Friction stir spot welding (FSSW), a solid state welding process, can join materials that are difficult to joint by fusion welding processes. In this study, similar FSSW of lightweight alloy sheets, such as aluminium and magnesium alloys, was performed using tools without thread. The histories of vertical plunge load applied to lapped sheets and the temperature distribution around the stirred surface of the upper positioned sheet during friction stirring were examined. Friction stir spot welding characteristics were discussed regarding the lap shear failure load and fractographic observations after tensile shear tests. A retracted tool produced the highest lap shear failure load for similar aluminium welds, while a pin tool produced the highest lap shear failure load for similar magnesium welds. The failure mode that brought a combination of shear and pullout fracture produced the highest lap shear failure load, which was closely related to the geometry of the tool that developed the enlarged fracture path.
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