Fatigue behavior of friction stir welded Al–Mg–Sc alloy

• Different tool travel speeds resulted in varying microstructural features.
• Only little effect of the tool travel speed on fatigue lifetime.
• Fatigue crack initiation nearly independent of local hardness distribution.
• Local microstructure in weld seam affects crack initiation and crack propagation.
• Crack initiation sites seem to depend on stress levels applied during fatigue.

The effect of Friction Stir Welding on the fatigue behavior of Al–Mg–Sc alloy has been studied. To reveal the influence of the welding parameters, different travel speeds of the welding tool have been used to provide weld seams with varying microstructural features. Crack initiation as well as crack propagation behavior under fatigue loading has been investigated with respect to the local microstructure at the crack initiation sites and along the crack path. Fatigue cracks were mostly initiated around the stir zone and the adjacent thermo-mechanical affected zone independent from hardness distributions in the weld seams. In some specimens, defect-like feature was observed at the crack origins, which shortened the fatigue lives. It has been found that while the effect of the tool travel speed on the fatigue lifetime seems to be little, the varying and complex local microstructure in the weld seam basically affects both the crack initiation sites and the crack propagation paths.