1. Selection of body materials
Lightweight vehicle body materials and structures are the requirements of almost all vehicles. Reducing the weight of the vehicle body can increase the load or improve the running speed. The use of light alloys and composite materials is the main way to reduce the weight of vehicles. The aerospace sector has widely used high-strength Aluminum alloys, titanium alloys and composites. Recently, high-speed trains have all used aluminum alloy all-welded structures, and automobiles are also using some aluminum alloy castings, forgings and composite parts.
(1) Classification and representation of aluminum alloys. As shown in Figure 1, according to the aluminum content of the aluminum alloy, it can be divided into 4 zones, which are produced by pressure processing and casting respectively. Zone 1 is deformed aluminum alloy, zone 4 is cast aluminum alloy, and zone 1 is divided into non-heat treatment. Strengthening class 2 and heat-treatable strengthening class 3; according to the alloy composition and composition, they are divided into many categories, which are respectively represented by alloy grades, and the post-processing state is also indicated in the aluminum alloy profile. For example, 6061T6 is the 6000 series artificial aging aluminum alloy.
(2) Composition and properties of aluminum alloy components. Different components of aluminum alloys have different structures. For example, the 2-zone aluminum alloy is a aluminum, which cannot be strengthened by heat treatment, and the 3-zone aluminum alloy is a+B aluminum. Similar to the same series of aluminum alloys, due to different compositions, the properties will be different. Even if the alloys with the same composition, due to different post-treatment states, the properties will be different. Take 6000 series aluminum alloys as an example (see Table 1 for its mechanical properties and chemical composition). ). The first two aluminum alloys in the table, the last two digits of the model are the same, indicating the same composition, but the heat treatment state T5 is hot working natural aging (T4 is solid solution natural aging), T6 is solid solution artificial aging, the two are due to different heat treatment states. The alloy structure is changed, and thus different properties are obtained. Aluminum alloys are generally not just binary alloys, but ternary or quaternary alloys to improve their mechanical properties and processing and heat treatment properties, such as aluminum-magnesium-zinc, aluminum-magnesium-zinc-copper, etc., and some improvements can also be added. Microelements such as titanium, zirconium, rare earth elements, etc. For example, the last three kinds of aluminum alloys in Table 1 are all 6000 series aluminum alloys. The main elements increase the content of magnesium and silicon or further increase the content of magnesium, silicon and manganese. Although they are all T6 (solid solution artificial aging), their performance has changed greatly. Zinc is added to the 70 series, and copper is added to the 20 series, which greatly improves the material strength and becomes a high-strength aluminum alloy or an ultra-high-strength aluminum alloy.
2. Examples of lightweight car body
(1) Examples of aluminum lightweighting for high-speed trains. The properties and lightweight effects of several high-speed train body structural materials are shown in Table 2. It can be seen from the table that the mechanical properties of different materials and the weight reduction effect of the car body are different. Due to the low density and high specific strength (strength/density) of lightweight materials, but low elastic modulus and poor stiffness, they must be extruded into hollow profiles to improve bending and torsional stiffness. The use of wide-width hollow profiles extruded into the same length of the train greatly reduces the workload of processing and welding. Although the material cost is greatly increased, the processing cost is reduced, and many new production processes and equipment for high-speed trains have occurred. Revolutionary change. It is convenient to use new welding technology and automation without increasing the total manufacturing cost, and at the same time avoids direct force transmission welds, so that the bearing capacity of the welded structure is significantly improved with the mechanical behavior of the original alloy structure of ordinary trains, and the quality of the whole vehicle is greatly reduced.
(2) Examples of lightweight automobile structures. Lightweight automobile body is also an inevitable requirement for development, and the way is to use aluminum alloy and composite materials. At present, automobile production is still dominated by fuel vehicles. The complex shape of the car body uses less aluminum alloy structure, which is mainly used for the casting of engines and components. Aluminum alloy and rolled aluminum alloy wheels. Taking several major automobile producing countries (the United States, Japan, and Germany) as examples, aluminum alloy castings account for 70% to 80%, rolled parts account for 18% to 28%, and forgings account for 0.7% to 3.2%. Automotive aluminum alloy wheels have been widely used, and their usage and weight reduction effects are shown in Table 3. It can be seen from the table that only the aluminum alloy wheel load can reduce the weight of the small car by 10kg, and the weight of the large car can be reduced by 100~185kg. In general, aluminum alloys for vehicles can reduce weight by 40%. For example, for every 100kg weight loss of a car, running 100km can save 0.5~0.8L of fuel. If a car uses 50kg of aluminum alloy, it can save 85L of fuel every year.
If the vehicle weight is reduced by 50%, the CO2 emission can be reduced by 13% from the reduction of fuel consumption. The table shows the analysis of automobile production cost, which shows that the material cost accounts for 53% of the total cost, and the weight reduction material of the car body is also the main factor, so the selection of materials is particularly important. If electric vehicles can be developed, the power plant and transmission mechanism can be simplified, the self-weight can be reduced, and the transmission efficiency and energy saving and emission reduction can be greatly improved.
3. Extruded hollow profiles to solve stiffness problems
Generally, sheets or bars are rolled, but the most used are hollow profiles, which are formed by extrusion.
(1) The closed hollow structure is replaced by an opening or a rib structure. The characteristic of the closed hollow structure is that the height can be increased to improve the bending rigidity, especially the torsional rigidity is greatly improved compared with the open structure. As shown in Figure 2, the simplest profiles are square tubes, round tubes or elliptical tubes, and can also be made into porous, wide, straight or curved hollow profiles, and can also be welded into a honeycomb structure by contact welding as required. Or foam aluminum or foam magnesium sandwich panels are made by aluminum processing plants.
(2) Examples of shaped hollow profiles. A shaped hollow structure extruded profile is shown in Figure 3. Direct or multiple pieces can be connected with longitudinal seam as a whole application.
(2) Calculation of strength and stiffness of materials and structures. The strength and stiffness calculation methods of materials and structures are shown in Figure 4.
(3) Calculation of shear strength and torsional stiffness of profiles. The calculation methods of shear strength and torsional stiffness of profiles are shown in Figure 5. It can be seen from the calculation formula in the figure that the bending stiffness and torsional stiffness of hollow profiles are much larger than those of general profiles.
(5) Comparison of torsional stiffness between hollow closed profiles and open profiles. See Figure 6 for calculation examples of open and closed square tubes. It can be seen from the figure that when several components have the same area, the force and tensile strength are the same, and the bending stiffness of the hollow profile increases a lot due to the increase in height. Two kinds of hollow profiles with the same shape and area are hundreds of times worse in torsional stiffness because one is open and the other is closed.
4. Torsional stiffness of the body structure
Different car body structure designs will get different torsional stiffness. Here are two examples of car body structures.
(1) Torsional stiffness of automobile structure. The torsional stiffness of different vehicle structures is shown in Figure 7. The torsional rigidity of the monolithic car is the largest, and the torsional rigidity of the canopy car or truck with only the underframe structure under force is the lowest.
(2) High-speed train structure. The structure of foreign high-speed trains is shown in Figure 8. Due to the light weight of the high-speed train body, most of the aluminum alloy welded structures are used. Due to the low modulus of elasticity of aluminum alloys, the stiffness is poor, so large extruded profiles with ribs, especially large hollow extruded profiles, must be used. In the early days of foreign countries, the car body structure with the outer skin of the aviation skeleton and the large extruded profile car body structure are shown in Figure 8(a) and (b). It is more reasonable to see the structure of large hollow extruded profiles as shown in Figure 8(c), which is currently used in high-speed trains produced in China and has high torsional stiffness. If the weight is to be further reduced, the bulk wall and ceiling of fiber-welded aluminum alloy honeycomb structure or foamed aluminum can be used as shown in Figure 8(d), which will further reduce the welding and assembly workload. In particular, the brazed aluminum alloy honeycomb structure or foam aluminum and fiber composite materials for the head structure have unique advantages.