Dynamic response of grain rotation and slipping system activity in Al6014 tailor heat treated blank after non-uniform loading via crystal plasticity finite element method

Plastic instability and premature failure during the forming of tailor heat-treated blank (THTB) can be probably inhibited by the introduction of a non-uniformly distributed pressure (NUDP) field due to the reduction of pressure localization on the heat-treated zone (HTZ). Nevertheless, the inherent complex thermal and mechanical loading conditions of such a process exhibit the underlying challenges in revealing its potential physical mechanisms. In this work, the deformation mechanisms of Al6014 THTB under NUPD are systematically investigated via the crystal plasticity finite element method, aiming to establish the dynamic response between the lattice structure, crystallographic orientation, and slip system activity at the microscopic level and the macroscopic plastic behaviors. The evolution of the slip system normal of the representative grains indicates that as compared to uniformly distributed pressure, NUDP can enhance grain rotation in the as-received zone and HTZ, leading to a higher fraction of grains with rotation angles (ξ) deviating from ξ = 0°. Besides, the evolution of slip system activity and cumulative shear strain rate suggests that when NUDP is introduced, the normal pressure on the HTZ is decreased, leading to the reduced increasing rate of slip system activation in each loading transient and the enhanced probability of simultaneous activation of multiple slip systems. This can promote the multi-directional slip activation within the HTZ, but maintain the limited dislocation movement and propagation rate so that significant grain rotation and intergranular deformation are suppressed. Accordingly, the duration of plastic shear strain accumulation is prolonged, which can mitigate the strain localization in the HTZ and improve the macroscopic forming limit. The new insights into the relationships between the microstructure evolution and mechanical response of Al6014 THTB under complex load conditions may facilitate process control.