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Dissimilar joints of copper to aluminium were produced by high power ultrasonic welding (USW). The interfacial reaction between copper and 6061 aluminium alloy as a function of welding time was studied. The intermetallic compound (IMC) layer is mainly composed of CuAl 2 and Cu 9 Al 4 . The thickness of the IMC layer increases with the welding time. For a relatively long welding time (0?7 s) in USW, the dendritic solidification microstructure was observed in local regions, owing to the occurrence of the eutectic reaction, a-AlzhRL, in the welding process. The lap shear load (or strength) of the joints first increases and then decreases with increasing welding time, and the failure of the joints occurred dominantly at the interface. This is mainly attributed to the development of IMC layer at the interface.

The effects of welding energy on the mechanical and microstructural characteristics of ultrasonic-welded pure copper plates were investigated. Complex dynamic recrystallization and grain growth occurred inside the weld zone during ultrasonic welding. At a low welding energy, a thin band of straight weld interfaces was observed and had an ultra-fine grain structure. With an increase in welding energy, the weld interface progressively changed from flat to sinusoidal, and eventually turned into a convoluted wavy pattern, bearing similarities to shear instabilities, as observed in fluid dynamics. The lap shear load of the joints initially increased and then remained stable as the welding energy increased. The tensile characteristics of the joints significantly depended on the development of plastic deformation at the interface. The influence of the microstructure on the hardness was also discussed.

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