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.

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.

Enhancing the low modulus and absolute strength of Mg-Li alloys remains a key research focus. This study employed an in situ-synthesis strategy to incorporate high-modulus phases, such as Mg₂Si, achieving a synergistic improvement in strength, plasticity, and modulus. The presence of Mg₂Si improved the microstructure of the matrix and promoted dynamic recrystallization, leading to a refined grain size of 2.9 µm. Through the combined effects of grain refinement strengthening and coefficient of thermal expansion mismatch strengthening, the Mg-8Li-3Al-2Zn-1Y-0.5Mn-0.5Gd-1Si alloy exhibited exceptional mechanical properties, including a yield strength of 248 MPa, an ultimate tensile strength of 273 MPa, an elastic modulus of 51 GPa, and a plasticity of 20.6 %. This study offers a new approach for designing and developing ultra-light, high-modulus Mg-Li alloys.

The ultra-light Mg–8Li–3Al–2Zn–0.5Y (LAZ832–0.5Y) thin wall parts with excellent performances were successfully fabricated by Gas Tungsten Arc Welding (GTAW) in this study. The microstructure of the top, middle and bottom regions of the thin wall fabricated under various conditions was examined and the mechanical properties of these thin walls were tested. The results showed that much finer microstructure was obtained by GTAW than that made by conventional casting method. In the as-deposited samples, the needle-like shaped α-Mg phase emerged at the top of the thin wall whereas the bar-shaped α-Mg phase showed up in the middle and bottom regions of the thin wall due to the complex thermal history. The Al₂Y phase was dispersed throughout both α-Mg and β-Li while the AlLi phase was mainly located in the β-Li. The best combination of ultimate tensile strength (UTS), yield strength (YS) and elongation to fracture of the as-deposited thin wall were 218.9 MPa, 171.4 MPa and 20.9%, respectively, which was manufactured under the optimal condition of 120 A 1800 mm/min 220 mm/min. After solid solution treatment at 350 °C for 4 h, the UTS increased slightly by 13% but the YS increased significantly by 65% compared with the samples before solid solution. The solution of the AlLi phase was believed to be the main strengthening mechanism. It is interesting to note that the UTS and YS of the as-deposited sample was better than those of the as-cast sample while the opposite situation took place after solid solution treatment.

In this work, the aging response and mechanism of dual-phase Mg-Li-Al-Zn alloy at various temperatures are investigated. The results show that the strengthening after quenching is primarily attributed to the immediate precipitation of the semi-coherent ~Mg₃Zn phase. The aging softening of the studied alloy is mainly caused by the rapid transformation of the strengthening ~Mg₃Zn phase to the softening MgLi(Al, Zn) phase, along with the coarsening of the α-Mg matrix and precipitates within β-Li matrix. Further analysis indicates that the quick precipitation and transformation of ~Mg₃Zn is a consequence of the high diffusion rate of solute atoms, resulting from dense vacancy concentration in the β-Li matrix. This research bridges a critical gap in the study of aging mechanism in the dual-phase Mg-Li-Al-Zn alloy, providing a theoretical basis for the development and application of high-performance and thermal-stable Mg-Li alloys.

A novel Mg-10Li-3Al (wt.%, LA103) matrix composite reinforced by ex situ micron TiB₂ particles was developed in the present study. The ball milling and cold pressing pretreatment of the reinforcements made it feasible to prepare this material under stir casting conditions with good dispersion. The microstructure and mechanical properties of the composites prepared by different pretreatment methods were analyzed in detail. The TiB₂ particles in the Al-TiB₂/LA103 composite using the pretreatment process were uniformly distributed in the microstructure due to the formation of highly wettable core-shell units in the melt. Compared with the matrix alloys, the Al-TiB₂/LA103 composite exhibited effective strength and elastic modulus improvements while maintaining acceptable elongation. The strengthening effect in the composites was mainly attributed to the strong grain refining effect of TiB₂. This work shows a balance of high specific modulus (36.1 GPa·cm³·g^-1) and elongation (8.4%) with the conventional stir casting path, which is of considerable application value.