Microstructure evolution and strengthening mechanism of electron beam welded dual-phase Mg-8Li-3Al-2Zn-0.5Y alloy

Nearly undamaged joints of electron beam welded (EBW) dual-phase Mg-8Li-3Al-2Zn-0.5Y alloy were achieved with joint coefficients exceeding 95%. All specimens were fractured at the base metal (BM), implying a significant departure from conventional fracture modes of welded joints. The fusion zone (FZ) consists of ultrafine acicular α-Mg and equiaxed β-Li, with grain sizes reduced by approximately 90% and 80%, respectively, compared to the base metal. This results in a significant increase in microhardness of about 40%. A unique multiphase mixture was observed in the heat-affected zone (HAZ), which mainly consists of lamellar eutectoid structures, fine precipitates zone, and numerous fine Mg₃(Al, Zn) particles. This mixture was transformed from typical Li(Al, Zn) (a common softening phase) undergoing atomic diffusion and solid-state phase transformation during welding. It introduces a synergistic strengthening effect, making the heat-affected zone no longer the weakest part of the joint. This study provides valuable insights into the electron beam welding technology for Mg-Li alloys and offers theoretical support for manufacturing high-quality joints.