The frictional behaviour of aluminium alloy is evaluated by a tapered plug penetration test under semidry conditions. Cemented tungsten carbide (WC) plugs at room temperature are penetrated into the cylindrical hollow workpieces of aluminium alloy. A small quantity of lubricant (2.0-3.0 g · m^-2) on the plug surface is effective in preventing the seizure of the workpieces. It is found that the seizure of the workpieces occurs when the inclination of the applied load exceeds a critical value, approximately 0.5 kN · mm^-1. To examine the deformation behaviour of the workpieces during plug penetration, finite element simulation is carried out. From the obtained simulation and experimental results, the critical coefficient of shear friction between aluminium and WC is estimated to be about 0.3 when the seizure occurs.