Ultrasonic metal welding is a rapid bonding technology that combines ultrasonic softening and high-frequency friction. However, the massive dislocation proliferation in high-strain-rate deformation is contradictory to rapid dynamic recovery induced by the ultrasonic vibration. To investigate the contribution of ultrasonic to the joint microstructure, the Cu single crystals with [001], [101], [111] orientations were selected to trace the abnormal deformation microstructure. It was indicated that the ultrasonic excitation broke the correlation between geometrically necessary boundaries and grain orientation, which resulted in a similar micro-hardness in the range of 112.5–116.0 HV near the interface in three single crystal joints. An analytical model of interfacial friction deformation was proposed to prove the abnormal enlargement of dislocation cells under ultrasonic excitation.
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