Effects of heat input on microstructures and mechanical properties of LAZ931 magnesium-lithium alloy by CO2 laser welding

Abstract

LAZ931 magnesium-lithium (Mg-Li) alloy is a novel ultra-light alloy material. It is added with important alloying elements aluminum and zinc to form a quaternary Mg-Li alloy, which has high strength and low density, and improved mechanical properties. Due to the natural aging softening of multiphase Mg-Li alloys, their strength will decrease at room temperature. The welding research of LAZ931 Mg-Li alloy is only friction stir welding, and there is no report on laser welding. The industrial circle has put forward an increasingly urgent demand for the welding technique of LAZ931 Mg-Li alloy. CO2 laser welding of LAZ931 Mg-Li alloy with 2.15 mm in thickness was studied to explore a reasonable heat input range. CO2 laser welding can make the welded seam zone(WSZ) grain significantly refined. The base metal(BM) softening phase disappears after laser welding. Dispersion strengthening of the precipitated phase in the heat-affected zone(HAZ), resulting in enhanced strength of the HAZ. The WSZ is mainly composed of fine needle-like α-phase, granular α-phase, β-phase, and granular precipitation phase, and the grain interior and the grain boundary distribution of α-phase along β-phase; The width of the HAZ is narrow, and it partially melted to form a coarse needle-like α phase. The tensile strength of the welded joints obtained by laser welding is all fractured in the BM, indicating that improved strength after laser welding, and with the increase in heat input, the ultimate tensile strength and elongation rates of the welded joints show a descending trend; The microhardness of the WSZ is the highest, followed by the HAZ, and the microhardness of the BM is the lowest, and the microhardness of the WSZ is higher than that of the BMs under different heat inputs. With the increase in heat input, the WSZ grain becomes coarse, which in turn makes the mechanical properties of the welded joint decrease. The research results can provide a theoretical basis and technical support for the engineering application of this type of ultra-light Mg-Li alloy.