1. Structure and materials of high-speed trains
After several years of great development, China’s high-speed railway has successfully developed 380A, 380B, CRH1, CRH2, CRH3, CRH5, CRH6 high-speed trains and intercity EMUs with speeds of 200km/h and 300km/h. It has become the country with the most complete high-speed railway system technology, the strongest integration capability, the longest operating mileage, the fastest running speed and the largest scale of construction in the world.
(1) The operation of high-speed trains.
High-speed trains operate at high speed, and the stress and environmental effects of the car body structure are complex, including vertical loads caused by self-weight load, and longitudinal tension, impact and alternating loads caused by traction and compression forces during high-speed operation of the train. . Under the combined effect of these complex loads and different air temperature and cavitation conditions, the welded structure needs to ensure sufficient strength, toughness and rigidity to ensure the safety and reliability of train operation.
(2) The structure of the high-speed train body.
It mainly includes chassis, side wall, roof, end wall, under-vehicle equipment compartment and roof equipment. Some of the car body underframes are composed of the front end of the underframe, the floor and the side beams of the underframe, and some are composed of traction beams, sleeper beams, side beams (side beams), end beams, beams and floors.
2. Structural requirements for high-speed operation
(1) Streamline the body shape.
Due to the need to overcome the air resistance when running at high speed, the train head must be streamlined according to the aerodynamic requirements. The head shape of a modern high-speed train is getting closer and closer to that of an airplane, as shown in Figure 1. In the same way, the shape of the car body is also quite different from that of ordinary steel cars. The bottom closed cover not only shields various equipment on the bottom of the vehicle but also reduces air resistance. The lower traction beam of the underframe of the car body is seated on the bogie through the core plate and runs in rolling contact between the wheel and the rail.
(2) Lightweight structure of motor car body and selection of materials.
Compared with the three materials, the specific gravity is the same, the elastic modulus is basically the same, but the yield strength is different. The higher the strength, the greater the weight reduction effect. The best weight reduction effect is aluminum alloy, because its yield strength is slightly higher than that of manganese-containing low-carbon steel, but its specific gravity is only 1/3, but its elastic modulus is only 1/3, and its stiffness is poor, so hollow profiles must be used to improve stiffness. In addition, when 6000 and 7000 series aluminum alloys with higher strength are used, low-strength welds are used to solve weld cracks, and a softening zone is formed in the heat-affected part. The joint strength can only reach about 80% of the base metal, and the plastic reserve is also It is far inferior to steel, so the weight can only be reduced by 35% to 50% according to different designs. In railway trains, manganese-containing low-carbon steel was originally used to make the car body, and later it was developed to use copper-manganese weathering steel, and further developed to use stainless steel. Now express and high-speed trains use all-welded aluminum alloy structures. The car body is made of 5000 series aluminum alloy (5083) and 6000 series aluminum alloy (6N01). Practice has proved that 5083 and 6N01 have good weldability and are ideal medium-strength welding structural materials. The traction beam part of the chassis adopts 7000 series aluminum alloy (7N01). 7000 series aluminum alloy has high strength and excellent heat treatment strengthening effect. After appropriate heat treatment process, the yield strength can reach 300~450MPa or more, but the general weldability is poor. Among them, A7N01 high-strength aluminum alloy has excellent extrusion properties and can be extruded into thin-walled profiles with complex shapes. It also has excellent normal temperature aging characteristics and welding performance, and has strong natural aging strengthening ability. Welded members are therefore valued as welded structural materials. A7N01 high-strength aluminum alloy profiles are generally used in load-bearing traction beams, bolsters and buffer beams, bases, door sills, car end walls and side construction skeletons. The research shows that A7N01, an Al-Zn-Mg series aluminum alloy, has higher fatigue strength of welded joints than 6N01, and the intergranular corrosion tendency is higher in the quenched natural aging state. The research on A7N01 has achieved more results, further The research on the mechanism and regularity of the welding and the research on the measures to reduce stress corrosion and improve the fatigue strength of welded joints are also in progress.
(3) Structural materials to further reduce the weight of the vehicle.
The use of magnesium alloys is the direction to further reduce the weight of the vehicle. The strength of magnesium alloy is comparable to that of aluminum alloy, its specific gravity is 2/3 of that of aluminum alloy, and 1/4 of that of steel, while its elastic modulus is only 65% of that of aluminum alloy and 22% of that of steel, and its stiffness is worse. The research on the performance needs to be further in-depth. The same is true for composite material applications, so it is currently only used in some interior parts, and has not been used in the body and underframe. Titanium alloys have high specific strength and specific elastic modulus, and have been used in aerospace vehicles, but they are expensive.
Vehicles are basically welded construction, so the weldability of the selected material is very important. The weldability of several light alloys is shown in Table 1. In the table, A, B, C, and D represent the weldability level of the difficulty of welding. A is excellent—it is easy to weld; B is good—good welding can be accomplished by optimized methods and processes for various structures; C is Yes – it is more difficult to weld, and some additional processes or measures must be used to complete good welding; D is inferior – this method is not recommended for welding. At present, the welding of light alloys is mostly MIG welding or TIG welding. In the table, A, B, C and D are the weldability for MIG welding or TIG welding. The weldability of various metals is different for different welding methods: for example, 2000 series and Except for 2219, 7005, and 7039, the 7000 series aluminum alloys have C-level weldability for MIG welding or TIG welding, but all contact welding has A-level; Contact welding, brazing and soldering are all grade A weldability, the rest are grade B, of which 6070 pairs of brazing are grade C. Due to advances in welding methods and technologies, some difficult-to-weld materials have also become easier to weld.