Mao Q, Coutris N, Rack H, Fadel G, Gibert J
Department of Mechanical Engineering, Clemson University, SC 29634, United States.
Department of Mechanical Engineering, Clemson University, SC 29634, United States.
Ultrasonics. 2020 Mar;102:106005. doi: 10.1016/j.ultras.2019.106005. Epub 2019 Sep 5.
Ultrasonic vibration has been observed to lower the flow stress necessary to initiate plastic deformation, a phenomenon known as "acoustic softening". This unique effect of ultrasound has been extensively applied in welding, machining, forming of metals, and ultrasonic additive manufacturing to lower the yield stress necessary to initiate plastic deformation, it nevertheless lacks fundamental investigation. Some prior studies showed experimental errors due to the design of experimental setups and the associated testing methods that have been introduced, leading to questions about their observations and conclusions. Therefore, an experimental setup described in this paper is designed to minimize the constraints identified from the setups in prior studies. Three types of aluminum are studied: Al 1100-O a commercially pure aluminum, Al 6061-O an aluminum alloy without precipitate strengthening, and Al 6061-T6 a precipitate-strengthened aluminum alloy. The acoustic softening and residual effect are compared based on the similarities and differences in microstructures of the three types of aluminum. In both acoustic softening and residual effect, linear relations are obtained between stress change and ultrasound intensities. The slope defined by the linear relations, i.e. the acoustic softening factor, depends on the microstructure of the specific material. The underlying mechanism of acoustic softening is associated with the activation of dislocations by ultrasonic energy and subsequently their interactions with other dislocations and precipitates, whereas the residual effects are attributed to the permanent changes in dislocation density due to dislocation annihilation, dynamic annealing, and dislocation-precipitate interaction.
超声振动已被观察到能降低引发塑性变形所需的流动应力，这一现象被称为“声学软化”。超声的这种独特效应已在焊接、加工、金属成型及超声增材制造中广泛应用，以降低引发塑性变形所需的屈服应力，然而它缺乏基础研究。一些先前的研究由于所引入的实验装置设计及相关测试方法而出现实验误差，这引发了对其观察结果和结论的质疑。因此，本文所描述的实验装置旨在将先前研究中装置所存在的限制降至最低。研究了三种类型的铝：商业纯铝Al 1100 - O、无析出强化的铝合金Al 6061 - O以及析出强化铝合金Al 6061 - T6。基于这三种类型铝微观结构的异同，对声学软化和残余效应进行了比较。在声学软化和残余效应方面，均获得了应力变化与超声强度之间的线性关系。由线性关系所定义的斜率，即声学软化因子，取决于特定材料的微观结构。声学软化的潜在机制与超声能量激活位错以及随后位错与其他位错和析出物的相互作用有关，而残余效应则归因于位错湮灭、动态退火以及位错 - 析出物相互作用导致的位错密度的永久性变化。
