The WUT team of Wuhan University of Technology is upgrading from traditional steel tube suspension to special tube structure suspension. The traditional way of connecting round tubes with threaded sleeves limits lightweighting. The new structure needs to have sufficient design flexibility to meet the stiffness requirements of each part and control the quality as much as possible. A one-piece control arm design based on metal 3D printing is proposed. The design adopts a hollow structure supported by a three-dimensional structure, and the shape is designed according to the load characteristics and the three-dimensional structure is used to ensure rigidity and control the quality. Metal 3D printing uses lighter materials such as aluminum and titanium to achieve weight reduction expectations and ensure performance.
1 Research Background
The China Formula Student Competition (FSAE) is a car design and manufacturing competition for teams of students majoring in automotive engineering or automotive-related majors in colleges and universities. Each participating team is required to design and manufacture a small single-seat racing car with excellent performance in acceleration, braking, and handling within 1 year in accordance with the competition rules and racing car manufacturing standards, and can successfully complete all or part of the competition.
With the popularity of FSAE racing and the increasing maturity of technology, teams are not only satisfied with stability and reliability. Lightweighting, as an effective means to improve vehicle performance, has become a common focus of all teams. Lightweighting of control arms reduces the weight of the entire vehicle while reducing the unsprung mass. When the unsprung mass increases, the initial peak value of the sprung mass acceleration decreases, but the overall acceleration amplitude changes and the change of the sprung mass acceleration becomes more drastic as the unsprung mass weight ratio increases. The tire contact force changes significantly, resulting in poor tire grip performance and affecting vehicle handling stability[1].
Many domestic and foreign teams, including the Wuhan University of Technology WUT team, have tried to use carbon fiber tubes bonded to aluminum alloy joints to make control arms. Some universities have taken the lead in using metal 3D printing technology to further reduce weight. The “Arian” of the Group team of the University of Leuven in Belgium is the world’s first racing car with most parts made using 3D printing. It participated in the 2012 Formula Student and achieved excellent results[2]. The Dresden University of Technology team uses metal 3D printing to make steering knuckles, while the Baden-Württemberg State Cooperation University team even uses metal 3D printed rocker arms.
Based on the characteristics of 3D printing, flexible design, free design of multi-section curved surfaces with large curvature changes, and lightweight, it is planned to use metal 3D printing as a production method for integrated control arm design.
2 Control arm design
In previous years, the Wuhan University of Technology WUT team used carbon fiber round tube glued aluminum alloy joint control arms and steel tube control arms. According to the force analysis of the steel control arm, it was found that the plane connected to the push rod produced a large stress, and the contact part of the lug and joint connected to the push rod had stress concentration; and the control arm cross arm strength redundancy means that additional mass is added. Therefore, the design is made for these two points during the design. After discussion and review of relevant data, it was found that the use of a skeleton combined with a stress skin is the best way to solve local torsional stress and vertical stress [3]. Therefore, it is proposed to adopt the design idea of giving priority to designing and optimizing the shape of the control arm, making a skeleton according to the shape and covering the stress skin.
2.1 Control arm shape design
This design uses the upper control arm connected to the push rod on the right rear side as the design object, and uses acceleration out of the left turn as the constraint condition. According to the suspension design of the 2021 season, a suspension model is established in VI-Grade, and an xml file is generated to establish a vehicle model. The vehicle dynamics simulation is carried out on the AutoCross track, and the displacement data of the rear suspension shock absorber is basically consistent with the design value. The shock absorber displacement data can be used to calculate the push rod force.
The method for calculating the push rod force Fp is as follows: See formula (1) in the figure
In formula (1): lp is the force of Fp on the rocker arm plane to the center distance of the rocker arm; Fs is the force of the spring on the rocker arm; ls is the force Fs to the center distance of the rocker arm.
It is known that the stiffness of the rear suspension shock absorber is 61 250 N/m, and the maximum compression of the spring is 21.958 mm. According to Hooke’s law, Fs=1 344.868 N, and the direction is along the shock absorber to the rocker arm. The push rod force Fp is directed along the push rod to the rocker arm, and the magnitude is unknown. By calculation, it is found that Fp=1 019.49 N.
The push rod force is decomposed into vectors in the X, Y, and Z axis directions to obtain the force vector coordinates (0, -599.668, 824.476) as the finite element analysis constraint conditions. The required design space is created in CATIA, and the functional holes of the hard points are reserved. The load condition is set on the substrate using the Ansys static analysis module, and the topology optimization module is used for preliminary optimization. The results are shown in Figure 1. It can be found that the positive stress below the lifting ear is large, and more structures are retained. The load on the rear cross arm is greater than that on the front cross arm, the structure is less removed, and the cross section changes little; the load on the front cross arm is smaller, the cross arm is thinner, and the cross section changes greatly. Because the control arm needs to avoid stress concentration due to sudden changes in the cross section, the control arm shape is designed as a smooth curved surface. The specific design ideas are as follows.
In CATIA generative shape design, import the model three views to trace the control arm contour. According to the contour, add details, replace the front and rear cross arms of the control arm with smooth curved surfaces, and bridge the part between the two curved surfaces. Cut off the redundant part of the curved surface connected to the ear in the middle, the ear and the bearing seat, bridge the unclosed part, merge all the curved surfaces into a whole, and use the closed surface operation to convert the whole into a solid whole. Model the bearing seat and the ear separately in the part design module, and finally assemble all the parts to complete the modeling, as shown in Figure 2.
2.2 Design of the internal skeleton of the control arm
The dynamic events in the FSAE competition include high-speed obstacle avoidance, endurance race, straight-line acceleration and figure-eight turn. Except for straight-line acceleration, the other events put the car in a turning condition. In the turning condition, the key to shortening the time is to pass the corner stably at a faster speed and with the smallest turning radius [4]. The endurance race and high-speed obstacle avoidance use the gymkhana track, which has many and continuous bends. During the race, the driver often needs to turn the steering wheel quickly and continuously to pass the track in the shortest time. During the race, the load transfer of the car is fast, so the force of the push rod on the control arm can be regarded as an impact load.
In summary, the internal skeleton of this design needs to reduce the negative impact of the impact load on the control arm. It should meet the requirements of small deformation under impact load, good stiffness, average safety factor of more than 3, minimum safety factor of not less than 1.3, and lighter weight than the steel tube control arm. The internal skeleton of the control arm, the preliminary design proposes two design ideas: adding beams directly as the skeleton or adding a lattice sandwich structure as the skeleton. This design intends to compare the stress distribution, quality and other indicators of the two design schemes by establishing a unit body, so as to determine the final design scheme.
2.2.1 Lattice sandwich structure vs. ribs
In the design space of 50 mm×50 mm×20 mm, typical cross ribs and lattices are established respectively, and the mechanical performance of the thin shell connected to the outer layer as the sandwich layer under various stress conditions is analyzed and compared. By applying vertical, lateral, bending moment, torque, far-end composite force and other loads, the two sandwich arrangements are analyzed. Taking the vertical load as an example, it can be found that under the same mass level, the ribs have greater strength, but the lattice structure makes the stress more evenly distributed on the load surface and the sandwich layer. The stress cloud diagram of the ribs and the lattice sandwich is shown in Figure 3.
The rib structure is very strong, but when faced with complex working conditions, stress concentration is prone to occur at the joints and bends, and it is difficult to handle the load applied to the gaps between the ribs, which is very easy to fail. The Lattice lattice structure can adapt to more complex stress conditions, and has uniform and continuous support for the sandwich surface, and has better mechanical properties. For the special-shaped control arm type parts with complex stress and special structure, the ribs are difficult to arrange and cannot play their advantages, while the lattice can fully adapt to the curved sandwich surface, change the unit size according to different stresses, make the stress distribution of the component more uniform, achieve lower quality, and give full play to the advantages of 3D printing. In summary, the lattice sandwich structure is more suitable for this design, and it is finally decided to use the lattice sandwich as the skeleton structure.
2.2.2 Ideas for establishing lattices
RADFORD et al. [5] pointed out that there are three stages in the impact process of sandwich structures: the first stage is the interaction between fluid and structure, the second stage is the compression of the core, and the third stage is the bending and stretching of the sandwich structure. This design does not need to consider the first stage.
According to the setting of the load condition, a single lattice arrangement obviously cannot meet the design goal. The design space is iterated using the Lattice optimization method of the Optimize module in SolidThinking Inspire to seek the best lattice unit condition setting. The lattice will have structural characteristics similar to trusses, not restricted by shape, and its diameter changes with the force condition within the set range to ensure better mechanical properties. Select minimize mass in the optimization module, and adjust the unit target length and diameter range respectively. Change the setting conditions according to the material properties and compare the results to select the best constraint conditions, as shown in Figure 4.
By analyzing the unit diameter range, the minimum unit diameter is 1 mm and the maximum unit diameter is 4.2 mm. Selecting a longer target unit length can obtain a lighter structure, and the optimal target length is 21 mm.
This setting can achieve the maximum lightweight while meeting the strength requirements of each position. It reduces the lattice density at the position with less stress, arranges a more slender lattice, and generates a denser and shorter lattice structure at the position with greater stress such as the lug, bearing seat and ridge to meet the strength requirements.
Commonly used metal materials for metal 3D printing include 304 stainless steel, 316L stainless steel, AlSi10Mg (indicated by al in the figure), and Ti6Al4V titanium alloy (indicated by ti in the figure). Their mechanical properties, density, price, and processing difficulty vary. It is necessary to modify the optimization conditions for different materials and compare and analyze the iterative optimization results, as shown in Figure 5.
It can be seen from Figure 5 that under the same strength and safety requirements, the optimization results of 304 steel tend to fill the large diameter structure, lose its flexible characteristics, have a large mass, and the lattice optimization results are not ideal. However, aluminum alloy and titanium alloy produce large lattice diameter changes during the optimization process, and maintain extremely low mass under the condition of achieving the minimum safety factor. Comparing the lattice structures of the two materials, aluminum alloy can achieve more optimized results in the short and thick unit body setting, while titanium alloy can obtain more slender and loose units due to its excellent mechanical properties. After several rounds of iterative optimization, it can be found that the optimized lattice structure of aluminum alloy material has a lighter mass and a lower price than titanium alloy. Aluminum alloy is also less difficult to finish the bearing seat than titanium alloy, so it is decided to use AlSi10Mg material. The final iteratively optimized lattice structure reaches a mass of 64.72 g while meeting the minimum safety factor of 1.3, which is 80.04% lighter than the original design space of 324.3 g. While meeting safety requirements, the weight reduction effect is as expected.
On the basis of the optimized lattice, a thin shell is added to obtain a smooth outer surface and better surface performance, as shown in Figure 6. The finally assembled control arm structure has a smoother appearance, allowing airflow to flow more smoothly, and has better mechanical properties under typical control arm stress conditions. After matching, the overall mass is finally fixed at 105.7 g, which is significantly lighter than the traditional control arm and has better mechanical properties. The final vehicle performance is shown in Figure 7.
3 Conclusion
Compared with traditional derivative design, 3D printed stress skin design can obtain a more complete and smooth appearance. Lattice sandwich has ever-changing microstructures and high porosity, and also has excellent properties such as light weight, high strength, explosion resistance, impact resistance, high efficiency heat dissipation and heat insulation, and absorption of electromagnetic waves and sound. In the student formula, the combination of 3D printing technology and lattice sandwich technology solves the problem of poor mechanical properties of standard parts. The processing problem of special-shaped structures allows college students to make more bold and radical designs, which promotes the development of student formula technology. In the manufacturing industry, Czinger’s 21C supercar uses metal 3D printing technology to manufacture the chassis and body. The plate-shaped cubic lattice structure energy absorption box has better impact resistance and energy absorption characteristics than the traditional energy absorption box, and has broad application prospects in the field of lightweight automotive passive protection [6]. In summary, both 3D printing technology and lattice sandwich technology have broad prospects. With their extremely high design flexibility, they can be applied to the body, power and chassis systems of automobiles, with a wide range of applications.
