A material model for warm forming of aluminium sheet

A material model has been developed to simulate the warm forming of Al–Mg sheet. Both the hardening behaviour, including temperature and strain rate effects, and the biaxial stress–strain response of the sheet are considered. A physically-based hardening model according to Bergström is used. This model incorporates the influence of the temperature and strain rate on the flow stress and on the hardening rate based on dynamic recovery. For deformations at constant temperature and strain rate, the Bergström model reduces to the well known Voce hardening model. The Bergström/Voce models can be fitted quite well to the results of monotonic tensile tests of an AA 5754-O alloy. The biaxial stress–strain response of the material is experimentally determined by uniaxial, plane strain, simple shear and equi-biaxial stress tests. It is demonstrated that the widely used Hill ’48 yield locus is inappropriate for simulation of deformation of aluminium. The low Rvalues for aluminium lead to a significant underestimation of the equi-biaxial yield stress. In the simulation of the deep drawing of a cylindrical cup this results in a much too thin bottom of the cup. The Vegter yield criterion is sufficiently flexible to accurately represent the shape of the yield locus and the anisotropy. 1