Page 189 - Materials Chemistry, Second Edition

176 3 Metals

Figure 3.14. The effect of matrix density on the compaction volume yielded from pressing.

The density of the bulk material following the pressing event is referred to as the
green density, coined more frequently for ceramic processing. It is most desirable to
have a powder with lower density, as this will undergo a greater change in volume
during compaction (Figure 3.14). The intimate pressing together, or alloying, of
metals during the pressing process is known as cold-welding. Sometimes, the
powder is too dense for efficient cold-welding; for these samples, such as heavy
metal alloys, a greater pressure (i.e., larger presses and stronger dies) is required.
It should be noted that powders under pressure do not behave as liquids; the pressure
is not uniformly transmitted and very little lateral flow takes place within the die.
The compacted powder will only be as pure as the initial components. The addition
of small impurities will cause dramatic differences in the resultant metallic material
following the pressing and sintering steps. The presence of bound vs. free impurities
may also result in observable differences in the compaction behavior for powders.
For iron powders, the presence of iron carbide (Fe 3 C) will increase the hardness of the
matrix, requiring higher pressures for compaction. However, free graphite particles
will act as a lubricant, increasing the pressing efficiency at lower pressures.
Unless handled under an inert atmosphere, metal powder grains will be coated
with a thin oxide film. Unless excessively strong SiO 2 or Al 2 O 3 films are produced,
the coating will rupture during the pressing process, exposing the clean underlying
metal surfaces. It should be noted that for alloying metals such as copper and zinc,
pressing is not necessary. The powders are simply placed in a mold and sintered, a
process aptly referred to as loose-powder sintering.
The major applications for powder metallurgical products center around the
automotive industry, specifically for engines, transmissions, and brake/steering
systems. Following pressing, the compacted metals may be injected into a mold,