Analysis of the response of thermal-affect and multiplex strengthening mechanisms of Mg-10Gd electron beam welded joint

Defect-free Mg-10Gd alloy welded joints with high tensile strength were obtained via electron beam weld. The microstructure of joints under different heat inputs was inconsistent. Sharp grain refinement occurred in the fusion zone due to the combination of the molten pool fluctuation and alloying elements. The divorced eutectic structures produced at grain boundary during grain growth and secondary Mg₅Gd(β_Ⅱ) located in the crystal were observed by transmission electron microscope. In the as-weld joint subjected to different heat-input, the fusion zone kept high strength while softening occurred in the heat-affected zone, which led the joint to fracture at the heat affected zone or base metal. With the triaxial tensile stress induced by the fusion zone, the mechanism of hindering effect of dislocation movement turned from the pining function of precipitates into a fixing effect from the triaxial tensile stress. The ultimate tensile strength of as-weld joints exceeded 93% of base metal. The tensile tests suggested that under the temperature below 200 °C, the fracture mechanism was quasi-cleavage, and when the temperature increased to 300 °C, the fracture mechanism changed to ductile fracture. Multiplex strengthening mechanisms existed in the as-weld joint, and the ratio of them varied in different regions. The dominant strengthening mechanism of base metal was precipitation strengthening, while grain boundary strengthening and solid solution strengthening were the primary strengthening mechanism of fusion zone and heat affected zone, respectively.