A linear vibration locus is conventionally used in ultrasonic metal welding. We have previously proposed the use of a nondirectional planar vibration locus as a means of improving the overall and orientation-dependent weld strength compared with the case of using a linear vibration locus. In this study, experiments in which a copper plate and an aluminum plate were used as welding targets were conducted and the weld strength under various conditions was measured to assess the welding characteristics. We found that for a short welding time, the strength of a weld produced using a planar vibration locus was 1.7-fold that produced using a linear vibration locus. We attribute this result to the aluminum plate vibrating more easily, and thus creating stronger vibrations at the interface between the copper and aluminum plates, when we used the nondirectional planar locus.