Some aspects of cold upset forming of sintered aluminium preforms using different lubricants

Cold upset forming and densification features in sintered aluminium preforms using different lubricants were investigated. Cylindrical preforms with an initial theoretical density of 91% and 0(.)75 aspect ratio were prepared using a suitable die, a punch and a die bottom insert on a 1(.)0 MN capacity universal testing machine. The preforms were well covered with dry fine silica sand and sintered in an electric muffle furnace at 520 +/- 10degreesC for a period of 1 h and then furnace cooled. Cold deformation experiments were carried out in several steps. Dimensions such as height, contact and bulged diameters and densities were measured for each test. In general, each compact was subjected to compressive loading in steps of 0(.)01 MN until fine cracks appeared on its free surface. Analysis of the experimental data showed an increasing trend in the value of exp(epsilon(z)-epsilon(theta)). Further, it was found that preform lubricated with lithium grease exhibited enhanced densification compared to that for MoS2 or graphite lubricant, provided that the initial preform density and aspect ratio were kept constant. The instantaneous strain hardening exponent n(i) and strength coefficient k(i) of the aluminium preforms were calculated and found to reach the peak value when the deformation or packing density was at low value. Further, it was observed that the value of n(i) and k(i) decreased and was found to be constant for both the lubricant preforms tested. It was established that the initial geometry of the PM preforms played a predominant role in influencing both n(i) and k(i).