Ultrasonic dynamic impact effect on deformation of aluminum during micro-compression tests

[Display omitted] •Ultrasonic dynamic impact is found as a different effect from acoustic softening.•Critical condition for impact effect is if vibrated punch detaches from materials.•Impact effect leads to much more force reduction and surface plastic deformation.•Deformation by impact effect is accumulated by the repeated impact step by step.•Impact effect is accompanied by more grain boundary rotation and/or local heating. Conventional ultrasonic-assisted forming in macro scale has been studied widely for decades. However, ultrasonic dynamic impact effect, more likely to occur in micro scale, has never been studied thoroughly. The purpose of this study is to investigate the material deformation behavior under ultrasonic vibration in micro scale considering the dynamic impact effect. In this study, based on the newly developed 60 kHz ultrasonic vibrator, compression tests with ultrasonic assistance were carried out on pure aluminum specimens with varying amplitudes. The effects of ultrasonic vibration on the force reduction, the improvement of surface finishing, and the variation of the contour shape of deformed specimens were analyzed and discussed. The results confirm that the ultrasonic dynamic impact and acoustic softening are two different mechanisms by ultrasonic vibration and the critical condition for ultrasonic dynamic impact effect is whether the vibrated punch is able to detach from the specimen or not in process. The deformation caused by ultrasonic dynamic impact effect is accumulated by the repeated impact step by step. Ultrasonic dynamic impact effect produces much more surface plastic deformation near both ends of specimens, which causes the significant contact surface expansion and can reduce the apparent force and surface roughness more effectively than acoustic softening and stress superposition. In addition, to confirm the proposed mechanism, the micro-structural evolution of specimens was investigated by using electron backscatter diffraction (EBSD). It is found that both acoustic softening and ultrasonic dynamic impact effect can produce severe plastic deformation, resulting in much finer grains. However, with ultrasonic dynamic impact effect, the low-angle grain boundaries on the top side (severer deformation zone) are even less than those on the center, which suggests the more significant grain boundary rotation and/or extremely local heating. The ratio of amplitude to specimen height is identified as an influencing index for occu