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Effects of Hot Isostatic Pressing on Copper Parts Additively Manufactured via Binder
Jetting
Ashwath Yegyan Kumar
Thesis submitted to the faculty of the Virginia Polytechnic Institute and State University in
partial fulfillment of the requirements for the degree of
Master of Science
In
Mechanical Engineering
Christopher Bryant Williams
Scott T. Huxtable
Hang Yu
14 February, 2018
Blacksburg, Virginia
Keywords: Additive Manufacturing, Binder Jetting, Hot Isostatic Pressing, Copper
Effects of Hot Isostatic Pressing on Copper Parts Additively Manufactured via Binder Jetting
Ashwath Yegyan Kumar
ACADEMIC ABSTRACT
Copper is a material of interest to Additive Manufacturing (AM) owing to its outstanding material
properties, which finds use in enhanced heat transfer and electronics applications. Its high thermal
conductivity and reflectivity cause challenges in the use of Powder Bed Fusion AM systems that
involve supplying high-energy lasers or electron beams. This makes Binder Jetting a better
alternative as it separates part creation (binding together of powders) from energy supply (post-
process sintering). However, it is challenging to fabricate parts of high density using this method
due to low packing density of powder while printing. This work aims to investigate the effects of
Hot Isostatic Pressing (HIP) as a secondary post-processing step on the densification of Binder Jet
copper parts. By understanding the effects of HIP, the author attempts to create parts of near-full
density, and subsequently to quantify the effects of the developed process chain on the material
properties of resultant copper parts. The goal is to be able to print parts of desired properties suited
to particular applications through control of the processing conditions, and hence the porosity.
First, 99.47% dense copper was fabricated using optimized powder configurations and process
parameters. Further, the HIP of parts sintered to three densities using different powder
configurations was shown to result in an improvement in strength and ductility with porosity in
spite of grain coarsening. The strength, ductility, thermal and electrical conductivity were then
compared to various physical and empirical models in the literature to develop an understanding
of the process-property-performance relationship.
Effects of Hot Isostatic Pressing on Copper Parts Additively Manufactured via Binder Jetting
Ashwath Yegyan Kumar
GENERAL AUDIENCE ABSTRACT
Additive Manufacturing (AM) is a technique of fabricating an object in a layer-wise fashion. The
layer-based approach provides opportunity for the manufacture of highly complex shapes. Binder
Jetting is an AM technology that creates parts by the selective jetting of a polymeric binder onto
successive layers of powdered material. In the case of metals, the printing process is followed by
sintering in an oven, which burns out the binder and densifies the part. However, this is typically
not enough to remove all the porosity in a specimen. While this enables the fabrication of a variety
of materials, the porosity in sintered parts can be a detriment to their properties. This work aims to
investigate the use of post-process Hot Isostatic Pressing (HIP) to eliminate the remaining porosity.
HIP is a technique of applying high pressures at high temperatures in an inert gas medium. The
goal of this research is to scientifically understand and quantify the effect of HIP on sintered parts
made via Binder Jetting. The research is carried out in the context of copper, which has unique
mechanical, thermal and electrical conductance properties that could be influenced by the presence
of pores. In this work, the effects of the Binder Jetting-Sintering-HIP process chain on the porosity,
and consequently the material properties, of copper parts are quantified. Resolving the issue of
porosity can enable the printing of copper parts for specialized applications from electronic
components to rocket engines. Developing a quantitative understanding can pave the way to design
specific processing conditions to fabricate not only fully dense copper parts with superior
properties, but also parts of a designed level of porosity that have specific target material
properties.
iv
ACKNOWLEDGEMENTS
I would like to express my most sincere gratitude to Dr. Williams for firstly having given me the
opportunity to work on this thesis, and for having been a constant source of guidance and advice,
for which I am indebted. Working under his supervision has been an extremely fruitful learning
experience that has not only taught me about various aspects of research, but also helped mold my
overall character. I would also like to thank the members of my committee, Dr. Huxtable and Dr.
Yu, for having provided invaluable advice and guidance towards my research.
I would next like to thank the National Science Foundation for providing the funding towards this
research (CMMI #1254287). I thank my colleague in the DREAMS lab, Yun Bai, who has been
instrumental in helping me out with running experiments and having provided countless advice
and suggestions for my research through the course of my stay here. I would also like to thank Dr.
Anders Eklund of Quintus Technologies for his knowledgeable inputs on Hot Isostatic Pressing
parameter development, which was instrumental in the success of our experiments. I thank Jue
Wang and Dr. Huxtable for having helped with running the conductivity measurement experiments
and helping me learn heat transfer concepts that were used in my research. I wish to thank Robert
Mills of the Extreme Environments, Robotics, and Materials Laboratory for having shared their
equipment for tensile testing, metallographic sample preparation and microscopy. I thank Matthew
Meeder and Dr. Al Wicks for the ideation and preliminary experiments for conductivity
measurements, which provided a platform for me to start my research.
I also thank all the members of the DREAMS lab for having given me a fantastic atmosphere to
work in, for all their inputs for my research, for having motivated me to grow as a researcher, and
for being great friends.
Last but far from the least, I express my sincerest thanks to my family back in India for having
sent me out here to learn and grow, and to several friends who have been encouraging and
supportive through my stay here.
v
Table of Contents Chapter 1: Introduction ........................................................................................................................... 1
Chapter 2: Influence of Hot Isostatic Pressing on the Density, Microstructure and Mechanical
Properties of Copper Parts Fabricated by Binder Jetting Additive Manufacturing ...................................... 3
Abstract ..................................................................................................................................................... 3
2.1 Introduction ................................................................................................................................... 3
2.1.1 Prior Work on Density Improvement in Binder Jetting ........................................................ 5
2.1.2 Hot Isostatic Pressing ............................................................................................................ 6
2.1.3 Research Objective ............................................................................................................... 7
2.2 Experimental Methods .................................................................................................................. 8
2.2.1 Materials Used ...................................................................................................................... 9
2.2.2 Processing Parameters ........................................................................................................... 9
2.2.3 Density Measurements ........................................................................................................ 10
2.2.4 Metallographic Analysis ..................................................................................................... 11
2.2.5 Tensile Testing .................................................................................................................... 12
2.3 Results ......................................................................................................................................... 12
2.3.1 Density ................................................................................................................................ 12
2.3.2 Porosity ............................................................................................................................... 13
2.3.3 Microstructure ..................................................................................................................... 17
2.3.4 Tensile Testing .................................................................................................................... 20
2.4 Closure and Future Work ............................................................................................................ 22
2.5 References ................................................................................................................................... 22
Chapter 3: Impacts of Process-Induced Porosity on Material Properties of Copper Made by Binder
Jetting Additive Manufacturing .................................................................................................................. 29
Abstract ................................................................................................................................................... 29
3.1 Introduction ................................................................................................................................. 29
3.1.1 Prior Work on Powder Bed Fusion of Copper .................................................................... 29
3.1.2 Binder Jetting of Copper ..................................................................................................... 31
3.1.3 Research Objective ............................................................................................................. 31
3.2 Modeling Porosity-Property Relationships ................................................................................. 32
3.2.1 Strength and Ductility ......................................................................................................... 32
3.2.2 Thermal and Electrical Conductivity .................................................................................. 34
3.2.3 Wiedemann-Franz Law ....................................................................................................... 35
3.3 Experimental Methods ................................................................................................................ 36
vi
3.3.1 Specimen fabrication ........................................................................................................... 36
3.3.2 Tensile Testing .................................................................................................................... 39
3.3.3 Thermal Conductivity Measurement: ................................................................................. 39
3.3.4 Electrical Resistivity Measurement..................................................................................... 40
3.4 Results ......................................................................................................................................... 41
3.4.1 Tensile Strength .................................................................................................................. 41
3.4.2 Ductility .............................................................................................................................. 42
3.4.3 Thermal Conductivity ......................................................................................................... 44
3.4.4 Electrical Conductivity ....................................................................................................... 47
3.4.5 Wiedemann-Franz Law ....................................................................................................... 48
3.5 Conclusions ................................................................................................................................. 49
3.6 References ................................................................................................................................... 50
Chapter 4: Closure ................................................................................................................................ 53
4.1 Conclusions ................................................................................................................................. 53
4.2 Summary of findings ................................................................................................................... 53
4.3 Contributions ............................................................................................................................... 54
4.4 Limitations and Future Work ...................................................................................................... 55
References (for Chapters 1 and 4)............................................................................................................... 56
Appendix A ................................................................................................................................................. 57
Appendix B ................................................................................................................................................. 67
vii
List of Figures
Figure 1.1: Schematic of the Binder Jetting process ..................................................................................... 1
Figure 2.1: A schematic of the Binder Jetting process.................................................................................. 4
Figure 2.2: Schematic of process chain ........................................................................................................ 9
Figure 2.3: Composite image showing porosity in part HIPed at 975°C for 4 hours ................................. 14
Figure 2.4: Porosity in 17µm parts ............................................................................................................. 15
Figure 2.5: Porosity in 25µm parts ............................................................................................................. 16
Figure 2.6: Porosity in bimodal parts .......................................................................................................... 17
Figure 2.7: Grain structure change upon HIP – 17µm parts ....................................................................... 18
Figure 2.8: Grain structure change upon HIP – 25µm parts ....................................................................... 18
Figure 2.9: Grain structure change upon HIP – bimodal parts ................................................................... 19
Figure 2.10: Samples of untested (top) and tested (bottom) tensile specimens .......................................... 21
Figure 3.1: A schematic of the binder jetting process ................................................................................. 31
Figure 3.2: Photograph depicting layout of parts in print bed .................................................................... 36
Figure 3.3: Schematic of process chain ...................................................................................................... 37
Figure 3.4: Porosity in 17µm specimen (YZ plane) : (a) Sintered – 16.37%; (b) HIPed – 14.17% ........... 38
Figure 3.5: Porosity in bimodal specimen (YZ Plane): (a) Sintered – 9.48%; (b) HIPed – 2.68% ............ 38
Figure 3.6: Electrical Resistivity measurement setup ................................................................................. 41
Figure 3.7: Tensile strength observed in comparison to Model Predictions ............................................... 42
Figure 3.8: Ductility – Observed vs. Model ................................................................................................ 43
Figure 3.9: Thermal Conductivity vs. Porosity: Observed data compared to models ................................ 44
Figure 3.10: Conductivity at full density (kfd) calculated for each processing condition .......................... 45
Figure 3.11: Grain Boundary Thermal Resistance calculated from Modified EMT Model ....................... 46
Figure 3.12: Electrical Conductivity vs Porosity: Observed data compared to models .............................. 47
Figure 3.13: Observed Thermal Conductivity vs. that calculated using the Wiedemann-Franz Law......... 48
viii
List of Tables
Table 2.1: Particle Size data for powders used (µm): ................................................................................... 9
Table 2.2: A comparison of density with and without oil impregnation .................................................... 13
Table 2.3: Grain size change upon HIP of various parts............................................................................. 20
Table 2.4: Tensile Strength and Ductility improvement upon HIP ............................................................ 21
Table 3.1: Powders and processing conditions used to achieve different porosities .................................. 38
1
Chapter 1: Introduction
Binder Jetting is a powder-bed based Additive Manufacturing (AM) technology that allows for a
cost-effective, scalable way of fabricating a wide variety of materials including metals, ceramics
and polymers. Part creation involves the selective jetting of a binder onto successive layers of
powdered material (Figure 1.1). This is followed by curing of the binder by supplying heat, and
depowdering any loose powder adhering to the surface by using compressed air. The ‘green’ part
thus formed is then subjected to sintering in a furnace for debinding and sintering for densification
[1]. However, it is challenging to create fully dense, homogeneous parts using this method as
sintering typically involves the infiltration of a secondary, lower melting alloy that fills the pores
by capillary action, minimizing shrinkage and helping in densification. This research attempts to
address these issues using Hot Isostatic Pressing (HIP), a technique of applying high pressures at
high temperatures to pre-sintered specimens, to remove residual porosity.
Figure 1.1: Schematic of the Binder Jetting process
The author conducts this research in the context of copper, which is a material of particular interest
for AM, owing to its excellent thermal and electrical conductivity properties. The realization of
complex geometries through AM can pave the way for novel designs for applications involving
advanced heat transfer, thermal management, electronics etc. The use of Binder Jetting for copper
helps circumvent challenges in its manufacturing using laser or electron beam based Powder Bed
Fusion (PBF) AM systems owing to its high thermal conductivity and optical reflectivity. Hence,
the overarching objective of this research is to be able to understand and quantify the effect of HIP
on Binder Jet, sintered copper parts and work scientifically towards realizing these applications
with desired thermal, electrical as well structural properties.
Towards achieving these goals, first, a proof of concept was established for the ability to fabricate
near-full density copper parts by the use of previously optimized powder configurations, printing
and sintering parameters [2] (Appendix A). Subsequently, in Chapter 2, a more detailed
2
investigation is carried out on the HIP of parts printed using three different powder configurations
in order to determine the influence of the process on the density (2.3.1), porosity (2.3.2),
microstructure (2.3.3) and mechanical properties (2.3.4) for different sintered densities. An
understanding of the effects of HIP on parts of differing levels of porosity can help in evaluating
the minimum density required for HIP to be effective in eliminating porosity, and aid in the
fabrication of parts with designed levels of target porosity.
Finally, in Chapter 3, an attempt is made to quantify the effect of such process-induced porosity
on the mechanical (3.4.1, 3.4.2), thermal and electrical properties (3.4.3 - 3.4.5) of copper. These
measurements are then positioned against models developed in the literature for Powder
Metallurgy (PM) and two-component structures, which can help develop intrinsic models
specifically suited to Binder Jetting porosity-property relationships. Such an understanding of the
process-porosity, and porosity-property correlations, can help in achieving the eventual goals of:
i. Realizing applications involving full density and the values of strength, ductility, thermal
and electrical conductivity being closest to that of pure copper; and
ii. Being able to achieve varying levels of ‘designed’ porosity and hence properties through
appropriate selection of processing conditions
3
Chapter 2: Influence of Hot Isostatic Pressing on the Density, Microstructure
and Mechanical Properties of Copper Parts Fabricated by Binder Jetting
Additive Manufacturing
Abstract
Hot Isostatic Pressing (HIP) is a technique of applying high pressures through a fluid medium at
high temperatures to enclosed powders, castings and pre-sintered metal parts to eliminate porosity.
Due to uniform photographic shrinkage expected from this process, it can be a useful post-
processing technique for complex-geometry parts fabricated using Additive Manufacturing
techniques. In order for the technique to work effectively, the parts are typically required to have
a minimum density of 92% where surface porosity is closed. While HIP has been used in
conjunction with other Powder Bed Fusion AM processes, its use for parts made using Binder
Jetting has not been investigated in detail due to the limitations of Binder Jetting in fabricating
high density parts. After previously reported success in its use in the context of Binder Jetting of
copper [1], an effort is made here to perform detailed investigations on the effect of HIP on three
different powder configurations that lead to varying levels of porosity in sintered copper
specimens. The effects of HIP on density, microstructure, tensile strength and ductility have been
investigated. The highest density achieved was 97.32% after HIP by using bimodal powders that
were printed and sintered to 90.52%. Both the tensile strength and ductility were found to improve
following HIP, which suggests that the reduction in porosity is predominant compared to the
detrimental effects of grain coarsening.
2.1 Introduction
Binder Jetting is a powder bed-based Additive Manufacturing (AM) technology that involves the
use of an inkjet printhead to pattern a binder onto a bed of powder in order to selectively bind
particles together (see Figure 2.1). This is followed by curing the binder to add sufficient strength
for part handling and depowdering. The green part can then be subjected to suitable post-
processing such as sintering or infiltration to densify and strengthen it further. Since the part
creation only involves binding powder particles, the technology can easily be adapted to a wide
variety of materials including metals, ceramics and polymers.
4
Figure 2.1: A schematic of the Binder Jetting process
The process offers a number of advantages in addition to its applicability for a wide range of
materials. Owing to its lack of reliance on any thermal processing during the primitive creation
process, Binder Jetting does not face issues with residual stresses, warping and shrinkage that are
faced in typical Powder Bed Fusion AM processes due to rapid thermal treatment. This eliminates
the need for designed support structures or anchors to be fabricated, allowing the loose powder
surrounding the printed part to act as an inherent support. Additionally, printing using Binder
Jetting is cheaper and more easily scalable than other powder bed AM processes, as scaling up
would involve merely increasing the size or number of printheads as compared to upgrading
expensive laser or electron-beam power sources. Binder Jetting thus also enables the processing
of thermally conductive and optically reflective materials such as copper [2], which can be difficult
to process using powder bed fusion technologies that employ high intensity energy sources.
One major disadvantage of the Binder Jetting AM process, however, is that the fabricated parts
often feature low density. This is due to the lack of any compaction of powder during printing,
resulting in the green parts essentially being loosely packed powder particles, held together by the
binder. This results in significant residual porosity after sintering (for e.g., 85.5% in the case of
copper [2]). One technique used to improve the density of metallic and ceramic parts is through
the infiltration of a secondary, lower melting material during sintering. This material can infiltrate
through the pores of the primary material’s matrix by capillary action after the binder pyrolyzes,
minimizing shrinkage and increasing density. This technique has been investigated in detail by
researchers for various material combinations [3 – 6]. The selection of an infiltrant material is
challenging as it involves finding a significantly lower melting material that is compatible in its
wetting properties with the parent material. The requirement that the infiltrant must have a
significantly lower melting point than the parent material also hinders the processing of
homogenous, non-alloyed structures for use in applications where the presence of a secondary
material can negatively influence the property of the overall structure. These issues with
5
infiltration necessitate research on other ways to minimize the porosity and shrinkage to achieve
enhanced material properties without infiltration.
2.1.1 Prior Work on Density Improvement in Binder Jetting
Binder Jetting involves several process and post-process sintering parameters that may influence
the density and other properties of fabricated parts. Substantial research has been conducted to
understand these relationships to achieve the best possible density and material properties. Turker
and co-authors were able to achieve 98.5% theoretical density by optimizing layer thickness and
sintering temperatures in the Binder Jetting of Inconel 718 superalloys [7]. Their process required
special process modifications such as drying after every layer is printed, and sintering in vacuum.
Gaytan et al. could achieve only as high as ~65% density in Barium Titanate using optimal binder
saturation and sintering temperatures [8], while Gonzalez and co-authors from the same research
group achieved up to ~96% density in binder jet alumina by optimizing the layer thickness, powder
particle size as well as sintering temperature [9]. Vaezi and Chua found that an increase in binder
saturation or decrease in layer thickness improved tensile strength but at the cost of surface quality
[10]. Miyanaji et al. performed an experimental design to optimize the binder saturation, drying
power and duration and the powder spread speed levels on the strength, shrinkage and dimensional
accuracy of binder jet dental porcelain [11]. They found that Z-accuracy is most significantly
improved by increasing the drying power, X/Y-accuracy by increasing spreading speed, strength
by increasing saturation and shrinkage by increasing the spreading speed. Shrestha and
Manogharan carried out a systematic experimental design to optimize binder saturation, layer
thickness, roll speed and feed-to-powder ratio in order to maximize the transverse rupture strength
of binder jet 316L Stainless Steel parts [12]. Bai and co-authors achieved 85.5% of theoretical
density in copper by optimizing powder type, binder saturation and sintering profile [2].
The use of bimodal powders in AM processes other than Binder Jetting has been explored by
various researchers [13 – 15]. In the case of Binder Jetting, Lanzetta and Sachs have studied the
influence of bimodal powders on improving the surface finish in alumina powder mixtures [16].
Verlee and co-authors [17] experimentally validated and modified the model for packing density
for Binder Jetting as presented in the context of powder metallurgy by German [18] to predict
sintered density of bimodal powder mixtures. In the case of Binder Jetting of copper, Bai and co-
authors used a bimodal powder mixture to achieve ~92% theoretical density [19]. The use of
6
nanosuspension binders has been investigated as a means to improve green density and sintering
performance of binder jet parts [20 – 22]. J. Bai and co-authors achieved up to ~89% theoretical
density in printing nanosilver suspensions in silver powder [23].
In addition to these methods, some other unique modifications to the process have been studied.
Grau et al. achieved up to 99% density in alumina using a technique called slurry-based 3DP. This
was done by dispersing sub-micron ceramic powders in a slurry, which is deposited on the build
piston instead of spreading a powder [24]. Li et al. used chemical vapor infiltration of Si3N4 to
improve the mechanical strength of porous Silicon Nitride [25]. Recently, Rabinskiy and co-
authors used a modified process involving a new binder composition, drying at high temperatures
and an additional step of mechanical compaction after the spreading of each layer, to reduce the
porosity and improve mechanical properties of Si3N4 [26].
2.1.2 Hot Isostatic Pressing
This study is concerned with investigating the use of Hot Isostatic Pressing (HIP) as a secondary
post-processing step after sintering to densify parts fabricated using Binder Jetting. HIP is a
technique of applying high, isostatic pressure using an inert gas medium (typically Argon) at high
temperatures to consolidate loose powders. Atkinson and Davies have presented the physics of the
process in detail [27]. The application of high pressures of the order of a few hundred MPa (tens
of thousands in PSI) causes the entrapped gases in the pores to overcome the surface-energy
driving force for pore closure, resulting in them dissolving in the matrix and on to the surface. In
addition to consolidation of encapsulated loose powders, the process finds other applications such
as the densification of castings and presintered powders. Photographic (dimensionally uniform)
shrinkage is expected to be observed due to the isostatic nature of fluid pressure applied. This
property makes the process potentially useful in densifying complex-geometry parts fabricated
using Additive Manufacturing. HIP is expected to work effectively when the density of the pre-
sintered parts is >92%, as this is the density at which all surface porosity is expected to be sealed
[28]. HIP has been employed in conjunction with various AM technologies to achieve high density
and improved material properties.
HIP has been successfully employed in reducing porosity of parts made using Directed Energy
Deposition methods such as Laser Engineered Net Shaping (LENS) for Ti-6Al-4V. Kobryn and
Semiatin noticed a reduction of anisotropy in Yield Strength from the stress-relieved to HIPed
7
Ti64 samples, along with a reduction in ultimate tensile strength but gain in ductility after HIP.
HIP was also found to have improved fatigue strength of these samples [29]. More recently, an
investigation by Qiu and co-authors also resulted in a reduction in porosity as well as strength after
HIP of Ti-64 made using LENS [30].
Among the Powder Bed Fusion (PBF) technologies, Selective Laser Sintering (SLS) is typically
unable to achieve the densities required for HIP to be effective. Agarwala and co-authors in the
University of Texas at Austin used HIP to densify Bronze-Nickel parts made using SLS, by
encapsulating them in glass to seal the surface connected porosity [31]. Researchers from the same
group later devised a method called SLS/HIP to achieve near-full density using containerless HIP
of SLS parts. This was done by tuning printing process parameters in such a way as to fabricate a
dense outer shell (to act as a container) of >92% density in order to avoid surface connected
porosity, while the internal bulk was processed to a density of ~80% [31 – 33]. Liu and co-authors
developed a process chain involving SLS followed by Cold Isostatic Pressing (CIP), degreasing,
sintering and finally HIP in order to achieve ~96% density in alumina parts [35].
Compared to SLS, Selective Laser Melt (SLM) is able to fabricate parts at higher densities (close
to 99%), which are better suited for HIP. Mower and co-author studied fatigue properties of HIPed
Ti-64 and 316L SS, fabricated by Direct Metal Laser Sintering (DMLS) [36]. Multiple authors
have studied the improvement in density and fatigue properties of Ti-6Al-4V fabricated using SLM
followed by HIP [36 – 38]. Similar studies were conducted for 316 Stainless Steel and Ti64 by
Leuders and co-authors [40] and for ASTM F75, a Co-Cr-Mo alloy, by Haan et al [41]. HIP has
also been studied in conjunction with Electron Beam Melting (EBM) extensively for Ti64 [41 –
46]. Other materials explored include Co-Cr-Mo alloy [48], Inconel 625 [49], Inconel 718 [45],
[50] and Ti-48Al-2Cr-2Nb, a Titanium Aluminide alloy [51]. In a vast majority of the studies
mentioned above, HIP is found to improve the density and fatigue strength and homogenize the
microstructure, but decrease the tensile strength due to coarsening/enlargement of grains caused
by high temperature processing.
2.1.3 Research Objective
There is limited reported literature on the use of HIP for Binder Jet parts, since the densities of
parts fabricated by Binder Jetting are typically less compared to other AM processes, and often
fail to reach the 92% density requirement to avoid surface-connected porosity. Kernan and co-
8
author achieved close to 100% density in WC-Co parts fabricated using slurry-based Binder Jetting
followed by sinter-HIP. Gonzalez noted an improvement the density of Binder Jet Inconel 625
from 96.51% as fabricated to 98.33% after HIP [52].
For Binder Jetting of copper, the authors of this article have previously achieved 99.47% density
using suitable HIP conditions [1]. This research work aims to further this prior study by evaluating
the HIP of parts made using various powder configurations and the effect of the process on the
density, porosity, microstructure and tensile strength of the parts fabricated by Binder Jetting.
Section 2.2 presents an overview of the experimental methods used in the investigations including
the materials (2.2.1), process parameters (2.2.2), density measurements (2.2.3), metallographic
analysis (2.2.4) and tensile testing (2.2.5). Section 2.3 presents the results of density measurements
(2.3.1), porosity analysis (2.3.2), microstructure analysis (2.3.3) and tensile testing (2.3.4), along
with brief discussions. Closure and future work are offered in Section 2.4.
2.2 Experimental Methods
The objective of this study is to evaluate the influence of HIP on the density, porosity,
microstructure and mechanical properties of Binder Jetting parts made using different powder
configurations and thus different sintered densities. While it is generally accepted that a minimum
of 92% density is required to be able to seal surface connected porosity and effectively remove
pores using HIP, the authors seek to understand the effect of HIP on parts that fall below this
density limit as well. A schematic of the process chain with the various control and experimental
variables is provided in Figure 2.2.
9
Figure 2.2: Schematic of process chain
2.2.1 Materials Used
The experiments were conducted on parts of various starting densities, by using three different
powder configurations for printing. The powders were all acquired from ACuPowder. The particle
size distributions of each of the powders used are provided in Table 2.1:
Table 2.1: Particle Size data for powders used (µm):
D10 D50 D90
17µm Powder 8 17 28
25µm Powder 16 25 37
5µm Powder 3 5.5 9
30µm Powder 15 30 37.5
The 17µm and 25µm powders were used as such, while the 5µm and 30µm powders were mixed
in the ratio of 27:73 by weight in a rotating drum for ~2 hours to create a bimodal powder mixture
which has demonstrated success in fabrication of high-density parts [1, 19].
2.2.2 Processing Parameters
The various printing, sintering and HIP parameters were set based on previous success in
fabricating and HIPing high density copper samples [1]. All tests specimens were printed using an
10
ExOne R2 3D printer. The binder used was the standard binder (PM-B-SR2-05) provided by
ExOne, as this has been known to bind well with the material of concern (copper) and leaves
minimal carbon residue after debinding [2]. The layer height for all powder types was kept constant
at 70µm. The binder saturation is defined as the ratio of binder per unit void space between powder
particles. It is an important machine parameter, which decides the X- and Y- spacing of adjacent
droplets being jetted from the nozzles. It is calculated based on user input values of powder packing
density and desired saturation percentage. The binder saturation value was set to 100%. All the
samples required for the different characterizations were fabricated in a single print in order to
maintain consistency among the samples used to study various properties.
Sintering was carried out in a box furnace, in a reducing Hydrogen atmosphere. The samples were
sintered at a temperature of 1075°C for 3h. Prior to this, debinding was carried out at 450°C for
30 minutes. All heating and cooling ramps were at 5°C/min. The sintering profile is depicted in
Figure 2.2.
All samples were post-processed using containerless HIP in a Quintus Technologies graphite
furnace equipped with the proprietary Uniform Rapid Cooling (URC®) technology to enable rapid
cooling which can aid in reducing grain growth at higher temperatures. The samples were HIPed
at a temperature of 1075°C under a pressure of 206.84 MPa for 2 hours in an Argon atmosphere.
Results have also been presented from an initial trial of HIP at 975°C under the same pressure for
4 hours.
2.2.3 Density Measurements
Density Measurements for the parts were carried out using an Archimedes Principle based
apparatus as recommended in the ASTM Standard B962-15 for measuring the density of sintered
Powder Metallurgy (PM) specimens [53]. For this purpose, multiple rectangular coupons were
printed for each powder configuration. Some of these were sent intact for HIP and some retained
for sintered part measurements with the assumption that parts within the same build are similarly
dense. Additionally, in each case, a few of the specimens were cut into two halves, with one half
retained and the other HIPed, to compare the improvement in density within the same part. The
density measurements were conducted both with and without oil impregnation (Section 2.3.1). Oil
impregnation helps avoid the formation of microbubbles on the surface parts when they are
immersed in water.
11
2.2.4 Metallographic Analysis
Sintered and HIPed specimens of each powder configuration were sectioned in the XY, YZ and
ZX cross sectional planes of each sample and polished in successively finer polishing materials in
order to smooth the surface. These polished sections were then photographed under an optical
microscope in order to study the internal porosity. The porosity was measured using a MATLAB
code written using the Image Analysis toolbox (Appendix B). This code converts the micrographs
into black and white images based on a user-defined threshold that helps distinguish the pores from
the rest of the specimen. The images loaded into this code were manually cropped in order to
exclude any regions outside the sample or large scratches that could be recognized by the software
as black regions, or pores. The code then calculates the ratio of the area of the black region, or
pores, to the total area of the section. By replicating this over several sections, a reasonable average
of the internal porosity may be obtained.
The polished samples, whose micrographic images were recorded for this analysis, were then
subsequently etched using nitric acid solution in order to study the microstructure and grain size.
In the case of 25 µm parts, the extensive porosity caused difficulties in polishing and etching due
to liquid entrapment inside the pores. This issue was overcome using a combination of the
application of a compressed air jet to clear the surface, subjecting the parts to vacuum, and allowing
the liquid entrapped to drain by gravity onto soft cloth.
Grain size calculations were performed based on the Saltykov Rectangle method, a modification
of the Jeffries planimetric analysis as described in the ASTM standard E112-13 [53, 54]. ImageJ
software was used to draw rectangles of known areas over metallographic images taken from
multiple sections. The number of grains completely enclosed (𝑛𝑖𝑛𝑠𝑖𝑑𝑒) and those intercepted by
the perimeter of the rectangle (𝑛𝑖𝑛𝑡𝑒𝑟𝑐𝑒𝑝𝑡𝑒𝑑) were counted and substituted into Equation 2.1 below
to obtain the number of grains per mm2, 𝑁𝐴 :
𝑁𝐴 =[𝑛𝑖𝑛𝑠𝑖𝑑𝑒+0.5𝑛𝑖𝑛𝑡𝑒𝑟𝑐𝑒𝑝𝑡𝑒𝑑+1]
𝐴 ( 2.1 )
where 𝐴 is the area of the selected region, which can be obtained using the pixel count in the
software, by appropriate scaling. The ASTM grain number 𝐺 is then calculated using Equation 2.2
below, from the ASTM standard:
𝐺 = 3.321928 𝑙𝑜𝑔(𝑁𝐴) − 2.954 ( 2.2 )
12
The average grain diameter 𝑑𝑔 is calculated as the square root of the average grain area 𝐴𝑔 per the
following equations:
𝐴𝑔 =1
𝑁𝐴 ( 2.3 )
𝑑𝑔 = √𝐴𝑔 ( 2.4 )
A 95% confidence interval was used to represent the variance in the grain number and grain size
data per the standard to account for the number of samples chosen and possible variations over the
different regions. The results from these analyses are presented in sections 2.3.2 and 2.3.3.
2.2.5 Tensile Testing
Tensile strength measurements were carried out in accordance with the ASTM standard B925 for
standard testing practices for PM specimen [56]. Flat unmachined tension test specimen
(“dogbones”) were designed and directly printed. All the specimens were printed laid flat on the
print bed (XY plane), causing the thickness of the sample to be along the build (Z) direction. Per
the ASTM Standard for Coordinate Systems nomenclature, the parts were printed oriented
randomly either in the XYZ or the YXZ direction [57]. The measurements were carried out on an
INSTRON machine with a 50kN load cell at a constant elongation rate of 0.381mm/min (strain
rate of 0.015 mm/mm/min as recommended by the standard) and a gauge length of 25.4mm (1
inch). A strain gauge was used to continually record the elongation. The tests were conducted until
the sample broke. The ultimate tensile strength and ductility results from these experiments are
presented in Section 2.3.4. Ductility is calculated based on elongation.
2.3 Results
2.3.1 Density
The density of sintered and HIPed parts (measured with and without oil impregnation) are reported
in Table 2.2 for each of the powder types.
13
Table 2.2: A comparison of density with and without oil impregnation
Part Type Density (%)
With Oil Impregnation Without Oil Impregnation
17 µm Sintered 83.63 ± 0.40 88.03 ± 2.40
HIPed 85.83 ± 0.19 86.72 ± 0.24
25 µm Sintered 77.73 ± 1.18 80.68 ± 1.96
HIPed 82.41 ± 0.33 90.24 ± 1.45
Bimodal Sintered 90.52 ± 0.30 91.53 ± 0.17
HIPed 97.32 ± 0.06 97.47 ± 0.31
These results indicate a higher variance and inaccuracy in the measurements without oil
impregnation, especially in the parts with lower density. This is due to the formation of
microbubbles on the surface of these parts due to the surface connected porosity and roughness of
binder jet parts, especially those of lower density. Henceforth in this article, the term ‘density’
shall be used in the context of values measured using oil impregnation only.
The results of improvement in density from Table 2.2 indicate that the effect of HIP on parts with
lower density is minimal. This is to be expected, as HIP has not been found to be effective in
removing porosity from parts with large amounts of porosity, especially surface porosity (Section
2.3.2). The technique is found to be effective in improving the density of the parts with higher
sintered density, namely those of the bimodal configuration. The authors have previously obtained
densities as high as 99.47 ± 0.28 % [1]. However, the results obtained in that study could not be
replicated here due to differences in the powder quality and issues with clogged inkjet nozzles
while printing samples in this study. Better quality control to replicate the same powder and
printhead conditions could help repeat those results in the future.
2.3.2 Porosity
In addition to bulk density measurements, samples of each type were sectioned at multiple
locations and polished in order to record the internal porosity using optical microscopy. Figure 2.3
shows a composite image created from three micrographs of a sample from an initial trial in the
HIP of bimodal parts at a lower temperature (975°C) for a longer holding period (4 hours) than the
rest of the samples. This image shows the presence of extensive surface porosity caused due to
14
infiltration of Argon gas into surface-connected pores in the sample, as opposed to dissolution of
entrapped gases into the matrix. This can happen in the presence of high surface porosity prior to
HIP, and with improper HIP parameters. This result shows a case where HIP failed to reduce the
porosity in binder jet parts.
Figure 2.3: Composite image showing porosity in part HIPed at 975°C for 4 hours
The parameters were then changed to a higher temperature of 1075°C and a shorter holding time
of 2 hours, which resulted in porosity reduction. Figures 2.4 and 2.5 show the polished micrographs
of 17µm and 25µm powder parts respectively. Images for each section depicted are from two
halves of the same part, one pictured after sintering and the other after HIP. These images indicate
a number of defects in the samples. For instance, a presence of excessive, non-uniform porosity
near the surface can be observed in nearly all sections. Weakly bound powder near the surface will
have been blown away during depowdering, resulting in low green density and relatively higher
surface porosity. The YZ sections of 17µm samples (Figure 2.4) indicate improper binding
between layers, which may be attributed to inefficiencies in powder spreading and packing during
printing. In some cases (e.g. 17µm – XY section, HIPed), the porosity can be observed to follow
linear patterns in the XY plane (plane of printing), along the Y direction (direction of printhead
traverse). This is due to clogged nozzles in the jetting head. It is seen that HIP does not remove
internal porosity effectively under these conditions.
15
These micrographs prove that there is a limit to the extent with which HIP can be useful in
eliminating porosity resulting from common process defects found in the Binder Jetting AM
process. Owing to the non-uniformity in the porosity of these sectioned parts, there was no fair
manner to sample sections to quantitatively determine the porosity in these parts. High-resolution
Computed tomography (CT) scanning is an alternative approach that may be used in the future to
quantify the three-dimensional porosity distribution in these cases.
Figure 2.4: Porosity in 17µm parts
16
Figure 2.5: Porosity in 25µm parts
In case of parts made using the bimodal powder, the issues mentioned above that were caused by
insufficient green part strength or powder packing were not observed. The porosity present was to
an extent that could largely be overcome by HIP, as can be seen in Figure 2.6. Due to the more
homogeneous nature of pore distribution in bimodal parts compared to the 17µm and 25µm parts,
a quantitative porosity measurement could be performed using image analysis (Section 2.2.4). The
porosity thus obtained was found to decrease from 2.90 ± 1.66 % as sintered, to 0.37 ± 0.21 %
after HIP. This quantitatively confirms the hypothesis of HIP improving the porosity in parts of a
sufficiently low starting porosity.
17
Figure 2.6: Porosity in bimodal parts
2.3.3 Microstructure
Sample micrographs for each part type and processing condition are presented in Figures 2.7 to
2.9. The grains in the case of both the 17µm (Figure 2.7) and the bimodal parts (Figure 2.9) are
largely equiaxed with significant twinning. HIP is clearly seen to coarsen this structure, as is
evidenced by the grain size measurements presented in Table 2.3.
The 25µm parts were found to form dendritic structures rather than the equiaxed grains otherwise
typically observed upon sintering (Figure 2.8). This is a natural microstructure for copper when it
is fully melted and rapidly solidified (especially in case of casting), whose formation depends on
various factors such as available surface area, oxygen content etc. [58]. While the formation of
these structures is not expected for the case of sintering and slow cooling, determining the exact
cause of their presence in these specimens with statistical accuracy requires further
experimentation that is beyond the scope of this study. However, it is observed that these dendritic
structures completely recrystallize to form equiaxed twinned grains upon HIP. The grain sizes
observed from the microstructures for these HIPed samples shows no immediately discernible,
qualitative difference in the grain sizes between the XY and YZ/XZ sections.
18
Figure 2.7: Grain structure change upon HIP – 17µm parts
Figure 2.8: Grain structure change upon HIP – 25µm parts
19
Figure 2.9: Grain structure change upon HIP – bimodal parts
The average grain sizes, as calculated using the Saltykov method along with the 95% confidence
interval, are presented in Table 2.3. The data shows that there is a significant coarsening of grains
upon HIP of 17µm and bimodal powder parts. This is to be expected due to high temperature
processing, however it is kept it at a minimal amount by rapid cooling at the end of the cycle. It is
particularly interesting to note that in the case of bimodal parts after sintering, the grain sizes are
well below those of the larger of the individual powder particles (30µm). The particle size of parent
powders in the cases of 25µm and bimodal parts are comparable to the grain sizes after HIP,
indicating only a reasonable amount of grain coarsening resulting from HIP.
20
Table 2.3: Grain size change upon HIP of various parts
Part
Type Plane
Sintered HIPed
% Theoretical
Density
Avg. Grain Diameter
± 95% CI
% Theoretical
Density
Avg. Grain Diameter
± 95% CI
17 µm XY
83.63 20.26 ± 3.01
85.83 37.05 ± 5.00
YZ/ZX 24.41 ± 5.85 30.89 ± 3.01
25 µm XY
77.73 -
82.41 24.88 ± 6.31
YZ/ZX - 23.38 ± 3.05
Bimodal XY
90.52 14.34 ± 1.34
97.32 24.08 ± 3.58
YZ/ZX 12.43 ± 2.23 27.43 ± 6.64
Differences in grain size between sections along the build plane (XY), and perpendicular to it
(YZ/ZX), were analyzed using a t-test for each part type. A statistically significant difference
between the orientations, at a 95% confidence interval, was found to be present only in the cases
of sintered bimodal specimens and HIPed 17µm specimens. This lack of a trend between sample
types could simply be a result of having had a limited number of samples (between 3 and 5 for
each) available for analysis. Randomizing the selection of regions for taking grain counts may
have brought about such a disparity. Further analysis will be required to derive physical meaning
from the grain orientation data.
2.3.4 Tensile Testing
HIP of Powder Bed Fusion AM processes typically results in a drop in strength as reported in
Section 2.1.2, due to a significant increase in the grain size. The data in Table 2.4 shows that HIP
improves the strength of all Binder Jet parts, indicating that the improvement in porosity is the
dominant factor compared to grain coarsening effects in determining strength in the case of Binder
Jetting. The maximum achievable strength from the literature for fully dense sintered copper
powder is 220 MPa [28], placing the tensile strength of 97.32% dense parts at only 80.16% of this
value. It is understandable that while HIP does improve the strength of Binder Jet parts, the
coarsened grains are going to result in a strength less than that achievable by sintering of loose
powders through traditional PM.
21
Figure 2.10: Samples of untested (top) and tested (bottom) tensile specimens
Figure 2.10 depicts a sample tension tested specimen to illustrate the scale of elongation. The
ductility is observed to increase significantly for the 25µm and bimodal powder parts, which
corresponds to an increase in their density. In case of the 17µm parts, the observed improvement
in ductility is minimal, which could be due to the comparatively smaller increase in density. The
ductility of similarly sintered pure copper powders is reported to be 45% [28]. Similar to the
strength, the ductility of all parts also falls well below this, with the most ductile specimens
(97.32% dense) at only 69.51% of this value in spite of grain coarsening.
Table 2.4: Tensile Strength and Ductility improvement upon HIP
Part
Type
Sintered HIPed
%
Theoretical
Density
Ultimate Tensile
Strength
(MPa)
% Elongation
%
Theoretical
Density
Ultimate Tensile
Strength
(MPa)
% Elongation
17 µm 83.63 115.84 ± 9.19 27.60 ± 2.58 85.83 135.31 ± 13.74 28.75 ± 1.27
25 µm 77.73 82.05 ± 5.34 12.07 ± 1.67 82.41 129.32 ± 0.94 30.38 ± 1.18
Bimodal 90.52 144.90* 17.87* 97.32 176.35 ± 6.48 31.28 ± 2.38
*Only one sample tested
The number of tested specimens was limited by available space in the print bed in addition to
damages during handling and depowdering of some green parts. The apparent premature fracture
of one set of samples (25µm, sintered) could later be traced back to a fabrication defect in these
specimens, caused by the presence of foreign particles in the powder. These results demand further
repetitions of the experiments to increase the sample size and make statistically significant
conclusions, as will be discussed in Section 2.4.
22
2.4 Closure and Future Work
Hot Isostatic Pressing (HIP) has been investigated as a secondary post-processing step in the
fabrication of high-density copper parts using Binder Jetting. The following conclusions may be
made based on the experimental results:
HIP improves density and porosity significantly only when the density of sintered parts is
at least 90%, which is achieved using a bimodal powder mixture of 30µm and 5µm powders
that has better packing properties than unimodal powders.
HIP causes dendritic structures to recrystallize into equiaxed grains, and previously
equiaxed grains to increase in size. The cause for the formation of these dendrites with the
most porous samples remains to be analyzed.
The tensile strength of samples increases upon HIP, indicating that the effect of reduction
in porosity outweighs that of grain coarsening in the case of Binder Jetting. The ductility
is found to improve when there is an increase in density due to HIP.
With careful control of powder quality and processing conditions, near-full density parts may be
printed. In future studies, copper parts will be characterized for various properties including
thermal and electrical conductivity. More work remains to be desired in order to get a complete
understanding of the mechanical properties improvement, particularly with regard to the effects of
part printing orientation on the tensile strength. The ductility improvement can be better
understood through a three-dimensional porosity distribution analysis that can be obtained using
techniques such as Computed Tomography (CT) scanning. An evaluation of the three-dimensional
pore distribution can give better insight into the nature of porosity and potentially explain the
observations regarding ductility increase. Lastly, a mathematical analysis of the variation of
material properties with porosity is part of the next phase of this study, as a step in the direction of
being able to tailor various processing parameters in order to achieve the density corresponding to
the desired properties.
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[50] W. J. Sames, K. A. Unocic, R. R. Dehoff, T. Lolla, and S. S. Babu, “Thermal effects on
microstructural heterogeneity of Inconel 718 materials fabricated by electron beam
melting,” J. Mater. Res., vol. 29, no. 17, pp. 1920–1930, 2014.
[51] M. Seifi, I. Ghamarian, P. Samimi, P. C. Collins, and J. J. Lewandowski, “Microstructure
and Mechanical Properties of Ti-48Al-2Cr-2Nb Manufactured Via Electron Beam
Melting,” in Proceedings of the 13th World Conference on Titanium, 2016, pp. 1317–1322.
[52] J. A. Gonzalez, “Characterization and Comparison of Metallic and Ceramic Parts Fabricated
Using Powder Bed-Based Additive Manufacturing Technologies,” The University of Texas
at El Paso, 2017.
[53] ASTM B962-13, “Standard Test Methods for Density of Compacted or Sintered Powder
Metallurgy (PM) Products Using Archimedes’ Principle,” 2013.
28
[54] ASTM E112-13, “Standard test methods for determining average grain size.” 2013.
[55] S. A. Saltykov, Stereometric Metallurgy, Part 2. 1961.
[56] ASTM B925-15, “Standard Practices for Production and Preparation of Powder Metallurgy
( P / M ) Test.” 2015.
[57] ISO/ASTM 52921, “Standard terminology for Additive Manufacturing - Coordinate
systems and test methodologies.” 2013.
[58] G. F. Vander Voort, R. N. Caron, R. G. Barth, and D. E. Tyler, “Microstructures of Copper
and Copper Alloys,” in ASM Handbook, Volume 9: Metallography and Microstructures,
Vol. 9, 2004, pp. 775–788.
29
Chapter 3: Impacts of Process-Induced Porosity on Material Properties of
Copper Made by Binder Jetting Additive Manufacturing
Abstract
Additive Manufacturing (AM) can be used to manufacture copper parts featuring complex
geometries for use in advanced thermal and electrical components. Binder Jetting is a relatively
cheap and scalable AM technology that is suited for manufacturing highly conductive or reflective
materials like copper. This study aims to quantify the effects of varying amounts of porosity on
the material properties of Binder Jet copper parts, and compare them with existing models in the
literature. Towards this, copper parts of porosities ranging from 2.68% to 16.37% were fabricated
via Binder Jetting, by varying powder morphology and post-process sintering and Hot Isostatic
Pressing (HIP) conditions. Their properties have then been compared against various models in
the literature for property-porosity relations for Powder Metallurgy (PM) copper, or for general
two-component structures. This can pave the way towards developing a scientific understanding
of the process-property-performance relationship in Binder Jetting of copper, which can help in
achieving desired properties through the choice of appropriate materials and processing conditions.
3.1 Introduction
Copper is an important material with high thermal and electrical conductivities that can be used in
applications involving enhanced heat transfer and electrical and electronic components. Additive
Manufacturing (AM) of copper parts can enable the fabrication of the complex geometries that
may be required for such applications such as heat exchangers and rocket engine components with
internal cooling channels [1]. However, its high thermal conductivity makes it difficult to control
the melt pool if fabricating by laser- or electron beam-based Powder Bed Fusion (PBF) AM
technologies, and its high optical resistivity limits the choice of wavelengths of laser that may be
used [2]. Hence, processing pure, unalloyed copper powder using such systems can be challenging.
3.1.1 Prior Work on Powder Bed Fusion of Copper
Singer and co-authors provide a review on the some of the significant work on AM of copper and
copper alloys [3]. Pogson and co-authors were able to produce low-resolution, thin-walled
structures of copper on a stainless steel substrate using relatively low-power laser (by Direct Metal
Laser Re-melting, or DMLR), but their methodology was unlikely to be successful in producing
high strength parts that are commercially realizable due to insufficient melting of powders [4].
30
Lykov and co-authors were able to achieve 88.1% density and a strength of only 149 MPa in
copper parts fabricated using Selective Laser Melting (SLM) [5]. Kaden et al. studied the use of
ultrashort laser pulses in the fabrication of both thin-walled and solid cuboid structures, but these
were porous owing to large grains formed due to the laser spot diameter being roughly the same
as the powder particle size [6]. In the industry, 3T RPD, a company based in the UK, claimed to
have produced pure copper parts using Direct Metal Laser Sintering (DMLS) by suitably
modifying the machine, calibrating process parameters, and modifying support structure designs
[7]. There is yet to be published work on the exact modifications applied, or the release of a
production-ready part. The Fraunhofer Institute for Laser Technology has announced work on the
development of a green laser light of 515nm wavelength that can be better absorbed by pure copper
and copper alloys for SLM [8].
Electron Beam Melting (EBM) is an alternative that can circumvent some issues faced with laser-
copper interactions. Ramirez et al. used EBM to fabricate open- cellular structures using precursor
powder (of 99.8% purity) containing Copper Oxide (Cu2O) precipitates, which reflected as
microstructural copper oxide arrays in the fabricated part [9]. Precipitation-dislocation
architectures were observed in parts fabricated via EBM using powders of relatively low purity of
98.5% [10]. Yang et al. fabricated auxetic structures made of pure copper using EBM, but were
unable to achieve consistent dimensional accuracy and strength owing to fabrication defects
arising due to the high thermal conductivity of copper [11]. Frigola et al. circumvented issues with
the high thermal conductivity of copper, resulting in high thermal gradients and hence
delamination, to form complex parts with internal cooling channels [12]. They mention the
importance of using high vacuum, and concerns with reusing powders that can result in detrimental
oxide formation. Terrazas et al. explored the AM of multi-material structures made of Ti-6Al-4V
and Copper in tandem using EBM [13]. The copper portion was observed to be misaligned,
warranting better part registration. The microstructure was observed to transition from columnar
grains away from the interface between the materials, to an equiaxed structure near the interface,
which could have acted as a substrate to facilitate epitaxial growth. Twinning was observed after
recrystallization caused by HIP. Lodes and co-authors fabricated up to 99.5% dense copper by
optimizing process parameters of EBM, but were yet to characterize the material properties [14].
In summary, a majority of these methods developed have had challenges in fabricating high-
density copper parts with minimal specialized modifications to the processes or the machines used.
31
3.1.2 Binder Jetting of Copper
Binder jetting is an alternative to these Powder Bed Fusion AM technologies that involves
selectively jetting a binder onto layers of powder to form a ‘green’ part by curing the binder (Figure
3.1). This is followed by debinding and sintering in a furnace for densification and strength. It
effectively circumvents issues caused by the high thermal conductivity and reflectivity of copper
by separating the part creation from energy supply. The primary disadvantage of this process,
however, is its inability to sufficiently densify the green parts. This is because the spreading of
layers does not involve any compaction or agitation, resulting in powder that is loosely packed.
The residual porosity in fabricated parts can be detrimental to their material properties. In the case
of copper, Binder Jetting has been used to achieve up to 99.47% density through the use of
optimized powder morphologies [15], printing parameters and sintering profiles [16], and post-
process Hot Isostatic Pressing (HIP) [17]. Various intermediate densities achieved using different
powders and post-processing conditions have been discussed in Chapter 2.
Figure 3.1: A schematic of the binder jetting process
3.1.3 Research Objective
In this article, the authors characterize the influence of this process-induced porosity on various
material properties, namely strength, ductility, and thermal and electrical conductivity. This is
done by analyzing the properties of specimens processed to achieve varying degrees of porosity
by varying powder morphology and post-process conditions. These properties are then compared
to equations and data in the literature in the context of porous copper prepared using powder
metallurgy, or to predictions based on models for two-component structures.
A comparison with PM copper is appropriate since Binder Jetting is closest in nature to PM
processes, albeit with loosely packed powders and no compaction. However, because of this
difference between the processes, it has been seen that powder metallurgy knowledge does not
directly translate to Binder Jetting. For instance, Bai and co-authors presented that the use of
32
bimodal powders in Binder Jetting yields higher density than when using small powders which is
contradictory to PM processes involving powder compaction, where finer powders may be
favorable [15]. Similarly, two-component system based models need not directly reflect Binder
Jet part properties either. These models are instead used in this study as a reference for comparison
to Binder Jetting, and to see if any of them can model this process well. The long-term goal is to
use these models as a foundation to develop process-structure-property relationships for Binder
Jetting.
Section 3.2 presents the various models that are used to study the measured properties, including
strength and ductility (3.2.1), thermal and electrical conductivity (3.2.2) and the Wiedemann-Franz
Law that provides a relationship between the two conductivities (3.2.3). Section 3.3 presents the
experimental methods used in fabricating the specimens of varying porosity (3.3.1), testing their
tensile (3.3.2), thermal (3.3.3) and electrical conductivity (3.3.4) properties. Section 3.4 presents
the results from the analyses of strength (3.4.1), ductility (3.4.2), thermal and electrical
conductivities (3.4.3 and 3.4.4) and the Wiedemann-Franz Law (3.4.5). Conclusions of the study
and suggestions for future work are presented in Section 3.5.
3.2 Modeling Porosity-Property Relationships
The goal of this work is to map the process-induced porosity to material properties, and hence lay
a framework for developing process-structure-property relationships for Binder Jetting of copper.
This is to be done by understanding the correlation between processing conditions and porosity,
which has been explored in Chapter 2, and that between process-induced porosity and material
properties as explored in this work. The models used in developing these porosity-property
relationships are presented here.
3.2.1 Strength and Ductility
The tensile strength of porous PM specimens can be estimated to vary with density by the Equation
3.1 [18]:
𝜎 = 𝜎𝑜𝐾 (𝜌
𝜌𝑡)
𝑚
( 3.1 )
where 𝜎 is the tensile strength of the porous material, 𝜎0 is the wrought strength of the same
material (220 MPa for copper), 𝜌 and 𝜌𝑡 are the density of the porous specimen and the theoretical
density of the material respectively. 𝐾 is equivalent to a stress concentration factor due to the
33
pores, and 𝑚 is the exponential dependence of strength on porosity. Both parameters depend on
the processing conditions, and the equation is valid in the absence of any microstructural problems.
German also presents data on the strength of PM copper with various degrees of porosity in his
book [18]. This data presented by German was fitted to the model he proposed in Equation 3.1 in
order to obtain the values of parameters 𝐾 and 𝑚 empirically to be 0.9926 and 2.5150 respectively
for PM copper (with an R2 value of 0.9998). Hence, the equation for sintered PM copper is found
to be as below:
𝜎 = 218.367 (𝜌
𝜌𝑡)
2.5150
( 3.2 )
Equation 3.3 gives the dependence of ductility on porosity as proposed by Haynes [19]:
𝑍 = {
(1−𝜀)1.5
(1+𝑐𝜀2)0.5 , 𝜀 < 0.15
(1−𝜀)1.5
(1+0.152𝑐)0.5 , 𝜀 > 0.15 ( 3.3 )
where 𝑍 is the relative ductility (ratio of observed elongation of porous material to wrought
material), 𝜀 is the porosity, and 𝑐 is a coefficient representing the sensitivity of the ductility to
porosity for the material under consideration. Fitting the ductility data provided by German [18]
for various porosities under 15% into the model in Equation 3.3, the value of 𝑐 is obtained as
210.237, at an R2 value of 0.9931. The elongation of wrought copper is taken as 0.45 based on the
same data. Hence, the empirical equation for the ductility dependence on porosity for PM copper
is found to be:
𝑍 = {
(1−𝜀)1.5
(1+210.237𝜀2)0.5 , 𝜀 < 0.15
0.4177(1 − 𝜀)1.5 , 𝜀 > 0.15 ( 3.4 )
These equations and calculated empirical constants are applicable for porosities achieved through
varying degrees of sintering of powders, and do not represent the same kind of processing as
Binder Jet parts. These values are hence used here as a reference to present where the ductility of
binder jet, sintered / HIPed copper parts stand in comparison to existing Powder Metallurgy data.
This may be used as a framework for future studies that may explore deriving new equations for
the processing conditions associated with binder jetting and sintering.
34
3.2.2 Thermal and Electrical Conductivity
There are a number of models in the literature that describe the variation of thermal conductivity
with porosity. A model proposed by Aivazov and Domashnev proposes the following relation for
the variation of thermal and electrical conductivity with porosity [20]:
𝐾 = 𝐾01−𝜀
1+𝑛𝜀2 ( 3.5 )
where 𝐾 is the thermal conductivity of the porous specimen, 𝐾0 that of pure wrought copper (388
W/m-K), 𝜀 is the porosity and 𝑛 is an experimentally determined constant. In regimes of porosity
less than 0.3, the relation may be approximated as follows [18]:
𝐾 = 𝐾0(1 − 𝜔𝜀) ( 3.6 )
where 𝜔 is a constant with a value between 1 and 2, also determined experimentally. Data
presented by German for his experiments on sintering of copper powders was fitted to this model
to give the value of for 𝜔 to be 1.1360 (at an R2 value of 0.9999) for thermal conductivity, and
1.1228 for electrical conductivity (at an R2 value of 0.9999). These values, although determined
for copper at a different kind of processing, are taken as a reference to compare against the
conductivity data obtained from Binder Jet copper.
Other models discussed in the literature [21 – 23] explore the behavior from a physical perspective
by modeling the porous material as a two-phase structure, and assuming various kinds of
dispersions and shapes of one constituent (pores) with regard to the other (parent material). In all
the equations described below, 𝐾 refers to the effective conductivity, 𝑘1 and 𝑣1 refer to the
conductivity and volume fraction of the parent material (copper) and 𝑘2 and 𝑣2 to those of the
dispersed medium (pores), respectively.
The Parallel model assumes that the phases are aligned parallel to the heat flow, providing
alternating parallel conduction pathways. This gives the upper bound to the effective thermal
conductivity, as given by Equation 3.7 [24]:
𝐾 = 𝑘1𝑣1 + 𝑘2𝑣2 ( 3.7 )
35
The Maxwell-Eucken model assumes a distribution of non-interacting spherical pores within the
copper medium, for which the effective conductivity is given by the equation below [21]:
𝐾 =𝑘1𝑣1+𝑘2𝑣2
3𝑘12𝑘1+𝑘2
𝑣1+𝑣23𝑘1
2𝑘1+𝑘2
( 3.8 )
The Effective Medium Theory (EMT) model assumes a random distribution of two phases, for
which the effective conductivity is given by Equation 3.9 [23]:
𝐾 = 0.25[(3𝑣2 − 1)𝑘2 + (2 − 3𝑣2)𝑘1 + {[(3𝑣2 − 1)𝑘2 + (2 − 3𝑣2)𝑘1]2 + 8𝑘1𝑘2}]0.5 ( 3.9 )
A modified form of the EMT has also been proposed that takes into consideration grain boundary
thermal resistance effects. The equation for this is as follows [23]:
𝐾𝑝𝑜𝑙𝑦 = [1
𝑘𝑠𝑖𝑛𝑔𝑙𝑒+ 𝑛𝑅𝑡ℎ]
−1
( 3.10 )
where 𝐾𝑝𝑜𝑙𝑦 is the thermal conductivity of the polycrystalline matrix, 𝑘𝑠𝑖𝑛𝑔𝑙𝑒 is that of pure, single
crystal material in the absence of grain boundaries, 𝑛 is the number of grain boundaries per unit
length, and 𝑅𝑡ℎ is the thermal resistance of the grain boundaries.
The observed thermal and electrical conductivity are compared against that predicted from each
of the equations 3.6 – 3.9 and presented in Section 3.4.3 and 3.4.4. Additional analyses are done
by using the observed thermal conductivity for each processing condition in the EMT and modified
EMT models to calculate the conductivity of the copper matrix in each case, and further estimate
the grain boundary thermal resistance, 𝑅𝑡ℎ to compare to what is estimated in the literature [23,
24]. This is presented in Section 3.4.3.
3.2.3 Wiedemann-Franz Law
The electrical and thermal conductivities are related by a semi-empirical relation called the
Wiedemann-Franz Law, which is given by Equation 3.11:
𝐾 = 𝐿𝑇
𝜌+ 𝑏 ( 3.11 )
where 𝐿 is the Lorenz function, 𝑇 is the temperature, and 𝜌 is the electrical resistivity. 𝐿𝑇
𝜌 and 𝑏
represent the material-dependent electron and lattice (phonon) contributions to thermal
conductivity respectively. In the case of copper, previous literature has proposed the values of 𝐿
36
and 𝑏 to be 2.39 x 10-8 and 0.075 [25 – 27]. Koh and Fortini in their study of porous PM copper
determined them to be 2.307 x 10-8 and 18.6, respectively [29].
One approach to a modification of the Wiedemann-Franz Law for the specific case of Binder Jet
copper could be to fit the conductivity data to determine a modified equation [30]. However, in
this study, the observed thermal conductivity data is directly compared to predictions using Koh
and Fortini’s model for OFHC copper powders sintered to varying porosities [29]. They state that
the constants are expected to independent of porosity and dependent only on the material. It is
assumed here that the difference in powder size and type, and the processing conditions in this
study as compared to theirs, has a negligible effect on these constants. The results from this
analysis are discussed in Section 3.4.5.
3.3 Experimental Methods
3.3.1 Specimen fabrication
Figure 3.2 shows a picture of the print bed during printing, indicating the layout of various
specimens printed for testing.
Figure 3.2: Photograph depicting layout of parts in print bed:
A: Rods for electrical conductivity measurements, B. Dogbones for Tensile Testing,
C. Rectangular coupons for density measurements, D. Discs for thermal conductivity measurements
37
Previous work by the authors on using different powder configurations to explore the HIP of parts
of varying sintered densities (Chapter 2) was used in achieving differing levels of porosity. A
schematic of the process chain is illustrated in Figure 3.3, showing the process variables and
control variables for each stage of the process chain (detailed explanations in Section 2.2). To
summarize, the illustrated parameters were used to print and sinter parts using powders with
median sizes of 17µm and 25µm and a bimodal combination of powders with median sizes 30µm
and 5µm, mixed in the ratio of 73:27 by weight. Some of these were HIPed at 1075°C, 206.84
MPa (30,000 psi) for 2 hours for analysis of HIPed specimens, and others retained for analyses of
sintered specimens. Different amounts of porosity were thus achieved.
Figure 3.3: Schematic of process chain
Density measurements were done using an Archimedes principle based apparatus to measure the
weight of parts in air and in water and calculating the relative density. These specimens were oil
impregnated for accuracy in measurements (Sections 2.2.3 and 2.3.1). The processing conditions
used to obtain specimens of each density and the corresponding porosity are presented in Table
3.1.
38
Table 3.1: Powders and processing conditions used to achieve different porosities
Powder Density (% Theoretical) Porosity (%)
Bimodal HIPed 97.32 2.68
Sintered 90.52 9.48
17 µm HIPed 85.83 14.17
Sintered 83.63 16.37
25 µm HIPed 82.41 17.59
Sintered 77.73 22.27
(a) (b)
Figure 3.4: Porosity in 17µm specimen (YZ plane) : (a) Sintered – 16.37%; (b) HIPed – 14.17%
(a) (b)
Figure 3.5: Porosity in bimodal specimen (YZ Plane): (a) Sintered – 9.48%; (b) HIPed – 2.68%
39
Optical porosity characterizations (Section 2.2.4) were done by polishing samples in successively
finer grit papers and polishing cloths to remove scratches on the surface, and observed under an
optical microscope. Three samples of each type were sectioned along all three planes for analysis.
Some representative images are provided in Figure 3.4 (17µm specimens) and Figure 3.5 (bimodal
specimens). These indicated that the porosity distribution is heterogeneous in the case of 17µm
and 25µm specimens, with more pores concentrated near the surface and between layers. Such a
porosity distribution is different from those assumed in the models discussed in Section 3.2. In the
case of bimodal parts, while the porosity distribution is more uniform, the processing conditions
and powders result in microstructural issues that are not accounted for in these models (Section
2.3.3). This invalidates the direct applicability of these models to Binder Jetting, and hence only a
comparison may be made between the data available and that expected based on those models.
3.3.2 Tensile Testing
Tensile testing data has previously been reported in Section 2.3.4. The same data has been used to
model the strength and ductility as a function of porosity. As reported in that work, the testing was
carried out per the ASTM Standard E8/E8-M -16a [31] using flat unmachined test specimens
(Section 2.2.5). The specimens were printed randomly in the XYZ or the YXZ orientations (Figure
3.2) [32]. The tests were run on an INSTRON machine using a 50kN load cell at a constant strain
rate of 0.015 mm/mm/min per the recommendation in the standard (elongation rate of 0.381
mm/min) for a gauge length of 1 inch (25.4 mm). The ultimate tensile strength at break is
measured, and the ductility is reported in terms of percentage elongation of gauge length.
3.3.3 Thermal Conductivity Measurement:
The thermal conductivity of these specimens of various densities was calculated by measuring the
thermal diffusivity using a laser flash diffusivity apparatus. The procedures followed were based
on the ASTM Standard E1461-13 [33]. The samples fabricated for these measurements were thin
cylinders with a sintered diameter of 10mm and a thickness of ~2 mm (Figure 3.2). These were
then sprayed with a thin layer of graphite in order to avoid the laser flashes from reflecting off the
surface. The laser pulse on one flat surface of the cylindrical specimen causes a rise in the
temperature of the entire part, with the temperature of the opposite face being monitored by the
apparatus. The time taken for the temperature of this face to reach half the maximum value is
40
calculated. This, along with the thickness of the specimen, 𝐿 gives the thermal diffusivity,
according to Equation 3.12 below:
𝛼 =0.13879𝐿2
𝑡12
( 3.12 )
where 𝛼 is the diffusivity, 𝐿 is the thickness of the sample and 𝑡1
2
is the time taken for the
temperature of the far side to reach half of the equilibrium value. The thermal conductivity can
then be calculated from the Equation 3.13 below:
𝜆 = 𝛼𝐶𝑝𝜌 ( 3.13 )
where 𝜆 is the thermal conductivity, 𝐶𝑝 is the specific heat capacity and 𝜌 is the density of the
specimen. Here, the specific heat capacity was measured using the Laser Flash apparatus and the
density from Archimedes Principle based measurements (3.3.1).
3.3.4 Electrical Resistivity Measurement
The resistivity of the samples was measured using a four-wire measurement apparatus as is
illustrated in Figure 3.6. For this purpose, the samples printed were thin cylindrical rods (Figure
3.2). A known current is passed through a measured length of the rod, and the corresponding drop
in voltage is measured. The resistivity can be calculated using Equation 3.14:
𝜌 =𝑅∗𝐴
𝐿=
𝑉∗𝐴
𝐼∗𝐿 ( 3.14 )
where 𝜌 is the resistivity, 𝑅 is the resistance, 𝑉 is the measured voltage drop, 𝐼 is the known current
passed, 𝐴 is the cross sectional area and 𝐿 is the length of the rod.
41
Figure 3.6: Electrical Resistivity measurement setup:
A: Power source, B: Multimeter, C. Tested sample, D: Connecting wires with alligator clips
Owing to the relatively low resistivity of copper, in order for the voltage to be measurable using
the available multimeter, the length of the rod and the current passed through it had to be
maximized, while the cross sectional area had to be minimal. Hence, a current of 10A was passed
using a high-current power source. Given the constraints of the printing process, and taking into
account that the as-printed green parts must be able to withstand handling and depowdering prior
to sintering, the parts were designed to have a diameter of 2mm and a length of ~150 mm. The
results from the resistivity measurements are presented in Section 3.4.4. Conductivity values are
presented either in S/m, or as a percentage of the International Annealed Copper Standard, or IACS
(100% IACS = 5.8001e7 S/m).
3.4 Results
3.4.1 Tensile Strength
Figure 3.7 compares the strength of the material obtained for each of the processing conditions, to
the values expected based on Equation 3.2 (Section 3.2.1).
42
Figure 3.7: Tensile strength observed in comparison to Model Predictions
It is evident that the tensile strength of both sintered and HIPed copper is below the value expected
based on the models. The disparity may be expected due to microstructural differences caused by
the difference in powder types and processing conditions as compared to the model. This results
in different grain structures for each of the specimens as compared to those considered in the
model. The powder size can dictate the grain sizes in the sintered parts, and the HIP can further
recrystallize or coarsen each of these (Section 2.3.3). A detailed evaluation of the grain structures
and nature of porosity in the sintered as well as HIPed samples using 3-dimensional visualization
techniques may provide further light into the significance of these findings. Additionally, studying
the influence of printing orientation rather than randomizing them as in this study may provide
further information regarding the isotropy of strength of parts fabricated using this process chain.
3.4.2 Ductility
Figure 3.8 presents the ductility of the fabricated specimen along with the predicted values based
on the model given in Equation 3.4.
43
Figure 3.8: Ductility – Observed vs. Model
The as-sintered 25µm specimens were observed to have failed prematurely, which was later
reasoned to have been due to a fabrication defect. In addition to this, the fact that there was only
one sample in the bimodal, sintered category available for testing, warrants further investigation
for statistically significant conclusions. Hence, without quantifying the degree by which the values
observed differ from those predicted, it may only be safe to make qualitative conclusions. The
printed parts have higher ductility than the values expected from the model in the case of the 17µm
and 25µm parts (~15% and higher porosity), while the model seems to over-predict the ductility
in the case of bimodal parts (<10% porosity). In the case of parts with open porosity (>~10%), this
could be due to the distribution of porosity and layer interface bonding defects in binder jet parts
being localized to near the surface as compared to the core as seen in the sample micrographs of
17µm specimens in the YZ plane (Figure 3.4). This is different from the uniform spread of pores
that may be expected from sintered powders, as assumed in the model.
In the case of lower porosity (i.e., bimodal) parts, the surface porosity is completely sealed, with
the majority of remaining porosity homogeneously distributed in the inner regions (Figure 3.5).
This porosity distribution may be closer to what is obtained by sintering of loose powders to similar
densities, and the difference in ductility may be due to factors such as difference in grain structure,
purity of the copper powders used, oxide formation etc. Detailed investigations in the future may
help formulate more accurate mathematical relations to express the variation of ductility with
porosity for these processing conditions.
44
3.4.3 Thermal Conductivity
The results from thermal conductivity obtained using the Laser Flash method are presented in
Figure 3.9 along with a comparison to the various models presented in Section 3.2.2.
Figure 3.9: Thermal Conductivity vs. Porosity: Observed data compared to models
It is seen that all the models overestimate the conductivity in comparison to the observed data.
This is due to a multitude of reasons. Firstly, there is a disparity in the shapes and distribution of
the pores assumed in comparison to each of the models, as explained in Section 3.2.2. In addition,
none of these models account for the presence of grain boundaries of varying types and sizes that
add to the thermal resistance, or for any processing defects in the processed samples.
The observed conductivity data fitted to a linear model shows a reasonably linear trend, with the
forecasted value of conductivity for fully dense parts predicted to be 335.18 W/m-K. This value
being lower than ideally expected (388 W/m-K) can be attributed to impurity of the copper
powders used, residual carbon after the binder burn-off, and the presence of thermal resistance at
the grain boundaries. However, since each of these data points represents a different powder /
processing condition, the trend and the forecast are to be taken merely as a guideline.
Hence, instead of using such a generalized linear trend to describe the data, the EMT model
(Equation 3.9) may be used to estimate the value of 𝑘1 by substituting the value of observed
conductivity for 𝐾. The equation for 𝑘1 expressed in terms of 𝐾 is as follows:
45
𝑘1 =2𝐾2+(𝑣1−2𝑣2)𝑘2𝐾
2𝑣1𝐾+𝑘2−𝑣2𝐾 ( 3.15 )
This expression returns the conductivity of the pore-free copper matrix for each processing
condition, accounting for effects of grain boundaries, impurities and any such non-porosity-related
effects. This calculated value of 𝑘1 will henceforth be referred to as 𝑘𝑓𝑑, for conductivity at full
density for a given powder and processing condition. The Figure 3.10 below plots the value of 𝑘𝑓𝑑
calculated for each of the processing conditions.
Figure 3.10: Conductivity at full density (𝑘𝑓𝑑) calculated for each processing condition
The plot shows that for most cases, the conductivity of the pore-free matrix lies between 335 and
350 W/m-K. This can be taken to provide a physical validation for the linear trendline-based
estimate. The comparatively high conductivity for the case of as-sintered 25µm parts can be
attributed to the difference in their microstructure (dendritic) as compared to that of the other
specimens (equiaxed, twinned) which may allow for differing mechanisms of conduction and
inaccuracies in the model prediction (Section 2.3.3).
As the next step in attempting to understand the physics of heat conduction in copper processed in
this manner, an attempt is made to calculate the grain boundary thermal resistance in each of these
kinds of parts, excluding the data from sintered 25µm specimen due their having dendritic
microstructure. This is done by taking the value of 𝑘𝑓𝑑 as that of 𝑘𝑝𝑜𝑙𝑦 in Equation 3.10. Re-writing
the equation here,
46
𝐾𝑝𝑜𝑙𝑦 = 𝑘𝑓𝑑 = [1
𝑘𝑠𝑖𝑛𝑔𝑙𝑒+ 𝑛𝑅𝑡ℎ]
−1
( 3.16 )
The value of 𝑛 is estimated based on the grain size calculations done in Section 2.3.3. The total
number of grains per unit length, is approximated as being equal to the total length (1 meter)
divided by the average grain diameter, 𝑑𝑔(in meters). The number of grain boundaries encountered
per unit length is then taken to be the next closest whole number to this value. For instance, if it is
estimated that there are an average of 10,000.23 grain boundaries per meter, then the value of 𝑛 is
taken to be 10,001 m-1 per the equation:
𝑛 = 𝑐𝑒𝑖𝑙𝑖𝑛𝑔 (1
𝑑𝑔) ( 3.17 )
Re-arranging Equation 3.16 for 𝑅𝑡ℎ yields the following equation:
𝑅𝑡ℎ =1
𝑛(
1
𝑘𝑓𝑑−
1
𝑘𝑠𝑖𝑛𝑔𝑙𝑒) ( 3.18 )
where 𝑘𝑠𝑖𝑛𝑔𝑙𝑒 is taken as 388 W/m-K. The results of calculating the grain boundary thermal
resistance for each processing condition are presented Figure 3.11 below.
Figure 3.11: Grain Boundary Thermal Resistance calculated from Modified EMT Model
The data shows that the thermal resistance of grain boundaries in the cases of Binder Jet copper
subjected to sintering / HIP is up to an order of magnitude greater than that estimated from the
literature [23, 24]. This is to be expected due to the difference in processing and measurement
47
conditions between the parts in this study and that in the literature. A physical interpretation of the
quantitative differences in the estimates is beyond the scope of this work.
3.4.4 Electrical Conductivity
A similar analysis is done for the resistivity measurements to compare with the same set of models
as in Section 3.4.3, and the results are presented in Figure 3.12.
Figure 3.12: Electrical Conductivity vs Porosity: Observed data compared to models
The electrical conductivity is observed to follow a trend similar to that reported for thermal
conductivity in that the models consistently overestimate their values. However, the electrical
conductivity seems to approach a value of 5.60e7 S/m (or 96.63% IACS), which is closer to the
theoretical value of 5.8001e7 S/m (100% IACS) at full density than the thermal conductivity
projection (86.39%). However, they are expected to be analogous in their variation. This may be
seen as due to differences in the printing and measurement orientations. The electrical resistivity
specimen were all rods printed parallel to the build plane on the printer (XYZ orientation, per
[32]), with the measurements taken along the length (Y-direction). Whereas, the thermal
conductivity specimens were disks printed with the thickness along the build direction (XYZ
orientation), which is the direction of measurement (Z-direction). Hence, layer interface bonding
issues in the higher-porosity specimens (Figure 3.4) may cause disparity in these values and their
relationships. This difference is further investigated in Section 3.4.5 using the context of the
Wiedemann-Franz Law.
48
3.4.5 Wiedemann-Franz Law
The thermal conductivity measured for various porosities is plotted against that calculated by the
Wiedemann-Franz Law as described by Koh and Fortini for PM copper (Section 3.2.3) to compare
and verify the applicability of the Law to the processes under consideration (Figure 3.13).
Figure 3.13: Observed Thermal Conductivity vs. that calculated using the Wiedemann-Franz Law
The figure shows that the model underestimates the thermal conductivity for higher porosity (lower
electrical conductivity) specimens and overestimates it for lower porosity specimens. This could
potentially be because of the differences in the powder material used as compared to their model.
However, since the parts do exhibit directional porosity (layer bonding defects in higher porosity
specimens) and the measurements of thermal and electrical conductivity were taken in different
directions, it is possible that this anisotropy may have caused such a trend. Further research into
the anisotropy in porosity may be done using three-dimensional porosity visualization techniques
such as Computed Tomography (CT) scanning. Investigating the potential directional variations
of thermal and electrical conductivity will require more detailed experiments that are beyond the
scope of this work.
49
3.5 Conclusions
The effects of porosity on various material properties of copper fabricated by Binder Jetting of
three different powder types, followed by sintering and Hot Isostatic Pressing, have been
investigated in comparison to models in the literature for copper parts with porosity achieved
through other processing methods. The major findings have been summarized below:
The strength of copper parts is found to be less than that predicted by the model in Equation
3.2 (Figure 3.7), which may be attributed to the presence of microstructural issues and
differences in the grain structure due to differences in powder material and/or
manufacturing techniques.
The ductility predictions based on Equation 3.4 overestimate the elongation in the low-
porosity regimes due to microstructural and compositional differences, and underestimate
the same in the high-porosity regimes where the porosity spread is localized to the surface
as compared to uniformly distributed pores in sintered Cu powders (Figure 3.8)
The thermal conductivity has been compared to various models in the literature, all of
which overestimate the influence of porosity. A generalized linear trend, while not
conclusive, predicts a maximum thermal conductivity of 335.18 W/m-K (Figure 3.9).
This value being less than that of PM copper (388W/m-K) is due to carbon residue from
binder pyrolysis, presence of oxides and differences in grain structure for Binder Jet
copper. A validation for this value has been performed by determining an approximate
range for the maximum possible conductivity for each processing condition using the EMT
model (Figure 3.10). Further, the grain boundary thermal resistance has been calculated for
each case and found to be greater than what may be assumed based on the literature (Figure
3.11).
The electrical conductivity has been found to compare similarly with models existing in
the literature. The prediction for electrical conductivity at full density, based on a
generalized linear trend similar to that employed for thermal conductivity, is 96.63% IACS
(Figure 3.12). Using these values to estimate the thermal conductivity using the
Wiedemann-Franz Law shows a variation in the disparity between the predicted and
observed thermal conductivity (Figure 3.13). This could potentially be due to the difference
50
in measurement direction of each conductivity relative to the layer interfaces, which is
influenced by the observed directionality in porosity (Figure 3.4).
To summarize, the properties analyzed are observed to consistently fall below the predictions
based on both Powder Metallurgy and two-component structural models. In the case of a
comparison with PM copper, this is attributed to the less uniform porosity distribution, differences
in purity of powders and in grain structures arising from different powders and processing
conditions for each level of porosity achieved using Binder Jetting. A more elaborate analysis of
achieving varying degrees of porosities through simply changing the sintering or HIP conditions
for each of the powders is required to derive relations similar to PM equations (Section 3.2) for
Binder Jetting. The predictions for thermal and electrical conductivity based on models for two-
component systems are also higher than the observed values, due to each of these models assuming
a different kind of porosity distribution than that observed for each of the Binder Jet specimens.
Better approximations for both the porosity distribution and the influence of grain boundary effects
and impurities can help develop accurate, physics-based two-component models for thermal and
electrical conductivity predictions in the future.
This study has placed results for AM of copper within the context of theoretical models for other
processing methods to achieve similar levels of porosity. This presents opportunities for future
work in developing specialized models for the particular cases of Binder Jetting, and more
elaborate studies of the effects of powder types, printing orientation, measurement direction,
processing parameters etc. Developing such models will enable one to print copper parts of desired
material properties using Binder Jetting by adjusting the material and processing parameters to
achieve the corresponding degree and type of porosity.
3.6 References
[1] T. McMahan, “NASA 3-D Prints First Full-Scale Copper Rocket Engine Part,” NASA Press
Office, 2015. [Online]. Available: https://www.nasa.gov/marshall/news/nasa-3-D-prints-
first-full-scale-copper-rocket-engine-part.html.
[2] F. Sciammarella, M. J. Gonser, and M. Styrcula, “Laser Additive Manufacturing of Pure
Copper,” in Rapid, 2013.
[3] F. Singer, D. C. Deisenroth, D. M. Hymas, and M. M. Ohadi, “Additively manufactured
copper components and composite structures for thermal management applications,” 16th
IEEE ITHERM Conf., pp. 174–183, 2017.
51
[4] S. R. Pogson, P. Fox, C. J. Sutcliffe, and W. O’Neill, “The production of copper parts using
DMLR,” Rapid Prototyp. J., vol. 9, no. 5, pp. 334–343, 2003.
[5] P. A. Lykov, E. V. Safonov, and A. M. Akhmedianov, “Selective Laser Melting of Copper,”
Mater. Sci. Forum, vol. 843, pp. 284–288, 2016.
[6] L. Kaden et al., “Selective laser melting of copper using ultrashort laser pulses,” Appl. Phys.
A, vol. 123, no. 9, p. 596, 2017.
[7] “3T produces Pure Copper Heat Exchanger using metal 3D Printing.” [Online]. Available:
http://www.3trpd.co.uk/3t-success-with-pure-copper-am-production/.
[8] “Green Light for New 3D Printing Process - Fraunhofer ILT.” [Online]. Available:
https://www.ilt.fraunhofer.de/en/press/press-releases/press-release-2017/press-release-
2017-08-30.html.
[9] D. A. Ramirez et al., “Open-cellular copper structures fabricated by additive manufacturing
using electron beam melting,” Mater. Sci. Eng. A, vol. 528, no. 16–17, pp. 5379–5386,
2011.
[10] D. A. Ramirez et al., “Novel precipitate-microstructural architecture developed in the
fabrication of solid copper components by additive manufacturing using electron beam
melting,” Acta Mater., vol. 59, no. 10, pp. 4088–4099, 2011.
[11] L. Yang, O. Harrysson, H. West II, and D. Cormier, “Design and characterization of
orthotropic re-entrant auxetic structures made via EBM using Ti6Al4V and pure copper,”
in Solid Freeform Fabrication Symposium, 2011, pp. 464–474.
[12] P. Frigola et al., “Fabricating copper components with electron beam melting,” Adv. Mater.
Process., vol. 172, no. 7, pp. 20–24, 2014.
[13] C. A. Terrazas et al., “Multi-material metallic structure fabrication using electron beam
melting,” Int. J. Adv. Manuf. Technol., vol. 71, no. 1–4, pp. 33–45, 2014.
[14] M. A. Lodes, R. Guschlbauer, and C. Körner, “Process development for the manufacturing
of 99.94% pure copper via selective electron beam melting,” Mater. Lett., vol. 143, pp. 298–
301, 2015.
[15] Y. Bai, G. Wagner, and C. B. Williams, “Effect of Particle Size Distribution on Powder
Packing and Sintering in Binder Jetting Additive Manufacturing of Metals,” J. Manuf. Sci.
Eng., vol. 139, no. 8, p. 81019, 2017.
[16] Y. Bai and C. B. Williams, “An exploration of binder jetting of copper,” Rapid Prototyp.
J., vol. 21, no. 2, pp. 177–185, 2015.
[17] A. Kumar, Y. Bai, A. Eklund, and C. B. Williams, “Effects of Hot Isostatic Pressing on
Copper Parts Fabricated via Binder Jetting,” in Procedia Manufacturing, 2017, vol. 10, pp.
935–944.
[18] R. M. German, Powder Metallurgy Science (Second Edition). 1994.
[19] R. Haynes, “Effect of Porosity Content on Ductility of Sintered Metals,” Powder Metall.,
vol. 20, no. 1, pp. 17–20, 1977.
52
[20] M. I. Aivazov and I. A. Domashnev, “Influence of Porosity on the Conductivity of Hot-
Pressed Titanium-Nitride Specimens,” Powder Metall. Met. Ceram., vol. 7, no. 9, pp. 708–
710, 1968.
[21] J. K. Carson, S. J. Lovatt, D. J. Tanner, and A. C. Cleland, “Thermal conductivity bounds
for isotropic, porous materials,” Int. J. Heat Mass Transf., vol. 48, no. 11, pp. 2150–2158,
2005.
[22] J. Wang, J. K. Carson, M. F. North, and D. J. Cleland, “A new approach to modelling the
effective thermal conductivity of heterogeneous materials,” Int. J. Heat Mass Transf., vol.
49, no. 17–18, pp. 3075–3083, 2006.
[23] C. Vincent, J. F. Silvain, J. M. Heintz, and N. Chandra, “Effect of porosity on the thermal
conductivity of copper processed by powder metallurgy,” J. Phys. Chem. Solids, vol. 73,
no. 3, pp. 499–504, 2012.
[24] G. C. J. Bart, “Thermal conduction in non homogeneous and phase change media,” 1994.
[25] B. C. Gundrum, D. G. Cahill, and R. S. Averback, “Thermal conductance of metal-metal
interfaces,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 72, no. 24, p. 245426, 2005.
[26] R. W. Powell, “Correlation of Metallic Thermal and Electrical Conductivities for Both Solid
and Liquid Phases,” Int. J. Heat Mass Transf., vol. 8, no. 7, pp. 1033–1045, 1965.
[27] C. S. Smith, “The Relation Between the Thermal and Electrical Conductivities of Copper
Alloys,” Phys. Rev., vol. 48, no. 2, pp. 166–167, 1935.
[28] F. H. Schofield, “The Thermal and Electrical Conductivities of Some Pure Metals,” in
Proceedings of the Royal Society of London, Series A, 1925, vol. 107, no. 742, pp. 206–227.
[29] J. C. Y. Koh and A. Fortini, “Prediction of thermal conductivity and electrical resistivity of
porous metallic materials,” Int. J. Heat Mass Transf., vol. 16, no. 11, pp. 2013–2022, 1973.
[30] S. J. Raab, R. Guschlbauer, M. A. Lodes, and C. Körner, “Thermal and Electrical
Conductivity of 99.9% Pure Copper Processed via Selective Electron Beam Melting,” Adv.
Eng. Mater., vol. 18, no. 9, pp. 1661–1666, 2016.
[31] ASTM Int., “Standard Test Methods for Tension Testing of Metallic Materials 1,” Astm,
vol. i, no. C, pp. 1–27, 2009.
[32] ISO/ASTM 52921, “Standard terminology for Additive Manufacturing - Coordinate
systems and test methodologies,” vol. 2013, pp. 1–13, 2013.
[33] E1461, “E1461. Standard test method for thermal diffusivity by the flash method,” ASTM,
West Conshohocken, PA, vol. i, pp. 1–11, 2013.
53
Chapter 4: Closure
4.1 Conclusions
The goal of this thesis research was to develop an understanding of the process-structure-property
relationship in the Binder Jetting of copper, by investigating the effects of post-process Hot
Isostatic Pressing on the porosity of Binder Jet copper (Chapter 2), and relating the porosity to
various material properties (Chapter 3). This investigation has been conducted by an evaluation of
the effects of HIP on the density, porosity, microstructure, tensile strength and ductility of Binder
Jet copper parts of three different sintered densities, achieved using three powder configurations
(Chapter 2). This resulted six different kinds of parts, each having been subjected to a different
processing condition, that were available for study. The strength, ductility, thermal and electrical
conductivities of these were then compared to models in the literature for PM copper and/or by
treating the pore distribution in copper as a two-component system using various assumptions
(Chapter 3). These analyses provided a means of understanding the correlation between process-
induced porosity and the material properties in a scientific manner. These studies have helped
understand the improvement the density and properties of Binder Jet copper using HIP, and
provided a good framework for future studies in developing quantitative models for the process-
structure-performance relationship in Binder Jetting.
4.2 Summary of findings
Detailed investigations have been carried out towards understanding the effects of Hot Isostatic
Pressing on the properties of copper parts fabricated using Binder Jetting. The major findings are
listed below:
Copper parts of 99.47% theoretical density have been fabricated using optimized powder
configurations, printing, sintering and HIP conditions [1] (Appendix A).
The effects of HIP on the porosity, microstructure and mechanical properties of parts of
three different powder configurations has been investigated. The following conclusions
may be drawn:
o HIP has been found to be effective in removing porosity in Binder Jet copper parts
to a reasonable extent, only when the sintered density is at least 90% (Section
2.3.1). More than 93% density seems to be required for approaching full-density
[1].
54
o The reduction in porosity has been determined to be the more significant factor in
affecting the tensile strength of Binder Jet specimens, rather than the associated
grain coarsening (Sections 2.3.3, 2.3.4).
o The ductility of HIPed parts has been found to increase with densification by HIP
(Section 2.3.4).
The dependence of material properties on process-induced porosity has been investigated
by comparison with existing models in the literature. The following are the major
conclusions:
o The tensile strength of Binder Jet parts is found to be less than the existing model
for sintered copper powder due to microstructural differences (Section 3.4.1).
o The ductility, being porosity-dependent, has different behaviors compared to
existing PM model for different levels of porosity (Section 3.4.2). The
heterogeneity of porosity distribution may play a significant role here.
o The thermal conductivity has been found to be less than that predicted by various
models in the literature that describe the dependence of properties on porosity. This
is due to the presence of impurities and to the grain boundary thermal resistance,
an estimate of which has been made to be greater than that predicted by the
literature for each of the processing conditions (Section 3.4.3).
o The electrical conductivity predicted for fully dense copper parts has been found to
approach closer to the theoretical value than the thermal conductivity, which may
indicate a potential anisotropy due to differing measurement directions (Sections
3.4.4, 3.4.5).
4.3 Contributions
This work has made the following novel contributions to the scientific literature in understanding
the Binder Jetting AM process:
Near-full density, high-purity copper parts have been fabricated for the first time using a
Binder Jetting-based process chain by using post-process HIP.
While most reported literature on HIP deals with parts that are at least 92% dense and have
no surface connected porosity, this work has contributed to understanding the impact of
55
HIP on Binder Jet parts, which typically feature surface porosity and fall below this density
requirement.
This work has explored the influence of porosity induced by different post-processing
conditions, on the mechanical, thermal and electrical properties of Binder Jet copper
The strength, ductility, and thermal and electrical conductivities of Binder Jet copper have
been situated in the context of conventional Powder Metallurgy and of two-component
structures, to help understand the extents of the properties that can be achieved using
Binder Jetting AM.
4.4 Limitations and Future Work
A comprehensive attempt has been made towards understanding the effects of HIP on Binder Jet
copper specimens. The findings of this work have presented scope for further work in this area in
order to fully understand the science of the process and to be able to design the process for a desired
porosity and material properties:
Careful control of powder quality and processing conditions can help print near full density
copper parts [1], which can be characterized to quantify the highest achievable values of
various material properties.
A more comprehensive study of the effects of part orientation on the mechanical properties
can help study the impact of HIP on anisotropic parts.
A three-dimensional porosity distribution analysis of Binder Jet copper can provide further
insight into understanding the influence of heterogeneity in pore distribution on the
properties analyzed.
The role of the measurement direction of various properties needs to be evaluated in
relation to the printing orientation and porosity distribution to develop accurate quantitative
process models.
The study of additional powder configurations and processing conditions can help in
obtaining more data for porosity in Binder Jet copper. This can help in developing more
refined models specific to Binder Jetting.
Lastly, although the scope of this work is restricted to printing pure copper, similar studies can be
carried out for other metals of interest to understand material-specific behaviors.
56
References (for Chapters 1 and 4)
[1] E. Sachs, M. Cima, and J. Cornie, “Three-dimensional printing: rapid tooling and prototypes
directly form a CAD model,” CIRP Ann. - Manuf. Technol., vol. 39, no. 1, pp. 201–204,
1990.
[2] A. Kumar, Y. Bai, A. Eklund, and C. B. Williams, “Effects of Hot Isostatic Pressing on
Copper Parts Fabricated via Binder Jetting,” in Procedia Manufacturing, 2017, vol. 10, pp.
935–944.
57
Appendix A
Publication: Effects of Hot Isostatic Pressing on Copper Parts Fabricated via Binder
Jetting
Available online at www.sciencedirect.com
ScienceDirect
Procedia Manufacturing 00 (2017) 000–000
www.elsevier.com/locate/procedia
45th SME North American Manufacturing Research Conference, NAMRC 45, LA, USA
Effects of Hot Isostatic Pressing on Copper Parts Fabricated via
Binder Jetting
Ashwath Kumara, Yun Baia, Anders Eklundb, Christopher B. Williamsa1
aDepartment of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24060, United States of
America bQuintus Tecnologies, LLC, Lewis Center, Ohio, 43035, United States of America
Abstract
Binder Jetting, an Additive Manufacturing process that fabricates parts via selective inkjet deposition of binder into a powder bed,
is capable of cost-effectively producing complex metal and ceramic components without the need for support structures or anchors.
Printed green parts are then sintered for added densification and strength. However, printed parts typically contain porosity due to
the use of coarse powders and a loosely packed powder bed. While researchers have investigated many techniques (e.g., process
parameter and powder morphology optimization) for achieving full theoretical density in binder jetted parts, 100% density is
difficult to achieve without infiltration of a secondary lower melting point material. In this work, the authors investigate the use of
Hot Isostatic Pressing (HIP) as a post-process heat treatment of sintered parts to evaluate its effect on density, porosity, and
shrinkage of parts printed in Binder Jetting. The authors conduct this investigation in the context of copper. It is demonstrated that
the use of HIP can improve the final part density from 92% (following sintering) to 99.7% of theoretical density.
© 2017 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of NAMRI/SME.
Keywords: Binder Jetting; Additive Manufacturing, Hot Isostatic Pressing, Copper
* Corresponding author. Tel.: +1-540-231-3422; fax: +1-540-231-9100.
E-mail address: [email protected]
58
1. Introduction
Binder Jetting is an Additive Manufacturing (AM) technology that involves binding together powder particles by
selectively jetting a binder layer-by-layer to form a printed green part. Binder Jetting is able to process a wide variety
of powdered materials, including polymers [1,2], ceramics [3,4] metals [5,6], foundry sand [7], and the active
ingredients for pharmaceutical applications [8]. The printing process begins with a counter-rotating roller that spreads
a layer of powder of a set thickness from a ‘feed box’ onto a ‘build box’. After each layer is deposited, an inkjet
printhead deposits patterns of binder according to the shape of the part’s cross-section based on the model data taken
from the ‘slice file’ fed to the printer. The binder droplets penetrate into the powder through the voids between the
particles, and stitch together the particles within a layer, as well as to the previously formed layer. This process is
repeated layer after layer to form the green part. The setup is illustrated in Figure 1. The printed part is then placed
into a low-temperature oven (~200C) to cure the binder. Subsequently, the loose powder particles adhering weakly to
the edges of the green part are removed using compressed air. Since the green part is composed of bound loosely
packed powder, it is not dense enough to be mechanically strong; hence the part is then sintered in a furnace where
the binder pyrolyzes, and the high temperature causes the powder particles to sinter through atomic diffusion. This
process results in the part shrinking into the voids left by the binder, causing the part to densify and hence strengthen.
Fig. 1. Schematic Illustrating Binder Jetting [7].
A unique feature of Binder Jetting metals is the separation of powder sintering from part creation, which
circumvents issues of residual stresses being induced in the part due to rapid melting/solidification of metal powder
as found in metal Powder Bed Fusion (PBF) processes. In PBF processes, these stresses and associated warpage are
dealt with using anchors or supports. Since Binder Jetting does not involve such stresses, the powder bed is sufficient
to act as a support, thereby providing the design freedom expected from AM technologies. The independence of the
part creation and energy application steps also makes the technology inherently scalable, as printing larger parts would
not have significant additional costs as compared to metal PBF processes where upgrading and adding energy sources
is expensive. Also, a wide array of materials can be printed with this technology, as the ability to print a given material
into green parts would simply involve choosing a material that can be made in the form of a powder that flows and a
compatible binder. Post process heat treatment can be tailored by choosing suitable sintering profiles. For example,
Binder Jetting has been found to be particularly suitable for copper [9], which is challenging to process via PBF due
to its high thermal conductivity (which makes it disperse any impinging energy onto surrounding particles, thereby
resulting in a lack of control of the melt pool) and reflectivity (which places limitations on the kinds of energy sources
that can be used) [10].
While this process presents a cost-effective and scalable means of fabricating complex metal and ceramic
components, the printed parts suffer from porosity when high strength and conductivity is required. The difficulty in
creating parts of near-full density is due to the powder particles being loosely packed with insufficient compaction by
the roller, which results in the sintered parts being porous due to reduction in the availability of sintering and neck-
forming sites. Infiltration is a technique that is often incorporated in conjunction with sintering in order to achieve full
density [11,12,13]. This involves the addition of a lower melting alloy that flows into the matrix of printed metal
through guided pathways, that fills into the pores by capillary action. This reduces the shrinkage during sintering and
increases the final density. However, due to the addition of a secondary substance, the material properties change
accordingly. Thus there is a requirement for an alternative process that does not affect the composition and properties
of the part, while still being able to achieve full density.
59
Eliminating porosity via solid state sintering without infiltration is a challenge, and hence has been a focal point of
research in Binder Jetting. Investigations have been made in this direction by optimizing process parameters or making
suitable modifications to the system. Turker et al. investigated the effect of layer thickness, powder particle size and
sintering cycles in the Binder Jetting of IN 718 superalloy [14]. Liquid phase sintering mechanism or optimized
sintering parameters have been successful in improving sintered density [4,15,16]. The effect of bimodal powder
mixtures as well as printing and sintering process parameters in improving sintered density has been studied for copper
by Bai and co-authors [17]. The use of bimodal powder mixtures of 316L Stainless Steel was studied by Verlee and
co-authors [18]. A design modification to a Binder Jetting machine was applied in order to incorporate powder
compaction during printing to achieve better green part densities by Gregorski [19]. Slurry-based Binder Jetting is
another technique that uses base material in the form of slurries instead of powders. The effects of this technique in
improving the density of printed parts has been studied by Grau and co-authors [20], and by Ables [21]. Another
promising technique for densification of printed green parts is the jetting of functional inks instead of a binder. Bai
and co-authors have investigated the use of nanosilver suspensions jetted onto a silver substrate in order to improve
sintering performance and dimensional accuracy [22]. Additionally, the effect of various printing and sintering process
parameters on mechanical properties have been investigated [23]. In the case of copper, these problems have been
overcome to the extent of achieving a sintered density of 87.1% by using a bimodal powder configuration [17]. Further
refinements of the post-process sintering cycle created parts with 92% theoretical density.
In this paper, the authors explore the use of Hot Isostatic Pressing (HIP) as means for further densifying metal parts
printed with Binder Jetting. HIP has been used in both powder metallurgy and additive manufacturing industries as a
means to create finished parts that are 100% dense (Section 2). The primary goal of this work is to explore the effects
of HIP on parts made via Binder Jetting. The authors’ research is conducted within the context of printed copper parts.
In this context, achieving full density through infiltration is not acceptable, as the presence of a secondary material
would drastically alter the desired thermal, electrical and mechanical properties. Thus a secondary goal of this work
is to explore a means for additive manufacture of fully dense copper parts. Prior art in using HIP for AM parts
(predominantly in metal PBF processes) is reviewed in Section 2. An overview of the authors’ experimental methods
to test the hypothesis that HIP of Binder Jetting parts will result in full theoretical density is presented in Section 3.
Results from these experiments are presented in Section 4, with closure offered in Section 5.
2. Hot Isostatic Pressing in Additive Manufacturing
HIP is a technique widely employed in Powder Metallurgy to consolidate loose powders into a desired shape by
applying isostatic pressure using an inert gas (usually Argon) at elevated temperatures. The physics of HIP have been
investigated in detail by Atkinson and co-author [24]. In summary, the process involves the application of sufficiently
high pressure for pores entrapped in a part to overcome the surface energy driving force for pore closure. These pores
then dissolve in the matrix and diffuse to the surface of the part. Due to the pressure being isostatic in nature, it acts
perpendicular to the surface irrespective of the geometry, and the shrinkage seen in the part after HIP is expected to
be ‘photographic’, which implies that the shape of the part is retained despite the dimensional shrinkage. Apart from
powder consolidation, HIP is also used in densifying castings and pre-sintered components. Before direct-metal PBF
systems were available, researchers looked to HIP to bring green parts, formed from two-phase powder mixtures, to
full density. Currently, HIP is often used to bring metal components made via direct-metal PBF processes to 100% of
theoretical density.
HIP was found to help approach full density in a bronze-nickel alloy printed using PBF technology [25]. In addition,
a specific process, ‘SLS/HIP’ was developed by Das and co-authors [26] for achieving high density parts made of
superalloys such as Inconel Alloy 625 and Ti-6Al-4V. The process involves sintering the boundaries of each layer to
high densities (>98% of theoretical density) while printing, thereby creating a solid shell with the enclosed portion
being of density as low as 80%. This shell can essentially act as a container so the part can be HIPed as printed. Liu
and co-authors presented another process chain featuring PBF, Cold Isostatic Pressing, degreasing, sintering, and HIP
for fabricating alumina parts [27]. The use of HIP resulted in a final density of 95.94%, as compared to 32.06% density
as printed. The effectiveness of HIP in pore closure in parts made by using Electron Beam Melting (EBM) was studied
by conducting X-Ray Computed Tomography scans before and after HIP [28].
The improvement in density has been found to translate into an improvement in mechanical properties. HIP has
been found to bring about an improvement in fatigue strength of ASTM F75, a cobalt-based alloy manufactured using
a PBF process [29]. The same was found to be true of the alloys AlSi10Mg and Ti-6Al-4V [30]. A HIP process
60
developed by Peter et al. has resulted in an improvement in grain structure of Ti-6Al-4V and Inconel 718 parts
fabricated using EBM [31].
The use of HIP for green parts made via Binder Jetting has not been widely documented in the literature. The only
prior art discovered by the authors was Kernan and co-authors’ use of sinter-HIP following slurry-based Binder Jetting
of Tungsten Carbide-Cobalt (WC-Co). In their work, the use of HIP provided a means of achieving density
approaching 100% [32]. HIP requires that the outer shell of the part is dense and devoid of pores, for the internal pores
to be effectively removed. This is the reason parts are either enclosed in a container or previously sintered before HIP.
Parts fabricated via Binder Jetting are generally expected to have surface connected porosity, which could render the
use of HIP to improve density difficult. This is a possible reason for the limited prior research in this space. The goal
of this work is to address this gap by, (i) furthering the understanding of the impact of HIP on parts made via Binder
Jetting, and (ii) to exploring techniques for enabling AM of fully-dense copper artifacts.
3. Research Methods
To explore the effect of HIP on parts made via Binder Jetting, the authors evaluated the density, porosity, and
shrinkage of printed parts in their printed green state following post-process sintering, and following a subsequent
HIP process. These experiments were conducted in the context of printing high purity copper components in Binder
Jetting. The experimental methodology employed in this work, including the parameters used to form the green part
and of the post-process sintering were based on prior work done by the authors [17], and are detailed in this section.
3.1. Materials
The copper powders used in this study were gas atomized to yield a spherical shape. This shape aids in good
packing as compared to irregularly shaped powder, and also in ease of powder recoating. Additionally, spherical
particles perform better when sintering, due to easier necking as compared to irregularly shaped particles. Based on
previous work on bimodal powder mixtures to achieve maximum sintered density [17], the two copper powders were
chosen to have median particle diameter of 30µm and 5µm, and mixed in the ratio of 73:27 by weight in a rotating
drum for ~2 hours to yield the desired powder configuration.
3.2. Binder Jetting Process Parameters
The experimental specimens were printed on an ExOne R2 Binder Jetting printer, using a standard off-the-shelf
binder provided by ExOne that has been found to be compatible with the copper powders used, and to leave a minimal
amount of residue upon pyrolysis during sintering. A layer thickness of 70µm was used to print the parts. The
saturation ratio (the ratio of the amount of space between the powder particles in the bed that is occupied by binder)
is an important parameter in achieving strong and dense green parts. To set the saturation, the binder drop volume and
packing density of the powder bed are measured and entered into the machine which then adjusts the amount of jetted
binder accordingly. The value of saturation ratio must be chosen so as to be high enough to allow for sufficient binder
penetration between layers, but not so high that binder seeps into more regions of the powder bed than intended,
thereby having detrimental effects on part accuracy as well as surface finish. Based on prior investigation considering
these factors, the saturation ratio was set to 100% [17].
The parts printed for measuring density were rectangular coupons of three different kinds of target dimensions:
A. 16x16x4 mm
B. 32x8x4 mm
C. 16x8x4 mm
Following printing, all samples were sintered as described in Section 3.3. One coupon each of Type A and Type B
were cut in half; one half of each was retained and the other half was subjected to HIP. The density and microstructure
of the halves of the specimens were then evaluated (Section 4) and compared. This was done in order to study the
effect of HIP on the density of the same coupon before and after HIP, thereby accounting for any minor variations in
density between two parts from the same print. This kind of variation was a possibility in the Type C coupons, of
which two were fabricated; with one subjected to HIP and the other retained for comparison.
61
3.3. Post-process Heat Treatment process parameters
Following printing, the samples were sintered in a box furnace in a hydrogen atmosphere. The sintering profile
used is shown in Figure 2. A constant heating rate of 5°C/min was used to first raise the temperature of the furnace to
450°C, where it was held for 30 min to burn off the binder. The temperature was increased at 5°C/min to a sintering
temperature of 1075°C, where it was held for 3 hours. The furnace was then allowed to cool at a rate of 5°C/min until
it reached room temperature. The choice of this sintering profile was based on previous research by Meeder et al. in
order to optimize sintered density [33].
Fig. 2. Sintering Profile Used.
Following sintering, a subset of the specimens was subjected to HIP in an argon atmosphere using a Graphite
furnace with Quintus Technologies’ proprietary Uniform Rapid Cooling (URC®) technology. This was used in order
to speed up the HIP cycle by reducing the cooling phase, thereby minimizing the amount of time the sample is held at
higher temperatures. Grain growth, which can lead to reduction in yield strength and toughness, has been found to be
greater at higher temperatures [24]. Hence this technology can minimize grain growth during the cycle, and potentially
result in better mechanical properties than can be obtained from a cycle with a slower cooling rate. The HIP was done
in a container-less fashion, as the sintered parts were observed to not have any significant surface connected porosity
that could affect the process. The HIP parameters used were a temperature set to 1075°C and a pressure of 206.84
MPa (30,000 psi), held for 2 hours in an Argon atmosphere. Conventionally, copper is HIPed at 800 to 950°C under
a pressure of 100 MPa [24]. The pressure value was chosen to be higher than this, based on preliminary trials that
indicated a better pore closure with higher pressure. The temperature could then be raised to a slightly higher value as
well, and an initial trial had been conducted at a HIP temperature of 975°C held for four hours. However, this caused
issues with remaining porosity. Hence, the temperature in the successful trial reported here was increased to 1075°C,
with the hold time reduced to two hours to minimize grain growth.
3.4. Shrinkage, Density Measurements
Part shrinkage was calculated by measuring the dimensions of the part after sintering and after HIP, then comparing
them to the dimensions of the green part. Linear shrinkage in each of the three directions was calculated. Density
measurements were carried out using an Archimedes Principle based apparatus that calculates the density of the part
using its weight measured in air and in water (ASTM Standard B962-15) [34]. Oil impregnation is to be carried out
per the standard in order to seal any surface connected porosity that is present in the sample to be measured. However,
in this case, it was not carried out in order to avoid contaminating the parts to be HIPed. Micrographs of sintered
specimens indicated the absence of any significant surface connected porosity, and this step could hence be avoided.
62
In addition to measuring the bulk density by this method, the porosity of the internal sections was measured, the details
of which are outlined in Section 3.5.
3.5. Porosity Measurements
For porosity measurements, the samples of each type were sectioned at multiple locations (across the X-Z and Y-
Z planes), then polished. Multiple images were taken of each section under a microscope, and a MATLAB code was
written to calculate the porosity from these images using the image processing toolbox. The images were first imported
and converted to a binary black and white image, with black regions corresponding to pixels turned ‘off’ and white
regions ‘on’. The threshold for differentiating black regions (pores), from white regions (solid copper) was set by
qualitative visual observation. The porosity, which is the ratio of the area of the black regions to that of the total area
of the image, is calculated by dividing the number of pixels in the black region (returned by the function, “bwarea”),
by the total number of pixels (returned by the function, “numel”). In order to ensure any stray scratches or the
background of the images do not get counted as black regions, these features were manually cropped out of the
microscope images before analysis.
4. Results and Discussions
4.1. Shrinkage and Density
Table 1 shows the total linear shrinkage in each direction after sintering and after HIP for each of the three part
types. The X and Y dimensional shrinkages were nearly identical, and have been grouped together for the purpose of
simplification.
Table 1. Linear Shrinkage in Parts after sintering and HIP
Part Type Shrinkage after Sintering (%) Total Shrinkage after HIP (%)
X/Y Z X/Y Z
A 14.35 16.91 15.52 18.77
B 14.14 18.57 15.56 19.30
C 14.25 16.91 16.16 19.70
These results indicate that there is a slightly more shrinkage in the Z-direction as compared to the X or Y directions.
This indicates an anisotropy in the distribution of porosity, i.e. there is more porosity between layers than within a
layer. This is expected, as the particles are not consolidated across layers as well as they are within a layer, due to
limited diffusion of binder in the bed. The shrinkage may also be attributed in part to gravity effects that come into
play during sintering, that may cause greater shrinkage in the Z-direction than X/Y directions. This variation goes to
show that it is important to design for parts to be manufactured using this process chain by keeping in mind that
different compensations in the geometric dimensions will be required for each of the directions. The shrinkage in these
is accompanied by an increase in the density. Figure 3 summarizes the improvement in density in each of the parts,
by comparing the green density, sintered density, and HIP density for each type.
63
Fig. 3. Density improvement upon HIP of parts.
The mean green density of the printed specimens was measured to be 55.00%, using dimensional measurements.
Following the sintering cycle detailed in Section 3.3, the density improved to an average of 93.90%. Following the
HIP process, the measured mean bulk density was 99.47%, using the Archimedes Principle apparatus. It is noted that
the geometry (as indicated by part “Type”) did not impact the density measurements.
4.2. Porosity
The results of the porosity analysis using optical micrographs are presented in Figure 4, comparing the porosity of
parts of each type after sintering and after HIP.
Fig. 4. Porosity improvement upon HIP of parts.
The results presented above show a general trend of an overall improvement in porosity as seen from the optical
micrographs. There is a visible consistency in the range of porosity in HIPed samples, irrespective of initial sintered
55.13 54.97 54.88
93.46 93.89 94.3499.24 99.78 99.38
0
20
40
60
80
100
120
Type A Type B Type C
Den
sity
(%
Theo
reti
cal)
Part Type
Density Improvement Upon HIP
Green Density Sintered Density HIPed Density
64
porosity. The numerical averages of the porosity as obtained from the all the micrograph images of each of the
specimen are summarized in Table 2 below.
Table 2. Comparison of average porosity between sintered and HIPed parts.
Part Type Sintered Sample Porosity (%) HIPed Sample Porosity (%)
A 0.26 0.10
B 0.26 0.09
C 1.29 0.10
A decrease in average porosity in all the samples is evident from the analysis. Figure 5 shows representative
micrographs (of Type C parts) that were used in the image analysis. The porosity of these images as calculated using
the MATLAB code are in given in the description.
Fig. 5. Sample Micrographs (Type C) indicating density improvement upon HIP (a) Sintered Part, 1.88% porosity; (b) HIPed Part, 0.13%
porosity calculated from image analysis.
From these images of sectioned parts, it is clear that HIP has assisted in densifying the part to nearly full density.
There is, however, some existing residual porosity as yet unremoved after HIP, visible in some of the micrographs. It
remains to be seen if there is a scope for further improvement by tuning the HIP process parameters. The extent of
detrimental effects of this residual porosity on material properties is also yet to be seen, but it may be safely assumed
that the properties of the HIPed parts will offer improvements from those of the as-sintered parts.
5. Conclusions and Future Work
A process chain has been developed for achieving near-full density copper parts using Binder Jetting followed by
sintering and Hot Isostatic Pressing. The HIP process was successfully able to increase the density of printed copper
parts from 92% to 99.7%. The use of HIP also significantly reduced the porosity of the printed parts. In addition, it
was observed that HIP resulted in an anisotropic shrinkage (~16-20% linear shrinkage) due to graded density in the
green part.
In addition to understanding how HIP affects the density of parts created via Binder Jetting, this research also
provides the base for establishing a set protocol for Additive Manufacturing of high-density copper parts. These results
provide promise for a means of fabricating complex copper parts for end-use applications that need to utilize the
unique thermal and electrical properties of the material. The major applications of printed copper are likely to be in
the areas of enhanced heat transfer applications, advanced electrical components etc.
Future work will focus on characterizing the mechanical, thermal and electrical properties of the parts following
HIP. The complexity in the geometry of products to be used in such applications can easily be achieved using Additive
Manufacturing, but the effect of this entire process chain on the dimensional stability of such parts is yet to be
investigated. It is expected in this case that the effects of HIP in particular, on complex geometry parts will not be
65
significant, owing to the expected photographic shrinkage associated with the isostatic pressure involved.
Additionally, the effect of sintered density in the effectiveness of HIP is a potential gap to address. One could thus
investigate the minimum sintered density for HIP to work effectively.
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Appendix B
MATLAB CODE FOR IMAGE ANALYSIS
% All image files are first manually cropped to exclude exterior regions,
scratches etc. These are saved as JPGs in subfolders for each cross section
of each specimen, with the file name as the serial number of the image within
the subfolder (e.g., 1.jpg, 2.jpg, etc.). This code works for this kind of
file/folder organization and is not generalizable without making appropriate
modifications
close all clear all clc
here = mfilename('fullpath'); [path, ~, ~] = fileparts(here); addpath(genpath(path));
% Extract images from subfolder for XY sections: cd(strcat(path,'\XY'));
PicsXY = dir(strcat(pwd,'\*.jpg')); for i=1:length(PicsXY) filename = sprintf('%d.jpg',i); fXY{i} = imread(filename); figure() imshow(fXY{i});
%convert to black and white images with threshold of 0.5, show image to
verify threshold selection
f1bw = im2bw(fXY{i},0.5); figure() imshow(f1bw);
%porosity calculation
atot1(i) = numel(f1bw); wtot1(i) = bwarea(f1bw); btot1(i) = atot1(i) - wtot1(i); por(i,1) = btot1(i)*100/atot1(i); end
%same code repeated for YZ and ZX sections; kept separate to allow for
different thresholding choices if required cd(strcat(path,'\YZ')); PicsYZ = dir(strcat(pwd,'\*.jpg')); for i=1:length(PicsYZ) filename2 = sprintf('%d.jpg',i); fYZ{i} = imread(filename2); figure() imshow(fYZ{i});
f2bw = im2bw(fYZ{i},0.5); figure() imshow(f2bw);
68
atot2(i) = numel(f2bw); wtot2(i) = bwarea(f2bw); btot2(i) = atot2(i) - wtot2(i); por(i,2) = btot2(i)*100/atot2(i); end
cd(strcat(path,'\ZX')); PicsZX = dir(strcat(pwd,'\*.jpg')); for i=1:length(PicsZX) filename3 = sprintf('%d.jpg',i); fZX{i} = imread(filename3); figure() imshow(fZX{i});
f3bw = im2bw(fZX{i},0.5); figure() imshow(f3bw);
atot3(i) = numel(f3bw); wtot3(i) = bwarea(f3bw); btot3(i) = atot3(i) - wtot3(i); por(i,3) = btot3(i)*100/atot3(i); end