△ Figure 1: The “Mg” shaped lattice structure prepared by laser powder bed melting technology (made of magnesium alloy WE43)

The author and others have systematically studied the evaporation phenomenon during the laser powder bed melting process of AZ91 alloy. In the temperature range between 870 K (AZ91 liquidus temperature) and 2000 K, the evaporation rate of Mg is about 4.2 times that of Al × 104~ 3.5 × 1010 times, 54-160 times of Zn, and 2.3 times of Mn × 105~3.5 × 109 times, and a numerical model was also established to predict the variation of the composition accuracy of AZ91 with the input energy density Ev, achieving an optimal input power density of 60 J/mm3. High or low Ev can lead to high melt pool temperature, and magnesium will preferentially evaporate, causing the alloy composition to deviate from Mg-9Al. On the premise of ensuring the accuracy of alloy composition, the processing window of magnesium alloy laser powder bed melting technology is relatively limited.

△ Figure 2 Laser Powder Bed Melting Technology for Cylindrical and Cubic Magnesium Alloy Samples

In addition to evaporation, porosity must also be considered. Figure 3 provides a schematic diagram of the relationship between different process parameters and related defects, as well as the relationship between product density and input energy density. There are differences in the optimal Ev values corresponding to the lowest porosity among different alloy systems. For Mg Al alloys, the optimal Ev is between 100 and 200 J/mm3. Mg-RE alloy has a larger processing window (50 to 250J/mm3) to achieve low porosity（

△ Figure 3 (a) Schematic diagram of processing window and related defects, (b) Functional relationship between relative density of magnesium alloy and input energy density of laser powder bed melting technology

It can be seen that the alloy composition is also crucial for achieving high density and low porosity. It is difficult to avoid pores in the production of magnesium alloys with additives, and a certain degree of porosity is acceptable, but hot tearing and cracking must be avoided. Compared to cast and forged alloys, there are much fewer commonly used magnesium alloy systems for additive manufacturing. The commercial magnesium powders currently used for additive manufacturing include pure magnesium, AZ91, and WE43, mainly due to their relatively high market demand, better printability, and the characteristics of structural and biomedical materials.

△ Table 1 Input energy density, grain size, tensile and electrochemical properties of laser additive manufacturing of magnesium and magnesium alloys

Table 1 summarizes the tensile properties of magnesium alloys produced by laser powder bed melting. The yield strength is usually above 200MPa, with some reaching 350MPa, which is sufficient for most structural applications. However, low ductility is a major issue, as most laser powder bed melting magnesium alloys have a ductility of less than 5%, and some alloys do not even have any ductility, making them difficult to apply as engineering materials. The high residual stress caused by rapid solidification and the formation of intermetallic compounds along grain boundaries are the root causes of ductility failure.

In addition, the sputtered powder or steam will redeposit on the surface of the sample, resulting in poor consolidation or weak bonding, which is also detrimental to the ductility of the sample. According to reports, the WE43 alloy magnesium alloy produced by laser powder bed melting has the highest ductility to date, reaching 12.2%. The improvement of sample ductility can be achieved through subsequent high-temperature annealing treatment, or by optimizing powder quality, composition, and processing technology. At present, the most promising application for additive manufacturing of magnesium alloys is biodegradable implants. The electrochemical corrosion resistance of pure magnesium and some magnesium alloys produced by laser powder bed melting is shown in Table 1. The corrosion resistance of WE43 alloy produced by laser powder bed melting is far inferior to that of cast alloys. In contrast, Mg Al alloys exhibit similar corrosion resistance to cast alloys. For the Mg Zn system, the corrosion current density and hydrogen evolution rate of ZK60 alloy produced by laser powder bed melting are better than those of cast ZK60 alloy, but the surface corrosion of the sample is more severe. In addition, considering the biocompatibility of laser powder bed melting to produce Mg alloy, a large number of studies have reported the biocompatibility of laser powder bed melting to produce WE43 alloy as an in vitro scaffold implant. No cytotoxicity from Mg-RE alloy has been found, but the intense hydrogen evolution reaction on the surface of the bare material leads to local pH changes that can damage cell metabolism. This problem can be solved through surface modification. In addition to WE43 alloy, there are also reports on the laser powder bed melting manufacturing of Mg Nd Zn Zr (JDBM) stent implants, similar to the research on WE43 alloy.

Non laser magnesium alloy additive manufacturing technology

Table 4 Process parameters, grain size, and mechanical properties of magnesium alloys manufactured using friction stir additive manufacturing

In contrast, laser additive manufacturing technology has demonstrated high dimensional accuracy and has prepared a series of high-strength magnesium alloys. Despite limited ductility, it has broad development prospects. Magnesium alloys made with non laser additive materials have moderate strength and considerable ductility compared to them. In addition, the laser powder bed melting technology of magnesium alloys is most suitable for biomedical applications, but this method is limited in product size. The scalability of adhesive spray additive manufacturing technology is conducive to large-scale production. For example, in large-scale industries such as the automotive industry, development is relatively slow, and more comprehensive research is needed to gain a deeper understanding of their printing and sintering behavior, microstructure evolution patterns, mechanical and electrochemical properties.

summary

In summary, magnesium alloy additive manufacturing has broad prospects. Through additive manufacturing technology, it can be achieved, including but not limited to: expanding the solubility limit of solute elements in magnesium and exploring the alloying behavior of previously insoluble elements (including transition metals); Realize direct production of thin-walled and rod components, and prepare ultra lightweight components; A micro and macro layered porous structure that can simulate the preparation of human skeletal structures, used for devices and intelligent components in biomedical applications.

At the same time, this review indicates that there are still a series of scientific, technical, and practical difficulties in the production of magnesium and magnesium alloy powders, such as the mechanism of dislocation density, residual stress, component segregation, and pores affecting performance (strengthening mechanism and adverse effects on ductility), the impact of raw material powder preparation process and state characteristics, additive manufacturing process parameters, and post-treatment on product performance, and the safety and consistency of magnesium and magnesium alloy powder production, Further research and resolution are urgently needed.