In this study, mechanical and thermal behavior of the mating interface during ultrasonic metal welding is investigated using molecular dynamics (MD) simulations. In ultrasonic welding process, the reciprocating motion of the sonotrode together with the application of the external pressure on the mating parts are the sources of friction heat generation, high temperature gradient at the interface and plastic deformations. The rapid process of ultrasonic welding, which takes a few seconds at the longest, involves coupled mechanical and thermal processes. Therefore, MD simulations have been employed to elucidate the nano-mechanics of this complex coupled process within the picosecond timescale. To this end, the atomic scale simulations of the microstructure at and in the vicinity of the mating interface have been carried out. This contribution addresses the interactive effects of the process parameters on the interface temperature evolution and the diffusion behavior of the interface atoms at the atomic scale. The results of this work are compared to the results from macro scale investigations.