Evolution of acoustic softening effect on ultrasonic-assisted micro/meso-compression behavior and microstructure
Evolution of acoustic softening effect on ultrasonic-assisted micro/meso-compression behavior and microstructure
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DOI：
10.1016/j.ultras.2020.106107
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发表时间：
2020-09-01
期刊：
影响因子：
4.2
通讯作者：
Yang, Ming
中科院分区：
文献类型：
--
作者：
Hu, Jun;Shimizu, Tetsuhide;Yang, Ming
超声振动技术的应用是解决微/细观成形工艺问题的有效方法。超声振动可以降低成形过程中的流动应力，称为超声体积效应。体积效应包含应力叠加导致表观平均应力降低、声软化和超声冲击导致真实的应力降低等多种机制。然而，材料变形过程中声软化的演化特征和机理尚不清楚。而以往的研究中，由于动态力传感系统的方便，只测量了平均应力而没有测量振荡应力，混淆了超声体积效应、声软化效应、应力叠加效应和超声冲击效应。本研究的目的是探讨声软化对细观/细观压缩行为和组织演变的影响。研制了一种基于动态力传感技术的超声辅助压缩试验系统。并结合EBSD显微组织分析，对C1100 O纯铜进行了一系列不同幅值的超声辅助细观压缩试验。通过对振动过程中应力波形的分析，成功地将声软化与应力叠加分离开来，并发现变形应变对声软化的影响很大。随着流动应变或超声振幅的增大，声软化引起的应力降低量增大。此外，在小应变和大应变之间存在着声软化率的演化过渡。当发生超声软化时，小角度晶界在晶粒中随机分布，而非超声辅助时的堆积分布，这意味着超声软化改善了小角度晶界或位错的运动，导致真实的应力降低。此外，在小变形应变下，晶粒细化为等轴晶，位错密度显著降低，这可能是由于超声诱导动态回复导致位错湮灭增加的结果。但随着变形量的增加，声硬化作用逐渐增强，导致声软化对位错密度降低的作用减弱。本研究结果提供了一个有益的理解超声辅助微/细观成形中的声学软化的潜在机制。
The application of ultrasonic vibration is an effective method to overcome the processing problems in micro/meso-forming. Previously it was observed that ultrasonic vibration could reduce flowing stress in the forming process, called ultrasonic volume effect. The volume effect contains multi-mechanisms such as stress superposition leading to apparent average stress reduction, acoustic softening and ultrasonic impact leading to real stress reductin. However, the evolutional characteristics and the mechanism of acoustic softening on material deformation is still not clear. And in most previous studies only the average stress but not the oscillatory stress was measured due to the convenience of dynamic force sensing system, which confused the different ultrasonic volume effects, acoustic softening, stress superposition and ultrasonic impact. The purpose of this study is to investigate the effects of acoustic softening on micro/meso-compression behavior and microstructure evolution. An ultrasonic-assisted compression test system with dynamic force sensing technology was developed. And a series of ultrasonic-assisted micro/meso-compression tests at different amplitudes were carried out on pure copper C1100O combining the microstructure analysis by EBSD technique. By analyzing the waveform of the oscillatory stress in the process, acoustic softening was successfully separated from the stress superposition and it was found that the deformation strain plays an important role on the effect of acoustic softening. The stress reduction by acoustic softening increases with the flowing strain or ultrasonic amplitude increasing. Besides, there is an evolutionary transition of acoustic softening ratio between small strain and large strain. When acoustic softening occurs, the low-angle grain boundaries distribute randomly in grains, compared to the piled distribution without ultrasonic assistance, implying motions of the low-angle grain boundaries or dislocation is improved by acoustic softening, resulting in the real stress reduction. In addition, with small deformation strain, the elongated grain becomes equiaxed and dislocation density is significantly reduced, which may be the result of the increased dislocation annihilation due to ultrasonic-induced dynamic recovery. However, with the deformation strain increasing to some extent, acoustic hardening gradually becomes significant, leading to much less effectiveness of acoustic softening on dislocation density reduction. The findings of this study provide an instructive understanding of the underlying mechanisms of acoustic softening in ultrasonic-assisted micro/meso-forming.