Ultrasonic welding phenomenon of pure aluminum sheets was observed using high speed video camera. The dynamic vibration behaviors of the edges of a welding tip, and a pair of aluminum sheets (upper specimen, lower specimen) were analyzed from the captured images using digital image correlation method. The following results were obtained. The welding process was comprised of following three stages. First, the upper specimen vibrated accompanying with small slipping between the edge and the upper specimen. The upper specimen thereby moved to one side along the vibration direction (Welding process I). In the second process, the vibration amplitude of the upper specimen decreased, while the friction between the edge and the upper specimen relatively increased (Welding process II). Then, the vibration amplitudes of the upper specimen and the lower specimen increased and the welding region plastically deformed (Welding process III). Joint strength increased with the welding time. In the fracture surface observation, partially welded regions were observed in the welding time corresponding to the welding process I, and growth of the welded regions was confirmed in the welding process II. The maximum strength was obtained in the welding process III, while adhesion to the welding tip and breakage of welded joints caused by compressive plastic deformation of welding region occurred in some specimens. The edges of a welding tip penetrated to the upper specimen with friction and plastic deformation of the upper specimen during the welding processes I and II. The penetrated depth saturated in the welding process II and the tip indentation forms on the surface of upper specimen.
