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Low temperature Properties of Aluminum Alloy

October 23, 2019

High-speed railway carriages are welded with aluminum, and some high-speed rail lines pass through the high cold zone of minus 30 to 40 degrees Celsius. Some of the instruments, equipment and living supplies on the Antarctic scientific research ship are made of aluminum and need to withstand the test of minus 60 degrees Celsius and 70 degrees Celsius. Some of the equipment on merchant ships from China to Europe via the Arctic is also made of aluminum, some of which are exposed and the ambient temperature is minus 50 or 60 degrees Celsius; can they operate in such a cold environment? No problem, aluminum alloy and aluminum is the least afraid of cold and heat.

Aluminum and aluminum alloys are the best low temperature materials, without low temperature brittleness, unlike ordinary steel, nickel alloy and so on. Although their strength properties increase with the decrease of temperature, However, plasticity and toughness decrease with the decrease of temperature, that is, there is obvious brittleness at low temperature. However, aluminum and aluminum alloys are very different. There is not a trace of low temperature brittleness. All their mechanical properties increase significantly with the decrease of temperature, regardless of the composition of the material, whether it is cast aluminum alloy or wrought aluminum alloy. Whether it is powder metallurgy alloy or composite material; It has nothing to do with the material state, whether it is in the state of processing or heat treatment, and it has nothing to do with the preparation process of the ingot, whether it is rolled with ingot, continuous casting with melt or continuous rolling. It has nothing to do with the extraction process of aluminum, electrolytic, carbothermal reduction, chemical extraction, all have no low temperature brittleness; It has nothing to do with purity, whether it's 99.50% to 99.79% pure aluminum, There is no low temperature brittleness in 99.80%-99.949% high-purity aluminum, 99.950%-99.9959% ultra-pure aluminum (Super Purity), 99.9960%-99.9990% ultra-pure aluminum (extreme purity), > 99.9990% ultra-pure aluminum and so on. Interestingly, the other two light metals, magnesium and titanium, are not as brittle at low temperatures as aluminum.

The relationship between mechanical properties and temperature of extruded aluminum alloy profiles for several commonly used high-speed railway vehicles is shown in the table below.

The aluminum products in high-speed railway compartments include 5005 alloy plates, 5052 alloy plates, 5083 alloy plates and profiles of Al-Mg system, 6061 alloy plates and profiles of Al-Mg-Si system, 6N01 alloy profiles and 6063 alloy profiles. Al-Zn-Mg is 7N01 alloy plate and profile, 7003 alloy profile. The standard states are O, H14, H18, H112, T4, T5, T6.

It is obvious from the data in the table that no matter what kind of aluminum alloy, its mechanical properties increase with the decrease of temperature, so aluminum is a kind of wonderful low temperature structure material. Rocket cryogenic fuel (liquid hydrogen, liquid oxygen) storage tanks, liquefied natural gas (LNG) carrier and shore-based storage tanks, cryogenic chemical product containers, cold storage, refrigerated vehicles and so on can be made of aluminum.

The structural parts of the carriages and heads of high-speed trains running on the earth, all the parts that can be made of aluminum alloy can be made of the current corresponding aluminum alloy. There is no need to find another way to study a kind of aluminum alloy with box structure operated in high and cold areas and its production process. Of course, if we can develop a 6XXX alloy whose properties are about 10% larger than 6061 alloy, or a 7XXX alloy which is about 8% larger than 7N01 alloy, That's a lot of work, of course.

In fact, the high-speed railway in Heilongjiang Province in China and in Inner Mongolia Autonomous region, Xinjiang Uygur Autonomous region, Tibet Autonomous region and Qinghai Province is not "high cold" for aluminum alloy structure, and there is no need for special treatment of alloy composition. There is no need to make special adjustments to the process parameters of material production.

As long as the properties of aluminum produced for high-speed rail or later for high-speed rail carriages operating in other cold areas meet the requirements of Chinese GB, European EN, Japanese JIS, American ASTM, etc., they are qualified products and no special measures need to be taken. So as not to increase the cost.

Development trend of Aluminum Alloy in car

At present, the plate alloys used in the manufacture and maintenance of rail vehicle body are 5052, 5083, 5454, 6061 and so on, and the extruded profiles are 5083, 6061, 7N01 and so on. In addition, some new alloys such as 5059, 5383, 6082 have also been used. They all have good weldability, the welding wire is 5356 or 5556 alloy, of course, it is best to use friction stir welding (FSW), not only high welding quality, but also no welding wire. Later, 7N01 alloy (Mn0.20~0.7, Mg1.0~2.0, Zn4.0~5.0, unit%) developed in Japan has been widely used in the manufacture of rail vehicles. In the manufacture of high-speed railway Trans Rapid carriages in Germany, 5005 alloy plates are used to make wallboards, and 6061, 6063 and 6005 alloys are used to extrude profiles. In short, these alloys are still basically used in high-speed rail vehicles made in China and other countries.

Aluminum alloy for 200km/h~350km/h train body

According to the train operation speed, the aluminum alloy body can be divided into: the vehicles with speed less than 200km/h can be called the first generation alloy, they are conventional alloys, they are mostly used to manufacture the box body of urban rail vehicles, such as 6063, 6061, 5083 alloy and so on. The second generation aluminum alloys such as 6N01, 5005, 6005A, 7003, 7005 alloys are used to manufacture high-speed railway vehicle body with speed of 200km/h~350km/h, and the third generation alloys are 6082 aluminum alloys containing scandium and so on.

In addition to 6N01 and 7N01 alloys (they are Japanese alloys, N stands for Nippon), there are 7003 alloys. Their Mg content is lower than that of 7N01 alloys. They are a kind of Al-Zn-Mg alloys with low Mg. Their weldability and strength are similar to those of 7N01 alloys. And has higher extrudable performance. A large number of 7003 and 7N01 alloys are used to produce wide profiles with wall thickness 3mm in Northeast Japan, Shinkansen Shinkansen and Sapporo subway.

6005A alloys (Si0.50~0.9, Mg0.40~0.7, Mn and Cr 0.12%) are registered in France. Compared with American alloy 6005, the content of Mg is equal to that of Si, but the Cr, of Mn is increased by 0.12% to 0.50%. It not only has the same extrudable properties as 6063 alloy, but also improves the strength properties. The same is true of 6N01 alloy. The wing side panels (wide 507mm, wall thickness 2.5mm), wide 558mm floors and large wide hollow profiles for manufacturing side plate hooks and eaves are extruded with 6N01 alloy.

Aluminum alloy for 350ktm/h~450km/h train body

This is a new generation of aluminum alloy for box body. the speed of the train is as high as 450m, and the vehicle has to withstand greater external forces and be subjected to stronger vibrations. Therefore, a new generation of lightweight high-speed railway aluminum alloy should be developed.

Aluminum alloy containing scandium. Scandium is the most effective grain refiner for aluminum and aluminum alloys, and it is also one of the effective elements to optimize the properties of aluminum alloys. The content of scandium is less than 0.5%. All alloys containing scandium are collectively called aluminum-aluminum alloys no matter how much they contain scandium. Al-Sc alloy has the advantages of high strength, good plasticity, excellent weldability and strong corrosion resistance. It is one of the new generation aluminum alloys selected in the fields of warship, aerospace, reactor, national defense military equipment and so on. Of course, it can also be used to make railway vehicle structures.

However, the improvement of microstructure and properties by adding scandium to the current aluminum alloy remains to be studied systematically. Scandium is a kind of rare earth element. China is a producer of scandium in the world and is in the forefront of Al-Sc alloy research. But on the whole, it is not as good as Russia, which is the leader in the research and application of Al-Sc alloy. Northeast Light Alloy Co., Ltd., Central South University, Institute of Aerospace Materials and Technology have done a lot of work on the research and development of Al-Sc alloy. Their research on "large size 5B70 aluminum-magnesium-scandium alloy sheet" won the first prize in science and technology of China's non-ferrous metal industry in 2017. there was also a second prize, which was "weldable high strength Al-Zn-Mg-Sc alloy for aerospace".

6082 alloy

Since the popularization of 6N01 alloy, 6082 alloy formed in 1972 has attracted the attention of railway equipment manufacturing department. The strength of this alloy is between the strength of 7N01 alloy and 6N01 alloy. The tensile strength Rm (spray on-line quenching) of 6082-T5 square tube meets the corresponding requirements of underframe beam. The basic experiments show that the alloy can be used in the field. However, a lot of work still needs to be done if it is to be widely promoted in the railway equipment department. For the fatigue strength of the assembly joints of aluminum trains, which were considered foolproof 30 years ago, due to changes in train load conditions and lightweight structure, they are no longer suitable for the current new type of high-speed trains, but this has nothing to do with the temperature in high and cold areas. Because minus dozens of degrees Celsius is really a "small test edge" for aluminum alloy, it is not a low temperature, at the same time, the lower the temperature, the stronger and tougher the aluminum structure appears.

Aluminum foam

High-speed train has the characteristics of light axle weight, frequent acceleration and deceleration and overload operation, which requires the car body structure to be as lightweight as possible on the premise of strength, stiffness, safety and comfort. Obviously, the properties of ultra-light aluminum foam, such as high specific strength, high specific mode, high damping and so on, are very consistent with these requirements. The application of aluminum foam in high-speed trains has been studied and evaluated in detail abroad. It is found that the energy absorption ability of steel pipe filled with aluminum foam is 35% ~ 40% higher than that of air pipe, and the bending strength is increased by 40% ~ 50%. As a result, the carriage posts and partition can be made stronger. Not easy to collapse; Filling the locomotive head buffer with aluminum foam can improve the ability of absorbing impact energy, and the weight of the sandwich plate made of 10mm thick aluminum foam and thin aluminum plate is 50% lighter than that of the original steel plate, but the stiffness is increased by 8 times.

At present, relevant units of high-speed rail in China are studying the feasibility of using foam aluminum sandwich panels to prepare high-speed rail car floors and doors. In order to speed up the solution of a series of major scientific and technological problems facing the next generation of high-speed railway, the railway department and the Chinese Academy of Sciences have jointly set up the Advanced Rail Transit Mechanics Research Center. In this paper, the material and structure reliability of high-speed train and the theory and technology of noise reduction are studied, among which a considerable part of the content is related to ultra-light aluminum foam.

With the continuous improvement of the running speed of high-speed trains, the impact of noise on passenger comfort and the surrounding environment has become one of the key constraints to the development of high-speed rail. Will likely become a major obstacle to the development of high-speed rail. In order to reduce the noise pollution of high-speed trains, barriers must be installed on both sides of the railway and in the tunnels passing through densely populated areas. One of the main functions of ultra-light open-cell aluminum foam is sound absorption, and this performance can be adjusted by changing the pass or sound structure. In addition, aluminum foam also has good anticorrosion, climate resistance and processing properties, so it is a good sound absorbing material for field sound barrier.

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