A friction model for loading and reloading effects in deep drawing processes

Abstract

Deep drawing is one of the most widely-used forming processes to manufacture automotive body parts from sheet metal. In order to simulate deep drawing processes, a finite element (FE) method was used to predict formability. The accuracy of the FE simulation depends on the material models, numerical techniques, and contact algorithms. Despite the fact that the contact conditions between the tool and sheet material influences the coefficient of friction in forming processes, the coefficient of friction is often treated as a constant Coulomb friction coefficient in FE simulations. However, a friction model based on local contact conditions and surface topography is required to improve forming predictability. There is growing interest in developing contact models to predict the nature of friction conditions for use in FE calculations. In deep drawing processes, the sliding contact predominantly occurs in the blank holder region between the tool and sheet material. The contact pressure in the blank holder is non-uniform due to bending and material compression which vary depending on tool geometry. The sheet metal surface is subjected to repeated contact during sliding, which in turn affects the local friction conditions. The objective of this paper is to develop a sliding friction model for mixed modes of surface deformation. The deterministic approach used in the current model includes the roughness of both the sheet material and the tool. The sheet material is subject to an asperity flattening process. Further, the tool surface indents into the sheet material under normal loading. The geometry of the asperities is characterized by an elliptical paraboloid shape to better calculate the load-dependence of friction. The model has been compared with data from experiments using a rotational friction tester under multiple loading conditions.