Investigating ultrasound-induced acoustic softening in aluminum and its alloys

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.