Fatigue performance of AA7075-T6 Friction Stir Welding Joints and effect of post weld treatments

Widespread usage of Friction Stir Welding (FSW) in the aircraft industry for manufacturing of aluminum airframe components is hindered by the limited knowledge of the fatigue resistance of FSW butt and lap joints. Published... [ view full abstract ]

Widespread usage of Friction Stir Welding (FSW) in the aircraft industry for manufacturing of aluminum airframe components is hindered by the limited knowledge of the fatigue resistance of FSW butt and lap joints. Published work suggests that the fatigue life of FSWed airframe structures is limited by a) the surface irregularities such as tool striations and burrs that can facilitate fatigue crack nucleation, b) the hardness drop or softening in the aluminum alloy weldments due to thermomechanical cycling of the precipitation-hardened material, and c) the thermomechanically-induced residual stresses and related sheet distortion that complicate assembly of the structure.

The fatigue properties of AA7075-T6 friction stir butt welded joints in the as-welded condition and after selected post-weld treatments were investigated in this work. At a stress corresponding to 106 cycles, the fatigue strength of the as-FSWed joints was 77 % of the 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. A simple brushing post-weld treatment on the top surface succeeded in removing most of the surface irregularities and 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 % of the base metal. The fatigue cracks nucleated in the middle of the weld or underneath in the HAZ, suggesting that the heat treatment was effective in restoring the precipitation hardening.