High-speed train carriages are welded using aluminum materials. Some high-speed train lines pass through frigid regions with temperatures as low as minus 30 to 40 degrees Celsius. Some instruments, equipment, and living supplies on Antarctic research vessels are made from aluminum materials and need to withstand temperatures as low as minus 60 to 70 degrees Celsius. Chinese cargo ships traveling from the Arctic to Europe also use some equipment made from aluminum materials, and some of it is exposed to temperatures as low as minus 50 to 60 degrees Celsius. Can they operate normally in such extreme cold? No problem, aluminum alloys and aluminum materials are not afraid of extreme cold or heat.

Aluminum and aluminum alloys are excellent low-temperature materials. They do not exhibit low-temperature brittleness like ordinary steel or nickel alloys, which show a significant decrease in strength and ductility at low temperatures. However, aluminum and aluminum alloys are different. They do not exhibit any trace of low-temperature brittleness. All their mechanical properties significantly increase as the temperature decreases. This is independent of the material's composition, whether it's cast aluminum alloy or wrought aluminum alloy, powder metallurgy alloy, or composite material. It is also independent of the material's state, whether it's in the as-processed state or after heat treatment. It is unrelated to the ingot preparation process, whether it's produced by casting and rolling or continuous casting and rolling. It is also unrelated to the aluminum extraction process, including electrolysis, carbon thermal reduction, and chemical extraction. This applies to all levels of purity, from process aluminum with 99.50% to 99.79% purity, high-purity aluminum with 99.80% to 99.949% purity, super-purity aluminum with 99.950% to 99.9959% purity, extreme-purity aluminum with 99.9960% to 99.9990% purity, and ultra-high-purity aluminum with over 99.9990% purity. Interestingly, two other light metals, magnesium and titanium, also do not exhibit low-temperature brittleness.

The mechanical properties of commonly used aluminium alloys for high-speed train carriages and their relationship with temperature are shown in the table below.

High-speed train carriages use aluminum materials such as Al-Mg series 5005 alloy plates, 5052 alloy plates, 5083 alloy plates, and profiles; Al-Mg-Si series 6061 alloy plates and profiles, 6N01 alloy profiles, 6063 alloy profiles; Al-Zn-Mg series 7N01 alloy plates and profiles, 7003 alloy profiles. They come in standard states: O, H14, H18, H112, T4, T5, T6.

From the data in the table, it is evident that the mechanical properties of aluminum alloys increase as the temperature decreases. Therefore, aluminum is an excellent low-temperature structural material suitable for use in rocket low-temperature fuel (liquid hydrogen, liquid oxygen) tanks, liquefied natural gas (LNG) transport ships and onshore tanks, low-temperature chemical product containers, cold storage, refrigerated trucks, and more.

The structural components of high-speed trains running on Earth, including carriage and locomotive components, can all be manufactured using existing aluminium alloys. There is no need to research a new aluminium alloy for carriage structures operating in frigid regions. However, if a new 6XXX alloy with performance about 10% higher than 6061 alloy or a 7XXX alloy with overall performance approximately 8% higher than 7N01 alloy could be developed, that would be a significant achievement.

Next, let's discuss the development trends of carriage aluminum alloys.

In the current manufacturing and maintenance of rail vehicle carriages, alloy plates such as 5052, 5083, 5454, and 6061 are used, along with extruded profiles like 5083, 6061, and 7N01. Some newer alloys like 5059, 5383, and 6082 are also being applied. They all exhibit excellent weldability, with welding wires typically being 5356 or 5556 alloys. Of course, friction stir welding (FSW) is the preferred method, as it not only ensures high welding quality but also eliminates the need for welding wires. Japan's 7N01 alloy, with its composition of Mn 0.200.7%, Mg 1.02.0%, and Zn 4.0~5.0% (all in %), has found widespread use in the manufacturing of rail vehicles. Germany used 5005 alloy plates to produce sidewalls for high-speed Trans Rapid carriages and employed 6061, 6063, and 6005 alloy extrusions for profiles. In summary, up to now, both China and other countries have mostly adhered to these alloys for high-speed train manufacturing.

Aluminum Alloys for Carriages at 200km/h~350km/h

We can categorize carriage aluminum alloys based on the operational speed of the trains. The first-generation alloys are used for vehicles with speeds below 200km/h and are conventional alloys primarily used for manufacturing urban rail vehicle carriages, such as 6063, 6061, and 5083 alloys. Second-generation aluminum alloys like 6N01, 5005, 6005A, 7003, and 7005 are used for manufacturing carriages of high-speed trains with speeds ranging from 200km/h to 350km/h. Third-generation alloys include 6082 and scandium-containing aluminum alloys.

Scandium-Containing Aluminum Alloys

Scandium is one of the most effective grain refiners for aluminum and is considered an essential element for optimizing aluminum alloy properties. Scandium content is typically less than 0.5% in aluminum alloys, and alloys containing scandium are collectively referred to as aluminum-scandium alloys (Al-Sc alloys). Al-Sc alloys offer advantages such as high strength, good ductility, excellent weldability, and corrosion resistance. They are being used in various applications, including ships, aerospace vehicles, reactors, and defense equipment, making them a new generation of aluminum alloys suitable for railway vehicle structures.

Aluminum Foam

High-speed trains are characterized by lightweight axle loads, frequent acceleration and deceleration, and overloaded operations, which require the carriage structure to be as lightweight as possible while meeting strength, rigidity, safety, and comfort requirements. Clearly, the ultra-light aluminum foam's high specific strength, specific modulus, and high damping characteristics align with these requirements. Foreign research and evaluation of the application of aluminum foam in high-speed trains have shown that aluminum foam-filled steel tubes have a 35% to 40% higher energy absorption capability than empty tubes and a 40% to 50% increase in flexural strength. This makes carriage pillars and partitions more robust and less prone to collapse. Using aluminum foam for energy absorption in the locomotive's front buffer zone enhances impact absorption capabilities. Sandwich panels made of 10mm thick aluminum foam and thin aluminum sheets are 50% lighter than original steel plates while increasing stiffness by 8 times.