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1.2 Gas protection method The gas protection method is to cover the surface of the magnesium alloy liquid with an inert gas or a gas which can react with magnesium to form a dense oxide film, thereby isolating the oxygen in the air. The main protective gas used is SF6, S02, CO2. Ar, N2, etc. In order to further improve the protection and reduce the amount of expensive SF6 gas, foreign countries generally mix air or Other dry gases such as CO: mixed gas in SF6 gas, but the following problems exist: 1) polluting environment, SF6 will produce S02 SF4 and other toxic gases, SF6 has a global effect of 24,900 times that of CO2; 2) complex equipment, requiring complex gas mixing devices and sealing devices; 3) corrosion equipment, significantly reducing the service life of cesium.
1.3 Alloying method In the past, people added magnesium in magnesium alloy to improve the flame retardancy of magnesium alloy, but the toxicity of bismuth is large, and the excessive addition will cause grain coarsening and increase the tendency of hot cracking, so it is added. The limit of quantity. Japanese scholars believe that the addition of a certain amount of calcium can significantly increase the ignition temperature of magnesium alloys, but the addition amount is too high, and the mechanical properties of magnesium alloys are seriously deteriorated. The addition of calcium and zirconium simultaneously has a flame retardant effect. Domestic research believes that the addition of rare earth lanthanum to magnesium alloy AZ91D can effectively improve the light-off temperature of magnesium alloy.
2 Magnesium alloy melt modification treatment The purpose of magnesium alloy smelting metamorphism is to change the microstructure of magnesium alloy. The process has a great influence on the grain size and mechanical properties of the alloy, and the oxidation inclusions in the magnesium solution are also Certainly affected. Studies have shown that for magnesium alloys without Al, the use of zirconium for metamorphism has a good grain refining effect. The principle of action is that Zr undergoes peritectic reaction and promotes grain refinement. After adding a suitable carbon material to the Mg-Al alloy, it is chemically reacted with the alloy liquid to form A1C4, which can act as an external nucleus to promote grain refinement of the magnesium alloy. Adding different content of mixed rare earth on the basis of AZ91 magnesium alloy has obvious effects on the microstructure and properties of as-cast and solid solution aging.
3 Magnesium alloy forming technology literature has elaborated on the progress of magnesium alloy forming technology. Magnesium alloy forming is divided into two methods of deformation and casting, and the casting forming process is currently mainly used. Magnesium alloys can be formed by sand casting, lost foam casting, die casting, semi-solid casting, etc. The new technologies of magnesium alloy die casting developed in recent years include vacuum die casting and oxygenation die casting. The former has successfully produced AM60B magnesium alloy automobile wheel and steering wheel. The latter has also been used to produce magnesium alloy parts on automobiles. Solving the problem of forming large and complex shaped parts of automobiles is the current direction for further development and improvement of magnesium alloy forming processing technology. Here is a brief introduction to the commonly used magnesium alloy casting methods.
3.1 Die Casting This method is to inject a molten magnesium alloy liquid into a precision metal cavity with high speed and high pressure to form it quickly. According to the way of feeding the magnesium liquid into the metal cavity, the die casting machine can be divided into a hot chamber die casting machine and a cold chamber die casting machine.
1) Hot chamber die casting machine. The pressure chamber is directly immersed in the magnesium liquid in the crucible, and is heated for a long time, and the injection member is mounted above the crucible. When the die casting is performed once per cycle, it is not necessary to intentionally supply the molten metal to the pressure chamber, so the production can be fast, continuous, and easy to realize automation. The advantages of the hot chamber die casting machine are simple production process and high efficiency; low metal consumption and stable process; the magnesium liquid pressed into the cavity is clean and the casting quality is good; Thin-walled parts. However, the pressure chamber, the die-casting punch and the crucible are immersed in the magnesium liquid for a long time, which affects the service life and requires high requirements for these hot work materials. 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 palm-sized computer casings, but because magnesium alloy hot chamber die-casting machine uses a punch to directly pass the magnesium alloy liquid through the closed gooseneck. The nozzle and the metal mold cavity are pressed, so that the supercharging pressure is small when the injection is performed, and generally it is not suitable for large-sized, thick-walled, and heavy-loaded parts such as automobiles and aerospace.
2) Cold chamber die casting machine. At each shot, the magnesium solution is injected into the injection sleeve either manually or through an automatic dosing machine, so that the casting cycle is longer than that of the hot chamber die casting machine. The cold chamber die casting machine is characterized by high injection pressure and high injection speed, so it can produce thin-walled parts or thick-walled parts with wide adaptability; the die-casting machine can be enlarged, and the alloy type can be easily replaced. Used in combination with aluminum alloys; consumables for die casting machines are less expensive than hot chamber die casting machines. In most cases, large-scale, thick-walled, stressed, and specially-designed die-casting parts are produced using a cold chamber die casting machine.
During the die casting of magnesium alloy, due to the high injection speed, when the magnesium liquid is filled into the mold cavity, there will inevitably be a turbulent flow of metal liquid and a gas-filling phenomenon, which will cause void defects in the interior and the surface of the workpiece, so that castings with high requirements are required. How to improve the yield is one of the main problems faced by magnesium alloy die casting.
3.2 Semi-solid forming technology Magnesium alloy semi-solid forming is a forming technology developed in recent years, which can obtain high-density magnesium alloy products and is a competitive magnesium alloy forming method. There are several methods for semi-solid forming.
3.2.1 Thixotropic casting Thixoforming is the quantitative cutting of the prepared non-dendritic bar material and reheating to the liquid-solid two-phase zone (solid phase volume fraction of 50%-80%), and then using die casting or die Forging process semi-solid forming, thixotropic casting does not use melting equipment, ingot reheating is convenient for transportation and heating, easy to automate; however, the preparation of preforms requires huge investment, and the key technology is monopolized by a few foreign companies. This results in high costs and is only suitable for the manufacture of critical parts requiring high strength.
3.2.2 Rheology Casting Rheology casting uses metal melt as raw material, and after cooling and stirring to produce semi-solid alloy slurry, it is directly formed into a die-casting machine by pipeline or container. For rheocasting, due to non-dendritic semi-solid alloy Slurry has difficulties in holding, state control and transportation, which limits its industrial application to a large extent, and is slower than the application of the thixotropic casting industry. With the development of semi-solid casting technology, the limitations of thixo casting in prefabricated material uniformity and cost, induction heating control and material consumption, reliability and repeatability of forming process, waste recycling, etc. are becoming more and more obvious, and its economic benefits are very Difficult to do so, so the development of rheology casting has once again received people's attention, Japan's Hitachi and UBE have developed a new rheological casting process and equipment. In short, rheocasting not only can produce high-quality formed parts at low cost, but also the production process will be significantly shorter than thixotropic casting, and it is easier to integrate with traditional die-casting technology and reduce equipment investment.
1 Flame Retardant Technology of Cast Magnesium Alloy Liquid 1.1 The flux protection method uses a compound with a low melting point to melt into a liquid state at a lower temperature, and spreads on the surface of the magnesium alloy to protect the magnesium liquid from contact with air. The flux currently used is mainly composed of anhydrous carnallite (MgCI2-KC), which is composed of some fluoride and chloride. The agent is convenient to use, has low production cost, good protection effect, and is suitable for the production characteristics of small and medium-sized enterprises. However, the agent should be re-dehydrated before use, and it will release a scent when used. Since the density of the flux will gradually sink, it needs to be added continuously. A large amount of harmful gases are released during use, which pollutes the environment and corrodes the plant. Therefore, it is an important subject to study a new type of magnesium alloy flux with good coverage and refining effect and no pollution.
February 15, 2019