Ultrasonic metal welding (USMW) has proven to be a suitable technique for joining electronic components such as battery cells. However, despite its widespread application, the USMW process still suffers from considerable quality fluctuations. The fluctuations largely originate in the lack of profound scientific knowledge and understanding of the interdependency between the tools and joining members. Therefore, this paper aims to investigate the in-situ oscillation behavior of the USMW process by using a high-speed image capturing. The recorded image-sequences are analyzed by a two-frame motion Gunnar Farnebäck algorithm. A two-dimensional vector field is obtained from which the displacement and relative movement between the tools and the joining members can be computed. Different welding parameters and metal alloys (AW-1050A, CW-008A) are set to study their effects on welding sequence. Microstructural investigations and T-peel tensile testing are used to validate the high-speed observations. Based on the measured amplitude profiles, USMW can be divided into six characteristic welding stages. Additionally, the measurements illustrate the dependence of the bond formation on the welding parameters and in particular on the welded materials. The results provide a more profound understanding of the thermo-mechanical mechanism during USMW and will introduce a prospective improvement in quality and process stability.