In ultrasonic welding, the interface plastic flow caused by the coupled effects of thermal and mechanical fields represents the fundamental interface behavior for the bond formation. The high strain rate and heat flux on the bonding interface are believed to produce the plastic flow that enhances the formation of a wavy interface (hydrodynamic instability). To understand the mechanism for bond formation, a 2-scale thermo-mechanical coupled FE model has been developed to study the interface plastic flow. Temperature-dependent material properties and experimentally determined Al-Al friction coefficients have been input to the global model which provides the boundary conditions on the interface for the local model. The local model has been built at different locations along the interface. A transient dynamic analysis is conducted on both the global and local thermo-mechanical models. Temperature and strain fields of the bond region as a function of time are obtained. The wavy-shaped interface plastic flow field has been predicted by the models. The simulation results are able to provide insights for understanding the experimental observations.