Since 1990s, magnesium, as a rapidly rising engineering metal material in the world, has maintained a high growth rate of 15% every year, far higher than that of aluminum, copper, zinc, nickel and steel, which is unprecedented in the application of modern engineering metal materials. Taking magnesium alloy die castings as an example, according to the estimation of magnesium international magnesium association and magnesium hydrogen company, the application of magnesium in magnesium alloy die castings in the world has reached 24,000 t in 199 1 year. Since then, it has steadily increased at the rate of 15%-20% every year, and 1997 reached 64 000t t. In 2000, it broke through the 100000 ton mark. By 2008, it may increase to 240,000 tons, of which 80% is used in the automobile industry.

1 application of cast magnesium alloy

1. 1 aerospace field

As far as aviation materials are concerned, structural weight reduction and structural load-bearing and functional integration are important directions for the development of aircraft airframe structural materials. Magnesium has long been used in aviation industry because of its low density and high specific strength. The economic benefits and performance improvement brought by weight reduction of aviation materials are very significant. The fuel cost savings brought by weight reduction of commercial aircraft and automobiles are nearly 100 times that of the latter. The fuel cost saved by fighters is nearly 10 times that of commercial aircraft. More importantly, the improvement of its mobility can greatly improve its combat effectiveness and viability. Because of this, the aviation industry will take various measures to increase the consumption of magnesium alloys.

1.2 military field

Magnesium alloy has the characteristics of light weight, good specific strength and stiffness, good vibration damping performance and strong electromagnetic interference shielding ability, which can meet the requirements of military products for weight reduction, sound absorption, shock absorption and radiation protection.

1.3 automobile field

Magnesium alloys used as automobile parts usually have the following advantages:

1) Improve the comprehensive standard of fuel economy and reduce the exhaust emission and fuel cost. According to estimates, 60% of the fuel used by cars is consumed by their own weight. Every time the car loses weight 10%, the fuel consumption decreases by 8%-10%;

2) Weight reduction can increase the vehicle's load and payload, and also improve the braking and acceleration performance;

3) It can greatly improve the noise and vibration of vehicles.

1.4 motorcycle park

In the past 50 years, through continuous technological innovation, the application of magnesium alloy in motorcycles has been expanding in breadth and depth. Application models have expanded from racing cars to sports motorcycles, light motorcycles and concept motorcycles, covering more than a dozen major motorcycle brands in Europe, America and Japan. Magnesium alloy application components cover more than 40 kinds of power systems, transmission systems and various motorcycle accessories, among which Dymay wheels in Britain alone have more than 400 kinds. At present, the application of magnesium alloy in motorcycles in China is still blank. Chongqing Longxin took the lead in trial-producing the "magnesium alloy green concept motorcycle" with model LXl50, which attracted wide attention in China. Now, three of the used 12 parts have been mass-produced.

1.5 3C field

3C products-Computer, Communication and consumer electronic products are the fastest-growing industries in the world today, and digital technology has led to the continuous emergence of various digital products. Magnesium alloy 3C products first appeared in Japan. 1998, Japanese manufacturers began to use magnesium alloys in various portable goods (such as PDA, mobile phone, etc.). ). At present, the most common 3C product using magnesium alloy is notebook computer, which was first introduced by Sony Corporation of Japan. Driven by the development trend of 3C products in the direction of light, thin, short and small, the application of magnesium alloys has been increasing in recent years.

2 melting technology of cast magnesium alloy

Flame retardant technology of cast magnesium alloy liquid

2. 1. 1 flux protection law

The compound with low melting point melts into liquid at low temperature and spreads on the surface of magnesium alloy to prevent magnesium liquid from contacting with air, thus playing a protective role. At present, the commonly used flux is mainly anhydrous carnallite (MGC L2-KC) with some fluoride and chloride added. The agent is convenient to use, low in production cost, good in protection and use, and suitable for the production characteristics of small and medium-sized enterprises. However, the medicine should be dehydrated again before use, which will release a choking smell when used. Due to the high density of flux, it gradually sinks and needs to be added continuously. A large number of harmful gases are released during use, which pollutes the environment and seriously corrodes the factory building. Therefore, it is an important subject to study a new type of magnesium alloy flux with good covering and refining effect and no pollution.

2. 1.2 gas protection law

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, thus isolating oxygen in the air. The main protective gases used are SF6, SO2, CO2, Ar, N2, etc. In order to further improve the protection effect and reduce the COnsumption of expensive SF6 gas, air or co: mixed gas and other dry gases are generally mixed into SF6 gas abroad to achieve good protection effect, but there are the following problems:

1) pollutes the environment, SF6 will produce toxic gases such as SO2 and SF4, and its effect on global warming is 24,900 times that of CO2;

2) The equipment is complex, requiring complex gas mixing devices and sealing devices;

3) Corrosion of equipment, significantly reducing the service life of crucible.

2. 1.3 alloying method

In the past, beryllium was added to magnesium alloys to improve their flame retardant properties. However, beryllium is toxic, too much beryllium will cause grain coarsening and increase the tendency of hot cracking, so it is limited by the addition amount. Japanese scholars believe that adding a certain amount of calcium can obviously improve the light-off temperature of magnesium alloys, but there are problems that the mechanical properties of magnesium alloys are seriously deteriorated due to too high addition amount. Adding calcium and zirconium at the same time has flame retardant effect. Domestic research shows that adding rare earth cerium to magnesium alloy AZ9 1D can effectively increase the light-off temperature of magnesium alloy.

2.2 Modification treatment technology of magnesium alloy melt

The purpose of melting modification of magnesium alloy is to change the microstructure of magnesium alloy. This process has a great influence on the grain size and mechanical properties of the alloy, and also has a certain influence on oxide inclusions in magnesium liquid. The results show that for magnesium alloys containing no Al, Zr modification has a good grain refinement effect, and its principle is that peritectic reaction of Zr promotes grain refinement. A 1C4 can be produced by adding suitable carbon material into magnesium-aluminum alloy and reacting with alloy liquid, which can play the role of heterogeneous crystal nucleus and promote the grain refinement of magnesium alloy. Adding different contents of mixed rare earth to AZ9 1 magnesium alloy also has obvious effects on its as-cast and solution aging microstructure and properties.

3 magnesium alloy forming technology

Magnesium alloy forming can be divided into two methods: deformation and casting. At present, casting technology is mainly used. Magnesium alloys can be formed by sand casting, lost foam casting, die casting and semi-solid casting. In recent years, new technologies of magnesium alloy die casting have been developed, including vacuum die casting and oxygen die casting. The former has successfully produced AM60B magnesium alloy wheels and steering wheels for automobiles, and the latter has also been used to produce magnesium alloy parts for automobiles. Solving the forming problem of large complex automobile parts is the direction of further developing and perfecting magnesium alloy forming technology. The common casting methods of magnesium alloys are briefly introduced here.

3. 1 die casting

In this method, molten magnesium alloy is injected into the cavity of precision metal mold at high speed and high pressure to make it rapidly formed. Die casting machines can be divided into hot chamber die casting machines and cold chamber die casting machines according to the way magnesium liquid is injected into metal cavities.

1) hot chamber die casting machine. Its pressure chamber is directly immersed in the magnesium liquid in the crucible and is in a heated state for a long time, and the injection part is installed above the crucible. In this way, it is not necessary to supply magnesium liquid to the pressure chamber in every cycle of die casting, so the production can be rapid and continuous, and it is easy to realize automation. The advantages of hot chamber die casting machine are simple production process and high efficiency; Less metal consumption and stable process; The magnesium liquid pressed into the cavity is clean and the casting quality is good; Magnesium hydraulic cavity has good fluidity and is suitable for pressing thin-walled parts. However, the pressing chamber, die-casting punch and crucible are immersed in magnesium liquid for a long time, which affects the service life and requires higher materials for these heating parts. Magnesium alloy hot chamber die casting machine is more suitable for producing some thin-walled parts with high appearance requirements, such as mobile phones and PDA housings. However, the hot chamber die casting machine for magnesium alloy uses a punch to directly press magnesium alloy liquid into the metal cavity through a closed gooseneck and nozzle, and the pressurization pressure during injection is small, so it is generally not suitable for large, thick and heavy-duty parts such as automobiles and aerospace.

2) Cold chamber die casting machine. At each injection, magnesium liquid is injected into the injection sleeve by manual or automatic quantitative feeder, so the casting cycle is longer than that of hot chamber die casting. Cold chamber die casting machine is characterized by high injection pressure and high injection speed, so it can produce thin-walled or thick-walled parts and has a wide range of applications. Die casting machine can be enlarged, alloy type can be easily changed, and aluminum alloy can also be used; Consumables of die casting machine are cheaper than hot chamber die casting machine. In most cases, large, thick-walled, stressed and special requirements die castings are produced by cold chamber die casting machine.

When magnesium alloy is die-cast, due to the high injection speed, when magnesium liquid is filled into the cavity, the turbulence of metal liquid and gas entrainment will inevitably occur, resulting in hole defects in the interior and surface of the workpiece. Therefore, how to improve the yield of high-demand castings is one of the main problems faced by magnesium alloy die casting.

3.2 Semi-solid forming technology

Semi-solid forming of magnesium alloy is a forming technology developed in recent years, which can obtain high-density magnesium alloy products and is a competitive magnesium alloy forming method. Semi-solid forming mainly has the following methods.

Thixotropic casting

Thixocasting is to quantitatively cut and reheat the prepared non-dendritic bar to the liquid-solid two-phase zone (solid volume fraction is 50%-80%), and then semi-solid forming is carried out by die casting or die forging process. Thixotropic casting does not use melting equipment, and the ingot is easy to transport, heat after reheating and realize automation. However, the preparation of preform requires huge investment, and the key technology is monopolized by a few foreign companies, which leads to its high cost and is only suitable for manufacturing high-strength key parts.

Rheological casting

Rheological casting takes metal melt as raw material, which is cooled and stirred to make semi-solid alloy slurry, and then transported to die casting machine through pipeline or container for direct molding. For rheocasting, the industrial application of non-dendritic semi-solid alloy slurry is limited to a great extent because it is difficult to maintain, control and transport the slurry, which is slower than thixotropic casting. With the development of semi-solid casting technology, thixotropic casting has more and more obvious restrictions on the uniformity and cost of prefabricated materials, induction heating control and material consumption, reliability and repeatability of forming process, waste recycling and so on. Its economic benefits are not satisfactory. Therefore, the development of rheocasting has once again attracted people's attention, and both Hitachi and UBE have developed new rheocasting processes and equipment. In a word, rheocasting can not only produce high-quality parts at low cost, but also significantly shorten the production process compared with thixotropic casting, which is easier to connect with traditional die casting technology and reduce equipment investment. Obviously, rheological casting technology will have greater application potential.

Research progress of high performance cast magnesium alloys

4. 1 heat-resistant magnesium alloy

Poor heat resistance is one of the main reasons that hinder the wide application of magnesium alloys. When the temperature rises, its strength and creep resistance are greatly reduced, so it is difficult to be widely used as the material of key parts (such as engine parts) in automobile and other industries. Rare earth elements (re) and silicon (Si) are the main alloying elements used in the developed heat-resistant magnesium alloy. Rare earth is an important element to improve the heat resistance of magnesium alloys. Magnesium alloys QE22 and WE54 containing rare earth have the same high temperature strength as aluminum alloys, but the high cost of rare earth alloys is a big obstacle to their wide application.

Mg-Al-Si (As) alloy is a die-casting magnesium alloy developed by Volkswagen. At 175 cC, the creep strength of AS4 1 alloy is obviously higher than that of AZ9 1 and AM60 alloy. However, due to the formation of coarse "Chinese-style" Mg2Si phase in the solidification process of magnesium alloy, its casting performance and mechanical properties are damaged. It is found that the addition of trace Ca can improve the morphology of Chinese-shaped MgaSi phase, refine Mg2Si particles and improve the microstructure and properties of AS-based magnesium alloys.

4.2 Corrosion-resistant Magnesium Alloy

The corrosion resistance of magnesium alloys can be solved from two aspects:

1) Strictly limit the content of impurity elements such as Pe, Cu and Ni in magnesium alloys. For example, the corrosion resistance of high-purity AZ9 1HP magnesium alloy in salt spray test is about 0/00 times that of AZ9/Kloc-0, which exceeds that of die-casting aluminum alloy A380 and is far superior to that of low-carbon steel.

2) Surface treatment of magnesium alloy. According to different corrosion resistance requirements, chemical surface treatment, anodic oxidation treatment, organic coating, electroplating, electroless plating, thermal spraying and other methods can be selected. For example, the corrosion resistance of magnesium alloy after electroless plating exceeds that of stainless steel.

4.3 Flame retardant magnesium alloy

Magnesium alloys are prone to violent oxidative combustion during melting and casting. Practice has proved that flux protection and gas protection methods such as SF6, SO2, CO2 and Ar are effective flame retardant methods, but they will cause serious environmental pollution, reduce alloy properties and increase equipment investment in application.

Adding calcium to pure magnesium can greatly improve the anti-oxidation combustion ability of magnesium liquid, but adding a lot of calcium will seriously deteriorate the mechanical properties of magnesium alloy, making this method unable to be applied to production practice.

Recently, the National Engineering Research Center of Light Alloy Precision Forming of Shanghai Jiaotong University has developed a flame-retardant magnesium alloy with good flame retardancy and mechanical properties for automobiles by adding various elements at the same time, and successfully carried out industrial tests on the automobile gearbox cover, producing mobile phone casings, MP3 casings and other electronic product casings.

4.4 magnesium alloy with high strength and toughness

The normal temperature strength and plastic toughness of existing magnesium alloys need to be further improved. Adding Ca and Zr to Mg-Zn and Mg-Y alloys can significantly refine the grains and improve their tensile strength and yield strength. Adding Ag and Th can improve the mechanical properties of Mg-Re-Zr alloy. For example, QE22A alloy containing Ag has high tensile properties and creep resistance at room temperature, and has been widely used as high-quality castings for aircraft and missiles. By means of rapid solidification powder metallurgy, high extrusion ratio and equal channel angular extrusion (ECAE), magnesium alloy grains can be treated very finely, so as to obtain high strength, high plasticity and even superplasticity.

5 General situation of application of magnesium alloy cast in China

5. 1 Production is limited by technology and equipment.

At present, the output of primary magnesium in China ranks first in the world. In 2000t, the national output was about 200,000 t, more than 80% of which were exported as primary raw materials at low prices, and the domestic consumption was about 20,000 t ... Only 2,000 t was used for Santana gearbox housing, and the rest was used for general purposes such as alloy preparation. Because the technical equipment, development and application of magnesium alloys are relatively backward, the domestic magnesium industry presents serious structural contradictions. China nonferrous metal die casting has a considerable foundation. At present, there are about 3,000 die casting factories and related enterprises, and about 20 die casting machine manufacturers with an annual output of 300,000 tons. Among them, aluminum die castings account for 75.5%, and magnesium die castings only account for about 1%. Shanghai Gantong Auto Parts Co., Ltd. has been producing magnesium alloy die-casting gearbox housing for Shanghai Santana car for many years. But generally speaking, compared with developed countries, the comprehensive quality of die castings in China is poor (large machining allowance, high rejection rate, low alloy utilization rate, poor basic conditions of casting technology and equipment, serious environmental protection and energy consumption problems, lack of specialized personnel and new technology and product development capabilities). As a result, the product price rises and lacks competitiveness. It can be said that our existing foundation can not meet the requirements of magnesium alloy industrialization at all. Although magnesium alloy is a very suitable metal material for die casting, the production practice shows that magnesium alloy die casting needs a high technical level and experience accumulation. Generally speaking, the production technology level of magnesium alloy die casting is still very low. Compared with aluminum alloy die casting, magnesium alloy die casting has poor stability in quality and output, and high rejection rate, which leads to high price of magnesium alloy products, which restricts the popularization and application of magnesium alloy products and the development of new products. In the process of realizing magnesium alloy industrialization in China, we should pay full attention to the research of related basic application work and the cultivation of magnesium alloy professionals.

5.2 The government attaches great importance to this.

During the Ninth Five-Year Plan period, the Ministry of Science and Technology carried out research on the application of magnesium alloy materials in automobiles, the development of flame-retardant magnesium alloys and the development of high-quality sacrificial magnesium anodes. The previous research work also includes the research, management and operation mechanism innovation of magnesium alloy standards. In 2000, the Ministry of Science and Technology launched the preliminary strategic research of "Development, Application and Industrialization of Magnesium Alloys", and now this project has been listed as a major national "Tenth Five-Year Plan" project, and is organizing joint research; In the national "863" plan, the research contents of new materials and new processes for magnesium alloys are also arranged. The State Planning Commission also listed magnesium alloy industrialization as a high-tech industrialization demonstration project this year. Weapons and other military industrial groups have also started corresponding research and development plans.

5.3 International cooperation has become increasingly active.

In May, 20001,a group of five people from Taiwan Province Industrial Research Institute of China visited Chinese mainland, and reached a memorandum of cooperation on magnesium alloy application technology development in Chinese mainland, Hongkong and Taiwan Province Province. The Hong Kong Productivity Council has also sent people to Beijing many times to discuss cooperation in this project. Hong Kong Lijin Co., Ltd. cooperated with Tsinghua University to establish "Die Casting High-tech Research Center"; Cross-Strait and Hong Kong have set up magnesium alloy project coordination groups. Tsinghua University established the Sino-Russian International Cooperation Laboratory of "Light Metal Materials"; From June 5, 2000 to 10, domestic experts were organized to visit and investigate the industrial application projects of magnesium alloys in Europe. Relevant departments of China and the United States are also actively negotiating and communicating; Ningxia Huayuan also signed contracts with Japan Huayuan Company and Japan Metal Co., Ltd. to develop heat-resistant magnesium alloys.

5.4 The corporate attitude is very positive.

Automobile companies such as Shanghai Automobile (Group) Company, FAW Group, Dongfeng Company, Chery, Chang 'an and Jiangling are all using magnesium alloy parts. Chongqing Magnesium Technology Co., Ltd., a joint venture between Chongqing Longxin Group and Southwest Aluminum Company, has developed 10 kinds of magnesium alloy motorcycle parts. More than 300,000 magnesium alloy parts have been installed. The amount of magnesium used for bicycles is 5 kS, and the total weight loss is about 3kg.