A novel spot welding process for dissimilar metal lap joints using a new tool with the tip made of spherical ceramics, i.e., "Friction Anchor Welding,'' was applied to three lapped sheets that consisted of an Al alloy (1.0-mm-thick A5052) and two steels (0.6-mm- and 1.0-mm-thick SPCC). Consequently, a steel projection was formed in the Al alloy sheet, and the sheets were welded at a plunge depth greater than 1.4 mm. The height of this steel projection increased with the plunge depth when the plunge depth was less than 2.2 mm; accordingly, the weld strength increased, and the tensile shear strength and the cross tensile strength reached approximately 3.8 kN/point and 2.5 kN/point, respectively. However, when the plunge depth was greater than 2.2 mm, the tip of the steel projection peeled off from the Al alloy, followed by cracks formation along the steel projection and the Al alloy interface, and the weld strength consequently decreased.

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The friction stir welding (FSW) of 5052-O and 5052-H34 aluminum alloys were performed to investigate the mechanical properties of the joints. The mechanical properties of the joints were evaluated by tensile tests and Vickers hardness distribution. The experimental results showed that tensile properties of the joints are significantly affected by the welding process parameters, such as welding speed and rotation speed. It was found that the joint strength of A5052-O joints could be made approximately equal to the strength of the parent metal by using a wide rotational speed and high welding speed. On the other hand, the joint strength of A5052-H34 joints was lower than that of the parent metal. When the welding speed was lower, the joint strength was the same as A5052-O joint. However, when the welding speed was increased, the joint strength was decreased rapidly. Therefore, the effect of friction stir welding on the heat treatment of 5052 aluminum alloys was clarified.

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7075-T651 aluminum alloy sheets with 5 mm in thickness were friction stir welded using a numerical controlled friction stir welder. The effects of tool inserting speed and preheating time on microstructures and mechanical properties of friction stir welded joint were investigated. Regardless of welding conditions, the microstructures of stir zone showed finer than that of the base metal. A Kissing bond was not found in the start position of the welding, however it was observed in the end position of the welding regardless of welding conditions. The hardness of stir zone and heat affected zone of the joint was decreased than that of the base metal. In case of the tool inserting speed of 2 mm/s and preheating time of 0.1 s, the softened area of the start position of the welding was narrowed. Maximum tensile strength at the start position of the welding under conditions of tool inserting speed of 2 mm/s and preheating time of 0.1 s showed 492 MPa which is 84％ of tensile strength of the base metal.

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