Abstract: Using the forged aluminum control arm as the prototype, the carbon fiber composite control arm was developed using a hybrid process used by carbon fiber prepreg and CF-SMC (carbon fiber sheet molding compound). The development process of carbon fiber control arm is introduced in detail from the aspects of material scheme, structural scheme and analysis, process scheme and manufacturing, and performance test. In the process of structural design, the mechanical properties are analyzed with maximum braking as a typical working condition, and after the analysis and iteration of the conceptual scheme, process scheme and final scheme, the quality of the carbon fiber control of the final design and manufacture is reduced by 21%. Finally, the performance test of the carbon fiber control arm is carried out, and the causes of defects and improvement ideas are analyzed in detail according to the test results.
Keywords: carbon fiber, control arm, hybrid process, lightweight1
Automobile lightweight is one of the important ways to save energy and reduce emissions of automobiles, carbon fiber composite materials have many performance advantages such as light weight and high strength, and are ideal materials for deep and lightweight automobiles. The intermediate materials before the molding of carbon fiber composite materials can be divided into continuous fiber type and discontinuous fiber type according to the fiber state, and the continuous fiber type has carbon fiber dry fabric, carbon fiber prepreg, etc., and then through resin transfer molding (RTM) or compression molding, the molded product has excellent mechanical properties; The discontinuous fiber type has carbon fiber reinforced sheet molding compound (CF-SMC), which can form a complex product structure through molded flow molding.
As a force transmission and guidance element of the automotive system, the control arm transmits various forces acting on the wheels to the body while ensuring that the wheels move according to a certain trajectory . At present, the control arm on the market is basically divided into two kinds of steel material and aluminum material according to the way of use, steel material generally adopts steel plate stamping and welding method, aluminum material generally adopts casting or forging method. The lightweight control arm can not only improve the lightweight level of the whole vehicle, but also significantly improve the handling performance of the vehicle, at present, the forged aluminum control arm is a more mainstream lightweight solution, widely used in high-end models.
From the perspective of the development trend of materials, carbon fiber composite materials are expected to be applied to chassis components such as control arms due to their excellent comprehensive properties to meet the needs of further lightweight. In this paper, based on the forged aluminum control arm of an SUV model (Figure 1), we develop a carbon fiber composite control arm that uses a hybrid process shared by carbon fiber prepreg and CF-SMC to meet the needs of strength, stiffness, and structural characteristics of special-shaped solids.
Figure 1 Carbon fiber control arm developed overseas
02 Technical route analysis
2.1 Foreign development
At present, there are relevant research and development cases in carbon fiber control arms abroad, but they have not been mass-produced and applied. Figure 1a is the carbon fiber control arm made by RTM process at the Fraunhofer Institute in Germany, which is 45% lower than the original steel control arm , Figure 1b is the control arm made by Magneti Marelli using CF-SMC molding process, which is 50% lower in mass than the original steel control arm, and Figure 1c is Lamborghini's control arm made by CFSMC molding process, which is 30% lower than the original forged aluminum control arm .
2.2 Technical Route
The prototype control arm selected in this article is used in SUV models and faces much higher load requirements than similar components in sports cars and general performance cars. In addition, the prototype of the control arm is made of forged aluminum, which has high strength and good integration, and the lightweight effect has been excellent, on the basis of which it seeks further lightweight space through composite materialization, which faces great technical problems. In this case, this paper abandons the foreign RTM scheme and CF-SMC scheme, and adopts the method of mixing continuous fiber prepreg and CF-SMC to meet its strength, stiffness and structural characteristics, while maintaining the original integration effect as much as possible. Through the investigation of material resources at home and abroad, the material scheme of prepreg and CF-SMC is screened and determined; Structural design is carried out according to the structural form, space assembly relationship and load requirements of the prototype of the control arm; According to the given load requirements, the finite element model of the product under the mixing process is established for mechanical property analysis; According to the product structure and the assembly method of the control arm assembly, the molding process plan and mold scheme are formulated in combination with the material characteristics and mixed process characteristics, and finally the trial production and performance test of the sample are carried out.
03 Material scheme
The carbon fiber prepreg adopts the fabric type, and the CF-SMC fiber mass fraction is about 50%, and the main material parameters are shown in Table 1.
Table 1 Material performance parameters
Note: "—" is CF-SMC and has no corresponding parameters.
04 Structural scheme and analysis
4.1 Load Cases
By simulating and calculating the typical limit working conditions of the front suspension system, the load conditions at each point of the control arm are obtained, and the specific load values are shown in Table 2.
Table 2 Load values of typical working conditions of control arm
Through comparison, it is found that the load strength of the maximum braking condition is much higher than that of other working conditions, so the maximum braking condition is used as the typical working condition for structural design and analysis and evaluation.
4.2 Conceptual Scheme
The conceptual scheme is shown in Figure 2, using the method of integral bonding of the upper plate and the lower plate, in which the upper and lower plates are jointly formed by prepreg and CF-SMC, respectively, and the fluidity of CF-SMC is used to inlay metal accessories such as ball joints, front bushings, and rear studs.
Figure 2 Concept of carbon fiber control arm
The overall idea is to use continuous fiber prepreg as much as possible to meet the high load requirements of the control arm.
4.3 Process Scheme
The design of the process concept is carried out according to the conceptual concept, as shown in Figure 3. Considering that the inlay method of the front bushing and ball joint is difficult to achieve in the conceptual scheme, it is changed to a bolted connection method, which can also strengthen the connection strength between the upper and lower plates and make up for the lack of simple gluing. In addition, considering that the application of CFSMC is mainly for the inlay of the rear stud and the overall stiffness of the control arm, the CF-SMC solid part is completely arranged on the upper plate, and the lower plate is only a prepreg molded structure, which simplifies the process, and the CF-SMC solid part is continuous in the Z direction to better provide stiffness support.
Figure 3 Process scheme of carbon fiber control arm
After several rounds of structural design and analysis optimization, the process scheme did not meet the requirements, as shown in Figure 4, which was mainly manifested in the failure of Tsai-Wu in the prepreg fabric part, and the excessive stress in the CF-SMC part and the structural adhesive part.
Figure 4 Process scenario analysis results
Through analysis, the reasons for the above failure are as follows.
a. Although the carbon fiber prepreg fabric has high strength, in order to meet the structural characteristics of the control arm, the fabric has a large amount of deformation, and it is easy to occur shear failure between layers under load, resulting in Tsai-Wu failure;
b. Although the strength of carbon fiber fabric is high, the strength of CF-SMC and structural adhesive is weak compared with carbon fiber fabric, and the stress cannot be greatly dispersed on the carbon fiber fabric through structural design during composite use, resulting in partial failure of CF-SMC and structural adhesive;
c. In the process of carbon fibrillation of the control arm, the original connection part needs to retain the metal structure, and at the same time steel bolts are required for mechanical connection, resulting in no obvious lightweight effect.
4.4 Final Proposal
The failure form of the process scheme was summarized, and the forged aluminum control arm was analyzed at the same time, and it was found that the main bearing parts of the control arm under load were three contour edges. Therefore, the idea was changed, and a structural scheme with CFSMC as the main body and carbon fiber prepreg fabric as the local reinforcement was formulated, and the scheme was shown in Figure 5, and the quality reduction effect reached 21%.
Figure 5 Final scheme of carbon fiber control arm
Solution idea: CF-SMC as the main body to avoid its failure due to thin structure and harsh load. By reinforcing the three contour edges of carbon fiber prepreg fabric, the high-strength performance of carbon fiber fabric prepreg can be exerted in the area where it really needs to be carried, and at the same time, the molding deformation of carbon fiber fabric can be greatly reduced, effectively avoiding Tsai-Wu failure. In addition, this solution does not require structural adhesive for bonding, and also avoids the risk of structural adhesive failure. At the same time, all the metal accessories of this solution adopt the method of in-one inlay molding, which eliminates the need for mechanical connection and additional metal structure parts due to connection, which can effectively improve the weight reduction effect.
The laying method and reference direction of carbon fiber fabric prepreg are shown in Figure 6. The prepreg has a single layer thickness of 0.22 mm and a layup of [(0/45)3/0]s, for a total of 14 layers with a total thickness of 3.08 mm.
Figure 6 Ply laying method
4.5 CAE analysis
Strength analysis of carbon fiber control arm at maximum braking conditions. In the finite element model, the CF-SMC body adopts solid elements, the carbon fiber prepreg adopts two-dimensional elements to give composite laminate properties, the CF-SMC unit and the prepreg unit adopt co-node treatment, the metal attachment adopts solid unit, and the CF-SMC unit adopts co-node processing. The load decomposition of the maximum braking condition is used as the load, and the constraint method adopts inertia release, and the finite element model is shown in Figure 7.
Figure 7 Finite element model
The final analysis results are shown in Figure 8. According to the analysis results, the strength of the material is also compared, and each part of the carbon fiber control arm meets the strength requirements. From the analysis of the cloud map, it can be seen that the three prepregs play an effective bearing role, and the failure degree of Tsai-Wu is controlled by reducing the amount of fabric deformation. In addition, with CF-SMC as the main body, the maximum stress of the CF-SMC part is controlled at <150 MPa. The overall results of the CAE analysis are in full agreement with the idea of the structural scheme.
Figure 8 Analysis results of the final solution
05 Process solution and manufacturing
The carbon fiber control arm body adopts a hybrid molding method, and the process scheme is shown in Figure 9.
Figure 9 Hybrid molding process
Specific manufacturing process: first the prepreg is preformed to form a preform, and then the preform is laid in the mold with CF-SMC, and then the prepreg, CF-SMC and metal accessories are molded in one piece to form the control arm body, and then the ball and bushing are installed to obtain a carbon fiber control arm, the specific process is shown in Figure 10.
Figure 10 Manufacturing process of carbon fiber control arm
06 Performance test
6.1 Non-destructive testing
The sample was inspected with CT (electronic computed tomography) equipment and the results were shown in Figure 11.
Figure 11 CT photo of carbon fiber control arm
Through non-destructive testing, it can be seen that there are local void defects, the first reason is that the product thickness is too large, but the product thickness is designed according to performance requirements, and there is not much room for optimization. Other causes of the appearance of defects: poor exhaust; High fiber content and poor fluidity; Insufficient molding pressure. Summarizing the above reasons, the following directions are formulated for subsequent process improvement: add vacuum device to the mold; Under the premise that the performance meets the requirements, the fiber content is slightly reduced; Tentatively increases molding pressure.
6.2 Static strength test
The static strength test of the carbon fiber control arm was carried out, the front point and the back point of the control arm were fixed, and the loading test was carried out in X+, Y- and Y+3 directions for the outer point of the control arm, and the test results were shown in Table 3, and the failure state was shown in Figure 12.
Table 3 Static strength test results
Note: "——" is a value that is not destroyed and therefore has no destructive power.
Figure 12 Carbon fiber control arm failure state
The carbon fiber control arm has no yield process and is brittle fractured. Among them, the loading breaking strength of X+ and Y- direction is about 10 kN, and both of them break at the ball head, which is slightly different from the ideal state. By observing the cross-section of CF-SMC, it is found that the fibers at the fracture have a large part of the peeling phenomenon, which does not play an effective bearing role. CF-SMC fiber orientation and paving method, material fluidity and structural form related, largely rely on the fluidity of the material for filling and welding, because the ball head belongs to the large attachment thin-wall inlay method, the paving method is limited, in the case of CF-SMC poor fluidity, the fibers at the weld can not be staggered, resulting in low strength. Improvements will be made in the direction of improving material flow, improving the spreading method and local reinforcement of the prepreg.
a. This paper takes the forged aluminum control arm as the prototype, uses the hybrid process of carbon fiber prepreg and CF-SMC to develop the carbon fiber composite control arm, analyzes the mechanical properties with the maximum brake as the typical working condition, and elaborates the iterative ideas and analysis process from the concept scheme to the process scheme to the final scheme, and the quality of the final carbon fiber control arm is reduced by 21%.
b. This paper elaborates the manufacturing process of carbon fiber control arm, and analyzes the causes of defects and improvement ideas in detail according to the subsequent performance test results.
c. The carbon fiber control arm developed in this paper is a forward-looking exploration of carbon fiber composite for chassis components, and the process and related technologies used in this paper have guiding significance for other chassis components.
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Source | Journal - Automotive Technology and Materials
Authors: Equation 1 Wang Changbin1 Jackie Chan 2 Li Jichuan2 (1.Institute of Materials and Lightweight of China FAW Co., Ltd., Changchun 130011, China; 2. R&D Institute of China FAW Co., Ltd., Changchun 130011, China)