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

Abstract

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.