Evaluation of surface topography parameters for friction prediction in stamping

The continued globalisation of the automotive industry, leading to increasing demands for competitiveness and escalating legislative requirements, is the main driving force of research activities of steel sheet surfaces. Recent studies on the stamping process have been carried out among others within AUTOsurf, a project funded by the European Community, and by Wihlborg and Crafoord. The purpose of this study was to evaluate the viability of the proposed parameters for friction prediction. Seventeen different surface topographies were investigated. The sheet materials were either, hot-dip galvanised, electrogalvanised or galvannealed, and electron beam or electric discharged textures. The frictional response was measured in a bending under tension (BUT) test under mixed lubricated conditions. This BUT test simulates the conditions of the die radius in a stamping tool. The laboratory test differs from the experimental work performed in AUTOsurf which simulated the conditions of the holding-down plate. In spite of the differences in test equipments in AUTOsurf, e.g. the rotational friction tester (RTF), on a comparison the correlation of frictional response was significant. But neither of the proposed parameters could predict the frictional response with sufficient accuracy in this study. In addition, the friction model in AUTOsurf describes peak lubrication as a dragging phenomenon on sliding surfaces. The movement eased friction in inverse proportion to the average peak area. However, the trend in this study showed the opposite, movement eased friction proportionally to the average peak area. The result indicates a switch of dominant friction mechanism when the sliding velocity is increased, i.e. from a dragging phenomenon at low velocities to micro-hydrodynamic wedge effects at high velocities.