Carbon Fiber Reinforced Polymer (CFRP) has the characteristics of high strength, high stiffness, high fracture toughness, corrosion resistance, high damping, etc., which can greatly improve the service life, fuel efficiency, safety and comfort of automobiles, and has been recognized as the most ideal in the automotive industry. Lightweight materials. However, since the traditional composite material molding process comes from the aviation industry with many varieties, small batches, and high-cost production, in order to meet the urgent needs of automotive CFRP for high-efficiency, low-cost, large-scale, and automated manufacturing technology, mainstream international car companies have combined body With the specific characteristics of flexible component design, uneven thickness and different complexity, many new and differentiated rapid prototyping processes have been developed to achieve the goal of maximizing the effectiveness of composite materials with the smallest amount of carbon fiber.
By comparing with other advanced lightweight materials such as high-strength steel, aluminum alloy and magnesium alloy, this article introduces the diversified characteristics and significant advantages of carbon fiber composite materials in terms of performance; combined with typical application cases of carbon fiber composite parts for vehicles, analysis It has developed the most promising differentiated rapid prototyping process.
Carbon fiber reinforced resin matrix composites have a series of advantages such as light weight, high strength, high fracture toughness, corrosion resistance, strong designability, easy forming, good vibration damping performance, etc., which can not only meet the design requirements of rigidity and light weight of components, Vehicle safety also has obvious advantages. At present, CFRP has become the most popular and promising new lightweight material in the automotive industry after high-strength steel, aluminum alloy, magnesium alloy, engineering plastics and glass fiber composite materials.
1 CFRP performance for vehicles
Judging from the current status and development trend of lightweight materials at home and abroad, although high-strength steel is still the most commonly used and mature automotive lightweight material at this stage, it will not be replaced for the time being. Aluminum alloys, magnesium alloys, engineering plastics and GFRP The application of CFRP has also shown a gradual increase trend, but CFRP has significant advantages in terms of light weight, high strength and high modulus, impact resistance, shock absorption and sound insulation performance, and corrosion resistance that other materials cannot compare.
1.1 Lightweight, high strength and high modulus
CFRP is composed of carbon fiber reinforced phase and resin matrix, and has the characteristics of light weight, high strength and high modulus. Its density is 1.5~2g/cm3, which is about 1/4 of high-strength steel and 2/3 of aluminum alloy, which is equivalent to GFRP and magnesium alloy. According to Audi's statistics, under the premise of not changing the shape, structure and function of the components, different lightweight materials are used. Aluminum and magnesium alloy parts are 40% and 49% lighter than high-strength steel parts, respectively. Quasi-isotropic CFRP And the weight loss percentage of unidirectional fabric CFRP can reach 52% and 76%. It can be seen that the weight reduction effect of CFRP is significant. If combined with an optimized structural design scheme, a better weight reduction effect can be obtained.
The tensile strength and tensile modulus of CFRP are affected by various factors such as fiber type, dosage, shape, layering method, and resin, and its performance varies. On the whole, the tensile strength, tensile modulus, especially the specific strength and specific modulus are significantly higher than that of metal materials, which is the core performance advantage of CFRP.
The Science and Technology Review pointed out that the specific strength and specific modulus of CFRP are several times higher than that of alloy materials, and its performance is outstanding. In particular, continuous fiber CFRP, due to its outstanding mechanical anisotropy, has the largest strength along the fiber direction and the smallest strength perpendicular to the fiber direction, which is a weak link. Therefore, the fiber orientation needs to be specially designed according to the load-bearing characteristics.
1.2 Good impact resistance and high fracture toughness
The high strength and high rigidity of CFRP also determine that CFRP components can absorb more energy from the outside than other material components when causing the same degree of deformation or even fracture damage. The energy absorption rate of CFRP during collision is 4 to 5 times that of steel and aluminum alloy, that is, CFRP has higher fracture toughness. Mercedes-Benz used the CFRP spire-type crumple column at the front end of the SLR McLaren sports car. It is woven from countless carbon fiber bundles. It not only has extremely high strength, but also when subjected to a frontal impact, the CFRP crumple column can It absorbs a large amount of impact energy by breaking into countless small fragments, which improves the safety of the vehicle.
At the same time, this form of damage is similar to tempered glass, which can effectively avoid the fatal damage that large-size CFRP parts may cause to the human body, and further improve the riding safety. In addition, CFRP components will not sag even if they are partially subjected to heavy point forces in column impact and side impact, and they also show high crash safety and structural reliability. However, GFRP parts, which are also composite materials, are not sufficiently safe due to factors such as low modulus, poor fatigue resistance, and weak energy absorption.
1.3 Vibration and noise reduction performance
The source of noise in the driving process of a car is complex. According to different sources, the four main types of noise are body structure noise, tire noise (tire noise), engine noise (machine noise) and aerodynamic noise. Therefore, in order to improve ride comfort, from the perspective of automotive components, on the one hand, it is necessary to reduce the vibration of the components themselves and between components, and on the other hand, to achieve effective isolation of external noise. The natural frequency of a material is proportional to the square root of its specific modulus. CFRP has a higher specific modulus, so the natural frequency of the material itself is relatively high. The vibration modes of various parts of the car body are related to the structure and material properties of the parts. Connection friction and so on are closely related.
The modal number of each part of the BMW i3 body is 40~90Hz, avoiding the 20~28Hz frequency range of the powertrain, effectively reducing component vibration and reducing body structure noise. At the same time, the interaction between the viscoelasticity of the resin polymer chain in CFRP and the fiber-resin interface also exhibits a significant damping effect, which enables the material to absorb vibration energy more effectively and the vibration attenuates rapidly. Comparing the vibration test results of the same size and shape aluminum alloy beam and carbon fiber composite beam, the former requires 9s to stop the vibration, while the latter only takes 2.5s.
The excellent damping characteristics enable various noises to be better isolated, realizing effective noise shielding. Of course, the damping characteristics exhibited by CFRP components have a very complicated mechanism. Vehicle vibration and noise reduction is also a vast system engineering, which requires material selection, structural design, and vehicle body sealing.
1.4 Corrosion resistance
A dense oxide film can be formed on the surface of aluminum alloy during use, which makes it more resistant to corrosion than high-strength steel and magnesium alloy. Therefore, in many cases, aluminum alloy exposed to the atmosphere can be used without surface treatment, while high-strength steel and magnesium alloy require surface protection such as painting and electroplating. However, aluminum alloy has poor resistance to electrochemical corrosion, and its acid resistance is not as good as steel. It can be said that the corrosion resistance of traditional lightweight alloy materials has its own strengths and weaknesses, and they are not all-round materials.
CFRP has excellent weather resistance such as seawater resistance, salt spray resistance, mechanical friction resistance, and chemical resistance properties such as acid and alkali resistance, organic solvent resistance, and industrial exhaust gas resistance. It is capable of withstanding acid rain, salt spray and other harsh weather and air pollution. Service environment under conditions. CFRP has better corrosion resistance than traditional lightweight metal materials, which is also an important consideration for choosing carbon fiber composite materials to manufacture body panels.
In addition, it should also be considered that the high polymer in the carbon fiber composite material can absorb light quanta under the action of ultraviolet rays, and trigger oxygen to destroy the matrix resin on the surface of the material, that is, photooxidative aging; the effect of visible light and infrared rays The polymer can also absorb energy and release heat to promote the oxidation reaction, that is, thermal ageing occurs. Therefore, it is necessary to protect the CFRP by improving the weather resistance of the resin matrix, surface painting, and attaching a protective film.
2 CFRP rapid prototyping process for vehicles
When the traditional automobile industry uses steel plates and aluminum alloy plates to manufacture parts, the stamping production line can stamp 10 to 14 parts per minute, and the 8-hour production capacity can reach 6000, which is efficient and fast. The traditional CFRP molding process comes from the aerospace and military fields of multi-variety, small batch, and high-cost production. It generally uses small-scale production technologies such as autoclaves. The complete curing cycle of a conventional epoxy CFRP part is usually greater than 4 hours. , The implementation cycle is long and the production efficiency is low, which cannot meet the urgent needs of high-efficiency, low-cost, large-scale and automated manufacturing technology for automotive CFRP.
Therefore, in order to achieve the purpose of maximizing the effect of CFRP under the minimum amount of carbon fiber, the international mainstream car companies have combined the specific characteristics of flexible body parts design, uneven thickness, and different levels of complexity, and focused on the development of many on the basis of the original conventional composite material molding process. Differentiated new rapid prototyping process.
At present, the most promising CFRP molding processes in the automotive industry include rapid RTM molding processes, rapid prepreg molding processes, sheet molding compounds, and long fiber reinforced thermoplastic resin composite materials.
2.1 Rapid RTM molding process
RTM molding process is the most important liquid molding technology. It has a short molding cycle, high fiber content, good surface finish, and high dimensional accuracy. Because there is no need to use prepreg and autoclave, the RTM process cost is relatively low, and it is widely used in the production of large structural parts in the aviation industry. However, the traditional RTM process, from fiber placement, resin injection, impregnation, curing, to final demolding, has a total time of more than 2 hours, which is difficult to meet the needs of the modern automobile industry for rapid manufacturing technology. Therefore, the rapid RTM technology is not only the first choice for the integrated molding of large and complex CFRP parts, but also the development direction of the future automotive CFRP molding process. High pressure RTM is an effective method to increase the injection speed by increasing the injection pressure.
Using this process, the injection pressure can reach several gigapascals, which ensures a higher clamping speed and pressing speed, greatly shortens the molding time of parts, and improves process efficiency. At the same time, increasing the pressure can prompt the resin to quickly fill the mold cavity, improve the fiber resin infiltration, reduce the number of resin injections, promote air discharge, and reduce the porosity of the finished product, thereby achieving excellent surface properties. If you choose to inject a low-viscosity resin system or a low-viscosity reactive mixture system at the same time, the injection speed can be further improved; the precise measurement of the reaction materials through high-pressure metering technology can also shorten the injection time.
In addition, due to the strong designability of the structure and performance of CFRP products, when HP-RTM is applied to the manufacture of large and complex structural parts, the advantages are more obvious. It can not only realize the integrated molding of the parts within 5 minutes, but also greatly reduce the parts and tightness. The number of firmware simplifies connection and assembly, greatly reduces energy consumption in the production process, and reduces production costs.
The CFRP parts of the BMW i3 body are largely produced using HP-RTM technology. The Leipzig and Landshut plants of BMW are equipped with 2 HP-RTM injection units for each 3000 t hydraulic press. When the automated production line puts the carbon fiber preforms into the steel accurately After the mold is closed and the mold is closed, the HP-RTM unit can inject resin into the mold with the help of high pressure, and complete the curing of the epoxy resin within 5 minutes [9]. The use of HP-RTM technology reduces the number of CFRP parts of the BMW i3 by 2/3 compared to the number of traditional metal parts, only about 150 [10].
2.2 PCM molding process
Compression molding is a molding method in which the stamped CFRP semi-finished product is placed in a mold in advance, and then heated and pressurized to form and solidify. Among them, the forming blank before hot pressing is the key to achieving rapid manufacturing.
In recent years, prepregs have been more and more widely used because of their precise fiber and resin ratio. The PCM molding process, as an ideal CFRP tank external hot pressing process, can not only greatly shorten the molding cycle and improve production efficiency, but also has high dimensional accuracy, good surface finish, relatively low production cost, and easy realization of complex structural parts. Molding and other characteristics. At the same time, due to the good fiber orientation in the product, the strength and stiffness of the product are relatively high, which has become an important molding process for CFRP for vehicles.
The rapid curing PCM molding process launched by Mitsubishi Corporation of Japan in 2012 uses two large-tow carbon fiber prepregs, 60kP330 and 50kWCF, and hopes to obtain good processability, excellent mechanical properties and high productivity similar to small tow CFRP. In 2014, Mitsubishi Rayon applied the PCM process to the manufacture of Nismo’s Nismo version of the GT-R trunk door. The weight is only 1/2 of the aluminum alloy product, and the molding cycle is shortened to about 10 minutes, which can be used for CFRP automotive parts. Mass production.
The rapid hot press molding process of thermoplastic CFRP prepreg developed by Ningbo Materials realizes the continuous operation of continuous fiber yarn/fabric film laminated melt prepreg process. It is used in the mass production of a certain model of Chery Automobile, and the molding efficiency reaches 8 pieces per hour, product quality meets safety collision standards.
2.3 Other molding processes
RTM molding process requires high mold manufacturing accuracy, long mold production cycle and high price. The material processing and transportation cost of prepreg is relatively high, and the cost of mold is not low. Therefore, these two molding processes have a large initial investment. Therefore, other composite material molding processes, such as sheet molding compound compression molding process, long carbon fiber reinforced thermoplastic material injection molding process, have also been widely used.
2.4 SMC compression molding process
SMC is a sheet-like molded material made of resin paste impregnated fiber or chopped fiber mat, covered with polyethylene film on both sides, and belongs to the scope of prepreg material. SMC has high molding efficiency, good surface finish, good dimensional stability, short molding cycle, low cost, suitable for mass production, suitable for the production of thin-walled products with small cross-section changes, and has been obtained in the field of GFRP automotive parts production. widely used.
At present, in terms of automotive CFRP molding process, SMC is mainly used for the production of sheet chopped fiber composite materials. Due to the discontinuity of fibers, the strength of the product is not high, and the strength is isotropic in the plane. The wettability of carbon fiber in the resin paste is an important issue for the SMC process. The necessary surface treatment of the carbon fiber and the use of appropriate wetting and dispersing agents can effectively improve the wettability and uniformity of the carbon fiber in the resin paste. . Carbon fiber SMC has also found many applications in the automotive industry. The 2003 Dodge Viper is the first model to use continuous carbon fiber and glass fiber hybrid reinforced vinyl SMC parts in batches. CFRP is mainly used for the manufacture of door and windshield structures.
The windshield strength of this model has been greatly improved compared with the original model. The strength of the new door is improved under the premise of reduced weight, and the amount of door sag is well controlled. In 2012, Japan's Asahi Organic Materials Industry Co., Ltd. used the conductivity of carbon fiber to produce components for absorbing radio waves for electric vehicles using the SMC process. The components are used for both electromagnetic shielding and structural materials.
In 2013, auto parts supplier Magna and large tow carbon fiber manufacturer Zoltek combined PANEX35 carbon fiber with Magna’s EpicBlendSMC formula and technology to jointly develop low-cost carbon fiber SMC technology and products for vehicles, which greatly improved Productivity. In addition, domestic car companies, such as SAIC, BAIC, Chery, FAW, etc. have also carried out a lot of research on SMC, and applied CFRP to the tailgate, new energy vehicle battery cover, engine hood, rear roof, front engine compartment cover, etc. On the outer cover of the car.
2.5 LFT injection molding process
In addition to thermosetting resins and carbon fiber fabrics, continuous fibers, thermoplastic resins and discontinuous carbon fibers also have many applications in the automotive field. The LFT molding process has excellent molding processability, high molding rate, high yield, relatively simple equipment, low process cost, the internal structure of the product is formed due to the long fiber length, so that the product has good impact resistance and rigidity Therefore, LFT products can be used for body parts that are subject to greater force. LFT has been widely used in automobile bodies, and it is also a molding process with great application potential. Compared with aluminum alloy and high-strength steel, carbon fiber-reinforced nylon 6 LFT composite material has the same specific modulus and 50% to 250% higher specific strength. It has considerable competition in the manufacture of automotive sub-structures.
In addition to thermosetting resins and carbon fiber fabrics, continuous fibers, thermoplastic resins and discontinuous carbon fibers also have many applications in the automotive field. The LFT molding process has excellent molding processability, high molding rate, high yield, relatively simple equipment, low process cost, the internal structure of the product is formed due to the long fiber length, so that the product has good impact resistance and rigidity Therefore, LFT products can be used for body parts that are subject to greater force. LFT has been widely used in automobile bodies, and it is also a molding process with great application potential. Compared with aluminum alloy and high-strength steel, carbon fiber-reinforced nylon 6 LFT composite material has the same specific modulus and 50% to 250% higher specific strength. It has considerable competition in the manufacture of automotive sub-structures.
3 Analysis and discussion
At present, CFRP has been used in the connecting rods of automobile engine systems, rocker tank bottom shells, transmission shafts, reducers, brake pads of brake systems, transverse beams, brackets, wheels, and leaf springs of chassis systems. Four doors and two covers of the body system, radiator cover, bumper, bottom plate, door and window frame and other components. Application models have also evolved from customized models such as early F1 racing cars, super sports cars, high-end cars, and concept cars to standardized mass-produced models represented by BMW i3. According to figures released by BMW, the CFRP cost of an i3 electric car is about 13,800 Euros, of which raw material costs account for 11% of the total CFRP cost, and 89% of the cost comes from the molding process. There is no doubt that how to obtain fast, efficient and low-cost automated molding technology has become a difficult problem for many automakers.
As mentioned earlier, RTM and PCM two molding processes require a large initial investment, while SMC and LFT are currently commonly used molding processes for GFRP automotive parts. Therefore, the latter two seem to be more convenient in terms of hardware conditions and existing foundations. However, the applicable parts of several molding processes are quite different. RTM and PCM products have better fiber orientation and better mechanical properties such as rigidity and strength. They are more suitable for manufacturing large sheet metal structures and frame structures on steel car bodies. Such as body frame, outer cover, etc.; SMC and LFT parts have weaker mechanical properties due to the use of discontinuous fibers, and are more suitable for the production of small shaped parts with complex structures, but the thickness of the parts can be larger.
Therefore, when selecting the molding process, the performance requirements of each component should be different according to the specific service conditions, combined with the specific appearance and structure characteristics of the component itself, and the cost, existing hardware foundation and other factors should be considered comprehensively, and a variety of molding should be used differentiated The process is more feasible for production.
The CFRP parts of the Lexus LFA super sports car are mainly manufactured by three molding processes: the body frame, side rails and front bulkheads and other hollow structure skeletons are molded with multi-axial fabric prepregs to obtain higher strength. , Rigidity and dimensional accuracy; the front impact energy absorbing box, the cab floor, the hood and the A-pillar-flat bracket are formed by the RTM process, and the roof components are produced by the vacuum-assisted RTM process to achieve large and multiple complex structures The integrated molding of the rear part, such as the speed control tail, and other parts that do not require high strength, are formed by the chopped carbon fiber reinforced SMC molding process, which can obtain a better surface finish.
Of course, it is not the ultimate goal of the manufacturer to obtain differentiated component performance by using a differentiated molding process, but to implement the results in the lightweight and low cost of the car body. According to reports from SAMPE China 2015 and JEC 2015, the more mature rapid CFRP molding process, automatic carbon fiber placement, automatic spinning technology, fabric setting technology, automatic continuous molding technology, and the large-scale application of sandwich structures will further satisfy the complex The performance requirements of auto parts have improved the quality and production costs of auto parts.
4 Conclusion
With the successive mass production of BMW i3 and i8 models, CFRP has received more and more attention as an advanced lightweight material. International mainstream car companies have developed and developed many new and differentiated rapid prototyping processes such as HP-RTM, PCM, SMC, LFT, etc., which are suitable for high-performance carbon fiber, and have achieved good results. Of course, the application of CFRP in the automotive field is not only a simple process of material processing and shaping, but also involves lightweight material selection, structural optimization, rapid prototyping, component connection technology, collision safety testing, and even carbon fiber recycling technology. Therefore, the realization of CFRP lightweight applications requires the development of related technologies in all aspects.