A critical condition for seizure initiation has been investigated under a relatively low extrusion speed using an extrusion-type friction testing machine. To examine the effects of lubricant oil film thickness on seizure initiation, the oil film thicknesses at the tool-workpiece interface were estimated theoretically, and paraffinic mineral oils with three grades of viscosity were tested with various thicknesses of oil film applied, using commercially pure aluminium as the workpiece. The seizure phenomenon was always near the die exit. The lower the viscosity grade of oil used and the thinner the oil film applied, the shorter the critical punch stroke for seizure initiation became. Consequently, it has been clarified that the critical oil film thickness for seizure initiation was about 0.2 μm.

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