Acoustic softening and hardening of aluminum in high-frequency vibration-assisted micro/meso forming

A hybrid micro/meso forming assisted by high-frequency vibration was experimentally investigated by upsetting aluminum. Experiments with various vibration amplitudes and durations were conducted. The high-frequency vibration resulted in both acoustic softening and hardening behavior. Results showed that the overall forming stress reduced by 30% when a transverse vibration of 9.3 kHz was applied, but the stress recovered once the vibration stopped. On the other hand, a hardening behavior was observed during the vibration and resulted in a permanent hardening of the material even after the vibration had stopped. The effects of acoustic softening and hardening were coupled during the vibration-assisted upsetting. It was found that larger vibration amplitude led to a more significant acoustic softening and hardening. The findings of this study provided a basis to understand the underlying mechanisms of vibration-assisted forming.