1 development of magnesium alloys
Magnesium alloy is the lightest metal structural material in practical application, but the research and development of magnesium alloy is not sufficient compared with aluminum alloy, and the application of magnesium alloy is still very limited. At present, the production of magnesium alloy is only 1% of the aluminum alloy. Magnesium alloy as the structure of the application of the maximum use is the casting, of which more than 90% are die castings.
The main problem of wide application of magnesium alloy is because magnesium is very lively, very easy to oxidation and burning of magnesium alloy in the process of smelting and processing, therefore, magnesium alloy production difficult; magnesium alloy production technology is not mature and perfect, especially the forming technology of magnesium alloys have poor corrosion resistance for further development; magnesium alloy; high temperature strength and creep properties of the existing industrial magnesium alloy low limit magnesium alloy at high temperature (150 ~ 350 DEG C) application; room temperature mechanical properties of magnesium alloy, especially the strength and toughness to be further enhanced; magnesium alloy series is relatively little, the research and development of magnesium alloy lag, can not adapt to different application requirements.
Magnesium alloy can be divided into casting magnesium alloy and wrought magnesium alloy. Magnesium alloy according to the different elements of the alloy is mainly Mg-Al-Zn-Mn series (Az), -Mn Mg-Al system (AM) and Mg-Al-Si-Mn system (As), Mg-Al-RE series (AE), n Mg-Zn-Zr (ZK), Mg-Zn-RE series (zE) and other alloys.
China has rich resources of magnesium, the primary magnesium production capacity, production and export are ranked first in the world. In the research and application of magnesium and magnesium alloys, the gap between China and Europe and the United States and other developed countries is still quite large "a hand, poor in our country on the quality of raw magnesium, magnesium alloy ingot quality is not satisfactory, lack of export competitiveness, as the application of structural material of magnesium in domestic consumption and rarely, only as the export of cheap raw materials, belonging to the typical resource exporting industry, at present, domestic magnesium metallurgy enterprise mostly in loss or face closure; on the other hand, the study and application of magnesium alloy are obviously weak. Therefore, how to make use of magnesium resource advantage of our country, the magnesium resources superiority transformation for the technical and economic advantages, to promote the development of the national economy and enhance the international competitiveness of the industry of magnesium derivatives in China, is an urgent task in front of us.2.New development of magnesium alloys
(1) heat resistant magnesium alloy
Poor heat resistance is one of the main reasons that hinder the application of magnesium alloy, its strength and creep resistance greatly declined when temperature increases, making it difficult as the key parts (such as engine parts) materials in automobile industry get more extensive application.
The main alloying elements used in heat resistant magnesium alloys are rare earth elements (RE) and silicon (Si). Rare earth is an important element to improve the heat resistance of magnesium alloy. QE22 and WE54 containing rare earth elements have high temperature strength, but the high cost of rare earth alloys is a major obstacle to their wide application.
Mg-Al-Si (AS) is a die-casting magnesium alloy developed by German Volkswagen. The creep strength of AS41 alloy was significantly higher than that of AZ91 and AM60 alloy at 175. However, the AS magnesium alloy can form a large Mg2Si phase in the solidification process, which can damage the casting properties and mechanical properties. The study found that the addition of trace Ca can improve the morphology of Chinese character Mg2si phase, refine the Mg2si particles, and grasp the structure and properties of AS series magnesium alloy.
Since 1980s, foreign countries are committed to using C to improve the tensile strength and creep properties of magnesium alloys. ZAC8506 (0.6Ca Mg-8Zn-5Al-) recently developed in the United States, as well as the Canadian study of Mg-5Al-0.8Ca and other magnesium alloy, its tensile strength and creep properties are better.
In 2001, Tohoku University, Japan, congenital waiting by rapid solidification method with the grain size of 100-200nm. High strength magnesium alloy Mg-2at% Y-1at% Zn and its strength is 3 times of super aluminum alloy, also with superplasticity, high heat resistance and high corrosion resistance.
(2) corrosion resistant magnesium alloy.
The corrosion resistance of magnesium alloys can be solved by two aspects: the content of Fe, Cu, Ni and other impurity elements in magnesium alloy is strictly limited. For example, the corrosion resistance of high purity AZ91HP magnesium alloy in the salt spray test is about 100 times that of AZ91C, which is better than that of die cast aluminum alloy A380, which is much better than that of the low carbon steel. Surface treatment of magnesium alloy. According to different corrosion resistance requirements, can choose the chemical surface treatment, anodic oxidation treatment, organic coating, electroplating, chemical plating, thermal spraying and other methods to deal with. For example, the corrosion resistance of the magnesium alloy is more than that of the stainless steel.
(3) flame retardant magnesium alloy.
In the process of smelting and casting, magnesium alloy is prone to violent oxidation. Practice has proved that the flux protection method and SF6, CO2, SO2, Ar gas protection method is an effective method of flame retardant, but they in the application will produce serious environmental pollution, and make the properties of the alloy are reduced, the equipment investment is increasing.
The addition of calcium in pure magnesium can greatly improve the oxidation resistance of magnesium liquid, but the mechanical properties of magnesium alloys can be severely deteriorated by adding a large amount of calcium, which can not be used in production practice. Hinge can prevent the magnesium alloy from further oxidation, but when the hinge content is too high, it will cause the coarsening of the grain and increase the tendency of hot cracking.
Recently, precision forming National Engineering Research Center of Shanghai Jiaotong University light alloy by also adds several elements developed a flame retardancy and mechanical properties were good car ignition proof magnesium alloy, successfully were car transmission cover the industrial test, and production of a mobile phone shell, MP3 shell of the electronic product shell.
(4) high strength and high toughness magnesium alloy.
The normal temperature strength and ductility of the existing magnesium alloys need to be further improved. In mg Zn and Mg-Y alloy adds the Ca, Zr can significantly refine the grain, improve the tensile strength and yield strength. The Canucks Ag and th can improve the mechanical properties of Mg re Zr alloy, such as those containing Ag QE22A alloy with high room temperature tensile properties and creep resistance properties, has been widely used as aircraft, missile quality castings; through rapid solidification powder metallurgy, high compression ratio and equal channel angular extrusion (ECAE) method, very fine grains of the magnesium alloy, to obtain high strength and high plasticity and superplasticity.
(5) wrought magnesium alloy.
Although the casting of magnesium alloy products amount than that of wrought magnesium alloy, but the wrought magnesium alloy can obtain higher strength, better ductility and more diverse mechanical properties can meet the requirements of the use of different occasions structure. Therefore, the development of deformation alloy, is the future development trend of more long-term.
The development of a new type of wrought magnesium alloy and its forming process has been highly valued by domestic and foreign material workers. The United States has successfully developed the magnesium alloy products of various series, such as by extrusion + after heat treatment of ZK60 high strength wrought magnesium alloy, its strength and fracture toughness of Al7075 alloy or Al7475 equivalent to the aging state, and by rapid solidification powder metallurgy (RS) + (PM) + hot extrusion process development of Mg- Al-Zn system EA55RS wrought magnesium alloy, be reported so far the best performance of magnesium alloy, its performance not only greatly exceeds the conventional magnesium alloy, strength even more than 7075 Aluminum Alloy, and superplasticity (300 C, 436%), and the corrosion rate of 2024-T6 Aluminum Alloy, also with SiCp reinforcement, and become a model of advanced magnesium alloy material. The deformation of magnesium alloy materials in Japan 1999 development of super-high strength IM Mg-Y, and magnesium alloy sheet cold pressing process. British developed Mg-Al-B extruded magnesium alloy for the Magnox nuclear reactor fuel tank. A deformed magnesium alloy with excellent mechanical properties and corrosion resistance has been developed by Israel, France and Russia.
(6) magnesium alloy forming technology.
Magnesium alloy is divided into two types of deformation and casting methods, the current main use of casting forming process. Die casting is the most widely used magnesium alloy forming method. In recent years, the development of magnesium alloy die casting technology of vacuum die casting and oxygenation Diecast, the former has successfully produced AM60B magnesium alloy automobile wheel Yi and the direction of plate, the latter also has begun for automobile production of magnesium alloy parts.
(Thixo-Molding) a new forming technology of magnesium alloy semi-solid thixo casting. In recent years by attention in the United States, Japan and Canada and other countries. Compared with the traditional die casting, thixotropic casting method without melting, pouring and gas protection, cleaner production process, safety and energy saving. Has developed magnesium alloy semi-solid thixo casting with die casting machine, by the end of 1998, around the world, there are more than 100 machines investment operation, about 40 kinds of standard of semi solid magnesium alloy products for the automotive, electronics and other consumer goods. But relatively speaking, the semi solid casting magnesium alloy material of the selectivity is small, at present the application of only AZ91D alloy, need to further develop suitable for semi-solid casting magnesium alloy system.
Magnesium alloy die casting, squeeze casting, low pressure die casting, squeeze casting, flow casting, vacuum casting, pressure casting, etc..