Dissimilar T-lap joint between aluminum alloy 7075-T651 and 5083-H116 was fabricated by friction stir welding. Special focus was paid to the defect formation and mechanical properties which were significantly affected by the welding parameters. The results showed that four typical types of defects were detected in the T-lap joint; tunnel, kissing bond, hook, and bonding line defects. The tunnel and bonding line defects could be avoidable by decreasing the welding rate. At low welding rate, however, the hook defect was formed and reduced the skin effective thickness. The kissing bond defect was too hard to be eliminated by using one welding pass only by changing the welding parameters. Among these defects, the hook defect played the important role in the mechanical properties of T-lap joint. It is a special finding that there was the optimum welding rate by which the maximum joint efficiency was attained. These findings were summarized into the table and map which can optimize the joint properties.

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The high temperature strength of the friction welded superalloy joint between cast polycrystalline Mar-M247 and the forged IN718 alloys has been investigated under low cycle and thermomechanical fatigue loadings, in comparison with those of the base materials. The experiments showed that the lives of the dissimilar joints were significantly influenced by the test conditions and loading modes. Not only the lives themselves but also the failure positions and mechanisms were sensitive to the loading mode. Especially under the loading conditions in which the creep-fatigue interaction was pronounced, some specific failure phenomena appeared in the DFW joint. The fracture behavior dependences on the loading modes and test conditions were discussed, based on analyses of the macroscopic elastic follow-up mechanism, the stress state by finite element method, and the inhomogeneous microstructure in the joint.

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Effects of tool offset and reversed metal flow on mechanical properties of dissimilar friction stir welded T-lap joints between AA7075 and AA5083 AbstractThe effects of tool offset and the reversed metal flow induced by a double-pass friction stir welding (FSW) on the welding interface characteristics were investigated in a dissimilar thickness T-lap joint between 7075-T651 and 5083-H116 aluminum alloys. The performance of the joint was evaluated by the skin and stringer tensile tests. The experimental results showed that the bonding interface could be improved by applying double-pass welding which the tool was offsetted toward advancing side. An employment of the double-pass welding significantly improved the strength so that the joint efficiency reached about 90% in comparison with the 5083 base metal strength while the tensile strength of the joint produced by the single-pass welding was quite low. The kissing bond defects were found to be the main culprit in the tensile behavior of the T-lap joint. The reversed metal flow and the gap distance between pin and die played a key role in the formation of the welding interface.

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