1 Introduction Magnesium alloy materials were introduced in 1808 and were used in industrial production in 1886. Magnesium alloy die-casting technology has successfully used magnesium alloys for die-casting since 1916, and has experienced more than 80 years of development. Humans have experienced a long history of understanding and mastering the production technology of magnesium alloys and their products. Since the introduction of high-strength MgAl9Zn1 in 1927, the industrial application of magnesium alloys has made substantial progress. In 1936, Volkswagen AG began to produce the engine drive system parts of the "Beetle" car with die-cast magnesium alloy. In 1946, the amount of magnesium alloy used in bicycles was about 18kg. In the United States between 1948 and 1962, the use of hot chamber die casting machines for the production of magnesium alloy die-casting parts for automobiles reached millions. Despite this, in the past, magnesium alloys were mainly used as structural materials in the aerospace industry. In other fields, the main use of magnesium in the world is to produce aluminum alloys, followed by steel desulfurization and ductile iron production.
In recent years, the use of die-cast magnesium alloys has increased significantly due to the increasing demand for lighter weight products, the continuous improvement of magnesium alloy performance and the significant advancement of die-casting technology. In particular, humans have put forward the requirements for further weight reduction, fuel consumption and emissions, and improved driving safety and comfort. The magnesium alloy die-casting technology is developing rapidly. In addition, magnesium alloy die castings have been gradually expanded to other areas, such as laptop casings, portable chainsaw casings, fishing hooks, video cassettes, mobile phone casings, communications equipment on aircraft and radar casings, and Some household appliances have and so on.
Magnesium is mainly refined from magnesium-containing ores. The magnesite reserves in Dashiqiao City, Liaoning Province, China account for more than 60% of the world's reserves, and the ore grade is as high as 40%. The magnesia and magnesia products produced in China are widely used for export. Make full use of China's abundant magnesia resources for deep 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 parts, is a project in front of China's foundry workers task.
2 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, which is slightly lower than the fiber reinforced plastic with the highest specific strength; its specific stiffness and aluminum Alloy and steel are equivalent, far higher than fiber reinforced plastic; its corrosion resistance is much better than low carbon steel, it has exceeded the die-casting aluminum alloy A380; its vibration damping and magnetic shielding are far superior to aluminum alloy; in view of the power of magnesium alloy The viscosity is low, the filling speed under the same fluid state (the Reynolds index is equal) is much larger than that of the aluminum alloy, and the melting point, specific heat capacity and phase change latent heat of the magnesium alloy are lower than that of the aluminum alloy, so the melting energy is low, the solidification speed is fast, magnesium The actual die casting cycle of the alloy can be 50% shorter than that of the aluminum alloy. In addition, the affinity between the magnesium alloy and iron is small, and the ability to dissolve iron is low, so that it is not easy to adhere to the surface of the mold, and the life of the mold used is 2 to 3 times higher than that of the aluminum alloy.
Most of the commonly used die-cast magnesium alloys are American grades 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. For the die-casting magnesium alloy, there are mainly the following aspects:
(1) High-temperature performance: At present, AZ and AM two series of magnesium alloy die-casting parts account for 90% of automotive magnesium alloy die-casting parts. The strength of these two series of magnesium alloys is significantly lower at 150 °C. AS series die-casting magnesium alloys with creep resistance above 150 °C have been developed, such as AS41A alloy (Mg43% Al1%Si0.35%Mn), which has better creep strength at 175 °C than AZ91D and AM60B, and has higher elongation. Long rate, yield strength and tensile strength. Volkswagen's Beetle engine crankcases have previously used AS41 and AS42, and a recently improved alloy AE42 has better creep properties at elevated temperatures. Some trace elements such as rare earth elements Y, Nd, Sr, etc., have obvious grain refinement effect on die-cast magnesium alloy, which can improve the strength and creep resistance of die-cast magnesium alloy, such as the creep resistance of recently developed AE42. It is superior to traditional MgAlSi alloy and can be used for a long time at 200-250 °C. However, the improvement of high temperature performance of AS and AE alloys is still limited, and the casting performance is worse than that of AZ and AE alloys. In addition, the high cost of rare earth elements makes the production and application limited.
(2) Extensibility: At present, magnesium die castings are growing very rapidly in applications requiring safety and high fracture toughness. In order to improve the ability to absorb energy under working conditions, the fracture toughness of the material should be improved. This can be done by reducing aluminum in the alloy. The AM60 and AM50 are widely used in safety components such as instrument panel brackets, steering wheel shafts and seats. The AM20 is currently also applied to the back frame of the seat. In addition, the relationship between elongation at break and temperature is also quite close, especially at temperatures above about 50 ° C, increasing with increasing temperature.
(3) Corrosion resistance of magnesium alloys: Corrosion resistance has also been a major obstacle to the expansion of magnesium alloys. Magnesium has high chemical activity, and magnesium-based alloys and composite materials are prone to micro-cell corrosion. Generally, low-purity die-cast magnesium alloys have poor corrosion resistance. High-purity die-casting magnesium alloys (such as AZ91D) with strict impurity elements such as Fe, Ni, Cu, and AE42 containing rare earths have higher corrosion resistance than the die-cast aluminum alloy A380, far better than low carbon steel [ 4]. Adjusting chemical composition, surface treatment, and controlling microstructure can improve corrosion resistance. Although there are many ways to improve the corrosion resistance of magnesium alloy parts, if the problem is not solved by the material itself, the poor corrosion resistance is always a technical obstacle for obtaining a large number of applications of magnesium alloy parts.
(4) Flame-retardant magnesium alloy: Adding Al (2.5%), Be alloy (Be added in an amount of 0.0005% to 0.03%) or a Ca-containing alloy to the magnesium alloy can also effectively prevent oxidation of the magnesium alloy liquid. At present, some researchers are engaged in the research of flame-retardant magnesium alloys [5]. Once this research is successful, magnesium alloys are smelted and cast like aluminum alloys, which has wider application prospects.
(5) Magnesium alloy matrix composites: Magnesium alloy matrix composites reinforced with particles such as silicon carbide have been researched and developed for many years. Although they have not yet reached the stage of commercial application in the field of die casting, sand casting, precision casting, etc. have been used. The method has been made into castings such as impellers, bicycle cranks, automobile cylinder liners, etc., and the combination of such composite materials and semi-solid casting is applied to the development trend in the fields of die casting and extrusion casting.
3 Magnesium alloy die-casting method Magnesium alloy can be die-cast by cold chamber or hot chamber die casting machine. At present, the improvement of the hot chamber die casting machine mainly includes: using the accumulator to pressurize, the injection speed of the plunger is up to 6m/s; the induction of the gooseneck tube and the nozzle to maintain the optimum temperature; Melt insulation, and use insulation and recirculation piping to accurately maintain the bath temperature. When the magnesium alloy is die-cast by a conventional cold chamber die casting machine, the injection molding system and the automatic feeding system of the die casting machine must be modified to make it suitable for the requirements of magnesium alloy die casting. The contents of the transformation include: (1) increasing the rapid injection speed of the injection system from 4 to 5 m/s to 6 to 10 m/s when die-casting aluminum alloy; (2) shortening the pressure-building time of the pressurization process; ) Improve the injection force; (4) Use electromagnetic automatic dosing device to prevent oxidation of magnesium alloy during casting; (5) If necessary special die casting process such as vacuum die casting, configure necessary equipment.
Like other die-casting alloys, the traditional die-casting technology fills the die-cast cavity with a high-speed turbulent flow and dispersion state. The gas is either dissolved under high pressure or dissolved in the die-casting alloy, or forms many high-pressure micro-dispersions distributed in the die-casting part. Stomata. Therefore, the magnesium alloy die-casting parts produced by the conventional die-casting method cannot be heat-treated and can not be used at a relatively high temperature. In order to eliminate such defects, improve the intrinsic quality of die-casting parts and expand the application range of die-casting parts, some new die-casting methods have been researched and developed in the past 20 years, including oxygen-filled die-casting, semi-solid metal rheology or thixotropic die casting and extrusion. Casting, as well as several undulating vacuum die casting.
Vacuum die casting eliminates or significantly reduces the pores and dissolved gases in the die casting by removing the gas in the cavity during the die casting process, thereby improving the mechanical properties and surface quality of the die casting. At present, the AM60B magnesium alloy automobile wheel hub has been successfully produced by vacuum die casting on a cold chamber die casting machine, and the AM60B magnesium alloy automobile steering wheel parts are produced on a hot chamber die casting machine with a lock type force of 2940 kN. The elongation of the casting is increased from 8% to 16%.
Oxygen die casting is also known as Pore-Free Die Casting Process (PF method). The method fills the cavity with oxygen or other reactive gas before the filling of the molten metal, and replaces the air in the cavity. When the molten metal is filled, the reactive gas reacts with the molten metal to form a metal oxide particle dispersed in the die casting. In the piece, the gas in the die casting is eliminated, so that the die casting can be heat treated and strengthened. Japan Light Metal Co., Ltd. produced a computerized AZ91 magnesium alloy integral head bracket by oxygenation die casting method, which replaced the original multi-layered bracket, which not only reduced the weight of the bracket, but also achieved great economic benefits. The company also produced AM60 magnesium alloy automotive wheels and motorcycle wheels in batches using oxygen-filled die-casting, which is 15% lighter than aluminum wheels.
In recent years, companies in the United States, Japan and the United Kingdom have successfully developed magnesium alloy semi-solid thixotropic injection casting machines. The magnesium alloy semi-solid thixotropic injection casting machine injects a semi-solid magnesium alloy into a die-casting mold under a certain pressure, and its working principle is similar to that of an injection molding machine. It feeds the pre-formed non-dendritic magnesium particles into a screw feed mechanism, heats the magnesium particles to a semi-solid state in a screw feed mechanism, and passes the semi-solid magnesium through the magnesium alloy slurry collection chamber at the other end of the screw feed mechanism The alloy slurry is fed into the injection chamber for injection molding. This casting forming method represents a development direction in the production of magnesium alloy castings.
4 Magnesium alloy smelting operation and safe production Since the magnesium alloy liquid is easily oxidized and the oxide film formed on the surface is loose, the density coefficient α is only 0.79, which cannot prevent the alloy from continuing to oxidize. The magnesium alloy liquid reacts with oxygen, water vapor and nitrogen in the atmosphere to form refractory MgO, Mg3N2 and other compounds which are not melted in the magnesium liquid, and is mixed into the mold to form "oxidation slag". Therefore, it is important to prevent oxidation when melting alloys. There are two main methods for melt protection of magnesium alloys, namely flux protection and gas protection.
Smelting with protective flux usually brings the following problems: (1) Chloride and fluoride salts are volatile at high temperatures to produce toxic gases such as HCl, Cl2, HF, etc.; (2) Due to the high density of the flux, some fluxes may accompany magnesium The liquid is mixed into the mold to cause "flux slag"; (3) the gas generated by the volatilization of the flux may penetrate into the alloy liquid, become a source of corrosion during the use of the material, accelerate the corrosion of the material, and reduce the service life.
At present, most manufacturers use gas protection, that is, using a mixture of two or four kinds of dry SF6, N2, CO2, and SO2 gases to form a dense continuous film on the surface of the magnesium alloy bath to prevent oxidation of the magnesium alloy liquid. SF6 is not a toxic gas, but its greenhouse effect on the earth is 24,000 times more serious than CO2, while the magnesium industry's SF6 dosage accounts for 7% of the world's total use (1996). In the future, it will inevitably limit its use and even stop its use, but it has not yet Find a suitable alternative to SF6. Studies have shown that SO2 formed by spraying sulfur powder on the surface of the molten pool has a protective effect on the magnesium alloy solution.
The dangers of the production of magnesium alloy die-casting parts are mostly caused by the faults in the processing and post-burial process. According to Japanese statistics, the dangers caused by the production of magnesium alloy die-casting parts, smelting accounted for 25%, casting accounted for 10%, processing accounted for 39%, storage and waste accounted for 16%, electrical accounted for 3%, and others accounted for 7%. Obviously, the risk of processing and post-treatment is more than three to four times greater than the die casting process. During the processing, whether it is sand blasting, turning, milling, polishing, etc., it will inevitably produce magnesium dust and sparks. For example, the phoenix in the factory is poor, the concentration of magnesium dust in the air is too large, once the spark and the air or the magnesium of the ground Dust contact, light burning, heavy explosion. Therefore, dust collectors must be installed in the plant and equipped with fireproof sand and fire protection facilities.
5 Die-casting design Because the chemical, physical and die-casting characteristics of magnesium alloys are very different from those of aluminum alloys, the design of the mold cannot fully apply the design principle of aluminum alloy die-casting [8].
Magnesium alloy liquid is easy to oxidize and burn. The tendency of hot cracking during casting is larger than that of aluminum alloy. It is more complicated than aluminum alloy die casting in terms of melting, casting and die-casting temperature control. The filling time of magnesium alloy is short, and the problem of exhaust gas is particularly prominent. The specific heat capacity and latent heat of phase change of magnesium alloy are lower than that of aluminum alloy. Therefore, partial crystallization of local (thin section) is easy to occur in the process of die casting, resulting in blockage of the supplementary channel. Insufficient defects. The magnesium alloy die-cast design mainly considers the following aspects:
(1) Die casting machine selection. The type of die casting machine used for production depends mainly on the wall thickness of the casting. Roland Fink in the process of researching the "optimization of magnesium alloy die-casting process", through the analysis of magnesium alloy die-casting economy, cold chamber die-casting and hot chamber die-casting process, generally less than 1kg of castings need to be hot chamber die-casting Machine to ensure the filling of thin-walled parts, large parts are recommended to use cold chamber die-casting machine.
(2) Process parameters. In the die-casting production process, selecting the appropriate process parameters is a prerequisite for obtaining high-quality castings to maximize the productivity of the die-casting machine, and is the basis for the correct design of the die-casting mold. During die casting, there are many factors affecting the filling of alloy liquid, including injection pressure, injection speed, filling time and die temperature. Due to the difference in wall thickness and complexity of die castings, the choice of process parameters varies widely. Compared with aluminum and zinc alloys, magnesium alloys have better fluidity, so the secondary injection speed can be larger. The punch speed of magnesium alloy is about 30% faster than aluminum alloy, and the maximum is even more than 10m/s. Since the casting properties of magnesium alloys such as fluidity are quite sensitive to the type temperature and pouring temperature, the magnesium alloy liquid is very easy to solidify during the filling process, and the gentle casting temperature must be precisely controlled, otherwise the waste product is easy to be discharged.
(3) Pouring system design. The gating system plays an important role in controlling the flow direction of the molten metal, the exhaust overflow condition, the temperature distribution of the mold, the pressure transmission, the length of the filling time, and the speed and flow state of the molten metal through the runner. Regulating effect. The gating system design is summarized as follows:
In-gate position: Since magnesium alloy is solidified faster in the cavity than aluminum, zinc and other alloys, and general magnesium alloy die-casting parts are thin-walled parts, the choice of the position of the gate must avoid direct impact on the cavity surface. Ensure that the flow path of the molten metal in the cavity is the shortest to prevent under- and cold-separation.
Filling speed: Generally speaking, due to the thermodynamic characteristics of magnesium alloy, the heat transfer rate of the alloy to the mold is very fast, and the solidification interval is large, and the fluidity is poor. Therefore, in order to avoid premature solidification of the runner magnesium liquid, the magnesium liquid should be made. High-speed and smooth filling into the cavity [9]. Generally, the flow rate of the inner runner is 90-100 m/s. For some thin-wall magnesium alloy die-casting parts, the gate runner speed is even as high as 20 m/s.
Inlet size: In many cases, the gate is removed by machining. The inner runner width should be less than 50% of the wall thickness to avoid damage to the casting during trimming. In order to obtain the minimum thickness of the gate and to ensure the thin wall of the magnesium die casting, the width of the gate should be as large as possible to ensure a proper cross-sectional area of ​​the gate.
Filling time: It is closely related to the speed of the gate runner and has a great influence on thin-walled castings with high surface quality requirements. The filling time is 0% less than that of the aluminum alloy, and is usually 10 to 100 ms.
Overflow tank design For thin-walled magnesium alloy die-casting parts, the optimal overflow tank inlet area is about 20% to 25% of the cross-section area of ​​the gate [9].
5 Research on Computer Simulation of Filling Process With the increasing application of magnesium alloy castings, higher requirements are imposed on the filling properties of die-cast magnesium alloys. At present, little is known about the filling rule of die-cast magnesium alloy, the relationship between filling performance and die-casting process parameters, and the critical wall thickness of filling. Therefore, systematic research is urgently needed. To this end, the computer simulation of magnesium alloy filling and solidification process should be vigorously carried out, and an expert system should be formed on the basis of this to guide the development of die-casting process, die-casting design, quality control of die-casting parts, and improve the qualification of magnesium alloy die-casting parts. Rate and die cast life.
Numerical simulation software is most commonly used in automotive magnesium die castings. Some automotive industries in Germany have successfully simulated seat frames, tangential fuel pumps, Audi 5x gearboxes, wheels, 4-cylinder engine blocks, etc. Magnesium alloy die-casting parts effectively shorten the product development cycle and greatly enhance the competitiveness of the enterprise market.
6 Application prospects in the automotive industry According to the literature, 98% of the world's engineering component magnesium alloy demand comes from the die-casting industry, and more than 70% of it is used in the automotive industry. Therefore, the die-casting process performance of magnesium alloys is in the industry. The development of the application plays a decisive role.
North America is the region with the largest amount of magnesium alloys, with an annual growth rate of 30%. Well-known car companies such as Ford, GM and Chrysler have been working on new magnesium alloy and magnesium alloy clutch housings, steering column frames, intake manifolds and lighting grippers for over a decade. Development and application. In 1996, the Ministry of Energy and the GM, Ford and Chrysler Group signed a cooperation program called “PNGV†(New Generation Vehicle), which aims to produce energy-efficient cars that meet market requirements. General Motors successfully developed magnesium alloy automotive wheels in 1997 and signed an agreement with the world's largest magnesium production and processing company, Hydro, to apply magnesium alloy die castings; Wisconsin Lindberg thixoforming development center for magnesium The alloy die-casting technology has been innovated to produce magnesium alloy racing clutch plates and automotive transmission parts using semi-solid die casting technology. The use of magnesium alloys in Europe is second only to North America, with an annual growth rate of 60%. The famous Mercedes-Benz automobile company first applied magnesium alloy die-casting parts to the car seat bracket. Audi Motor Company first introduced the magnesium alloy die-casting automobile instrument panel. It can be said that Germany is the pioneer and main force to promote the development of magnesium alloy die-casting. In 1997, Germany was led by the Federal Ministry of Science and Technology (BMBF), and together with more than 50 companies including Volkswagen and six universities and research institutes such as the Technical University of Munich, invested 25 million marks for a three-year "MADICA". "(Magnesium alloy die casting) development project. Toyota Motor Corporation first manufactured magnesium alloy automotive wheels, steering shaft systems, cam covers and other components; Mitsubishi worked with the Australian Ministry of Industry and Technology to develop ultra-lightweight magnesium alloy engines. At present, various automobile companies in Japan produce and apply a large number of magnesium alloy shell-type die-casting parts. Figure 1 shows the demand for magnesium alloy die-casting parts in various regions of the world from 1991 to 2006. Figure 2 shows the 10 largest auto companies in the world in terms of magnesium alloys in 1997.
In recent years, humans have put forward requirements for further weight reduction, fuel consumption and emissions, and improved driving safety and comfort. Since magnesium is 75% lighter than steel, replacing the steel with magnesium alloy die casting can reduce the weight of the car. 10%, and the fuel consumption per 100 kilometers can be reduced by 0.7 per 100kg of body weight. Therefore, the output of magnesium alloy die castings for automobiles has shown a sustained and rapid growth in industrialized countries. Ford Motor Company's goal is to strive for a fuel consumption of up to 3 liters per 100 kilometers for medium-sized cars in a few years; the total weight of bicycle magnesium alloy die-casting parts of the AudiA6 sedan has reached 14.2kg. Its future goal is to increase the total weight of bicycle magnesium alloy die castings to 50-80kg. Japan's Mitsubishi Corporation is working with the Australian Ministry of Industry and Technology to develop an ultra-lightweight magnesium engine.
At present, the components of the car using magnesium alloy include some components on the seat, knee pads, steering column components, steering wheel, brake and clutch pedal bracket, airbag restraint, pedal bracket, and small radiator frame for car audio. , mirror brackets and some components of the openable roof; engine body, cylinder head, intake manifold, oil pump housing, auxiliary device bracket and electrical wiring device in the car engine; and gearbox, clutch housing, steering Plate, bonnet, valve plate, window, motor housing, oil filter joint, intake manifold, mirror cover, headlight holder, bracket for brake anti-lock system, wheel, fuel tank door. In the future development, on some large-scale die-casting parts, it will also expand its applications, such as roof, engine cover, rear hatch, inner door panel, instrument panel and so on. Other parts that require safety and high fracture toughness will also be the areas of magnesium alloy expansion applications, such as seat frames, steering columns, body protection panels, radiator grille reinforcements in front of the engine and some body structure supports. . At present, the use of magnesium die-casting parts in China's automobiles is only the transmission case and cover of the Santana sedan.
Cars are moving toward safer, lighter, more material and more energy-efficient, less environmentally polluting and more comfortable. They require a new concept design for the car, which is a new way for the application of magnesium alloy in the automotive industry. Broad field. The application of magnesium alloy die castings in automobiles will increase at a faster rate as the production of automobiles increases.
7 Conclusion Magnesium alloy die-casting is a systematic project integrating design, manufacture and research. Magnesium alloy die-casting workers should have more comprehensive knowledge, experience and research and development capabilities than those engaged in zinc and aluminum alloy die-casting.
Magnesium alloys for automobiles are mainly die-casting products. Magnesium die-casting parts are used to reduce the quality of automobiles, reduce fuel consumption and reduce exhaust gas emissions. Magnesium alloy die-casting parts have the advantages of noise reduction and vibration damping performance and high casting precision, and have comprehensive economic benefits. It is the most promising material for lightweight vehicles and has broad application prospects.
China is a country with extremely rich magnesium resources. However, due to its backward position in magnesium alloy die-casting technology and application, most of its magnesium resources are exported to foreign countries in the form of primary products. The export production of magnesium is much larger than domestic consumption. With the rapid development of the domestic automobile industry and the industries of computer, information, communication, instrumentation, aerospace and other industries, China's magnesium alloy die-casting industry will also have a place in the world market of strong hands such as Lin.
Cosmetic Tube,Plastic Soft Tube,Plastic Tube
Feiyi Cosmetic Trading Co., Ltd. , http://www.stfycosmetic.com