Application of lightweight and variable cross-sect

2022-08-15
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Automobile lightweight and the application of variable cross-section thin plate

the connotation and approach of automobile lightweight

automobile lightweight can not only improve the speed, but also reduce fuel consumption, reduce exhaust emissions and improve safety (shorten braking distance and reduce collision inertia). However, car lightweight is not simply miniaturization. First of all, we should keep the original performance of the car unaffected. We should not only reduce the weight of the car, but also ensure the safety, crashworthiness, vibration resistance and comfort of the car. At the same time, the cost of the car itself should not be increased, so as not to cause economic pressure to customers

automotive lightweight technology includes the reasonable design of automotive structure. Mr. Jeff Hrivnak, global business manager of Solvay healthcare, explained two aspects of Solvay's latest advanced polymer solutions for medical devices and equipment and the use of lightweight materials. In terms of structural design, front wheel drive, high rigid structure and ultra light suspension structure can be used to achieve the purpose of lightweight. In terms of materials, materials can be replaced or new materials can be used to achieve the purpose of automotive lightweight. All the world's automobile manufacturing powers attach great importance to and compete to carry out various forms of automobile lightweight research programs and cooperative research

at the end of last century, the ultra light steel auto body (ULSAB), a coalition of 35 steel manufacturers from 18 countries, jointly formulated a new industry standard for automotive body design, with the goal of reducing the weight of automotive steel body and maintaining the same performance and cost of vehicles. Some research projects are also committed to proposing new lightweight design concepts, including research projects such as ultra light steel frame (ulsac) and ultra light steel suspension system (ulsas); Some scientific research departments and colleges and universities in China are also studying the manufacture of body parts with laser tailor welded blanks, the design and manufacture of laminated springs for body suspension systems with variable cross-section spring steel beams, in order to maintain the vibration resistance of the body, save materials and reduce costs

the main way to realize vehicle lightweight

according to statistics, the three major parts of vehicle body, chassis (including suspension system) and engine account for more than 65% of the total weight of a car. Among them, the weight of outer and inner body panels ranks first. Therefore, reducing the weight of the car body in white has a dual effect on reducing the power consumption of the engine and reducing the total weight of the car. To this end, we should first look for a breakthrough in the body in white manufacturing materials. Specifically, there are several schemes as follows:

1) use light materials with low density and high strength, such as aluminum magnesium light alloys, plastic polymer materials, ceramic materials, etc

2) use high-strength steel with the same density, the same elastic modulus and good process performance, and the section thickness is thin

3) use lightweight structural materials based on new material processing technology, such as continuously extruded variable cross-section profiles, metal matrix composite plates, laser welded plates, etc

schemes 1) and 2) achieve the purpose of vehicle lightweight by changing the type of body materials. Among them, aluminum alloy has a series of advantages such as high strength, corrosion resistance, good thermal stability and easy forming, and has been successfully applied in the manufacturing of body, chassis, suspension system, engine, wheel and other components. However, due to the high content of silicon and iron in aluminum alloy, its recycling and reuse has become a new problem, which affects the larger scale use of aluminum alloy. Magnesium is lighter than aluminum, and can be used as the best substitute for aluminum. With the development of automotive lightweight technology, more than 60 kinds of automotive parts have been made of magnesium alloy, and the consumption of magnesium materials in the world is increasing. However, on the earth, magnesium is just a scarce metal, and its price is expensive. Needless to say, coupled with the difficulties of magnesium alloy in processing and forming, its wide application is limited. Plastic polymers, such as continuous glass fiber and thermoplastic resin modified polypropylene composites (GMT), are the best lightweight materials for vehicles, and their density is only 1/5 of that of metals. The energy consumption of making auto parts with plastic is only 1/2 of that of steel. It also has the characteristics of easy processing, good formability, corrosion resistance and so on. At present, there are more than 800 parts using GMT materials on cars, mainly including engine hood, instrument panel frame, battery bracket, seat frame, car front-end module, bumper, luggage rack, spare wheel, fender, fan blade, engine chassis, roof liner, etc. In addition to being used to make parts, it is also expected to be applied to the whole body manufacturing, that is, the so-called "all plastic body". However, the properties of different kinds of plastic polymer materials vary greatly, and the strength, impact resistance, creep resistance and aging resistance of plastics are also their insurmountable weaknesses; And scheme 2) will also lead to an increase in the cost of the car body

in contrast, at least at present and for a long time to come, steel is still the best choice for automobile body manufacturing materials: steel not only has excellent performance price ratio, but also has long-term accumulated metallurgical technology and molding processing experience, so that it still occupies an unshakable dominant position in the automotive industry. Scheme 3) is from the point of view of material processing, which greatly improves the bearing performance, molding performance or other aspects of the steel plate material after special processing

variable cross-section sheet and its application in body manufacturing

there are two kinds of variable cross-section sheet used in body manufacturing, one is laser tailor welded blanks (TWB), and the other is continuous variable cross-section panels (TRB) obtained by flexible rolling production process

twb production process and its application

twb uses laser welding technology to weld metal sheets of different thickness, different surface coatings and even different raw materials together according to the strength and stiffness requirements of body design, and then stamping. In this way, the stamping engineer can customize an ideal splicing sheet for a certain body part in advance according to the actual stress and deformation of each part of the body, so as to save materials, reduce weight and improve the performance of body parts. In some automobile manufacturing powers, TWB has become a standard process in the automobile manufacturing industry, which is mainly used to manufacture body parts such as car body side frames, door inner panels, body chassis, motor spacing guide rails, middle column inner panels, mudguards and crash boxes

because TWB can be spliced arbitrarily according to needs, it has great flexibility, and can optimize the design of some body parts with the same thickness according to the concept of equal strength, and convert them from the original forging processing to stamping processing, which not only improves the processing efficiency, but also saves processing energy

trb is a continuous variable cross-section sheet obtained by a new rolling process flexible rolling technology. The flexible rolling technology is similar to the longitudinal rolling process in the traditional rolling processing method, but its biggest difference is that in the rolling process, the spacing of rolls can be adjusted and changed in real time, so that the rolled sheet has a customized variable cross-section shape along the rolling direction (see Figure 1). In the process of flexible rolling, the distance between rolls can be adjusted automatically and continuously through the real-time control of the rolling mill by computer, so as to realize the rolling from equal thickness coil to TRB coil. This requires that the actual stress and deformation of each part of the steel plate and the load-bearing condition of the whole car body in the subsequent forming and processing must be considered in advance when designing the car body, and the sheet metal profile conducive to the subsequent processing must be selected before rolling. In the current design field, there are quite mature and powerful cad/cam/cae software. This kind of optimization design can be realized by means of dfm/dfa (Design for manufacturing and design for assembly)

trb continuously changing cross-section provides the possibility to facilitate subsequent molding processing. For example, if the finite element analysis or digital simulation technology is used in advance to judge the parts of the body covering that may have cracks or high material fluidity in the stamping process, then a large sheet thickness can be pre allocated to a certain part of a part in the body design stage, so as to effectively avoid the occurrence of waste products in the tensile test space between the lower beam and the upper beam

comparison between TWB and TRB

weight reduction effect

the application of TWB and TRB is to achieve the purpose of vehicle lightweight. Based on the basic theory of bearing capacity of thin-walled beams in engineering mechanics, if the structural members made of equal thickness plates, TWB and TRB have the same stiffness, the weight reduction effect is shown in Figure 2

trb has excellent weight reduction effect due to its continuously changing section shape, that is, the body structural parts made of TRB material with the least weight can achieve the same stiffness as the other two kinds of plates

mechanical properties and application effects

due to the sudden change of TWB thickness and the influence of weld, and there must be some differences in material properties between the welding added metal material and the welded substrate, the hardness of TWB along the length direction will also change in a jumping manner, as shown in Figure 3 A). This will bring a very adverse impact on the subsequent molding processing. Moreover, the weld of TWB cannot be completely covered up even with any coating measures from the appearance, so it is not suitable to be used as the material of the outer covering parts of the car body, and it is generally only used to make the inner covering parts or supporting structural parts. In contrast, TRB has better mechanical properties, its hardness changes gently along the length direction, and has better molding properties without the hardness and stress peaks like TWB; The thickness of parts made of TRB can be continuously changed to meet the load-bearing requirements of various parts of the car body; Its surface change is continuous and smooth, so it can be used to make all kinds of body outer covering parts. See Figure 3B)

process complexity

twb can be spliced arbitrarily through laser welding process, which has great flexibility. However, because it uses the butt joint or lap joint of plates with different thickness, the thickness of the plate at the splice has a sudden change; In addition, local hardening will occur in and around the weld, which requires a heat treatment process to eliminate the hardening effect, thus increasing the complexity of the process. TRB relies on the flexible rolling process to form a continuous and slowly changing transition zone between plates of different thickness, and there is no weld problem of TWB. However, its disadvantage is that due to the limitation of rolling technology and rolling equipment, its thickness change can only occur in the initial rolling direction of the sheet; In addition, the existing rolling process cannot "roll" plates of different metal materials on a whole plate, that is, it is not as flexible as TWB

from the above comparative analysis, it can be seen that TWB and TRB have their own characteristics and shortcomings in weight reduction, mechanical properties, manufacturing process, etc. from the perspective of comprehensive indicators, TRB has greater advantages. Therefore, in order to achieve the purpose of automobile lightweight, it seems that there is a better scheme proposed: that is to combine TRB and TWB to make "tailored blanks" in the true sense, so as to obtain a new type of automobile lightweight materials

trb application examples

in some automotive manufacturing powers (such as Germany), TRB has begun to be put into practical application in the automotive industry. Figure 4 a) shows a TRB prototype part used in the "Mercedes Benz" E-class car. This "side frame" stamped by TRB

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