Published in： 2021-01-15 14:02:16, published via： cyanbat | clicks: 0 | Information Sources：
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Cast magnesium alloy has the following characteristics: large crystallization temperature interval, large volume shrinkage and linear shrinkage, small eutectic amount, specific heat capacity, latent heat of solidification, density and liquid head in the structure, low fluidity, and general tendency to crack and shrinkage Much larger than cast aluminum alloy.
Development historyedit
Casting magnesium alloy can be roughly divided into three stages:
(1) The first stage is a basic stage, mainly adding aluminum and zinc to magnesium, namely Mg-A1-Zn alloy. Such alloys can obtain tensile strength similar to that of cast aluminum alloys. ZM5 in my country, L121 in the United Kingdom, and AM80A in the United States belong to this type of alloy. The main additive element is aluminum, while the content of zinc is low, mainly because the increase in zinc content is prone to microporosity.
(2) The second stage is an improvement stage. Zirconium is added to magnesium. Common zirconium-containing alloys include Mg-Zn-Zr and Mg-RE-Zr. The main role of zirconium in magnesium alloys is to refine the grains of magnesium alloys, thereby improving the yield strength of magnesium alloys, and making magnesium alloys have good fatigue resistance and lower notch sensitivity. The disadvantage is still due to the wide crystal spacing, which is prone to microporosity and hot cracking. Therefore, the most widely used is Mg-Al, a die-cast magnesium alloy without zirconium. In addition, in order to improve the high-temperature creep resistance of magnesium alloys, magnesium alloys with rare earth elements as the main component have been produced.
(3) The third stage is the improvement stage. Adding silver to the magnesium alloy can enhance the aging strengthening effect after silver alloying, which greatly improves the high temperature strength and creep resistance of the magnesium alloy, but it will reduce the corrosion resistance of the alloy.
Application editor
1. Aerospace
As far as aviation materials are concerned, structural weight reduction and the integration of structural load-bearing and function are important directions for the development of aircraft body structural materials. Magnesium has been used in the aviation industry for a long time due to its low density and high specific strength. The economic benefits and performance improvements brought about by the weight reduction of aviation materials are very significant. The fuel cost savings brought about by the same weight reduction of commercial aircraft and automobiles, the former is nearly 100 times that of the latter. The fuel cost savings of fighter jets is nearly 10 times that of commercial aircraft. More importantly, the improvement of its maneuverability can greatly improve its combat effectiveness and survivability. Because of this, the aviation industry will take various measures to increase the amount of magnesium alloy.
2. Military field
Magnesium alloy has the characteristics of light weight, good specific strength and rigidity, good vibration reduction performance, and strong electromagnetic interference shielding ability, which can meet the requirements of military products for weight reduction, noise absorption, vibration reduction, and radiation protection.
1. Automotive industry
Magnesium alloy used as auto parts usually has the following advantages:
1) Improve comprehensive fuel economy standards and reduce exhaust emissions and fuel costs. According to estimates, 60% of the fuel used by cars is consumed by the car's own weight. For every 10% reduction in car weight, fuel consumption will be reduced by 8%-10%;
2) Weight reduction can increase the vehicle's loading capacity and effective load, while also improving braking and acceleration performance;
3) It can greatly improve the noise and vibration of the vehicle.
4. Motorcycle field
For more than 50 years, after continuous technological innovation, the application of magnesium alloys in motorcycles has been continuously expanded in breadth and depth. The application models have expanded from racing cars to sports motorcycles, lightweight motorcycles, and concept motorcycles, covering Europe, America and Japan. Several major motorcycle brands, magnesium alloy application parts covering more than 40 kinds of power systems, transmission systems and various motorcycle accessories, of which only the British Dymay wheels are used in as many as 400 models. The application of magnesium alloy in domestic motorcycles is still blank. Chongqing Loncin took the lead in trial-manufacturing the "magnesium alloy green concept motorcycle" model LXl50, which has attracted widespread attention in China. Now 3 of the 12 components used are Realized large-scale production.
5. 3C field
3C products-Computer, Communication, Consumer Electronic Product (computer, communication, consumer electronic products) are the fastest growing industries in the world today. Digital technology has led to the continuous emergence of various digital products. Magnesium alloy 3C products first appeared in Japan. In 1998, Japanese manufacturers began to use magnesium alloy materials in various portable products (such as PDAs, mobile phones, etc.). Nowadays, the most common 3C products using magnesium alloy are notebook computers, which are also manufactured by Japan. Sony was the first to introduce it. Driven by the development trend of 3C products in the direction of light, thin, short and small, the application of magnesium alloys has continued to grow in recent years. [2]
Smelting technologyedit
1. Flame retardant technology of casting magnesium alloy liquid
1.1 Flux Protection Law
The low melting point compound is used to melt into a liquid state at a lower temperature and spread on the magnesium alloy liquid surface to prevent the magnesium liquid from contacting the air, thereby playing a protective role. The flux commonly used now is mainly composed of anhydrous carnallite (MgCI2-KC), with some fluoride and chloride added. The agent is more convenient to use, low production cost, good protection and use effect, and is suitable for the production characteristics of small and medium-sized enterprises. However, the agent must be dehydrated before use, and it will release a choking odor when used. Due to the high density of the flux, it will gradually sink, so it needs to be added continuously. A large amount 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 covering, refining effect and pollution-free.
1.2 Gas protection law
The gas protection method is to cover the surface of the magnesium alloy liquid with a layer of inert gas or a gas that can react with magnesium to form a dense oxide film 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 more expensive SF6 gas, foreign countries generally mix SF6 gas with air or other dry gases such as CO: The mixed gas has a good protection effect, but there are the following problems:
1) Polluting the environment, SF6 will produce toxic gases such as S02 and SF4. The effect of SF6 on global change is 24900 times that of CO2;
2) The equipment is complex, requiring complicated gas mixing devices and sealing devices;
3) Corrosion equipment significantly reduces the service life of the crucible.
1.3 Alloying method
In the past, people used the addition of beryllium to magnesium alloys to improve the flame-retardant properties of magnesium alloys, but beryllium is more toxic, and excessive addition of beryllium will cause coarsening of crystal grains and increase the tendency of thermal cracking, so the addition is limited. Japanese scholars believe that adding a certain amount of calcium can significantly increase the ignition point temperature of magnesium alloys, but there is an excessive amount of addition that seriously deteriorates the mechanical properties of magnesium alloys. Adding calcium and zirconium at the same time has a flame retardant effect. Domestic research believes that adding rare earth cerium to magnesium alloy AZ91D can effectively increase the ignition temperature of magnesium alloy.
2. Modification technology of magnesium alloy melt
The purpose of smelting and metamorphism of magnesium alloy is to change the structure of magnesium alloy. This process has a greater impact on the grain size and mechanical properties of the alloy, and also has a certain impact on the oxide inclusions in the magnesium liquid. Studies have shown that for magnesium alloys that do not contain Al, the modification of zirconium has a good grain refinement effect. The principle of action is that Zr undergoes peritectic reaction to promote grain refinement. After adding suitable carbon materials to the Mg-Al alloy, it chemically reacts with the alloy liquid to form A1C4. This compound can act as a foreign crystal nucleus and promote the grain refinement of the magnesium alloy. On the basis of AZ91 magnesium alloy, adding different contents of mixed rare earths has obvious effects on the structure and properties of the as-cast and solid-solution-aged.
3. Magnesium alloy forming technology
Magnesium alloy forming is divided into two methods: deformation and casting. Currently, casting forming process is mainly used. Magnesium alloys can be formed by sand casting, lost foam casting, die casting, semi-solid casting and other methods. New magnesium alloy die casting technologies developed in recent years include vacuum die casting and oxygenated die casting. The former has successfully produced AM60B magnesium alloy automobile wheels and steering wheels. , The latter has also begun to be used in the production of magnesium alloy parts for automobiles. Solving the forming problem of large and complex shape parts of automobiles is the direction of 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
The method is to inject molten magnesium alloy liquid into a precise metal cavity at high speed and high pressure to make it rapidly shaped. According to the method of sending magnesium liquid into the metal cavity, the die casting machine can be divided into two types: hot chamber die casting machine and 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 in a heated state for a long time, and the injection part is installed above the crucible. In this way, there is no need to supply magnesium liquid to the pressure chamber specially every time die-casting cycle, so the production can be fast and continuous, and it is 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 better; the magnesium hydraulic pressure is good in fluidity and suitable for pressing Thin-walled pieces. However, the pressure chamber, die-casting punch and crucible are immersed in the magnesium liquid for a long time, which affects the service life, and requires higher materials for these hot work pieces. Magnesium alloy hot chamber die-casting machine is more suitable for the production of thin-walled parts with higher appearance requirements, such as mobile phones and palmtop computer shells, but because magnesium alloy hot chamber die-casting machines use punches to directly pass the magnesium alloy liquid through the closed gooseneck And the nozzle presses into the metal mold cavity, so the boost pressure is small during injection, and it is generally not suitable for large, thick, heavy load parts such as automobiles, aerospace and aviation.
2) Cold chamber die casting machine. In each injection, the magnesium liquid is injected into the injection sleeve manually or through an automatic quantitative feeder, so the casting cycle is longer than that of the hot chamber die casting machine. The characteristics of the 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; the die-casting machine can be large-scaled, and the alloy type can be easily replaced. Used with aluminum alloy; consumables for die casting machine are cheaper than hot chamber die casting machine. In most cases, cold-chamber die-casting machines are used to produce large-scale, thick-walled, stressed and special requirements.
When magnesium alloy die-casting, due to the high injection speed, when the magnesium liquid is filled into the mold cavity, turbulence and entrainment of the molten metal will inevitably occur, resulting in hole defects inside and on the surface of the workpiece. Therefore, for castings with high requirements , How to improve its 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. It can obtain high-density magnesium alloy products and is a competitive magnesium alloy forming method. There are mainly the following methods for semi-solid forming.
3.2.1 Thixocasting
Thixocasting is the method of quantitatively cutting the prepared bar with non-dendritic structure and reheating it to the liquid-solid two-phase zone (the volume fraction of the solid phase is 50%-80%), and then semi-solid forming by die casting or die forging process. Thixotropic casting does not use melting equipment, the ingot is easy to transport and heat after reheating, and it is easy to realize automation; however, the preparation of prefabricated billets requires huge investment, and the key technology is monopolized by a few foreign companies, resulting in high costs , Only suitable for manufacturing key parts requiring high strength.
3.2.2 Rheological casting
Rheological casting uses metal melt as raw material. After cooling and stirring to produce a semi-solid alloy slurry, it is transported to the die-casting machine in a pipeline or container for direct forming. For rheological casting, the non-dendritic semi-solid alloy slurry is kept in Difficulties in control and transportation have largely limited its industrial application, which is slower than the industrial application of thixotropic casting. With the development of semi-solid casting technology, thixotropic casting has more and more obvious limitations in terms of uniformity and cost of precast materials, induction heating control and material consumption, reliability and repeatability of the forming process, and waste recycling. Its economic benefits are very obvious. It is difficult to satisfy everyone, so the development of rheological casting has once again attracted people's attention. Japan's Hitachi and UBE have developed new rheological casting processes and equipment. In short, rheological casting can not only produce high-quality formed parts at low cost, but also the production process will be significantly shorter than thixotropic casting, easier to integrate with traditional die casting technology, and reduce equipment investment. Obviously, rheological casting technology will have greater application potential.