Powder Metallurgy

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Powder Metallurgy: the creation of metal parts by compacting fine
metal powders in suitable dies and subsequent heating without melting.
Typical products made this way include gears, cams, cutting tools, balls
for ball point pens, oil impregnated bearings and bushings, piston rings,
valve guides, connecting rods, hydraulic pistons, brake pads, motor
Reasons to use powder metallurgy:
a) Its a technique for making parts from high-melting-point refractory
metals, which may be difficult or uneconomical to produce by other
b) It offers high production rates on relatively complex parts, and can be
c) It offers good dimensional control and can reduce the amount of scrap
d) It is possible to obtain special mechanical and physical properties
such as stiffness, damping, hardness, density, toughness, and specific
electrical and magnetic properties. Some of the highly alloyed new
super-alloys can only be manufactured by powder metallurgy methods.
e) It offers the capability for impregnation and infiltration for special
Limitations to use of Powder Metallurgy:
a) high cost of powder compared to other raw material forms
b) high cost of tooling and equipment, especially for small runs.
c) limitations on part size and complexity.
d) strength properties are generally lower than those of forged parts.
Common materials used for metal powder: iron, copper, aluminum, tin,
brass, nickel, titanium, and refractory metals. Alloys can also be
produced such as brass, bronze, and steels, where the powder is a
prealloyed powder (each powder particle is itself an alloy).

Steps in creating a part by Powder metallurgy.
1) Powder Production
2) Blending

3) Compaction
4) Sintering
5) Finishing Operations.
(additional processing may be done to a metal powder part including
coining, sizing, forging, machining, infiltration, or re-sintering)
1) Powder production:
The starting material in powder metallurgy is metal powder. Powder may
be produced by quite a wide variety of methods. Powder size is in the
order of 0.1 m to 1000 m.
Note that how well the powder performs under the compaction and
sintering will be dependent upon how it is made.
Some of the ways powder is produced include

Characteristics which affect how the powder behaves
Particle Size
Size Distribution
Particle Shape
Material Type (density, flow properties, compressibility)
2) Blending: mixing of the metal powder
Mixing is completed to
a) to produce a uniform mix of different powder sizes and shapes
b) to mix different metals or materials to impart specific mechanical or
physical properties.
c) to add lubricants to improve flow characteristics of the material.

d) to add other chemicals for binding, deflocculants, and sintering aids
Powders can be overmixed which can alter the shape of particles and
can work harden material.
Metallic powders are often explosive and can be hazardess to work with.

3) Compaction: when the blended powder is compressed in dies under
high pressure to obtain the required shape, density, and particle-toparticle contact. This process provides sufficient strength to be further
Compacted metal powder is called a green compact since it does not
possess full material strength.
Pressure used: 25 to 50 tons/in2 for iron and alloys
10 to 20 tons/in2 for copper and alloys
Compaction is important in determining the density of the final product.
Generally, the final part elastic modulus and other strength properties
are related to the material density.
Methods of compaction include:
mechanical and hydraulic presses
hot and cold isostatic pressing
metal injection molding
pressureless compaction
ceramic molds
spray deposition

4) Sintering: the process of heating a compacted metal powder form to
a temperature below the melting point, but sufficiently high enough to
allow bonding of the individual particles.

Usually consists of 1) low temperature burn off of volatile lubricants
2) high temperature for sintering powder material
3) cooling chamber

The bond depends upon diffusion, plastic flow, evaporation of volatile
materials, recrystallization, grain growth, and pore shrinkage. So the
actual process is quite complicated.
--principle controlling factors: Time and Temperature.
--typically 70 to 90% of melting temperature.
--as little as 10 min. for Fe or Cu -- up to 8 hours for tungsten and
--this can be done in an automated continuous process
--material will maintain greater porosity
--this is the process which provides the major strength of the metal
powder process.

5) Finishing Processes: Finishing processes can include forging,
coining, rolling, machining, infiltration, impregnating, resintering, heat
treating, grinding, plating, and repressing.
forging and coining: used to impart dimensional accuracy, improve
strength, improve surface finish, and increase density of material.

impregnation: immersion of the part in heated oil which allows the pores
of the sintered material to fill with lubricant.
infiltration: when a lower-melting-point metal is introduced and drawn
into the porous sintered part by capillary action. This can improve
hardness and tensile strength of the material. Can also lower frictional
characteristics. Commonly used materials include lead and copper.

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