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Application and development of magnesium alloys

magnesium alloy casting technology

1.1 die cast magnesium alloys

since the introduction of high-strength mgal9znl in 1927, the industrial application of magnesium alloys has made substantial progress. In 1936, chegong began to use die-casting magnesium alloy to produce engine transmission system parts of "beetle" cars. By 1980, the use of magnesium alloy die-casting parts reached 380000 tons. The German government has formulated a magnesium alloy research and development plan with an investment of 25million German marks, which mainly studies the pressure alloy process, rapid prototyping and tool manufacturing technology and semi-solid forming process, so as to improve Germany's ability in the application of magnesium alloys. Magnesium alloy has good mold filling ability, rapid solidification speed after mold filling, and small thermal impact on die-casting mold. It can be used for die-casting thin-walled parts without hot cracks, under casting and other defects, which does not change the structure and working state of the original machine equipment. Magnesium alloy reduces the thermal fatigue of die-casting mold, prolongs the service life of die-casting mold, shortens the mold retention time of die-casting, and the die-casting production rate is about 75% faster than that of aluminum alloy. Magnesium alloy basically does not react with iron and does not erode the steel crucible and die-casting mold, which not only avoids the pollution of the crucible to the magnesium alloy liquid, but also prolongs the service life of the steel crucible and die-casting mold, so that magnesium alloy can be used not only in cold chamber die-casting machine like aluminum alloy, but also in hot chamber die-casting machine with higher efficiency. The shrinkage of magnesium alloy is uniform, and the stripping force is 20% - 30% lower than that of aluminum alloy. Therefore, magnesium alloy die castings not only have high dimensional accuracy, but also the cleaning energy consumption of die castings is less than 60% of that of aluminum alloy die castings. Die casting magnesium alloys generally take Al as the main alloy element, and there are three series commonly used:

AZ (mg Al Zn Mn), am (mg Al Mn), as (mg Al Si Mn) system. The main problem affecting the development of magnesium alloy die casting is that the alloy liquid is easy to oxidize and burn, which brings a series of difficulties to smelting, and causes pollution to the environment and the alloy itself, corrosion to smelting equipment and increase of die casting production cost. At present, some researchers are engaged in the research of flame retardant magnesium alloys with properties close to existing magnesium alloys

1.2 magnesium alloy semi-solid casting

the semi-solid injection molding process developed by Dow Chemical Company of the United States integrates the characteristics of plastic injection molding and metal die casting processes. The internal and surface quality of die castings produced by this process are significantly improved, the dimensional accuracy, mechanical properties and corrosion resistance are improved, and there is no need to configure a furnace, and the working environment is clean. However, its forming pressure is high, about three times that of hot chamber die casting, and magnesium alloy needs to be prefabricated into chips, which increases the production cost by 10%, and its equipment investment is relatively expensive

1.3 squeeze casting

the development trend of magnesium alloy squeeze casting technology is to combine with other casting molding technologies. Toshiba machinery company of Japan has studied the leomacs system, which is characterized by the combination of squeeze casting with low-pressure casting and electromagnetic quantitative pouring pump, which can not only reduce the uneven solidification of castings, but also integrate into one. It has the following characteristics:

(1) it shortens the residence time of molten metal from feeding to extrusion, and shortens the cycle of casting production

(2) during the production process, the metal liquid is isolated from the air, which effectively prevents the oxidation and combustion of magnesium alloy liquid

(3) magnesium alloy liquid can be kept warm and poured at a lower temperature. Therefore, the equipment can produce thin-walled magnesium alloy castings with complex shapes. However, the high investment of this equipment limits its wide industrial application

this casting process can realize the mixing, injection and extrusion of melt in the same working unit. The electromagnetic stirring device installed on the periphery of the injection chamber is used to stir the melt in the cooling process. When the stirring time required by the specific solid fraction is reached, the injection punch presses the slurry into the cavity and keeps it warm. This process makes full use of the advantages of the two casting methods, overcomes the limitations of rheological casting itself, and can improve the service life of the mold

1.4 other magnesium alloy casting and forming methods

low pressure casting and differential pressure casting have been actually used to produce magnesium alloy parts of automobiles. The characteristic of these two casting methods for magnesium alloys is that the pressurization system is effectively combined with the gas protection of the alloy during the casting process. Lost foam casting is one of the most advanced casting processes in the world at present. The advantages of magnesium alloy lost foam casting are

(1) using dry sand avoids the combustion problem of magnesium alloy caused by water in ordinary sand casting, and the reducing atmosphere formed by lost foam gasification can inhibit the oxidation and combustion of magnesium alloy

(2) the shrinkage of magnesium alloy is 1.2 times that of aluminum alloy, with a large tendency of hot cracking and good yielding of dry sand, which can effectively solve this problem. Magnesium alloy lost foam casting is a new technology combining advanced materials and advanced technology, which will show great advantages in the production of complex automotive parts. At present, the research and application of magnesium alloy lost foam casting technology has just started and needs further development

smelting technology of magnesium alloy

2.1 flame retardant technology of alloy liquid

flux protection method is to use low melting point compounds to melt into liquid at a lower temperature and spread out on the liquid surface of magnesium alloy, which plays a protective role by preventing the contact between magnesium liquid and air. At present, the commonly used flux is mainly composed of anhydrous carnallite (MgCl2 KC) and some fluorides and chlorides. The agent has the advantages of convenient use, low production cost and good protective effect. It is suitable for the production characteristics of small and medium-sized enterprises. However, the agent should be dehydrated again before use, which will release a choking smell when used. Because the density of flux is relatively high, it gradually sinks and needs to be added continuously. A large number of harmful gases are released during use, which pollutes the environment and corrodes the plant seriously. Therefore, it is an important subject to study a new type of magnesium alloy flux with good coverage, refining effect and pollution-free

in developed countries, magnesium alloys are usually smelted by gas shielding method and good results have been achieved. The gas protection method is to cover the surface of magnesium alloy liquid with a layer of inert gas or gas that can react with magnesium to form a dense oxide film, so as to isolate oxygen in the air. The main protective gases used are SF6, SO2, CO2, AR, n2 Etc. In order to further improve the protective effect and reduce the consumption of expensive SF6 gas, foreign countries generally mix air or other dry gases such as CO2 in SF6 gas The protection effect of mixed gas is good, but there are the following problems:

(1) polluting the environment, SF6 will produce SO2, SF4 and other toxic gases, and the effect of SF62 on global warming is 24900 times that of CO2

(2) the equipment is complex, requiring complex gas mixing devices and sealing devices, (3) corroding the equipment, significantly reducing the service life of the crucible

another practical method to improve the flame retardancy of magnesium alloy liquid is alloying. Long ago, people used to add beryllium to magnesium alloys to improve the flame retardancy of magnesium alloys, but beryllium is toxic, and too high an amount will cause grain coarsening and increase the tendency of hot cracking, so it is limited by the amount of beryllium added. Japanese scholars believe that adding a certain amount of calcium can significantly improve the ignition temperature of magnesium alloys, but there are problems that the amount of calcium is too high and seriously deteriorate the mechanical properties of magnesium alloys. The addition of calcium and zirconium at the same time has flame retardant effect. Domestic research shows that adding rare earth and high content of beryllium into magnesium alloys at the same time can obtain magnesium alloys that can be directly melted in the atmosphere and have better casting properties and mechanical properties. We found another element that has flame retardant effect on magnesium alloy. The test results show that adding a small amount of this element and combining with other elements can significantly improve the ignition temperature of magnesium liquid. This element also has beneficial effects on grain refinement and strengthening of magnesium alloys, and improves the mechanical properties of magnesium alloys

2.2 melt modification technology

the purpose of magnesium alloy smelting modification is to change the microstructure of magnesium alloy. This process has a great impact on the grain size and mechanical properties of the alloy, and also has a certain impact on the oxidation inclusions in magnesium liquid. The research shows that for magnesium alloys without Al, zirconium modification has a good effect on grain refinement. The action principle is that Zr peritectic reaction occurs to promote grain refinement. After adding suitable carbon materials into mg Al alloys, they can react with the alloy liquid to form alc4. This compound can play the role of foreign crystal nucleus and promote the grain refinement of magnesium alloys. Using cccl6 is one of the process measures. Recently, Japan adopted the process of mixing argon with high-purity carbon powder to add carbon, and achieved a good modification effect. The effects of addition temperature, treatment time and other factors on the modification effect were investigated. Our experimental research shows that under the condition of containing Mg, Al and other alloy elements, overheating and high temperature treatment of industrial magnesium alloy solution has obvious grain refinement effect. When the melt of high-temperature magnesium alloy is quickly reduced to low temperature for pouring, the casting can still retain the effect of grain refinement if the basic range is not correct, which shows that the microstructure of magnesium alloy is hereditary. Our other research also shows that the treatment of magnesium alloy liquid with carbonate also has better modification

research and development of three new types of High-performance Magnesium Alloys

in order to improve the heat resistance of magnesium alloys, domestic and foreign researchers have also developed a series of magnesium alloys containing rare earth, including Zr high-temperature magnesium alloys, mg Al Si based alloys, mg Zn Cu based alloys, etc. Particle reinforced and graphite fiber reinforced magnesium matrix composites were studied. Another use of magnesium alloys is to prepare amorphous alloy materials. The results show that magnesium based alloys have strong amorphous forming ability. Through rapid solidification, amorphous materials with high strength and toughness can be made. At present, rod-shaped magnesium based amorphous materials with a diameter of 9mm have been produced in new consumer markets such as yachts and ice and snow sports equipment. Magnesium based alloys are one of the future development directions of bulk amorphous alloys. It is also reported that as a hydrogen storage functional material, magnesium based alloy has a series of advantages, such as strong hydrogen storage capacity and cheap raw materials. Magnesium can store 7.7% hydrogen. Practical magnesium based hydrogen storage functional materials have been produced by mechanical alloying method. At present, they are being further tested and studied in the laboratory. We have carried out the research on magnesium based amorphous materials by rapid solidification method. We believe that Mg Ni RE alloy and mg Al CA alloy have strong amorphous forming ability. The formation of amorphous is related to many factors, especially the alloy composition has an important influence on the amorphous forming ability, and the amorphous forming ability is also related to the viscosity of the melt. The performance test shows that the corrosion resistance, toughness and hardness of the base amorphous material are significantly improved. With the continuous improvement of the service life of electrical appliances and pipelines, it is of great practical significance to study more effective anode sacrificial materials. The most commonly used magnesium anode material is az63a (mg Al Zn) alloy. At present, manganese containing magnesium alloy anode materials have been developed, which have higher electronegativity and current efficiency and are widely used abroad. We are studying magnesium based corrosion control materials with higher current efficiency

four prospects

there is no doubt that magnesium alloy castings will be widely used in the future world. The abundant magnesium mineral resources in China provide a material basis for the development of magnesium alloys. Only one example is given to illustrate the application prospect of magnesium alloy: experts predict that the fuel consumption of cars will generally be reduced to 3l/100km in 2005, and the carbon dioxide emission is required to be reduced to 120mg/kg. When the dead weight of the car is reduced by 100k and the fuel consumption can be reduced by 0.4ml per 100km, the car adopts more magnesium alloy

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