Mechanisms of stick-slip vibration observed during ejection of warm compacted iron powder

The stress and friction coefficients on the die wall in warm compaction (W/C) were investigated during ejection of iron powder compacts by focusing on stick-slip phenomenon. Friction coefficients were measured during the ejection process by comparing W/C with cold compaction (C/C). In both cases, static friction coefficients were higher than kinetic friction coefficient that has been mentioned as the condition to generate the stick-slip, but the friction vibration was observed only in W/C not C/C. To confirm the mechanism of the stick-slip vibration generated specifically in W/C, static friction coefficients were measured in different ejecting velocities. The static friction coefficients in W/C increased prominently with decrease in ejecting velocity in comparison with C/C. This result indicates that local plastic flow of iron powder induced by heating results in broadening and producing stiffly adhesions between iron powder and the microscopic tips dispersed on the die surface during slower ejection, and the stick-slip might be attributed to such the adhesions. Consequently, it was proposed that blocking to forming the stiffly adhesions by modifying a microscopic morphology or materials of the die wall surface is an effective measure to suppress the stick-slip.