Magnesium alloy material available 1808, 1886 for industrial production. Magnesium alloy die-casting technology from 1916 to the successful use of magnesium alloy die-casting count, has also experienced eighty years of development. Humans have gone through a long course of exploration in understanding and controlling the production technology of magnesium alloy and its products. From the introduction of high-strength MgAl9Zn1 in 1927, substantial progress has been made in the industrial application of magnesium alloys. 1936 German Volkswagen Automotive began production of die cast magnesium alloy, "Beatles" engine parts of the engine transmission system, in 1946, the use of bicycles magnesium alloy weighing about 18kg. The United States in 1948 ~ 1962 with hot chamber die casting machine magnesium alloy die casting car parts for millions. In the past, however, magnesium alloys were used primarily as structural materials in the aviation field. In other areas, the main use of magnesium in the world was to produce aluminum alloys, followed by steel desulphurization and ductile iron production. In recent years, due to the increasingly urgent demand for lightweight products, the continuous improvement of the properties of magnesium alloys and the remarkable progress of die-casting technology, the amount of die-cast magnesium alloy has increased remarkably. In particular, human beings have put forward requirements on automobiles to further reduce weight, reduce fuel consumption and emissions, improve driving safety and comfort, magnesium alloy die-casting technology is rapidly developing. In addition, magnesium alloy die-casting parts have been gradually expanded to other areas such as laptop housings, portable saw chains, automatic retractable hooks, video cassette housings, mobile phone housings, communications equipment on aircraft and radar housings, and Some home appliances and so on. Magnesium is mainly refined by magnesium ore. Magnesite reserves in Dashiqiao City, Liaoning Province, China account for more than 60% of the world's reserves, with ore grades of over 40%. China's magnesia and magnesia products produced in large quantities for export. Make full use of our country's rich magnesia resources for further development, combined with the development of China's automotive, computer, communications, aerospace, electronics and other emerging industries to promote the production and application of magnesium alloy die-casting, is placed in front of a foundry workers in our country task. Research on die-casting magnesium alloy The density of magnesium alloy is less than 2g / cm3, which is the lightest metal structural material at present, its specific strength is higher than that of aluminum alloy and steel, slightly lower than that of fiber-reinforced plastic with the highest specific strength. And steel, much higher than the fiber-reinforced plastic; its corrosion resistance is much better than low-carbon steel, aluminum die-cast aluminum alloy has exceeded the A380; its damping, magnetic shielding is far superior to the aluminum alloy; given magnesium alloy kinetics Low viscosity, the same fluid state (Reynolds index equal) under the charge filling speed is much larger than the aluminum alloy, combined with the magnesium alloy melting point, specific heat capacity and phase transition latent heat are lower than the aluminum alloy, so the melting energy less, solidification speed, magnesium alloy The actual die-casting cycle can be 50% shorter than aluminum. In addition, the affinity of magnesium alloy with iron is small, the ability of solid solution of iron is low, so it is not easy to adhere to the mold surface, the mold life of its use is 2 to 3 times higher than the aluminum alloy. Die-cast magnesium alloys commonly used in the United States are AZ91, AM60, AM50, AM20, AS41 and AE42, which belong to the four series of Mg-Al-Zn, Mg-Al-Mn, Mg-Al-Si and Mg-Al-RE. Right and die-casting magnesium alloy, at present mainly in the following aspects of research: (1) high temperature performance: AZ and AM two series of magnesium alloy die casting automotive magnesium alloy die casting accounts for 90% of the two series Of the magnesium alloy at 150 ℃ above the intensity were significantly decreased. AS series die-cast magnesium alloys, such as AS41A alloy (Mg43% Al1% Si0.35% Mn), have been developed to withstand creep at temperatures above 150 ° C, exhibiting a 175 ° C creep strength better than AZ91D and AM60B, Length, yield strength and tensile strength. Volkswagen Beetle engine crankcase has been used AS41 and AS42, recently adopted an improved alloy AE42 creep performance is better at high temperatures. Some trace elements such as rare earth elements Y, Nd, Sr, etc., have obvious grain refinement effect on die-cast magnesium alloy and can improve the strength and creep resistance of die-cast magnesium alloy, such as the creep resistance of the recently developed AE42 Better than the traditional MgAlSi alloy, 200 ~ 250 ℃ long-term use. However, the improvement of high temperature performance of AS and AE alloys is still limited. The casting performance is worse than that of AZ and AE alloys. In addition, the high cost of rare earth elements limits the production and application. (2) ductility: At present, magnesium die casting in the need for safety and high fracture toughness of the use of very rapid growth. In the case of work to improve the ability to absorb energy, it should improve the fracture toughness of the material. This can be done by reducing the aluminum in the alloy. The AM60 and AM50 are widely used in safety components such as instrument panel brackets, steering wheel shafts and seats, and the AM20 is currently being applied to the seat back frame. In addition, the relationship between elongation at break and temperature is quite close, especially at temperatures above about 50 ° C, with increasing temperature. (3) corrosion resistance of magnesium alloys: corrosion resistance has also been a major obstacle to the expansion of the application of magnesium alloys. High chemical activity of magnesium, magnesium-based alloys and composite materials prone to micro-cell corrosion, corrosion resistance of low-purity die-casting magnesium alloy is generally poor. Strictly defined Fe, Ni, Cu and other impurity elements of high purity die-casting magnesium alloy (such as AZ91D), and AE42 containing rare earths, salt spray test corrosion resistance than aluminum die-casting A380, much better than low-carbon steel. Adjust the chemical composition, surface treatment and control of microstructure can improve its corrosion resistance. Although there are many ways to improve the corrosion resistance of magnesium alloy parts, poor corrosion resistance is always a technical obstacle for obtaining large quantities of magnesium alloy parts without solving the problem from the material itself. (4) Flame Retardant Magnesium Alloys: The addition of Al (2.5%), Be alloy (Be added in an amount of 0.0005% ~ 0.03%) or Ca containing alloy to the magnesium alloy can also effectively prevent the oxidation of the magnesium alloy liquid. At present, some researchers are engaged in the study of flame-retardant magnesium alloys. Once the research is successful, the magnesium alloys will be melted and cast just like aluminum alloy, obtaining a wider application prospect. (5) Magnesium alloy-based composite material: The magnesium alloy-based composite material reinforced by particles such as silicon carbide has been researched and developed for many years. Although not yet reached the stage of commercial application in the die-casting field, sand casting and precision casting have been used Method made of impeller, bicycle crank, automotive cylinder liner and other castings, and this combination of materials and semi-solid casting combination, used in the field of die-casting and extrusion casting trends. Vacuum die-casting through the die-casting process to remove the gas cavity to eliminate or significantly reduce the porosity and dissolved gas die-casting gas, improve die-casting mechanical properties and surface quality. Has been successfully in the cold chamber die casting machine with vacuum die casting method to produce AM60B magnesium alloy car wheels, locking force of 2940kN hot chamber die casting machine to produce AM60B magnesium alloy car steering wheel parts, casting elongation increased from 8% to 16%. Oxygenated die-casting, also known as non-porous die-casting (Pore-Free Die Casting Process, PF method). The method of filling the molten metal before the oxygen or other reactive gas filled cavity, replacing the cavity of the cavity, filling the molten metal, the reactive gas and the molten metal reaction to form metal oxide particles dispersed in the die-casting Piece, thereby eliminating the gas inside the die-casting, so die-casting heat treatment can be strengthened. Nippon Light Metal (strain) with oxygen die-casting method of production of computers AZ91 magnesium alloy magnetic head stent, instead of the original multi-layer laminated stent, not only to reduce the weight of the stent, and achieved great economic benefits. The company also used mass production of AM60 magnesium alloy car wheels and motorcycle wheels in batches with an oxygen-based die-casting process, which is 15% lighter than aluminum wheels. In recent years, the United States, Japan and the United Kingdom and other countries have successfully developed a magnesium alloy semi-solid thixotropic injection molding machine. Semi-solid magnesium alloy thixotropic injection molding machine to a certain pressure will be semi-solid magnesium alloy into the die-casting and make it shaped, its working principle is similar to the injection molding machine. It prefabricated non-dendritic magnesium particles into the screw feeding mechanism in the screw feeding mechanism will be heated to a semi-solid magnesium particles, and the other end of the screw feeding mechanism by the magnesium alloy slurry collection chamber will be semi-solid magnesium Alloy slurry into the injection chamber for injection molding. This casting method represents a magnesium alloy casting production development. Magnesium alloy smelting operations and safety of production Because the magnesium alloy liquid is easy to oxidize, and the surface oxide film is loose, the density coefficient α value of only 0.79, can not prevent the alloy to continue oxidation. The magnesium alloy liquid reacts with oxygen, water vapor and nitrogen in the atmosphere to form refractory MgO, Mg3N2 and other compounds that do not melt in the magnesium liquid, and forms "oxidation slag" after being mixed into the mold. Therefore, the oxidation of the alloy melting is crucial. There are two main methods for melt protection of magnesium alloys: flux protection and gas protection. Melting with a protective flux usually brings about the following problems: (1) Chloride and fluoride salts are easily volatilized at high temperatures to produce toxic gases such as HCl, Cl2, HF, etc .; (2) Due to the higher density of flux, part of the flux will follow the same magnesium Liquid mixed into the mold resulting in "flux slag"; (3) volatilization of flux generated gas may infiltrate into the alloy liquid, a corrosion source of materials used to accelerate corrosion and reduce service life. At present, most manufacturers use gas protection, that is, 2 to 4 kinds of mixed gases in dry SF6, N2, CO2, and SO2 gases form a dense continuous film on the surface of the magnesium alloy molten pool to prevent the oxidation of the magnesium alloy liquid. SF6 is not a toxic gas, but its global warming effect on the earth is 24,000 times more severe than that of CO2, while that of the magnesium industry accounts for 7% of the world's total (1996) and will inevitably limit its use and even its use in the future, but not yet Find a suitable alternative to SF6. The results show that the use of sulfur powder sprinkled on the surface of the molten pool of magnesium oxide solution has a protective effect. The dangers of magnesium alloy die-casting production mostly caused by the process of processing and post-buried in the process of fault caused. According to Japanese statistics, magnesium alloy die-casting production process in the risk of melting 25%, casting 10%, processing 39%, storage and waste accounted for 16%, 3% of electrical, other accounted for 7%. Obviously, the risk of processing and post-processing process than the casting process 3 to 4 times. During the processing, whether blasting, turning, milling, polishing, etc., will inevitably produce magnesium dust and sparks, such as factory poor phoenix, magnesium dust concentration in the air is too large, once the spark and air or ground magnesium Dust contact, ranging from burning, heavy explosion. Therefore, dust collector must be installed in the plant and equipped with fireproof sand and fire prevention facilities. Die-casting design Magnesium alloy chemical, physical parameters and die-casting properties and aluminum are very different, so the design of the mold can not be fully applied aluminum die-casting design principles. Magnesium alloy liquid is easily oxidized and burned, hot cracking tendency is larger than that of aluminum alloy during casting, and is more complicated than aluminum alloy die-casting in melting, pouring and temperature control of die-casting. Magnesium alloy filling time is short, the exhaust problem is particularly prominent, the specific heat capacity and phase transition latent heat of magnesium alloy are lower than the aluminum alloy, so prone to partial (thin section) pre-crystallization phenomenon in the die-casting process, leading to blockage of the supply channel Imperfect defects. Magnesium alloy die-casting design mainly consider the following aspects: (1) die-casting machine selection. What type of die-casting machine to produce mainly depends on the wall thickness of the casting. Roland Fink in the "magnesium alloy die-casting process optimization," the issue of research process, through the magnesium alloy die-casting economy, cold chamber die-casting and hot chamber die-casting process analysis, under normal circumstances, less than 1kg of castings require the use of hot chamber die-casting Machine, in order to ensure the fullness of thin-walled parts, large-size is recommended cold chamber die-casting machine. (2) process parameters. In the die-casting production process, the choice of appropriate process parameters is to obtain high-quality casting casting machine to play a maximum productivity of the prerequisite for the correct design of die casting basis. Die casting, the impact of alloy liquid filling molding many factors, including the main injection pressure, injection speed, filling time and die temperature and so on. Due to the different thickness and complexity of the die casting, the range of process parameter selection varies greatly. Magnesium alloy compared with aluminum, zinc alloy, better fluidity, so the secondary injection speed can be greater, magnesium alloy punch faster than aluminum about 30%, the maximum even more than 10m / s. Due to the magnesium alloy casting properties such as fluidity is very sensitive to the temperature of the pouring temperature is very easy to solidification of magnesium alloy liquid filling process, you must accurately control the type of pouring temperature, otherwise easy to waste. (3) pouring system design. The pouring system plays an important role in controlling the flow direction of liquid metal, the conditions of exhaust gas overflow, the temperature distribution of molds, the transfer of pressure, the filling time and the speed and flow of molten metal through the runner. Regulatory effect. Gating system design is summarized as follows: Within the runner position: Because the magnesium alloy in the cavity than aluminum, zinc and other alloys solidification faster, and the general magnesium alloy die-casting parts for the thin-walled parts, the choice of gate position must be avoided Direct impact on the cavity surface, to ensure the liquid metal in the cavity flow path is the shortest, in order to prevent the phenomenon of inadequate pouring and cold isolation. Filling speed: In general, due to the thermodynamic properties of magnesium alloy, alloy heat transfer to the mold quickly, and the large range of solidification, poor fluidity, so to prevent premature solidification of the runner magnesium solution, magnesium solution should be High-speed smooth filling cavity. In general runner flow rate of 90 ~ 100m / s, for some thin-walled magnesium alloy die-casting parts, runner speed up to 20m / s. In runner size: In many cases, the runner is removed by machining. The gate width should be less than 50% of the wall thickness to avoid damage to the castings during trimming. In order to obtain the minimum thickness of the gate, while ensuring the thin-walled magnesium die-casting requirements, gate width should be taken to ensure adequate cross-sectional area of ​​the gate. Filling time: it is closely related to the runner speed, the surface quality of high-impact thin-walled castings have a great impact. Filling time than aluminum 0% less, usually taken as 10 ~ 100ms. Overflow tank design For thin-walled magnesium alloy die-casting parts, the optimal overflow tank inlet area is about 20% to 25% of the runner cross-sectional area. Research on filling process Computer simulation With the growing field of application of magnesium alloy castings, higher requirements for the filling performance of die-cast magnesium alloy are put forward. At present, there is a poor understanding of the filling and molding laws of the die-casting magnesium alloy, the relationship between the filling performance and the die-casting process parameters, and the critical wall thickness of the filling. Therefore, systematic research is urgently needed. To this end, we should vigorously carry out filling and solidification of magnesium alloy computer simulation study, and on this basis, an expert system to guide the development of die-casting process, die-casting design, die-casting quality control and improve the quality of magnesium alloy die- Rate and die-casting of the service life. Numerical simulation software is most commonly used in automotive magnesium die castings. Some German automotive industries have successfully simulated automotive applications such as seat frames, thixotropic fuel pumps, Audi 5x gearboxes, wheels, and 4-cylinder engine blocks Magnesium alloy die-casting, which effectively shorten the product development cycle, which greatly enhance the market competitiveness of enterprises. Application prospects in the automotive industry According to reports in the literature, the world engineering components now 98% of magnesium alloy demand from the die-casting industry, of which more than 70% for the automotive industry, the magnesium alloy die-casting process performance in the industry The development of application plays a decisive role. North America is the largest amount of magnesium alloy area, its annual growth rate of 30%. Companies such as Ford Motor Company, General Motors and Chrysler, which have been dedicated to the automotive components of new magnesium alloy and magnesium alloy clutch housings, steering column mounts, intake manifolds and lighting holders for more than a decade Development and application. In 1996, the government Department of Energy signed a cooperation program with the three major groups GM, Ford and Chrysler entitled "PNGV" (next-generation transport), which aims to produce energy-saving sedans that meet the market requirements. General Motors successfully developed the magnesium alloy wheels in 1997 and signed an agreement with Hydro, the world's largest magnesium production and processing company, for the application of magnesium alloy die-castings. The Lindberg thixotropy development center in Wisconsin, for magnesium Alloy die-casting technology has been innovative, using semi-solid die casting technology to produce magnesium alloy racing clutch plates and automotive transmission parts. The magnesium alloy in Europe is second only to North America, its annual growth rate of 60%. The famous Mercedes-Benz car company will be the first magnesium alloy die-casting parts used in car seat bracket, Audi Motor Company first introduced magnesium alloy die-casting car dashboard, it can be said that Germany is to promote magnesium alloy die-casting development and the main force. In 1997, Germany also led by the Federal Ministry of Science and Technology Education (BMBF), more than 50 companies such as Volkswagen and 6 universities and research institutes such as Munich University of Technology, invested 25 million DM conducted a three-year "MADICA "(Magnesium alloy die-casting) development project. Toyota Motor Corporation first created magnesium alloy car wheels, steering shaft systems, cam covers and other components; Mitsubishi and the Australian Department of Industrial Technology to develop super-lightweight magnesium alloy engine. At present, various car companies in Japan produce and apply a large number of magnesium alloy shell die-casting.