1. The basic characteristics of magnesium alloys
A magnesium alloy is an alloy formed by adding other elements to a metal magnesium base. The density of magnesium (Mg) is 1.8g / cm3, which is 2/3 of aluminum (Al), only 1/4 of iron (Fe), which is the smallest density among the metals used. At the same time, Mg has high specific strength, elastic modulus, and good thermal conductivity. After being made of magnesium alloy, compared with aluminum alloy, magnesium alloy can bear higher impact load. At present, magnesium aluminum alloy is the most widely used and has been widely used in aerospace, rail transportation, chemical industry and other fields.
The main features of magnesium alloys include:
① The strength of magnesium aluminum alloy is much higher than that of steel, and the density is equivalent to that of aluminum alloy.
②Excellent shock absorption and anti-noise performance. Magnesium alloys are subjected to impact loads in the elastic range, and their absorbed energy is greater than aluminum alloys. Therefore, the shock absorption capacity is 100 times that of aluminum and 300 to 500 times that of titanium alloys.
③ Good mechanical processing and casting performance. The cutting resistance of magnesium alloy is small, and it is an alloy material with very low machining difficulty. And the surface is smooth after cutting without post-processing. The size of magnesium alloy die-casting can improve the machining accuracy by 25% compared with aluminum alloy, and reduce the energy consumption by 50%.
④ Good thermal conductivity and electromagnetic shielding performance. Its thermal conductivity is higher than that of aluminum alloy, and it can also be made thinner and more conducive to heat dissipation. It has good electromagnetic shielding properties and can be used in applications with high requirements for anti-electromagnetic interference standards.
The main components of magnesium alloys include elements such as Mg, Al, manganese (Mn), zinc (Zn), silicon (Si), copper (Cu), nickel (Ni), and Fe. According to the composition range, it can be divided into magnesium aluminum alloy, magnesium manganese alloy and magnesium zinc alloy. According to different forming methods of magnesium alloy, it can be divided into two types: die-cast magnesium alloy and deformed magnesium alloy. Among them, the die-cast magnesium alloy occupies most of the cast magnesium alloy. Magnesium alloys such as AZ31B, AZ61A, and AZ91D have been widely used in industrial production.
2. Application of magnesium alloys in rail transit equipment
Modern high-speed trains must meet safety design indicators, energy conservation and environmental protection design indicators, and comfort indicators. Magnesium alloys can absorb energy such as shocks and shocks. For example, if they are used to make high-speed rail train seats, they can absorb more vibration energy after a train crash, thereby reducing injuries to passengers. The magnesium alloy has a low density, and has obvious advantages compared with other materials in terms of train weight reduction. Because of its good workability, it can be made into any complex shape, so it can make a seat that conforms to ergonomics and aesthetic design. According to research and test results of magnesium alloy profile components for high-speed trains by domestic and foreign research institutions and OEMs, it is concluded that magnesium alloys have great application potential in the lightweight upgrade of high-speed rail trains. The use of AZ92D magnesium alloy instead of small table brackets made of polyamide (PA) plastic in motor vehicles has significantly improved the load-bearing capacity, elongation at break and impact toughness of small tables.
Magnesium alloys have appeared on many occasions in high-speed trains abroad. The German ICE high-speed trains from Germany and the French TGV Duplex double-deck high-speed trains use magnesium alloys to make seats. Among them, the French TGV Duplex train produced a total of 45,000 seats, and produced small tables, armrests, pedals and side parts of the seats. Compared to traditional aluminum seats, the weight of each double seat is reduced by 6kg, which is 1/5 of the total mass. The seats and supports, backrests and armrests, floor mats, and bases of the Japan Shinkansen N700 series high-speed trains have been made of magnesium alloys. Because the cost of magnesium alloys is comparable to that of aluminum alloys, the energy consumption of trains is reduced. Overall, the cost of trains is reduced. Korea KTX express train seats used glass fiber reinforced plastic and aluminum alloy as materials. After using magnesium alloy sheet metal parts, the weight of a seat is reduced by 5kg compared with the previous one, and the material cost is also reduced by 8% to 10%.
The application of magnesium alloys in China's rail transit equipment is also rapidly advancing. At present, magnesium alloys are mainly used in China for air-conditioning vent grills, window protection railings, seats and floors, sleeper bed frames and brackets, luggage rack frames, internal dashboard frames and other components. Magnesium alloy models commonly used in rail transit equipment include AZ31B, AZ61A, AZ91D, ZK60, AM60B and so on. The demand for the bearing capacity of magnesium alloy parts is increasing, and the main bearing parts have begun to be manufactured. Therefore, an important direction for the future development of magnesium alloys is to develop high-strength, high-toughness, fatigue-resistant and shock-absorbing railway vehicle parts.
In China, Tangshan Railway Passenger Car Co., Ltd. has also applied magnesium alloy to its first domestic practical low-speed magnetic levitation train. Its internal structural parts and lamp body are made of magnesium alloy to achieve the purpose of overall weight reduction.
BGJT Magnesium Alloy Materials Technology Group has used its magnesium alloy with independent intellectual property rights to prepare new magnesium alloy materials. The magnesium alloy has very high mechanical strength and toughness. They also optimized the extrusion die at the same time, so that the magnesium alloy can deform uniformly and violently during the extrusion process in the die, and induce it to produce in the crystallization process. The recrystallized structure formed has a very fine grain size, is uniformly distributed in the magnesium alloy material, and has a strong basal texture. After the low temperature T5 aging treatment, Al12Mg17 phase pinning was formed at the grain boundaries due to extrusion. This multi-scale onlooking structure has greatly increased the key mechanical strength indicators such as tensile strength, yield strength, and elongation of magnesium, reaching 396MPa, 289MPa and 11.5%, while successfully reducing the weight of parts by 25%, fully meet the mechanical performance requirements of high-speed rail seat frames.
In December 2017, the extruded side wall profiles and floor guide profile magnesium alloys produced by CDTZ Lightweight Technology Co., Ltd. were successfully accepted and delivered to the "Fuxing" high-speed train. The total length of this batch of products reached about 10,000 extension meters, which was successfully installed on 11 trains in this batch. The company also delivered two batches of approximately 10,000-meter-long extruded magnesium alloy profiles, which are expected to be installed on 25 subway trains. Tianzhi Company successfully opened the full technical process of material design, extrusion molding simulation, mold optimization design, extrusion process parameter optimization, on-site fine regulation and control, orthopedic sizing of profiles. Alloy precision extrusion profiles have made important progress and breakthroughs in magnesium alloy precision extrusion technology. Fuxing is a high-speed train with the highest operating speed in the world. After the installation of magnesium alloy profiles, the lightweight interior of the EMU has been upgraded and the important goals of energy saving and weight reduction have been achieved.
Chongqing University of Technology has developed the third-generation LOOP, which uses flame-retardant magnesium alloy as the body material. The body uses a circular wing design to form a frame that slopes backwards on three dimensions. Compared with the first and second generation car bodies, the design has a lot of improved airflow stability. The ring wing design can increase the train's transportation capacity by 30% to 40% while maintaining the same track width. The world's first high-speed pneumatic levitation train line is scheduled to open in Japan in 2025. At that time, an underground tunnel from Narita Airport to Haneda Jiyang will be completed, and the train will run at a speed of 400km per hour. It is expected to have 3 cars, each of which will carry 120 people at a time.
3. Analysis of constraints on larger-scale application of magnesium alloys
Although magnesium alloys have many advantages not available in other materials, they still have certain shortcomings that limit their application to larger scale applications. mainly include:
① Because the magnesium alloy has a hexagonal lattice structure, after the processing such as rolling or directional extrusion, it is easy to cause anisotropy of the material and cause deviations in the mechanical properties of the parties.
② The properties of magnesium alloys are relatively lively and the chemical electronegativity is very strong. Its electrode potential is -2.37V, which is lower than -1.71V of aluminum materials, which is prone to oxidative corrosion. For example, the contact surface with other metals is prone to electrochemical corrosion; it is also easily oxidized by oxygen in the air to indicate the formation of an oxidized thin layer. The oxidized layer has a loose texture, indicating that it is porous and cannot block the air and accelerate the oxidation. And, the higher the temperature, the faster the oxidation process. Magnesium alloys, especially in the liquid state, are extremely susceptible to ignition and the burning process is extremely violent. Therefore, magnesium and magnesium alloys must be smelted and cast in a solvent-covered or protective atmosphere or in a vacuum. The melting point of magnesium metal is 650 ° C, and the melting point of magnesium alloy is lower. As long as it exceeds 600 ° C, the magnesium alloy will easily melt into a liquid, which will cause uncontrollable and violent combustion. Therefore, after the magnesium alloy parts are loaded on the vehicle, a combustion test must be done to ensure that it will not burn.
③ Welding of magnesium alloys cannot be performed by conventional methods. We must use new solid-phase welding methods (friction stir welding, etc.) for welding. At the same time, the performance of the welded product needs to be tested for further certification.
④ Limited by the current stage of technology, it is difficult to extrude magnesium alloys with wide widths, and it is difficult to produce large-section extrusion profiles. At present, as long as the composition ratio of magnesium alloy meets the standard and strict control of processing technology parameters can make the physical properties of magnesium alloy reach the standard, it is impossible to actually control the material size error, especially the side bend value and stability. At present, China is able to produce large-scale thin-walled hollow aluminum profiles required for rail transportation equipment. A 250MN extruder has been built. The physical properties of the large-scale extruded aluminum of the car body are comparable to or even better than similar products from Germany and Japan. But there is still a gap between dimensional error and stability.
⑤ The mechanical properties of magnesium alloys will decrease to a large extent at high temperatures. At present, most magnesium alloy materials can only maintain normal performance below 150 ° C. Research is currently underway to add rare earth elements to magnesium alloys to produce heat-resistant magnesium alloys, but currently the cost of rare earth magnesium alloys cannot be reduced to commercial levels.