Carbon fiber is generally combined with epoxy resin to form a composite material. This composite material inherits a series of advantages such as higher specific strength, specific modulus, fatigue strength, and shock resistance of carbon fiber itself. At the same time, it inherits epoxy. The resin formulation is flexible and versatile, and its application is highly targeted. Compared with aluminum alloy structural members, the weight reduction effect of carbon fiber composite materials can reach 20% to 40%. Compared with steel metal components, the weight reduction effect of carbon fiber composite materials can even reach 60% to 80%. The use of carbon fiber composite materials This not only reduces the overall vehicle quality, but also affects and changes the automobile manufacturing process to some extent.
1 Process Type
Carbon Fiber Reinforced Polymers (CFRP) refers to a composite of carbon fibers as a reinforcing phase and a thermoplastic or thermosetting resin material. The manufacturing technology of CFRP composite materials mainly includes prepreg forming and liquid forming processes. The comparison and analysis of the process types of carbon fiber reinforced polymer matrix composites are shown in Table 1.
2 Automotive Assembly and Assembly Technology
The combined assembly of composite automotive parts and the connection between composite parts and metal parts is an inevitable problem. The composite material is anisotropic, with low interlaminar strength and low ductility, making the design and analysis of the joints of composite materials much more complex than metals. The connection between traditional metal parts in the automotive industry is not suitable for composite materials. Connecting, therefore, it is crucial to understand and improve the way in which automotive composite materials are connected and fixed, and to make reasonable choices.
Due to the continuity of the fibers broken by the openings, local stress concentrations are caused. The joints of composite materials are usually the weakest link in the whole structure. Therefore, it is critical to ensure the strength of joints in the structural design of composite materials. Composite materials are divided into three major categories, namely, glued connections, mechanical connections, and hybrid connections between the two. For thermoplastic composites, there are welding techniques. Composite material connection technology design needs to be determined according to the specific use conditions and design requirements of the components.
2.1 glued connection
Compared with the mechanical connection, the main advantages of the bonding technology are stress concentration caused by no openings, reduced structural quality, fatigue resistance, good vibration and insulation properties, smooth appearance, simple bonding process, and no electrochemical corrosion problems. However, there are some shortcomings in the bonding technology, such as difficult control of bonding quality, relatively large dispersibility of bonding strength, lack of reliable inspection methods, and strict requirements on surface treatment and bonding of bonding surfaces. For carbon fiber composite body, bonding is the main connection.
2.2 Mechanical connection
Mechanical connection is generally used rivets and bolts, is the most commonly used connection. The main advantage of the mechanical connection is the high reliability of the connection, which can be repeatedly dismantled and assembled during maintenance or replacement, does not require surface treatment, and has a relatively small impact on the environment. The main disadvantage of mechanical connections is the increase in mass, the stress concentration, and the electrochemical corrosion of metals and composites. The comparison of rivet connections and bolt connections is shown in Figure 1.
2.3 Hybrid Connection
In order to improve the safety and integrity of the connection, in some important connection sites, a hybrid connection method of bonding and mechanical connection is usually adopted at the same time, and the advantages of the two connection methods are fully utilized to ensure that the connection site has sufficient strength and high reliability.
Welding technology is mainly applied to thermoplastic composite parts. The basic principle is to heat the resin on the surface of the molten thermoplastic composite, and then lap the press to make it integrated. Welding mainly includes ultrasonic welding, electric induction welding and resistance welding. The advantages of welding are good connection and short cycle, no surface treatment, high connection strength, low stress, etc.; the inadequacies are difficult to disassemble and need to add conductive materials or metal wires. In addition, during the molding of the composite structural member, the metal connector can be embedded in the fiber preform, and the composite material and the metal embedded member are integrated after the molding, and the composite parts can be connected through the metal embedded member to avoid Machining damage composites.
3 Application advantages for automotive
There are a number of factors to consider when selecting automotive materials, such as mechanical properties, light weight, material stability, material designability, and processability. Each of these factors will have a non-negligible impact on the design, production, sales, and use of automobiles. In recent years, Carbon Fiber Reinforced Polymers (CFRP) has become a new automotive material attracting attention due to its unique performance characteristics. Compared with other automotive materials, carbon fiber reinforced polymer matrix composites have the following advantages.
3.1 Excellent mechanical properties
The density of carbon fiber reinforced resin matrix composites (CFRP) for vehicles is 1.5 to 2g/cm3, which is only 1/4 to 1/5 of common carbon steel, and is about 1/3 lighter than aluminum alloy, but the carbon fiber composite material The comprehensive mechanical properties are obviously better than metal materials, and its tensile strength is 3 to 4 times that of steel. The fatigue strength of steel and aluminum is 30% to 50% of tensile strength, and CFRP can reach 70% to 80%. At the same time, CFRP also has better vibration damping characteristics than light metals, such as light alloy requires 9s to stop vibration, The carbon fiber composite material 2s can be stopped and has higher specific strength and specific modulus.
The design of carbon fiber composite material is strong, and the matrix material can be reasonably selected according to the performance requirements, the arrangement of the fibers can be designed and the structure of the composite material, and the product design can be carried out flexibly. For example, by arranging the carbon fibers in the direction of force, the anisotropy of the strength of the composite material can be fully exerted, thereby achieving the purpose of material saving and quality reduction. For products that require corrosion resistance, a base material with good corrosion resistance can be used during design.
3.3 can achieve integrated manufacturing
Modularization and integration are also trends in the automotive structure. When the composite material is formed, it is easy to form a curved surface of various shapes to achieve integrated production of automobile parts and components. Integrated molding can not only reduce the number of parts and molds, reduce the number of components and other processes, but also greatly shorten the production cycle. For example, if the front-end module of a car is made of carbon fiber composite material, it can be integrally formed and integrated to avoid localized stress concentration caused by subsequent welding and subsequent processing of metal parts, while reducing product accuracy and improving performance while reducing auto parts. Quality, reduce manufacturing costs.
3.4 Energy Absorption and Impact Resistance
Carbon fiber reinforced resin matrix composites (CFRP) have a certain degree of viscoelasticity, and there is a slight local relative movement between the carbon fiber and the matrix, which can generate interfacial friction. Under the synergetic effect of viscoelasticity and interfacial friction, CFRP parts have better energy absorption and impact resistance. On the other hand, the specially-absorbed carbon fiber composite crashes into small fragments in high-speed collisions, absorbs a large amount of impact energy, and its energy absorption capacity is 4 to 5 times higher than that of metal materials, which can effectively improve vehicles. Security, protect members' safety.
3.5 Good corrosion resistance
Carbon fiber-reinforced polymer matrix composites are mainly composed of carbon fiber tow and resin materials, and have excellent acid and alkali resistance properties. Auto parts made of them need no surface antiseptic treatment, and their weather resistance and aging resistance are good. Their service life is good. 2 to 3 times that of steel.
3.6 high temperature performance
The performance of carbon fiber at temperatures below 400 °C remains very stable, and there is no significant change at 1 000 °C.
3.7 Good fatigue resistance
Carbon fiber reinforced materials have an inhibitory effect on the fatigue crack propagation due to the fiber, and its fatigue resistance can reach 70% to 80%. The structure of carbon fiber is stable. After the fatigue life of the composite material is millions of cycles, its strength retention rate There are still 60%, while steel and aluminum are 40% and 30%, respectively, and fiberglass is only 20% to 25%. Therefore, the fatigue resistance of carbon fiber composites is suitable for a wide range of applications in the automotive industry.
4 Economic Analysis for New Energy Passenger Vehicles
Because of the use of carbon fiber, the body can be reduced by more than 50%. Taking the weight loss of 100kg on a typical A-class vehicle as an example, the significance of lightweighting of the vehicle is very obvious. It can be explained from the following aspects: 1 For one station For a passenger car with 300 km and a charging capacity of 45 kW·h, the same driving range can be reduced by 3.6 kW·h, as calculated by the industry expert, “100 kg per 100 kg, plus 8% increase in driving range.” The battery saving cost is about 0.6 million yuan; 2 The average life cycle of driving 400,000 kilometers and the electricity cost are calculated according to 0.9 yuan/kW·h. The electricity cost of the entire vehicle can save 400000/100×1.2×0.9=0.43 million. 100km saves 1.2kW·h electricity.) 3Because of the application of carbon fiber materials, taking the production scale of 50,000 vehicles as an example, the saved process investment and equipment investment are converted into the economic equivalent of electric vehicles, and each vehicle is Amortization saved about 2,000 yuan; 4 because the process is streamlined, personnel costs at least save 1,000 yuan / Taiwan.
The above items sum up to an average savings of 0.6+0.432+0.2+0.1=13.3 million yuan per vehicle, but these costs are not sufficient to offset the increase in the cost of the material itself due to the introduction of carbon fiber. It can be seen that there are still major problems in the application of carbon fiber bodies. If you want to promote lightweight body, you can only start from reducing the input of the process and equipment. The above items sum up to an average savings of 0.6+0.432+0.2+0.1=13.3 million yuan per vehicle, but these costs are not sufficient to offset the increase in the cost of the material itself due to the introduction of carbon fiber. It can be seen that there are still major problems in the application of carbon fiber bodies.
If you want to promote lightweight body, you can only start from reducing the input of the process and equipment.
If the car achieves mass production of carbon fiber bodies, the cost of the carbon fiber material itself will also be greatly reduced, the entire industry effect will be quite large, and the economic benefits will also become more obvious. These are only from the perspective of carbon fiber analysis, if you consider the aluminum alloy car body weight reduction factor of 50kg, according to the same reason positive stack, the economic effect is self-evident.
5 Development Trends for Vehicle Body
Given the characteristics of carbon fiber reinforced composites, this type of material is increasingly favored by automotive manufacturers. It is estimated that in the automotive sector, the use of carbon fiber is growing at an average annual rate of 34% and will reach 23,000 tons by 2020. Figure 2 shows the roadmap for the development of carbon fiber reinforced composites for bodywork.
At present, carbon fiber reinforced composites are mainly applied to body panels, body trims, and structural components. For example, BMW has used a large number of carbon fiber composite materials in the development of a variety of models to manufacture body structural parts. This has become an important moment for the application of carbon fiber composite materials in automobile manufacturing. At the same time, BMW has further cooperated with SGL in Germany, investing 100 million euros in the research and development of low-cost carbon fiber, and increasing the carbon fiber production from 3,000 tons per year to 9000 tons to meet the growing BMW i-series electric vehicles and others. Demand for models.
In summary, carbon fiber-reinforced resin matrix composites (CFRP) have become an important development direction for new automotive materials in the future with its unique performance advantages. However, in order to promote the use of this material in the automotive field, it is necessary to start collaborative research and development of production, learning, and research from the following aspects: (1) Further search for lower-cost carbon fiber precursors; (2) Develop new carbon fiber manufacturing processes, such as stabilization of precursor materials. Technology; 3 Optimize carbon fiber manufacturing process parameters or use nano-carbon fiber to further improve the performance of CFRP composite materials; 4 Develop rapid and effective CFRP parts molding and manufacturing technologies, such as rapid solidification molding technology and composite material flow control technology; 5 Use computer simulation analysis technology (CAE) to select different carbon fiber composite materials and optimize molding process parameters.