Tensile Shear Properties of the Friction Stir Lap Welded Joints and Material Flow Mechanism Under Pulsatile Revolutions
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N stir welding (FSW) is a solid-state joining process invented by The Welding Institute in the United Kingdom.[1] Compared to traditional fusion welding, FSW is characterized by fewer metallurgical defects and residual distortion, and such a solid-state welding technique has been highlighted to fabricate the joints of aluminum alloys.[2–4] Furthermore, lap joints of aluminum alloy are widely adopted in aerospace, shipbuilding, railway, and automobile industries,[5–7] including the fabrication of aircraft skins, ship decks, train sidewalls, and car pillars.[8–11] Thus, a great deal of research on friction stir lap welding (FSLW) of aluminum alloys has emerged in response.[12–14] Taking the principle of FSW into consideration, a lap interface is more difficult to completely eliminate than a butt interface.[15] Apart from groove and cavity defects, the hook defect (HD) and ‘‘cold lap defect’’ (CLD) are two kinds of typical interface defects that emerged in the FSLW joints. The original interface on the advancing side (AS) of a lap joint that folds upward along the boundary of the stir zone (SZ) is defined as HD. The original interface on the retreating side (RS), first
YANYING HU, HUIJIE LIU, and SHUAISHUAI DU are with the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted March 1, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
extending upward and then penetrating toward the SZ, is defined as the CLD.[7] The HD and CLD can significantly reduce the effective sheet thickness (EST) and effective lap width (ELW) of a lap joint, hence leading to poor tensile shear properties.[16] In essence, the tensile shear properties of the FSLW joints are determined by the size of the interface defect. The effects of rotational speed, welding speed, and pin length on the microstructure, tensile shear property, and fatigue resistance were reported in the literature.[17–19] Moreover, the published results show that the pin profile significantly influences the HD and CLD morphologies and then strongly affects the joint strength and the failure mode. Salari et al.[20] welded 5456 aluminum alloy sheets using four kinds of tools and found that the highest tensile shear property of the lap joint was obtained by the stepped conical threaded pin. Buffa et al.[18] found that the cylindrical–conical pin was more appropriate for making lap joints than a straight cylindrical pin. Al clad sheets of 2014-T4 aluminum alloy were friction stir lap welded by Babu et al.[5] using a triangle pin and a conical threaded pin. They found that the conical threaded pin can obtain stronger lap joints. Although these studies cover several kinds of tools, a systematic analysis is still lacking and, thus, the contributions of detailed structures, such as threads and planes on the pin, have not been comprehensively investigated. The previous analyses of tensile shear properties are limited to the size and orientation of interface defects. The rela
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