Fatigue properties of friction stir welded joints and their residual stress distributions--Studies on characteristics of friction stir welded joints in structural aluminum alloys

摘要

Fatigue characteristics and residual stress distribution for the Friction Stir Weld (FSW) joints of an intermediate aluminum alloy base material with a thickness of 8 mm for structures was investigated and the following results were obtained. The fatigue strength for 5XX was roughly the same as that of the base material regardless of the implementation method and had a higher level than MIG welded joints. Also, a durability limit of 10~7 times exceeds the proof stress requirement of 125 MPa of the base material by 0.2 percent. Incidentally, in cases where there were extremely small imperfections in the route area, fatigue strength was lowered but was at a higher level than for MIG welded joints. The fatigue strength for 6XX was at a lower level than that of the base material but had a higher level than MIG welded joints. Incidentally, when flush shape forming is performed, a large stress concentration occurs in the toe flash area and fatigue strength is reduced to roughly the same level as that of MIG welded joints. In the case of 71 base material with high fatigue strength, the fatigue strength for 7XX joints was at a lower level than that for the base material but was at a higher level than that for MIG welding. In, the case of 72 base material with standard fatigue strength, implementation method 4 produced a weld with roughly the same fatigue strength as that of the base material where implementation method 1 had a fatigue strength lower than all the others but still at a higher level when compared to MIG welding. Incidentally, in cases where there were extremely small imperfections in the route area, fatigue strength was lowered to approximately the same level as that for MIG welded joints. Sample joints that were confirmed to be safe through a penetrant test and ultrasonic phased array flaw detection test were used but there were some found to have reduced strength, as noted above, and additional investigation regarding clarification of the strength reduction mechanism as well as a non-destructive inspection method is needed. The residual stress distribution in the 5XX joints was measured using a dissection method and x-ray diffraction method. As a result, a good match was obtained from both of these except with a particular section. The residual stress in the weld direction was a tensile stress of roughly 90 MPa in the stirred zone which reduced rapidly from here along the thermally affected area and changed to compress stress at a position roughly 10 mm from the toe flash. The residual stress in the direction perpendicular to the weld direction was a compression stress of roughly 20 MPa. However, in most of the fatigue test sample shapes, this has been released and it is thought that there was no effect of residual stress on these fatigue tests. From what has been described, the effect of imperfections and the effect of residual stress using large test samples need to be investigated.