Abstract:The multi-layer and multi-pass MIG welding with Al-Mg alloy welding wire has been used to weld 10 mm thick hot rolled aluminum magnesium manganese alloy plate, followed by a detailed study on the microstructure and mechanical properties of welded joints by means of metallography, scanning electron microscopy (including energy spectrum and electron backscatter diffraction). The results show that the microstructure and mechanical properties of the welded joints in different regions show certain inhomogeneity due to the different effects of welding thermal cycle, cooling rate and such heat stabilized phase particles as Al3Zr. The weld zone is a typical casting microstructure, and the heat-affected zone is a recrystallized structure which with some deformation characteristics. Due to the effect of the inhomogeneous nucleation of Al3Zr particles on the boundary of the molten pool, the fine grain thin layer with little strengthening effect is formed, with the microhardness value smaller than that of the weld zone as well as the heat affected zone. However, the grain at the interface of the two passes grows along the direction of heat dissipation, with an obvious coarsening and the lowest degree of microhardness.