Constitutive modelling and its application to stress-relaxation age forming of AA6082 with elastic and plastic loadings

A novel constitutive model has been proposed in this study that predicts the different stress-relaxation ageing (SRA) behaviour of AA6082-T6 with elastic and plastic loading strains, extending the applications of stress-relaxation age forming (SRAF) from conventional elastically loaded panels to complex-shaped panels under plastically loaded conditions. The particular contributions of loading strain levels in elastic or plastic regions on the evolution of microstructural variables (i.e., inter-particle spacing, dislocation density, and precipitate length), yield strength and stress-relaxation behaviour during SRA process are concurrently modelled. The decreasing creep threshold stress and the increasing dislocation recovery effect from annealing with increasing initial strain in the plastic region have been proposed and introduced in the model. TEM analysis has been performed to quantify the effect of loading strain and ageing time on the evolution of β″ precipitates, and further calibrate the material model. Furthermore, the established model has been implemented into FE simulation to optimise the tool surface design of a train body panel component with complex and large curvatures, and corresponding SRAF tests have been conducted with the optimised tool surface. The maximum shape deviation from the objective shape of a component with a dimension of 820*300*3 mm3 has been controlled within 3 mm, demonstrating the feasibility of the developed material model for SRAF in industrial applications, especially for highly demanded complex-shaped components