Recently, fatigue tests have been performed to investigate the fatigue behavior of friction stir butt-welded aluminum alloy joints. However, the joints of more than 10 mm thick plates have rarely been tested, nor has the effect of the inclined angle of the friction stir welding line to the applied stress been studied. In this research, fatigue tests were carried out on friction stir butt-welded joints with or without welding flash. The joints had a friction welding line that was inclined angles of 0°, 30°, 60° and 90° to the perpendicular direction of the applied stress. Observations of the fatigue failure modes and fracture surfaces indicated that fatigue cracks tended to initiate from the base material or the heat-affected zone when the inclined angle was 0°, regardless of the presence of welding flash. When the inclined angle was 30°, 60° or 90°, most cracks in the joints with welding flash initiated at the stirred zone, whereas cracks in joints without welding flash tended to initiate from the base material or the stirred zone. A comparison of the S-N data indicated that the change in fatigue strength was small, regardless of the inclined angles of the friction stir welding line or the presence of welding flash.
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