1 material
The new stainless steel car body adopts ultra-low carbon (C
The main materials of aluminum alloy car body are A 1-Mg series (5000 series), A 1-Mg-Si series (6000 series) and A 1-Zn-Mg series (7000 series). The characteristic of aluminum alloy car body is that the relative volume mass of aluminum is about 1/3 of that of ordinary steel, so as to reduce the weight of car body. The dead weight of aluminum alloy car body can generally reach 1/2 of that of ordinary steel car body. The weakness of aluminum alloy car body is that the longitudinal elastic modulus of aluminum is small, which is about 1/3 of ordinary steel, so the rigidity of car body tends to decrease. The rigidity of aluminum alloy car body is generally less than that of ordinary steel car body and stainless steel car body. This is an important reason for increasing the thickness of aluminum alloy car body and increasing the cross section of car body as much as possible to improve the bending stiffness of car body.
2 manufacturing process
The structure of stainless steel car body adopts plate-beam combined full-load and all-welded structure. Because the plates used are thin (0.4 ~ 1.2 mm for the body outer panel and 0.8~3 mm for the beams and columns), a large number of thin plates (generally 0.8 ~ 3 mm) must be rolled into reinforced (rigid) profiles and connected with the outer panel by spot welding to form a cavity, so as to improve the rigidity and strength of the outer panel. This is one of the structural characteristics of stainless steel cars. In order not to reduce the strength and deformation of the plate, spot welding should be used as far as possible. Especially for materials with high strength grade, any form of welding is not allowed. Plane or three-dimensional connection and spot welding are adopted between beams and columns. Plate splicing adopts lap seam welding. Replacing arc welding with spot welding is another feature and technical key of stainless steel car. In recent years, due to the development of new technology, laser welding has been used instead of spot welding.
Aluminum alloy car bodies can be divided into plate beams, large open profiles, large hollow closed profiles and their combinations.
At present, the widely used structure of aluminum alloy car body is large truss hollow profile assembly welding (usually automatic arc welding). When manufacturing large hollow profiles to assemble and weld the car body, it is only necessary to continuously and automatically butt weld the profiles along the length of the car body. Due to the small number of car body parts, less welding workload and easy automation, the car body manufacturing cost is greatly reduced and the product quality is improved. In terms of sound insulation, some panels and ribs are also pasted with shock-proof and sound-absorbing materials, or filled with (semi-filled) polyurethane foam (foamed when the profile is extruded), which greatly improves the shock-proof and sound-insulation effect.
As far as manufacturing technology is concerned, aluminum alloy car body is simpler than stainless steel car body.
3 Lightweight
The specific gravity of aluminum alloy is 2.7 1 g/cm3, and that of stainless steel is 7.85 g/cm3. Although the specific gravity of aluminum alloy is much smaller than that of stainless steel, its tensile strength is poor, ranging from 270 to 310 MPa, while stainless steel can reach 690 to 930 MPa, which is 2 to 3 times that of aluminum alloy. Moreover, the rigidity of aluminum alloy is low, and its elastic modulus is only 0.7 1E 10N/mm2, while that of stainless steel is 2.06E 10N/mm2, which is three times that of aluminum alloy. Therefore, in order to ensure sufficient bearing strength and rigidity of subway vehicles, aluminum alloy vehicles must adopt large hollow profiles and their assemblies. In order to improve the section coefficient of aluminum alloy car body and increase the bending strength, it is necessary to increase the plate thickness, with a wall thickness of 2 ~ 6.5 mm, while stainless steel car body can adopt a fully welded structure with a plate-beam combination, with a plate thickness of 0.8 ~ 3 mm and an outer plate thickness of 0.6 ~ 1.2 mm, so as to realize the lightweight of the car body (see table 1). The stainless steel car body basically adopts spot welding or laser welding technology, and only a few parts adopt arc welding, with little welding filler; The aluminum alloy car body completely adopts arc welding technology, and the weld is filled with a lot of welding materials, which increases the weight of the car body.
Stainless steel body is generally uncoated body, without the weight of coating material; The surface of aluminum alloy car body is coated with polyester putty, primer, topcoat and other coating materials, which increases the weight of car body.
Generally speaking, the weight of lightweight stainless steel car body relative to aluminum alloy car body is 0.5 ~ 1t/ car.
4 Appearance quality
Although the stainless steel car body does not need anti-corrosion protection or painting in the manufacturing process, in order to improve the decoration, the plate is decorated with lines or pear skins. Vehicle manufacturers can do appropriate decoration, or decorate with colored film, or spray water-based paint. Because the surface decoration of the car body is mostly carried by raw materials, it should be protected by film during the processing before welding. Because the external wall panel is very thin (generally 1.5 mm), it is very bright and allergic to unevenness. As long as there is unevenness of 0.2 mm, it will be uncomfortable to the naked eye after reflection and refraction; If the spot welding process is used for welding, the spot welding trace cannot be eliminated; If the laser welding process is used for welding, the solder joint is beautiful and elegant.
The corrosion resistance of aluminum alloy car body is poor, but the hollow aluminum profile is straight, and different decorations and colors can be selected according to user's requirements. Therefore, it gives people a solemn and beautiful feeling and is easily accepted by the majority of passengers. Another unsatisfactory aspect of aluminum alloy body is its poor corrosion resistance, and it can't be painted like stainless steel. Although there are some aluminum alloy car bodies that are not painted, after a period of use, due to corrosive conditions in the atmosphere (such as water, detergents, contact with metal dust in the use environment), the surface will always appear corrosion, pitting, discoloration and other phenomena, which will affect the appearance, so most cars are painted.
In terms of appearance quality, aluminum alloy looks good, but stainless steel can also achieve the appearance effect of aluminum alloy after painting through the latest technology and water-based paint.
5 security
The yield strength of aluminum alloy material is 200~260 MPa, and that of stainless steel material is 345MPa-685 MPa. The strength of stainless steel material is obviously higher than that of aluminum alloy material, and its bearing capacity is stronger.
The tensile strength of aluminum alloy is 270 ~ 3 10 MPa, and the ratio of yield strength to tensile strength (referred to as yield strength ratio) is only about 0.84. In the event of an accident, aluminum alloy will break soon after yielding and deformation. The tensile strength of stainless steel is 690~930 MPa, and the yield ratio is 0.5 ~ 0.74. In the process from yield deformation to fracture, the material itself will produce greater plastic deformation and can absorb more impact energy.
The stainless steel car body mainly adopts spot welding technology. In the case of accidental impact, the structure will be folded and deformed like an accordion, and the solder joints will be destroyed one after another along the stress direction. When the aluminum alloy car body adopts arc welding technology, it will tear along the weak area of weld or parent metal when it is accidentally hit, which is the main reason why the aluminum alloy car body is more likely to be damaged, and the stainless steel car body is only partially deformed when it is accidentally hit.
Melting point of stainless steel 1500℃, melting point of aluminum alloy 660℃, softening and deformation above 300℃. The heat resistance of aluminum alloy is only 44% of that of stainless steel. In the event of a serious fire, the aluminum alloy car will melt quickly, which will bring disastrous consequences.
As far as safety is concerned, stainless steel car body is far superior to aluminum alloy car body.
6 conclusion
To sum up, stainless steel car body and aluminum alloy car body have their own advantages in the selection of subway vehicles. The above problems should be considered comprehensively in vehicle selection in a city, especially the influence of body selection on local industrial chain and local cultural characteristics. Especially in some covered depots, it is necessary to consider the advantages and disadvantages of aluminum alloy and stainless steel vehicles from the perspective of environment and humanistic care, and combine factors such as economy, safety and appearance to unify multiple lines in the same city as far as possible in car body selection, so as to facilitate network resources.