Surface flattening during aluminum sheet forming

Predicting friction in sheet metal forming is essential for accurate modelling of the process and therefore its optimization. Estimation of real contact area is prerequisite for reliable prediction of friction. In this paper, effects of normal load, as well as normal load combined by strain in aluminum sheet metal on the real area of contact are investigated. An earlier setup is improved to probe combined normal load and bulk strain effects on the real area of contact. Experiments are carried out on two grades of aluminum, AA6016 and AA5182 sheets with electro discharge surface textures (EDT). Fractional real area of contact at different contact pressure and strain levels is measured using confocal microscopic images of the deformed surfaces. A semi-analytical model is used to predict real area of contact evolution due to normal load using a hardness database of different asperity geometries. To account for flattening of asperities due to combined normal load and bulk strain, a new model is developed and calibrated based on the experiments. The experimental results show that extent of increase in real area of contact due to only normal load or combined normal load and strain depends on the surface texture, the material properties and contact conditions. Predictions of the new models were validated using different textures of the same aluminum grade. The results show that the models capture the evolution of real area of contact with reasonable accuracy. These models will be used to enhance friction modelling in aluminum sheet forming.