Multi-scale contact modeling of coated steels for sheet metal forming applications

Friction in sheet metal forming is a local phenomenon which depends on continuously evolving contact conditions during the forming process. This is mainly influenced by local contact pressure, surface textures of the sheet metal as well as the forming tool surface profile and material behavior. The first step for an accurate prediction of friction is to reliably estimate real area of contact at various normal loads. In this study, a multi-scale contact model for the normal load is presented to predict asperity deformation in coated steels and thus to estimate the real area of contact. Surface profiles of the zinc layer and steel substrate are modelled explicitly obtained from confocal measurements. Different mechanical properties are assigned to the zinc coating and the steel substrate. The model was calibrated and validated relative to lab-scale normal load tests using different samples of zinc coated steel with distinct surface textures. The results show that the model is able to predict the real area of contact in zinc-coated steels for various contact pressures and different surface textures. Current multi-scale model can be used to determine the local friction coefficient in sheet metal forming processes more accurately.