High joint efficiency achieved in ultra-light Mg-Li alloy by electron beam welding with controlled heat input

For ultra-light Mg-Li alloys with a high Li content, fusion welding is a challenge due to the relatively active main alloying elements Mg and Li. In this study, electron beam welding technology was applied for the first time to join 8-mm-thick forged Mg-12Li-3Al-2Zn-1Si-1Y alloy plates. By controlling the heat input, defects in the welded joints and elemental evaporation were minimized. However, for the Mg-12Li-3Al-2Zn-1Si-1Y alloy, the Mg₂Si eutectic phase segregates at the fusion-zone grain boundaries during rapid solidification of the molten pool, thereby promoting the precipitation of coarse Mg₃(Al, Zn) phases along the boundaries. This segregation weakened intergranular atomic bonding in the fusion zone, leading to reduced deformability. Consequently, tensile fracture of the welded joints occurred in the fusion zone. With increasing heat input, the precipitation of α-Mg phases around Mg₃(Al, Zn) phases at the grain boundaries improved the deformability of the grain boundaries. As the fusion zone and heat-affected zone were strengthened, fracture shifted to the base material, which became the weakest region. Under these conditions (with the heat input ranging from 123.4 to 164.6 J/mm), the joint efficiency exceeded 95 %, and the ultimate tensile strength was approximately 280 MPa. When the heat input was further increased, grains in the heat affected zone coarsened, creating a softened zone where fracture occurred. This study provides a theoretical basis for electron beam welding of high-Li-content Mg-Li alloys, and offers a solution for their joining applications.