Powder metallurgy - General

Unlike conventional metallurgical processes, which always involve the solidification of a molten metal, powder metallurgy starts with a metal powder and uses a consolidation process known as sintering. This can be defined as a reaction between particles of a powder mass, resulting in the formation of a coherent continuous solid.

Sintering is not specific to powder metallurgy; it has been used since time immemorial by ceramists, who largely preceded metallurgists.

In the manufacturing processes that are the most important in terms of tonnage, the chain comprises three essential operations:

• production of compressible metal powders; the powders may be pure metals or alloys, they may be mixed with each other or with other powders, non-metals or metal compounds such as oxides or carbides; the mixture further comprises a small proportion of a solid lubricant powder;

• Cold pressing of the powder in tools to produce an agglomerated, manageable part of precise shape and dimensions; the uniaxial pressure is between 200 and 800 MPa;

• sintering, which involves heating the tablets to a high temperature, but well below the melting point of the pure metal or alloy obtained; in most cases, a controlled, reducing atmosphere is required.

In this process, the shape and dimensions of the parts vary very little, and the final material therefore has significant residual porosity. In exceptional cases, dimensional shrinkage during sintering can lead to zero porosity.

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