Experimental investigation on fatigue crack initiation and propagation mechanism of friction stir lap welded dissimilar joints of magnesium and aluminum alloys

At present, the effect of the microstructure of friction stir lap welded (FSLW) dissimilar joints of Mg and Al alloys on fatigue crack initiation and propagation is not clear. In this article, the effects of the microstructure of FSLW dissimilar joints of AZ31 Mg and Al-Zn-Cu-Mg alloys are studied. The grain orientations of the crack initiation and propagation region were characterized by electron backscattering diffraction analysis. And the models for fatigue crack propagation in the different grain orientations were established. Based on the above experimental results, the initiation mechanisms of fatigue cracks were clarified, and the effects of grain orientations on crack propagation were revealed. The results showed that the crack initiation was caused by the simultaneous activation of the (0002, 2110] basal slip and twinning in various directions by fatigue loading. The direction of crack propagation was dominated by the (0002)[2110] basal slip with (1012)[1011] twinning adjusting in the crack propagation zone. The cyclic effect of concentration and release of stress on the activation and termination of twins was the main reason for the zigzag path of crack propagation.