Energy efficient methods for joining aluminium to steel have potential for major applications in the automobile industry. Results are reported where 1 mm gauge 6111 aluminium and DC04 steel automotive sheet have been successfully joined using the novel process of high power ultrasonic spot welding. The microstructure and performance of the joints were examined, as a function of the welding conditions. A maximum failure load of 2·8 kN was obtained in lap shear tests, compared with 2·9 kN for comparable AI-AI joints. The failure mode changed from interfacial debonding to nugget pullout and back to interfacial failure as the weld energy increased. A thin <2 μm reaction layer of FeAI3 and Fe2Al5 formed at the interface with increasing weld energy. The factors determining the joint characteristics, interface reaction, and the microstructure evolution in the welds are discussed.

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