Research on ultra-high-cycle fatigue properties of 7050 aluminum alloy FSW welded joints

Ultra high cycle fatigue tests were used to test the fatigue property of 7050 aluminum alloy friction stir welding joint in the very high cycle regime. The results show that fracture still occurs in the welded joints after 10⁷ cycles, and the S-N curves present multi-stage decreasing shapes with the turning point at about 10⁸ cycles. The fatigue fracture morphology of specimen was observed by scanning electron microscope. It is found that fatigue cracks always initiated at the surface under the condition of high stress. However, fatigue crack initiation sites were inclined to transform from surface to subsurface or interior as cyclic loading decreased. The actual initiation sites depend on the competition mechanism of crack initiation from surface, subsurface or interior. Most of the specimens fractured at thermo- mechanical affected zone (TMAZ) and heat affected zone(HAZ). The fracture location is closely related to inhomogeneous microstructure of FSW joint through the EBSD and hardness analysis .