Effect of Post Weld Treatments on the Fatigue Behavior of AA7075-T6 Friction Stir Welded Butt Joints

Friction stir welding (FSW) is increasingly an alternative approach to traditional manufacturing of aluminum airframe components. Widespread usage of this technology by the industry is however hindered by the limited fatigue resistance knowledge of FSWed joints. FSW of integral airframe structures is also limited by surface irregularities that can facilitate fatigue crack initiation. Also, the thermomechanically-induced residual stresses and related sheet distortion complicate assembly of the structure and reduce its effective fatigue life. Moreover, the hardness drop in aluminum alloy weldments constitutes a metallurgical shortcoming that limits the properties of the assembly.

The tensile and fatigue properties of AA7075-T6 butt welded joints in as-FSWed condition and after the application of selected post-weld treatments were investigated in this work. At the stress corresponding to 10⁶ cycles, the fatigue cracks initiated in the region of the tool striations on the weld crown surface and the fatigue strength of the as-FSWed joints was 76.5 % of base metal. Deep rolling was efficient for reducing sheet deformation, but produced an unexpected reduction in the fatigue strength to about 40 % of the base metal. However, a simple brushing post-weld treatment on the top surface improved the fatigue strength to about 90% of the base metal, with cracks originating in the low hardness location of the heat affected zone (HAZ) on the root surface of the weld. Finally, a post-weld heat treatment after brushing improved the fatigue strength to 96.2 % of the base metal. The fatigue cracks initiated in the middle of the weld or underneath in the HAZ, suggesting that the heat treatment was effective in restoring the precipitation hardening.