Galling initiation due to frictional heating

The lifetime of sheet metal forming (SMF) tools is often limited by galling, a wear type that affects the surface quality of the products and the lifetime of SMF tools. Since SMF tools represent high economical value, it is clear that improvement and prediction of tool life is of high industrial importance. Therefore, models are required that can predict friction and wear related phenomena in SMF-processes, in particular galling. This paper demonstrates the application of a wear model, which is able to predict the initiation of galling in lubricated sheet metal forming processes, to laboratory results gained with the TNO slider-on-sheet tribometer. Experiments are conducted with different (coated) tool surfaces, in sliding contact with stainless steel sheet, using two lubricants. By comparing the critical temperature of the generated boundary layer with the flash temperature at the interface of the sheet and individual tool summits, it is possible to predict whether or not galling initiation will occur. It is shown that the laboratory results are in good agreement with the predicted results of the presented wear model. Galling initiation in lubricated sheet metal forming processes, can be avoided by the application of smooth tool surfaces with enhanced thermal conductivity and lubricants which form boundary layers with a high critical temperature.