High Cycle Rotating Bending Fatigue of 6061-T6 Friction Stir Welded Extrusions

Fatigue testing was done on friction stir welded joints of 6061-T6 aluminum extrusions. Tests were run using a rotating bending fatigue machine at stresses from 111.5 to 138.7 MPa. Failures occurred on the order of 105 to 107 cycles, and an S-N curve was generated based off of the failure results. After the samples failed, the location of the failure and the number of cycles to failure were noted. Fatigue samples were designed in SolidWorks with a tapered 2 inch reduced section. The 2 inch reduced section will include the entire weld region as seen from the microhardness profile of the weld, and the taper allows the aluminum samples to have equal stress along the surface of the sample across the reduced section, allowing fatigue to target the weakest area of the joint. Strain gauge testing was done to prove the model, which states that the reduced section was under equal stress. There was only a 1 MPa difference when comparing the stresses on either side of the weld. The strain gauge testing was also used to convert the applied moment that is set on the RBF machine into a stress value that will be used in the S-N curve. Fatigue strength for both the base metal and the FSW samples were higher than that stated in the literature. Micrographs were taken to find the location and grain structure of where failure occurred in the samples. SEM analysis was done on samples that failed at a low number of cycles to check for abnormalities in the material and find reasons for premature failure. The friction stir welded aluminum had failures on both sides of the stir zone between the heat affected zone and thermal mechanical affected zone