastern Ohio, and West Virginia. This effort is funded with a $40M private match to the initial $30M federal investment. NIST’s Measurement Science for Advanced Manufacturing program (MSAM) has funded NCDMM with $5M for research to ensure quality parts are produced from LS of metal powders [25]. The NCDMM mission is to build a national network for additive manufacturing and 3D printing technologies in the U.S. [5].

Currently, AmericaMakes is funding projects for ME processing of high-temperature commercial polymers, ME for complex shape composite tooling, and AM manufactured composite tooling for hydroforming. The University of Texas at El Paso has formed the W.M. Keck Center for 3D Evaluation which has one project to develop integrated technologies for multi-material structures [4]. In addition, materials characterization, quality control, data sets for process-property validation, and tailored materials have been identified as critical topic areas in both project calls from AmericaMakes.

Additive manufacturing has led to several centers of development within academia. The first freeform manufacturing center was started in 1988 at The University of Texas at Austin. Pennsylvania State University started the Center for Innovative Materials Processing through Direct Digital Deposition (CIMP-3D) for metallic AM with the following government partners: DARPA, ONR, and Battelle. This consortium is developing an advanced


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process modeling and analysis framework for metal manufacturing. This framework includes incorporation of material science principles (microstructure, kinetics, and thermodynamics) to predict and control the final properties of the additive manufactured part. The Consortium for Additive Manufacturing Materials (CAMM) was started in 2013 as part of the NIST AMTech program ($500K) and will develop a comprehensive roadmapping effort for new types of additive materials [26]. The American Lightweight Materials Innovation Institute is a partnership between the Edison Welding Institute, The Ohio State University, and the University of Michigan. The University of Connecticut has recently opened the Pratt & Whitney Additive Manufacturing Innovation Center to focus on metals. The Michigan Technological University hosts the Open Sustainability Technology Laboratory. This laboratory supports the development of open source software and hardware for all forms of additive manufacturing. The goal is to make additive manufacturing technology fully accessible to any user. North Carolina State University hosts the Center for Additive Manufacturing and Logistics. Northern Illinois University was awarded $2.4M through the NIST MSAM program to develop physics-based AM models for process control and quality assurance [25]. Many of the academic efforts focus on the development of new materials, improvement of existing materials, development of instrumentation and test methods, and process material-property relationships for aerospace and biomedical applications. There is overlap within projects, but they are directed at improving the AM process and final products.

Additive manufacturing has recently been demonstrated for more than small parts and devices. Oak Ridge National Laboratory has partnered with Cincinnati Incorporated to develop a large-scale polymer additive manufacturing system, with a goal of increasing speed by 500 times and building components that are 10 times larger (> 1 m3) than typical AM parts [27, 28]. Local Motors is a company that specializes in the development of large area additive manufacturing machines. This company additively manufactured an automobile during the International Manufacturing Technology Show within the exhibition hall [29]. They have recently opened an office in the National Harbor area of Washington D.C.; the large scale printer and polymers used for this demonstration were developed through collaborative research supported with ORNL. Andrey Rudenko additively manufactured a 2-story concrete home in the shape of a castle out of concrete [30]. While these applications remain a technical curiosity, they represent the desire to push the industry to stronger materials and applications critical to life safety.



One of the advantages of additive manufacturing is the accessibility of the technology to individuals, especially for soft materials. A large community of do-it-yourself consumers has grown up to provide open source plans for building machines, control software, start new small manufacturing businesses, and release of downloadable CAD drawings. The ease of technology transfer has spurned innovations in manufacturing at all levels. The growth of sub-$5K consumer-level printers is expected to increase [31]. Further information on advances in the open source additive manufacturing market may be found at RepRap project [32] and Makerspace [33] websites.

The significant investment in AM from public-private partnerships, entrepreneurs, and the general public is expected to lead to new manufacturing technologies, better materials, and new markets for AM. Engineers require science-based standards to support design and validate mechanical performance. Machine manufacturers require methods to predict performance based on processing parameters (e.g. extrusion temperature, extrusion nozzle shape, extrusion velocity, etc.), which requires incorporation of polymer physics, constitutive equations, and improved process control methodologies. Standards will be critical to supporting competitiveness as these efforts mature to widely available manufactured products.

4. MATERIALS Polymers are critical for AM because they represent the greatest market

penetration and user accessibility [31, 32, 33]. The importance of polymers to materials in this community has been documented through a survey of “commons based peer production” of users available through the web [34], where polymers represent the major source of printable materials. This was a survey conducted within the open source community, mentioned earlier, to gauge their materials and manufacturing needs. Figure 1 shows the breakdown of materials used in AM printing by tonnage used in 2013. Photopolymers generated over $239M in market revenue and the market is expected to reach $470M in 2022 [13]. Laser sintering equipment is not yet easily obtained by most non-manufacturing consumers because of the cost of the equipment and the difficulties in handling dispersible metal powders. Although polymers lag metals in the development of structural mechanical properties, the potential to impact many more markets from consumer products, sustainable applications, advanced manufacturing, and biomedical devices is far greater for polymers [15].


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Figure 1. Pie chart showing the types of material utilized for AM as fraction of material consumed by AM from a user survey [15].

There are a number of polymeric materials available from machine manufacturers and choices are dependent on the methodology used to create objects. A survey of websites from major equipment manufacturers, Statasys [35], 3D Systems [36], and Makerbot [37], reveal material choices that include acrylonitrile-styrene-butadiene (ABS), polycarbonate (PC), polylactide (PLA), toughened polystyrene, nylon, toughened polycarbonate, and polyurethane. Many of these materials are toughened to improve impact and fracture performance, but it is not clear whether AM takes full advantage of these properties. These materials are mainly employed for prototyping designs and the creation of low performance parts, but the demand for new higher performance polymer materials and composites is growing.

There are several examples of advances in materials, machines, and control strategies to support these advances. Equipment manufacturers offer materials that perform at high temperatures, and are chemically resistant such as polyphenylene sulfide (PPS), polyetherimide (PEI), polyphenylsulfone (PPSU), and polyether ether ketone (PEEK). There are composites systems based on glass fibers, carbon fibers, and dispersed nanomaterials [38]. The car produced via material extrusion at the International Manufacturing



Technology show utilized a discontinuous carbon fiber composite developed at ORNL through the DOE Manufacturing Demonstration Facility [39]. There are additional technologies to print both discontinuous and continuous carbon fiber composites [39, 40]. Arevo Labs has announced, through a press release, the capability to print multiple high performance polymers using a combination of Solvay polymers paired to advanced instrumentation control, with a claim to deliver parts designed to achieve predicted performance [41, 42]. The Army Research Laboratory (ARL) has demonstrated the capability to produce multi-functional composites using an electric Field-Aided Laminar Composite (FALCom) processing technique [43]. This process utilizes electric fields to align nano- and micro- particles into chain-like structures that are cured into place in the photopolymer. This increases strength and provides conductive pathways for multifunctional performance. Finally, a survey of recent meeting abstracts and industry press releases show the industry is moving towards printing multiple materials, colors, and functionality within the same part. This will increase the complexity in predicting the final properties of the materials.

While the choices for materials and manufacturing processes continue to improve, the ability of stakeholders to compare materials/machines for development of part design and performance has not followed a parallel path. Performance property information within the technical data sheets for these materials is not standardized. There are no defined standards to classify materials or standardized processing/manufacturing parameters used to specify the properties of a final part. A search of the technical data sheets for materials used in additive manufacturing that includes fused deposition and direct printing materials provides an idea of the important mechanical properties and the standard tests used to specify those properties. The standards are ASTM D638 (tensile strength, elongation at break, modulus of elasticity) [44], ASTM D790 (flexural strength, flexural modulus) [45], ASTM D256 (Izod Notched Impact) [46], and various hardness scales. According to ASTM D5592 [47], several of the above-referenced ASTM standards are applicable to the development of engineering design properties for load-bearing plastic components. These include ASTM D638 (tensile), ASTM D695 (compression) [48], ASTM D2990 (creep) [49], ASTM D3418 (transition temperatures for semi-crystalline polymers) [50], ASTM D4473 (dynamic mechanical for cure behavior) [51], D5045 (plane-strain fracture toughness) [52], ASTM D5279 (dynamic mechanical properties in torsion). ASTM D5592 [47] was used to identify the ASTM standards typically used in engineering design of plastics for this survey.


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While manufacturers provide some traceability to mechanical property testing, the information is not complete. For example, some manufacturers provide printing parameters and others provide no printing parameters. The quality assurance provided on the raw materials prior to printing is not defined and polymers remain a black box. Manufacturers control their interactions with suppliers and the resins are tailored for their machines. Variability within molecular weight and distribution, dispersed phase volume concentration, viscosity, void content, crystallinity, range of additives and other performance qualities required for a specification are not reported. These materials are still fabricated for existing, conventional manufacturing processes such as compression or injection molding. The range of melt parameters, e.g. melt viscosity, for a commercial extrusion/injection process may not be compatible for the AM process and the lack of standardization has led to material disclaimers provided on technical datasheets, for example:

"The performance characteristics of these materials may vary according

to application, operating conditions, or end use. Each user is responsible for determining that the Stratasys material is safe, lawful, and technically suitable for the intended application, as well as for identifying the proper disposal (or recycling) method consistent with applicable environmental laws and regulations. Stratasys makes no warranties of any kind, express or implied, including, but not limited to, the warranties of merchantability, fitness for a particular use, or warranty against patent infringement. The information presented in this document are typical values intended for reference and comparison purposes only. They should not be used for design specifications or quality control purposes. End-use material performance can be impacted (+/-) by, but not limited to, part design, end-use conditions, test conditions, color, etc.

Actual values will vary with build conditions. Tested parts were built on Fortus 400 mc @ 0.010” (0.254 mm) slice. Product specifications are subject to change without notice.”[35] As will be demonstrated in the specific standards discussion below,

process variables and manufacturing design can lead to final parts with anisotropic, inconsistent or substandard performance.



5. MECHANICAL PROPERTIES FOR DESIGN Accurate mechanical property measurements are required to select

materials and design a structure for its intended application. Engineers utilize this knowledge to make material decisions in both safety-critical and non-safety critical designs. These properties are determined using accepted measurement standards, certified databases, or reference materials. Applicable test standards are determined based on the final usage of the material, inherent weakness in the design, durability requirements, and safety factors. The Department of Defense Composite Materials Handbook has a useful reference that illustrates the staged process of a “building block approach” for determining the properties of the material and transitioning that information to the performance of the system [53]. Figure 2 shows the building block approach utilized to minimize the risk of new material insertion into aerospace structural systems. The building blocks to safely incorporate new materials into structural design rely on increasing the scale of testing from coupon tests to more complex component tests and finally full scale tests. This approach will be used to illustrate where standards for mechanical property measurements of AM manufactured parts are needed to incorporate new materials or designs into systems.

The parameters that define the system are collected within the design considerations (lower block, purple in figure). Engineers require estimates of the loading, temperature, moisture environment of the application, and material information such as mechanical properties, long term stability, susceptibility to damage, and manufacturing cost prior to designing a part. Design considerations for AM follow the same requirements as any other manufacturing process. Supporting technologies (left block, yellow in figure) are the technologies to obtain, measure, and validate the material information. In AM, this can be difficult because material property information is often controlled by manufacturers and dependent on AM processing. The engineer must decide how material properties will be validated through standard test methods, how statistical analysis will be done to determine the properties of the population, estimate the needs for post-processing of the part, and determine whether non-destructive techniques are required to validate internal structural dimensions. If these properties are dependent on material source and machine manufacturer such as in AM, it becomes difficult to estimate the level of Supporting Technologies required. The Building Blocks bring all of these considerations together (center block, green in figure). The building blocks allow engineers to understand material performance, joining or bonding


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performance, performance of components working together, and finally full incorporation into a full scale system test. In AM this may require significant effort for coupon level tests such as printing a single bead of material to optimize extrusion parameters, moving to ASTM dog bone geometries to identify optimal print layouts and finally incorporating the AM part into the other elements and components for further testing. In today’s modeling intensive world, many of these concepts are validated via simulation. Therefore, science-based standardized testing is required for AM materials to support accurate simulations. This testing requires confidence in material mechanical properties and performance, which is an area that AM is lacking.

Figure 2. Building block integration for the development of composite structures [53].

The building block integration approach (Figure 2) may be further expanded to highlight the type of information gathered from each of the building blocks. Figure 3 shows the building block approach for a commercial aircraft composite primary structure to provide an estimate of the level of experimental effort anticipated at each stage of the building block. The use of aggressive environments (Environment) allows engineers to scale allowed loading or strain limits for the material based on the effects of temperature, moisture, or cycling. Coupon level testing relies on simplified sample and test geometries that deliver specific information such as mechanical properties (modulus, strength, etc.), interlaminar properties, adhesive properties, and durability (highlighted Coupons and Elements). These tests may easily number



in the thousands to build statistical confidence in performance. Success beyond the coupon level leads to manufacturing smaller structures with increasing complexity for assembly (inclusion of joints and bolts) and loading (stress concentrations, off-axis loading). The costs per subcomponent increase, therefore, sampling is smaller. The coupon level and element level require a firm understanding of the impact of manufacturing processes on performance, and demand a science-based standardized testing framework to increase success in the final subcomponent stage. For any processes, weaknesses must be identified at this time to prevent costly engineering choices in the final structure and before manufacture and testing of large panels or subcomponents. An additive part will readily cross between the coupon and element level because performance may be defined by internal fusion joints, print directions, manufacturer, additive method, and material supplier. Large panels permit the validation of the design concepts and analysis methods are fully validated. Failure or success of the structure can validate the material design values developed from earlier testing. Each level of testing is used to support structure performance models to reduce test costs, but models rely on accurate material and interface property measurements. Accurate material and interface property measurements rely on a solid framework.

Figure 3. Building block approach for the support of composites structures in the 777 aircraft [53]. In this graphic durability is defined through moisture stability, temperature stability, creep resistance, and fatigue resistance.


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From the stratified view of the building block manufacturing or qualification approach, there are identifiable challenges for AM. The first challenge is the identification of test methods for characterizing the mesostructure of the AM part at both the structural level and the molecular level. The mesostructure is defined at the structural level by anisotropy in the axial direction of extruded material, the presence of voids, and the degree of curing between reacted or melted layers during the deposition process. Many AM systems allow for complete infill (no voids) for ME and voids may not be as prolific in sintering, this does not eliminate anisotropy in mechanical performance. The mesostructure at the molecular level is defined by the degree of mixing between polymers with differing thermal histories, degrees of dispersion in composites, the coefficient of thermal expansion mismatches, and the adhesion between dissimilar materials. Both length scales lead to anisotropy in performance. Figure 4 is a cross-section of an ME produced part that highlights the potential mesostructure present from incomplete bonding between lines of extruded material. The majority of literature concerning material extrusion processes addresses the impact of different void space between lines of extruded material, orientation of beads to load direction, and the influence of build direction. The current focus for this overview will be the combination of material direction and void space that leads to the structural level mesostructure. These types of porous structures represent a significant challenge for engineers to determine safe design parameters. Engineers need to understand the strength of the fusion, strength of the bead, and the micromechanics of a porous structure. Standards provide the framework of scientifically grounded test geometries and methodologies to address the challenges of AM and facilitate a smooth transition between boxes to produce design parameters.

Given the complicated mesostructure of AM parts, the scope of this standard review was expanded to include standard tests used for fiber composite materials. Fiber composites are lightweight structural materials that contain high strength fibers (E~70 GPa to 250 GPa) embedded in a polymer matrix material (E~ 3 GPa to 4 GPa). The fibers carry any significant loads and the matrix is designed to provide rigidity to the fiber structure. Since the fibers are stronger in the axial direction, the resulting composites can have significantly anisotropic properties. This anisotropy has been addressed in standards by specifying the directionality of the fibers. Figure 5 shows an example of fibers oriented in different x,y directions along the z-plane. If this rotation is done symmetrically, the composite laminate may be considered quasi-isotropic in material properties in the plane of the fibers. While



geometry guidelines are critical for fiber composites, it is not clear whether these are valid for AM materials since stress distribution through the AM mesostructure will differ greatly from a fiber composite.

Figure 4. Mesostructure of an ME-ABS material illustrating the porous structure that may be achieved based on the raster angle and bead overlap [54].

Figure 5. Graphic illustrating the directionality of fibers, through z, in a fiber composite laminate. The orientation and number of fibers controls the performance of the composite.


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Mechanical properties: These properties are often defined in terms of the behavior at loads that do not produce failure or failure behavior itself. For solids, the first case is generally characterized by moduli that are defined as the stress divided by the strain. Moduli can be measured with a number of different loading modes: tension, compression, flexure, shear, or torsion [55], and in the linear region, they are proportionality constants independent of strain. The moduli of polymeric materials are functions of temperature and time. These dependencies, particularly temperature, are important. For many polymers used in additive manufacturing, these properties are often reported in the technical specification sheet in tension (or compression) and shear loading modes. Since two parameters are needed to model behavior in different loading directions, an alternative is to report Young’s Modulus and Poisson’s Ratio. Poisson’s Ratio is the negative ratio of transverse to axial strain.

Figure 6. Three modes of crack surface displacements Mode I (opening or tensile mode), Mode II (sliding mode), and Mode III (tearing mode) [56].

Failure Properties: As the stress increases, materials begin to fail via plastic deformation (nonlinear stress vs. strain) or brittle fracture. One approach to quantify failure behavior is by determining yield strength, ultimate strength, and impact strength. Each one of these may be defined in relationship to the mode of loading: tension, compression, flexure, shear, or torsion [55].



For many polymers used in additive manufacturing, these properties are often reported in the technical specification sheet in tension or compression loading modes. On the other hand, these parameters do not adequately characterize materials that fail by the propagation of cracks. To describe this behavior requires fracture parameters that are often not reported in the technical sheet [53]. The fracture toughness or fracture energy of the material is [55] defined in three modes (I: crack opening, II: in-plane shear, and III: out of plane shear) as shown in Figure 6. Standards are available to test in Mode I, Mode II, and mixtures of Mode I and II, although the complex shapes of many AM parts may increase the importance of Mode III. Fracture toughness is a critical factor for ME and LS created polymer parts and given the complex mesostructure and anisotropy, it may be difficult to induce purely one loading Mode for testing.

The terms used to define failure properties are given below:

Yield Stress/Yield Strength Ultimate Strength

Elongation at Yield Elongation at Break Fracture Toughness

Fracture Energy Impact Strength Bearing Strength

Open Hole Compression Strength Crack Growth Resistance Curves

Bearing and open hole tests evaluate the ability of the material to perform

with an engineered flaw such as a bolt or pin in the structure. Interlaminar properties are important for fiber reinforced composites. As

shown in Figure 5, the fibers lie in a plane and provide high strength and stiffness in that direction. Although designers plan structures so the loads are in the fiber direction, unexpected events can produce loads perpendicular to this plane, which causes damage. This can significantly reduce the performance in the fiber direction, particularly compressive strength. Often the fiber-matrix interface represents the weak point of the composite because it relies on bonding between the fiber filler and the compliant matrix. Interlaminar test methods allow the user to understand the susceptibility of the material to damage between fiber layers. Unfortunately, interlaminar properties do not provide explicit engineering limits on the maximum load that


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a composite material can bear in off-axis loading, but tests like interlaminar fracture and short beam strength can indicate general ranking. For composites, standard test methods for interlaminar properties are:

Short beam strength Shear modulus

Ultimate shear stress strength Durability: Durable material properties may be quantified using several

standards depending on the usage of the material. In the context of this review, durability is addressed for mechanical durability, specifically creep and fatigue. There are standards available in ASTM and ISO that define exposure conditions for moisture, temperature, and artificial sunlight exposure on materials. Creep is important for understanding the ability of the material to withstand long-term static loading. Fatigue properties are important for understanding the ability of the material to withstand cycling loading during usage. Finally, impact strength is a measure of the material to withstand high strain rate loading and evaluate the ability of the material to absorb energy and resist damage. Mechanical properties that define mechanical durability are:

Creep Modulus

Creep Rupture Fatigue life (S-N plots/R-N plots)

Standards are important for mechanical and failure properties because they

are the language that mitigates risk within the design and allows engineers to build structural and safety critical parts with a known performance window. The specific ASTM and ISO standards that are applicable to quantify the material properties listed above will be addressed individually.

6. CHALLENGES FOR MECHANICAL PROPERTY CHARACTERIZATION

While AM provides the opportunity to quickly go from design to product

especially for parts that have difficult or impossible to machine features, challenges remain for predicting mechanical performance. Many AM processes differ from traditional polymer processing in that not all of the



material is melted and homogenized. The AM process of depositing layers of polymeric material results in parts with anisotropic properties, residual stress, and this is a significant challenge. Researchers and Original Equipment Manufacturers (OEMs) must establish the standardized methods to determine material properties from AM processing rather than the mechanical properties of a particular design. Peer-reviewed literature has begun to highlight complexity in relating material properties, AM mesostructure, and part design for standardized testing. Despite the importance of this problem for the success of AM as a critical manufacturing process, the literature available in this area is not significantly large. Figure 7 shows the typical geometrical variables related to ME deposition geometry.

Figure 7. Graphical representation of process variables related to the build geometry for additive manufacturing. a) raster angle, b) extruded filament height and layer width, c) air gap between extruded filaments, d) combinations of variables (T, air gap, width, height, velocity) can increase the coalescence between filaments, e) the build direction can affect the load transfer between filaments and interfaces.

The majority of studies have focused on the impact of raster angle, air gap, filament width, layer height, and build orientation to major part axis (i.e. x, y, or z-direction). Build orientation allows one to capture interacting factors such as filament length, temperature gradients, and nozzle velocity on mechanical properties. Many additive systems that rely on material extrusion processes allow for full infill of the deposited part, but it was not possible to compare


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literature studies directly. Evidence for mechanical anisotropy was more readily identified in the literature for powder bed fusion processes.

The impact of processing for anisotropic mechanical properties in material extrusion has been identified by multiple authors with ME processing of ABS polymers as a major focus [54, 57-68]. In the case of ME, AM parts exhibit inferior mechanical properties compared to the as-received polymer filament that feeds the AM extruder head and compression molding or injection molded parts constructed from this same as-received filament. Raster angle is a process parameter that influences anisotropy and strength in AM parts. Rodriquez showed reductions in modulus can range from 11% to 37% [65]. In general, parts were stronger when the beads were aligned in the loading direction for tensile loading [57, 64], mixed angles for flexure [57, 58], and orthogonal to loading for compression [67]. Ahn used twelve layers of ABS oriented in [0°], [45° /- 45° ], [0° /90° ] and [90° ] to investigate the effect of raster angle [57]. Alignment of long fibers with the loading path increased strength, but gaps between bead layers reduced strength. Similar work in unidirectional deposition of materials supports this idea. Es-Said found that increasing raster angle lowered strength, but had little effect on modulus [59]. They also found that failure tended to occur in the deposition lines. As the raster angle increases the tensile properties of the material are reduced [57, 65]. Huang has shown that tensile and shear properties of unidirectional ABS parts reach a minimum around raster angles of 50º [61]. Similar results have been found for ME production of PC materials. Masood could achieve approximately 80% of an injected molded material by aligning the beads with loading [63]. Hill conducted a systematic study of raster angle on unidirectional parts with similar density and found that aligning the beads in the axial direction produced the best failure properties [60]. The strength could be modeled using micromechanics to account for the fusion strength.

The air gap influences void percentage, which plays a large role in determining mechanical properties. The strength of the AM part is not only derived from lines of extruded filament, but the interaction between filaments is important. The presence of voids and sharp corners increase stress within the part and failure can initiate at these features [58]. In general, minimizing the gap between filaments increases the contact area between filaments and leads to a stronger fusion interface. Rodriguez has shown that a combination of process parameters may be used to control the void space, and he could model the resulting structure using laminate theory [65]. ME relies on the thermal gradients between neighboring filaments to allow the thermoplastic polymers to diffuse and form a stronger fusion interface. These fusions have



been compared to the matrix in a fiber reinforced composites because they hold the filaments together [65]. The time required to maximize bond strength between successive filaments is not immediately known from technical data sheets, but it is related to the structure of the polymer (molecular weight, glass transition temperature, crystallinity, etc.) and the thermal gradient between the new filament and the previous filaments. A guideline was developed to minimize the air gap and optimize the bead width or height to increase contact area in order to improve bond strength. Sun observed temperature profiles in ME and bead interface formation to quantify interface formation. He found that envelope temperature and convection in the build chamber have a significant effect on mesostructure [69]. Sood et al. used a response surface developed from design of experiments coupled to neural network analysis to demonstrate the non-linear behavior of printing parameters and utilized neural network modelling to predict compressive properties [67, 70]. Another empirical study led to six build rules to help designers maximize mechanical performance [57]. These studies have highlighted the complicated relationship between the build parameters, part design, and final properties.

There are examples of unintended consequences from not accounting for interacting parameters. Decreasing the bead width reduces residual stress in the filament and can increase diffusion length, but requires more laps to create the part. Residual stress is caused by the contraction of the polymer filament on cooling. In the case of semi-crystalline materials, the contraction of volume from crystallization increases residual stress at interfaces. In order to fill the space with a smaller filament width, the nozzle must complete a higher number of long passes and short passes for direction reversal. The extruding nozzle will change speed to accommodate the printing process; the corollary for sintering is laser raster speed and energy. The successive thermal cycling caused by changes in nozzle speed impacts thermal diffusion because speed changes impact residence time [70]. Another example is maximizing the temperature and filament height can lead to part distortion and surface roughness that negatively impacts appearance and mechanical properties. Orientation of the polymer during extrusion will reduce strain to failure up to 33% [58] and increase the time for polymer diffusion, but orientation has not been sufficiently addressed in the literature. Failure in these materials is often brittle and driven by build construction. Larger filament widths, oriented polymer within beads, and poor diffusion are all suspected to lead to inter-laminar failures facilitated by defects and surface roughness at the fusion lines. These factors need to be accounted for in models, validated through


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standardized testing, and incorporated into the process workflow to support AM.

It is difficult to isolate the impact of build direction on anisotropy because this is often confounded with other build parameters. Riddick et al. combined xz, yz, and xy build directions with changes in bead length and raster angle combinations of [0º], [0º /90º] and [90º ] and found that the xz direction had the highest modulus (2.67 GPa) and strength (15.26 MPa) due to a combination of void filling and optimal raster geometry [64]. Others have found that build paths that maximize alignment of fibers in the loading direction increased strength [58]. There are a small number of studies investigating the durability of ME parts. One study showed that a PEI/PC blend aged at room temperature and water was stable over 52 weeks and relatively insensitive to short time exposure to aerospace solvents [71].

Similar challenges exist for powder bed fusion processing and concern the impact of thermal gradients and void space in the part. If the layer heights are too large, the powder acts as a thermal insulator which prevents heat transfer to the lower powder layer. This can reduce the consolidation of molten material into the gaps below and inter-diffusion of polymer species [7]. The crystallization temperature of the material is important for the increase of bonding between the molten beads [72]. Shrinkage has been shown to depend on different build parameters based on laser raster direction, where laser power and scan length are critical for x-direction while hatch spacing and part bed temperature are critical for the z-direction [73]. Recycled powder presents a challenge for laser sintering. There is potential for polymer aging and crosslinking for a material that has been previously processed, but not sintered. This aging increases melt viscosity and prevents flow, which creates weak interfaces and defects within the part. Thermal control of the powder bed is also important. Improper powder bed temperature leads to interruption of the thermal gradients during the sintering process which can lead to reduction in performance [72]. Leigh found that increasing the size of the polymer interface, characterized by an h/r ratio (see Figure 6b and 6d) increased the yield stress and ultimate tensile strength [7], but there was limited information found on durability studies for LS parts. One study showed that these materials exhibited a slight drop in mechanical properties when subjected to aggressive automotive fluids, similar to the performance of the base polymers [74].

In the case of material jetting, the kinetics of the photo-polymerization reaction and thermal annealing of the part must lead to uniform cross-linking between successive layers. Lee found that 3D printing produces anisotropic materials with much lower compressive strength [62]. Additional challenges



include the presence of additives, printing on an over-cured surface, and the proprietary knowledge of formulations. Similar to thermoplastic technologies, the print resin formulations are not known to users. Therefore, the impact of reactants, catalysts, and monomer molecular weight are difficult to separate. The photochemistry literature of dental resins, photolithographic patterning, and radiation curing is expected to contain a wealth of prior art concerning optimization of formulations and processing. A nanocomposite deposition system that produced composite parts, but requires a machining step after successive layers had mechanical performance closer to ME produced parts, although the materials remained anisotropic. Ang et al., investigated the impact of trapped volume (voids) and trapped material for various rapid prototyping metrologies [10]. They found that these artifacts affect the functionality and dimensional stability of the final part, but they did not measure the mechanical properties.

Research is moving towards a better understanding of raw material microstructure, the physics and chemistry of polymer fusion and photopolymerization, and utilizing composite modeling to understand anisotropic properties as a function of the AM mesostructure. Fusion and coalescence have been presented in terms of a Frenkel-Eshelby formulation [61]. Huang was able to combine a force model with an understanding of the magnitude of coalescence to predict the performance of ABS. Composite models have been used to understand the deformation of these materials with an eye toward design variables and identifying fusion contributions to properties [58, 60, 65]. Many of these studies have been done on industrial or consumer-grade equipment [68], and it is not clear how the structure of the raw materials and tolerances of the process variables were controlled. There is much work that is required to drive current research towards industrially relevant AM standards.

7. OVERVIEW OF STANDARD TESTING METHODS There are two standards groups addressing AM. In ASTM committee F42

has jurisdiction over Additive Manufacturing Technologies and subcommittee F42.05 addresses materials and processes specifically [75]. TC 261 promulgates standards in the AM fields for the ISO. Both of the groups currently address mechanical testing of AM materials and parts via reference to established standards [76]. The following analysis describes the


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applicability of existing standards to mechanical testing of polymer AM materials and parts.

7.1. Tension These standards are classified for plastics (ASTM D638 [44], ISO 527-2

[77]) and composites (ASTM D3039 [78], ISO 527-4 [79, 80]). The standards utilize dog-bone or end tab specimens whose geometry is based on the thickness of the sample or the type of composite. Tension measurements provide Young’s modulus, Poisson’s ratio, Yield Stress, Strength, and Elongation to Break. The standards for composites address the orientation of fibers within a composite, but the applicability of such standards to AM materials has not been thoroughly described in the literature. Ahn found that the ASTM D638 type I sample geometry caused premature failure of specimens. Early failure was caused by a stress concentration within the radius of the dog bone near the gauge length. This area of specimens contained the ends of filaments, which caused excessive shear. The authors switched to the ASTM D3039 geometry to alleviate the problem. However, this was the only reference found with this problem. ISO 458 [81], which was not reviewed, provides the test standard for the stiffness of a material under torsion.

7.2. Flexure ASTM D790 [45] and ISO 178 [82] are equivalent standards that utilize a

three-point bend method to measure the flexural modulus, flexural strength, flexural stress and strain at break within a 5% strain limit. If the strain limit is not met, then ASTM D6272 [83], which is a four-point method, is used to increase the chance of achieving a failure measurement. This test reduces the stress concentration associated with the center roller in a three point test. These standards are applicable for unreinforced and reinforced materials. For composites containing high modulus fibers, ASTM D7264 [84] should be used for testing. The standard does not address the specific challenges for AM materials that may have anisotropic properties.



7.3. Compression The applicable standards for compression measurements are ASTM D695

[48] and ISO 604 [85]. ASTM D3410 [86] and ISO 14126 [87] are specific to compression of a fiber reinforced composite in-plane direction. The standards provide measurement of compressive modulus, compressive yield stress, compressive strength at failure, and compressive strain at failure. There are geometrical restrictions for the diameter and height of the sample.

7.4. Shear There are a number of different standard tests to measure the shear

modulus and strength of materials. The fiber reinforced composite standards (ISO 14129 [88], ISO 14130 [89], ASTM D2344 [90], and ASTM D3518 [91]) are not directly applicable to AM. These methods are developed for polymers reinforced with high strength fibers or textiles in specific orientations to the loading direction. The standards require determination of specific inteiaminar failure modes between the aligned fibers. These types of samples are not typically manufactured in AM. There are also two notched specimen standards ASTM D7078 [92] and ASTM D3846 [93] for measuring shear properties. These standards utilize specimens with specific notch geometries and defined alignment of the fiber reinforcement. These test methods may not be directly applicable to AM manufactured materials for two reasons. First, AM produced parts do not possess the large ratio of moduli and failure strength in different directions that is found in fiber composites, therefore the mechanics of load distribution and crack propagation will be different in an AM material. Second, composite laminates may be manufactured with sharp initiation cracks in the matrix between the fiber layers. These cracks increase the precision of the test used to measure a material failure property. The current thermal processing methods in AM are not conducive to intentionally producing a well-defined, sharp initiation crack, and this will hinder the ability to characterize failure behavior. There are methods to introduce a sharp crack into a material. Fatigue, which is used for metals, does not work for polymeric materials, and the use of a sharp edge, such as razor blades, may not produce a proper crack in AM parts.

There are only two shear standards that are directly applicable to AM. These are ASTM D4255 [94] and ISO 15310 [95] to determine the shear modulus of plastics and fiber reinforced materials, respectively. These


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standards allow for testing isotropic materials, but there is no guidance for testing materials constructed via AM.

7.5. Creep Creep measurement standards provide the methodology to measure

dimensional changes in samples under load as a function of different exposure environments such as temperature, aqueous, or surfactant solutions. There are a variety of loading environments: tensile, compression, flexure, and solutions. ASTM D2990-09 [49] references ASTM D543 Practices for evaluating the resistance of plastics to chemical reagents for testing in environmental conditions [96] that specifies solution composition for sample immersion. The ISO standard equivalent is ISO 899 [97, 98]. There are restrictions on the ratio of the length to the cross-section. The method recommends testing at a minimum of two different test temperatures within the use range of the material to understand the effect of temperature. Loading at seven stress levels to produce creep-rupture at different times up to 3000 h provides a measure of long term performance. Design data for creep is obtained by testing materials at different stress levels to produce 1% strain in 1000 h. There is no guidance provided for anisotropic samples, such as fiber composites.

7.6. Fatigue ASTM D7774 [99] is the standard for uniaxial loading with no equivalent

in ISO. The test frequency can range between (1-25) Hz, but less than 5 Hz is recommended. This frequency reduce the chances of heat generation in the sample. The test method allows generation of a stress or strain as a function of cycles, with the fatigue limit characterized by failure of the specimen or reaching 107 cycles. The 107 cycle value is chosen to limit the test time, but depending on the applications this may or may not be the best choice. The maximum and minimum stress or strain levels are defined through an R ratio. The R ratio is the ratio of minimum to maximum stress or displacement that the material is cycled through during testing. Testing is conducted within the elastic limit of the material. For this standard, samples may be loaded in either tension or compression.

ASTM D7791 [100] and ISO 13003 [101] are the test methods for flexure fatigue of plastics. The subject matter of the tests is similar, but technically



different. In both tests, the loading is sinusoidal. ASTM D7791 utilizes either a three-point or four-point loading with cycling occurring in positive and negative directions. Control occurs in either stress or strain versus cycle number. The R ratio is -1 and the stress or strains do not exceed the proportional limit. The test ends at either failure or reaching 107 cycles.

ISO 13003 calculates the ultimate tensile/flexural strength for fatigue loading rate. In strain control, the end of the test is listed as the damage level related to specimen stiffness reduction of 20%. Four fatigue levels are tested in accordance with the fatigue life of interest or the range of stress/strain of interest. Similarly, strain or stress vs. the number of cycles is reported. Neither standard addressed the challenges associated with AM processing inducing anisotropy within the materials.

There are two standards that relate to fatigue delamination or crack propagation. These are ISO 15850 [102] and ASTM D6115 [103]. Both of these standards are specifically applicable to the measurement of fracture energy in the interlaminar region of a fiber composite. Similar to other composite specific standards, it is not clear whether AM materials would meet the fracture mechanics assumptions that support these standards.

7.7. Fracture Toughness Fracture toughness measurements are used to determine the energy

required to initiate crack propagation from a precrack within a material or composite. These values are used for designing parts and developing materials. These standards often require the development of a sharp pre-crack within a material, application of load, and monitoring of the load, displacement, and crack progression. A linear elastic fracture mechanics analysis was used to develop measurements of fracture energy (GiC) and fracture toughness (KiC), where i indicates the mode of loading I, II, or III, see Figure 6. In composites, these measurements are used to determine the fracture toughness between plies containing high modulus fibers or textiles. In polymers, this test provides material properties for engineering design. These often require the insertion of a pre-crack material to create the sharp crack, which may be problematic given the dimensional limitations for AM processes.

ISO 15024 [104] and ASTM D5528 [105] are specifically for fiber reinforced composites. These standards are used to generate crack growth resistance curves (R-curves), which are measures of delamination resistance through the composite. They are not directly applicable to current AM


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manufacturing because continuous fibers are not present. It is not immediately clear whether these concepts could work for evaluation of the mesostructure or fusion zone in an AM material. ISO 29221 [106] is a standard for the plane-strain crack-arrest toughness in a compact tension specimen. This requires a sharp initial crack and a groove in the sample to limit the location of crack growth across the specimen. The challenges in AM relate to the dimensional resolution of the AM process and whether the impact of build direction on the propagation of the crack.

ISO 13586 [107] applies to the measurement of rigid and semi-rigid thermoplastics and discontinuous fiber composites. The biggest question is the influence of the mesostructure created during the AM process to measure a material property. An amendment to ISO 13586 [108] provides testing injected molded composites with directions for testing longitudinal and transverse processing directions. This would be an acceptable place to start for determining the applicability of this test standard to AM parts.

ASTM D6068 [109] is for the development of J-R curves for plastic materials. This is a specific method to develop an understanding of parameters for cohesive zone modeling of crack propagation. The method requires optical measurement of crack growth. This method may be applicable to specific types of deposition processes such as powder bed fusion of clear thermoplastics or material jetting of clear photopolymers, but it would still require the machining of a crack into the sample.

7.8. Impact ISO 179 [110, 111] and ASTM D6110 [112] describe the method for the

Charpy impact test. ISO 180 [113] and ASTM D256 [46] are the methods for Izod impact testing. Impact testing is mentioned with technical data sheets for many AM relevant polymers. It is not clear how the AM material is prepared and oriented for impact testing from the material data sheets. The main differences between the tests are the material position and notch placement. The load is applied rapidly by hitting the sample with a heavy striker. In the Izod test, the material is in a vertical position and the notch faces the striker. In the Charpy test, the material is horizontal with the notch away from the striker. The Charpy notch may be a V or a U-shaped notch. Similar to fracture toughness testing, it is not clear whether the notch may be sufficiently deposited into the AM part or should be machined at a later date.



7.9. Bearing Strength and Open Hole Compression These tests represent functional strength tests for composites that will be

fastened using bolts. It can also provide some insight into the effects of a damage region on performance. The specific standards are ASTM D953-10 [114], ASTM D5961 [115], and ASTM D6484 [116]. In ISO, the specific standards are ISO 12815 [117] and ISO 12817 [118]. The tests develop design parameters for integrating composites materials together and within structures. Currently, no demand was observed in the literature for these types of measurement standards by the AM community, which is not to say the demand does not exist. It is feasible that as AM materials are integrated with other structures such as in the human body, the community will need to understand the impact of part design on load carrying capability and whether the part will deform over time. Current standards should be directly applicable, but there should be comments provided on material isotropy.

8. STANDARDS APPLICABILITY ASSESSMENT In the following charts, ASTM and ISO standards were evaluated to

determine their applicability to additive manufacturing. One of three classifications is given for each standard. If special consideration should be noted for the standard, specific notes are provided. The list covers standards for plastic materials and composites. The following labels are used for assessing applicability of the standard:

• YES – The standard should be applicable for additive manufacturing

with very minimal or no modifications. • YES WITH GUIDANCE – The standard should be generally

applicable for additive manufacturing, but there may be limits on its applicability, and some modifications or additional considerations are probably needed. These include: • Geometrical limits on test specimens • Required post-processing such that specimens built via additive

manufacturing meet the requirements of the standard; this typically includes surface finish, dimensional requirements, or pre-crack requirements.




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• Material isotropy requirements. AM specimens often have inherent anisotropy. The measurement methods that specify applicability for isotropic materials may still work, but the measured results may have larger uncertainties. This includes consideration of separating material properties from part properties.

• Application specific considerations, such as elevated testing temperatures or immersion environments.

• NO – The standard requires specimens that cannot be built via AM, even major modifications may not be adequate, another method is better suited for the measurement or the measurement simply is not applicable.

In some cases standards were identified and listed in the applicability

index, but they were not specifically reviewed. 9. POLYMER ADDITIVE MANUFACTURING AT NIST

An emerging program for polymer AM started in MML in 2014. This

program is not a component of the current EL additive manufacturing effort focused on metal AM, but the two laboratory efforts are collaborating to leverage expertise. As mentioned earlier, thermal fusion is a critical component to the strength of AM parts. Maximizing adhesion across the fusion zone requires a balance between diffusion time, residual stress, and maintaining dimensional stability. There are 9 projects addressing challenges in metal and polymer AM. The polymer centric projects are shown below:

FY2015 Projects Division Project Leader (s) Polymer Focus: Mechanical Strength of Additive Manufactured Polymeric Materials

647 Jason Killgore

Interfaces and Bonding During Additive Manufacturing of Polymeric Materials

642 Kalman Migler; Kathryn Beers

Metrology of Defects and Distortion at Interfaces in Additive Manufactured Polymeric Materials

642 Kalman Migler; Ronald Jones

Metrology of Non-metallic Precursors and Relationships to Final Product Quality and Performance

643 Greg Gillen; Michael Verkouteran



FY2015 Projects Division Project Leader (s) Nanomechanical Property Measurement of Surfaces and Interfaces for Polymeric and Metal Additive Manufacturing Materials

643 Richard Gates

For material extrusion AM processes, the focus is on the interfaces of the

deposition line and developing molecular level measurements to determine microstructure-processing relationships within this fusion zone. These projects utilize AM process variables such as temperature, speed, and extruded filament size to maximize the width of the diffusion zone between model polymer materials. Polymer structure (chain-length, branching, chemistry) is systematically varied along with process variables. The diffusion zone is directly measured using spectroscopic rheology and neutron reflectivity to determine width, orientation, degree of entanglement, and mechanical properties. The neutron reflectivity studies will be jointly supported through the Additive Manufactured Polymeric Materials project led by Dr. Ronald Jones. Experimental results will be used to generate constitutive models for fusion bond strength and process parameters in collaboration with Georgetown University. The Mechanical Strength of Additive Manufactured Polymeric Materials project is led by Dr. Jason Killgore. Dr. Killgore is utilizing sophisticated elastic and viscoelastic measurements based on scanning probe microscopy and frequency shifts during contact with a material to determine mechanical properties through the fusion line. Future efforts in this project will develop localized adhesion measurements for assessing the strength of the fusion line and potential defects caused by processing parameters. Dr. Richard Gates is utilizing a different set of nanomechanical measurement tools to investigate the mechanical properties of the polymer fusion interfaces. The final project is led by Dr. Greg Gillen and Dr. Michael Verkouteran titled Metrology of Non-metallic Precursors and Relationships to Final Product Quality and Performance. This project investigates the impact of resin properties on the final product properties for material jetting processes. In addition, the impact of photopolymer precursor structure and formulation on final properties will be investigated. These efforts are important for developing the measurement science to predict fusion interface strength as a function of processing parameters. The MML effort has a strong focus on structure-property relationships for molecular level performance. This is an important step, but this work would be greatly enhanced with additional work to transition molecular-level knowledge to testing and qualification of AM parts.



There are several areas where NIST laboratories may leverage core expertise to accelerate the measurement science delivered to the AM community. The mechanical properties of any AM part will be a function of the composite and includes the material properties, build geometry (raster angle, filament size, gap, temperature, etc.), voids and defects, and roughness. There are many ASTM and ISO test standards available for AM materials, but a technical basis is required to provide the guidance for testing additive parts to account for anisotropy and develop the models required to predict final mechanical properties. This technical basis will transition the molecular structure-property measurements from MML into standards for testing coupon level samples in the purview of ASTM and ISO standards. Standardizing how materials are built and tested in order to determine mechanical and failure performance for parts in tension, compression, flexure, fatigue, and fracture is key to deploying engineered AM structures with confidence. Further, the development of composite mechanics models for properties of AM parts will inform the MML effort on molecular scale measurements. This engineering effort would close the measurement loop across multiple length scales and provide a route to translate the innovations from molecular scale measurements into the advancement of AM part quality and strength. This is an area where EL and MML may leverage their respective AM programs to improve measurements science, standards, and the AM community. Similar examples of successful collaboration exist in the metal AM programs. A second area of collaboration would be the development of standard reference materials for printing to calibrate the performance of different AM machines. Ultimately, the constitutive equations, standards, and design decisions developed for polymer AM need to be brought into the AM workflow. This is a third area of collaboration between the EL and MML Additive Manufacturing efforts.

CONCLUSION AND PATH FORWARD Polymer AM is gaining attention from many press-worthy applications

that has helped drive advances in materials and manufacturing equipment. There has been significant investment from private industry and government to foster AM growth. Polymer AM equipment is becoming more accessible to the general public further increasing the possibility for distributed manufacturing of high quality parts and increasing innovation in design. Advances in machine design have led to AM with high performance semi-crystalline polymers and



composites, which are creating a demand for better mechanical property measurements. AM process parameters exhibit a non-linear effect on mechanical properties and one of the challenging aspects is maximizing the fusion bond and relating process parameters to polymer structure for thermoplastics, semi-crystalline materials, composites, and thermosets.

This report helped to outline specific measurement and standards recommendations:

1) Engineers require the ability to compare the rheological, thermal, and

size distribution of AM raw materials. In addition, engineers should be able to compare mechanical properties of as received material with material properties of a extruded material. This will alleviate some of the unknowns that users face with printing materials from different lots or printing machines. A classification system should be developed for AM materials so that materials may be classified based on expected mechanical performance. This would allow engineers to identify the physical properties of the polymer such as melt viscosity, molecular weight, and heat capacity, so that material deposition can be optimized.

2) An effort should be made to identify standard reference polymers that could be used to validate AM machine performance.

3) Constitutive equations are required to predict the effect of print parameters on filament geometry and inter-diffusion of polymers.

4) A suite of standard test methods should be developed to support the measurement of material properties for engineering design. The test geometries developed in the standards should be supported by mechanical models to provide an understanding of anisotropy.

5) A better understanding of the durability of AM materials to environmental stresses such as weather, fatigue, creep, and thermal cycling is needed. The presence of residual stress, buried flaws, kissing bonds, and other stress risers will be increase the probability of failure under external stresses.

6) Constitutive equations for polymer structure-property relationships and intelligent printing design to account for intended load paths should be incorporated into the AM workflow to better predict AM final properties.

7) Finally, the AM community of designers and engineers should be supported in the critical aspects of structural properties of polymers



and composites to prevent the treatment of soft materials based on past experience with metallic materials.

International standards organizations, namely ASTM and ISO, have

developed more than 25 standards each (55 combined) for testing mechanical properties of polymers and composites. A few of the composites standards are intended for materials containing high modulus fibers and are not directly applicable to samples made with current AM processes. On the other hand, studies have shown certain composite standards actually improve test consistency on AM materials compared to geometries for non-composite materials. The majority of existing standards are applicable to testing AM produced parts, but guidance is required to specifically address engineering property measurements from AM processes. This report is intended to provide a brief survey of the landscape for polymer AM and summarize current efforts at NIST to identify technical gaps. The growing application of AM to the biomedical industry will require similar support for standards and material classification. There is an opportunity to leverage the efforts in MML to bridge the length scales to provide measurement science, bulk characterization, and standards to support the growing AM industry.

ACKNOWLEDGMENTS The author would like to thank John Slotwinski (The Johns Hopkins

University Applied Physics Laboratory), Don Hunston (NIST/EL), Stephanie Watson (NIST/EL), Jon Seppala (NIST/MML), and Kalman Migler (NIST/MML) for their helpful comments, suggestions, and discussions concerning the development of this manuscript.

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[82] ISO 178:2010, Plastics – Determination of flexural properties, ISO/TC 61/SC 2, Ed. Switzerland: International Standards Organization, 2010.

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[89] ISO 14130:1997, Fibre-reinforced plastic composites – Determination of apparent interlaminar shear strength by short-beam method, ISO/TC 61/SC 13, Ed. Switzerland: International Standards Organization, 2012.

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[102] ISO 15850:2014, Plastics Determination of tension-tension fatigue crack propagation – Linear elastic fracture mechanics (LEFM) approach, ISO/TC 61/SC 2, Ed. Switzerland: International Standards Organization, 2014.

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INDEX

A

absorption spectra, 46 absorption spectroscopy, 36 accessibility, 77 accounting, 91 acrylonitrile, 78, 118 additives, 80, 93 adhesion, 84, 108, 109 adhesive properties, 82 adjustment, 15 advancement, 110 aerospace, 4, 20, 76, 81, 92 age, 71 agencies, 68 amplitude, 9, 10, 14, 54, 55 anisometry, 29 anisotropy, 13, 51, 55, 71, 74, 75, 84, 87,

90, 92, 97, 108, 110, 111 annealing, 92 arrest, 11, 98, 122 assessment, vii, 2, 3, 12, 19, 21, 22, 43, 50,

59, 74 atomic force, 74 atoms, 36, 37

B

base, 43, 92 bending, 10, 19, 56, 58

biomaterials, 74 biomedical applications, 76 bonding, 81, 84, 87, 92, 119 bonds, 111 bone, 82, 94 branching, 70, 109 brass, 25 breakdown, 77 building blocks, 81, 82 businesses, 77 butadiene, 78, 118

C

CAD, 68, 70, 74, 77 CAM, 68, 70, 74 carbon, 37, 78, 115, 116 ceramic, 23 certification, 75 challenges, 67, 70, 71, 72, 74, 84, 88, 92,

94, 97, 98, 108 chemical, 13, 23, 35, 36, 37, 44, 46, 96 chemical bonds, 37 circulation, 26 classes, 70 classification, 28, 29, 31, 50, 111, 112 classroom, 114 cleavage, 11, 57, 58 CO2, 37 collaboration, 109, 110


Index 126

colleges, 69, 75 color, 80 commercial, 51, 53, 70, 75, 80, 82, 115 community, 69, 75, 77, 99, 110, 111, 115 competitiveness, 77 complexity, 34, 79, 83, 89 composites, 68, 70, 73, 74, 78, 83, 84, 87,

91, 94, 95, 96, 97, 98, 99, 111, 112, 116, 117, 120, 121, 122, 123

composition, 9, 13, 23, 26, 35, 36, 44, 96 compounds, 43 compression, 3, 4, 5, 6, 9, 53, 59, 79, 80, 86,

90, 95, 96, 110, 123 computer, 69 conductivity, 39, 40, 41, 47 configuration, 4 consensus, 2, 12, 22, 43, 50, 74 consolidation, 92 constant load, 52 constituents, 35 construction, 91 consumers, 77 containers, 24 cooling, 91 cost, 77, 81 covering, 3, 12, 38 cracks, 8, 12, 71, 87, 95 creep, 10, 12, 14, 15, 53, 55, 59, 79, 83, 88,

96, 111, 121 crystalline, 37, 71, 91, 111 crystallinity, 80, 91 crystallization, 91, 92 cure, 71, 79 cycles, 10, 14, 54, 56, 96, 97 cycling, 10, 55, 56, 82, 88, 91, 97, 111

D

data set, 75 database, 20 defects, 71, 91, 92, 109, 110 deformation, 3, 4, 6, 7, 8, 10, 14, 54, 55, 93 dental resins, 93 Department of Defense, 81, 117

deposition, 69, 70, 79, 84, 89, 90, 93, 98, 109, 111, 117, 118, 119

depth, 14, 15, 25 designers, 87, 91, 111 detection, 36, 46 diffraction, 25, 36, 37 diffusion, 71, 91, 92, 108, 109, 111 diffusion time, 108 diffusivity, 41, 47 directionality, 84, 85 dispersion, 24, 84 displacement, 4, 9, 11, 12, 14, 57, 58, 59,

61, 96, 97 distribution, 26, 37, 80, 85, 95, 111 ductility, 61 durability, 81, 82, 83, 88, 92, 111

E

elastic deformation, 4, 8, 14, 61 elastic fracture, 97, 122 electric field, 79 electrical resistance, 40 electron, 25, 35, 36, 37 electrons, 36, 37 elongation, 15, 16, 29, 79 emission, 36, 37, 46 energy, 13, 26, 35, 36, 37, 39, 71, 74, 87,

88, 91, 97 engineering, 4, 16, 68, 72, 75, 79, 83, 87,

97, 110, 111, 112 England, 45 entrepreneurs, 77 environment(s), 8, 35, 81, 82, 96, 108 environmental conditions, 96, 119 environmental stress(s), 111 equipment, 3, 70, 77, 78, 93, 110 excitation, 7, 37 expertise, 108, 110 exposure, 37, 88, 92, 96 extrusion, 69, 70, 77, 78, 80, 82, 84, 89, 90,

91, 109


Index 127

F

Fabrication, 47, 113, 116, 119 feedstock, 22 fiber(s), 73, 74, 78, 79, 84, 85, 87, 90, 91,

92, 94, 95, 96, 97, 98, 112, 115, 116 fidelity, 35 filament, 70, 89, 90, 91, 109, 110, 111 fitness, 80 flame, 36, 37, 46 flaws, 111 flooding, 36 fluid, 26 fluorescence, 36, 37, 46 force, 5, 9, 10, 11, 12, 14, 15, 16, 18, 54, 55,

56, 57, 58, 59, 61, 93 formation, 10, 55, 91 fracture toughness, 3, 4, 9, 11, 19, 56, 57,

58, 59, 79, 87, 97, 98, 122 friction, 38 funding, 75 fusion, 22, 36, 37, 69, 70, 71, 72, 83, 84, 90,

91, 92, 93, 98, 108, 109, 111

G

gamma rays, 37 geometry, 7, 9, 54, 71, 85, 89, 92, 94, 110,

111 glass transition, 70, 91 glass transition temperature, 71, 91 glue, 27 google, 113, 117 grants, 69 graphite, 37 growth, 3, 4, 9, 10, 11, 12, 18, 55, 56, 59,

72, 77, 97, 98, 110, 113 growth rate, 10, 55 growth time, 12 guidance, vii, 6, 7, 8, 9, 10, 11, 13, 21, 24,

50, 51, 52, 53, 54, 65, 96, 110, 112 guidelines, 57, 68, 85

H

hardness, 3, 5, 7, 8, 14, 16, 17, 18, 51, 60, 61, 74, 79

heat capacity, 41, 111 heat loss, 40 heat transfer, 40, 92 height, 8, 15, 71, 89, 91, 95 helium, 16, 53 high strength, 72, 73, 84, 87, 95 House, 115 human, 34, 99 human body, 99 hydrogen, 37

I

ideal, 15 identification, 37, 70, 84 image, 27 immersion, 96, 108 impact energy, 11 impact strength, 86, 88, 123 implants, 113 indentation, 3, 7, 8, 14, 15, 18, 60, 61, 74 individuals, 77 industry(s), 2, 4, 21, 22, 50, 67, 68, 69, 71,

72, 74, 75, 76, 79, 110, 112 infrastructure, 69 initiation, 12, 57, 95 insertion, 40, 81, 97 institutions, 68 integration, 82 interface, 70, 83, 87, 90, 92, 109 interference, 37 investment, 75, 77, 110 ions, 36, 37 issues, 51, 55

J

joints, 83 jurisdiction, 93


Index 128

K

kinetics, 76, 92

L

laminar, 26, 91 landscape, 112 laws, 26, 80 laws and regulations, 80 lead, 77, 80, 84, 91, 92 ligament, 12, 59 light, 25, 26, 36, 37, 41 light beam, 26 light scattering, 25, 26 linear dependence, 71 linear function, 34 low temperatures, 6, 7, 52, 54

M

machinery, 69 magnetic field(s), 26 magnitude, 8, 15, 93 majority, 84, 89, 112 manufacturing, vii, 1, 2, 3, 12, 13, 21, 22,

25, 27, 34, 38, 44, 49, 50, 51, 56, 57, 59, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 79, 80, 81, 83, 84, 86, 87, 89, 98, 99, 108, 110, 113, 114, 115

market penetration, 77 mass, 7, 25, 35, 36, 37, 38, 41, 42, 43 mass spectrometry, 36 material surface, 5 matrix, 73, 84, 87, 91, 95, 117 matter, 96 measurement(s), vii, 2, 3, 5, 6, 7, 8, 11, 12,

13, 14, 21, 22, 23, 26, 29, 39, 44, 49, 51, 60, 61, 67, 72, 73, 74, 81, 83, 94, 95, 96, 97, 98, 99, 108, 109, 110, 111, 112, 114

mechanical properties, vii, 1, 3, 12, 13, 18, 35, 61, 67, 68, 71, 72, 73, 74, 77, 79, 81, 82, 89, 90, 91, 92, 93, 109, 110, 111, 112, 113, 117

mechanical testing, 3, 68, 74, 93 media, 116 medical, 72, 113 melt, 69, 70, 71, 80, 92, 111, 119 melts, 37, 70, 71 metallurgy, vii, 49, 52, 65 metals, 4, 5, 9, 11, 12, 37, 51, 53, 57, 59, 67,

72, 76, 77, 95 meter, 38, 42, 43 methodology, 78, 96 microscopy, 25, 27, 74, 109 microstructure, 76, 93, 109 mission, 68, 69, 75 mixing, 84 modelling, 91, 118, 119 models, 72, 75, 76, 83, 91, 93, 109, 110,

111 modifications, 50, 51, 99, 108 modulus, 3, 4, 5, 6, 7, 8, 15, 54, 56, 79, 82,

88, 90, 92, 94, 95, 97, 112, 120, 121 moisture, 81, 82, 83, 88 mold, 70 molecular structure, 110 molecular weight, 70, 80, 91, 93, 111 molecules, 43 molybdenum, 63 morphology, 13, 23, 27, 28, 44 MSF, 118 MTS, 115

N

nanomaterials, 78 nebulizer, 37 neural network(s), 8, 61, 91 nitrogen, 37 non-metals, 3 nonprofit organizations, 69 nucleation, 10, 55

O

opportunities, 73, 114 optimization, 67, 72, 93


Index 129

organs, 72 overlap, 76, 85 oxygen, 37

P

parallel, 30, 79 partial differential equations, 26 pathways, 79 permit, 83 Philadelphia, 44, 117 photons, 37 photopolymerization, 93 physical properties, 111 physics, 68, 71, 76, 77, 93 Pie chart, 78 plastic deformation, 4, 5, 15, 16, 86 plastics, 79, 94, 95, 96, 122, 123 point load, 97, 121 polycarbonate, 78, 118 polyether, 78 polymer(s), 3, 67, 68, 70, 71, 72, 73, 75, 76,

77, 78, 79, 80, 84, 86, 87, 88, 90, 91, 92, 93, 94, 95, 97, 98, 108, 109, 110, 111, 112, 119

polymer aging, 92 polymer composites, 68, 72, 73 polymer materials, 78, 109 polymer matrix, 73, 84 polymer structure, 70, 111 polymeric materials, 70, 73, 78, 86, 95 polymerization, 92 polystyrene, 78 polyurethane, 78 population, 81 porosity, 13 preparation, 6, 12 President, 75 principles, 27, 40, 44, 76, 114, 119 probability, 111 probe, 40, 109 process control, 76, 77 profit, 68, 75 project, 2, 21, 22, 49, 75, 77, 109 propagation, 87, 95, 97, 98, 122

proportionality, 4, 86 public-private partnerships, 77

Q

quality assurance, 57, 76, 80 quality control, 8, 60, 75, 80 Quartz, 46 quasi-static loading, 12, 59

R

radiation, 36, 37, 93 radius, 32, 94 raw materials, 22, 67, 72, 80, 93, 111 reactants, 93 reagents, 96 recommendations, 74, 111 recycling, 80 reflectivity, 109 reinforcement, 95 relief, 11, 56 reproduction, 28 requirements, 51, 52, 53, 54, 62, 81, 99, 108 researchers, 27 resins, 80 resistance, 5, 7, 8, 12, 14, 19, 56, 58, 83, 96,

97, 123 resolution, 98 response, 8, 91, 120 restrictions, 95, 96 revenue, 67, 72, 77 rheology, 109 rings, 40 risk, 72, 81, 88 rods, 59 room temperature, 5, 6, 7, 16, 52, 53, 54, 92 roughness, 54, 56, 60, 71, 91, 110 rules, 91

S

safety, 51, 76, 81, 88 scale system, 82


Index 130

scattering, 26, 46 science, 2, 21, 49, 68, 76, 77, 82, 83, 109,

110, 112 scope, 67, 73, 84 sedimentation, 25, 26 semi-crystalline polymers, 79, 110 sensitivity, 2, 13, 22, 58 shape, 7, 11, 27, 28, 33, 39, 40, 41, 45, 75,

76, 77 shear, 3, 4, 7, 15, 54, 59, 86, 88, 90, 94, 95,

120, 121 shear strength, 121 showing, 78 signals, 26 silhouette, 31, 32, 33 simulation(s), 82 sintering, 69, 71, 73, 77, 84, 91, 92, 113,

119 smoothness, 31 software, 70, 76, 77 solid surfaces, 10, 56 solution, 26, 37, 96 solvents, 92 species, 63, 92 specifications, 25, 64, 80 spectroscopy, 35, 36, 37, 46 spin, 24 stability, 81, 83, 93, 108 stable crack, 12, 19, 58 stakeholders, 72, 79 standardization, 80 standardized testing, 82, 83, 89, 92 state(s), vii, 2, 8, 13, 21, 22, 25, 37, 39, 40,

113 steel, 11, 12, 19, 25, 42, 51, 59 STM, 6 storage, 24 stress, 4, 8, 10, 11, 12, 13, 14, 15, 16, 56,

58, 61, 71, 83, 85, 86, 88, 89, 90, 91, 92, 94, 95, 96, 97, 108, 111, 120

stress intensity factor, 12, 58 stress test, 8 stretching, 4 structural dimension, 81

structure, 35, 37, 70, 74, 81, 82, 84, 85, 87, 90, 93, 109

style, 73 styrene, 78, 117, 118 substrate, 70 Sun, 91, 119 supplier(s), 3, 80, 83 surface area, 14, 16, 38 surfactant, 96 susceptibility, 81, 87 sustainability, 74 Switzerland, 114, 119, 120, 121, 122, 123 symmetry, 27, 37

T

tanks, 24 tantalum, 63, 64 techniques, 2, 7, 8, 22, 23, 25, 26, 27, 35,

39, 44, 81 technology(s), 68, 69, 75, 76, 77, 79, 81, 93,

113 technology transfer, 77 TEM, 25 temperature, 9, 10, 16, 37, 39, 40, 41, 52,

53, 55, 56, 70, 71, 75, 77, 81, 82, 83, 86, 88, 89, 91, 92, 96, 109, 110

tensile strength, 3, 6, 79, 92 tension, 3, 4, 5, 6, 9, 12, 15, 51, 52, 53, 59,

61, 86, 96, 98, 110, 120, 122 test data, 6 testing, vii, 1, 2, 4, 5, 6, 7, 8, 9, 10, 11, 12,

15, 16, 17, 18, 19, 22, 49, 51, 52, 53, 54, 56, 57, 58, 59, 61, 64, 68, 72, 73, 80, 81, 82, 87, 92, 94, 96, 98, 108, 109, 110, 112, 122

textiles, 95, 97 thermal energy, 37 thermal expansion, 84 thermal properties, 23, 39, 44 thermodynamics, 76 thermoplastics, 98, 111 thermosets, 111 torsion, 4, 79, 86, 94, 120 transducer, 7


Index 131

transition temperature, 79 transmission, 25 treatment, 112 tungsten, 7, 63 tungsten carbide, 7, 63 twist, 15, 121

U

uniaxial tension, 5, 58 uniform, 6, 10, 15, 26, 40, 53, 92 United States, 68 universities, 69, 75 USA, 46

V

vacuum, 36, 43 validation, 75, 83 variables, 80, 89, 93, 109 variations, 2, 22, 54 velocity, 26, 77, 89 vibration, 7 Vickers hardness, 18, 61 viscosity, 13, 26, 80, 92, 111

visions, 116 visualization, 28, 34

W

Washington, 45, 76, 117 water, 92 wavelengths, 37 weakness, 81 wealth, 93 web, 77, 115 websites, 77, 78 wires, 51 workflow, 67, 72, 92, 110, 111

X

X-ray diffraction, 46 X-ray photoelectron spectroscopy, 36

Y

yield, 3, 4, 6, 86, 92, 95


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3- Book.ID 8364 Additive-KDs1.iran-mavad.com/pdf96/Additive Manufacturing... · ISBN: (eBook) XXX JSBO NBWBE DPN. CONTENTS Preface vii Chapter 1 Mechanical Properties Testing for