Clinching is one of a remarkable joining method for aluminum sheets because of its high productivity, recyclability and possibility to reduce costs. We have studied about the effects of ultrasonic vibrations on the improvement of peeling strength of clinched joint. Through this study, we found a remarkable phenomenon named stirring phenomenon by ultrasonic stress vibrations; in case ultrasonic stress vibrations were applied to clinching tools, vertical material flows were created and materials were stirred and unified finally. This phenomenon can be a core technology for a new joining method and a new grain refinement method. Therefore, clarification of generation mechanism of this phenomenon is required. In this study, thin aluminum sheets were clinched under various conditions of ultrasonic vibrations. And we observed metallographic changes in the earlier stage of the vortex initiation. As a result, the location where the driving force of stirring effect was generated was revealed. And mechanism for generation of this driving force was discussed considering a unique metallographic structure.
