Aluminum (Al) acts as the primary alloying element in Mg-Al-Zn alloys, so its content is of crucial importance for the samples fabricated by wire arc additive manufacturing (WAAM). However, microstructure and property fluctuations of the alloys induced by varying WAAM processes hinder the extracting of consistent Al content-microstructure-property relationships from existing studies. In this study, Mg-xAl-Zn (x = 3, 6, 9, 12) alloys were fabricated via WAAM using consistent process parameters to focus on the influence of Al content alone. The results indicate that all of the four alloys primarily consist of α-Mg grains and Mg₁₇Al₁₂ phase. As Al content increases, the α-Mg grain size decreases from 43.1 μm in Mg-3Al-Zn to 19.7 μm in Mg-12Al-Zn. The content of Mg₁₇Al₁₂ phase increases from near 0% in Mg-3Al-Zn to 7.78% in Mg-12Al-Zn, and a sudden increase observed when Al content reached 12 wt%. The microhardness and yield strength of WAAM Mg-xAl-Zn alloys increase linearly with the increasing of Al content, which is attributed to the synergistic effects of grain refinement strengthening, solid solution strengthening, and second-phase strengthening induced by Al content. However, when the Al content reaches 9 wt%, the elongation and tensile strength decreased because of the excessive Mg₁₇Al₁₂ phase. Among the investigated alloys, Mg-6Al-Zn achieves the optimal strength-ductility balance, with a tensile strength of 266.3 ± 0.4 MPa and an elongation of 13.9 ± 1.4%. Thus, alloys with an Al content of approximately 6 wt% exhibit favorable performance and can be used without heat treatment, while those with Al content exceeding 9 wt% require heat treatment to optimize microstructure and improve ductility.

Gadolinium (Gd) is one of the most effective strengthening elements for magnesium alloys. The development of commercially available Mg-Gd alloys with high Gd content and the optimization of their preparation processes have been a major focus in magnesium alloy research. In this study, a Mg-23Gd-2Zn-0.4Zr alloy with ultra-high Gd content is designed, and high-quality fabrication is achieved using laser-directed energy deposition (LDED) technology. Through heat treatment and microstructure control, a balance between tensile strength (425 MPa) and elongation (3.4%) is achieved. The ultra-high strength of the LDED-T6 VZ232K alloy is primarily attributed to precipitation strengthening caused by the ultra-high density (2.4 × 10⁴ µm^-2) of β′ phase. The high ductility is mainly due to the modification of the fracture mode, facilitated by the introduction of a substantial number of stacking fault structures during solution heat treatment. The extended hardness plateau (exceeding 138 Hv) and high yield strength (exceeding 300 MPa) are associated with the three-directional cross-interlocked structure of the β′ phase in the over-aged state at 220 ℃ and 250 ℃. The analysis of the LDED-VZ232K alloy indicates that reduced heat input during the additive manufacturing (AM) process is critical for the defect-free fabrication of alloys with ultra-high Gd content.

The magnesium matrix composites (MMCs) formed by introducing reinforcements to magnesium alloys overcome the limitations of the mechanical properties to a certain extent, presenting unique and excellent properties that any component does not have, such as high specific stiffness and specific strength, good dimensional stability, outstanding shock absorption performance, excellent electromagnetic shielding and hydrogen storage characteristics, etc. As an emerging manufacturing technology, additive manufacturing (AM) is based on the design of three-dimensional (3D) data model to obtain 3D objects through layer-by-layer processing, which possesses the advantages of short manufacturing cycle, high material utilization rate, high degree of design freedom, excellent mechanical properties and the ability to fabricate complex structural components. Combining the high stiffness and high strength properties of MMCs and the technical advantages of AM forming complex structural parts with high performance, the prepared AM MMCs have huge potential advantages and broad application prospects in new high-tech industries such as automobile, aerospace, consumer electronics and biomedicine, etc. This paper reviews the research progress in the field of AM MMCs, mainly introduces the main AM technologies, including selective laser melting (SLM), electron beam selective melting (EBSM), laser engineered net shaping (LENS) and wire and arc additive manufacturing (WAAM). The formation mechanism and control methods of the typical defects including balling effect, porosity, poor fusion, loss of alloy elements and cracks produced during AM are discussed. The main challenges of AM MMCs are proposed from the aspects of composition design and the preparation of powder raw material. The relationship between the microstructure and mechanical properties, corrosion performance and biocompatibility of AM MMCs are elaborated in detail. The application potential of AM MMCs in various fields at present and in the future is introduced. Finally, the development direction and urgent problems to be solved in the AM MMCs are prospected.