Fatigue crack propagation behaviors of friction stir welded sheets and profiles AA7050-T7451 aluminum alloy joints: Microstructure dependence

Sheets and profiles 7050-T7451 aluminum alloy were butt welded by friction stir welding. Electron backscatter diffraction, transmission electron microscope and fatigue crack growth testing were conducted to evaluate and deeply elucidate the fatigue crack propagation behavior and mechanisms in the transverse direction (TD) and longitude direction (LD) to the welding. Results indicated that the fatigue crack growth rate (FCGR) of TD was higher than that of LD in the case of low stress intensity factor (ΔK) ranges based on the Paris fitting curves. Inverse tendency appeared with the increase of ΔK values. The fatigue crack propagation path of the TD was in the sequence of sheet-thermo-mechanically affected zone (TMAZ, stir zone (SZ), profile-TMAZ, profile-heat affected zone (HAZ). Fatigue crack were dominant in the SZ and ended in the sheet-TMAZ of the LD specimen. The size of crack tip plastic zone can be characterized by the kernel average misorientation (KAM) value distribution, indicating the microstructure dependence.