Micro-hardness of annealed and work-hardened copper polycrystals

Abstract

Strengthening effects in materials bonded by the high strain rate plastic deformation process, Ultrasonic Additive Manufacturing (UAM), were investigated. Aluminum (Al 6061) was pretreated by tempering and annealing prior to bonding through UAM. Following UAM, multiscale material characterization was performed. Tensile testing in the rolling (x) direction demonstrated the material became harder after the UAM process, and nanoindentation demonstrated the foil-foil interfaces became harder than the bulk foil material. The strengthening effects are a result of microstructure changes at the interfaces and in the bulk foil regions which were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. These microstructure changes result from dynamic recrystallization, dynamic recovery, adiabatic heating, and precipitate dissolution. This study signifies the metallurgical features creating mechanical strength increases, rather than decreases, in UAM builds.