Microstructure evolution of Al6061 and copper during ultrasonic energy assisted compression

Metal forming is often performed at elevated temperatures, typically above the recrystallization temperature of metals, which results in a reduction in yield stress during forming. Similar reduction in yield stress can also be achieved by simultaneous application of ultrasonic energy during deformation. The resulting softening of metals causes a reduction in the forming forces and increased ductility. A thorough understanding of the effect of ultrasonic energy irradiation during deformation on the microstructure is lacking which seems to be inhibiting wider adoption of ultrasonic energy in more manufacturing processes. In this manuscript, a detailed analysis of the microstructure of aluminum 6061 alloy and copper, which have identical Faced-Center Cubic crystal structure but different stacking fault energies, has been presented. Focus has been placed on the differences between their microstructure evolution behaviors after deformation under simultaneous application of ultrasonic energy. These differences have been distinctly highlighted using Electron Backscatter Diffraction (EBSD) and TEM analysis. EBSD analysis revealed assistance to subgrain formation due to ultrasonic energy. It was also observed that due to lack of dynamic recovery, copper underwent discontinuous dynamic recrystallization (DDRX) during ultrasonic energy assisted deformation. On the other hand, the recrystallization mechanism in Al6061 alloy was found to be geometric dynamic recrystallization (gDRX). The insights obtained in this work potentially advance the understanding of mechanisms governing microstructure evolution during ultrasonic energy assisted deformation and establish analogy between the mechanisms governing thermomechanical processing and ultrasonic energy assisted deformation.