Mechanical behaviors and microstructure characteristics of W-tempered and peak-aged 7075 alloy sheets under low frequency vibration-assisted tension

Low frequency vibration-assisted forming (LFVF) with high excitation force and dynamic response is beneficial because of the low load and high forming quality. To reveal the mechanism of LFVF, low frequency vibration-assisted tension (LFVT) with different amplitudes was performed on W-tempered (WT) and peak-aged (T6) 7075 alloy sheets, and the influences of low frequency vibration on the mechanical behaviors and microstructure evolution were investigated. The application of low frequency vibration created a remarkable vibration-softening effect, and a transition from vibration hardening to vibration softening was observed in the LFVT of 7075-T6. Besides, the stress-overshooting phenomenon occurred after vibration excitation in the LFVT of 7075-T6. The vibration-softening effect and stress-overshooting phenomenon were enhanced when the vibration amplitude was increased from 0.078 to 0.243 mm. The ductility of 7075-WT and 7075-T6 sheets could be improved with a vibration amplitude of 0.165 mm. The fracture elongation of 7075-WT and 7075-T6 increased from 17.93% and 13.62% to 20.14% and 14.45%, respectively. An increase in the vibration amplitude resulted in a slight increase in the hardness of the 7075-T6 samples, whereas naturally aged 7075-WT samples exhibited the opposite effect. In addition, the low-angle grain boundaries (LAGBs) and dislocation density generally increased with the superimposition of vibration for 7075-WT and 7075-T6, because the vibration stress provided additional energy for the initiation and migration of dislocations. For the LFVT of 7075-T6, the dislocation tangles enhanced by the interaction between precipitates and dislocations resulted in increased hardness and stress overshooting. The LFVT of 7075-WT with a large vibration amplitude resulted in the peak values for the LAGBs and dislocation density because of dislocation annihilation and absorption to the grain boundaries. These results provide a fundamental understanding of LFVF in high-strength aluminum alloys.