Density of magnesium alloy is about 67% of aluminum and 23% of steel. It also has high specific strength/specific stiffness, good dimensional stability, easy machining, good thermal and electrical conductivity, damping and vibration reduction, good electromagnetic shielding and easy to recycle. With characteristics such as recycling, it has broad application prospects in aviation, aerospace, automobiles and other fields.
Plastic deformation processing of magnesium alloys is relatively difficult due to hexagonal close-packed crystal structure of α-Mg, and casting is often main method. Among them, die casting has become main production method of magnesium alloy products due to its characteristics of easy mass production and near net shape. However, during die-casting process, metal liquid is filled at high speed, which easily generates turbulence and entrains gas, resulting in a large number of pore defects inside casting and reducing toughness of casting. Starting from strengthening mechanism of magnesium alloys, characteristics of die-casting, gravity casting and thermal processing and other preparation processes are compared to explore effective ways to improve strength of magnesium alloys under die-casting conditions. Combined with research work on high-strength and tough die-cast magnesium alloys in recent years, we explore development direction of strengthening and toughening die-cast magnesium alloys. Role of various strengthening mechanisms of magnesium alloys was analyzed, effective ways to improve strength of die-cast magnesium alloys were discussed based on strengthening characteristics of magnesium alloy gravity casting and hot processing preparation methods. This paper summarizes current research progress of die-cast magnesium alloys, discusses them from aspects of component development, impact of die-casting technology on microstructure and properties, microstructure refinement, and aging strengthening research. In addition, development direction and prospects of high-strength and tough die-cast magnesium alloys are discussed.

Cooling rate of gravity casting is slow, as-cast structure has coarse grains, and second phase is also continuously coarse. However, through subsequent heat treatment, continuous coarse second phase can be solid dissolved into matrix, then through aging treatment, fine and dispersed precipitate phases can be precipitated, especially nanophases with sizes below 0.1 μm, thereby significantly improving strength of casting.
Thermal processing significantly refines grains through plastic deformation and breaks continuously distributed coarse second phase, making it dispersed and distributed in matrix. Effects of fine grain strengthening and second phase strengthening are obvious. Ding Wenjiang et al. found that yield strength of NZ30K magnesium alloy extruded at 350℃ can reach 227MPa, mainly relying on grain fineness and dispersion strengthening. In addition, after aging treatment of NZ30K extruded at 350℃, yield strength can be increased to 290MPa. Although magnesium alloy materials prepared by thermal processing have excellent properties, extrusion, rolling and drawing are mostly suitable for production of magnesium alloy profiles, plates and bars.
Compared with gravity castings, die castings have a dense structure and refined grains and second phases. At present, under die-casting conditions, grain size of magnesium alloy can be less than 10 μm, second phase volume fraction is between 5% and 16%. It is difficult to refine second phase particles with a volume fraction of more than 10% to less than 0.5 μm through cooling speed control. Aging precipitate phase precipitated through heat treatment is small in size, up to nanometer level, and can be significantly strengthened without a large volume fraction. Therefore, it is necessary for die-cast magnesium alloys to combine cooling rate refinement and aging precipitation to jointly regulate the second phase, forming a synergistic strengthening effect of micron-scale solidification precipitates and nano-scale aging precipitates. In recent years, there have been many studies on age-strengthenable die-cast magnesium alloys, which is one of main directions of current research on high-strength and toughness die-cast magnesium alloys.

At present, research on high-strength and tough die-cast magnesium alloys mainly includes: develop high-strength and tough magnesium alloy systems and components suitable for die-casting process, which requires that developed alloys have good casting properties and are not prone to hot cracking; study impact of die-casting process on structure and properties, focusing on study of pre-crystallized structures (ESCs: External Solidified Crystals) and pore defects in pressure chamber; study grain refinement and structure regulation through cooling rate control or adding refiners under die-casting conditions; study age-strengthenable die-cast magnesium alloys and their age-strengthening effects, and heat treatment by eliminating pore defects through vacuum die-casting.

At present, die-cast magnesium alloys researched and developed are mainly divided into Mg-Al series and Mg-Al-RE series. Al is beneficial to casting performance of magnesium alloys, while RE is beneficial to improving strength and high-temperature properties of magnesium alloys. Therefore, Al and RE have become main alloying elements of die-cast magnesium alloys. On this basis, Ca, Sr, Si, Sn and other elements are usually added.
Mg-Al system is the most commonly used die-casting magnesium alloy system, such as AZ91D and AM60 alloy. Among them, AZ91D has excellent casting performance but poor plasticity, while AM60 has better plasticity, is often used to manufacture shock-absorbing and impact-resistant safety components such as dashboard supports and car seat frames. Among them, AXT710 (X:Ca, T, Sn) has the best comprehensive mechanical properties, trace amounts of Sn play an important role in it. After adding a trace amount of Sn, it simultaneously plays roles of solid solution strengthening, second phase strengthening and fine grain strengthening.

Table 1 Room temperature mechanical properties of Mg-Al die-cast magnesium alloy

 

Figure 1 SEM structure of AX71 and AXT710
Table 2 shows mechanical properties of Mg-Al-RE die-cast magnesium alloy. RE in AE44 is added in the form of natural mixed rare earths, which contain rare earth elements such as La, Ce, and Nd. Due to high price of Nd, La and Ce are cheap. Therefore, it is necessary to study individual effects of La, Ce, Nd, etc. on performance of AE44, so as to adjust ingredients to obtain a better balance between performance and price. After research, it was found that La has the best strengthening effect, while Nd has the worst strengthening effect. From both cost and performance perspectives, priority is given to using La or La/Ce mixture (a by-product of Nd extraction from natural mixed rare earths) instead of natural mixed rare earths to add AE44.

Table 2 Mg-Al-RE die-cast magnesium alloy
Room temperature mechanical properties
ASm44 prepared by completely replacing La with Sm has an elongation of 21%, but strength is lower. Using ASm44 as benchmark alloy and introducing dispersion strengthening phases such as Mg2Si for strengthening, there is a lot of room for strength improvement. After adding Cu, two strengthening phases, Al4Cu9 and Mg2Cu6Al5, were formed, and Mg17Al12 phase continuously distributed along grain boundaries was reduced. Therefore, strength and plasticity of ASm62-3Cu alloy are greatly improved compared to ASm62-1Cu alloy. However, excessive Cu will coarsen brittle Mg2Cu6Al5 and form network connections, seriously affecting plasticity. Elongation of ASm62-5Cu will be greatly reduced, and tensile strength will also be reduced. Since Al content in ASm62 is relatively excessive (compared to Sm), a large number of Mg17Al12 phases are formed along grain, so performance is far inferior to ASm44. By conducting relevant research on the basis of ASm44, it is expected to obtain high-strength and tough die-cast magnesium alloy materials.

In the general cold chamber die-casting process, when melt flows in pressure chamber, it contacts with cold wall of pressure chamber and causes dissipation of superheat, some crystals will nucleate or even grow, and enter mold cavity together with melt. This part of crystals will nucleate and grow up in advance during final solidification process. It is prone to form coarse grain structures during final solidification process. These coarse grain structures are called pressure chamber pre-crystallized structures (ESCs), and their distribution is affected by turbulence of high-speed injection. Defects such as ESCs and pores have a great impact on performance of castings, can easily cause problems such as stress concentration and defect zones, reducing the strength and toughness of castings.

Some researchers studied effect of cooling rate on structure and properties of AZ91 and AM series die-cast magnesium alloys by preparing die-cast samples with different wall thicknesses. Results show that as cooling rate increases, average grain size of die-cast specimens is significantly refined, tensile strength and elongation are improved. In addition to cooling rate control, adding refiners is another effective way to refine grains in conventional casting processes. For aluminum-free magnesium alloys, grains are refined mainly by adding Zr element, such as Mg-Zn and Mg-RE systems. At present, it is mainly added in the form of Mg-Zr master alloy. During solidification process, Zr atoms preferentially precipitate in α-Zr phase. α-Zr and α-Mg have a close-packed hexagonal structure, lattice constant (a=0.323nm, c=0.514nm) is very close to Mg (a=0.320nm, c=0.520nm), which can serve as heterogeneous nucleation points for magnesium grains, thereby refining grains. It is generally believed that nucleation efficiency is related to size and interface of nucleating particles.

 

Figure 2 Microstructure of AM50 die-cast magnesium alloy

In order to exert effect of aging strengthening to improve strength of die castings, vacuum die casting is usually used to reduce porosity so that it can be subjected to high-temperature heat treatment. In recent years, researchers have conducted many studies and optimizations on vacuum die-casting processes based on AZ91D and AM60B, found that porosity and ESCs can be greatly reduced by reasonably setting segmented slow injection and adding liquid flow buffer packages. Vacuum die-cast Mg-8Gd-3Y-0.4Zr rare earth magnesium alloy castings were heat treated (T6 state: 500℃×2h solid solution treatment, 225℃×16h aging treatment), tensile strength can reach 275MPa, yield strength can reach 241MPa, and elongation is 7.2%. Compared with die-cast state without heat treatment, mechanical properties are significantly improved. However, vacuum die-casting greatly increases cost of equipment configuration and difficulty of control technology. Another aging strengthening idea is to lower solution temperature and shorten solution time, or direct aging treatment (T5 treatment) to avoid blistering and deformation of die castings caused by long-term high temperature.

In research on age strengthening of Mg-Al-RE die-cast magnesium alloy, it was found that adding a trace amount of Mn can make Mg-Al-RE alloy precipitate a nano-precipitate phase during aging at 200℃ without solid solution treatment, thereby improving strength of alloy. This overcomes disadvantages that die-cast magnesium alloys cannot undergo high-temperature solution treatment due to pore defects, and precipitation strengthening effect is limited. Solidification precipitate phase in Mg-Al-RE series die-cast alloy is refined by controlling cooling rate and adding refiners, combined with aging treatment, nano-precipitate phase is precipitated, which can further improve strength of die-cast magnesium alloy. At the same time, by controlling die-casting process, defects such as pores in die-casting parts can be reduced and toughness can be improved. Based on this, it is expected to prepare high-strength and tough die-cast magnesium alloy products, replace more aluminum and steel parts, reduce weight of automobiles, and promote development of lightweight automobiles.