Microstructures and fatigue properties of friction stir lap welds in aluminum alloy AA6061-T6

Microstructural features and fatigue properties of friction stir lap-welded joints for AA6061-T6 alloy are investigated and the influences of single pass welding (SPW) or double pass welding (DPW), hooking defects and fatigue stress ratio on the fatigue properties are analyzed. It is found that the fatigue strengths of FSW lap-welded joints are obviously lower than that of the fusion lap-welded joints of IIW FAT22 and only approximately correspond and close to the IIW FAT12 design curve. The higher fatigue stress ratio R will lead to the decrease of the fatigue strength of lap-welded joints. The existence of hooking defects is the key factor to reduce the fatigue strengths. Fatigue cracks always initiate at the tip of hooking defect in the RS of SPW and AS of DPW joints respectively. The severity of hooking defects and the quality of lap-welds could not be improved by the DPW process as compared with the SPW process. The fatigue strength Delta sigma(m) and Delta sigma(k) (R = 0.1) of DPW joints will be 21.26% and 17.88% lower than that of SPW joints respectively. The fracture of lap-welded joints exhibits multiple crack initiations from the tips of the hooking locations, and the crack propagation shows some brittle fracture and is mainly characterized by the fatigue striations. The locations of crack initiation and the amount of secondary cracks in DPW joints are more than that 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 deep-hole type dimples in the SPW joints but shallow-hole type dimples in the DPW joints. (C) 2011 Elsevier Ltd. All rights reserved.