Conventional high power ultrasonic vibration has been widely used to improve manufacturing processes like surface treatment and metal forming. Ultrasonic vibration affects material properties, leading to a flow stress reduction, which is called ultrasonic volume effect. The volume effect contains multi-mechanisms such as stress superposition due to oscillatory stress, acoustic softening by easier dislocation motion and dynamic impact leading to extra surface plastic deformation. However, most researches ignored the stress superposition for the convenience of measurement, and few studies considered ultrasonic dynamic impact since the relatively low ultrasonic energy in macro scale. The purpose of this study is to investigate the characteristics and mechanisms of different ultrasonic volume effects in micro-forming. A 60 kHz longitudinal ultrasonic-assisted compression test system was developed and a series of ultrasonic-assisted compression tests at different amplitudes on commercially pure aluminum A1100 in micro-scale were carried out combining the surface analysis by SEM, EDX and micro-hardness test. Three different ultrasonic volume effects, stress superposition, acoustic softening and dynamic impact, were confirmed in the ultrasonic-assisted compression tests. In order to quantitatively predict stress superposition, a hybrid model for stress superposition is developed considering the elastic deformation of experimental apparatus in practice, the evolution of the modeling results fitted well with the experimental results. With low ultrasonic amplitude, stress superposition and acoustic softening occurred because vibrated punch contacted with the specimen all the time during compression. However, with higher amplitude, due to the extra surface plastic deformation by larger ultrasonic energy, forming stress was further reduced by the ultrasonic dynamic impact. A possible method to distinguish the effects of dynamic impact and acoustic softening is to analyze the waveform of the oscillatory stress in the process. In the case of ultrasonic dynamic impact effect, a higher amount of oxidation was observed on the specimen surface, which could be the result of local heating by surface plastic deformation and surface friction when the vibrated punch detached from the specimen. The findings of this study provide an instructive understanding of the underlying mechanisms of volume effects in ultrasonic-assisted micro-forming.

常规的高功率超声振动已被广泛用于改善制造工艺，例如表面处理和金属成型。超声波振动会影响材料性能，导致流应力降低，这称为超声波体积效应。体积效应包含多种机制，例如由于振荡应力导致的应力叠加，易位错运动引起的声软化以及导致额外的表面塑性变形的动态冲击。但是，为了方便测量，大多数研究都忽略了应力叠加，并且由于宏观尺度上相对较低的超声能量，很少有研究考虑超声动态冲击。这项研究的目的是研究微成形中不同超声体积效应的特征和机理。研制了60 kHz纵向超声辅助压缩试验系统，结合SEM，EDX表面分析和显微硬度试验，对商用纯铝A1100进行了一系列不同幅度的超声辅助压缩试验。在超声辅助压缩测试中，证实了三种不同的超声体积效应，即应力叠加，声软化和动态冲击。为了定量地预测应力叠加，在实践中考虑了实验装置的弹性变形，建立了应力叠加的混合模型，其建模结果的演化与实验结果吻合得很好。超声波振幅低 由于在压缩过程中振动的冲头始终与试样接触，因此发生了应力叠加和声音软化。但是，在更大的振幅下，由于更大的超声能量会引起额外的表面塑性变形，因此通过超声动态冲击会进一步降低成型应力。区分动态冲击和声软化效果的一种可能方法是分析过程中的振荡应力波形。在超声动态冲击效应的情况下，在试样表面观察到较高的氧化程度，这可能是当振动的冲头从试样上脱离时，由于表面塑性变形和表面摩擦而引起的局部加热的结果。