One, magnesium alloy casting molding technology
1.1 die casting magnesium alloy
Since the introduction of high strength MgAl9Znl in 1927, the industrial applications of magnesium alloys have gained substantial progress. 1936 car began to use die-casting magnesium alloy production "beetle" car engine transmission system parts, to 1980 magnesium alloy die-casting parts with up to 380 thousand tons. German government has drawn up a investment 25 million reichsmarks magnesium alloy research and development plan, mainly study pressure alloy technology, rapid prototyping and manufacturing technology and semi - solid forming process, to improve the ability of Germany in the application of magnesium alloy. Magnesium alloy has good filling capacity, and the solidification rate is high, and it can be used for die casting thin wall parts without thermal cracking and less casting. Magnesium alloy to reduce the die casting of thermal fatigue phenomenon, prolong the life of die casting type, shorten the retention time of die casting, die casting production rate is about 75% faster than aluminum alloy. Magnesium alloy and the iron did not react, no corrosion to steel crucible and casting type, both to avoid the crucible for magnesium alloy liquid pollution, and prolongs the life of crucible steel and die casting type, makes not only as aluminum alloy can be used for cold chamber die casting machine, die casting magnesium alloy, and can use efficiency higher hot chamber die casting machine casting. The shrinkage rate of the magnesium alloy is uniform and uniform, and the stripping force is lower than that of the aluminum alloy 20%-30%. Therefore, the magnesium alloy die casting parts not only have high dimensional accuracy, but also can not clean the aluminum alloy die casting parts by 60%. Die casting magnesium alloy in general to Al as the main alloying elements, commonly used in 3 Series: AZ (Mg-Al-Zn-Mn), AM (Mg-Al-Mn), AS (Mg-Al-Si-Mn) system. Influence the development of magnesium alloy die casting is the main problem alloy liquid oxidation combustion, which to the melting bring a series of difficulties, and cause the pollution to the environment and alloy and smelting equipment erosion and die casting production cost increase. At present, some researchers are engaged in the study of the properties of magnesium alloys which are close to the existing magnesium alloys.
1.2 magnesium alloy semi solid casting
The development of the Chemical DOW company in the United States is a semi solid fusion technology, which combines the characteristics of plastic injection molding and die casting process. With this process, the internal and surface quality of die casting is improved obviously, the dimension precision, mechanical property and corrosion resistance are improved, and the furnace is not needed. But the forming pressure is high, about 3 times of the hot chamber die casting, and the magnesium alloy need to be prefabricated chip, so that the production cost increased by 10%, and its equipment investment is more expensive.
1.3 squeeze casting
The development trend of casting technology of magnesium alloy extrusion is combined with other casting technology. Japan's Toshiba Machine Co LEOMACS system is studied, which is characterized by the squeeze casting and low pressure casting and electromagnetic pump quantitative pouring knot, uneven solidification, can not only reduce the casting together, has the following characteristics:
(1) from the feed to shorten the residence time of liquid metal extrusion and shortens the cycle of casting production;
(2) isolated from the metal liquid and air in the production process, and effectively prevent the oxidation of magnesium alloy liquid combustion;
(3) magnesium alloy liquid insulation at low temperature and pouring. So the equipment can be thin-walled magnesium alloy castings with complex shape. But the equipment investment is very high, application limits its extensive industrialization.
The casting process can be in the same unit of work in the melt mixing, injection and extrusion pressure applied. Installed in the pressure injection chamber of the periphery of the electromagnetic stirring apparatus for stirring and cooling process of melt when reaches a certain solid fraction required mixing time, the plunger will slurry pressure into the cavity and thermal insulation. This process makes full use of the two types of casting strengths, overcome the limitation of rheological casting, and can improve the life of mold.
1.4 other magnesium alloy casting forming method
Low pressure casting and differential pressure casting have been used in the production of magnesium alloy parts. The two casting methods used in the magnesium alloy are characterized by the effective combination of the pressure system and the gas protection in the casting process. Lost foam casting is one of the most advanced casting technology in the world:
(1) the dry sand, to avoid the ordinary sand casting due to moisture induced magnesium alloy combustion problems, and EPC gasified to form a reducing atmosphere, can inhibit the oxidation and burning of magnesium alloy;
(2) magnesium alloy shrinkage rate is 1.2 times Aluminum Alloy, hot crack, dry sand collapsibility is good, can effectively solve the problem.
Magnesium alloy lost foam casting is a new technology combined with advanced materials and advanced technology, which will show great superiority in the production of complex parts for the production of automobiles. At present, the research and application of magnesium alloy lost foam casting technology has just started, and needs to be further developed.
Two, magnesium alloy smelting technology
Flame retardant technology of 2.1 alloy liquid
The flux protection method using low melting point compounds in low temperature molten into liquid and spread on the surface of magnesium alloy, due to prevent the magnesium liquid and air contact so as to protect. Now commonly used flux by anhydrous carnallite (MgCl2-KC), add some fluoride and chloride. The agent has the characteristics of convenient use, low production cost, good protection and good use effect, and is suitable for the production of small and medium enterprises. However, before the use of the agent to re new dehydration, the use will release the smell of choking. The flux density is the gradually sinking, constantly add. The use of the release of a large number of harmful gases, pollute the environment, corrosion plant serious. Therefore, study on the new type of coverage, the refining effect is good and pollution-free magnesium alloy flux is 1 important projects.
In developed countries, it is usually used in the process of smelting magnesium alloy with the gas protection method, and has achieved good results. Gas protecting method is in the surface of the magnesium alloy liquid covered a layer of inert gas or gas reacted with magnesium to form a compact oxide film, so as to isolate the air of oxygen, the main protective gas SF6, SO2, CO2, AR and N2. In order to further improve the protection and reduce the amount of SF6 gas more expensive, foreign general in the SF6 gas mixture of air or other dry gases such as CO2. mixed gas protection effect is good, but there are the following problems:
(1) pollute the environment, SF6 will produce SO2, SF4 and other toxic gases, SF62 on the global change is 24900 times the role of CO2;
(2) the equipment is complex, requires complex gas mixing device and sealing device, (3) corrosion equipment, significantly reduce the life of the crucible.
The other way is to improve the fire resistance of magnesium alloy. Long ago, people used in magnesium alloy added beryllium element to improve the flame retardant properties of magnesium alloy, but beryllium toxicity, and adding amount of high caused by grain coarsening and increase the hot cracking tendency, so by adding amount of restrictions. Japanese scholars believe that adding a certain amount of calcium can significantly improve the ignition temperature of magnesium alloy, but there is a excessive dosage, and serious evil of the mechanical properties of magnesium alloy. The addition of calcium and zirconium has the effect of flame retardant. It is believed that magnesium alloys with high content of rare earth and high content of beryllium in magnesium alloys can be obtained directly in the atmosphere, and have good casting properties and mechanical properties. We have found an additional 1 elements that play a role in magnesium alloys. Test results show that adding trace amounts of the element, and through and other elements combined effect can significantly improve the ignition temperature of the liquid magnesium. The element also has the advantage of grain refinement and strengthening, and improves the mechanical properties of magnesium alloy.
2.2 melt treatment technology
The purpose of magnesium alloy melting and modification is to change the microstructure of the magnesium alloy, which has a great influence on the grain size and mechanical properties of the alloy, and also has a certain effect on the oxidation of the magnesium alloy. The results show that, for the magnesium alloy without Al, the zirconium modification has good effect on grain refinement, which is the effect of Zr on the occurrence of the package crystal reaction and the promotion of grain refinement. After adding suitable carbon materials in mg Al alloy and the chemical reaction AlC4 and alloy liquid, the compounds can play to the role of exotic nuclei, prompting the grain refinement of magnesium alloy, the CcCl6 is one of technical measures. Recently, Japan by argon mixed with process of high pure carbon powder is added in the carbon, obtained better modification effect, and the effects of adding temperature, the impact of treatment time and other factors on the modification effect. Our experimental study shows that under the condition of Mg, Al and other alloying elements, the high temperature treatment of industrial magnesium alloy has obvious grain refining effect. The high temperature magnesium alloy melt can be reduced to low temperature for casting, the casting can still retain the effect of grain refinement, and show that the microstructure of magnesium alloy is hereditary. In addition, we also showed that the treatment of magnesium alloy with carbonate has a good effect.
Three. Research and development of new type high performance magnesium alloy
In order to improve the heat resistant properties of magnesium alloy, domestic and foreign researchers have developed a series containing rare earth magnesium alloy, the development of the high temperature magnesium alloy containing Zr, mg Al Si alloy, mg Zn Cu based alloy etc., the effects of the particle reinforced and graphite fiber reinforced magnesium matrix composites. Another application of magnesium alloy is to prepare amorphous alloy material. The research shows that the Mg based alloys have strong amorphous forming ability. By means of rapid solidification can be made with high strength and toughness of amorphous materials, at present, the system has a 9mm diameter rod like magnesium amorphous. Mg based alloys are one of the future development directions of bulk amorphous alloys. A series of advantages, the other reported that magnesium alloys as hydrogen storage materials with strong hydrogen storage capacity, raw materials cheap, magnesium can store 7.7% hydrogen, using the mechanical alloying method has produced practical magnesium based hydrogen storage materials, at present, is a laboratory for further test experiments. We adopt method of rapid solidification magnesium based amorphous alloys is studied, that Mg Ni RE alloy, mg Al CA alloy with stronger amorphous forming ability and non crystal formation is related to many factors, especially the alloy composition of amorphous forming ability has an important influence, the amorphous formation also depends on the viscosity of the melt is associated. Performance test showed that the increase of magnesium based amorphous materials corrosion resistance, toughness and hardness. With the continuous improvement of the service life of electrical equipment and pipelines, it is of great practical significance to study more effective anode sacrificial material. The most commonly used magnesium anode materials for AZ63A (Mg-Al-Zn) alloy, has been studied and developed a manganese magnesium alloy anode material, which has a higher electrical and current efficiency, and more widely used in foreign countries. We are studying the higher current efficiency of magnesium based corrosion control materials.
Four, prospects
The future of the world, magnesium alloy castings will be a large number of applications that can not be questioned. The abundant magnesium resources in our country provide the material foundation for the development of magnesium alloy. To name just a case that the application prospect of magnesium alloy: experts predict, in 2005 car fuel consumption generally to drop to 3L/100Km, carbon dioxide emissions reduced to 120mg/Kg. When the car to reduce the weight of 100k, 100km per driving, can reduce fuel consumption 0.4ml, so the car uses more magnesium alloy castings are taken for granted. Expected in 2030 the world will have 1 billion cars, the amount of magnesium alloy castings can be imagined. The research and application of magnesium alloys in China has just started, and there is a great gap between China and foreign countries. It is believed that magnesium alloys will be widely used in the automotive industry and other products as the most promising metal materials. The understanding to the international development trend and China's research and application status, we should make a difference, the trend of the era catch, catch up with and surpass the advanced world level, to meet the arrival of the magnesium alloy application climax do theoretical and technical preparation, contribute to the development of magnesium alloy materials science.