Powder Metallurgy

1. Powder Metallurgy Processes

2. Introduction
• In powder metallurgy (PM) process, metal powders are compacted into desired and often
complex shapes and sintered (heated without melting) to form a solid piece
• One of its first modern uses was in the early 1900s to make the tungsten filaments for
incandescent light bulbs
• The availability of a wide range of metal–powder compositions, the ability to produce parts to net
dimensions (net-shape forming), and the overall economics of the operation give this unique
process its numerous attractive and expanding applications
• The most commonly used metals in PM are iron, copper, aluminum, tin, nickel, titanium, and the
refractory metals

3. Introduction
Components made by powder-metallurgy techniques
• Balls for ballpoint pens
• Automotive components (which now constitute about 70% of the PM market)
such as piston rings, connecting rods, brake pads, gears, cams, and bushings
• Tool steels, tungsten carbides, and cermets as tool and die materials
• Graphite brushes impregnated with copper for electric motors
• Magnetic materials
• Metal filters and oil-impregnated bearings with controlled porosity ; surgical
implants, and several others for aerospace, nuclear, and industrial applications

4. Production of
Metal Powders
The powder-metallurgy process
typically consists of the
following operations, in
sequence:
1. Powder production
2. Blending
3. Compaction
4. Sintering
5. Finishing operations
Outline of processes and operations involved in producing
powder-metallurgy parts

5. Production of
Metal Powders
• Particle shapes in metal
powders, and the processes
by which they are produced
• Iron powders are produced
by many of these processes

6. 1. Methods of Powder
Production
• There are several methods of producing metal powders,
and most of them can be produced by more than one
method
• The choice depends on the requirements of the end
product
• The microstructure, bulk and surface properties, chemical
purity, porosity, shape, and size distribution of the particles
depend on the particular process used
• These characteristics are important because they
significantly affect the flow and permeability during
compaction and in subsequent sintering operations
• Particle sizes produced range from 0.1 to 1000 μm

7. 1. Methods of Powder Production
1. Atomization
• Atomization involves a liquid-metal stream produced by injecting molten metal through a
small orifice
• The stream is broken up by jets of inert gas or air or water known as gas or water
atomization. The size and shape of the particles formed depend on the temperature of the
molten metal, rate of flow, nozzle size, and jet characteristics
• The use of water results in a slurry of metal powder and liquid at the bottom of the
atomization chamber. Gas atomization usually results in more spherical particles
• In centrifugal atomization, the molten-metal stream drops onto a rapidly rotating disk or cup,
so that centrifugal forces break up the stream and generate particles
• In another variation of this method, a consumable electrode is rotated rapidly in a helium-
filled chamber. The centrifugal force breaks up the molten tip of the electrode into metal
particles

8. 1. Methods of
Powder
Production
Methods of metal-powder
production by atomization:
(a) gas atomization; (b) water
atomization; (c) centrifugal
atomization with a spinning
disk or cup; and (d)
atomization with a rotating
consumable electrode

9. 1. Methods of Powder
Production
2. Reduction
• The reduction of metal oxides (i.e., removal of oxygen) uses gases,
such as hydrogen and carbon monoxide, as reducing agents
• By this means, very fine metallic oxides are reduced to the metallic
state. The powders produced are spongy and porous and have
uniformly sized spherical or angular shapes
3. Electrolytic Deposition
• Electrolytic deposition utilizes either aqueous solutions or fused
salts. The powders produced are among the purest available
4. Carbonyls
• Metal carbonyls, such as iron carbonyl and nickel carbonyl are
formed by letting iron or nickel react with carbon monoxide
• The reaction products are then decomposed to iron and nickel, and
they turn into small, dense, uniformly spherical particles of high
purity

10. 1. Methods of Powder Production
5. Comminution
• Mechanical comminution (pulverization) involves crushing milling in a ball mill, or grinding of
brittle or less ductile metals into small particles
• A ball mill is a machine with a rotating hollow cylinder partly filled with steel or white cast-
iron balls
• The powder or particles placed into a ball mill are impacted by the balls as the cylinder is
rotated or its contents are agitated
• This action has two effects: (a) the particles periodically fracture, resulting in smaller
particles, and (b) the morphology of the particles is affected

11. 1. Methods of Powder Production
6. Mechanical Alloying
• In mechanical alloying, powders of two or more pure metals are mixed in a ball mill
• Under the impact of the hard balls, the powders fracture and bond together by diffusion,
entrapping the second phase and forming alloy powders
• The dispersed phase can result in strengthening of the particles or can impart special
electrical or magnetic properties to the powder

12. 1. Methods of Powder Production
7. Miscellaneous Methods
Other, less commonly used, methods for making powders are as follows:
• Precipitation from a chemical solution
• Production of fine metal chips by machining
• Vapor condensation

13. 1. Methods of Powder Production
Particle size
• It is usually is controlled by screening—that is, by passing the metal powder through screens
(sieves) of various mesh sizes
• Screen analysis is achieved by using a vertical stack of screens, with the mesh size becoming
finer as the powder flows downward through the screens
• In addition to screening, several other methods are available for particle-size analysis:
• Sedimentation, which involves measuring the rate at which particles settle in a fluid.
• Microscopic analysis, which may include the use of transmission and scanning electron
microscopy
• Light scattering from a laser that illuminates a sample consisting of particles suspended in a liquid
medium. The particles cause the light to be scattered, and a detector then digitizes the signals
and computes the particle-size distribution
• Optical methods (such as particles blocking a beam of light), in which the particle is sensed by a
photocell
• Suspending particles

14. 1. Methods of Powder Production
Particle Shape
• A major influence on processing characteristics, particle shape usually is described in terms of
aspect ratio or shape factor
• Aspect ratio is the ratio of the largest dimension to the smallest dimension of the particle. This
ratio ranges from unity for a spherical particle to about 10 for flakelike or needlelike particles
Shape Factor
• Also called the shape index, shape factor (SF) is a measure of the ratio of the surface area of the
particle to its volume—normalized by reference to a spherical particle of equivalent volume
• Thus, the shape factor for a flake is higher than that for a sphere
Size Distribution
• The size distribution of particles is an important consideration, because it affects the processing
characteristics of the powder.
• The distribution of particle size is given in terms of a frequency-distribution plot

15. 2. Blending Metal Powders
• Powders of different metals and other materials can be mixed in order to impar special physical
and mechanical properties and characteristics to the PM product. Mixtures of metals can be
produced by alloying the metal before producing a powder, or else blends can be produced. Proper
mixing is essential to ensure the uniformity of mechanical properties throughout the part
• Even when a single metal is used, the powders may vary significantly in size and shape; hence,
they must be blended to obtain uniformity from part to part. The ideal mix is one in which all of the
particles of each material (and of each size and morphology) are distributed uniformly
• Lubricants can be mixed with the powders to improve their flow characteristics. They reduce
friction between the metal particles, improve flow of the powder metals into the dies, and improve
die life. Lubricants typically are stearic acid or zinc stearate in a proportion of from 0.25 to 5% by
weight
• Other additives, such as binders (as in sand molds), are used to develop sufficient green strength
and additives also can be used to facilitate sintering

16. 2. Blending Metal Powders
• (a) through (d) Some common
bowl geometries for mixing or
blending powders.
• (e) A mixer suitable for blending
metal powders. Since metal powders
are abrasive, mixers rely on the
rotation or tumbling of enclosed
geometries, as opposed to using
aggressive agitators

17. 3. Compaction of Metal Powders
• Compaction is the step in which the blended powders are pressed into various shapes in dies
• The purposes of compaction are to obtain the required shape, density, and particle-to-particle
contact and to make the part sufficiently strong for further processing
• The powder (feedstock) is fed into the die by a feed shoe, and the upper punch descends into the
die
• The presses used are actuated either hydraulically or mechanically, and the process generally is
carried out at room temperature, although it can be done at elevated temperatures
• The pressed powder is known as green compact, since it has a low strength

18. 3. Compaction of Metal
Powders
(a) Compaction of metal powder to form a
bushing. The pressed-powder part is
called green compact
(b) Typical tool and die set for compacting
a spur gear

19. 3. Compaction of Metal Powders

20. 3. Compaction of Metal
Powders
3.1 Isostatic Pressing
Green compacts may be subjected to hydrostatic
pressure in order to achieve more uniform
compaction and, hence, density
a) Cold isostatic pressing
• The metal powder is placed in a flexible rubber
mold typically made of neoprene rubber, urethane,
polyvinyl chloride, or another elastomer
• The assembly then is pressurized hydrostatically in
a chamber, usually using water. The most common
pressure is 400 Mpa, although pressures of up to
1000 MPa may be used
Schematic diagram of cold isostatic
pressing. Pressure is applied
isostatically inside a high-pressure
chamber. (a) The wet bag process to
form a cup-shaped part. The powder
is enclosed in a flexible container
around a solid-core rod. (b) The dry
bag process used to form a PM
cylinder

21. 3. Compaction of
Metal Powders
b) Hot isostatic pressing (HIP)
• The container generally is made of a high-
melting-point sheet metal and the
pressurizing medium is high-temperature
inert gas or a vitreous (glasslike) fluid
• Common conditions for HIP are pressures
as high as 100 MPa although they can be
three times as high—and temperatures of
1200°C (2200°F)
• The main advantage of HIP is its ability to
produce compacts having almost 100%
density, good metallurgical bonding of the
particles, and good mechanical properties.
• Consequently, it has gained wide
acceptance in making high quality parts

22. 3. Compaction of Metal Powders
The HIP process is used mainly to produce superalloy components for the aircraft and aerospace
industries and in military, medical, and chemical applications
Advantages of hot isostatic pressing:
• Because of the uniformity of pressure from all directions and the absence of die-wall friction, it
produces fully dense compacts of practically uniform grain structure and density
• HIP is capable of handling much larger parts than those in other compacting processes
The limitations of HIP are as follows:
• Wider dimensional tolerances are needed than those obtained in other compacting
• Processes
• HIP requires higher equipment cost and production time than are required by other processes
• HIP is applicable only to relatively small production quantities, typically less than 10,000 parts per
year

23. 3. Compaction of Metal Powders
3.2 Powder-injection Molding
• In this process, also called metal-injection molding (MIM), very fine metal powders are blended
with a 25 to 45% polymer or a wax-based binder
• The mixture then undergoes a process similar to die casting. It is injected into the mold at a
temperature of 135° to 200°C (275° to 400°F). The molded green parts are placed in a low-
temperature oven to burn off the plastic (debinding), or the binder is removed by solvent
extraction. The parts then are sintered in a furnace at temperatures as high as 1375°C
• Generally, metals that are suitable for powder-injection molding (PIM) are those which melt at
temperatures above 1000°C
• Typical parts made are components for watches, small-caliber gun barrels, scope rings for rifles,
door hinges, impellers for sprinkler systems, and surgical knives

24. 3. Compaction of Metal Powders
Advantages of powder-injection molding
• Complex shapes having wall thicknesses as small as 5 mm (0.2 in.) can be molded and then
removed easily from the dies
• Mechanical properties are nearly equal to those of wrought products
• Dimensional tolerances are good
• High production rates can be achieved by using multicavity dies
• Parts produced by the PIM process compete well against small investment-cast parts, small
forgings, and complex machined parts. However, the PIM process does not compete well with
zinc and aluminum die casting or with screw machining
The major limitations of PIM are the high cost and limited availability of finemetal powders

25. 4. Sintering • Sintering is the process whereby green compacts
are heated in a controlled atmosphere furnace to a
temperature below the melting point, but
sufficiently high to allow bonding (fusion) of the
individual particles
• The green compact is brittle, and its green strength
is low. The nature and strength of the bond
between the particles and, hence, that of the
sintered compact, depend on the complex
mechanisms of diffusion, plastic flow, evaporation
of volatile materials in the compact,
recrystallization, grain growth, and pore shrinkage
• The principal variables in sintering are temperature,
time, and the furnace atmosphere.
• Sintering temperatures are generally within 70 to
90% of the melting point of the metal or alloy
• Sintering times range from a minimum of about 10
minutes for iron and copper alloys to as much as 8
hours for tungsten and tantalum
Mechanisms
Sintering mechanisms are complex and depend
on the composition of the metal particles as well
as on the processing parameters. The sintering
mechanisms are diffusion (solid-state
bonding), vapor-phase transport, and liquid-
phase sintering

26. 4. Sintering
Because PM applications usually
involve medium-to-high production,
most sintering furnaces are designed
with mechanized flow-through
capability for the work parts. The
heat treatment consists of three
steps, accomplished in three
chambers in these continuous
furnaces:
• preheat, in which lubricants and
binders are burned off
• sinter
• cool down

27. 5. Secondary and Finishing Operations
a) Coining and sizing are compacting operations performed under high pressure in presses. The purposes of these
operations are to impart dimensional accuracy to the sintered part and to improve its strength and surface finish by
further densification
b) Preformed and sintered alloy-powder compacts subsequently may be cold or hot forged to the desired final shapes and
sometimes by impact forging
c) Powder-metal parts may be subjected to other finishing operations, such as
• Machining: for producing various geometric features by milling, drilling, and tapping (to produce threaded holes)
• Grinding: for improved dimensional accuracy and surface finish
• Plating: for improved appearance and resistance to wear and corrosion.
• Heat treating: for improved hardness and strength
d) Infiltration is a process whereby a slug of a lower-melting-point metal is placed in contact with the sintered part. The
assembly is then heated to a temperature sufficiently high to melt the slug. The molten metal infiltrates the pores by
capillary action and produces a relatively pore-free part having good density and strength
e) Electroplating can be applied on PM parts, but special care is required to remove the electrolytic fluid, since it presents
health hazards. Under some conditions, electroplating can seal a part and eliminate its permeability