Temperature dependent micromechanics-based friction model for cold stamping processes

Temperature rise in cold stamping processes due to frictional heating and plastic deformation of sheet metal alters the tool-sheet metal tribosystem. This is more prominent in forming advanced high strength steels and multi-stage forming operations where the temperature on the tool surface can rise significantly. The rise in temperature directly affects the friction due to break down of lubricant, change in physical properties of tribolayers and material behavior. This can result in formability issues such as workpiece-splitting, etc. Therefore, it is important to account for temperature effects on friction in sheet metal forming analyses. In this study, the temperature effect was included in a micromechanics-based friction model which allows calculation of local friction coefficients as a function of contact pressure, bulk strain and relative sliding velocity. The temperature influence on friction was introduced through material behavior of sheet metal, viscosity of lubricant and shear strength of boundary layer in the micromechanics-based model. The model validation has been done by comparing the calculated fractional real contact area with the experimental results. The model can be used in formability analyses and to predict optimum stamping press parameters such as the blank holder force and the press speed.