Analysis of Restrictions on Large-scale Application of Magnesium Alloys

Today, Alloy WIKI will talk to you about the basic characteristics of magnesium alloys. Magnesium alloys are alloys formed by adding other elements to the metal magnesium base. Magnesium (Mg) has a density of 1.8g/cm3, which is 2/3 of aluminum (Al) and only 1/4 of iron (Fe), which is the lowest density among metals used. At the same time, Mg has high specific strength, elastic modulus, and good thermal conductivity. After being made into magnesium alloy, magnesium alloy can carry higher impact load than aluminum alloy. At present, magnesium-aluminum alloys are widely used and have been widely used in aerospace, rail transit, chemical engineering and other fields.

The main features of magnesium alloy include:

• ①The strength of magnesium-aluminum alloy is much higher than that of steel, and the density is equivalent to that of aluminum alloy.
• ②Excellent shock and noise resistance. Magnesium alloy is subjected to impact load in the elastic range, and the energy absorbed is greater than that of aluminum alloy, so the shock absorption capacity is 100 times that of aluminum and 300 to 500 times that of titanium alloy.
• ③Good machining and casting performance. Magnesium alloy has low cutting resistance and is an alloy material with very low machining difficulty. And after cutting, the surface is smooth and no post-processing is required. The size of magnesium alloy die castings is 25% higher than that of aluminum alloys, and the processing energy consumption is reduced by 50%.
• ④Good thermal conductivity and electromagnetic shielding performance. Its thermal conductivity is higher than that of aluminum alloy, and it can be made thinner, which is more conducive to heat dissipation. It has good electromagnetic shielding properties and can be competent for occasions with high requirements for anti-electromagnetic interference standards.

The main components of magnesium alloys include Mg, Al, manganese (Mn), zinc (Zn), silicon (Si), copper (Cu), nickel (Ni), Fe and other elements. According to the composition range, it can be divided into magnesium-aluminum alloy, magnesium-manganese alloy and magnesium-zinc alloy. According to the different forming methods of magnesium alloy, it can be divided into two types: die-cast magnesium alloy and deformed magnesium alloy. Among them, 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.

Modern high-speed trains must meet safety design indicators, energy conservation and environmental protection design indicators, and comfort indicators. Magnesium alloy can well absorb energy such as shock and collision. For example, using it to make high-speed rail train seats can absorb more vibration energy after train collision, thereby reducing passengers’ injuries. Magnesium alloy has a low density and has obvious advantages compared with other materials in terms of train lightweight. Due to its good workability, it can be made into any complicated shape, so it can make a seat that conforms to ergonomic and aesthetic design.

According to a number of 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 huge application potential in the lightweight upgrade of high-speed rail trains. Using AZ92D magnesium alloy to replace the small table support made of polyamide (PA) plastic in the high-speed train, the load-bearing capacity, elongation at break and impact toughness of the small table have been significantly improved.

Magnesium alloys have appeared on many occasions in foreign high-speed trains. Both the ICE high-speed trains of Germany’s Siemens and the French TGV Duplex double-decker high-speed trains use magnesium alloy seats. Among them, the French TGV Duplex train produced a total of 45,000 seats, and produced parts such as small tables, armrests, pedals and seat sides. Compared with traditional aluminum alloy seats, the weight of each double seat is reduced by 6kg, accounting for 1/5 of the total weight. The seats and brackets, backrests and armrests, floor mats, bases, etc. of Japan’s Shinkansen N700 series high-speed trains have all been made of magnesium alloy.

As the cost of magnesium alloy is comparable to that of aluminum alloy, the energy consumption during train operation is reduced, and overall the operating cost of trains is reduced. South Korea’s KTX express train seats used FRP and aluminum alloy as materials in the past. After the use of magnesium alloy sheet parts, the weight of the 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 alloy in my country’s rail transit equipment is also advancing rapidly. At present, magnesium alloys are mainly used in air conditioning vent grilles, window protection railings, seats and floors, sleeper bed frames and brackets, luggage rack frames, internal instrument panel frames and other components in China.

Commonly used magnesium alloy models in rail transit equipment include A Z31B, A Z61A, A Z91D, Z K60, AM60B, etc. The demand for the load-bearing capacity of magnesium alloy parts is increasing, and the main load-bearing parts have been manufactured. Therefore, an important direction for the development of magnesium alloys in the future is to develop high-strength, high-toughness, fatigue-resistant and shock-absorbing railway vehicle parts. In my country’s Tangshan Railway Passenger Car Co., Ltd., magnesium alloy is also applied to the domestic first practical low- and medium-speed maglev train manufactured by it. Its internal structural parts and lamp bodies are made of magnesium alloy to achieve the goal of overall weight reduction.

The Automobile Steel Institute of Baoshan Iron and Steel Co., Ltd. used its BGZ8 type magnesium alloy with independent intellectual property rights to prepare a new type of magnesium alloy material. The magnesium alloy has very high mechanical strength and toughness. They optimized the extrusion die at the same time, so that the magnesium alloy can undergo uniform and severe deformation during the extrusion process in the die, and induce it to occur in the crystallization process. The formed recrystallized structure has a very fine grain size, and is evenly distributed in the magnesium alloy material, and has a strong {0001} basal texture.

After the low temperature T5 aging treatment, the Al12Mg17 phase pinning is formed due to extrusion at the grain boundary. This multi-scale onlooker structure greatly increases the magnesium tensile strength, yield strength and elongation and other key mechanical strength indicators, reaching 396MPa, 289MPa and 11.5%, while successfully reducing the weight of parts by 25%, it fully meets the mechanical performance requirements of the high-speed rail seat frame. In December 2017, the extruded side wall profile and floor guide channel profile magnesium alloy produced by Chengdu Tianzhi Lightweight Technology Co., Ltd. (“Tianzhi Company”) was successfully accepted and delivered to the “Fuxing” train.

The total length of this batch of products reached about 10,000 extension meters, and was successfully installed on 11 trains of this batch. The company also delivered 2 batches of magnesium alloy extruded profiles with an extension of about 10,000 meters, which are expected to be installed on 25 subway trains. Tianzhi Company has successfully completed the entire technical process of material design, extrusion molding simulation, mold optimization design, extrusion process parameter optimization, on-site fine control, and profile correction to length. It can supply large-scale, thin-walled, and complex-section magnesium in large quantities. 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, it has realized the lightweight upgrade of the internal parts of the EMU and completed the important goal of energy saving and weight reduction. Chongqing University of Technology has developed the third-generation pneumatic suspension train LOOP, which uses flame-retardant magnesium alloy as the body material. The body adopts a ring-shaped wing design, forming a rearwardly inclined frame on a three-dimensional surface. Compared with the previous first and second generation car bodies, this design has greatly improved airflow stability. The ring-shaped wing design can increase the train transportation capacity by 30% to 40% while keeping the track width unchanged. The world’s first high-speed pneumatic suspension train line is planned to be opened in Japan in 2025. An underground tunnel from Japan’s Narita Airport to Haneda Kiyang will be built, and the train will run at a speed of 400km per hour. It is expected to be 3 carriages, each carrying 120 people at a time.

Analysis of Restrictions on Large-scale Application of Magnesium Alloys

Although magnesium alloy has many advantages that other materials do not possess, there are still certain shortcomings that restrict its large-scale application. Mainly include:

• ①Because magnesium alloy has a hexagonal lattice structure, it is easy to cause material anisotropy after rolling or directional extrusion and other processing treatments, leading to deviations in mechanical properties in all directions.
• ②The nature of magnesium alloy is relatively active, and its chemical electronegativity is very strong. Its electrode potential is -2.37V, which is lower than -1.71V of aluminum material, which is prone to oxidation and corrosion. For example, the contact surface with other metals is prone to electrochemical corrosion; it is also easy to be oxidized by oxygen in the air to indicate the formation of a thin oxide layer. The texture of the oxide layer is urged and loosened, indicating that it is porous and cannot block the air and accelerate the oxidation process. And, the higher the temperature, the faster the oxidation process.
• Magnesium alloys, especially in the liquid state, are very easy to catch fire, and the combustion process is extremely violent. Therefore, the smelting and casting of magnesium and magnesium alloys must be carried out in a solvent cover or in a protective atmosphere or vacuum. The melting point of metallic magnesium is 650℃, 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, causing uncontrollable and violent combustion. Therefore, after the magnesium alloy parts are loaded into the car, a combustion test must be done to ensure that they will not burn.
• ④Welding of magnesium alloys cannot be carried out by conventional methods, and new solid-phase welding methods (friction stir welding, etc.) must be used for welding. At the same time, the performance of the welded product needs to be tested for further certification.
• ④ Due to the limitation of the process at this stage, wide-width extrusion of magnesium alloy is difficult, and it is difficult to produce large-section extruded profiles. At present, as long as the magnesium alloy composition ratio meets the standard and the processing process parameters are strictly controlled, the physical properties of the magnesium alloy can meet the standard, but the material dimensional error, especially the side deflection value and stability can not be actually controlled. At present, the country can produce large-scale thin-walled hollow aluminum profiles required for rail transit equipment. A 250MN extruder has been built. The physical properties of the large-scale extruded aluminum material for the car body are equivalent to or even better than similar products in Germany and Japan. But there is still a considerable gap between dimensional error and stability.
• ⑤The 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 under 150℃. Currently, research is underway to add rare earth elements to magnesium alloys to make heat-resistant magnesium alloys, but the cost of rare earth magnesium alloys cannot be reduced to a commercial level.

Concluding remarks

my country is rich in magnesite and dolomite resources, and the reserves of magnesium resources have reached more than half of the global total. Therefore, there are huge resources and cost advantages in the research and application of magnesium alloys. The development speed of my country’s rail transit is leading the world, and the requirements for lightweight rail transit equipment are becoming stronger. With the research and development of magnesium alloy smelting and processing technology, the performance of magnesium alloy will continue to be improved, and the stability will be further strengthened. It is expected that new applications of magnesium alloys will emerge in endlessly in the future. The gradual development of applications from non-load-bearing parts to load-bearing parts of high-speed trains is the key to the application of magnesium alloys in high-speed trains to achieve lightweight upgrades.