Surface strengthening phenomenon and mechanism of 6061 solid solution alloy after ultrasonic stress relief

Abstract

The evolution of residual stress, microstructure and mechanical properties, and their correlations of supersaturated 6061 solid solution alloy were investigated after ultrasonic stress relief. It is found that following ultrasonic stress relief, the residual stress of 6061 solid solution aluminum alloy is gradually released with the hardness of the alloy rising first to a peak and then decreasing gradually to a stable value. After ultrasonic stress relief, the peak hardness of the alloy is capable of reaching 81.8 ± 1.95HV, which is about 45% higher than 56HV of the initial state. When the residual stress was released by about 80%, the hardness of the alloy came to a stable value of 63.75 ± 1.65HV. It is proved that the ultrasonic stress relief could not induce a precipitation of the strengthening phase in the supersaturated 6061 solid solution alloy. The hardening of the alloy was mainly caused by the evolution of dislocation during the residual stress relief process. The superposition of ultrasonic dynamic stress and the internal residual stress in the alloy first prompted the multiplication and entanglement of dislocation, and consequently increase the hardness of the alloy. With further relief of the residual stress, the dislocation multiplication rate slows down and the annihilation rate accelerates, and then the corresponding dislocation density and entanglement are obviously reduced, resulting in a decrease in the hardness of the alloy to a constant.