Ultrasonic vibration-induced severe plastic deformation of Cu foils: effects of elastic-plastic stress wave bounce, acoustic softening, and size effect.

In this work, the unique plastic deformation behavior of T2 rolled copper foils under ultrasonic impact (UI) and ultrasonic rolling (UR) has been studied systematically. The numerical simulation and test results show that with the decrease of the thickness of copper sheet, the equivalent plastic strain (PEEQ) value and plastic strain εp formed by UI or UR increase exponentially, and its distribution changes from gradient to uniform on the cross section, which is distinctive from the commonly observed gradient structure in ultrasonically treated bulk materials. Correspondingly, characterized by EBSD and TEM, the 100-μm-thick copper foil treated by UR shows the overall strengthening and forming a sub-micron grain structure with significant refinement and uniform distribution on the cross section. The microstructure homogenization of copper foil is attributed to the rigid wall effect facilitating the elastic-plastic stress wave bounce. The exponential increase of PEEQ and εp is explained as that with the decrease of the thickness of ultrasonic object, on the one hand, the acoustic softening and size effect which can reduce the elastic limit of copper foil are enhanced; on the other hand, the plastic deformation mode changes from stress superposition to dynamic impact. Through the establishment of a new theoretical model, the criterion of ultrasonic dynamic impact effect is deduced and verified by experiments. [ABSTRACT FROM AUTHOR]