A comparative study of mechanical and microstructural characteristics of aluminium and titanium undergoing ultrasonic assisted compression testing

The mechanism of ultrasonic softening effect has been widely investigated and used in the metal material processing area. Deficiency discussion, however, has been focused on the residual effect of ultrasonic vibration on the plasticity of metals only. In this study, ultrasonic vibration assisted compression (UAC) experiments were carried out using commercially pure aluminium and titanium samples. Despite the similar ultrasonic softening effect, different ultrasonic residual effects were observed in the aluminium and titanium samples. Aluminium exhibited a residual hardening phenomenon, whereas in titanium the initial residual hardening effect transferred to a residual softening effect with an increase in amplitude of the applied ultrasonic vibration. Electron backscatter diffraction (EBSD) characterizations revealed that substructure multiplication, texture evolution and dynamic recrystallization caused by the ultrasonic vibration are the main causes of the residual hardening effect of aluminium. The boundary strengthen mechanism based Hall–Petch relationship was used to explain the mechanism and to predict the influence of the ultrasonic residual effect for aluminium. For the titanium, it was found that the ultrasonic vibration reduced the density of deformation twinning in the specimen, and the evolution of residual effect was analysed from the aspects of grain refinement, deformation twinning induced strain hardening. By quantitative study, the main reason for the residual hardening effect of UAC aluminium sample is identified to be the ultrasonic vibration induced grain refinement, while for the titanium, the weakened strain hardening effect induced by deformation twinning should be the main factor accounting for the residual softening of titanium. Also the ultrasonic vibration could have a similar influence as that of the thermal treatment to the titanium sample, resulting in the yield point elongation (YPE) phenomenon.