Abstract 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 occurrence of ultrasonic dynamic impact effect for the same material, confirming the promising prospect of ultrasonic vibration in micro forming. The findings in this study can help to provide a basis to understand the underlying mechanisms of ultrasonic-assisted micro forming and a guideline for designing the process in next step.

中文翻译：

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微压缩试验中超声动态冲击对铝变形的影响

摘要 几十年来，宏观尺度的常规超声辅助成形得到了广泛的研究。然而，超声动态冲击效应更可能发生在微观尺度上，却从未被彻底研究过。本研究的目的是在考虑动态冲击效应的情况下，在微观尺度上研究超声振动下的材料变形行为。在这项研究中，基于新开发的 60 kHz 超声波振动器，对不同振幅的纯铝试样进行了超声波辅助压缩试验。分析和讨论了超声振动对受力降低、表面光洁度提高以及变形试样轮廓形状变化的影响。结果证实，超声动态冲击和声学软化是超声振动的两种不同机制，超声动态冲击效果的关键条件是振动冲头在加工过程中是否能够从试样上脱离。超声波动态冲击效应引起的变形是通过逐步重复冲击而累积的。超声动态冲击效应在试样两端附近产生更多的表面塑性变形，导致接触面显着膨胀，比声软化和应力叠加更能有效地降低表观力和表面粗糙度。此外，为了证实所提出的机制，通过使用电子背散射衍射 (EBSD) 研究了样品的微观结构演变。研究发现，声学软化和超声动态冲击效应都会产生严重的塑性变形，从而产生更细的晶粒。然而，在超声动态冲击作用下，顶部（变形区）的小角度晶界甚至小于中心的小角度晶界，这表明晶界旋转更显着和/或极端局部加热。振幅与试样高度的比值被确定为同一材料发生超声动态冲击效应的影响指标，证实了超声振动在微成形中的广阔前景。本研究的发现有助于为理解超声辅助微成形的潜在机制提供基础，并为下一步设计工艺提供指导。