Revealing ductile/quasi-cleavage fracture and DRX-affected grain size evolution of AA7075 alloy during hot stamping process

Abstract

The microstructural evolution, including ductile fracture modes and grain size at elevated temperature, exhibited significant effects on the macroscopic rheological behaviour and restricted the formability of the high-strength AA7075 alloy employed in the hot stamping process. In this study, AA7075 alloy was subjected to uniaxial hot tensile tests at deformation temperatures of 350–450 ℃ and strain rates of 0.01–1 s−1 to analyse the thermal rheological behaviour, damage and grain size evolution, in tandem with the preferred grain orientations affected by intergranular and intracrystalline evolution. A set of improved viscoplastic damage constitutive models was established, which consider the combined interactions of dislocation density and grain size as well as the ductile and quasi-cleavage fracture modes, to elaborate and reveal the fracture modes and grain size evolution of AA7075 alloy during the hot stamping process. Particularly, the evolution of dislocations accumulation was established incorporated with DRX-affected grain refinement. Subsequently, the material parameters were determined and calibrated based on a genetic algorithm, and the feasibility of the constitutive model was verified by simulations of uniaxial tension experiments. Finally, a finite element (FE) simulation of a hot formed T-shaped specimen was carried out. The mesoscopic and macroscopic deformation behaviours, including the evolution of grain size, damage and thickness distribution, were predicted, and the influences of grain size and damage on the formability were explained. Especially it was found that the grain size of the T-shaped components at the punch fillet was the largest, while the grain size at the bottom of the T-parts was the smallest. In addition, severe thinning region of T-shaped parts possess the largest normalized grain size. This study provides theoretical guidance and a scientific basis for the application and forming control of AA7075 alloy in hot stamping processes.