1. Research Background and Significance
1.1 Background
In the past century, the global automotive industry has witnessed significant growth. By the end of June 2021, China’s vehicle ownership had reached nearly 300 million, ranking first in the world. As the national economy continues to develop, China’s car sales are still increasing year by year. To address energy conservation and greenhouse gas emission reduction, automotive lightweighting has become a crucial research direction for automotive companies and research institutions. Door bodies, which account for a significant portion of the vehicle’s mass, play a vital role in achieving lightweighting goals. Traditional door manufacturing methods, such as stamping and welding, are complex and not conducive to automotive lightweighting. In recent years, aluminum alloy has emerged as a promising material for door manufacturing due to its low density, high specific strength, and good corrosion resistance. However, the welding of aluminum alloy door panels presents challenges such as oxidation slag inclusion and increased material redundancy. Against this backdrop, the aluminum alloy integrated investment casting process for door bodies has emerged as a potential solution.
1.2 Significance
This research proposes an aluminum alloy door investment casting process that combines the advantages of low-pressure casting and investment casting. By using aluminum alloy with low density and high specific strength, along with the precision investment casting process for thin-wall castings, this method can effectively reduce the weight of the door body while meeting the performance requirements of the vehicle. The integrated casting process simplifies the manufacturing process, shortens the production cycle, and improves the quality of the castings. It also reduces the need for subsequent machining, lowers manufacturing costs, and contributes to energy conservation and emission reduction.
2. Research Status at Home and Abroad
2.1 Door Manufacturing Process Classification and Research Status
2.1.1 Stamping Forming Technology
Stamping is a widely used forming method in the automotive industry. It involves applying external force to sheet metal to obtain the desired shape and size. However, stamping has limitations such as the production of burrs, wrinkles, cracks, and springback. Additionally, it can only produce parts with simple shapes and uniform thickness, leading to increased part count and longer assembly times, which are not conducive to automotive lightweighting.
2.1.2 Extrusion Casting Forming Technology
Extrusion casting combines casting and forging processes by applying pressure to the molten metal in the mold cavity. While it can produce complex-shaped parts, it requires large extrusion casting machines and has issues such as rapid filling speed, which can lead to gas entrapment and turbulence, resulting in casting defects and poor microstructure.
2.1.3 Pressure Casting Forming Technology
Pressure casting offers high production efficiency and good casting accuracy and mechanical properties. However, the high injection pressure can cause gas entrapment, leading to porosity in the castings and affecting their mechanical properties and surface quality. Moreover, the equipment cost is high, and the castings cannot be heat-treated.
2.1.4 Advanced Manufacturing Process Research
In recent years, there has been a growing interest in combining low-pressure and investment casting processes to produce complex aluminum alloy parts. This approach has shown potential in improving casting quality and reducing defects.
2.2 Door Structure Optimization Research Status
With the development of computer technology, finite element analysis software has been widely used in door structure optimization. By simulating the performance of the door under different working conditions, engineers can optimize the structure to reduce weight while maintaining performance.
2.3 Investment Casting Research Status
Investment casting is a precision net-shape forming process widely used in industries such as aerospace and automotive. It has a long history and has evolved from technology introduction to independent innovation in China. Recent advancements include the development of advanced equipment and process technologies, enabling the production of high-precision castings.
2.4 Casting Numerical Simulation Technology Research Status
Numerical simulation has become an essential tool in the casting industry. It allows engineers to predict casting defects, optimize the gating system, and adjust process parameters to improve casting quality and reduce production costs.
3. Research Contents and Methods
3.1 Structural Performance Analysis of the Car Door Body
3.1.1 Finite Element Model Establishment
The car door body is modeled using Hypermesh for meshing and then imported into Ansys for analysis. The geometric model is preprocessed by cleaning and repairing to ensure the quality of the mesh. The material properties of the ZL114A alloy are defined, and the finite element model is generated.
3.1.2 Modal Analysis
Modal analysis is performed to evaluate the dynamic characteristics of the door body. The results show that the first-order natural frequency of the door body is 42.2Hz, which is greater than the excitation frequencies of the engine and road surface, ensuring that resonance does not occur during vehicle operation.
3.1.3 Stiffness Analysis
The stiffness of the door body under different loading conditions, including sinking, torsional, window frame, and waistline stiffness, is analyzed. The results indicate that the stiffness values meet the requirements of the enterprise standards, ensuring the normal operation of the door and the comfort of passengers.
3.2 Wax Pattern Molding Process Design
3.2.1 Mold Flow Analysis and Gate Location Selection
Mold flow analysis is conducted using Moldflow software to evaluate different gate location schemes. Based on the analysis of filling time, flow front temperature, volume shrinkage rate, weld line, and pressure, the optimal gate location scheme is determined.
3.2.2 Design of Pouring, Cooling, and Venting Systems
The pouring system is designed to ensure smooth filling of the mold cavity. The cooling system is optimized to reduce the cooling time and improve the quality of the wax pattern. The venting system is designed to prevent air entrapment during injection.
3.2.3 Single-Factor and Orthogonal Experiments
Single-factor experiments are carried out to study the effects of melt temperature, mold temperature, injection time, and holding pressure on the quality of the wax pattern. Orthogonal experiments are then designed to optimize the process parameters. The results show that the optimal process parameters are: melt temperature of 56°C, mold temperature of 33°C, injection time of 8s, and holding pressure of 60% of the filling pressure, with a volume shrinkage rate of 3.863%.
3.3 Low-Pressure Investment Casting Process Design and Optimization
3.3.1 Process Parameter Calculation
The process parameters for low-pressure investment casting, including casting slope, dimensional tolerance, and casting shrinkage rate, are determined based on the characteristics of the door body. The gating system is designed, and the cross-sectional areas of the runner and ingate are calculated.
3.3.2 Numerical Simulation and Optimization
Procast software is used to simulate the filling and solidification processes of the casting. The initial pouring scheme is analyzed and optimized based on the simulation results. The effects of pouring temperature, mold preheating temperature, and filling time on the casting quality are studied using single-factor and orthogonal experiments. The optimal process parameters are determined as: pouring temperature of 720°C, mold preheating temperature of 480°C, and filling time of 16s.
3.3.3 Process Feeding Optimization
To reduce shrinkage porosity defects, a liquid nitrogen forced cooling method is proposed. The cooling location and liquid nitrogen usage are calculated, and the process is optimized. The results show that the volume of shrinkage porosity is reduced to 4.9 cm³, significantly improving the casting quality.
4. Results and Discussions
4.1 Structural Performance of the Car Door Body
The finite element analysis results demonstrate that the designed car door body meets the performance requirements in terms of modal and stiffness characteristics. The optimized structure ensures the door’s functionality and durability while reducing its weight.
4.2 Wax Pattern Molding Process Optimization
The optimized wax pattern molding process results in a high-quality wax pattern with a minimum volume shrinkage rate. The selection of the optimal gate location and process parameters significantly improves the quality and dimensional accuracy of the wax pattern.
4.3 Low-Pressure Investment Casting Process Optimization
The optimized low-pressure investment casting process produces castings with minimal shrinkage porosity defects. The combination of process parameter optimization and the use of liquid nitrogen forced cooling effectively improves the casting quality and meets the requirements of automotive applications.
5. Conclusions and Future Work
5.1 Main Research Achievements
- The structural performance of the car door body is analyzed, and the finite element model is established and verified.
- The wax pattern molding process is optimized, and the best process parameters are obtained.
- The low-pressure investment casting process is designed and optimized, and the casting quality is improved.
5.2 Innovations
- The integrated investment casting process for the car door body is proposed, reducing the manufacturing process and weight.
- The venting holes in the wax pattern are utilized for gating and connection, improving the casting process.
5.3 Future Research Directions
- Further optimization of the door body structure, including strength and crash analysis.
- Exploration of other parting surface and gate location schemes for the wax pattern molding process.
- In-depth study of casting defects such as grain structure and oxidation slag inclusion.