At present, the lightweight of metal products in aerospace, automobile transportation and other industries has become a research hotspot. Magnesium alloys is an alloy composed of magnesium as the matrix and other elements. It is the lightest in practical metals and has rich reserves. Magnesium alloys have great advantages in light weight and have attracted much attention. Magnesium alloys have many advantages, such as high specific strength, good thermal formability, good machinability and biocompatibility, excellent damping and electromagnetic shielding, and degradability. They have great potential in weight reduction of metal products, and are also highly valued in biomedicine. The main problems in the development of magnesium alloys at present: (1) due to the inherent close hexagonal structure of magnesium, the plastic forming ability of magnesium alloys is poor, and it is difficult to form complex parts; (2) Magnesium alloy has poor corrosion resistance, which greatly limits its application in various fields. How to improve the corrosion resistance of magnesium alloys is worth further study.
1 Magnesium alloys classification
At present, magnesium alloys mainly contain aluminum (AL), zinc (Zn), manganese (MN), zirconium (Zr) and other alloy elements, as well as rare earth metal elements such as rhenium (RE). Different kinds and contents of alloying elements added to magnesium alloys will have a great impact on the properties of magnesium alloys. Generally, when the content of zzn does not exceed 22%, the creep resistance of magnesium alloy castings can be improved; An appropriate amount of MMN elements can improve the heat resistance and corrosion resistance of magnesium alloy, and remove the harmful element FFE to a certain extent in the smelting process; Proper amount of AAL elements can effectively improve the casting properties of magnesium alloys; The addition of appropriate rare earth metals can generally improve the fluidity of the alloy, reduce shrinkage porosity, refine the grain size of the alloy, and improve the strength and plasticity. According to the types of alloys added, magnesium alloys can be roughly divided into the following series:
- (1) Mg Al Zn series. This series of alloys are the first magnesium alloys explored and applied by people. They can be used not only for casting, but also for deformation processing. When the alloy is cast and formed, the content of Al element is 77%~9%. When it is used for deformation processing, the content of AAL element should be kept at 33%~5%.
- (2) Mg Al Mn series. Adding a certain amount of MMN to magnesium aluminum alloy will not have a great impact on the mechanical properties of the alloy, but will slightly reduce the plasticity of the alloy. Adding 11% - 25% MMN will greatly improve the stress corrosion resistance of the alloy, and significantly improve the corrosion resistance and welding performance of the alloy.
- (3) Mg Al RE series. This series of alloys have excellent deformation processability. The strength and plasticity of as cast alloys can be greatly improved by extrusion. The addition of rare earth element RRE can reduce the grain size, and the comprehensive mechanical properties of alloys can be significantly improved by appropriately increasing the content of AAL.
- (4) Mg Zn Zr series. The content of zzn elements in this series of alloys is generally controlled at about 66%, which plays the role of alloy solution strengthening and heat treatment strengthening. Zzr in the alloy can significantly refine the alloy grains and improve the strength, plasticity and heat resistance of the alloy. The content of Zr is generally 00.5% - 0.9%. It is often used to manufacture structural components with large stress in aircraft and aerospace vehicles.
- (5) Mg Zn RE series. According to the different content of rare earth metal RRE, the alloy will develop from solid solution strengthening to the second phase to strengthen the matrix. The addition of re atoms will cause lattice distortion, hinder the movement of dislocation in the structure or strengthen the grain boundary
Although alloys have many advantages, such as high specific strength, high-speed cutting, biocompatibility and so on, the magnesium alloys that can be obtained at present still have problems such as cutting combustion, high temperature oxidation and so on. How to prepare high-quality magnesium alloys that meet various industry standards still needs in-depth research. The quality of magnesium alloys generally depends on the type, content and processing technology of elements. This paper summarizes the development of magnesium alloys from the aspects of manufacturing technology, surface and internal strengthening mechanism and cutting technology.
2 Forming process
According to the physical properties of the constituent elements of magnesium alloy and the shape, size and application of the target component, the following 33 different forming processes are generally adopted:
- (1) Gravity casting: use molten magnesium alloy to cast components by its own gravity. The operation methods include: metal mold casting, semi-metal mold casting, shell mold casting, investment mold casting and sand mold casting. Among them, sand casting cost is low, but the operation is relatively cumbersome, which is suitable for small batch production of large castings
- (2) Pressure casting: use a certain pressure to help the alloy melt fill the mold cavity and complete the casting process. According to the pressure, it can be divided into high-pressure casting and low-pressure casting. High pressure casting is to squeeze liquid or semi-solid magnesium alloy melt into the mold cavity at high speed and solidify it. The size of the alloy obtained by this method is more accurate and the outline is clear; Low pressure casting is to use a lower pressure to raise the liquid of the alloy melt and fill the mold smoothly, so that it can solidify and crust in sequence, then crystallize under pressure, and finally release the pressure to obtain castings. This method can also obtain high-quality alloy castings. In the solidification link of the die casting process, if the cooling rate is increased, the grain will be significantly refined and the tensile strength and elongation of the casting will be improved [9-11]. Magnesium alloys are suitable for pressure casting due to their low melting point (pure magnesium is about 6650 ℃), rapid solidification, low alloy liquid viscosity and good fluidity. At present, vacuum die casting, semi-solid die casting and oxygen filled die casting are often used for magnesium alloy pressure casting.
- (3) Semi solid forming: magnesium alloy is heated to the coexistence of solid and liquid by cyclic induction heating, and then the material is formed directly. Compared with liquid forming, it has the advantages of low forming temperature, long die life, low defects and high compactness. This technology has the advantages of both casting and solid-state plastic forming. The forming temperature is lower than the liquid state, and the deformation resistance is lower than the solid state. The resulting alloy components have high precision. At present, it has been used in some magnesium alloy automotive parts and aerospace industrial instruments.
In order to refine the grain size and improve the comprehensive mechanical properties, as cast magnesium alloys are usually rolled, extruded, forged and other plastic processing. Because the lattice type of matrix magnesium belongs to close hexagonal structure, and its constituent alloy is generally the distortion result of close hexagonal structure, with less slip system and weak plastic deformation ability, magnesium alloys often use rolling process. Considering the poor cold deformation ability of magnesium alloys, hot rolling or warm rolling is generally used.. The rolling temperature depends on the type and content of alloy elements, which is generally about 555 ℃ below the alloy solidus. Appropriate temperature and strain rate can refine grains, reduce deformation resistance and improve processing efficiency.
3 Application of magnesium alloy
(1) Aerospace field
With the rapid development of aerospace and national defense industry, the demand for high-performance lightweight structural materials in China has increased sharply, and the lightweight of aerospace materials has become more and more important. On the basis of meeting various performances, using magnesium alloy to make aerospace components not only saves resources, but also is conducive to the subsequent use of spacecraft and prolongs its service life. In addition, it is also conducive to ensuring the safety of spacecraft in the return process. At present, China has set corresponding lightweight targets in the aerospace field.
(2) Rail transit field
Material lightweight is of great significance to the development of rail transit industry. Flame retardant magnesium alloy is used in the weight reduction of rail transit transit transit directional frame, but at present, the application of magnesium alloy in this field has not been promoted, mainly because some key technologies have not been broken through, and it still needs a period of time to repeatedly test it to ensure that it meets the requirements of all aspects.
(3) Automobile industry
Magnesium alloy can make the vehicle achieve the goal of lightweight and improve the shock absorption effect, which is favored in the field of vehicle weight reduction. It can be used to make automobile wheel hubs, gearbox boxes, cylinder head covers, steering wheels and steering brackets, etc. to reduce self weight, reduce fuel consumption and protect the environment. However, magnesium alloys are not widely used because of their poor corrosion resistance and flammability.
(4) Medical field
Magnesium alloy has good biocompatibility, degradability and close to human bone density, and can be used in medical treatment. However, only some magnesium alloys are suitable for the medical field, and the properties will change after the alloy is formed. Magnesium can promote the absorption of calcium ions by the human body, so as to accelerate the growth of bone tissue and wound healing, and shorten the treatment cycle of patients.
(5) Military field
The application of magnesium alloy in military equipment can reduce the quality of military equipment and enhance the mobility and combat effectiveness of troops. It can be used in the gearbox, butt body, aiming device and other parts of military firearms. It can also be used in the hub, seat frame, gearbox box and other parts of armored vehicles, as well as the traction device, ammunition supply box and gunner's mirror of artillery and ammunition.
(6) Electronic products
Magnesium alloy has the advantages of light weight, non-magnetic, good shock absorption and so on. It has a bright future in the field of electronic products. Because 33c (computer), represented by notebook computers, mobile phones and digital cameras,
Communication, consumption) products have been developing in the direction of light, thin and small, and magnesium alloys are more and more outstanding. Magnesium alloy electronic products not only have excellent appearance and touch, but also have good thermal conductivity and rigidity. The thickness of the thinnest 33c product shell is only 0.4 mm.
(7) Other areas
Magnesium alloy is relatively stable in gasoline and lubricating oil, and can be used to make oil pipes, gear boxes and other parts.
Magnesium alloy has many advantages, such as low density, high specific strength, good biocompatibility and so on. It can achieve better results than other metal or plastic materials, and has a broad application prospect. At the same time, we should also pay attention to the problems existing in the manufacturing and use of magnesium alloys. For example, some rare earth metals that form alloys with magnesium are expensive and difficult to commercialize; Magnesium alloy is easy to corrode during use; It is easy to burn when cutting. The above problems limit the development of magnesium alloys and need further exploration and solution by relevant scientific researchers.
