Carbon fiber composite material (CFRP) has the characteristics of high strength, high stiffness, high fracture toughness, corrosion resistance and high damping, which can greatly improve the service life, fuel efficiency and safety and comfort of automobiles. Quantify material. 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 high-efficiency, low-cost, large-scale and automated manufacturing technology for automotive CFRP, international mainstream car companies combine body Due to the specific characteristics of flexible design of parts, uneven thickness and different degrees of complexity, many differentiated new rapid prototyping processes have been developed to achieve the purpose of maximizing the efficacy of composite materials with the minimum amount of carbon fiber.
In this paper, by comparing with other advanced lightweight materials such as high-strength steel, aluminum alloy, magnesium alloy, etc., this paper introduces the diversified characteristics and significant advantages of carbon fiber composite materials in terms of performance; It has developed the differentiated rapid prototyping process with the most development potential.
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 molding, good vibration damping and damping performance, etc. There are also obvious advantages in vehicle safety. 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 Performance of automotive CFRP
Judging from the current status and development trend of lightweight materials at home and abroad, although high-strength steel is still the most widely 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 is also showing a gradually increasing trend, but the significant advantages of CFRP in terms of light weight, high strength and high modulus, impact resistance, shock absorption and sound insulation performance, and corrosion resistance are unmatched by other materials.
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 statistics, on the premise of not changing the shape, structure and function of the parts, using different lightweight materials, aluminum alloy and magnesium alloy parts are 40% and 49% lighter than high-strength steel parts, quasi-isotropic CFRP and unidirectional The weight loss percentage of fabric CFRP can reach 52% and 76%. It can be seen that the weight reduction effect of CFRP is significant. If combined with the optimized structural design scheme, a better lightweight effect can be obtained.
The tensile strength and tensile modulus of CFRP are affected by many factors such as fiber type, dosage, shape, layup method and resin, and the performance varies. Overall, tensile strength, tensile modulus, especially specific strength and specific modulus are significantly higher than those of metal materials, which are the core performance advantages of CFRP.
The specific strength and specific modulus of CFRP are several times higher than those of alloy materials, and the performance is outstanding. In particular, continuous fiber CFRP, due to the outstanding anisotropy of mechanical properties, has the highest strength along the fiber direction, and the smallest strength perpendicular to the fiber direction, which is a weak link. Therefore, fiber orientation needs to be specially designed according to load-bearing properties.
1.2 Good impact resistance and high fracture toughness
The high strength and high stiffness characteristics of CFRP also determine that CFRP components can absorb more energy from the outside world than other material components when causing the same degree of deformation or even fracture damage. The energy absorption rate of CFRP in the collision process is 4 to 5 times that of steel and aluminum alloys, that is, CFRP has higher fracture toughness. The energy-absorbing area at the front end of some sports cars adopts CFRP spire collapse columns, which are woven from countless carbon fiber bundles, which not only have extremely high strength, but also can be broken into numerous The small fragments absorb a large amount of impact energy, 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 injury that may be caused to the human body by large-sized CFRP components, further improving ride safety. In addition, CFRP components will not dent even if they are partially subjected to heavy point force in column impact and side impact, and also show high crash safety and structural reliability. However, the GFRP components, which are also composite materials, are not safe enough due to factors such as low modulus, poor fatigue resistance, and weak energy absorption.
1.3 Vibration and noise reduction performance
The sources of noise during driving are 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, the vibration of the components themselves and between components should be reduced, and on the other hand, the external noise should be effectively isolated. The natural frequency of the material is proportional to the square root of its specific modulus. CFRP has a high specific modulus, so the natural frequency of the material itself is relatively high; and the vibration modes of various parts of the body are related to the structure of the components, material properties and Connection friction and so on are closely related.
The modal number of each part of the car body is 40~90Hz, avoiding the frequency range of 20~28Hz of the powertrain, effectively reducing the vibration of the components and reducing the noise of the body structure. At the same time, the interaction between the viscoelasticity of the resin polymer chains in CFRP and the fiber-resin interface also showed a significant damping effect, which made the material absorb vibration energy more effectively and the vibration rapidly attenuated. Comparing the vibration test results of the gold beam and the carbon fiber composite beam in the same size and shape of the aluminum alloy technology report, the former needs 9s to stop the vibration, while the latter only needs 2.5s.
Excellent damping characteristics make various noises better isolated, and achieve effective noise shielding. Of course, the damping characteristics exhibited by CFRP components have a very complex mechanism. Vehicle vibration and noise reduction is also a huge systematic project, which requires the cooperation of material selection, structural design, and vehicle body sealing.
1.4 Corrosion resistance
The surface of aluminum alloy can form a dense oxide film during use, making it more resistant to corrosion than high-strength steel and magnesium alloys. Therefore, in many cases, aluminum alloys exposed to the atmosphere can be used without surface treatment, while high-strength steels and magnesium alloys require surface protection such as painting and electroplating. However, the electrochemical corrosion resistance of aluminum alloys is poor, and the acid resistance is not as good as that of steel. It can be said that the corrosion resistance of traditional lightweight alloy materials has different strengths and weaknesses, and they are not all-round materials.
CFRP has excellent weather resistance such as seawater resistance, salt spray resistance, and mechanical friction resistance, as well as chemical resistance properties such as acid and alkali resistance, organic solvent resistance, and industrial waste gas resistance. condition of service environment. Compared with traditional lightweight metal materials, CFRP has better corrosion resistance, which is also an important consideration for choosing carbon fiber composite materials to manufacture body panels.
In addition, it is also necessary to consider that the high polymer in the carbon fiber composite material can absorb light quantum under the action of ultraviolet rays, and cause the damage of oxygen to the matrix resin on the surface of the material, that is, photo-oxidative aging; the effect of visible light and infrared rays Under high pressure, the polymer can also absorb energy and release heat to promote the oxidation reaction, that is, thermal aging occurs. Therefore, it is necessary to protect CFRP by improving the weather resistance of the resin matrix, painting the surface, 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 production capacity can reach 6,000 in 8 hours, which is efficient and fast. The traditional CFRP molding process comes from the aerospace and military field of multi-variety, small-batch, and high-cost production. It generally adopts small-scale production technologies such as autoclave. 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 automotive CFRP for high-efficiency, low-cost, large-scale, and automated manufacturing technology.
Therefore, in order to maximize the effect of CFRP with the minimum amount of carbon fiber, the international mainstream car companies have focused on the development of a number of advanced composite materials based on the original conventional composite material molding process based on the specific characteristics of flexible design, uneven thickness and different levels of complexity of body parts. Differentiated new rapid prototyping process.
At present, the most potential CFRP molding processes in the automotive industry include rapid RTM molding process, prepreg rapid compression molding process, sheet molding compound 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 short molding cycle, high fiber content, good surface finish and high dimensional accuracy. Since prepreg and autoclave are not required, the RTM process is relatively inexpensive and is widely used in the production of large structural parts in the aerospace industry. However, the traditional RTM process takes more than 2 hours from fiber placement, resin injection, impregnation, curing, to final demolding, which is difficult to meet the needs of the modern automobile industry for rapid manufacturing technology. Therefore, rapid RTM technology is not only the first choice for the integrated molding of large and complex CFRP components at present, but also the development direction of CFRP molding technology for vehicles in the future. 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 gigapascals, which ensures high clamping speed and pressing speed, greatly shortens the part molding time, and improves the 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 material system at the same time, the injection speed can be further increased; the precise metering of the reaction material 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 components, the advantages are more obvious. The number of firmware, simplifying connection and assembly, greatly reduces the energy consumption of the production process and reduces the production cost.
The CFRP parts of the automobile body are produced by HP-RTM technology in large quantities. The factory is equipped with 2 HP-RTM injection units for each 3000 t hydraulic press. The resin can be injected into the mold with the help of high pressure, and the curing of the epoxy resin can be completed within 5 minutes [9]. The use of HP-RTM technology reduces the number of CFRP parts by 2/3 compared to traditional metal parts, to only about 150.
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 realizing rapid manufacturing.
In recent years, prepreg has been more and more widely used because of its precise fiber and resin ratio. The PCM molding process, as an ideal hot pressing process outside the CFRP tank, can not only greatly shorten the molding cycle and improve the production efficiency, but also has the advantages of high dimensional accuracy, good surface finish, relatively low production cost, and easy realization of one-time production of complex structural parts. At the same time, due to the good fiber orientation in the product, the strength and rigidity of the product are relatively high, and it has become an important molding process of CFRP for vehicles.
The rapid curing PCM molding process uses two kinds of 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. The PCM process is applied to the manufacture of the trunk door of the car, the weight is only 1/2 of the aluminum alloy product, and the molding cycle is shortened to about 10min, which can be used for the mass production of CFRP auto parts.
The thermoplastic CFRP prepreg rapid hot pressing molding process realizes the continuous operation of the continuous fiber yarn/fabric film lamination melting prepreg process, which is used for the mass production of a certain model of Chery automobile bumpers, and the molding efficiency reaches 8 pieces per hour. The product quality meets the safety collision standard.
2.3 Other molding processes
The RTM molding process has high requirements on mold manufacturing precision, long mold production cycle and high price. However, the material processing and transportation costs of prepreg are high, and the cost of mold is not low. Therefore, these two molding processes have a large initial investment. Therefore, other composite molding processes, such as sheet molding compound molding process and long carbon fiber reinforced thermoplastic material injection molding process, have also been widely used.
2.4 SMC molding process
SMC is a sheet-like molding material made of resin paste impregnated fiber or chopped fiber mat and covered with polyethylene film on both sides, which belongs to the scope of prepreg. SMC has high molding efficiency, good surface finish, good dimensional stability, short molding cycle and low cost. It is suitable for mass production and thin-walled products with little change in cross-section. It has been obtained in the field of GFRP auto parts production. widely used.
At present, SMC is mainly used for the production of sheet-shaped chopped fiber composite materials in the CFRP molding process for vehicles. Due to the discontinuity of the fibers, the strength of the products is not high, and the strength has the characteristics of in-plane isotropy. The wettability of carbon fibers in the resin paste is an important issue faced by the SMC process. The necessary surface treatment of the carbon fibers and the use of appropriate wetting and dispersing agents can effectively improve the wettability and uniformity of the carbon fibers in the resin paste. . Carbon fiber SMC has also found many applications in the automotive industry. CFRP is mainly used in the manufacture of door and windshield structures.
The windshield strength of the car has been greatly improved compared to the original model. The strength of the new door has been improved under the premise of reducing the weight, and the sag of the door has been well controlled.
Many manufacturers have also carried out a lot of research on SMC, and applied CFRP to the tailgate, new energy vehicle battery box cover, engine cover, rear top cover, front cabin cover and other automotive exterior covers.
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, and low process cost. Due to the long fiber length inside the product, a skeleton structure is formed, which makes the product have better impact resistance and stiffness. , so LFT products can be used for vehicle body parts with greater stress. LFT has been widely used in automobile bodies, and it is also a forming process with great application potential. Compared with aluminum alloy and high-strength steel, the LFT composite material of carbon fiber reinforced nylon 6 has the same specific modulus and 50%~250% higher specific strength, and has considerable competition in the manufacture of automotive secondary load-bearing structural parts.
3 Analysis and discussion
At present, CFRP has been used in the connecting rod of the automobile engine system, the rocker tank bottom shell, the transmission shaft of the transmission system, the reducer, the brake pad of the braking system,Cross rails, brackets, hubs, leaf springs of the chassis system, four doors and two covers of the body system, radiator covers, bumpers, bottom plates, door and window frames and other components. The application models have also developed from customized models such as early F1 racing cars, super sports cars, high-end cars, and concept cars to standardized mass-produced models.
As mentioned above, the initial investment of RTM and PCM is relatively large, while SMC and LFT are the commonly used molding processes for GFRP auto parts. Therefore, in terms of hardware conditions and existing foundations, the latter two seem to be more convenient. However, the applicable parts of several molding processes are quite different. RTM and PCM products are more suitable for the manufacture of large sheet metal structures and frame structures on steel bodies due to their better fiber orientation and better mechanical properties such as rigidity and strength. Such as body frame, outer cover, etc.; while SMC and LFT parts have slightly weaker mechanical properties due to the use of discontinuous fibers, and are more suitable for the production of small special-shaped parts with complex structures, but the thickness of the parts can be larger.
Therefore, when selecting the molding process, it should be based on the different performance requirements of each component according to the specific service conditions, combined with the specific shape and structure characteristics of the component itself, plus the cost, existing hardware foundation and other factors, and differentiated using a variety of molding. The production process is more feasible.
The CFRP parts of a super sports car are mainly manufactured by three molding processes: the body frame, side rails and front bulkheads and other hollow structures are molded with multi-axial fabric prepregs to obtain higher strength. , rigidity and dimensional accuracy; front impact energy-absorbing box, cab floor, hood and A-pillar-flat bracket are formed by RTM process, and vacuum-assisted RTM process is used to produce roof parts to realize large and multiple complex structures The integrated molding of the rear part, such as the speed control rear wing, etc., which does not require high strength, is formed by the short-cut carbon fiber reinforced SMC molding process, which can obtain a better surface finish.
Of course, the use of differentiated molding processes to obtain differentiated component performance is not the ultimate goal of manufacturers, but to implement the results into lightweight and low-cost car bodies. According to reports, the more mature rapid CFRP molding process, automatic carbon fiber laying, automatic yarn spreading technology, fabric setting technology, automatic continuous forming technology, and large-scale application of sandwich structures will further meet the performance requirements of complex auto parts, making The quality and production costs of auto parts are improved.
4 Conclusion
With the successive mass production of some models, CFRP has attracted more and more attention as an advanced lightweight material. International mainstream car companies have developed and developed many differentiated new rapid prototyping processes such as HP-RTM, PCM, SMC, and LFT suitable for high-performance carbon fibers, and have achieved good results. Of course, the application of CFRP in the automotive field is not only a simple process of material processing, but also involves the selection of lightweight materials, structural optimization, rapid prototyping, component connection technology, crash safety testing, and even carbon fiber recycling technology. Therefore, the realization of CFRP lightweight application requires related technologies in all links to go hand in hand and develop together.
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