Effect of texture and banded structure on the crack initiation mechanism of a friction stir welded magnesium alloy joint in very high cycle fatigue regime

Friction stir welding (FSW) produces heterogeneous microstructure in nugget zone of its joint, which should affect the fatigue responses. This work revealed the unique texture and banded structure of the Mg-FSW joint and their decisive roles on crack initiation behaviors. At higher stress levels, failures occur along the AS NZ/TMAZ boundary (AS: advancing side, NZ: nugget zone, TMAZ: thermo-mechanically affected zone) due to the inhomogeneous deformations near the boundary. Severe plastic deformations concentrate in the softer layers of banded structure. However, at lower stress levels, plastic deformations become weak and localized, but accumulate in the alternate layers of banded structure simultaneously due to the same soft texture orientation, and induce random failures in banded structure. The failure mechanisms are verified by semi-quantitative mechanical models.