Abstract: A selective laser melting formed Al-Mg-Sc alloy sheet and 5A06 with O heat treatment sheet with the thickness of 1.2mm were welded by tungsten inert gas （TIG） welding with 5B71 filler wire. The microstructure evolution and refinement characteristics of the welded joints were investigated. The results showed that a large number of pores with the diameter of 0.2mm distributed near the front surface of the weld zone and the fusion line of the SLM Al-Mg-Sc and 5A06 with O heat treatment sheet weld sample with appropriate TIG welding parameters.The number and size of pores and inclusions near the fusion line of SLM side were larger than that of 5A06 side and base metals in the welded joint. The microstructure of weld zone was equiaxed grains, which was optimized by the micro-alloying elements of Sc and Zr. The subcooled zone of molten pool was ultrafine grain region composed of the average finer grain with a diameter of about 20μm. Meanwhile, the other zones of molten pool mainly consisted of equiaxed grains with a diameter of 40～50μm. It was found that some concentrated pores, no shear lip and some dimples were present on the surface of fracture sample. Furthermore, some strengthening phases were observed in the dimples. It was indicated that the fracture mode was brittle-ductile mixed.