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1
CHAPTER 13
POWDER METALLURGY
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INTRODUCTION
Powder Metallurgyis a manufacturing method to produce components by bringing a powder of the starting material into desired end shape
The essential feature is that the bond between particles is produced without total melting
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PROCESSING STEPS IN PM
IF NECESSARY
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PROCESSING STEPS IN PM
1. Particles of desired size are produced (production and characterization)
2. Blend particles to ensure even distribution (mixing)
3. Compact particles to impart desired shape (compaction)
4. Sinter parts to create strong, permanent bonds between particles (consolidation)
5. Finishing operations
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POWDER PRODUCTION
POWDER MANUFACTURINGPOWDER MANUFACTURING METHODSMETHODS
MECHANICALMECHANICALCOMMINUTIONCOMMINUTION
CHEMICALCHEMICALREACTIONSREACTIONS
ELECTROLYTIC ELECTROLYTIC DEPOSITIONDEPOSITION
METALMETALATOMIZATIONATOMIZATION
Machining
Milling techniques
High purity powder
deposition at the cathode of
electrolytic cells
Decomposition of solids by gas
reduction, precipitation from gas or a
liquid, or solid-solid reactive
synthesis
Gas Atomization
Liquid atomization
Centrifugal
Melt Explosion
Plasma
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MILLING TECHNIQUES
JAR MILLING ATTRITION MILLING
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ATOMIZATION TECHNIQUES
Disintegration of melt into droplets that freeze into particles
Production rates as high as 400Kg/min
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ATOMIZATION TECHNIQUES
GAS ATOMIZATION WATER ATOMIZATION
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ATOMIZATION TECHNIQUES
(a) 5-10 kg capacity water atomiser (a) 5-10 kg capacity water atomiser (b) 30Kg capacity inert gas atomiser(b) 30Kg capacity inert gas atomiser
(a)
(b)
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POWDERS CHARACTERIZATION (MORPHOLOGY)
» Particle Shape (Spheroidal, nodular, irregular, polygonal, ligaments, flakes)
» Particle Size (too large may not display the desired structure and desired densities might not be obtained. Too small particles are difficult to handle and tend to agglomerate)
» Particle Size Distribution (different processes are used to do the analysis such as sieve analysis, sedimentation, electron microscopy, and diffraction techniques)
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POWDERS CHARACTERIZATION (MORPHOLOGY)
Rounded and irregular, stainless steel, atomized
Sponge, palladiumelectrolytic
Porous & cubic Nickel, carbonyl decomposition
Crushed ribbon,Iron-based metallic glass
Irregular, titanium sodium reduced & milled
Angular, Niobium hydridemilled
Acicular,
tellurium, milled Spherical & agglomerated
Fines, Iron, atomized
polygonal Aggregates, Tungsten, Gas Reduced
Rounded & ligamental
Tin, Atomized
Spherical, Iron alloy,
centrifugally atomized Flake, tinSplat quenched
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POWDERS PHYSICAL PROPERTIES
» Specific Surface Area Indicates the surface available for bonding and also the area on which adsorbed contaminant may be present (cm2/gm)
» Densities » Theoretical Density: Density when there is no porosity (actual
reported density of material)» Apparent Density: Density when powder is in a loose state in die
» Tap Density: Highest density achieved by vibration of powders in die
» Green Density: Density of powders after compaction in die
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POWDERS PHYSICAL PROPERTIES
» Flow Properties given by flow rate and angle of repose
» Compressibility
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BLENDING OF POWDERS
» To mix the powders in order to obtain uniformity
» In order to impart special properties, powders of different materials may be mixed
» To mix the powders with some type of lubricant to reduce die friction and aid ejection of the product from the compaction mold
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COMPACTION
» Purposes
1. To obtain the required shape, density, and particle-to particle contact
2. To impart sufficient strength for further handling of the part
» The pressed powder is known as “green compact”
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COMPACTION
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COMPACTION
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COMPACTION
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COLD COMPACTION
Dry powders, which may be coated with lubricant or dry binder; are compacted by the application of pressure to form the so-called GREEN BODY
The density of the green body is function of:» The applied pressure» Powder shape (spherical powders compact to a higher
density)» Powder size
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COLD COMPACTION
SO WHAT ARE THE SOURCES OF GREEN STRENGTH?
» Sliding combined with pressure promotes adhesion (sometimes cold welding)
» Mechanical interlocking (especially with irregular shapes)
» Bonding agents are used in the absence of previous mechanisms (ceramics)
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COLD COMPACTION
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DIE PRESSING
Widest application for net-shape (or near-net-shape) parts.
(a) Density is higher under the punch when compacting with a single punch in a fixed container; better uniformity is obtained with (b) a single punch and
floating container or (c) with 2 counteracting punches
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DIE PRESSING
)exp(0
0 A
kApp fr
l
For a single acting punch with applied pressure p0 the pressure at l depth in the body is:
Where: Is wall friction
Afr is the frictional surface area.
A0 is the compacted areaAnd k is a factor representing radial to axial stress ratioFor an elastic solid:
1a
r
p
pk
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DIE PRESSING
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DIE PRESSING
Uniform fill density can be assured with the use of multipunch dies
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DIE PRESSING
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COLD ISOSTATIC PRESSING
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COLD ISOSTATIC PRESSING
» Powder is placed in deformable (reusable rubber) mold
» Assembly is hydrostatically pressurized by means of a hydraulic fluid inside a pressure vessel (see figure 6.5)
» No need to use lubricants or binders
» Common pressure applied is between 300 MPa (45 kpsi) to 550 MPa (80 kpsi)
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HOT ISOSTATIC PRESSING
» Container made of high melting point sheet metal
» Pressurizing medium is inert gas or vitreous (glasslike) fluids
» Common conditions are 100 MPa and 1100oC
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POWDER INJECTION MOLDING
Taken from plastics technology
MIM Metal Injection Molding
CIM Ceramics Injection Molding
Typically 40% binder (70% paraffin wax + 30% polypropylene)
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POWDER INJECTION MOLDING
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SINTERING
The green compact is heated to attain the required final properties. In this course of heating several changes take place
» Drying: liquid constituents are driven off at lower temperatures
» Sintering: At higher temperatures (0.7 – 0.9 Tm) sintering takes place
» Shrinkage:From the law of conservation of mass
3/1
shrinkageLinear
shrinkage Volumetric
))(())((constantMass
sintered
green
sintered
green
green
sintered
sinteredsinteredgreengreen
V
V
VV
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SINTERING
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SINTERING
3 important variables:
1. Atmosphere2. Temperature3. Time
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FINISHING OPERATIONS
»Repressing»Re-sintering
»Forging»Extrusion»Rolling»Machining»Heat Treatment
»Coining (Resizing)Increase density and improve dimensional tolerance»ImpregnationImmersion in heated oil; capillary action fills the pores.»InfiltrationImpregnation with a metal.
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DESIGN CONSIDERATIONS
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ADVANTAGES & DISADVANTAGES OF PM
Advantages Availability of a wide range of composites to obtain special mechanical and physical properties, such as stiffness, damping characteristics, hardness, density, toughness, and electrical and magnetic properties. Some of the highly alloyed new superalloys can be manufactures into parts by P/M processing. A technique for making parts from high-melting-point refractory metals, which would be difficult or uneconomical to make by other methods. High production rates on relatively complex parts, with automated equipment requiring little labor. Good dimensional control and, in many instances, elimination of machining and finishing operations, thus eliminating scrap and waste and saving energy. Capability for impregnation and infiltration for special applications.
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ADVANTAGES & DISADVANTAGES OF PM
Disadvantages
Size of parts, complexity of shape of parts, and press capacity. High cost of powder metals compared to other raw materials. High cost of tooling and equipment for small production runs Mechanical properties, such as strength and ductility, that are generally lower those obtained by forging. However, the properties of fully-dense P/M parts made by HIP or additional forging can be better than those made by other processes.
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TRENDS IN PM