Mechanical properties and microstructure evolution of 5A06 aluminum alloy under two-step dynamic tension

With the development of aluminum alloy thin-walled components towards larger sizes, the application of multi-step high-velocity forming (HVF) for such components are becoming increasingly widespread. This study focused on the two-step dynamic deformation of 5A06 aluminum alloy (AA5A06). Based on quasi-static, fully dynamic, and two-step dynamic uniaxial tensile tests, the mechanical properties of AA5A06 sheets were investigated. The test results show that dynamic pre-forming can strengthen the strength and ductility enhancements of AA5A06 during subsequent dynamic forming. As the dynamic pre-strain increased, both the ultimate tensile strength and total fracture elongation of AA5A06 also improved. Under a dynamic pre-strain of 16.7 %, the material exhibited excellent mechanical properties, with an ultimate tensile strength of 418 MPa and a total fracture elongation of 54.1 %, respectively. Compared to quasi-static and fully dynamic tensions, the ultimate tensile strength under two-step dynamic tension increased by 23.3 % and 16.4 %, respectively, while the total fracture elongation increased by 114.7 % and 20 % respectively. Subsequently, the microstructure analysis at fracture revealed that the main texture components after two-step dynamic deformation were both copper texture {112}<111> and Brass texture {110}<112>, which indicated that the dislocation-slip pattern may be primarily dominated by cross slips during two-step dynamic deformation. Moreover, increasing the dynamic pre-strain did not change the texture components during two-step dynamic deformation, but it increased the intensity of copper texture while weakening the intensity of brass texture. The high dislocation density within the grain boundaries generated by two-step dynamic deformation led to higher strength. The ductility enhancement was attributed to two main factors. First, the uniform distribution of dislocations during dynamic deformation prevented local strain concentration. Second, the two-step dynamic deformation resulted in original second-phase particles being cut through into two or more sections, and larger particles becoming more prone to crack. A large number of piled-up dislocations can continue to move after cutting through the particles, allowing for release of local shear stress, which in turn improved the material ductility. From the fracture micro-morphologies, it can be concluded that the ductile fracture was predominant in the fracture mode of two-step dynamic deformation, and the fracture exhibited a higher degree of plastic deformation level. This study provides a basic research for the application of multi-step HVF for large aluminum alloy thin-walled components.