At the "2019 (Fourth) New Energy Vehicle Lightweight Technology Symposium", Di Emei, a technical expert of China's carbon fiber material automotive component leader SZHT Carbon Fiber Components Technology Group, provided insights on automotive carbon fiber composite materials:
Everyone should know something about carbon fiber. The frequency of entering the field of vision in the past two years is very high. Carbon fiber has a very long history of application in automobiles, but because our country ’s new industrial development is a little later than abroad, and our domestic sports car culture is relatively weak, it ’s generally not easy to understand some high-end sports cars. Materials. The car that is closer to us is a passenger car. Now carbon fiber is slowly approaching our ordinary people. It is intended to be used in mass production cars, but why can carbon fiber be used in cars? Share it with you today.
First of all, let's talk about racing, that is, speed and passion. It is to achieve the highest maneuverability of this vehicle under the safety index. Everyone knows that if equipped with the same engine, its output power is constant. The weight of the car is Its maneuverability will have a particularly large impact. The lighter the car, the faster it will be. So far, carbon fiber should be said to be an imperial material for racing cars. It must be used because it can achieve the required dual performance of power and safety. It is also the comprehensive optimal choice. At present, only carbon fiber composite materials can Meet the requirements of racing.
Carbon fiber, in terms of strength and modulus, is compared with the performance of traditional basic materials. Its tensile strength can reach 2000Mpa, high-strength steel can reach 700Mpa, better can reach 1000Mpa, and aluminum alloy can reach 360Mpa. Its tensile modulus is similar to that of high-strength steel, which is three times higher than that of aluminum alloy. However, its advantages are also obvious. Its density is very low, which has a great effect on the lightweight and technical performance of this car. Big advantage.
The second is composite material, which is also a widely used material. The fatigue load coefficient of traditional base materials may only reach about 50% of the design load, but composite materials can reach 70% to 80% of the design load, and composite materials The crack propagation is very slow, and the basis of any expansion is completely different, mainly depending on the structure of the material itself. Like Lego bricks, composite materials have a very high degree of freedom in modeling and many forming processes, which can produce the entire passenger compartment structure of a racing car at one time, which greatly improves the safety performance of the passenger compartment. In addition, the failure mode of its impact performance is not the same. Carbon fiber composite material is a combination of fiber and resin, and its impact performance mainly depends on fiber fracture and layer breaking.
It should be said that sports cars originate from racing cars. At present, carbon fiber is only used in top sports cars or limited edition sports cars. The main reason for using carbon fiber materials is because carbon fiber has more molding methods. Its production investment, compared to The investment in the production of parts for traditional cars is much less. Second, the user experience of carbon fiber is good, and its appearance is very beautiful. In addition, carbon fiber composite materials can improve comfort, because the lower the center of gravity of a sports car, the more stable it is in handling. After carbon fiber reduces the weight of the car, the entire center of gravity of the car moves down.
The reason why today's mass-produced cars also need to raise the weight of the car to such a high level is also urgency, which is also derived from our country's environmental protection requirements and energy security considerations. The biggest problem with new energy vehicles at present is the mileage, especially electric vehicles. The weight reduction of the entire vehicle by 10% can bring many benefits.
From this we think of how to achieve weight reduction? At present, battery capacity, material weight reduction, wind resistance coefficient, and other methods all involve cost issues. For the present, one of the most cost-effective ways for car companies to reduce vehicle weight is to start with materials. From metal materials to composite materials to reduce the weight of the entire vehicle. The application of carbon fiber in mass production vehicles reduces costs and improves production efficiency.
At present, the application methods of carbon fiber in mass production vehicles are mainly all-carbon fiber composite materials components, mixed use of multiple materials, carbon fiber connectors, integration of various types of carbon fiber parts, and the use of interior and exterior decoration. The application of carbon fiber in mass production vehicles needs to improve the following four aspects: First, the cost of carbon fiber itself is relatively high, and the use of multiple materials can achieve the goal of weight reduction while the cost is relatively increased. Second, independent technological innovation. I would like to emphasize that for mass-produced cars, because of the large production batches and process limitations of composite materials, it may not be easy to meet the quality requirements of mass-produced cars at this time. This requires us to further To develop, it can meet the technological requirements of mass production vehicle production. The third is to reduce costs. At present, carbon fiber is also working on low cost research. Fourth, the degree of manual participation of composite materials in the production process is still very high, so how to increase the automation level of the entire production line is also a bottleneck for the mass application of composite materials in automobile plants.
In the BMW A3 or A8, the entire body compartment is completely replaced with composite materials. The only difference between it and racing cars and top sports cars is that its body is made of multiple composite parts. . Later, the BMW 7 Series changed its approach and began to make basic reinforcements. Carbon fiber has a high degree of structural design freedom, which is also one of its application advantages. It can integrate complex structural parts and reduce costs from another direction. Secondly, aluminum alloy has its own unique connection method, but after the combination of carbon fiber and aluminum alloy, it is a very good connector; in addition, the application of carbon fiber in interior decoration.
Everyone knows that the future car K50 should be said to be a classic case of mass production of carbon fiber cars in China! We have accumulated a lot of experience in the process of supplying carbon fiber parts and components to the future cars. I hope to share with you here. We have refined some experience throughout the entire production process, and at the same time we have formed a lot of corporate standards. At present, Huate Carbon Fiber is mainly dedicated to high-end composite materials such as prepreg molding and autoclave.
We have also made in-depth technical accumulation for the application of carbon fiber in the body of BMW. In fact, we also want to explore. Is this carbon fiber process route of BMW correct in terms of performance? We also did process verification for this process. In fact, in my personal opinion, this carbon fiber process route of BMW is feasible. The main problem is that the molding of composite materials is a chemical reaction, which includes the combination of two components: resin and fiber, which improves its integrity. To achieve its design performance. The biggest problem with this process route is that the applicability is too narrow, and the molding time of complex parts is still very long. Everyone knows that the side wall of the BMW A3 is spliced together by many carbon fiber composite material preforms, and then put into a mold to cure and shape, but this process route is very easy to use.
SZHT carbon fiber also has its own experience in the application of composite materials from structural design to manufacturing. The first is cost. First, the cost of raw materials is already very high, so our cost control in the entire process from raw materials to manufacturing is very strict. At present, the coating cost is the largest in the total cost ratio. One direction we are committed to reducing costs is curing molding. While doing process optimization, we have developed a set of production process routes suitable for mass production. Compared with the route, it has been greatly optimized and improved.
The yield of carbon fiber is relatively low, but one of the benefits of composite materials is repairability. Some molding defects can actually meet the requirements of use through post-repair. The cost of carbon fiber preforming is mainly due to labor costs. At present, preforming is the most needed link for automated production. We hope that everyone can work together to increase the level of automation in the preforming of composite materials.
A composite material is not so much a simple material as it is a structure in itself, which belongs to the category of near-net-shape. Because of the combination of fibers and resins, fibers have different forms of existence, simply because the production of fibers is unidirectional, and it can be made by textiles to make fabrics. Fabrics are divided into plain weave fabrics, twill weave fabrics, and a variety of items. Fiber types are also divided into long fibers, short-term fibers, powders, etc. Therefore, composite materials are a more complex structure because of their various combinations. The continuous fiber layer and ply structure need to pay attention to during the design process. It is different from the traditional structure in the structural design. It is a laminated structure and requires different ply forms and ply angles. It is actually It's a process of building blocks.
In the whole structure design and manufacturing process, the first step is material selection. According to the technical requirements of this part, the fiber reinforcement and carbon fiber have poor impact resistance, so it must be carried out with other high-performance materials. Combination to improve impact resistance. From the perspective of cost, we must pay special attention to different auto parts. What kind of fiber to use, what kind of performance to achieve, and what kind of testing goals to achieve, we have to consider a comprehensive cost; we do outer coverings. There is also a need to meet the user experience requirements. Second, the carbon fiber composite material has another problem. Its coating is completely different from that of traditional metals. Its coating cost accounts for about 20% of the entire manufacturing cost. One way to reduce the overall cost is to improve its surface quality, which is the only thing we can do.
Therefore, the choice of raw materials must consider the overall cost of the entire production link and the design link. There are also a variety of ways to reinforce the composite fiber and matrix resin, which results in different processes. The general process is mainly divided into the wet process of fiber and resin in the curing process, and the composite material will The resin is made into an intermediate product, and the prepreg is then manufactured in the later stage. This has resulted in a variety of molding methods, different equipment requirements, different labor cost requirements, different quality control, and different material properties, which have resulted in products. The depth is different, so in the production process of composite materials, we must choose the process according to the goals of the car company.
Composite products have their own unique features in structural design. Although composite materials have a relatively high degree of freedom in modeling, from the perspective of production and overall qualification, we have very high requirements for design. Based on the characteristics of composite materials, I summarize the following points. Its structural design should simplify structural features, integrate parts as much as possible, and minimize the weakening of some rib structures like traditional sheet metal. By varying the thickness of the design, it can be made into a sandwich structure or a variety of types. Structural formula to increase its strength.
Another advantage of composite materials is that it is very inclusive. It can be integrally formed with other metal materials during the molding process. That is to say, the composite material can be embedded in the middle of the production process and can be carried out. With variable thickness design, it can also be manufactured three-dimensionally from a single part, such as intermediate foam sandwiches, which reduces the subsequent steps such as the bonding and strengthening method. We also need to consider maintainability when designing the structural design of composite materials. In general, as far as possible, the mechanical connection of the hybrid connection method is used where the load is large. This should be the most reliable connection method at present.
When the vulnerable parts are deformed, the damaged parts should be mechanically connected or riveted as far as possible. Convenient for later disassembly and repair.
In the use of composite materials in automotive parts, there is a relatively large challenge. Design is mold design, because of the thermal expansion coefficient of the composite material and the accumulation of the database, the complexity of the material itself will cause the mold design deformability and portability. Less controllable phenomenon. We generally perform calculations at the beginning of the design to simulate the distribution of the mold temperature field.
The most important point of composite material is the design of its fixtures. The design of fixtures is as simple as possible. When designing fixtures, we should minimize the profiling surface. The performance analysis of composite materials, I want to emphasize one point is that composite materials will have different defects in the process of prototype production, which is also an inherent problem of this material and there is no way to solve it.
Below I will briefly introduce its application direction. Since 1982, composite materials have continued to this day. Its main application concept is the accumulation of aerospace technology. For composite materials brands, we are also witnessing a reduction in the cost of composite materials.
Carbon fiber is another direction of domestic development, or a short-term application direction. From the perspective of composition and technology accumulation, I think it is currently more suitable for structural reinforcement and connectors. We It is relatively easy to implement, and it is also acceptable in terms of cost. New energy vehicles are a breakthrough in the field of carbon fiber applications. The current challenge is mainly insufficient technology accumulation. The research on this material itself is not deep. The establishment of the database has not been completed. Secondly, everyone is still confused about how to use this material. The most important thing is It ’s still a matter of talent training, thank you all!