Modelling of single-asperity ploughing in aluminum sheet metal forming

In sheet metal forming, there are direct interactions between tool and workpiece asperities in boundary lubrication regimes. Under these conditions, the friction force results from ploughing of hard tool surface asperities through softer workpiece material and shearing of the boundary layers which form on workpiece and tool surfaces. To predict friction, it is important to understand these two frictional mechanisms. The boundary-layer shear strength was characterized experimentally and incorporated into finite element simulations of a single-asperity ploughing model. These simulations were validated using single-asperity ploughing experiments. After validation, additional simulations with a wider range of asperity geometries and contact conditions were conducted. Results show boundary-layer shear dominates for blunt asperities where the ratio of asperity height to its radius in the sliding direction is small. As this ratio increases, the contribution of material deformation to friction becomes dominant. Current results show that the assumption that friction is a simple additive contribution of these two mechanisms is inaccurate for relatively sharp and wide asperities. For ploughing aligned with the major radius of elliptical asperities, the height to major radius ratio has the strongest influence on the friction response. Asperities with higher ellipticity exhibit higher coefficients of friction as the sliding direction deviates from the major radius orientation due to an increased attack angle. The developed single-asperity model accounts for the micro-scale deformations at the sheet metal–tool interface and provides the foundation for a multi-scale model capable of predicting the friction coefficient under various contact conditions in aluminum sheet forming.