(Step
4)
-
The
final and most critical step is performed in a high
temperature furnace where the material achieves its
final properties and dimensions.
A
microprocessor controls a complex run of
temperatures, times, and atmospheres. A
combination of reactive and inert gases is utilized
to tailor the atmosphere to the special requirements
of each process sequence. The five (5) basic
sequences are:
- PURGE
- DECARBURIZATION
- REDUCTION
- SINTER
- COOL
DOWN
During
sintering, temperatures approach 85% of the alloy's
melting point (solidus temperature).
As the furnace reaches the solidus
temperatiure, diffusional flow causes densification
of the part. When
densification occurs, pores are eliminated from the
part and the part shrinks. Since the size and shape of the original powder is rigidly
controlled, the shrinkage is uniform along all axis
and is very predictable. The finished part retains
the original complex shape of the molded part, and
close tolerances can be achieved. Part shrinkage
varies between 15% to 25% depending upon the alloy.
The tooling must be precisely oversized and
compensated so that the sintered part shrinks to the
desired dimensions.
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