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. At the same time, it inherits epoxy. The resin formulation design is flexible and diverse, and the application is highly targeted. Compared with aluminum alloy structural members, the carbon fiber composite material can achieve a weight reduction effect of 20% to 40%. Compared with steel metal components, the weight reduction effect of carbon fiber composite materials can be achieved. Can even reach 60% to 80%. The use of carbon fiber composite materials 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 the reinforcing phase and a thermoplastic or thermosetting resin material. The manufacturing technology of CFRP composites mainly includes prepreg forming and liquid forming. Process, Comparative analysis of carbon fiber reinforced polymer matrix composite process types are shown in Table 1.
2 Automotive Assembly and Assembly Technology
The assembly of composite automobile parts and the connection between composite parts and metal parts are unavoidable problems. The composite material is anisotropic, with low interlaminar strength and low ductility, making the design of composite parts a joint. Analysis is much more complicated than metal, and the connection between traditional metal parts in the automotive industry is not suitable for the connection of composite materials. Therefore, it is crucial to understand and improve the connection and fixing methods of automotive composite materials.
Due to the continuity of the fiber breakage caused by the open hole, local stress concentration is caused. The joint of the composite material is usually the weakest link in the entire structure. Therefore, the connection strength is the key to the structural design of the composite material. The connection mode of the composite material is mainly divided into There are three major categories, namely, glued connections, mechanical connections, and hybrid connections between the two. For thermoplastic composites, there are also welding techniques. The design of the composite connection technology needs to be determined based on the specific use of the components and design requirements.
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, no electrochemical corrosion problems, etc. However, the bonding technology also has some disadvantages, 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, adhesive The main connection method.
2.2 Mechanical connection
Rivets and bolts are commonly used for mechanical joints. They are the most common type of connection. The main advantage of mechanical connections is high reliability of the connection. They can be repeatedly disassembled and assembled during maintenance or replacement, and do not require surface treatment. Small, etc. The main disadvantage of the mechanical connection is the increase of mass, stress concentration, and electrochemical corrosion caused by contact between the metal and the composite material. The comparison between the rivet connection and the bolt connection is shown in Fig. 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 modes are fully utilized to ensure that the connection site has sufficient strength and high reliability.
2.4 Welding
The 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 pressurizing to make it integrated. There are three main types of welding: ultrasonic welding, electric induction welding and resistance welding. Methods. The advantages of welding are good connection and short cycle time, no surface treatment, high connection strength, low stress, etc.; the inadequacies are difficult to disassemble, need to add conductive materials or wire, etc. In addition, in the composite structure forming During the process, the metal connectors can be embedded in the fiber preforms. After the molding, the composite materials and the metal embedded parts are integrated, and the composite parts can be connected through the metal embedded parts to avoid machining damage to the composite materials.
3 Application advantages for automotive
When selecting automotive materials, there are a number of factors that need to be considered, such as mechanical properties, light weight, material stability, material designability and processability, etc. Each of these factors will result in the design, production, sale, and use of the vehicle. The effects of neglect. In recent years, Carbon Fiber Reinforced Polymers (CFRP) has become a new automotive material attracting people's attention due to its unique performance characteristics. Compared with other automotive materials, carbon fiber reinforced polymer (CFRP) enhances polymerization. Material matrix composites have the following advantages.
3.1 Excellent mechanical properties
The density of carbon fiber reinforced resin matrix composites (CFRP) for vehicles is 1.5~2g/cm3, which is only 1/4~1/5 of common carbon steel, and is 1/3 lighter than aluminum alloy, but the carbon fiber composite material The comprehensive mechanical properties are obviously superior to 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 the tensile strength, and the CFRP can reach 70% to 80%. CFRP also has better vibration damping characteristics than light metals. For example, light alloys require 9 seconds to stop vibration, while carbon fiber composites can stop for 2 seconds, with higher specific strength and specific modulus.
3.2 Designable
The carbon fiber composite material has strong designability, and can reasonably select the matrix material according to the performance requirements, design the arrangement of the fibers and the structure of the composite material, and flexibly carry out product design. For example, the carbon fibers can be arranged in the direction of force and can be fully The anisotropy of the strength of the composite material can be exploited to achieve the purpose of material saving and quality reduction. For the products requiring corrosion resistance, the base material with good corrosion resistance can be selected during design.
3.3 can achieve integrated manufacturing
Modularization and integration are also trends in the development of automotive structures. Composites are easy to form into curved surfaces of various shapes when they are molded, enabling integrated manufacturing of auto parts products. Integrated molding can not only reduce the number of parts and molds The number of components and the process of reducing component connections can also greatly shorten the production cycle. For example, if the automobile front end module is made of carbon fiber composite material, it can be integrally formed and integrated, avoiding the subsequent partial welding and subsequent processing of the metal parts. Stress concentration, while ensuring product accuracy and improving performance, reduce the quality of auto parts and 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 action of viscoelasticity and interfacial friction, CFRP parts have Better energy absorption and impact resistance. On the other hand, the specially-absorbed carbon fiber composite crash energy absorbing structure is fragmented into smaller pieces in high-speed collisions, absorbs a large amount of impact energy, and has higher energy absorption capacity than metal materials. 4 to 5 times, can effectively improve vehicle safety and ensure the safety of members.
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 the steel.
3.6 high temperature performance
The performance of carbon fiber below 400 °C remains very stable, and it does not change much at 1 000 °C.
3.7 Good fatigue resistance
Carbon fiber reinforced materials have an inhibitory effect on fatigue crack propagation due to the fiber's fatigue resistance of up to 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%~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 weight of the car body can be reduced by more than 50%. As an example, the weight loss of a typical A-class vehicle is 100kg. The significance of the lightweighting of the vehicle is obvious. It can be explained in terms of the following aspects: 1 For 1 station For a passenger car with a capacity of 45 kW·h and a driving capacity of 300 km, it is calculated by the industry expert 'increasing driving mileage by approximately 8% per 100kg, and the same driving range can reduce the electricity consumption of 3.6kW·h. The battery-saving cost is about 0.6 million yuan; 2 The life cycle of driving 400,000 kilometers, the average electricity cost is calculated according to 0.9 yuan / kW • h, the entire vehicle life cycle can save electricity costs 400,000/100 * 1.2 * 0.9 = 0.432 million yuan (According to 100km to save 1.2kW·h electric power calculation); 3Because of the application of carbon fiber materials, with the production scale of 50,000 vehicles as an example, the process investment saved, equipment investment is converted into the economic equivalent of electric vehicles, each vehicle Amortization saved about 2,000 yuan; 4 because the process is streamlined, personnel costs at least save 1,000 yuan / Taiwan.
For the above items, the average cost per vehicle can be saved at 0.6+0.432+0.2+0.1=13.3 million yuan, but these costs are not enough to offset the increase in the cost of the material itself due to the introduction of carbon fiber. This shows that carbon fiber There is still a big problem in the application of the car body. If we want to promote a lightweight car body, we can only start from reducing the input of the process and equipment. The above totals, each car can save an average of 0.6+0.432+0.2+0.1=1.332 Million yuan cost, but these costs are not enough to offset the increase in the cost of the material itself caused by the introduction of carbon fiber. It can be seen that the application of carbon fiber car body is still a big problem.
If you want to promote a lightweight body, you can only start by reducing the investment in process and equipment.
If the car mass production of carbon fiber body, the cost of carbon fiber material itself will be greatly reduced, the entire industry effect is also quite large, the economic benefits will become more obvious. These are only analyzed from the perspective of carbon fiber, if you consider aluminum alloy car body reduction The factor weighing 50kg is superimposed on the same reason and 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 automakers. It is estimated that in the automotive sector, carbon fiber usage is growing at an average annual rate of 34% and will reach 23,000 tons by 2020. 2 is a roadmap for the development of carbon fiber reinforced composites for car bodies.
At present, carbon fiber reinforced composites are mainly applied to body panels, trims and structural parts on automobiles. For example, BMW has used a large number of carbon fiber composite materials in the development of various models to manufacture body structure parts. This has become Carbon fiber composites are used in important moments in automotive manufacturing. At the same time, BMW and SGL further cooperate with each other to invest EUR 100 million in the development of low-cost carbon fiber and increase the carbon fiber production from 3,000 tons per year to 9000 tons. Meet the growing demand for BMW i-series electric cars and other models.
6 Conclusion
In summary, carbon fiber-reinforced resin matrix composites (CFRP) has become an important development direction for future automotive new materials with its unique performance advantages. However, in order to promote the application of this material in the automotive field, it also needs to start from the following aspects: Carry out collaborative R&D in production, research, and research: 1 Further seek for lower cost carbon fiber precursors; 2 Develop new processes for carbon fiber manufacturing, such as stabilization of precursor materials; 3 Optimize carbon fiber manufacturing process parameters or use nano-carbon fibers to further improve CFRP composites Performance; 4 R&D rapid, effective CFRP part molding manufacturing technologies, such as rapid solidification molding technology, composite material flowability control technology, etc.; 5 using computer simulation analysis technology (CAE) to select different carbon fiber composite materials, and optimize the molding process parameter.