Source: Fugu Magnesium Association

The demand for lightweight materials in important fields such as aerospace and weapon equipment is becoming increasingly urgent. Magnesium alloy, as the lightest metal structural material, has gradually received widespread attention, and the additive manufacturing of magnesium alloy has also begun to receive more and more attention in the material industry.

Magnesium alloy, as the lightest metal structural material, has a density of only 1.74g/cm3, which is about 2/3 of aluminum alloy, 1/3 of zinc alloy, 1/4 of steel, and 2/5 of titanium alloy, equivalent to most engineering plastics. Not only that, magnesium alloys also have many excellent properties, such as excellent specific strength and stiffness, excellent damping performance, thermal stability, and electromagnetic radiation resistance, which have been widely used in aviation, aerospace, automotive, electronic communication and other fields.

With the further improvement of industrial requirements for product comprehensive performance, the design concepts of lighter components such as flow channels and topologies have begun to emerge. However, at present, the forming methods of magnesium alloys mainly rely on traditional casting, powder metallurgy, and plastic forming. These traditional processing techniques are difficult to process the internal parts of integrated components, and cannot construct fine flow channel structures or topological structures within the components, which limits the advantages of lightweight and the potential of complex structural parts forming of magnesium alloys. In this case, additive manufacturing has broken through the limitations of traditional manufacturing and has the characteristics of high precision, high design freedom, high utilization rate, and energy conservation. By designing process parameters, the microstructure and performance of alloys can be regulated, maximizing the ability of synergistic design of alloy materials, and producing complex structural products that cannot be achieved by traditional manufacturing through net forming. This expands the application of magnesium alloys in fields such as biomedical, automotive, and consumer electronics

Classification of magnesium alloys

3D printing technology has been widely used in the manufacturing of complex samples such as stainless steel, titanium alloy, aluminum alloy, and has been successfully used in engine casings, heat dissipation pipelines, and weight reducing structural components. In recent years, with the increasing understanding of the flammability of magnesium alloys during processing, research on additive manufacturing of magnesium alloys has gradually been carried out, in order to break through the limitations of traditional magnesium alloy preparation processes on the lightweight advantages of magnesium alloys. At present, researchers have successfully utilized Selective Laser Melting (SLM) technology, Wire Arc Additive Manufacturing (WAAM) technology, Friction Stir Additive Manufacturing (FSAM) technology, and Laser Melting Deposition (LMD) technology to prepare topologically optimized designs, The production has produced a series of magnesium alloy parts that cannot be manufactured using traditional processing methods, greatly expanding the application potential of magnesium alloys in lightweight and complex components.

Common Magnesium Alloy Composition and Classification

Pure magnesium is rarely used directly due to its low strength. Magnesium alloys commonly used in additive manufacturing are classified into AZ series (AZ31, AZ61, AZ80, AZ91), ZK series (ZK60, ZK61), and WE series (WE43, WE54, WE93) according to their grades.

The AZ series (Mg Al Zn) magnesium alloy is developed based on the Mg Al series magnesium alloy. The appropriate addition of Zn element can improve the creep resistance of the specimen and reduce the adverse effects of impurities such as Fe and Ni on the corrosion performance of the magnesium alloy. It has balanced mechanical properties and certain corrosion resistance, and is currently the most widely used magnesium alloy in additive manufacturing research.

The ZK series (Mg Zn Zr) magnesium alloy is developed by adding Zr element on the basis of the Mg Zn series magnesium alloy. Research has shown that adding Zr element to magnesium can effectively refine grain size, and has a strong solid solution strengthening effect, improving the mechanical properties of magnesium alloys. It is a promising biomedical material for research.

The WE (Mg RE) series magnesium alloys belong to rare earth magnesium alloys, and magnesium alloys with added rare earth elements exhibit good creep resistance and tensile properties at room temperature. However, the cost of rare earth elements is relatively high. Currently, research on additive manufacturing mainly focuses on AZ series magnesium alloys, and there is less research on additive manufacturing of other series alloys, especially rare earth magnesium alloys. Developing low-cost and high-performance rare earth magnesium alloys is of great significance for the research on additive manufacturing of magnesium alloys.

Magnesium alloy 3D printing technology

The additive manufacturing process of metal materials is closely related to the characteristics of melting heat sources. Based on the progress of advanced connection technology, the additive manufacturing of metal materials has developed rapidly. At present, the mainstream magnesium alloy additive manufacturing technologies on the market can be divided into SLM, WAAM, and FSAM according to the melting heat source, and their respective schematic diagrams are shown in the following figure.

Selective Laser Melting Technology

Selective Laser Melting Technology (SLM) uses laser as a heat source to scan metal powder layer by layer to obtain designed metal parts. It is suitable for manufacturing small volume, complex structures, and parts with high precision requirements. Laser energy density is high, and the temperature at the center of the spot is much higher than the boiling point of magnesium alloy. Magnesium alloy evaporation and element burning often occur during the forming process. In addition, the experimental results are easily affected by various conditions (powder shape and size, experimental system, environment), narrow process windows, and improper parameter selection can lead to poor surface quality and defects such as spheroidization and evaporation.

At present, the research on magnesium alloy SLM at home and abroad is still in the initial stage of development, and almost all research is exploring suitable process parameters through a large number of experiments, comparing their microstructure and mechanical properties. The relevant research is not yet mature. Due to the close correlation between the optimal process parameters in each experiment and factors such as the experimental system and hardware equipment, the repeatability of the experiment is low, which makes the practical value of the optimal process parameters in each experiment unclear. The existing experimental results are difficult to establish accurate theoretical models, and deepening research in modeling and simulation will contribute to the widespread application of magnesium alloy SLM.

Arc Fusion Deposition Technology

Arc Fused Wire Deposition Technology (WAAM) relies on welding arc melting to deposit welding wire, which has the advantages of low cost and high deposition efficiency, and is suitable for additive manufacturing of large volume and complex structures. The arc heat source has a large heat input and is prone to thermal cracks and pores during the WAAM forming process, resulting in severe heat accumulation effects. The lower layer of the sample often undergoes grain coarsening and changes in grain orientation due to high-temperature heat accumulation and multiple thermal cycles, while material deformation caused by thermal stress can lead to a decrease in forming accuracy. At present, research mainly focuses on the formation and microstructure properties of single layer and single layer multi-layer surfacing welding.

金属丝是WAAM工艺的主要输入材料，高性能的WAAM镁合金工件对于丝材有一定的要求，如下表所示。

Solid state stirring friction additive technology

Solid State Stirred Friction Additive Manufacturing (FSAM) is the process of plastic deformation and fusion of materials through the rotation and movement of the stirring head and the friction heat generated by the laminated thin plates. It has the advantages of high manufacturing efficiency and excellent performance, and is suitable for additive manufacturing of larger volume components. In the traditional magnesium alloy manufacturing process, many problems are often encountered, such as coarse grains, hot cracks, pores, oxidation, and evaporation. Compared with traditional manufacturing techniques, the FSAM process has fewer heat inputs, narrower heat affected zones, and can obtain ultrafine grains based on the stirring friction dynamic recrystallization process, effectively reducing defects in traditional manufacturing techniques, making the FSAM process one of the most suitable processes for magnesium alloy additive manufacturing. However, there are still certain problems in using FSAM technology to manufacture magnesium alloys, as the pores, banded structures, and hook shaped defects in the samples after additive manufacturing cannot be effectively resolved. There are significant differences in the applicable conditions, manufacturing efficiency, heat source energy input, and microstructure of the added components among the three magnesium alloy additive manufacturing processes. The comparison of their process characteristics is shown in the table below.

In addition, safety issues in additive manufacturing processes are crucial. In SLM processes, due to the rapid heat accumulation and large surface area of magnesium powder materials used, they cannot fully dissipate heat from each other, and are prone to combustion and explosion in contact with oxygen. It is necessary to strictly follow the standardized operations of magnesium alloy powder during storage and use, and safety hazards are particularly prominent; In the WAAM process, due to the use of magnesium alloy welding wires as raw materials, the manufacturing process is less prone to combustion and explosion, and has high safety; In the FSAM process, magnesium alloy plates, wires, or powders are usually selected as raw materials. However, as a type of solid-state additive manufacturing process, the manufacturing process has a lower temperature and only heats the material to a thermoplastic state rather than a molten state. Additionally, the size of the manufactured parts is large and heat dissipation is good, making the manufacturing process safer.

Microstructure and Properties of Magnesium Alloy Produced by Additives

（1） The SLM process of magnesium alloy is influenced by various process and material parameters, and changes in these parameters can lead to significant changes in the chemical composition, mechanical properties, and geometric shape of the manufactured components.

At present, research on SLM of magnesium alloys mainly focuses on exploring the influence of experimental parameters (powder characteristics, laser power density, scanning speed, pulse frequency, etc.) on sample forming. Therefore, identifying and paying attention to important parameters is crucial. Research has shown that laser power and scanning speed are important factors determining the forming quality of magnesium alloys prepared by SLM. The use of low energy density (such as low laser power and scanning speed) cannot completely melt magnesium alloy powder, forming powder sintering, resulting in high porosity and spheroidization phenomenon; As the energy density increases, the sample forming improves, but higher energy density can cause severe burning and severe evaporation of the magnesium alloy. The following table shows the comparison of forming processes using SLM technology for magnesium alloy additive manufacturing.

With the increasing range of use and service environment of magnesium alloys, their shortcomings have gradually been exposed. One of the most limiting factors for their widespread use is their low corrosion resistance, which makes them unable to serve for a long time in humid environments. One important reason for the poor corrosion resistance of magnesium alloys is that impurities in the alloy are prone to form micro galvanic corrosion with the matrix. When the impurity content in the alloy decreases by less than 0.05%, the corrosion rate will decrease by 90%. The SLM process uses high-purity powder forming to avoid the introduction of impurities in the raw material stage, which is beneficial for improving the corrosion performance of the alloy. The higher cooling rate during the SLM process is beneficial for refining the alloy structure and improving the corrosion resistance of the material. In addition, heat treatment (HIP, etc.) or other work hardening methods can further improve the mechanical properties of SLM magnesium alloys. The following table summarizes the mechanical properties of high-strength SLM magnesium alloys in recent years

However, during the SLM forming process of magnesium alloys, defects such as pores, hot cracks, and unmelted pores are also prone to occur. The formation of pores is mainly due to the small hole effect generated under high laser power and low scanning speed, forming a deeper molten pool. As the molten pool moves forward and solidifies, steam does not have time to precipitate, forming pores. Large laser input energy can be used without evaporating the magnesium alloy, reasonably reducing the dynamic viscosity of the molten metal, ensuring full diffusion of the molten metal, reducing powder splashing, thereby improving interlayer wettability and reducing porosity in the components. The boiling point and melting point of magnesium are only 440 ℃, leading to severe evaporation of the magnesium alloy during the SLM process, resulting in problems such as powder splashing and element burning. The high affinity of magnesium for oxygen is another major issue with SLM of magnesium alloys, as oxidation of magnesium alloys may hinder interlayer bonding and lead to spheroidization. In addition to introducing pollution, if the oxide layer is damaged by a laser beam, it will accumulate along the grain boundaries of the magnesium alloy sample, leading to the generation of microcracks. At present, the main way to reduce defects in magnesium alloy SLM forming components is to adjust process parameters, among which laser power and scanning speed are important factors determining the forming quality of magnesium alloy prepared by SLM. Appropriate process parameters can effectively reduce defects such as spheroidization, element burning loss, and porosity.

（2） At present, research on magnesium alloy WAAM mostly adopts Tungsten Inert Gas Welding (TIG) and Melt Inert Gas Welding (MIG). The arc heat input is large, which is prone to heat accumulation during additive manufacturing. As the number of deposition layers increases, the fluidity of the upper magnesium alloy increases, and the sample generally exhibits a "wide top and narrow bottom" forming characteristic, The following table shows the comparison of magnesium alloy forming using different WAAM processes.

In the magnesium alloy WAAM process, the solubility of hydrogen in the magnesium alloy decreases with the decrease of temperature. Due to the low density of magnesium and the rapid cooling of the WAAM process, gas cannot quickly move up and escape from the molten pool during solidification, forming pores. Similar phenomena have also been observed in aluminum alloy WAAM. By properly storing the welding wire, cleaning the base metal before welding, and adjusting welding parameters, the gas escape and dissolution in the molten pool can be controlled. For example, by appropriately increasing the welding current and welding speed, the gas escape conditions in the molten pool can be better than the dissolution conditions, and the generation of gas pores can be reduced.

（3） During the FSAM process, the material undergoes plastic flow, making it difficult to control the sample forming. Generally, the heat input is controlled by changing the speed of the stirring head and welding speed to improve the macroscopic forming of the sample.

The higher the speed of the stirring head, the greater the heat generated, resulting in greater thermal stress during the cooling process. However, the lower the speed of the stirring head, the less frictional heat generated is sufficient to cause the material to flow. Welding speed too high or too low can cause defects in the sample. Therefore, a well formed magnesium alloy FSAM specimen requires appropriate stirring head speed and welding speed.

Pores, banded structures, and hook shaped defects often occur in FSAM formed magnesium alloy components. When there is insufficient heat input during the additive manufacturing process, it can cause insufficient plastic deformation of the deposited metal, poor material fluidity, and incomplete closure of the sample interior, forming pores; When the heat input is too large, it will cause the material on the forward side of the mixing head to expand and overflow, resulting in insufficient backfilling and the formation of pores; When using a cylindrical or conical stirring head without threads, it can also cause insufficient plastic deformation of the material and form pores. Hook shaped defects and banded structures often occur at high heat input, and the bending direction of hook shaped defects is consistent with the direction of material movement around the stirring head. The following figure shows the forming defects in different areas of the longitudinal section of magnesium alloy FSAM specimens at higher rotational speeds. The welding heat input can be adjusted by reasonably controlling the welding speed and stirring head rotation speed, or selecting appropriate stirring head geometry to avoid defects.

As mentioned above, the microstructure directly affects the performance of the sample, and there are significant differences in microstructure among different manufacturing processes. The following table compares the microstructure (grain size, phase composition) of magnesium alloy additive manufacturing samples formed by different processes. From the table below, it can be observed that due to the wide variety of magnesium alloys, their composition varies greatly, and the phase composition varies after additive manufacturing. In addition, compared to WAAM, magnesium alloy SLM and FSAW have smaller grains, mainly due to rapid laser cooling and dynamic recrystallization during stirring friction, while WAAM has coarser grains due to severe heat accumulation.

Problems and Prospects in Additive Manufacturing of Magnesium Alloys

In recent years, with the efforts of domestic and foreign scholars, magnesium alloys have achieved some results in additive manufacturing. Compared to traditional forming technologies, their advantages are very prominent and their prospects are very broad. However, many problems have also been exposed during the research process, which restrict the further application and development of magnesium alloy additive manufacturing processes:

(1) There is a lack of basic research theory, due to the lack of a control model for the relevant heat source energy input in the magnesium alloy printing process, especially the difficulty in simulating the splashing formed by the overheated melt during the SLM forming process under high-energy laser input during back stamping, as well as the simulation research and theoretical analysis of the microstructure evolution during the rapid cooling process. The research on residual stress and processing defects in additive manufacturing processes often uses low-cost and more mature steel, aluminum alloy, or titanium alloy as research samples, with less attention paid to magnesium alloy systems, making it difficult for existing research to achieve breakthroughs in the principles and methods of multifunctional integrated optimization design in magnesium alloy additive manufacturing;

(2) Due to safety factors, the manufacturing experience of magnesium alloy additives is still relatively scarce. There are often certain defects inside the sample, such as hot cracks, pores, etc. Currently, only partial elimination of defects can be achieved through hot isostatic pressing after molding. Magnesium alloy is a thermosensitive material, and during the manufacturing process of additive materials, there are often strong physical and chemical changes as well as complex physical metallurgical processes, accompanied by complex deformation processes. The above processes are influenced by many factors, including materials, structural design, process, post-treatment, and many other factors. This also makes it difficult to accurately grasp the process structure performance relationship in the alloy additive manufacturing process, Resulting in the inability to fully utilize the performance of additive manufacturing of magnesium alloys. In addition, due to the active nature of magnesium alloys, which are prone to splashing and cracking during the additive process, it is necessary to develop specialized SLM machines and equipment for magnesium alloys;

(3) At present, there are no specialized magnesium alloy raw materials (wire and powder) suitable for additive manufacturing. The existing products are mostly commercial cast magnesium alloys, which cannot fully utilize the high-temperature and rapid cooling characteristics of additive manufacturing processes. It is particularly important to design and develop other magnesium alloy composition systems suitable for additive manufacturing and processing;

(4) Insufficient attention has been paid to the production of magnesium based composite materials or magnesium based gradient alloys with additives. Due to the unique nature of additive manufacturing processes, components or composite materials with gradient composition can be produced and processed through different powder bunkers or wire feeders. This idea has been successfully attempted in nickel alloys, titanium alloys, high entropy alloys, and iron aluminum alloys. The successful development of additive manufacturing magnesium based composite materials and magnesium based gradient materials will undoubtedly further leverage the weight reduction advantages of magnesium alloys and broaden their application scenarios.

With the gradual maturity of magnesium alloy additive manufacturing technology, its innovative role is becoming increasingly prominent. Additive manufacturing technology can effectively assist and accelerate the improvement of China's level in magnesium alloy manufacturing and research and development. For a large magnesium alloy resource country like China, this will be a key opportunity to improve China's magnesium technology. Only by being at an advanced level in technology can China's magnesium resource advantages be fully utilized, Take the lead in international industry competition and accelerate the transformation into an industrial powerhouse.