Forming limit prediction for hot stamping processes featuring non-isothermal and complex loading conditions

Abstract

An intrinsic feature of the hot stamping process, in which a hot blank is quenched and formed between water cooled dies, is the severe thermo-mechanical deformation that the blank experiences under the combined influences of non-isothermal and non-proportional loadings. This results in challenges for conventional forming limit prediction models to accurately predict material behaviour. In this paper, a novel viscoplastic-Hosford-MK model was developed to predict the forming limits of an Al–Li alloy under hot stamping conditions. The effectiveness of the developed model was verified by the demonstration of accurate responses to cold die quenching, strain rate and loading path changes, enabling the developed model to reveal a realistic critical material response under complex deformation conditions. Finally, by applying the developed model to the hot stamping of an AA2060 component, its accuracy was successfully validated. It was indicated that the onset of necking during hot stamping of the component did not necessarily occur at the maximum thinning region, and this was due to the comprehensive effects of varying loading path, strain rate and temperature. A detailed mathematical analysis of the developed M-K model was also conducted, and it was found that the incremental work per unit volume ratio (W˙b/W˙a=σ¯bdɛ¯b/σ¯adɛ¯a) between Zone b (where a thickness inhomogeneity exists) and Zone a (the remainder of the material) was a significant parameter that determined the formability of AA2060 under hot stamping conditions.