Effect of laser parameters on fatigue crack growth behavior of AZ31B magnesium alloy TIG welded bionic joint

The purpose of this study was to evaluate the effect of laser parameters on the fatigue crack growth (FCG) behavior of AZ31B Magnesium alloy TIG welded bionic joints with ‘soft-hard’ alternating structures prepared by laser bionic treatment (LBT). The changes of laser parameters promoted the growth of fine equiaxed grain with random texture, and average grain size was 5.12 ± 0.51–8.19 ± 1.02 μm, which significantly increased isotropy and microhardness. The average microhardness was 66.5 ± 1.68–70.85 ± 1.41 HV0.1, which increased by more than 23.11% compared with untreated welded joints. For bionic welded joints, the heat-affected zone (HAZ) had higher FCG resistance and cycle times than weld metal (WM), and the specimens with notch in the HAZ showed obvious crack deflection. It was found that FCG behavior showed significant microstructure sensitivity by comparing the FCG rate curve and crack opening displacement. The Paris parameters had a significant linear relationship and effectively reflect the FCG resistance of the different specimens. The fatigue fracture showed that the crack growth zone was mainly dominated by typical cleavage steps and secondary cracks. The fatigue fracture region contains quasi-cleavage and ductile fracture. A series of strengthening mechanisms were induced by LBT and could significantly affect FCG behavior, which promoted crack deflection and branching to increase the toughness of the joint, while the laser remelting zone increased the strength of the joint. Therefore, the synergistic effect of strength-toughness of the joint was obtained by LBT, which provided a new idea for optimizing the bionic design and improving the fatigue property of welded joints.