Present situation and Prospect of Automotive Lightweight Technology


Lightweight: the necessity of green manufacturing

Up to now, the global car ownership is approaching 1 billion 200 million, and the energy consumption and emission problems caused by automobiles are becoming increasingly prominent. Low emission and low fuel consumption have become indispensable to the long-term development of conservation oriented society. Automobile lightweight is the key technology to achieve this goal. Chinese and foreign car manufacturers have also developed a series of lightweight new products.

Relevant literature data show that the quality of vehicle car if the reduction in 10%, fuel utilization efficiency can be increased by 6% ~ 8% in the original basis; if the rolling resistance is reduced by 10%, fuel efficiency can be increased by 3%; if the transmission efficiency of automobile axle, transmission mechanism of the 10% increase in fuel utilization efficiency can be increased by 7%. It is obvious that the prominent advantage of the light weight reduction of automobiles is to reduce fuel consumption and reduce pollutant emissions.

Vehicle lightweight implementation approach

Automotive lightweight technology can be divided into three main aspects: structural optimization design, lightweight materials application and advanced manufacturing technology. Among them, the structure optimization design including optimization, shape optimization, topology optimization and multidisciplinary design optimization of automobile structure size; lightweight materials including high strength steel, Aluminum Alloy, magnesium alloy, titanium alloy, carbon fiber, plastics and composite materials; advanced manufacturing process includes, hydroforming and laser welding etc..

1. reasonable structure design

In 1970s, due to the progress of computational mechanics and computer technology, the United States, general motors and other companies pioneered the use of finite element method, the purpose is to explore car design. After 1990, large computer aided engineering software (CAE) gradually matured and began to be applied in the development of automotive components and vehicles, and showed an increasing trend year by year. After nearly ten years of development, CAE has been widely used in the design of automotive structures, and has also been applied to the design of automotive components.

(1) dimensional optimization. There are design parameters for structural dimension optimization. The shape and size of automobile parts are usually variables to satisfy the stiffness, vibration, strength and energy absorption under various working conditions. For the problems of linear statics and linear vibration in automobile design, the traditional numerical optimization algorithm can be used to design the light weight directly. In the automobile design will face a variety of problems, linear elastic problem is designed to face first, therefore, to design methods based on linear elastic size optimization has been widely used in the lightweight design, including the optimization of car service parts and losing the weight of the car.

Automotive design not only considers both vibration and linear static problems, but also considers nonlinear problems. Because the automobile collision process is very complex, it covers the high-speed movement and collision, large deformation and nonlinear problems, it is often difficult to get information about the sensitivity of collision response, so there is no good way to direct application of traditional optimization algorithm. In order to solve this problem, the car designers used approximate model method, or the crash occurred as a process, load each time node in the process of using the linear equivalent load, followed by the linear load as the basis to optimize the design of lightweight.

At present, the typical approximate methods are response surface function (response, surfacemethod, RSM), radial basis function (radial, basis, function, RBF) and Kriging model method. Taking the approximate model method as the support, the vehicle collision problem can be analyzed and the lightweight and safety design of the vehicle can be realized.

(2) shape optimization. The shape optimization that is appropriate to change the shape of the car, the purpose is to make the structure more uniform stress, concrete measures of automobile structure or external local shape is optimized, so that the material can play a greater potential. Shape optimization method can be used to parameterize the geometry of a structure with regular geometric shapes, so that the shape optimization can be changed to the dimension optimization problem. But from the structure of the car itself, many of which are irregular geometric shape, for it is difficult to use the parametric method to describe the car geometry, this is a difficult problem to solve. At this stage, people have come up with a solution to this problem: adopting the parametric optimization method, the advantage of this method is that it does not need to consider the dimension parameters.

(3) topology optimization. To optimize the topology optimization for the object is material distribution prior to have specified the design space, and then the topology optimization algorithm for optimization, automatic transmission path optimization of power, in order to achieve the goal of saving materials as much as possible. Topology optimization has been widely recognized as one of the most practical methods, and topology optimization has its limitations. It is mainly used in the conceptual design phase of automobile development. Many kinds of topology optimization methods have appeared, and the variable density method developed by researchers such as Bendsoe is one of the most important topological optimization methods. However, the variable density method also has its limitations. Its calculation results only provide the main material distribution of the automobile structure, and need the designer to do the design again. Therefore, in order to get the effect of obvious quality reduction feasibility, structural design, designers can work hard, that is, to optimize the use of topology optimization, shape optimization and size optimization and other optimization methods.

(4) multidisciplinary design optimization. Automotive lightweight is not a simple problem, it involves many subjects of comprehensive design optimization. For example, the lightweight design of a car body can not be considered only one aspect, including the strength, stiffness, impact, safety, comfort, fatigue and other disciplines. However, because of the existing problems, they can not be considered comprehensively and comprehensively during an optimization. In lightweight design, developers have found a feasible approach, that is, "decomposition coordination" scheme, which is a multidisciplinary optimization approach. This method is called multilevel optimization method, the typical method of parallel subspace method (concurrent subspaceoptimization CSSO), collaborative optimization (collaborative optimization CO), the target pass method (analyticaltarget cascading, ATC).

2. application of new materials

The weight reduction of car body is a big way for car enterprises to make light weight:

(1) adopt high strength steel. Compared with other materials, the high strength steel can achieve the effect of reducing the thickness of the cross section under the same density, the same elastic modulus and good technological performance. Because of the limitations of lightweight materials such as aluminum and magnesium alloys, the number of steel used in automobile production is still in the leading position at this stage. In the future, the most widely used material is still high strength steel. Because of its own advantages, the high strength steel can realize the thin-walled structure of the automobile, and the high strength steel can reduce the quality of the whole car. According to JDDRG statistics, in 1980 the proportion of high strength steel is 8.7%, after a lapse of 12 years after 1992 has risen to 23.3%, now use high strength steel ULSABAVC concept car developed in the project has accounted for 97%. The Swedish SSAB company has developed ultra high strength cold rolled steel sheet material yield strength up to 00MPa; the Posco Co Ltd has successfully developed the automobile lightweight steel plate forming good; China has also made some achievements in the development of ultra-fine grain steel, Baosteel, Panzhihua, Zhujiang Steel and Wuhan Iron and steel has developed a series of new fine crystallization low carbon high strength steel. But at present our country still lags behind the international advanced level in this respect.

(2) adopt aluminium alloy. International Aluminum Association statistics show that in 2006, the average amount of aluminum alloy in a single car was 121kg, 150kg in 2015, and it is expected to reach 180K g in 2020. At this stage, the main part of automotive aluminum alloy is cast aluminum, which accounts for about 80% of the amount of aluminum used in automobiles. The mechanical properties of wrought aluminum are better. It has also been used in automobiles, such as horizontal steering fork and forged aluminum wheels, which have been applied in Audi, A8, A4 and other models. The deformation of aluminum, aluminum Aluminum Alloy in 1980s from the front fender, body hair cover plate and the top cover application start, then it has been applied to the trunk lid, doors, floor structure, bumper, heat exchanger and aluminum body etc.. In addition, Aluminum Alloy foam is a kind of ultra light porous materials, is Aluminum Alloy the foam obtained, with vibration isolation, good energy absorption, sound absorption characteristics, can also be used as lightweight materials in automotive.

(3) magnesium alloy. In automotive lightweight materials, magnesium alloy has been recognized as the most promising material. As early as 80 years ago, the VW Beetle was made of magnesium alloy material. In 1982, with the decrease of magnesium alloy price and the improvement of corrosion resistance, Ford automobile company applies magnesium alloy to various parts such as transmission, clutch, brake system and steering column again. Major special research on magnesium alloy in our country "fifteen plan", in the part of the model has been used for applications, such as project achievements can be used in the transmission direction of magnesium alloy wheel manufacturing center frame etc.. The application of the magnesium casting on the car is now more extensive. There are nearly 70 kinds of alloy parts used in automobiles, such as transmission case, steering wheel frame and so on.

(4) titanium alloy. With the 90s sports car, luxury cars, car racing market size has expanded every year, titanium alloy parts have been rapid development. Titanium alloy parts are mainly used as follows: engine connecting rod, valve spring seat, engine valve, titanium alloy spring, exhaust system and silencer, turbocharger, car body frame, etc.. But its high cost restricts the application of titanium alloy in automobile, and the forming and welding problem of titanium alloy have not been solved at the same time.

(5) carbon fiber composite. As early as 1992, General Motors Corporation developed the concept of ultra light car, the body used carbon fiber composite material, the overall quality of the body is only 191kg. In order to further reduce the quality of the car, Ford developed a carbon fiber composite trunk lid for fuel cell vehicles in 2008. The development of new energy vehicles and a large tow carbon fiber manufacturers in Germany and Japan three high carbon fiber manufacturers to develop the full CFRP car. Domestic car prices have begun to carry out research on carbon fiber composite materials, such as SAIC technology center in cooperation with the Tongji University, to explore the feasibility of application of carbon fiber composite material in automobile, especially in body outer panel.

(6) plastic. Plastics are the most widely used non-metallic materials on automobiles. World car in 2000, the average amount of plastic per vehicle has reached 105kg, in the total quality of the car accounted for about 8% to 12%. In recent years, the proportion of plastic parts in automobile in China has reached 12% to 18%. In recent years, the use of plastics around the engine and on the parts of the body panels has continued to increase, accounting for 10% to 15% of the total mass of the car, especially in the United States, Europe and germany. For example, the Mercedes Benz Smart car, Elise car and lotus Renault Espace used plastic body; the 1998 CCV concept developed by Dai. Four pieces of thermoplastic body panels, count plate connectors, white car body weight is only 95kg in the whole body of thermoplastic created a milestone, headlamps and taillight glass will also abandon the traditional natural materials, instead of using general thermosetting plastics.

(7) other lightweight materials. Fine ceramic materials are third kinds of materials developed after metal and plastic. Their development history is only 20 short years. Ceramic materials not only to weight loss, because of its excellent corrosion resistance, heat resistance and wear resistance for automotive engine heat exchanger and the combustion chamber and other parts, increase power, greatly reducing fuel consumption. Honeycomb sandwich material is a new material widely used in aircraft. Its main characteristics are high specific strength, high rigidity and low density. At present, there are few examples of applications in the automobile, but the continuous application research, I believe will be more applications in the future.

3. lightweight manufacturing process

In addition to the above mentioned lightweight materials and structures to achieve lightweight vehicles, manufacturing process and also have been applied, such as body structure for connecting adhesive and glue welding process, there are some other new forming and connection technology, such as laser welding, laser welding and hydraulic forming etc..

(1) hydraulic forming. German Benz and BMW is the earliest in the car with hydraulic forming part of the car, Mercedes Benz set up shop hydroforming in 1993, "ULSAB plan" was launched in 1994 to promote the development of hydroforming technology. The United States related institutions and enterprises have joined forces to form a "hydraulic forming technology research and Development Association" to speed up the application and development of the technology. In 2000, China FAW Group and the combination of Harbin Institute of Technology developed the first domestic hydraulic forming equipment, the equipment can not produce large parts, can only be used to produce small shape parts, such as the Audi A6 car rear axle arm. China FAW in 2001 explored the application of hydraulic forming technology to produce M6 sedan sub frame. At present, hydraulic forming products have been widely used in automobiles. They have been used in more than 50% of automobile chassis.

(2) laser welding. Laser welding can be made of different materials, different thickness, different stamping properties, different strength and different surface treatment of the slab together welding, used for large panels pressing. Use this stitching slab not only can make the reasonable stamping strength and can make the quality loss, and can make the parts and moulds to reduce the number of dimensions to improve the accuracy, improve the structural strength and stiffness; and to improve the efficiency of design and development, shorten the cycle, avoid the waste of materials, to achieve the purpose of reducing the cost of. As early as 1985, the German Volkswagen welded laser welded blanks on the chassis of Audi models. At present, almost all enterprises have applied laser welding technology. Another important application of laser welding in automobile manufacturing is the laser welded body frame. Laser welding is a continuous connection, which can improve the strength and stiffness of the body. At the same time, the use of laser welding technology can reduce the use of steel plate, and reduce the quality of the body. For example, the German Volkswagen in 2004, the fifth generation Golf body, laser welding reached 70m.

Automotive Lightweight Technology Outlook

Future automotive lightweight should focus primarily on the development of the following technologies:

(1) structural optimization design technology. We should further research and improvement of multi discipline, automobile structure multi-objective optimization design method and topology optimization method and topology optimization, shape optimization and size optimization and other optimization methods combined structure optimization scheme feasibility of hope, in order to achieve significant weight loss of car.

(2) application of lightweight materials. To expand the wrought magnesium alloy, titanium alloy, plastic and fiber reinforced composite material in automobile; research on theory and method of design process and the corresponding mixed structure of various materials, using appropriate materials in various parts of the car, make the best use of materials, optimize material and parts to design the theory and method of the integration of the multi material.

(3) advanced manufacturing process: to promote the application of hydroforming and laser welding in automobile manufacturing, and further develop the application technology of hot forming process and variable thickness plate.

In addition, the total quality of all parts and components of automobile accounts for 3/4 of the whole vehicle quality, and the lightweight technology research of automotive components must be paid attention to. In a word, we should give full play to the advantages of different lightweight technologies, and realize the systematization and integration of automotive lightweight technology.