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The Er-containing Al-Mg alloy was welded by laser beam welding. Microstructures and mechanical properties of the joints were analyzed, and the effects of erbium on the microstructure and properties of the weld and the heat affected zone were investigated. The results show that due to the addition of rare earth element Er, the grain size of the weld center reduces to about 30 μm; a “fine grain layer” appears near the fusion line; no significant grain growth occur in the heat-affected zone; the micro-hardness in the cross section distributes in W-shape. The effect of Er on the tensile strength of welded joints is not obvious, and the tensile strength is 70% of that of the base material.