Abstract: lightweight technology has become an important way to achieve energy conservation and emission reduction. Carbon fiber composites provide an important material basis for automotive lightweight. Due to the particularity and complexity of material characteristics and manufacturing process, many key technologies need to be overcome when using carbon fiber composites to realize automobile lightweight. Combined with the characteristics of automotive products, this paper expounds the key technologies of carbon fiber composites in automotive lightweight application from the aspects of low-cost carbon fiber technology, material structure performance integration technology, high-efficiency molding technology, multi-material connection technology and recycling technology, and looks forward to the development trend of automotive carbon fiber composites in the future.
Key words: automotive lightweight carbon fiber composites low cost carbon fiber efficient molding multi material connection
introduction
In recent years, the continuous increase of automobile production and ownership has exacerbated the problems of energy, environment and security. Energy saving and environmental protection has become the primary problem facing the global automobile industry. Therefore, the EU has set the most stringent vehicle emission control target in the world, that is, from January 1, 2020, the average carbon dioxide emission level of 95% of new vehicles sold in the EU must be reduced from the current 130g / km to less than 95g / km. China will also fully implement the more stringent national V emission standard from January 1, 2018. In the current situation, to realize the sustainable development of the automobile industry, we must develop new technologies in energy conservation and emission reduction to reduce the energy consumption and environmental pollution of automobiles.
Research shows that vehicle lightweight is one of the most effective measures to reduce energy consumption and emissions. Every 100kg reduction in vehicle mass can save fuel by 0.3 ~ 0.5L / (100km), reduce CO2 emission by 8 ~ 11g / (100km), improve acceleration performance by 8% ~ 10%, and shorten braking distance by 2 ~ 7m. The lightweight of automobile structure can also reduce the mass of transmission system accordingly. The research of the Automotive Research Institute of Aachen University of technology shows that the final vehicle weight can be reduced to 785kg due to the direct weight reduction and the indirect chassis "secondary weight reduction" of the reference vehicle with a curb weight of 1.229t. In addition, vehicle lightweight is also conducive to improving vehicle power, braking and operational stability.
Automobile lightweight can be realized by lightweight materials, lightweight structure and lightweight manufacturing technology. Among many lightweight materials, carbon fiber composites have excellent comprehensive mechanical properties: specific strength (ratio of strength to density), specific modulus (ratio of modulus to density) and specific energy absorption (energy absorbed per unit mass within the effective failure length). Under the design principle of equal stiffness or equal strength, the weight of carbon fiber composite structure is reduced by more than 50% compared with low carbon steel structure, Compared with magnesium / aluminum alloy structure, the weight can be reduced by 30%, and the number of assembly parts can be reduced by 70%; It has good fatigue resistance, corrosion resistance and high service life of parts. However, the high cost and complex manufacturing process of carbon fiber greatly restrict the application of carbon fiber composites in automobiles. The development of key technologies for the design and manufacturing of carbon fiber composite auto parts has become a strategic measure for energy conservation, environmental protection, transformation and upgrading of China's auto industry. In made in China 2025 issued by the State Council in 2015, it has been clear that carbon fiber composite auto parts technology is an important development direction in the field of energy saving and new energy vehicles.
Combined with the characteristics of automobile parts, this paper expounds the key technologies of carbon fiber composites in automobile from the aspects of low-cost carbon fiber technology, material structure performance integrated design technology, high-efficiency molding technology, multi-material connection technology and recycling technology, in order to expand the application scope of carbon fiber composites in automobile To improve the application level of carbon fiber composites in automobiles.
1 low cost carbon fiber technology
As automobile is a bulk product, reliability and economy should be considered at the same time. The high cost of carbon fiber seriously restricts the large-scale application of carbon fiber composites in automobiles. In 2010, the cost of carbon fiber was more than $30 / kg, while that of low carbon steel was less than $1 / kg, and that of aluminum alloy was (2.4 ~ 2.6) dollars / kg. Only when the cost of carbon fiber is reduced to (11 ~ 15.4) US dollars / kg, the tensile strength is not less than 1.72gpa and the elastic modulus is not less than 172gpa, can carbon fiber composites be used in automobiles on a large scale. Therefore, it is urgent to reduce the cost of carbon fiber.
1.1 precursor Technology
At present, commercial carbon fibers are mainly prepared from three types of precursor materials: polyacrylonitrile (Pan), asphalt and rayon. PAN based carbon fiber has been widely used because of its superior comprehensive properties. As shown in Figure 1, in the composition of PAN based carbon fiber, precursor usually accounts for 51% of the total cost, so reducing the cost of precursor is the most direct way to reduce the cost of carbon fiber.
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Using low-cost raw materials other than polyacrylonitrile (such as asphalt, lignin, low-density polyethylene, etc.) to prepare carbon fiber precursor is an ideal measure to reduce the cost of carbon fiber. Asphalt is rich in resources, low price and high carbon content, which makes the cost of asphalt based carbon fiber low and has broad development prospects. Asphalt based carbon fiber can be divided into general-purpose asphalt based carbon fiber (isotropic asphalt based carbon fiber) and high-performance asphalt based carbon fiber (mesophase asphalt based carbon fiber). At present, the main companies producing asphalt based carbon fibers worldwide include Japan Wuyu chemical company, Japan Mitsubishi Chemical Company and American Amoco company. Although asphalt has cost advantages, and asphalt based carbon fiber has high elastic modulus and excellent thermal conductivity, there are some problems, such as low tensile strength and compressive strength, difficult to control the preparation process and high cost of asphalt purification.
Lignin has been widely concerned because of its wide source, low price, high carbon content, renewable and rich aromatic components in molecular structure. Lignin based carbon fibers can be prepared by melt spinning, dry spinning and electrospinning. Shichao Yang et al. Prepared the precursor from lignin / polylactic acid mixture by melt spinning process, and obtained lignin / polylactic acid based carbon fiber by thermal stabilization and carbonization process. Its tensile strength is 258.6 ~ 159.2mpa and tensile elastic modulus is 1.7 ~ 11.6gpa. Meng Zhang et al. Prepared precursor fibers from acetylated softwood sulfate lignin by dry spinning process, and thus prepared lignin based carbon fibers. The tensile modulus, tensile strength and elongation at break were (52 ± 2) GPA, (1.04 ± 0.10) GPA and (2.0 ± 0.2)% respectively. However, there are a large number of different chemical bonds and irregular structures in lignin, which need relatively harsh conditions (temperature, catalyst, etc.) for decomposition. At the same time, there are serious problems that are easy to produce coke in lignin thermal chemical decomposition. Therefore, there are still many problems to be solved when lignin is widely used in the preparation of carbon fiber.
Because the carbon content of polyethylene is 86% and that of polyacrylonitrile is 63%, the yield of carbon fiber can be increased from 50% to 70% by using polyethylene to prepare carbon fiber. At the same time, the raw material cost of polyacrylonitrile precursor is $3 / lb and that of polyethylene precursor is only $1 / lb. In addition, the spinnability of the melt is higher than that of polyacrylonitrile, which makes the process economy of polyethylene better. Therefore, the preparation of carbon fiber from polyethylene has a certain prospect. Jarod M. younker et al. Used n-heptane-4-sulfonic acid (H4S) sulfonated polyethylene and prepared carbon fiber by thermal cracking. Jongwonkim et al. Obtained linear low density polyethylene by dry wet spinning, and used a series of sulfuric acid treatment at temperature, pressure and time to obtain cross-linked fiber. After carbonization, carbon fiber was prepared, and its performance was equivalent to that of commercial carbon fiber (tensile strength 1.65gpa, tensile modulus 110GPa). In the process of preparing carbon fiber from polyethylene, impregnation method needs to be used for acid treatment, which leads to poor mechanical properties of carbon fiber. At present, polyethylene based carbon fiber is difficult to realize commercial production. From the above analysis, it can be seen that although the use of materials other than polyacrylonitrile to prepare carbon fiber has the advantage of low raw material cost, due to the immature preparation process, resulting in complex process and unstable product performance, it will be difficult to replace the use of polyacrylonitrile to prepare carbon fiber in a short time.
In addition to using new raw materials, the cost can be reduced by improving the preparation process of existing polyacrylonitrile (Pan) precursor. At present, PAN precursor is usually prepared by solution spinning. Carbon fiber preparation by melt spinning can avoid the use of toxic chemical solvents, lower cost and higher production efficiency. Amit K. naskar et al. Used the copolymer of acrylonitrile, methyl acrylate and acryloyl benzophenone for melt spinning, and carried out pre oxidation and carbonization under ultraviolet light to prepare carbon fiber. Professor Yu muhuo of Donghua University has prepared PAN precursor by melt spinning, and the spinning rate is greatly improved, and the surface quality of the fiber is better. Although melt spinning has many advantages, the preparation process is complex and difficult, and the quality of precursor is difficult to ensure. At present, commercial PAN based carbon fibers are mostly prepared by solution spinning.
1.2 pre oxidation process
Pre oxidation not only controls the quality of carbon fiber, but also restricts the output of carbon fiber. As shown in Figure 2, if the whole process of producing carbon fiber takes 88min, the pre oxidation time needs about 80min. At present, the production process of carbon fiber can be improved by pretreatment of PAN precursor and changing the process method in the pre oxidation process. The pretreatment of PAN precursor can be divided into chemical method and physical method. The chemical method is mainly to chemically modify pan with KMnO4, c6h5cooh, k2cr2o4 and other reagents to reduce the cyclization temperature, improve the pre oxygenation speed and reduce the cost; The physical methods are mainly ultraviolet, electron beam, X-ray γ Ray. The activation energy of cyclization reaction can be reduced by chemical and physical treatment, so as to reduce the pre oxidation time. Huiwu yuan et al. Irradiated the PAN precursor with different doses of electron beam. The results show that the radiation treatment can reduce the activation energy in the pre oxidation process, expand the reaction temperature range and reduce the initial and intermediate temperature of the reaction. Changing the pre oxidation process mainly includes changing the pre oxidation process parameters (temperature, time, applied tension), gas atmosphere, humidity, etc. to improve the performance of carbon fiber. Jeong hyeonyun et al. Optimized the pre oxidation process with different treatment temperature and tensile force. The results showed that prolonging the heat treatment time could increase the tensile strength of the fiber, and increasing the tensile force could improve the elastic modulus of the fiber, but had no significant effect on the tensile strength.
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1.3 large tow carbon fiber technology
Large tow carbon fiber refers to the carbon fiber with more than 24K carbon fibers in each bundle. The performance of tow carbon fiber is reduced. It is a general-purpose carbon fiber and is used in general industry. The preparation of large tow carbon fiber is a low-cost production technology. Its selling price is only 50% ~ 60% of that of small tow carbon fiber. The performance price ratio (strength, modulus, specific strength and specific modulus per unit price) is much higher than that of small tow carbon fiber. For example, the large tow carbon fiber c30t050 produced by German SGL Group has the same performance as T300, and the cost is only 1 / 4 of it. However, because the tow of large tow carbon fiber is large, it is easy to gather and the yarn spreading effect is not good, resulting in poor wettability of resin in large tow carbon fiber and manufacturing defects such as pores in monofilament. At the same time, disordered yarn and broken yarn are easy to occur in the process of yarn spreading, resulting in large dispersion of mechanical properties, which increases the manufacturing difficulty of large tow carbon fiber. At present, large tow carbon fiber thin-layer technology is being developed at home and abroad to reduce defects and improve product quality.
1.4 hybrid fiber technology
Mixing carbon fiber with other fibers can complement each other in performance and effectively reduce the production cost. For example, carbon fiber has high cost and excellent comprehensive properties, while glass fiber has low cost, but has the problem of low elastic modulus; Aramid fiber has low cost and good comprehensive performance, but its performance decreases significantly under hot and humid conditions. Therefore, the optimal hybrid fiber structure can be obtained by using different manufacturing processes, fiber types and percentages, fiber direction, ply sequence and laminate form. Jun heesong compared the effects of different laying methods on the tensile properties, section shape, bending properties and stress transfer path of carbon fiber / glass fiber and carbon fiber / aramid fiber laminates. Jin Zhang et al. Studied the tensile, compressive and flexural properties of carbon fiber / glass fiber laminates under different hybrid ratios and paving layers. Meisam jalalvand et al. Analyzed the failure forms of local area and overall range of carbon fiber / aramid fiber under different lamination methods. Mehdikalantari et al. Took the hybrid ratio of glass fiber and carbon fiber as the design variable, took the bending strength, cost, quality and robustness of glass fiber / carbon fiber laminates as the objective function, and transformed the multi-objective optimization problem into a single objective optimization problem by using the weighted sum method, so as to obtain a hybrid fiber structure with more stable and reasonable performance.
2 material structure performance integrated design technology
Traditional automotive steel often needs to get stamping parts from coil steel through flattening, cutting, blanking, stamping and other processes, and then connect the sheet stamping parts into an overall structure through welding and other processes. However, for carbon fiber resin matrix composites, the material and structure are formed at the same time, and the composite materials are no longer processed into composite components, and the shape and size of components are less constrained by the manufacturing process. Therefore, it is difficult to give full play to the potential of carbon fiber composites by using traditional design and manufacturing ideas. At the same time, from the perspective of material, structure and performance, developing the material structure performance integration technology of carbon fiber composites will not only reduce the number of parts and simplify the manufacturing and assembly process, but also give full play to the material performance, which is an important direction for the development of composite auto parts in the future.
The 2003 dodgeviper uses a carbon fiber composite baffle bracket composed of only four parts. Compared with the original steel structure, the number of parts of the structure is reduced by 15 ~ 20, the weight is reduced to 6.1kg, and the weight is reduced by up to 18kg. Mercedes Benz SLR adopts the front-end integral structure made of two conical carbon fiber composite longitudinal beams to effectively improve the manufacturing and assembly process of the structure and ensure the crashworthiness of the structure. BMW has adopted a large number of hybrid structures composed of carbon fiber composites and steel or aluminum alloy in the A-pillar, B-pillar, C-pillar and threshold of the 2016 7-series car, so as to realize lightweight and ensure the mechanical properties of the overall structure. Do hyoungkim et al. Used the hybrid fiber structure of glass fiber / carbon fiber to replace the traditional glass fiber structure on the automobile bumper, and used micro genetic algorithm to optimize the structure of three different parts of the bumper, which not only improved the collision performance of the structure, but also reduced the weight by 33%. M. R. bambach conducted NCAP frontal collision test on two models Yaris and Silverado equipped with carbon fiber composite reinforced steel energy absorbing tube (as shown in Figure 3). The results show that the impact resistance in frontal collision will not be affected when the mass increases by 140kg and 560kg. They also carried out the finite element simulation and experimental analysis of the side crushing process of the roof structure composed of CFRP steel composite circular pipe. The strength / mass of the composite structure is about twice that of the original structure. Under the condition that the strength of the structure remains unchanged, the weight can be reduced by 37% ~ 68% instead of the original steel structure. Liu Qiang and others designed the integral electric vehicle body structure with two-dimensional braided composites, and established the finite element model of carbon fiber braided composite body from meso scale to macro scale (as shown in Figure 4). Compared with the original glass fiber composite structure, the weight of the structure is reduced by 28%, and the crashworthiness in side crushing and collision is improved accordingly.
Using carbon fiber composites to meet the structural and performance requirements at the same time is the remarkable feature of carbon fiber composites. In the process of structural design and manufacturing of carbon fiber composites, the characteristics of lightweight and high strength, good energy absorption, good vibration absorption and excellent process performance of carbon fiber composites should be fully utilized to realize the integrated material structure performance manufacturing of carbon fiber composites auto parts. This is very important to reduce the structural quality, reduce the number of components, simplify the assembly process and meet the relevant performance requirements.
