Flow curve approximation of high-strength aluminium alloys in heat-assisted forming processes

Predicting the flow behaviour at high strain values beyond uniform elongation is a critical factor in sheet metal forming, especially in the context of heat-assisted forming processes of lightweight materials. Based on uniaxial tensile tests conducted at the temperature profiles of the Hotforming, W-Temper, and Warmforming process routes this study presents the determination of the flow curve for EN AW-7021 sheet material using five commonly used empirical approximation models. A support point obtained from layer compression tests was introduced to analyse the accuracy of the extrapolation models at higher strain values. All models considered were found to be in good agreement with the experimental tensile test data in the area of uniform elongation. However, the empirical approximation models show significant differences in strain hardening behaviour when higher strain values and varying temperature profiles are considered. For the Hotforming and W-Temper process routes, the Hockett-Sherby model estimates the flow behaviour with the highest accuracy. When using the Warmforming process, all approximation models considered seem to estimate the flow behaviour with reasonable accuracy, however the Ludwik-Hollomon modified equation shows the best correlation with the experimental data.