Low-cycle fatigue behaviour of magnesium alloy thick plate joints fabricated via differential double-shoulder friction stir welding

Magnesium (Mg) alloy thick plates welded by friction stir welding (FSW) often exhibit poor formability and uneven microstructures, leading to a significant deterioration in the mechanical properties. In this work, AZ31 Mg alloy plates with a thickness of 20 mm were successfully welded by differential double-shoulder friction stir welding (DDS-FSW). The low-cycle fatigue response behaviour was characterised, and the damage mechanism of the joints were studied. The results indicated that the fatigue life of the DDS-FSW joints in both parameters exceeded that of the FSW joint at 0.3 % strain amplitude, with increases of 200 % and 82 %, respectively. The predominant deformation mechanisms are basal slip and {10 1¯ 2} twinning. Basal slip is more likely to be activated in the FSW joint, whereas {10 1¯ 2} twinning is more easily activated in the DDS-FSW joints. Furthermore, the fatigue toughness (W0) and damage transition exponent (β) could be increased by improving the asymmetric distribution of the stir zone in the joint, the strength of “orientated micro-interface”, and the proportion and distribution uniformity of twins.