Microstructure and fatigue crack growth behavior in welding joint of Al-Mg alloy

•The weld is dominated by cellular dendrite with some bubbles and microcracks.•The defects of bubble, interface, and unglued particle hole generate crack origins.•The crack propagation appear deflection, bifurcation, bridging connection, etc. In this paper, the microstructure and fatigue crack growth behavior of Al-Mg alloy welding joint by metal inert-gas welding (MIG) were studied by means of metallographic microscope, scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The results show that the welded joint is characterized by obvious heterogeneous microstructure. The weld zone is dominated by equiaxed cellular dendritic crystals, with a small amount of welding bubbles and microcracks. The position of welding joint with bubble, grain boundary, interface between hard phase particles and matrix, and hole of coarse impurity particles debonded tend to generate fatigue crack origins, and further develop into the main crack. Due to the influence of stress state, grain orientation, welding bubble and coarse phase particle, the fatigue crack propagation in the weld zone will appear deflection, bifurcation, convergence, bridging connection and other path choices, which will affect the direction and rate of fatigue crack propagation.