Influence of deformation and roughness on friction in sheet metal forming process

It is important to control the friction during Sheet Metal Forming (SMF). Friction measurements have been performed on two apparatus, a SMF-simulator and a reciprocating tribometer. Under controlled conditions tests have been performed on the SMF-simulator to investigate the influence of bulk deformation on the friction. It was found that pre-deformation of the sheet results in an increase of the friction in the mixed lubrication regime compared to no bulk deformation tests. If, however, the coefficient of friction is plotted as a function of the lubrication number used and is adjusted for the increase of roughness and pressure due to straining, the Stribeck curves for the undeformed and the pre-deformed case are exactly the same. However, under conditions of simultaneously sliding and bulk deformation, the corrected value of the coefficient of friction in the mixed lubrication plotted in the Stribeck curve is higher than the one measured in the undeformed case. The coefficient of friction in the other regimes remains the same (the mixed lubrication regime becomes narrower). On both the SMF-simulator as the tribometer the roughness of the tool was varied in order to investigate this influence on the friction in the boundary lubrication regime. For low roughness values friction was independent on roughness. With increasing tool roughness friction increased due to increasing ploughing effects until the number of ploughing asperities on the tool does not increase any more leading again to a constant but higher coefficient of friction.