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Powder metallurgy parts
are formed by a sequence of processes. The first steps are the selection
of the suitable powder form, the production
of the powders, and the weighing and mixing
of the them. The blended powders are then pressed or compacted
into the desired shapes in dies. The green compacts are sintered.
To improve the properties of the sintered products, finishing operations
are carried out.
Powder Production
Bulk metals and
alloys, ores, salts, and other compounds can be processed into metal
powder for sintering through several different methods. Metal powder
design parameters such as shape, size distribution, porosity, chemical
purity, and bulk/surface characteristics are dependent on the method
used.
- Atomization
Atomization forms metal powder by injecting molten metal through
an orifice and then having the particles broken up by an inert
gas, air, or water to form metal particles. The powder size depends
on the metals temperature, flow rate through the orifice, nozzle
size, and jet characteristics.
- Reduction
Reduction forms metal powder by removing oxygen from metal oxides.
The powder formed from this method are either uniform-sized spheres
or angular shapes that are typically spongy and porous.
- Electrolytic deposition
Electrolytic deposition uses aqueous solutions or fused salts
to produce an anode/cathode cell. The metal powders made by this
method have high chemical purity.
- Carbonyls
Metal Carbonyls, such as Iron Carbonyls (Fe(CO)5)
or Nickel Carbonyls (Ni(CO)4), are made
to react with carbon monoxide (CO). After this, the product of
the reaction are decomposed to either Iron or Nickel. The powders
produced are generally small, dense, pure spherical particles.
- Comminution
Comminution are crushing/grinding operations performed on brittle
or less ductile metals to produce metal powders. The powders made
from brittle materials tend to have angular shapes, while the
powders made from ductile metals tend to be flaky and inappropriate
for some applications.
- Mechanical Alloying
Mechanical alloying produces powder by crushing/grinding 2 different
metals together such that the metals bond by diffusion.
- Other methods
Other methods of metal powder production include 1) precipitation
via chemical solution, 2) machining metal chips, and 3) vapor
condensation of chips.
Blending
The blending of metal powder, which is done under controlled conditions
(i.e. air, inert atmosphere, or liquid) to avoid contamination and
deterioration, fulfills several purposes:
- Produces an uniform
distribution of particle sizes and shapes
- Allows different metals
to be mixed to obtain specific physical properties
- Improve metal powder
interaction and prolongs the life of dies used when metal powder
is blended with lubricant
Compaction
The blended powders are compacted to form the shape of the desired
part. The density after compaction
(also called green density) depends on the compaction pressure,
dimensions of the compacted part, and powder hardness. Some of the
different methods that can be used to compact the powder include:
- Cold isostatic pressing
(CIP)
- Hot isostatic pressing
(HIP)
- Forging
- Rolling
- Extrusion
- Injection
Molding
- Pressureless compaction
Sintering
Sintering follows the
process of compacting and shaping the powdered material. Sintering
is the process of heating the material to a temperature below the
melting temperature but high enough to allow bonding or fusion of
the individual particles. Continuous sintering furnaces are used
for most production. These furnaces have three chambers:
- A chamber to volatilize the lubricants in the green compact
in order to improve bond strength and prevent cracking. It is
called the burn-off chamber. It slowly raise the temperature of
the compound in a controlled manner.
- A high-temperature chamber for sintering. It is the site of
actual solid-state diffusion and bonding between the powder particles.
The time during the second stage of sintering must be sufficient
to produce the desired density and final properties.
- A cooling chamber.
The furnace atmosphere should be properly controlled to obtain
successful sintering and optimum properties.
During the metal sintering process a wide variety of physical,
chemical, and metallurgical phenomena occur within the mass of metal
powder particles. These phenomena are influenced by the sintering
conditions, such as time, temperature, and atmosphere, and the chemical
composition of the powder mass. For a more detailed discussion of
these phenomena, see Theory.
Sintering is an important
process for compressed powdered metal shapes, called the green compacts.
After compaction, the density and strength of the material are low.
Sintering, then, increases both properties of the material.
Finishing Operations
Finishing operations are often times performed after sintering.
For better dimensional accuracy, different machining operations
such as coining and turning are performed. Fluid impregnation is
done for bearings and bushings that are internally lubricated. When
low porosity is desired, infiltration is done by heating a low-melting
metal slug next to the sintered part to fill the pores through capillary
action. Heat treating (quench and temper, stream treat) the sintered
part will improve its strength, hardness, and wear resistance. Finishing
operations are performed to improve the surface characteristics
of the part.
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