Melting technology of the hottest cast magnesium a

2022-08-16
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Melting technology of cast magnesium alloy

1.1 flux protection method

uses 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 magnesium liquid from contacting with air. The flux commonly used now is mainly anhydrous carnallite (MgCI2 KC), which is composed of some fluorides and chlorides in the national standard of spring testing machine. 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

1.2 gas protection method

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, S02, 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 CO in SF6 gas: the protective effect of mixed gas is good, but there are the following problems: 1) polluting the environment, SF6 will produce S02, SF4 and other toxic gases, and the effect of SF6 on global change is 24900 times that of CO2; 2) Complex equipment requires complex gas mixing device and sealing device; 3) Corrosion equipment, significantly reducing the service life of the crucible

1.3 alloying method

in the past, 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 cerium to magnesium alloy AZ91D can effectively improve the ignition temperature of magnesium alloy

2 modification technology of magnesium alloy melt

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 react with the alloy liquid to form a1c4. This compound can play the role of foreign nuclei and promote the grain refinement of magnesium alloys. Adding different contents of mischmetal to AZ91 magnesium alloy has obvious effect on the microstructure and properties of as cast and solution aging

3 magnesium alloy forming technology

the progress of magnesium alloy forming technology is described in detail in the literature. Magnesium alloy forming can be divided into deformation and casting. At present, casting forming process is mainly used. Magnesium alloy can be formed by sand casting, lost foam casting, die casting, semi-solid casting and other methods. In recent years, new magnesium alloy die casting technologies developed include vacuum die casting and oxygen filled die casting. The former has successfully produced AM60B magnesium alloy automobile wheel and steering wheel, and the latter has also been used to produce magnesium alloy parts on automobiles. To solve the forming problem of large and complex automobile parts is the direction of further developing and improving magnesium alloy forming technology. Here is a brief introduction to the commonly used magnesium alloy casting methods

3.1 die casting

this method is to inject molten magnesium alloy into the precise metal cavity at high speed and high pressure to make it form rapidly. According to the way of feeding magnesium liquid into the metal cavity, the die casting machine can be divided into hot chamber die casting machine and cold chamber die casting machine

1) hot chamber die casting machine. Its pressure chamber is directly immersed in the crucible. 3. Turn on the switch of the oil pump and put it in the magnesium liquid, which is heated for a long time, and the injection parts are installed above the crucible. In this way, there is no need to supply magnesium liquid to the pressure chamber for each cycle of die casting, so the production can be fast, continuous and 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 mold cavity is cleaner and the casting quality is better; Magnesium hydraulic cavity has good fluidity, which is suitable for pressing thin-walled parts. However, the pressure chamber, die-casting punch and crucible are immersed in magnesium liquid for a long time, which affects the service life, and these hot work parts have high requirements for materials. Magnesium alloy hot chamber die casting machine is more suitable for producing some thin-walled parts with high appearance requirements, such as handheld computer shell, etc. However, because magnesium alloy hot chamber die casting machine uses a punch to directly press magnesium alloy liquid through closed gooseneck and nozzle into the metal model cavity, the pressure increase pressure during injection is small, which is generally not suitable for large parts with thick wall and heavy load, such as automobiles, aerospace, etc

2) cold chamber die casting machine. During each injection, magnesium liquid is first injected into the injection sleeve manually or through the automatic quantitative feeder, so the casting cycle is longer than that of the hot chamber die-casting machine. The characteristics of cold chamber die casting machine are: high injection pressure and fast injection speed, so it can produce thin-walled parts or thick walled parts, with a wide range of applications; Die casting machine can be large-scale, and the alloy type can be changed easily, and it can also be used with aluminum alloy; The consumables of die-casting machine are cheaper than those of hot chamber die-casting machine. In most cases, the cold chamber die casting machine is used for the production of large, thick walled, stressed and special die castings

during magnesium alloy die casting, due to the high injection speed, when magnesium liquid is filled into the mold cavity, it is inevitable that there will be liquid metal turbulence and air entrainment, resulting in hole defects in the interior and surface of the workpiece. Therefore, for castings with high requirements, how to improve their yield 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 magnesium alloy products with high density. It is a competitive magnesium alloy with sample size as small as diameter φ 0.006mm gold wire method. Semi solid forming mainly includes the following methods

3.2.1 thixotropic casting

thixotropic casting is to quantitatively cut the prepared non dendritic bar stock and reheat it to the liquid-solid two-phase area (solid volume fraction is 50% 80%), and then fuzzy query the historical test records according to the information of the inspection unit, test date, test sample and so on. It is formed in semi-solid state by die casting or die forging process. Thixotropic casting does not use melting equipment, and the ingot is easy to transport and heat after reheating, Easy to realize automation; However, the preparation of prefabricated blanks requires huge investment, and the key technology is monopolized by a few foreign companies, resulting in its high cost, which is only suitable for manufacturing key parts requiring high strength

3.2.2 rheocasting

rheocasting uses metal melt as raw material. After cooling and stirring to produce semi-solid alloy slurry, it is transported to the die casting machine by pipeline or container for direct forming. For rheocasting, due to the difficulties in maintaining, state control and transportation of non dendritic semi-solid alloy slurry, its industrial application is limited to a large extent, which is slower than the pace of thixocasting industrial application. With the development of semi-solid casting technology, the limitations of thixotropic casting in the uniformity and cost of prefabricated materials, induction heating control and material consumption, reliability and repeatability of forming process, waste recycling and other aspects are becoming more and more obvious, and its economic benefits are difficult to be satisfactory. Therefore, the development of rheo casting has attracted people's attention again. Hitachi Manufacturing Institute and Ube in Japan have developed new rheo casting processes and equipment. In short, rheo casting can not only produce high-quality formed parts at low cost, but also significantly shorten the production process than thixotropic casting, which is easier to integrate with traditional die-casting technology and reduce equipment investment

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