Acoustic residual softening and microstructure evolution of T2 copper foil in ultrasonic vibration assisted micro-tension

The softening effect induced by ultrasonic vibration is widely studied due to its advantages in high product quality and processing efficiency, while only a few investigations focus on the residual effect of ultrasonic vibration, especially for its micro-mechanism. In this work, ultrasonic vibration assisted micro-tension of T2 copper foil with thickness of 200 μm was carried out. Both acoustic softening and acoustic residual softening were observed through stress-strain characteristics. Micro-hardness and fracture morphologies were analyzed to demonstrate the deformation behavior. The temperature on tensile specimen was also monitored and insignificant temperature rise was observed revealing that the thermal effect of ultrasonic vibration can be ignored. Furthermore, electron back-scattered diffraction (EBSD) examinations were performed to reveal the micro-mechanism of acoustic residual softening effect. It was found that acoustic residual softening is more closely related to ultrasonic duration other than to ultrasonic amplitudes. Microstructure examinations showed that low-angle grain boundary (LAGB) fraction, kernel average misorientation (KAM) and dislocation density in the specimen experiencing acoustic residual softening were all decreased, leading to the reduction in deformation resistance. Finally, a physical model considering dislocation annihilation and LAGB transformation was proposed to describe the micro-mechanism of acoustic residual softening observed in the experiments.