THE DRIVING FORCE FOR SHRINKAGE IN COPPER POWDER COMPACTS DURING THE EARLY STAGES OF SINTERING

Abstract

The shrinkage behaviour of compacts from irregular copper powder during the initial stages of sintering has been determined by a dilatometric method. The effects of compacting pressure and of external load during sintering at a constant heating rate of 3°C/min upon shrinkage were observed. The residual stresses present on the surface of compacts heated at the same rate to temperatures of 200, 300, 400, 500, and 600°C were also measured. It was observed that shrinkage starts at temperatures where considerable residual stresses in the surface of the compacts are still present, and that this temperature also depends upon the external stress applied during sintering. Residual and externally applied stresses complement each other in shifting the temperature of start of shrinkage to lower values with increasing stress. It is concluded that, in the low-temperature range up to 400°C, residual and externally applied stresses, rather than surface-tension forces, cause shrinkage.