Microstructure, texture and fatigue performance of friction stir welded dissimilar magnesium alloy joints

The stress-controlled fatigue of dissimilar friction stir welded AZ80/AZ61 and AZ80/AZ31 magnesium alloy joints was studied. Fatigue testing targeting the critical stir zone — base metal interface revealed endurance limits of 90 MPa for AZ80/AZ61 welds and 70 MPa for AZ80/AZ31 welds. Within the range 60–90 mm/min, welding speed did not affect the endurance limits, though lower speeds produced more homogeneous microstructures and reduced data scatter in AZ80/AZ61 joints. The superior fatigue resistance of AZ80/AZ61 welds is attributed to their stronger and more plastically uniform microstructure, which supports higher stress levels and sustains more cycles before crack nucleation. Cyclic stress–strain analysis shows that AZ80/AZ31 joints exhibit higher hardening rates under cyclic loading compared to AZ80/AZ61 joints, however, both joint types demonstrate significantly reduced hardening compared to monotonic tensile deformation. This behaviour is attributed to low plastic strains during cycling, texture effects inhibiting basal slip, and reversible twinning–detwinning mechanisms. Electron backscatter diffraction analysis revealed substantial texture evolution in the stir zone (SZ), thermomechanically affected (TMAZ) and heat-affected zones (HAZ), with mechanical twinning contributing to grain refinement and mechanical anisotropy. Transmission electron microscopy revealed a complex fatigue dislocation microstructure characterized by networks of basal and non-basal dislocations, with fine dislocation loops and debris present at all stress amplitudes. The density of these defects increased systematically with stress amplitude, providing insight into the cyclic deformation mechanisms governing fatigue life. • Alloy compatibility across dissimilar joints dictates fatigue mechanisms. • Cyclic hardening lower than monotonic due to twinning–detwinning influence. • Microstructure analysis and modelling reveal divergent evolution pathways in joints. • Dislocation networks scale with stress, representing a novel fatigue damage metric.