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Several major issues of the magnesium alloy die-casting technology and its application prospects

1 magnesium alloy capacity of 18kg or so. The United States between 1948 and 1962, production of hot chamber die casting machine car up to one million of magnesium alloy die castings. Nevertheless, magnesium alloys as structural materials mainly for the aviation sector in other areas, the main purpose of the world's magnesium is the production of aluminum, followed for the desulfurization of steel and ductile iron production.
  In recent years, lightweight products are becoming more urgent, the continuous improvement of the performance of magnesium alloy die-cin 1808 of the magnesium alloy material available, beginning for the industrial production in 1886. Magnesium alloy die-casting technology date from 1916, the magnesium alloy for die castings, has also experienced 80 years of development. Human understanding and control of the production technology of magnesium alloy and its products, has experienced a long exploration history. From 1927 high strength MgAl9Zn1 began, the industrial applications of magnesium alloys substantial progress. 1936 German Volkswagen produced using die cast magnesium alloy engine drive system of the "Beetle" car parts, 1946 bicyclesasting technology significantly advances, a significant increase by the amount of die cast magnesium alloy. In particular, the human vehicle to further reduce weight, reduce fuel consumption and emissions, improve driving safety and comfort requirements for magnesium alloy die-casting technology are developing fast. In addition, the magnesium alloy die castings has been gradually expanded to other areas, such as the portable computer housing, portable saws chassis, the hook automatically closing lines cartridge, VCR shell, mobile phone cases, the communications equipment on the aircraft and radar chassis, as well as some household appliances.
  Magnesium is mainly composed of magnesium ore refining. Dashiqiao City, Liaoning Province of China in the vicinity of magnesite reserves account for more than 60 percent of world reserves, ore grades up to 40%. A large number of our production of magnesia and magnesia products for export. Take full advantage of rich resources of magnesite in-depth development, combined with the development of emerging industries of automotive, computer, communications, aerospace, electronics, etc., to promote the production and application of magnesium alloy die castings, is placed in front of the China foundry workers a task.
2 die-cast magnesium alloy
  Density of magnesium alloys is less than 2g/cm3 is the lightest structural metallic materials, and its strength is higher than aluminum and steel, slightly lower than the highest strength fiber reinforced plastic; its quite specific stiffness and aluminum and steel. much higher than the fiber reinforced plastics; its corrosion resistance is much better than the low-carbon steel, more than die-casting aluminum alloy the A380; its vibration damping, magnetic shielding is far superior to aluminum alloy; low view of the kinematic viscosity of the magnesium alloy, the same filling velocity of the fluid state (equal to the Reynolds) is much larger than the aluminum alloy, combined with magnesium alloy melting point, low heat capacity and latent heat than aluminum, so the melting energy consumption, fast solidification, magnesium alloy die-casting actual comparable cycle aluminum alloy short of 50%. In addition, the affinity of magnesium alloy and iron small, low capacity of the solid solution of iron, and therefore not easy adhesion to the mold surface, they use the life of the die 2 to 3 times higher than aluminum.
  The die-casting magnesium alloy is mostly the U.S. grades of AZ91 of AM60, AM50 Magnesium Alloy AM20, AS41 and AE42, respectively, belong to the Mg-Al-Zn, Mg-Al-Mn, Mg-Al-Si and Mg-Al-RE-four series. And die-casting magnesium alloy, the following aspects:
The elevated temperature performance: (1) AZ and AM two series of magnesium alloy die castings accounted for the car with 90% of the magnesium alloy die castings, magnesium alloy of the two series in the strength of above 150 ℃ were significantly decreased. AS Series die-cast magnesium alloy has been developed out of the creep resistance above 150 ℃, such as the AS41A alloy (Mg43% Al1% Si0.35% Mn), the creep strength of 175 ℃ is better than AZ91D and AM60B, and higher stretch rate, yield strength and tensile strength. Volkswagen Beetle engine crankcase had previously been using AS41 and AS42, the recent introduction of an improved alloy AE42 creep properties at elevated temperatures is even better. Some trace elements such as rare earth elements Y, Nd, Sr, die-casting magnesium alloy has a significant role in grain refinement, can improve the strength and creep resistance of die cast magnesium alloy, such as the recently developed creep resistance of AE42 better than the traditional MgAlSi alloy, long-term use 200 ~ 250 ℃. AS and AE alloys for high temperature performance improvement is still limited, and its casting properties than AZ and AE alloys worse, coupled with the high cost of rare earth elements, so that the production and application subject to certain restrictions.
(2) Scalability: At present, the magnesium die castings require the use of safe and high fracture toughness increased very rapidly. Under working conditions, to improve the ability to absorb energy, it should improve the fracture toughness. By reducing aluminum in the alloy, can be achieved. AM60 and AM50 are widely used in the instrument panel bracket, steering wheel shaft and seating safety components, the AM20 is also applied to the seat back frame. In addition, the elongation and temperature relationship is quite close, especially to increase with increasing temperature above about 50 ℃.
  (3) The corrosion resistance of magnesium alloys: corrosion resistance also is a major obstacle to expand the application of magnesium alloy. High chemical activity of magnesium, magnesium-based alloys and composites are prone to micro-cell corrosion, corrosion resistance is generally low purity of the alloy. Stringent requirements of the Fe, Ni, Cu and other impurity elements in high purity die cast magnesium alloy (eg, of AZ91D), as well as the AE42 containing rare earth salt spray test the corrosion resistance of more than die-casting aluminum alloy the A380 is much better than the low-carbon steel [ 4]. Adjust the chemical composition, surface treatment and control of microstructure can improve its corrosion resistance. Despite the increase of magnesium alloy corrosion resistance of the method to solve the problem, but if not from the material itself, the poor corrosion resistance is always a technical obstacle to the magnesium alloy to obtain a large number of applications.
  (4) flame-retardant magnesium alloy: Al (2.5%) added to the magnesium alloy and Be alloy (Be the amount of 0.0005% to 0.03%) or with Ca alloys can also be effective in preventing the oxidation of the magnesium alloy liquid. , Some researchers are currently engaged in a flame retardant properties of magnesium alloys [5], this study once like aluminum alloy, magnesium alloy melting and casting, to obtain a more broad application prospects.
  (5) magnesium alloy matrix composites: such as silicon carbide particulate reinforced magnesium alloy matrix composites have been years of research and development, at present, although not yet reached the stage of the die-casting business applications in the field, but with sand casting, precision casting, etc. The method for producing the casting of the impeller, bicycle crank, automotive cylinder liner, and this composite material with semi-solid casting a combination, used in die casting and squeeze casting the field of development trends.
3 magnesium alloy die-casting
  The magnesium alloy used die casting cold chamber or hot chamber die casting machine. Improvement of the hot chamber die casting machines, including: the accumulator pressurized injection plunger, injection speeds of up to 6m / s; induction heating gooseneck and nozzle, so that it remains the most appropriate temperature; use of dual-furnace melted insulation, and insulation devices and recirculation piping, to accurately maintain the bath temperature. With ordinary cold chamber die casting machine, die-casting magnesium alloy die-casting machine injection system and automatic feeding system to make the necessary transformation to the applicable requirements of magnesium alloy die-casting. The content of the transformation include: (1) the injection system of the fast injection speed increased by die-casting aluminum alloy 4 to 5m / s to 6 to 10m / s; (2) shorten the pressurization process time pressure built; (3 configure the necessary equipment) to improve the injection force; (4) the use of electromagnetic automatic dosing device to prevent the oxidation of magnesium alloys in the casting process; (5) If using vacuum casting and other special die casting process.
  And other die-casting alloy, die-casting technology to make the liquid filling the high-speed turbulence and dispersion state of magnesium alloy die-casting cavity, the gas under high pressure or dissolved in die-casting alloy, or the formation of high-pressure micro-dispersed in the die casting stomata. Production of traditional die-casting magnesium alloy die-casting can not be heat strengthened, can not be used at higher temperatures. To eliminate this defect, to improve the intrinsic quality of the die-casting, and to expand the scope of application of the die casting, the past 20 years research and development of a number of new die-casting methods, including die casting, semi-solid metal rheological and thixotropic die-casting and extrusion casting, as well as the ups and downs of the vacuum die-casting.
The vacuum die by pumping gas in addition to the cavity in the die casting process to eliminate or significantly reduce the die casting pores and dissolved gases, improve the mechanical properties of the die-casting and surface quality. Currently has succeeded in the cold chamber die casting machine with vacuum die casting method production AM60B magnesium alloy automobile wheel hub, the locking force is 2940kN the hot chamber die casting machine production out AM60B magnesium alloy automobile steering wheel parts, 8% of castings elongation rate increased to 16%.
  Die casting, also known as non-stomatal Die Casting (Pore-Free Die Casting Process, the PF method). The liquid metal filling, the filling of oxygen or other reactive gases into the cavity, replacement air cavity, the liquid metal filling, reaction of reactive gases and the filling liquid metal to form metal oxide particles dispersed in the die casting pieces, thereby eliminating the gas within the die casting, die castings can be heat treated to strengthen. Nippon Light Metal (strain), the overall head bracket AZ91 magnesium alloy die casting production computer, instead of the original multi-layer composite bracket, not only to reduce the weight of the stent, and has made great economic benefits. The company also mass-produced by die casting of AM60 magnesium alloy automotive wheels and motorcycle wheels, aluminum wheels, weight reduction of 15%.
  The United States, Japan and the United Kingdom in recent years companies have successfully developed a magnesium alloy semi-solid thixotropic injection pressure casting machine. Magnesium alloy semi-solid thixotropic injection pressure casting machine to a certain pressure, semi-solid magnesium alloy making it launched into die-casting type forming, and its working principle is similar to injection molding machines. It will be prefabricated in non-dendritic state Magnesium spiral feeding into the bodies in the spiral feeder institutions in magnesium granules heated to a semi-solid, and the other end by spiral feeder institutions magnesium alloy slurry collection chamber will be semi-solid magnesium alloy slurry into the injection chamber for injection compression molding. This Casting method represents a direction of development of the production of magnesium alloy castings.
4 magnesium alloy smelting operations and production safety
  Magnesium alloy liquid is very easily oxidized, and the surface of the oxide film is a loose, its dense coefficient α is only 0.79, can not prevent the alloy continue to oxidation. Magnesium alloy liquid and atmospheric oxygen, water vapor, nitrogen response generated refractory compounds such as MgO, Mg3N2 does not melt the magnesium liquid, mixed with the mold after the formation of oxidized slag. Therefore, the melting alloy oxidation is critical. Melt protection of magnesium alloy, there are two main methods, the flux protection and gas protection.
  Protection flux melting usually will bring the following question: (1) chloride and fluoride salts at high temperatures, volatile produce toxic gases such as HCl, Cl2, HF; (2) As the flux density is larger, some flux will be accompanied by magnesium liquid mixed with mold caused by the flux slag; (3) The flux of volatile gas may seep into the liquid alloy has become the source material used in corrosion, accelerated corrosion, reduce the service life.
  The majority of manufacturers use gas protection, and four kinds of dry SF6 and of N2, of CO2, SO2 gas mixture, the magnesium alloy pool surface to form a dense continuous film in order to prevent oxidation of the magnesium alloy liquid. SF6 is not toxic gases, but it is the greenhouse effect serious 24 000 times more than CO2, and the magnesium industry SF6 with the world total amount of 7% (1996), the future is bound to limit the dosage and even stop its use, but not yet find suitable substitutes for SF6. Studies have shown that the formation of SO2 in the pool surface, sprinkle sulfur powder has a protective effect of magnesium alloy liquid.
  The risk of production of magnesium alloy die castings is mostly caused by negligence in the processing and in covering. According to Japanese statistics, the magnesium alloy die-casting production process caused by the risk of melting of 25%, casting accounted for 10 percent, accounted for 39% of processing, storage and waste accounted for 16%, electrical 3%, other 7%. Clearly, processing and risk of process 3 to 4 times more than die-casting process. Process, sand blasting, turning, milling, polishing, etc., are inevitably will produce magnesium dust and sparks, such as the plant through the phoenix bad, too much concentration of magnesium dust in the air, spark and air or ground magnesium dust exposure, ranging from burning, while in the explosion. The dust collector must be installed within the plant and are equipped with fire sand and fire facilities.
2) process parameters. In die-casting production process, select the appropriate process parameters is a prerequisite for high-quality casting to play the largest productivity of die casting machine is the basis of the proper design of die-casting mold. Die-casting, the impact of alloy liquid filling forming factors, the main injection pressure, injection speed, filling time, and die-casting mold temperature. Due to the different die casting wall thickness and complexity of the process parameters vary widely. Magnesium alloys with aluminum, zinc alloy, better mobility, two injection speed can be greater, the speed of the punch of the magnesium alloy by about 30% faster than aluminum, even more than 10m / s. Magnesium alloy casting properties such as fluidity is very sensitive to the type of temperature and pouring temperature, the magnesium alloy liquid in the filling process can easily solidified, must be precisely controlled temperature and pouring temperature, otherwise Yi out of waste.
(3) gating system design. Gating system in the direction of the liquid metal, exhaust overflow conditions, the mold temperature distribution, pressure transmission, filling the length of time and the liquid metal through a runner at the speed and flow state, plays an important control regulatory role. Gating system design are summarized as follows:
  The ingate Location: magnesium alloy than the aluminum, zinc alloy solidification in the cavity faster, and magnesium alloy die castings for the thin-walled parts, therefore runner position of choice must be to avoid a direct impact on the cavity surface, shortest flow path of the liquid metal in the cavity, in order to prevent the phenomenon appears poured cold shut.
  Filling velocity: In general, the thermodynamic properties of magnesium alloy, the alloy to the mold heat transfer speed quickly, and the solidification range, less liquid, so as to avoid the runner magnesium liquid premature solidification, magnesium liquid high-speed smoothly filled into the cavity [9]. General ingate flow rate of 90 ~ 100m / s, for some thin-walled magnesium alloy die castings, ingate speed even up to 20m / s.
  The ingate size: in many cases within the runner machined to remove. The ingate width should be less than 50% of wall thickness to avoid the process of trimming castings damage. In order to obtain minimum ingates thickness, while ensuring that the requirements of thin-walled magnesium die castings, The ingate width should be taken to ensure that the inside runner cross-sectional area.
  Filling time: it is closely related with the speed of the inner runner for the high surface quality requirements of thin-walled castings impact. Filling time 0% less than the aluminum alloy, usually taken as 10 ~ 100ms.
  The overflow tank is designed for thin-walled magnesium alloy die castings, best overflow tank entrance area of ​​approximately 20% of the runner cross-sectional area within ~ 25% [9].
5 filling process of computer simulation
  With the increasingly vast field of application of magnesium alloy castings, the filling performance of the alloy has a higher demand. Currently filling pattern of the alloy, the filling performance and the relationship of the casting process parameters, the filling of critical thickness and other poorly understood, therefore, an urgent need to carry out systematic research. Therefore, we should vigorously carry out the filling of magnesium alloy and the solidification process of computer simulation study, and on the basis of this expert system, the formulation of guiding the die casting process, die casting design, die casting, quality control, improve the qualified of the magnesium alloy die castings rate and the service life of die-casting type.
5 die casting design
  Magnesium alloy chemistry, physical parameters and die casting characteristics and aluminum alloys are very different, so the mold design can not fully apply the principles of aluminum alloy die-casting design [8].
  Easily oxidized burning magnesium alloy liquid, casting hot cracking tendency than aluminum, than aluminum alloy die-casting complex in terms of melting, casting and die-casting-type temperature control. Filling time of the magnesium alloy, the exhaust is particularly prominent, magnesium alloy is lower than the heat capacity and latent heat than aluminum, thus prone to local (thin cross section parts in the die casting process) early crystallization phenomenon, resulting in feeding channel blockage, resulting in pouring a shortage of defects. The magnesium alloy die-casting design to consider the following aspects:
  (1) die casting machine choice. What form of die-casting machine to produce mainly depends on the casting wall thickness. The Roland Fink is in the process of magnesium alloy die-casting process optimization through analysis of the economy, cold chamber die casting and hot chamber die casting process of magnesium alloy die, under normal circumstances, less than 1kg casting hot chamber die-casting machine, in order to ensure full of thin-walled, large recommend the use of cold chamber die casting machine.
Numerical simulation software is the most common application in the automotive magnesium die castings, some of the automotive industry in Germany has successfully simulated the seat frame, THIXOFORMED fuel pump, Audi 5-speed gearbox, wheels, 4-cylinder engine block and other automotive Magnesium Die Casting, effectively shortening the product development cycle, greatly enhance the market competitiveness of enterprises.
Application prospects in the automotive industry
  According to the literature, 98 percent of the world engineering components of magnesium alloy demand from the die casting industry for more than 70% of which for the automotive industry, so the magnesium alloy die-casting process performance in industry development plays a decisive role.
  North America is the region with the largest amount of magnesium alloy, 30% annual pace of development. The famous car companies such as Ford, GM and Chrysler and other companies in the past ten years has been committed to a new magnesium alloy and magnesium alloy clutch housing, steering column, rack and into the manifold and lighting holder, auto parts development and application. Government Department of Energy and General Motors, Ford and Chrysler Group signed in 1996 a project called "the PNGV (a new generation of transport) of the cooperation program, the program aims to produce energy-saving cars that meet market requirements. General Motors Corporation in 1997 to develop a magnesium alloy car wheels, and with the world's largest magnesium production and processing companies - Hydro Company signed a protocol of the application of magnesium alloy die castings; Wisconsin Lindberg thixotropic forming the Development Center of magnesium alloy die-casting technology innovation, the use of semi-solid casting technology to produce magnesium alloy racing clutch plates, automotive transmission parts. The amount of magnesium alloy in Europe second only to North America, the annual pace of development is 60%. Mercedes-Benz car company first magnesium alloy die castings used in automotive seat bracket, Audi launched the magnesium alloy die-casting car dashboard, it can be said that Germany is a pioneer and the main force to promote the development of magnesium alloy die. In 1997, Germany led by the Federal Science and Technology Ministry of Education (BMBF), the joint Volkswagen, more than 50 companies and the Technical University of Munich six universities and research institutions, investment of 25 million marks a three-year MADICA (magnesium alloy die-casting) development projects. Toyota Motor Corporation first to produce magnesium alloy car wheels, steering shaft system, cam cover and other parts; Mitsubishi Corporation and the Australian Industry Ministry of Science and Technology, to develop ultra-lightweight magnesium alloy engine. At present, the various car companies in Japan are the production and application of die-casting magnesium alloy shell class. Figure 1 is from 1991 to 2006 in various regions of the world magnesium alloy die castings demand situation. Figure 2 is a 1997 magnesium alloy in the world's largest amount of 10 car companies.
In recent years, human beings on the car to further reduce weight, reduce fuel consumption and emissions, and improve driving safety and comfort requirements, Magnesium 75% lighter than steel, magnesium alloy die castings to replace the single pieces of steel can make car to reduce weight 10% of body weight reduced by 100kg 100 km fuel consumption decreased by 0.7 automotive magnesium alloy die casting the momentum of sustained and rapid growth of production in industrialized countries. Ford Motor Company's goal is to strive to several years after the mid-size car 100 km fuel consumption of as little as three liters; AudiA6 car bicycle magnesium alloy die casting weight has reached 14.2kg. The future goal is to cycling magnesium alloy die castings gross weight increased to 50 ~ 80kg. Japan's Mitsubishi Aluminum Company is working with the Australian Industry Ministry of Science and the quality of the development of a ultra-light magnesium engine.
At present, the magnesium alloy components on the car seat parts, knee pads, steering column parts, steering wheel, brake and clutch pedal bracket, airbags limiting device, the pedal bracket, car audio small radiator frame , mirror bracket, as well as open roof components; car engine in the engine block, cylinder heads, intake manifold, pump housing, assistive devices, stents, and electrical wiring devices; gearbox, clutch housing, steering plate columns, valve cover, valve plates, windows, motor cases, the joints of the oil filter, intake manifold, mirror cover, headlight cage bracket anti-lock braking system, wheels, fuel tank door. Future development on some large-scale die-casting, will expand its application, such as roof, engine cover, and then hatch inside the door frame, dashboard, etc.. Other parts, such as the need for safety and high fracture toughness of magnesium alloy to expand the application areas, such as the framework of the seat, steering column, body protection panels, engine radiator grille to strengthen the board and body structural support . At present, China's auto magnesium die castings of Santana's transmission box and lid.
  Car towards a safer, lighter, more efficient, more energy efficient, environmental pollution, lighter, more comfortable in the direction the development of new conceptual design requirements for automotive applications for magnesium alloys in the automotive industry to open remote more wider area. Magnesium alloy die castings in the car with the gradual growth of the automobile production rose at a faster rate.
7 Conclusion
  The magnesium alloy die-casting is a design, manufacturing and systems engineering research in one, the magnesium alloy die-casting workers and personnel engaged in the zinc and aluminum die casting compared to more comprehensive knowledge, experience and research and development capabilities.
  Automotive magnesium alloy die-casting products, the use of magnesium die castings make the quality of the car to reduce fuel consumption reduction to reduce exhaust gases; and magnesium alloy die castings with high precision for NVH performance and casting, the overall economic efficiency. Vehicle weight reduction is the most promising material, has broad application prospects.
  China Magnesium is extremely rich country, but because of the magnesium alloy die-casting technology and applications in a backward position, magnesium resources in the form of primary products exported to foreign countries, magnesium export production is much larger than domestic consumption. With the rapid development of the domestic auto industry and the computer, information, communications, instrumentation, aerospace and other industries, China's magnesium alloy die-casting industry will also on a multitude of strong world market!

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