Processing of powders to form metallic articles

In order to make a dense material it is necessary to eliminate pores which form as a result of sintering. Pores can be moved drrough a compact by grain boundary dragging, so that the adhesion of the pores to the boundaries 

The mean distance that particles will migrate is given by tire classical expression 

An example where one metal melts before the densihcation process, is the formation of bronze from a 90 10 weight percentage mixture of copper and tin. The tin melts at a temperature of 505 K, and the liquid immediately wets the copper particles, leaving voids in the compact. The tin then diffuses into the copper particles, leaving further voids due to dre Kirkendall effect. The compact is therefore seen to swell before the hnal sintering temperature of 1080 K is reached. After a period of homogenization dictated by tire criterion above, the alloy shrinks on cooling to leave a net dilatation on alloy formation of about 1%. 

The elimination of pores comes about by tire combined effects of grain growth and Ostwald ripening in the hnal stages of sintering. 

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