EFFECTS OF ASPERITY SHAPE ON THE TRIBOLOGICAL BEHAVIOUR DURING STAMPING OF ZINC COATED STEEL SHEETS FOR CAR BODIES

摘要

This work was initially focused to analyze the onset of systematic fractures of car body parts (Ford Focus model) during stamping. Fractures were attributed to unexpected high values of friction between the tools and the steel sheets. All the sheets that failed fulfilled the required standard mechanical properties and topographical surface requirements (Ra between 1.0 and 1.7 μm and peak count PC > 50 peaks/cm, according to the German SEP 1940 standard), and were processed with the same lubricant. A detailed study of the surface features showed that, having similar and correct values of the conventionally used parameters for surface texture requirements, such as Ra and peak count PC, those sheets that failed had some different asperity features, related to the asperity peak shape. Experimental work (measurement of real values of surface contact Sc and friction behavior under boundary lubrication conditions) confirmed that these differences in the asperity shape have an important effect on the friction behavior, controlling the rate of surface roughness flattening and therefore determining the real values of surface contact area and the remaining valley areas able to carry lubricant. The differences in asperity shape were reproduced in single roughness models and the flattening process of the asperities was simulated by finite element analysis. The agreement between simulation and experimental observation was excellent. The influence of asperity shape was found to be very important, justifying the real differences found in the values of real surface in contact and hence in the friction behavior during stamping real car body parts.