To examine the seizure mechanism in cold extrusion and forging, a bifurcating extrusion type friction testing machine has been manufactured by the authors. This friction testing method was devised to simulate the frictional surface in the process accompanied with a remarkable surface expansion which influences markedly the seizure phenomena in extrusion and forging. Using a commercially pure aluminium as the workmetal, the frictional characteristics, and in particular the seizure mechanism, have been investigated under a lubricated condition with a mineral oil of higher viscosity and a dry frictional condition. Under the lubricated condition, the frictional surface was remarkably expanded more than 26 times; nevertheless no seizure was observed throughout the extruded travel. On the other hand, under the dry frictional condition, the seizure is initiated at the region where a virgin surface is exposed or the relative slip displacement exceeds a critical value. It was observed that even in the dry frictional condition, a smooth burnished metal surface could be obtained if the surface expansion ratio and the relative slip displacement are lower than a critical value.

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