Understanding the Effect of Acoustoplasticity on the Microstructural Evolution and Hardness of Copper

Acoustoplasticity has been used to join thin layers of metallic materials and therefore it is considered to be a welding technique. In this work, acoustoplasticity was performed on 6mm thick pure copper plate, with an average grain size of 20um, to study the effects of frequency (varied from 1000-4000Hz), downforce (varied from 4000-6000N), and power (varied from 4000-6000W) on the surface hardness and microstructural evolution. Scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and Vickers hardness characterization revealed at least three identifiable microstructural zones within the top 600um from the acoustoplastically-deformed surface, depending on the acoustoplacitiy conditions (i.e power, downforce, and frequency). The upper zone contained refined equiaxed grains ranging between 5um to 15um. The intermediate zone exhibited a crystallographic reorientation zone containing wavy elongated grains. The third zone contained a separate crystallographic reorientation region. The hardness was greatest within the grain refined region and reduced gradually with increased distance from the surface until it plateaued to the values obtained for the as-received copper plate. The results suggest that acoustoplasticity may be used on bulk samples for grain refinement applications other than welding and joining.