With the recent increase in the demand for net shape forming, numerical simulations are being widely used to increase the efficiency and effectiveness of the design of bulk metal forming processes. Consideration of tribological problems at the contact interface between the tool and material is very important in such numerical simulations. Lubrication and friction play crucial roles in metal forming by influencing metal flow, forging load and die wear.
In this study, a new friction test based on backward extrusion is proposed in order to evaluate the shear friction factor in metal forming operations. In this measuring method, the workpiece material is primarily positioned at a predetermined groove location within the forming die of the extrusion tool-set and is pressurized by a punch. This process forms a sharp tip at the backward extruded end of the extruded product. Thereafter, the perpendicular distance from the external side surface of the extruded product to the tip is measured and used to calculate the shear friction factor. Experiments were carried out using annealed aluminum alloy AL6061-O as the workpiece material with different kinds of lubricants. The relation between the tip distance and the shear friction factor was obtained through simulations using CAMPform, which is a thermal-rigid viscoplastic FEM program, in the backward extrusion process for a reduction ratio of 64%. The shear friction factor, which has a range of 0.0~1.0, is calculated using the this linear relationship between the shear friction factor and the tip distance.