Ultrasonic Vibration Induced Softening And Residual Effects On The Lightweight Metals: Aluminium And Titanium

Ultrasonic softening effect refers to the phenomenon where the superimposed ultrasonic vibration can reduce the forming load during the metal plastic manufacturing process. Apart from the ultrasonic softening effect, ultrasonic residual softening and residual hardening effect have also been reported. Although the ultrasonic vibration technology has been successfully applied in industries for years, unanimous understanding of the mechanism of ultrasonic softening effect and its residual effects is yet to reach. We investigated the influence of ultrasonic vibration on the plasticity of lightweight metals by ultrasonic assisted compression (UAC) experiments. Aluminium and titanium were chosen as the research examples, due to their distinguishing crystal structures which may result in different responses to the ultrasonic vibration stimulation and their extensive industrial utilisations. The ultrasonic softening effect was observed in the UAC tests for both aluminium and titanium samples. It was found that the transient ultrasonic vibration can cause localized deformation in these samples. In terms of dynamic impacting, the ultrasonic softening effect can be explained in this way: the ultrasonic vibration functions like high frequency dynamic impact, which induces localized deformation, resulting in an unloading phenomenon to the samples. Nanoindentation test was carried out to characterize the localized deformation and to validate the proposed ultrasonic softening mechanism. Through electron backscatter diffraction (EBSD) analysis, we found that continuous ultrasonic vibration could refine the grain size for both aluminium and titanium samples. Besides, ultrasonic vibration induced grain boundaries tend to be parallel to the vibration direction, which implies that ultrasonic vibration promotes the dislocation propagation along vibration direction. In terms of acoustic softening, vibration induced dislocation multiplication is also expected to contribute to the ultrasonic softening effect. Therefore, the ultrasonic softening mechanism can be summarised as the coupling effect of dynamic impacting and acoustic softening. The ultrasonic vibration also induced residual hardening effect of aluminium.While for titanium, a residual softening effect was observed during UAC test. The residual hardening effect of aluminium was explained and predicted by grain boundary strengthening modelling. While for titanium, the ultrasonic vibration induced grain refinement was not obvious, but ultrasonic vibration did reduce the fraction of the twinning boundaries. Considering that the twinning boundary is a strengthening factor, the reduction of twinning boundaries can decrease the strength of titanium, which is the supposed interpretation for the residual softening effect of titanium. Besides, the phenomenological power-law constitutive model built in DAMASK was modified to incorporate the ultrasonic softening effect and the crystal plasticity based finite element analysis was conducted to provide guidance to the simulation of ultrasonic vibration assisted manufacturing (UAM) process. This study explored the mechanisms of the ultrasonic softening and the residual effects. The results obtained can be used as guidance to parameter selection during UAM. What’s more, discussions about the ultrasonic vibration induced microstructure evolution and the corresponding change of the mechanical properties provides hint to the service performance prediction of the products manufactured with the assistance of ultrasonic vibratio