Low-pressure casting is one of important processes for producing large, complex, high-performance magnesium alloy castings. In recent years, it has shown a high application prospect and has attracted more and more attention from researchers. This paper introduces development status, forming process and characteristics, main material classification, strengthening mechanism, heat treatment scheme and common defects of low-pressure casting magnesium alloys, and looks forward to the future development direction of low-pressure casting magnesium alloys.

Magnesium is structural metal with the lowest density. It is rich in resources and has advantages of high specific strength and specific stiffness, good vibration damping, strong electromagnetic shielding ability, easy cutting and easy recycling. It is widely used in the fields of automobiles, aerospace, electronic products and national defense military products. Magnesium is an emerging structural metal material developed after steel and aluminum alloys. Its lattice structure is different from that of steel and aluminum. It is known as green engineering material of 21st century. Magnesium alloys have strong applicability in casting technology. Among several technologies for preparing magnesium alloy materials and components, casting magnesium alloys has the longest development history, the most mature technology and equipment. With continuous development of high-tech, demand for high-performance magnesium alloys is increasing, casting process has also been improved and upgraded. At present, high-pressure casting is the most commonly used forming method for producing magnesium alloy parts, but die casting is only suitable for production of large quantities of thin-walled parts. Investment cost of its equipment and molds is high, and it is easy to have casting defects such as air entrainment, which affects quality of finished product. Low-pressure casting is a casting method between high-pressure casting and gravity casting. It has stable filling and is more suitable for forming large and complex magnesium alloy castings. Castings can be strengthened by heat treatment, and mechanical properties are further improved. Low-pressure casting magnesium alloys have shown good application prospects in recent years, have attracted more and more attention.

Low-pressure casting refers to a casting process in which molten metal is filled from bottom to top under action of low gas pressure and solidified under pressure to form a casting. During filling process of magnesium alloy liquid, fluidity is improved, it is easy to obtain castings with different wall thicknesses and complex structures. At the same time, during filling process, its pressure is controllable, which can effectively alleviate tumbling and splashing of magnesium liquid during filling, make magnesium liquid fill mold smoothly, reduce formation of casting defects such as oxidation inclusions and looseness, thus obtain high-quality castings. Common low-pressure casting methods include the most commonly used top-type gas pressure low-pressure casting, vacuum low-pressure lost foam casting, and electromagnetic low-pressure casting. Principle of commonly used top-type gas pressure low-pressure casting technology is shown in Figure 1. This method is to pass 0.02~0.06 MPa compressed gas into a sealed crucible, so that metal liquid slowly fills mold cavity from bottom to top. After filling is completed, pressurization is stopped and pressure is maintained until casting is completely solidified, then pressure is released to allow residual metal liquid to flow back to crucible, and finally casting is demolded. In this process, tumbling, impact and splashing of molten metal are significantly reduced, thereby greatly reducing generation of oxide slag; at the same time, its pressure filling method ensures high fluidity of alloy liquid and density of casting, which is suitable for production of large and complex thin-walled castings. It is worth mentioning that due to simple pouring and riser system designed by this process, metal recovery rate can reach more than 90%, which greatly saves production costs. Process control of low-pressure casting has a great influence on performance of final casting. Taking low-pressure casting magnesium alloy wheel hub as an example, in casting process, filling order of magnesium alloy liquid is usually wheel center-spoke-rim-rim, where rim are thin-walled areas away from gate, so solidification order should be rim-rim-spoke-wheel center, which is sequential solidification. If wheel hub does not achieve sequential solidification during solidification process, solidification defects are prone to occur, such as thermal cracking defects at spokes and rims, coarse grains at wheel center, shrinkage cavities and shrinkage on rim, etc. Therefore, in order to avoid occurrence of casting defects as much as possible, it is necessary to adjust mold temperature by local cooling or adjust pressure holding process to optimize casting process. Vacuum low-pressure lost foam casting technology is a new casting process that integrates vacuum lost foam casting and low-pressure casting. Its schematic diagram is shown in Figure 2. Process steps are: first, put lost foam casting pattern in bottom injection sand box, add molding sand and compact it, and start vacuuming; then add magnesium liquid into pouring furnace, and at the same time, inert gas is introduced into pouring furnace. Magnesium liquid flows into sand box under pressure, high temperature brought by it can gasify lost foam casting pattern and finally complete pouring. This process has less equipment investment than die-casting process, castings can be strengthened by heat treatment, and casting cost performance is improved; castings produced by sand casting have higher precision and better surface quality, production cycle is improved, and final castings have higher comprehensive performance. Process requires a low pouring temperature and a small loss of pouring temperature. Pouring system of alloy casting is simple and effective, with a high yield rate. It can be used for liquid forming of complex magnesium alloy components, can obtain high-precision, low-surface roughness and high-performance magnesium alloy castings, which improves defects of insufficient pouring and cold shut that are easy to occur in traditional casting processes. Zhang Dafu et al. applied this technology to liquid precision forming of AZ91D magnesium alloy, and successfully prepared complex magnesium alloy castings such as motor housings and exhaust pipes, effectively improving defects of insufficient pouring and cold shut of magnesium alloys, and greatly improving performance of castings.

 

Figure 1 Schematic diagram of top-mounted gas pressure low-pressure casting technology
Conventional magnesium alloy low-pressure casting process mainly uses compressed air to fill mold and maintain pressure to complete production of castings. Electromagnetic low-pressure casting technology is a casting method that uses Lorentz force to make molten metal rise and fill mold, and its schematic diagram is shown in Figure 3. It has high production efficiency, continuous and nearly zero-residue precision forming, and has unique advantages in the field of magnesium alloy low-pressure casting. In this process, an electric current is passed through liquid metal, and liquid metal moves in a directional manner due to Lorentz force. When a higher current and magnetic field strength are reached, force generated can meet requirements of conventional low-pressure casting. Liquid metal is filled under computer control to ensure that liquid level rises steadily at different cross sections. Liquid metal is filled into mold, and after a certain solidification time, it is pressurized and maintained to finally obtain a high-quality casting. Studies have shown that aluminum alloy wheels of motorcycles produced by electromagnetic pump low-pressure casting devices have obvious performance advantages over those produced by traditional low-pressure casting. However, compared with aluminum alloys, magnesium alloys have characteristics of low specific heat and low latent heat of solidification, which leads to a fast cooling speed of magnesium alloy liquid in mold cavity. Therefore, it is necessary to adjust process parameters, such as rapid filling and strict control of mold temperature.

 

Figure 2 Schematic diagram of vacuum low-pressure lost foam casting technology for magnesium alloy

 

Figure 3 Schematic diagram of electromagnetic pump low-pressure casting process
In addition, Wu Guohua and others combined coating transfer core making technology, crucible liquid metal sealing technology and low-pressure casting technology to innovatively develop a new process for precision low-pressure casting of large magnesium alloy castings, ensuring high purity and high cleanliness of magnesium alloy liquid. Manufactured magnesium alloy castings have dense structure, high dimensional accuracy and good surface quality, have ability to develop and conduct small batch trial production of 100 kg magnesium alloy castings.

As early as beginning of 20th century, Britain proposed low-pressure casting technology and applied it to magnesium alloy production, but it was limited by technology and equipment at that time and has not been widely used. After World War II, automobile industry began to develop rapidly, demand for automobile wheels and other parts continued to increase. Such parts are also suitable for low-pressure casting production, and technology has developed rapidly, but equipment and technology are only in the hands of a few developed countries. After mid-20th century, countries began to invest manpower and material resources to conduct in-depth research on principles and processes of low-pressure casting technology. With progress of globalization and technological development, low-pressure casting equipment has been continuously upgraded, and its application range has become wider. It is now widely used in aerospace, automobile and various mechanical industries.
Take low-pressure cast aluminum alloy wheels as an example. Its market share in the 1980s was only 4%, but by the beginning of 21st century, its market share had expanded to more than 60%. At present, low-pressure casting has become the most important process for producing aluminum alloy wheels. In order to further accelerate lightweighting process of automobiles, manufacturers and researchers in various countries are accelerating research and development and production of magnesium alloy wheels to replace aluminum alloys to achieve deeper lightweighting. Research mainly focuses on new materials and new processes. Research shows that to obtain same stiffness as aluminum alloy wheels, wall thickness of magnesium alloy wheels only needs to be increased to 1.17 times that of aluminum alloy wheels, and its weight is only 78% of that of aluminum wheels, which has a significant lightweight effect; using magnesium alloy wheels instead of aluminum alloy wheels on new energy vehicles can increase vehicle endurance by more than 8%, acceleration performance and vibration reduction effect of vehicle are also significantly improved. Currently, casting processes used for magnesium alloys include gravity casting, high pressure casting, extrusion casting and low pressure casting. Among them, low pressure casting has unique advantages in manufacture of magnesium alloy parts due to its characteristics such as stable filling and sequential solidification, and has received widespread attention in industrial field. However, production of magnesium alloy wheels through low pressure casting technology still needs to be further studied and improved, technology is not mature yet and has not been widely promoted and applied.
Since 21st century, with development of computer technology, numerical simulation technology has been widely used in development and optimization of low pressure casting processes. By using software to numerically simulate casting process of magnesium alloys, location and causes of defects such as pores, looseness and slag inclusions in castings can be predicted in advance, shortening product process development cycle, reducing number of mold trials, and reducing production costs. Li Jingyi et al. used casting simulation software EasyCast and NovaCast to numerically simulate low-pressure casting process of magnesium alloy partition castings. It was found that solidification process of partition castings did not follow principle of sequential solidification and shrinkage cavities were generated. By adding cold iron to improve process, shrinkage compensation effect was effectively enhanced and shrinkage cavity defects were improved. Zhang Hua et al. analyzed casting process of AZ91D magnesium alloy wheels based on AnyCasting casting simulation software, accurately located location of casting defects and found out causes of defects, then optimized casting process, reducing number of mold trials, thereby greatly reducing costs; results of numerical simulation also provide theoretical guidance and basis for low-pressure casting of magnesium alloy wheels, promote process development and production process of low-pressure casting magnesium alloy wheels.
In recent years, rapid development of new energy vehicles has put forward higher requirements for lightweighting in automotive industry. Made in China 2025 proposes an overall plan for automobile development. Lightweighting has become one of latest core development goals of automobile manufacturers around world. Figure 4 shows weight reduction effect of magnesium alloy on automobile parts. For every 10% reduction in weight of pure electric vehicles, cruising range can be increased by about 6%. Weight of battery pack system of vehicle accounts for more than 20% of vehicle, and cost accounts for 30% to 60% of vehicle. In power battery system, battery shell accounts for about 2% to 30% of the total weight of system and is main structural component. Therefore, under premise of ensuring functional safety of battery system and the overall safety of vehicle, lightweighting of battery shell has become one of main improvement goals of battery system. "Research Report on Market Status of Aluminum-Magnesium Alloy Casting Industry" points out that magnesium alloy castings currently have three major conditions for replacing aluminum alloy castings in the field of lightweight automobiles, namely: density of magnesium alloy is about 2/3 of that of aluminum alloy, and its specific strength is higher than that of aluminum alloy and steel. Use of magnesium alloy to replace aluminum alloy can achieve a weight reduction rate of 25% to 35%; price ratio of magnesium to aluminum falls back to 1.3 range, and "magnesium replacing aluminum" is more cost-effective economically; magnesium alloy has advantages in vibration resistance, heat dissipation, and strength. In 2022, domestic use of magnesium alloy for a single vehicle will be only 3 to 5 kg, compared with use of more than 100 kg for aluminum alloy single vehicles, and development potential is huge. Full set of electric drive system housings for Zhiji L7 model is the first mass production application of magnesium alloy materials on new energy electric drive housings. This is also one of the largest magnesium alloy components currently used in mass production in automotive field at home and abroad. Subsequently, SAIC also developed a number of magnesium alloy new energy electric drive assembly products and successfully mass-produced them, which accelerated the expansion of new application scenarios for magnesium alloys and also heralded a bright application prospect for magnesium alloys in new energy vehicles. As performance of new energy vehicles gradually improves, requirements for lightweight and performance of components such as electric drive housings and battery trays have also increased, demand for low-pressure cast heat-treatable and strengthened magnesium alloy components has become more urgent.

 

Figure 4 Weight reduction effect of magnesium alloy on automotive parts

Pure magnesium has poor strength, so pure magnesium is usually alloyed to obtain magnesium alloys. Alloying elements directly strengthen mechanical properties of alloy through precipitation hardening, fine grain strengthening and solid solution strengthening. Table 1 briefly introduces the most common alloying elements and their effects on magnesium alloys. Casting magnesium alloys can be mainly divided into Mg Al series, Mg-Zn series and Mg-RE series according to main alloying elements. At present, the most widely used low-pressure cast magnesium alloy is Mg-Al-Zn (AZ) series.

Mg⁃Al series is the earliest magnesium alloy material used, Al is the most favorable element for improving the casting performance and mechanical properties of magnesium alloys. From Mg-Al binary phase diagram (as shown in Figure 5), it can be seen that solid solubility of Al in α-Mg matrix changes significantly with temperature, its limit solid solubility is 12.7% (mass fraction, 436 ℃), and solid solubility at room temperature is about 2% (mass fraction). With increase of Al content in magnesium alloy, crystallization range of alloy becomes narrower, fluidity of alloy is improved, tendency of hot cracking is improved, defects such as shrinkage are reduced, and mechanical properties are also significantly improved. When Al addition of alloy exceeds 6% (mass fraction), it can also be heat-treated and strengthened. Adding alloying elements such as Zn and Mn on the basis of Mg-Al alloy (as shown in Table 2) can further improve casting performance, mechanical properties, high temperature properties and corrosion resistance of magnesium alloy, and is widely used in industrial production.
Limit solid solubility of Zn in Mg-Al-Zn (AZ) alloy is 6% (mass fraction). Zn mainly plays a role in solid solution strengthening. After heat treatment, solid solution structure can be obtained to improve yield strength and creep resistance of alloy. However, when mass fraction of Zn is too high, it is not good for corrosion resistance of alloy. Therefore, Zn addition amount in magnesium alloy is generally controlled below 2% (mass fraction). Among many AZ alloys, AZ91D is the most widely used in low-pressure casting. This alloy has excellent formability and high mechanical properties, and can be applied to a variety of forming processes to manufacture complex structural parts. However, a low-melting-point brittle phase β-Mg17 Al12 will form in this series of alloys. With increase of alloy load, microcracks are easily formed on β-Mg17 Al12 phase and β-Mg17 Al12 /α-Mg phase interface. These microcracks gradually connect to form main cracks until the alloy breaks and fails, as shown in Figure 6. In addition, β-Mg17 Al12 phase is easy to soften and coarsen at high temperatures, resulting in low high-temperature strength and poor high-temperature creep performance. Enrichment on grain boundary will reduce creep resistance of alloy, so that alloy can only be used in working conditions below 125 ℃, which limits development of this series of alloys. In recent years, in order to improve the overall properties of alloy, researchers have tried to add micro-alloying elements to AZ91 alloy. Among them, adding trace amounts of rare earth elements has the most significant improvement in alloy properties. Wang et al. studied effect of adding different amounts of Gd on properties of AZ91 as-cast alloy. Results showed that adding a trace amount of Gd can improve strength and plasticity of AZ91 alloy, and α-Mg grains and Mg17 Al12 phase are significantly refined, because alloy A dispersed spherical Al8 Mn4 Gd phase is formed in phase, which has a strengthening effect as second phase; in addition, this phase can serve as a heterogeneous nucleation point for α-Mg grains and Mg17 Al12 phase, also plays a role in refining grains and structure. Chemical effect improves mechanical properties of alloy. Boby et al. added Sn and Y to AZ91 alloy, found that adding Sn formed a new Mg2 Sn phase, which inhibited nucleation and growth of discontinuous phase Mg17 Al12; when adding Y and Sn at the same time, a new Al2 Y phase appears, volume fraction of Mg17 Al12 decreases, grain size is refined, room temperature and high temperature tensile properties and corrosion resistance of alloy are improved. Optimal addition amounts of Sn and Y are 0.5% and 0.9% (mass fraction) respectively. Cai et al. found that adding Ce, Y and Gd can refine Mg17 Al12 grain size, optimize its distribution, reduce content of Mg17 Al12 phase in alloy, thereby improving alloy's mechanical properties and corrosion resistance, among which Ce has the most significant effect. Followed by Y and Gd. Up to now, addition of microalloying elements such as Ce, Nd, Y, Ca, Ti, B, Sr, Sb and Bi to AZ91 magnesium alloy has been widely studied, among which Ce, Nd, Y, Bi and Sb have the best effect on improving mechanical properties.

Table 1 Common alloying elements in magnesium alloys and their effects on alloys

 

Figure 5 Mg-Al binary phase diagram

 

Table 2 Composition of commonly used Mg-Al low-pressure casting magnesium alloys
Ultimate solid solubility of Mn in Mg-Al-Mn (AM) series alloy is 3.4% (mass fraction). Strength of this series alloy is lower than that of AZ series alloy, but it has excellent toughness and plasticity and is suitable for applications that need to withstand impact loads and requirements. In places with high safety, such as seat frames, equipment dashboards and automobile wheels, AM50 and AM60 alloys are the most widely used in low-pressure casting. Japan, Italy, United States and other countries use AM60A and AM60B to manufacture excellent performance die-cast magnesium alloy wheels; China also produces AB60B automobile wheel castings through metal mold gravity casting, with a process yield rate of up to 65%, which has accumulated experience for low-pressure casting production of magnesium alloy wheels.
In addition to above AZ series and AM series magnesium alloys, directly adding alloying elements such as Ce, Nd, Y, Sb, Bi, Ca, Ti, B and Sr to Mg-Al series alloys can achieve grain refinement and Melt purification, solid solution strengthening and second phase strengthening effects. Adding 2%~4% (mass fraction) Ce element to Mg-4Al alloy can form Al11 Ce3 and reduce formation of Mg17 Al12. AE44 alloy obtained by design optimization is suitable for high temperature working conditions, such as Corvette engine frame, whose operating temperature is as high as to 150 ℃. However, about 16% (mass fraction) of Ce is required to completely suppress formation of Mg17 Al12 phase in Mg-4Al alloy, which is too costly for most automotive applications, so Mg-Al-Ca alloy (AX series) is designed as a lower-cost alternative to AE series alloy. Simulation results of solidification path of Mg-Al-Ca by Luo et al. show that with increase of Ca content, fraction of Mg17 Al12 phase decreases, while fraction of (Mg, Al)2 Ca phase increases; when mass fraction of Ca reaches 3% (AX53 alloy), Mg17 Al12 phase is completely suppressed by binary Mg2 Ca and ternary (Mg, Al)2 Ca phases. Room temperature yield strength, yield strength and creep strength of cast alloy are 186, 151 and 74 MPa, respectively, which far exceed performance of AM50 cast alloy and have good application prospects in automotive parts.

In Mg-Zn cast magnesium alloys, Zn mainly plays role of solid solution strengthening and aging strengthening, and alloy can be strengthened by heat treatment; in addition, Zn can also effectively avoid deterioration of corrosion performance caused by impurity elements such as iron and nickel in magnesium alloys. Mg-Zn-Al (ZA) cast magnesium alloy has good casting performance and excellent creep resistance, its mechanical properties even exceed those of traditional AZ and AM systems. Cast magnesium alloy ZA-based alloy has good age hardening response, and its aging precipitation sequence is: SSSS (supersaturated solid solution) → GP zone → β1 ʹ (Mg4 Zn7) → β2 ʹ (MgZn2) → β (MgZn or Mg2 Zn3). Taking ZA64 alloy as an example, after solution and aging treatment, its tensile strength increased from 183 MPa in cast state to 260 MPa, and elongation increased by more than 1 times. Further adding alloying elements, such as Cu, Sn, Sr, Ca, Sb and RE, etc., to Mg-Zn cast magnesium alloys is the most commonly used method to improve mechanical properties of ZA series alloys. However, ZA series magnesium alloys have a high Zn content, alloy is prone to severe hot cracking and has poor casting performance, which limits its industrial application. This series of alloys is still in research stage and awaits further commercial application.

Mg-RE series (magnesium-rare earth series) cast magnesium alloys are currently an important direction in development of high-performance magnesium alloys. Rare earth (RE) elements have a certain degree of purification effect on magnesium alloy melts, can also improve casting performance, optimize structure, improve alloy mechanical properties and creep resistance, etc. Adding RE elements can increase strength of magnesium alloys by 100% to 250% %, ultimate operating temperature is increased to 350 ℃, corrosion resistance is significantly improved, and application fields of magnesium alloy materials are expanded. As the solid solubility of rare earth elements in magnesium alloys increases, its strengthening effect will become more obvious. Figure 7 shows difference in performance and density of Mg-RE, Mg-Al, and Mg-Zn casting alloys. At present, Mg-Y, Mg-Nd, and Mg-Gd series alloys have strong age hardening effects and great potential for practical application. They have become the most widely researched and applied Mg-RE series cast magnesium alloys.
Solid solubility of Y element in α-Mg is high, so alloy has strong aging strengthening ability. Its aging precipitation sequence is: SSSS (supersaturated solid solution) → βʹʹ (DO19-Mg3 Y) → βʹ (cbco-Mg7 Y) → β1 (fcc-Mg3 Y) → β (fcc-Mg5 Y). A large number of studies have shown that peak age strengthening effect of Mg-Y alloys mainly comes from dispersed and precipitated nanoscale metastable βʹ phase. Orientation relationship between this phase and α-Mg matrix is: (100) βʹ // (1-210) Mg, [001]βʹ //[0001]Mg, its three-dimensional structure is shown in Figure 8. In order to further improve alloy strength and creep resistance, researchers added other rare earth elements to Mg-Y alloy binary alloy to form an Mg-Y-RE ternary alloy system. Among them, WE43 is one of the most typical magnesium rare earth cast magnesium alloys. After proper solid solution and aging treatment, strength of alloy cast by metal mold is greatly improved, tensile strength can reach 345 MPa, and it can still maintain good mechanical properties at 300 ℃; adding trace amounts of Zn to WE43 alloy can promote precipitation of dispersed fine precipitates and produce long period ordered structure (LPSO) phases, thus improving age hardening response speed and mechanical properties of alloy, especially elongation.

 

Figure 8 Aging precipitation strengthening model of Mg-Y series alloy
Zn, Y, Nd and other alloying elements are added to Mg-Gd cast magnesium alloy to form a ternary or multi-component alloy, which has higher mechanical properties and tensile strength can even exceed 350 MPa. When Zn element is added to Mg-Gd system, aging precipitation sequence of alloy will change. When mass fraction of Gd in alloy is less than 6%, the sequence is: SSSS→ γʹʹ (hcp-Mg70 Gd15 Zn15 ) → γʹ (hcp -MgGdZn) → γ(hcp Mg12 GdZn). γʹ phase is formed on α-Mg basal surface, which can increase peak aging hardness of alloy. When Zn is added to an alloy with a high Gd content, precipitation strengthening comes from composite effect of βʹ phase precipitated on matrix prism surface and γʹ phase precipitated on basal surface, and mechanical properties of alloy are further improved. After Rong et al. added 1% Zn (mass fraction) to the Mg-15Gd-0.4Zr alloy, yield strength of peak-aged alloy increased from 232 MPa to 288 MPa, and tensile strength increased from 296 MPa to 403 MPa. This is because γʹ and βʹ phases are relatively vertically distributed in matrix, which produces a composite strengthening effect on matrix. Structure morphology is shown in Figure 9. In addition, Shanghai Jiao Tong University developed a high-strength and heat-resistant casting Mg-10Gd-3Y-0.5Zr (GW103K, JDM2) alloy based on Mg-Gd alloy, which has better fluidity and resistance to hot cracking during low-pressure casting or gravity casting. After solid solution and aging treatment, mechanical properties of alloy at room temperature are greatly improved, with a yield strength as high as 240 MPa and an elongation of 6%. Yield strength can still maintain a high level at a high temperature of 300℃, which is useful for aerospace, military, etc. It has made great contributions to lightweighting in the field and has been used in preparation of lightweight missile casings and radar components.
Ultimate solid solubility of Gd element in α-Mg is higher than that of Y. When temperature is cooled from 548 ℃ to 200 ℃, its solid solubility drops from 23.5% to 4% (mass fraction), so when Mg-Gd alloy is subjected to aging treatment, a large number of nanophases can be precipitated from matrix, bringing significant age hardening effect. Aging precipitation sequence of Mg-Gd is exactly same as that of Mg-Y series. In peak aging state, βʹ and βʹʹ or βʹ and β1 phases coexist mainly. Alloy has strong room temperature yield strength.
Ultimate solid solubility of light rare earth element Nd in α-Mg is about 3.6% (mass fraction), which can also produce a strong precipitation strengthening effect on magnesium alloys. Aging precipitation sequence of Mg-Nd series alloy is: SSSS → βʹʹ (DO19-Mg3 Nd) → βʹ (fcc-Mg3 Nd) → β (bct Mg12 Nd). βʹʹ is main strengthening phase in peak aging state of alloy. This phase It is completely consistent with magnesium matrix, and orientation relationship between the two is: (0001) βʹʹ // (0001) Mg, βʹʹ //Mg. βʹʹ phase has high thermal stability, can improve high-temperature stability and heat resistance of Mg-Nd alloy. However, growth limiting factor Q value of Nd element in Mg is small and does not have an obvious refining effect on magnesium alloys. Therefore, cast grains of Mg-Nd alloys are usually coarse and cannot be used as structural materials. Usually Additional elements such as Zr or Zn need to be added to refine grains and improve casting performance. Shanghai Jiao Tong University adjusted Nd/Zn ratio based on high-strength and heat-resistant ZM6 alloy, developed Mg-3Nd-0.2Zn-0.4Zr (JDM1) magnesium rare earth alloy with excellent performance and low cost. This alloy has good casting performance and mechanical properties, has been used in automobile wheels, engine brackets and other parts.
Above-mentioned common types of cast magnesium alloy series and their performance characteristics are summarized in Table 3 and Table 4. In actual production, it is necessary to select appropriate alloy material according to usage conditions of casting. There are some performance and application bottlenecks in existing alloy series. Therefore, optimizing alloy compositions and developing new alloys has become one of important research directions for cast magnesium alloys.

 

Figure 9 TEM bright field images and corresponding SAED images of peak aged G15K and GZ151K alloys

Although density of magnesium alloys is low, mechanical properties of currently commonly used magnesium alloy materials are still far behind those of steel and aluminum. Therefore, in order to improve comprehensive performance of cast magnesium alloys, researchers mainly start from aspects of magnesium alloy composition design and optimization, microstructure control, process optimization, heat treatment optimization, etc., to determine strengthening and toughening mechanism of magnesium alloys, develop new high-performance cast magnesium alloys. Magnesium alloys have a close-packed hexagonal structure, deformation mechanisms are mainly dislocation slip and twinning. Grain size, solid solution atoms, and second phase will all affect mechanical properties of cast magnesium alloys.

Fine-grain strengthening refers to improving strength of metal through refinement of grain size in polycrystalline alloys. Hall-Petch formula can be used to describe contribution of fine-grain strengthening to strength σgb: σgb = σ0 + kD−1/2 ( 1). In formula, σ0 is yield strength of initial grain structure of pure metal, ki is Hall-Petch constant, and D is average diameter of grains. k value of magnesium alloy is very large, 280~320 MPa·μm1/2, which is more than 4 times that of aluminum alloy. Therefore, grain refinement is more effective in improving mechanical properties of magnesium alloy. In addition, fine-grain structure is more uniform, and stress concentration cracking is less likely to occur during deformation, which improves plasticity and toughness of alloy. During liquid forming process, fine-grained structure can also significantly improve fluidity of melt and improve quality of alloy ingot. For low-pressure cast magnesium alloys, due to slow filling speed and low melt solidification speed of process, alloy grain structure is often coarse and mechanical properties are also low. Therefore, it is particularly important to study grain refinement method and principle of low-pressure cast magnesium alloys.
Currently available as-cast grain refinement technologies for magnesium alloys can be divided into two methods, which are to achieve grain refinement through melt treatment (such as inoculation, alloying) or stirring (such as ultrasonic treatment) during casting process. Role of solute elements in grain refinement has been widely discussed, and researchers generally believe that component supercooling caused by solute segregation is main cause of grain refinement. Supercooling of components can inhibit continued growth of original grains by promoting nucleation of new grains, thus achieving a tissue refinement effect. Adding some solute elements to magnesium alloys can effectively inhibit grain growth. This effect can be quantitatively described by growth restriction factor Q (Growth Restriction Factor, GRF). In a binary alloy system, its expression is as follows:

 

In formula, m is slope of liquidus line, C0 is solute concentration in alloy, and ki is distribution coefficient of solute element. Size and shape of grains in binary magnesium alloys are directly related to type and content of solutes present in alloy: the larger Q value, the greater tendency of solute to form a supercooled zone at dendrite/liquid front, resulting in finer grains. Effect is more significant. In binary system cast magnesium alloys, Q values of common solute elements are shown in Table 5. It should be noted that Q value listed in the table is only for binary system, while in actual alloys it is often a ternary or even multi-component system, which is not applicable to this equation, but it still has reference significance.

Table 3 Commonly used cast magnesium alloy series and performance characteristics

Table 4 Mechanical properties of commonly used cast magnesium alloys at room temperature

 

Table 5 Values of growth limiting factors Q for common binary magnesium alloys
It can be seen from Table 5 that adding solute elements with higher Q values such as Zr, Ca and Si can effectively refine magnesium alloy grains. Although Fe element has the highest Q value, it has a negative impact on corrosion resistance of magnesium alloys. Therefore, Fe is generally not considered as an alloying element of magnesium alloys. Q value of Zr element is second only to Fe, and its crystal structure is a close-packed hexagonal lattice. Adding Zr to magnesium alloys that do not contain Al and Mn (such as Mg-RE, Mg Zn-RE, etc.) can significantly refine grains; Zr reacts easily with elements such as Al, Mn, Si, Fe, etc. to form stable intermetallic compounds. These compounds are usually formed in the later stages of solidification and cannot serve as nucleation cores to refine structure; additional consumption of Zr leads to a further reduction in its refinement effect, dual effects limit application of Zr in magnesium alloys containing Al and Mn. Q value of Ca element is also high, Ca and Al will react in situ to generate Al2Ca, which has a high lattice match with Mg and can become an effective heterogeneous nucleation core for magnesium matrix. It has great application potential as a grain refiner in Mg-Al alloys and has been widely studied in recent years. In addition to elements listed in Table 5, Wang et al. added 0.01% Ti (mass fraction) to AZ31 magnesium alloy, grain size was significantly reduced; Q value of Ti was calculated to be 59500, which is higher than that in Mg-Zr system. Growth limiting factor is 4 orders of magnitude higher, indicating that extremely high compositional supercooling ability of Ti as a solute element leads to efficient grain refinement of AZ31 alloy.
There are also some elements that will react with other solute elements in magnesium alloy melt to form stable intermetallic compounds. Some of these particles can serve as nucleation cores during solidification process, while others will gather at grain boundaries to hinder grain growth. Finally, grain refinement effect is achieved, such as adding Mn and RE elements. Mn is one of few elements that can undergo a peritectic reaction with Mg. Therefore, although Mn has a very low Q value, it is still widely studied as a refinement element for magnesium alloys. However, Mn is usually removed after melt is overheated. In order to have a refining effect, adding a small amount of Mn to AZ91 alloy can significantly refine grains after overheating reaction. Adding some rare earth elements to cast magnesium alloys can often have a significant grain refinement effect. After Wang et al. added La, Pr and Ce to Mg-Al system, ingot grains were significantly refined, which was mainly attributed to restriction of grain growth by RE element segregation at frontier of solid-liquid interface. This hypothesis is based on RE-containing intermetallic compounds observed at grain boundaries in cast magnesium alloys. Chia et al. also support this view. They added La, Ce and Nd to Mg and obtained similar grain refinement results.
In addition to adding alloying elements, adding solid particles as grain refiners or in-situ generated particles to low-pressure casting magnesium alloy melt can increase number of nucleated grains during solidification process to achieve effect of grain refinement. For Mg-Al alloys, adding C is an effective method of grain refinement and has been maturely used in industry, but its grain refinement mechanism is still unclear. This method is only effective in magnesium alloys with added aluminum (usually Al content greater than 2%), presence of Be, Zr, Ti and rare earth elements has a negative impact on grain refinement. Adding SiC particles to MgAl alloys can reduce grain size, but mechanism is not yet clear. Some researchers believe that when SiC is added to melt, Al4 C3 is formed and acts as a nucleating agent; other researchers believe that Al2 MgC2 acts as a nucleating agent; others believe that Mg particles are directly in SiC Ongranular nucleation. Liu et al.’s research found that in Al-Ti-C master alloy, TiC particles always adhere to Al4 C3 particles, forming Al4 C3 + TiC particle clusters. Tests show that this particle cluster has a significant refining effect on pure magnesium. In addition to above, Al-Ti-B refiners, NbB2, ZrB2, Mg24 Y5, TiB2, other particles and rare earths also show good grain refining effects in aluminum-containing low-pressure casting magnesium alloys, but grain refining effect needs to be further improved.
In recent years, rapid solidification technology, ultrasonic grain refinement technology and high-strength melt shear grain refinement technology suitable for low-pressure casting of aluminum-containing and aluminum-free magnesium alloys have also been reported. Rapid solidification technology can speed up cooling rate of castings during solidification, thereby inhibiting growth of grains. The faster cooling rate of castings, the shorter growth time of crystals, and the smaller grain size. Therefore, low-pressure casting rapid solidification technology can effectively refine grain structure of castings. High-strength melt shear grain refinement technology effectively refines grains by producing a large amount of MgO as a nucleating agent in melt.
In summary, despite decades of research efforts, there is still no cost-effective and reliable grain refinement technology available for industrial production. Therefore, it is necessary to conduct more research work, such as further understanding influence of impurities such as iron and manganese on grain refinement of aluminum-containing magnesium alloys, studying relationship between grain refinement efficiency of different methods and purity of magnesium alloys, etc., to find new methods for microstructure refinement of magnesium alloys to expand application of magnesium and its alloys.

Solid solution is an important form of existence of alloying elements in magnesium alloys. When solute atoms are dissolved in Mg matrix, energy of system will increase, and resistance to slip deformation will increase, which will eventually increase strength of matrix and play a solid solution strengthening effect. Solid solution strengthening effect of magnesium alloys is determined by size mismatch degree εb, chemical mismatch degree εSFE and solute atom concentration Cs, where εSFE is calculated based on change in generalized stacking fault energy (Stacking Fault Energy). Since main slip mode under hexagonal close-packed (hcp) metal is basal plane slip, solid solution strengthening effect Δσss of magnesium alloy can be approximately expressed according to the first principle:

 

In formula, Azckss(o001) is increment of critical shear stress on (0001)Mg; M is Taylor factor, which ranges from 4 to 6.5. For magnesium alloys, the most commonly used solid solution elements are Al and Zn, but solid solution strengthening effect they bring is limited; rare earth elements Gd, Y, etc. have a large solid solubility in Mg matrix, difference between atomic radius and Mg is more than 12%, resulting in a large lattice distortion and a strong solid solution strengthening effect.
In cubic metals, especially steel and aluminum alloy materials, solid solution strengthening is often accompanied by a decrease in plasticity of alloy. However, in recent years, some abnormal rules have been discovered in study of solid solution strengthening of magnesium alloys. Research by Liu Tingting and others found that certain specific atoms, such as Al, Y, Mn, Gd, Zn and Er, etc., solid solution in magnesium hinder basal plane dislocation slip, but at the same time reduce basal plane and non-basal plane slip. Difference in resistance to movement is conducive to activation of non-base surface sliding, strength and plasticity of magnesium alloy are simultaneously improved. In response to these findings, team proposed theory of "solid solution strengthening and plasticization" of magnesium alloys, and based on this, successfully developed a variety of new high-performance magnesium alloys.
As mentioned earlier, solid solubility of rare earth elements such as Gd and Y in Mg matrix is relatively large, difference between atomic radius and Mg exceeds 12%, resulting in large lattice distortion and strong solid solution strengthening effect. Some of low-pressure casting Mg-RE alloys currently developed have been successfully used in parts such as automobile wheels, engine brackets, and helicopter casings.
In addition, some magnesium alloy series can also be strengthened through heat treatment. After proper solution treatment (T4) of low-pressure cast magnesium alloy, strength, plasticity, toughness and impact resistance of material can usually be improved at the same time.
Solid solution amount of alloy elements not only directly affects degree of solid solution strengthening of elements, but also affects tendency of each element to participate in formation of alloy phase, affects size of alloy grains, etc., thus indirectly affecting properties of alloy. Some solute atoms that cannot be solid-soluble will combine with Mg matrix or other solute atoms to form intermetallic compounds, which will precipitate in matrix to achieve second phase strengthening effect.

Solid solution strengthening effect of cast magnesium alloys is often limited, making it difficult to meet performance requirements of modern industrial applications for metal structural materials. Second phase strengthening is an important strengthening mechanism in magnesium alloys. Fine second phase particles in alloy are evenly distributed in matrix or on grain boundaries, interacting with dislocations through "cutting through" and "bypassing" mechanisms. Increase critical shear stress, thereby significantly increasing strength of alloy. Second phase strengthening is divided into two types: precipitation strengthening and dispersion strengthening. For aging precipitation phase of alloy, dislocation is mostly based on "cut-through" mechanism, such as Mg17 Al12 phase that is easily formed in Mg-Al binary alloy; For artificially added dispersed phase particles, due to their large size, dislocations usually adopt Orowan "bypass" mechanism, such as reinforced phase SiC particles added to magnesium matrix.
Strengthening effect of second phase of magnesium alloy is comprehensively affected by its distribution, size, shape, volume fraction and crystal structure. Nie compared differences in strengthening effects of spherical precipitates, basal lamellar precipitates, basal columnar precipitates, and cylindrical lamellar precipitates relative to matrix. Research shows that strengthening effect of each precipitate phase on matrix from strong to weak is: cylindrical flaky precipitate phase > basal surface columnar precipitate phase > spherical phase > basal surface flaky precipitate phase.
Generally speaking, artificial aging (T6) of low-pressure cast magnesium alloys (especially Mg-RE series) can precipitate aging strengthening phase and improve strength of magnesium alloy. Precipitation behavior of alloys has been introduced in detail in previous chapter and will not be repeated here.

Heat treatment is an important method to further adjust and improve microstructure and mechanical properties of magnesium alloys. According to type of alloying elements, heat-treated and strengthened cast magnesium alloys mainly include Mg-A1-Mn system (such as AM100A), Mg-A1-Zn system (such as AZ63A, AZ81A, AZ91C and AZ92A, etc.), Mg-Zn-Zr system (such as ZK51A and ZK6A, etc.), Mg-RE-Zn-Zr system (such as EZ33A and ZE41A), Mg-Ag-RE-Zr system (such as QE22A) and M-Zn-Cu system (such as ZC63A). Statistical results of conventional heat treatment types of cast magnesium alloys in different systems are shown in Table 6. Due to slow diffusion rate of alloying elements in magnesium alloys, commonly used heat treatment processes such as solution and aging treatment require long-term heat preservation, followed by cooling in still air or artificial forced airflow.
After solid solution treatment (T4), cast magnesium alloy can simultaneously improve strength, plasticity, toughness and impact resistance of material. Wall thickness of magnesium alloys cast by sand molds is larger and solid solution time required is often longer; solid solution time of metal molds and thin-walled castings is shorter. Peng Liming et al. studied effect of solid solution treatment on microstructure and mechanical properties of AZ91D-RE alloy. SEM results showed that second phase of cast AZ91D-RE alloy was mainly composed of network Mg17 Al12 phase, rod-shaped Al11 Er3 phase and granular Al10 Ce2 Mn7 phase composition. After alloy has been solid solution treated at 410℃/35 h, networked Mg17 Al12 phase is completely dissolved in α-Mg matrix, which plays a significant solid solution strengthening effect on alloy and greatly improves strength of alloy; rare earth compounds are not dissolved in matrix, but morphology changes from long rods to short rods with a tendency to spheroidization. This change reduces damage of compounds to matrix and improves plasticity of alloy.

 

Note: T4-solid solution treatment; T5-artificial aging; T6, T61-solid solution treatment plus artificial aging
Table 6 Conventional heat treatment types of cast magnesium alloys
Strength of magnesium alloys can be improved by solution treatment followed by artificial aging (T6). This process is mainly used in Mg-Al-Zn and Mg-Er-Zr alloys. However, plasticity of magnesium alloys tends to decrease after T6 treatment. Table 7 summarizes typical aging precipitation phases of several types of cast magnesium alloys.
Workpiece size and cross-sectional thickness of magnesium alloy castings, proportional relationship between workpiece size and furnace volume capacity, and placement of castings in furnace are all major factors that affect metal heat treatment process. Magnesium alloys have strict requirements on temperature control accuracy during heat treatment, and maximum temperature fluctuation range allowed during solution treatment is ±5℃. Therefore, it is necessary to accurately control furnace temperature of heat treatment furnace and ensure uniform temperature distribution; at the same time, furnace is required to have good sealing properties. When heat treatment temperature is higher than 400℃, a protective atmosphere must be used to prevent magnesium alloy surface from oxidizing and burning.

 

Table 7 Typical aging precipitation phases of cast magnesium alloys
In addition to above-mentioned conventional heat treatments, there are currently some special heat treatment processes used to improve the overall properties of magnesium alloys. Hot isostatic pressing treatment can improve density, mechanical properties and stability of magnesium alloy castings. Hot isostatic pressing process is to place material in a closed container, apply equal pressure to material in all directions, and apply high temperature at the same time. Under action of high temperature and high pressure, shape and external dimensions of workpiece do not change, but microscopic plastic deformation occurs in internal pores, closing shrinkage pore defects, increasing material density, and improving material properties. Zhou et al. used hot isostatic pressing technology to close shrinkage holes in cast Mg-6Gd-3Y 0.5Zr alloy, greatly reducing casting defects, significantly improving mechanical properties of material. WE43 alloy prepared by Tian Xiaoying and others using a low-pressure casting-hot isostatic pressing process exhibits excellent mechanical properties in the range from room temperature to 250℃. Reason is that alloy has a fine grain structure, eliminates shrinkage and shrinkage cavities. At present, there are few domestic reports on hot isostatic pressing of magnesium alloys, and research is still in its infancy.

Low-pressure casting magnesium alloy mold filling speed is slow and mold filling is relatively stable. Therefore, defects such as pores and shrinkage holes in castings can be greatly reduced, and surface quality of castings is also improved. However, with increasing demand for high-performance large and complex magnesium alloy castings in aerospace, automobile and other fields, alloying degree of magnesium alloys has been gradually improved, solidification range has become wider, and fluidity has become worse. Some casting defects such as porosity, component segregation, and thermal cracking are common during solidification process.

Porosity is a coarse spongy structure that appears inside castings. It is widely distributed in magnesium alloy castings and makes feeding difficult. Solidification temperature range of magnesium alloy is wide and solidification shrinkage is large. As a result, micropores formed by volume shrinkage cannot be replenished by alloy liquid, and eventually become loose. In severe cases, shrinkage cracks may occur due to excessive intergranular tensile stress, leading to alloy failure.
In cast magnesium alloy shells, loose defects mainly occur in thickness transition zone of casting. In process design, in order to reduce loose defects, shell casting process should follow principle of sequential solidification as much as possible to ensure smooth casting feeding channels, give full play to process advantages of low-pressure casting. In addition, appropriately lowering pouring temperature is conducive to reducing porosity, but too low a temperature will reduce shrinkage compensation effect; a lower filling speed can avoid air entrainment phenomenon during filling process, while extending insulation time of runner, thereby improving solidification quality. On this basis, increasing holding pressure between 30 and 60 kPa can fill micro-shrinkage pores between dendrites, improve shrinkage compensation ability of alloy, and improve internal quality of magnesium alloy castings.

Composition segregation is the most common segregation defect in magnesium alloys, which can lead to direct scrapping of castings in severe cases. In order to reduce component segregation of castings, methods such as adjusting refining temperature and chilling capacity, lowering pouring temperature can usually be used in actual casting production.
Alloying elements added to magnesium alloys usually have higher melting points and are mostly heavy metal elements. Increasing refining temperature can make heavy metal elements more fully melt into melt, effectively reducing component segregation of castings. In addition to alloy composition, component segregation is also related to cooling rate of castings. Parts with slower cooling rates are more likely to produce component segregation. Strengthening chilling in these parts can effectively prevent segregation in castings; in addition, appropriately lower pouring temperature to reduce alloy overcooling. degree to prevent precipitation of heavy metal elements during solidification process, and can also greatly improve segregation of components in alloy.

As magnesium alloy castings gradually develop into large-sized, thin-walled complex structural parts, tendency of hot cracking also increases. Hot cracking is a very serious casting defect for castings, showing straight or zigzag patterns on castings. Gaps and cracks are prone to occur at intersection of thickness and thickness of boss, at lower frame. Formation mechanism of hot cracking can be explained by liquid film theory: in later stage of solidification of magnesium liquid, a metal liquid film will form between closed dendrites. When casting continues to solidify, volume continues to shrink, and tensile stress will form between dendrites. Under this force, tearing eventually occurs, forming thermal cracks. Occurrence of hot cracking defects is affected by process design, melt quality and cold iron. During design process, when castings are designed with multiple bosses that are close together, reasonable anti-cracking ties should be designed to effectively eliminate crack defects. Melt purification and refinement can improve melt quality, reduce local stress at grain boundaries, reduce oxidation inclusions in magnesium liquid, thereby reducing tendency of hot cracking. Appropriate setting of cold iron will also improve hot cracking tendency of alloys, especially for rare earth heat-resistant magnesium alloys.

Oxidized inclusions often appear in low-pressure casting magnesium alloys. This inclusion is actually a film of MgO, mixed with MgO/MgS and some intermediate compounds, mostly distributed on the surface of casting or transition part of casting and various parts inside casting. Inclusion surface is usually rough and irregularly shaped. Oxidized inclusions are usually caused by imperfect or incorrect processes. Main factors and solutions are as follows:
(1) When adding liquid magnesium to crucible during continuous production, oxidized inclusions on liquid surface are washed into rising liquid pipe, and are brought into mold during pouring. Therefore, after adding magnesium liquid, slag in liquid riser tube should be completely removed.
(2) Oxide scale caused by repeated rise and fall of liquid level in liquid riser pipe can be determined by using a filter at the mouth of liquid riser pipe or inner runner of mold according to actual casting conditions.
(3) Pressurization speed is too fast, causing splashing and oxide scale. Therefore, filling speed should be strictly controlled to ensure that molten metal rises smoothly without impact or splashing.
(4) Non-metallic inclusions caused by falling off of casting materials and coatings. It is necessary to check whether coating layer has peeled off, and dust, sand, and debris in mold cavity must be thoroughly cleaned; in addition, refining agents need to be added during smelting to absorb various non-metallic impurities (such as oxides, nitrides, and chlorides). etc.), allowing it to sink to the bottom of crucible and be removed after standing.

Insufficient pouring and cold shutoff defects are one of defects that are prone to occur in low-pressure casting of magnesium alloys. They mainly occur far away from gate and at thin wall of casting. Insufficient pouring refers to defect that molten metal fails to fill mold cavity during mold filling process, resulting in incomplete casting formation.
Cold-separation defects refer to obvious discontinuity defects where two strands of metal meet due to incomplete fusion. Main causes of insufficient pouring and cold shut defects are related to filling process of molten metal, such as low pouring temperature and poor fluidity of molten metal, too slow filling speed, poor mold venting, poor air permeability, insufficient molten metal, etc. Corresponding to these factors, only by strictly controlling casting process can we effectively avoid these defects and obtain magnesium alloy castings with complete morphology.

Casting interlayer refers to occurrence of one or more defect areas dominated by gas or bubbles on the surface or body of casting. It is caused by failure of gas present in melt to be completely discharged during solidification process. Interlayers may be formed directly, or composite defects may form during solidification process. Usually, interlayers in magnesium alloy castings mostly occur during die-casting process. Main reasons are as follows:
(1) Gas inclusions during pouring. During pouring process of alloy castings, due to fast flow rate of magnesium liquid, gas, sand core and other impurities are easily brought in, resulting in generation of interlayers.
(2) Pouring temperature is too low or uneven. Low pouring temperature or uneven temperature distribution will cause surface of magnesium alloy castings to freeze and form an outer interlayer.
(3) Mold design and manufacturing issues. Unreasonable structural design of mold or poor manufacturing process can also lead to occurrence of interlayers.
(4) Poor smelting process. Improper control of magnesium alloy smelting process, such as too high a smelting temperature, too long holding time, etc., will increase gas and impurities in magnesium liquid, resulting in interlayers.
Above are some reasons for interlayers in magnesium alloy castings. Corresponding control measures can be taken to address these reasons, such as optimizing pouring process, improving mold design and manufacturing process, controlling smelting process, etc.
To sum up, in low-pressure casting of magnesium alloy shells, in order to ensure production of high-quality castings, every link such as raw materials, casting process and refinement and refining must be strictly controlled.

China has more than 70% of world's magnesium resources. At the same time, China is also a major producer of magnesium materials and products. Over past decade, China's output of magnesium and its alloys has exceeded 80% of world's total output, making it country with the most say in the world. As requirements for range of new energy vehicles and energy saving and emission reduction of traditional oil vehicles increase, lightweighting has become focus of current development of automobile industry. As the lightest commercial metal engineering material, demand for cast magnesium alloy is expected to grow rapidly. Therefore, in low-pressure casting of magnesium alloys, following issues deserve attention.
(1) Research and application of cast magnesium alloys, especially low-pressure casting products, is currently insufficient. When converting from small-scale laboratory to trial production of medium-sized and medium-scale industrial products, whether stability of sample and production efficiency can reach expected level , how big is difference between cost analysis and actual situation? Some technical difficulties may not be expected yet, scientific research and production personnel need to work together to eliminate application bottlenecks of cast magnesium alloys.
(2) Public has insufficient knowledge of magnesium alloy products. “Magnesium alloys have reactive chemical properties, are prone to corrosion, and have potential safety hazards”—this is often public’s first impression of magnesium alloy products. Non-professionals do not pay much attention to actual performance test results of magnesium alloy castings. Therefore, it is necessary to improve public's awareness of magnesium alloy products, understand advantages and disadvantages of magnesium alloy products, their actual flame retardant and corrosion resistance properties, so as to better promote application of magnesium castings.
(3) Stable raw magnesium prices and production bases are needed: In production cost of magnesium alloy castings, material prices generally account for about 50%, so fluctuation of raw magnesium prices has a huge impact on production cost of magnesium wheels. Relatively stable raw magnesium prices will further enhance users' determination to use and promote them. At the same time, in order to improve product mass production efficiency and qualification rate, a certain scale of production base is needed to jointly guarantee mass production and application promotion of magnesium alloy castings. In addition, although domestic vehicle manufacturers have expressed their expectation that magnesium alloy enterprises will provide them with high-performance magnesium alloy automotive parts products, replacement of parts materials requires a period of time for product design evaluation, testing and verification, and there is a conversion period for demand intentions to be converted into market orders; current relative price of raw magnesium is still relatively high, which makes domestic automobile manufacturers worry about high cost pressure.
(4) Technical difficulties in equipment and metallurgical structure in low-pressure casting need to be further addressed. How to further control defects such as oxide inclusions, shrinkage cavities, and segregation in low-pressure casting structure through alloy design, equipment optimization, effective process improvement is the key point that restricts whether low-pressure casting process and its products can play an important role in process of expanding application of magnesium.
Therefore, although potential market for application of high-performance low-pressure cast magnesium alloys in China is huge, there is still great uncertainty about whether potential market demand can be transformed into actual market demand.