| In this study, we selected the aluminum alloys of 2024-T4, 6061-T6 and 5A06-H112 as the object. Microstructural features and fatigue properties of friction stir lap-welded joints were detailed investigated. Firstly the microstructures and the hardness distributions of the lap joints were studied. Especially the hooking defects existing in lap joints and the effects of which on the fatigue properties of FSW lap-welded joints were investigated. Then fatigue specimens of the three aluminum alloys made by SPW and DPW lap joints were tested under load control with different fatigue stress ratio (R). The fatigue strengths of FSW lap joints were compared with that of fusion lap-welded joints of the IIW recommendations. Besides the influences of fatigue stress ratio R, single pass welding (SPW) or double pass welding (DPW), and base materials on the fatigue properties were analyzed. At last the fatigue fracture morphologies were analysed by using SEM to research the fatigue fracture mechanism of lap joints.Metallographic test results show that two notch-like unwelded zones are present at overlap ends, namely the hooking defects, which is a significant difference from FSW butt joints. The hooking defect is an adverse interface reorientation. It causes sharp discontinuities along the interface and creates acute tip where the stress becomes highly concentrated. As the consequence of the tip of hooking interface extending into top sheet, the EST of the lap joint is obviously reduced. Meanwhile the higher fatigue strength was achieved at the larger EST in aluminum alloys. So the thinner of top sheet thickness for lap joints caused by the hooking defect should decrease fatigue performance. Fatigue cracks always initiate at the tip of the hooking defect with respect to the loading side of the upper sheet and then propagate into the NZ leading to the final fracture of fatigue specimens. As a whole, the severity of hooking defects and the quality of lap joints could not be improved by the DPW process as compared with the SPW process.Fatigue test results show that the fatigue strengths of friction stir lap-welded joints are obviously lower than those of the fusion lap-welded joints of IIW FAT22, and only approximately correspond and close to the IIW FAT12 design curve. For friction stir lap-welded joints in aluminum alloy 2024, the largest calculated characteristic valueΔσk is 20.04MPa when R is 0.1. Only when R=0.3 theΔσk is 8.32MPa and others are all larger than the FAT12 design value. For friction stir lap-welded joints in aluminum alloy 6061, theΔσk are between 10.65MPa and 15.27MPa. But the valueΔσk of friction stir lap-welded joints in aluminum alloy 5A06 is only 7.18MPa which has the worst fatigue property. Besides the higher fatigue stress ratio R will lead to the decrease of the fatigue strength of lap joints. The effects of base materials on the fatigue properties of friction stir lap-welded joints are little or can be ignored in the lower stress range. But aluminum alloy 2024 lap joints with better tensile strength have larger fatigue strength than aluminum alloy 6061 lap joints with better plasticity.Fatigue fracture analyses show that the fracture of lap joints exhibits multiple crack initiations from the bottom of the upper plate corresponding to the hooking locations, and the crack propagation shows some brittle fracture and is mainly characterized by the fatigue striations in part of the joints. The locations of crack initiations and the amount of secondary cracks in DPW joints are more than those of in the SPW joints which leading to the lower fatigue properties. The ductile fracture mode is obviously shown in the final fracture zone with dimples and obvious tear ridges. |
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