Characterization of Acoustic Softening of Aluminum 6061 within a Plasticity Framework

Ultrasonic additive manufacturing (UAM) is a rapid prototyping technology that features a metal joining process through ultrasonic welding. The bonding mechanisms and mechanics of UAM have been investigated for decades. Meanwhile, the plastic deformations of metals were extensively studied by many researchers for their significant roles in bond formation. However, most of these research efforts considered solely the surface frictional effects on plastic deformation whereas the volumetric effects of ultrasound were rarely considered. This paper investigates the effects of ultrasound on deformation of Aluminum 6061 through experimental studies and highlights the volumetric effects of ultrasound, i.e. the “acoustic softening”: a stress reduction on the stress-strain relation of Aluminum 6061 upon application of ultrasounds. Based on observations obtained from a designed experimental setup, a phenomenological model is proposed to characterize the acoustic softening effects in terms of the ultrasonic intensities. Additionally, by modifying Hockett’s plasticity model, a plasticity frame work is established to characterize the deformation of Aluminum 6061 in UAM. The acoustic softening model is then incorporated into the plasticity framework. The complete model is then validated by comparing its predictions with experimental measurements.