近年，地球温暖化防止の観点から，環境保護や省エネ ルギーが強く意識されるようになり，特に自動車分野で は排ガス規制の強化や燃費向上の点で，車体の軽量化が 必要となっている。このため，軽量部材の適材適所化は 拡大傾向にあり，特にアルミニウム合金と鋼の異種金属 接合は回避できない課題となっている。%Friction stir spot welding (FSSW) was applied to dissimilar lap joint between aluminum alloy sheet and mild steel sheet without formation of brittle intermetallic compound at the weld interface. A sufficient strength of the dissimilar joint was obtained by stirring aluminum alloy near the interface without inserting a welding tool to steel surface from the aluminum alloy side. Consequently, joint strength of FSSW was higher than that of resistance spot welds, and the all of FSSW joints showed a plug fracture through aluminum alloy parent sheet. Moreover, it was suggested that joint strength was related to area of stir zone at the weld interface. An oxide film was observed partially in the stir zone by sweeping up from the interface. No brittle intermetallic compound was observed at the weld interface. But only the thin amorphous layer of several nm in thickness including oxygen was found at the weld interface.
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