Ultrasonic metal welding (USMW) is widely used in assembling Lithium-Ion battery packs due to its advantages in joining dissimilar and conductive materials. However, the welding process and mechanisms have not been well understood. In this research, the weld quality of two dissimilar materials, namely, aluminum (AW-1050A) with copper (CW008A, R240) sheets were investigated under different process parameters. The lap shear tests were conducted to evaluate the mechanical performance of Al/Cu joint. Signals from the ultrasonic generator were recorded and used to describe the welding process. In addition, the temperature development of the welding zone was measured using thermometry and infrared thermography during USMW. A high–speed image capturing was applied to investigate the in-situ oscillation behavior of the sonotrode, joining parts, and anvil. The collected data from the welding system as well as external measurement are correlated to the weld quality to characterize the joint formation process. The results give some new insights into the fundamental understanding of USMW and provide guidance to improve the weld quality and process stability.