Effects of Surface Roughness on Tribological Characteristics in Micro Double Cup Extrusion of Aluminum

This study used the micro double cup extrusion of aluminum to investigate the effects of the surface roughness of workpiece and die on the tribological conditions involving mixed lubrication and plastic deformation. The aluminum billets with the diameter of 3 mm and the height of 4.5 mm in different surface roughnesses were prepared by grinding processes and used in the experiments under conditions of dry and molybdenum disulfide lubrication. To estimate the friction factors, the heights of the extruded cups were measured and compared with those predicted by finite element simulations using the law of constant shear stress. The results show that smaller workpiece surface roughness resulted in low and nearly constant friction in the early stage of the micro extrusion under the dry and lubricated conditions, which were reflected in the estimated friction factors. In the later stage of the extrusion processes, the workpiece surface finish became close to the one of the die for all tested conditions. The friction was clearly reduced in the dry condition but it became greater in the cases with the lubricated condition. The surface condition in the later stage of the micro extrusion process was believed to allow the lubricant to escape from the contact interface.