Magnesium alloy die casting technology must pay attention to several issues

1, the magnesium alloy die-casting is a set of design, manufacturing and research in one of the systems engineering, workers and magnesium alloy die-casting of zinc, aluminum die-casting of staff compared to should have a more comprehensive knowledge, experience and research and development capacity.


Mainly automotive die-casting magnesium alloy products, the use of magnesium die-casting to reduce vehicle mass, fuel consumption and reduced emissions reduced; and magnesium alloy die casting and casting with the NVH performance of high precision, good overall economic efficiency , is the most promising lightweight car key material, and has broad application prospects.


China is a country extremely rich magnesium resources, but our country is currently in the magnesium alloy die-casting technology and applications in a backward position, magnesium resources are mostly in the form of primary products exported, the export production of magnesium is much larger than domestic consumption. With the domestic auto industry and the computer, information, communications, instrumentation, aerospace and other industries, the rapid development of China's magnesium alloy die-casting industry is also bound to the multitude of strong world market place.


2, die-cast magnesium alloy


The density of magnesium is less than 2G/CM3, is the lightest metal structure material, its specific strength than aluminum and steel, slightly lower than the highest strength fiber-reinforced plastic; its stiffness and aluminum and steel equivalent much higher than the fiber-reinforced plastic; its much better corrosion resistance than mild steel, more than die-casting aluminum A380; its vibration damping, magnetic shielding is much better than aluminum; view of the dynamic viscosity of low-magnesium alloy , the same fluid state (Reynolds index equal to) the filling speed is much greater than under the aluminum, combined with magnesium alloy melting point, heat capacity and latent heat than aluminum is low, so the melting energy consumption, fast solidification, magnesium alloy actual aluminum die casting cycle shorter than 50%. In addition, the affinity of magnesium alloy and iron small, low capacity iron solution, which is not easy adhesion surface of the mold, they use aluminum molds longer than 2 to 3 times higher.


Most commonly used in the United States die-cast magnesium alloy grades AZ91, AM60, AM50, AM20, AS41 and AE42, respectively, belong to MG-AL-ZN, MG-AL-MN, MG-AL-SI and MG-AL-RE four series . And casting of magnesium alloy, the present mainly in the following research:


(1) high temperature performance: at present two series of AZ and AM magnesium alloy die-casting magnesium alloy automotive accounted for 90% of die castings, magnesium alloy of these two series above 150 intensity decreased significantly. Has been developed creep resistance above 150 the AS series magnesium alloy die-casting, such as AS41A alloy (MG43% AL1% SI0.35% MN), its 175 creep strength than AZ91D and AM60B, and a higher elongation, yield strength and tensile strength. Volkswagen BEETLE engine crankcase previously been used AS41 and AS42, recently introduced an improved alloy AE42 creep properties at high temperatures even better. Some trace elements such as rare earth elements Y, ND, SR and so on, die-casting magnesium alloy has a significant role in grain refinement can improve the strength of magnesium alloy die-casting and creep resistance, such as the recent development of AE42 anti-creep change capacity than traditional MGALSI alloy, in the long-term use of 200 ~ 250 . However, AS and AE alloys for high temperature performance improvement is still limited, and its casting alloy performance than AZ and AE worse, coupled with the high cost of rare earth elements, the production and application subject to certain restrictions.


(2) Scalability: Currently, magnesium die castings and high in the need for safety purposes on the fracture toughness of very rapid growth. In the case of work to improve the ability to absorb energy, it should improve the fracture toughness. By reducing the aluminum alloy, can be done. AM60 and AM50 bracket in dashboard, seats, steering wheel shaft and safety components are widely used, AM20 is still applied to the seat back frame. In addition, elongation at break is also very close relationship with temperature, especially above at about 50 , with temperature increases.


(3) The corrosion resistance of magnesium alloys: corrosion resistance of magnesium alloys is also a major obstacle to expanding the application. High chemical activity of magnesium to magnesium-based alloy and composite materials prone to micro-cell corrosion, generally low-grade die-cast magnesium alloy corrosion resistance is poor. Strict requirements of the FE, NI, CU and other impurities in high purity magnesium die-casting alloys (such as AZ91D), and with rare earth AE42, the salt spray test the corrosion resistance of more than die-casting aluminum A380, far better than the low-carbon of steel. Adjusting the chemical composition, surface treatment and control of microstructure can improve its corrosion resistance, etc.. Despite the increase in corrosion resistance of magnesium alloy parts many methods, but if not from the material itself solve the problem, poor corrosion resistance of magnesium alloy parts is always a large number of applications received technical obstacles.


(4) flame-retardant magnesium alloy: add in the magnesium alloy AL (2.5%), BE alloy (BE adding the amount of 0.0005% to 0.03%) or CA-containing alloys can also be effective in preventing the oxidation of magnesium alloy liquid. Currently, some researchers are working with a flame retardant magnesium alloy, once the study is successful, the same magnesium alloy as aluminum smelting and casting, to obtain a wider range of applications.


(5) magnesium alloy based composite materials: silicon carbide particle reinforced magnesium alloy matrix composites have many years of research and development, at present, although not yet reached commercial application in the field of die casting stage, but have to use sand casting, precision casting made of the impeller and other methods, bicycle crank, auto cylinder castings, and have this kind of composite material with a combination of semi-solid casting, die casting and squeeze casting used in the field of trends.


3 ways magnesium alloy die-casting


Magnesium alloy can be cold chamber or hot chamber die casting machine casting. The current hot chamber die casting machine improvements include: the use of pressurized accumulator, injection plunger injection rate of up to 6M / S; induction heating gooseneck and nozzle, so that it remains the most suitable temperature; dual melting furnace insulation, and recycling of used equipment and pipe insulation, to keep accurate bath temperature. When an ordinary cold chamber die-casting magnesium alloy die-casting machine, the machine must die injection systems and automatic feeding system to make the necessary transformation to apply to the requirements of magnesium alloy die-casting. Transformation include: (1) the injection system of the fast speed of the die-casting aluminum injection when 4 ~ 5M / S to 6 ~ 10M / S; (2) shorten the time to build pressure pressurization process; ( 3) increase the injection power; (4) the use of electromagnetic automatic dosing devices, to prevent the magnesium oxide alloy in the casting process; (5) If the use of vacuum casting and other special casting process, the configuration of the necessary supporting equipment.


And other die-casting alloys, the traditional magnesium alloy die-casting technology enables high-speed fluid turbulence and dispersion state of die cavity filling, or dissolved gases under high pressure within the die casting alloy, or the formation of many dispersed in the die casting the high-pressure micro-pores. Therefore, the traditional method of production of magnesium alloy die-casting can not be heat strengthened, it can not be used at higher temperatures. To eliminate this defect, to improve the intrinsic quality die casting, die casting applications to expand the scope of research and development of the past 20 years some new casting methods, including oxygenation casting, semi-solid metal casting thixotropic rheology or and squeeze casting, and the ups and downs of the vacuum die-casting and so on.


Vacuum die casting process by pumping the gas cavity to eliminate or significantly reduce the die-casting and dissolved gases within the pores, to improve the mechanical properties of die-casting and surface quality. Has been successful in the cold chamber die casting machines, vacuum casting method to produce AM60B magnesium alloy car wheels, the locking force of 2940KN hot chamber die casting machine to produce AM60B magnesium alloy steering wheel parts, castings elongation rate of 8 % to 16%.


Oxygenation die casting, also known as non-porous (PORE-FREEDIECASTINGPROCESS, that is P. F method). The law before filling in the liquid metal, oxygen or other reactive gas filled cavity, the cavity displacement of air, liquid metal filling, the filling of metal active gas and liquid reaction of metal oxide particles dispersed in the pressure casting, the casting in order to eliminate the gas, so that casting can be heat strengthened. Nippon Light Metal (strain) the production of die-casting machine with the oxygenation of the whole head AZ91 magnesium alloy frame, instead of the original multi-layer composite stent, the stent will not only reduce weight, and has made great economic benefits. The company also used to mass produce the oxygenation method AM60 die casting magnesium alloy car wheels and motorcycle wheels, and aluminum wheels compared to 15% weight.


In recent years, the United States, Japan and the United Kingdom and other countries have successfully developed the company's magnesium alloy semi-solid thixotropic injection pressure casting machine. Thixotropic semi-solid magnesium alloy casting machine injection pressure to a certain pressure injected into the semi-solid magnesium alloy die-casting type of shape that make them, it works similar to the injection molding machine. It pre-state non-dendritic grains of magnesium into the screw feeder institutions, bodies in the spiral feed grains will be heated to a semi-solid magnesium, and through the screw feeder institutions other side of the room will be collected magnesium alloy semi-solid slurry of magnesium alloy into the slurry pressure injection chamber for injection molding. This approach represents the casting forming the production of a magnesium alloy casting direction.


4 magnesium alloy smelting operations and safety


As the liquid is very easily oxidized Lost wax casting  magnesium alloy and the oxide film formed on the surface is loose, and its dense coefficient value is only 0.79, can not prevent the alloy to oxidation. Magnesium alloy liquid and atmospheric oxygen, water vapor, nitrogen, the reaction does not melt in liquid magnesium refractory MGO, MG3N2 other compounds, mixed with mold after the formation of "oxide slag." Therefore, the melting alloy oxidation is essential. Magnesium melt protection is mainly in two ways, namely, protection and gas flux.


With the protection of flux melting usually will have the following questions: (1) chloride and fluoride salts at high temperatures produces volatile toxic gases such as HCL, CL2, HF, etc.; (2) Since the flux density, part of the flux will be accompanied by magnesium fluid mixed with mold caused by "flux slag"; (3) flux of volatile gases may be generated by liquid infiltration alloy, a material used in the process of corrosion of the source, accelerated corrosion of materials, reduced life.


At present, most manufacturers use gas, that is dry SF6, N2, CO2, SO2 gas in 2 to 4 kinds of gas mixture composition, the magnesium alloy pool surface form a dense continuous film of liquid to prevent oxidation of magnesium alloys. SF6 gas is not toxic, but it was the Earth's greenhouse effect than CO2 24,000 times serious, and SF6 with magnesium industry accounts for 7% of the total volume of the world (1996), the future is bound to limit their usage as well as to stop its use, but has not yet SF6 find suitable alternatives. Studies have shown that, with the sulfur powder sprinkled on the molten pool formed on the surface of magnesium alloy liquid SO2 has a protective effect.


Magnesium alloy die casting process and the risk of most of the post office by the buried fault caused by the process. According to Japanese statistics, magnesium alloy die-casting production process of the hazards, accounting for 25% of smelting, casting, 10%, 39% of processing, storage and waste 16%, electrical, 3%, other 7%. Clearly, processing and post-processing than the risk of 3 to 4 times the die casting process. Process, whether it is sand blasting, turning, milling, polishing, etc., will inevitably produce magnesium dust and sparks, such as factories through phoenix bad, magnesium dust concentration in the air too much, once sparks of magnesium with air or ground dust exposure, ranging from burning, while in the explosion. So dust collector must be installed inside the plant and equipped with fire prevention and fire safety facilities of sand.


5 die design


As the magnesium alloy with the chemical, physical parameters and aluminum die casting with very different characteristics, so the mold design is not fully apply the aluminum die-casting design principles.


Easily oxidized magnesium alloy liquid combustion, casting large hot tearing tendency than aluminum, in melting, casting and casting-type temperature control, etc. than the aluminum die-casting complex. Magnesium alloy filling time is short, the exhaust is particularly prominent, specific heat capacity of magnesium alloy and the latent heat than aluminum is low, and thus prone to local die-casting process (thin-section area) early crystallization, resulting in feeding channel blockage, lack of defects produced water. Magnesium alloy die-casting design mainly consider the following aspects:


(1) die casting machine selection. What form of die casting machines for production depends on the thickness. ROLANDFINK in the "magnesium alloy die-casting process optimization," to undertake a study of the process, through economy of magnesium alloy die-casting, cold chamber and hot chamber die casting process analysis suggested that under normal circumstances the casting needs less than 1KG hot-chamber die casting machine, to ensure full of thin-walled, large is recommended for cold chamber die casting machine.


(2) process parameters. In casting the production process, select the appropriate process parameters is to obtain high-quality die-casting machine castings play a prerequisite for maximum productivity, is the correct basis for die-casting mold design. Casting, the impact of alloy liquid filling forming many factors, including the main injection pressure, injection speed, filling time, and die-casting mold temperature, and so on. As the complexity of the casting wall thickness and the different process parameters chosen vary widely. Magnesium alloy with aluminum, zinc alloy compared to flow better, so two injection speed can be larger, magnesium aluminum punch faster than about 30% faster, even more than the maximum 10M / S. As the magnesium alloy casting properties such as pouring liquid on the type of moderate temperature sensitive, in the process of filling liquid easily solidified magnesium alloy, Investment casting must be precisely controlled mild pouring temperature, otherwise it easily out of waste.


(3) gating system design. Gating system for liquid metal flow direction, the exhaust overflow condition, the mold temperature points