Fracture mechanism and model of AA7075 multi-cavity component formed under complex stress states

During the multi-directional loading forming process of AA7075 aluminum alloy multi-cavity component, the stress states of material are complex and cracking defects are easy to occur, which seriously restricts the smooth process implementation and the improvement of forming quality. In this paper, based on the characteristic structure of AA7075 multi-cavity component, the material flow and stress states were studied. A trapezoidal groove shape specimen and test device were ingeniously designed. Then the fracture morphology, microstructure and mechanism of AA7075 alloy under different stress states are revealed. The results show that when the stress state changes from tensile-shear to pure shear and compression-shear, the fracture is easy to occur at the second phase particles, inclusions, the interface of elongated and equiaxed grains. The fracture modes are the mixture of surface shear fracture and core tensile fracture, but the pro-portion of shear plane is increasing. The grain morphology at the fracture surface is long grain with slightly increased axial diameter ratio, equiaxial grain with sufficient dynamic recrystalli-zation, and slender shear band with large angle deflection, respectively. Further, a thermoforming fracture model under complex stress states in the process of multi-directional loading forming of multi-cavity components is established.