Microstructure evolution and its influence mechanism on strength-plasticity of Al7075 sheets during hot multi-DoF forming

Al7075 sheets are usually manufactured by plastic deformation process owing to its effective strength enhancement, but the high strength and low plasticity restrict the application of plastic deformed Al7075 sheets. This study applies hot multi-DoF (degrees of freedom) forming process to manufacturing Al7075 sheets, and investigates the microstructure and mechanical property evolution of Al7075 sheets during the forming process. The results show that grain size decreases from 21.37 μm to 7.27 μm, which is mainly because that the strong multi-directional metal flow could lead to intensive lattice rotation and dislocation movement throughout all matrix grains and cause rapid grain boundary formation. The Cube texture and Goss texture mutually transform in initial deformation stage and both textures ultimately transform into Copper texture in final deformation stage. The 50% deformation amount is the turning point of mechanical property transformation: (1) When deformation amount is below 50%, tensile strength/yield strength linearly increases and plasticity linearly decreases, which is due to the severe dislocation multiplication and dominant work hardening in this deformation stage. (2) When deformation amount exceeds 50%, the strength-plasticity collaboratively improve, which is mainly because that on one hand, effective grain refinement promotes strong grain boundary strengthening, and on the other hand, increasing grain boundaries promote the coordinated deformation between matrix grains and delay crack initiation and propagation during tensile deformation.