PROCESS OF POWDER METALLURGY

In Powder Metallurgy the articles are produced by pressure and heat. Usually the pressure and heating stages are separate and are termed compacting and sintering stages respectively. The compacting is also called briquetting and compressing.

The manufacture of parts by powder metallurgy process usually involves a series of steps as follows :

* The manufacture of powders

A number of secondary operations such as

sizing,

coining,

machining,

impregnation,

infiltration,

plating and

heat treatment.

The essence of powder metallurgy is that a mixture, composed of specially selected and prepared powders, is compressed in dies under pressures of 10.2 to 102 Kgf / mm2 (100 to 1000 MN / m2). The half finished object obtained ( the pressing) has a strength which, although insufficient for the articles to be used, permits transportation to the next technological operation. The final mechanical strength of the materials is achieved only as a result of a high temperature treatment sintering, which is conducted at a temperature below the melting point of the basic metal which goes into the mixture (66 to 75 percent of melting point).

As a result of this technique , the particles in powder materials are always in contact with each other , but the nature of this contact is different from that between the grains in continuous bodies. In a continuous metal, the area of contact is almost independent of outside pressure, whilst in powders the area of contact surface increases in proportion to pressure applied.

The increase of contact area can be accomplished by changing the shape of the particles by action of external forces, i.e., by pressing and sizing or by drawing the atoms in the metals towards the contact regions, a phenomenon which is caused by the movement of atoms at an elevated temperature (sintering). As a result of the increase in contact surface, the bonding between the particles grows. Consequently, the strength of the products is raised and their properties are altered. The contact surface of bodies is defined as the size of the contact areas of contiguous particles, separated by distance less than the range of molecular forces, i.e., the size of the contact areas through which the stresses are transmitted between contiguous particles.

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