English
Automobile lightweighting is the core technology and important development direction of the automobile industry. It has become the development strategy of the national manufacturing industry. "Made in China 2025" mentioned automobile lightweighting many times, involving lightweight materials, optimized design, vehicle lightweighting technology and other aspects. "Energy-saving and New Energy Vehicle Lightweighting Technology Roadmap" also regards automobile lightweighting as one of the development strategic priorities. Many national ministries and commissions have also issued a number of technical standards, laws and regulations and R&D projects that benefit the automobile lightweighting industry. The support of the government provides opportunities for the development of automobile lightweighting, which is conducive to improving the core competitiveness of my country's automobile lightweighting industry.
Carbon fiber reinforced resin-based composite materials (CFRP) have a series of advantages such as light weight, high strength, high fracture toughness, corrosion resistance, strong designability, easy forming, and good vibration damping performance. It can not only meet the design requirements of component rigidity and lightweight, but also has obvious advantages in vehicle safety. At present, CFRP has become a popular and promising lightweight material in the automotive industry after high-strength steel, aluminum alloy, magnesium alloy, engineering plastics and glass fiber composite materials (GFRP).
The tensile strength and tensile modulus of CFRP are affected by many factors such as fiber type, dosage, morphology, lamination method and resin, and the performance varies. The performance comparison of carbon fiber and other lightweight materials is shown in Table 1. Overall, the tensile strength, tensile modulus, especially the specific strength and specific modulus are significantly higher than those of metal materials, which are the core performance advantages of CFRP. In the automotive industry, it has always received widespread attention from major automobile companies.
At first, due to high cost and long forming time, carbon fiber was only used in sports cars and a small number of expensive models. With the development of technology, the price of raw materials has been reduced, the manufacturing process has been optimized, and the forming cycle has been shortened. Carbon fiber reinforced composite materials are increasingly used in their vehicle parts. In addition, major OEMs have been developing carbon fiber bodies and parts. With the continuous research and application of carbon fiber in the automotive field, the cost of carbon fiber will further decrease, and high-strength and lightweight carbon fiber reinforced parts will be more and more widely used. In recent years, with the rapid development of the electric vehicle industry, due to the lightweight of electric vehicles, electric vehicles with the same number of batteries can travel farther, or electric vehicles with the same range can install fewer batteries, so the application prospects of carbon fiber in electric vehicles are more extensive.
Carbon Fiber Reinforced Composite Body
Carbon Fiber Reinforced Composite is an ideal non-metallic material for automotive covering parts. Its high strength can greatly reduce the weight of the car body while ensuring the anti-collision performance of the car. BMW is a pioneer in the application of carbon fiber. Since 2009, it has joined forces with Mitsubishi Rayon and SGL to try to use carbon fiber composite materials in its i series models. Through the successful attempt in the BMW i series models, it has been proved that carbon fiber is not only suitable for small-batch models, but can also be used in mass-produced models. This also proves that the potential for lightweight design of automobiles (for new energy vehicles, especially electric vehicles) by using carbon fiber composite materials is very large. In 2015, the sales of BMW i3 were close to 24,000 units, and the sales of i8 were close to 5,500 units. The experience accumulated by BMW in the i series carbon fiber body and parts has been successfully transferred to other BMW models. In the new BMW 7 Series exhibited at the 2015 European Body Conference, carbon fiber and high-strength steel are combined in the B-pillar, roof longitudinal beam/front cross beam, door sill beam, central channel and C-pillar to improve the torsional stiffness of the body and reduce the weight of the body. In 2016, the BMW 7 Series (Figure 1) achieved sales of 64,000 units, and the long-term sales target is expected to reach 100,000 units.
BMW has sorted out the patents involved in each production link of the i3 and carried out important patent protection. It can be seen from Table 2 that Mitsubishi Rayon, as a carbon fiber supplier, pays more attention to the patent layout in the upstream of the industrial chain; SGL, as a supplier of composite materials, mainly focuses on the fields of design, forming process and automotive parts; and BMW, as a vehicle manufacturer, has a large number of patent layouts in connection, quality inspection, repair and recycling, and has also applied for a small number of patents in carbon fiber bundles and carbon fiber fabrics.
Domestic carbon fiber reinforced composite body is still in the small batch trial production stage. Chery Arrizo 7 and Jiangsu Aoxin e25 both produce all-carbon fiber body, with obvious lightweight effect.
Carbon fiber reinforced composite parts
Due to its good lightweight advantages, carbon fiber reinforced composite materials are also widely used in wheel hubs, brake systems, transmission systems, interior and exterior decoration, and engine fields.
⑴ Carbon fiber wheels: Models with carbon fiber wheels are shown in Table 3. Carbon fiber can greatly reduce the mass of the wheels, thereby helping to reduce the moment of inertia. Since the weight reduction effect is approximately equal to a 6kg reduction in the weight of the sprung mass for every 1kg reduction in the unsprung mass, the carbon fiber wheels will significantly improve the vehicle's handling performance. At the same time, carbon fiber is strong and tough, and carbon fiber wheels are conducive to steering response and the transmission of road feel.
⑵ Braking system: Carbon fiber is also widely used in braking systems and transmission systems. Carbon fiber brake discs have excellent high temperature resistance, can withstand temperatures of 2500℃, and can also ensure stable performance. The Porsche 918 spider uses carbon fiber brake discs, which can reduce the speed from 300km/h to 50km/h within 50 meters; carbon fiber drive shafts can not only reduce weight by more than 40%, but also have very good durability. The carbon fiber drive shaft of the Toyota 86 weighs only 5.53kg, which is 50% lighter than the steel drive shaft.
⑶ Engine periphery: Carbon fiber composite materials as materials for automobile intake systems can reduce weight and achieve lightweight effects on the one hand; on the other hand, carbon fiber materials are easy to process into various curved shapes, and the surface is relatively smooth. Carbon fiber intake manifold (Figure 3) can effectively improve intake efficiency.
⑷ Transmission system: Since the stress conditions of the automobile transmission system are relatively complex, it has to withstand a large torque. Carbon fiber has high specific strength and low specific modulus, and is anisotropic, which can well replace the transmission shaft of traditional metal materials. In addition, the carbon fiber transmission shaft can reduce weight by more than 50%, and also has excellent fatigue resistance and durability.
Forming process of automotive carbon fiber
Traditional carbon fiber forming processes are mostly derived from the aerospace field, with many varieties, small batches, high production costs, and long production cycles. The curing cycle of traditional epoxy resin carbon fiber parts is usually more than 4 hours, which cannot meet the requirements of large quantities and low costs in the automotive industry. Therefore, for the automotive industry, mainstream carbon fiber companies and automobile companies in the world have jointly developed many rapid prototyping processes. At present, the carbon fiber forming processes with more applications and development potential include RTM, PCM, SMC, LFT, etc.
⑴ Rapid RTM forming: RTM forming process is the main liquid molding forming technology. It has a short forming cycle, high fiber content of products, good surface finish, high dimensional accuracy and relatively low cost. However, since the traditional RTM process cannot meet the requirements of the automotive industry in terms of forming speed, the rapid RTM forming technology has been developed for the automotive industry and has become a forming technology for large and complex carbon fiber parts.
⑵ High-pressure RTM forming: High-pressure RTM is an effective method to increase the injection speed by increasing the injection pressure. The injection pressure of this process can reach several thousand MPa, which ensures a high mold closing speed and pressing speed, greatly shortens the forming time of the parts, and improves the process efficiency. The BMW i3 body mainly adopts high-pressure RTM forming technology, and the curing time is shortened to less than 5 minutes, and the number of parts is only 1/3 of the number of traditional metal parts.
⑶ PCM forming: PCM forming is to put the semi-finished carbon fiber parts after stamping into the mold, and then form them by heating and pressurizing. Due to the good fiber orientation in the product, the strength and stiffness of the product are relatively high, and it has become one of the most widely used forming processes for automotive carbon fiber in recent years. However, the sensitive decomposition temperature of thermosetting carbon fiber composites is much higher than their curing temperature. How to recycle will be a bottleneck in the development of thermosetting carbon fiber.
⑷SMC compression molding: SMC is widely used in glass fiber reinforced composites. It has high molding efficiency, good surface quality, short molding cycle, low cost, and is suitable for mass production. At present, the SMC process is mainly used for the production of sheet-like chopped fiber composites in carbon fiber molding. Due to the discontinuity of the fiber, the product strength is not high, and the strength has the characteristics of in-plane isotropy. The wettability of carbon fiber is relatively poor, and the carbon fiber needs to be subjected to necessary surface treatment. The use of appropriate wetting dispersants can effectively improve the wettability and uniformity of carbon fiber in the resin paste.
⑸LFT molding: The LFT molding process has excellent molding processability, high molding rate, high yield rate, relatively simple equipment, and low process cost. The skeleton structure is formed inside the product due to the long fiber length, so that the product has good impact resistance and stiffness. Therefore, LFT products can be used for body parts with large forces. LFT has been widely used in automobile bodies and is also a molding process with great application potential.