Abstract
This thesis focuses on the design and optimization of the investment casting process for car door bodies. By combining low-pressure casting with investment casting, the manufacturing process of car door bodies can be simplified, production cycles shortened, and product quality improved. The castings produced have high dimensional accuracy, low surface roughness, retained surface dense layers, reduced stress concentrations, dense internal structures, and high process yields. This thesis provides a comprehensive analysis of the door body structure, and low-pressure investment casting process, aiming to optimize the casting process parameters and improve product performance.
1.1 Research Background and Significance
With the rapid development of the automotive industry, the demand for lightweight, high-strength, and high-precision car components is increasing. The car door body, as an important component of the automobile body, requires not only high structural strength and stiffness but also good appearance quality. Traditional manufacturing processes such as stamping and welding have limitations in achieving these requirements. Investment casting, with its ability to produce complex shapes and high-precision castings, has become an important manufacturing method for car door bodies.
1.2 Research Status at Home and Abroad
1.2.1 Classification and Research Status of Door Body Manufacturing Processes
Currently, door body manufacturing processes can be broadly classified into stamping, welding, and casting. Stamping processes have high material utilization rates and good production efficiency but are limited in terms of complexity and accuracy. Welding processes can achieve higher integration but may suffer from welding defects and stress concentrations. Casting processes, especially investment casting, can produce complex shapes with high precision and good surface quality.
1.2.2 Research Status of Door Body Structure Optimization
Many researchers have focused on optimizing door body structures to improve their performance. For example, Azim A used CAE software to analyze the performance of aluminum alloy door bodies and optimize their structures to reduce weight. Lee et al. optimized the door inner panel using topological optimization, size optimization, and laser welding technology.
1.2.3 Research Status of Investment Casting
Investment casting is widely used in the production of complex and precision metal parts. It has the advantages of high dimensional accuracy, low surface roughness, and the ability to retain surface dense layers. Researchers have continuously optimized the investment casting process to improve casting quality and reduce production costs.
1.2.4 Research Status of Casting Numerical Simulation Technology
Numerical simulation technology has become an important tool in casting process design and optimization. Software such as ProCAST can simulate the filling and solidification processes of metal liquids, predict defects, and optimize casting process parameters.
2. Structural and Performance Simulation Analysis of Door Body
2.1 Types of Door Structures
Car door structures can be classified into stamped door bodies, extruded door bodies, and cast door bodies. Stamped door bodies are the most commonly used, but they have limitations in terms of complexity and precision. Extruded door bodies have higher integration and strength but are difficult to produce in large quantities. Cast door bodies can achieve complex shapes and high precision, making them an ideal choice for lightweight and high-performance door bodies.
2.2 Lightweight Materials
Lightweight materials such as aluminum alloy and magnesium alloy are widely used in car door bodies to reduce weight and improve fuel efficiency. ZL114A aluminum alloy is commonly used in investment casting due to its good castability and mechanical properties.
2.3 Structural Characteristics of Integrated Door Body
Integrated door bodies, which combine multiple components into a single casting, have the advantages of reduced assembly work, improved dimensional accuracy, and increased structural strength.
2.4 Finite Element Modeling of Door Body
High-quality finite element models of door bodies were established using Hypermesh software and imported into Ansys software for modal and stiffness analysis. The results showed that the door body met performance requirements.
Table 2.1 Material Properties of ZL114A Alloy
| Alloy Code | Poisson’s Ratio | Density (g/cm³) | Elastic Modulus (GPa) |
|---|---|---|---|
| ZL114A | 0.3 | 2.65 | 69 |
3. Wax Molding Process Design and Optimization for Car Door Body
3.1 Gate Location Analysis
Gate location is crucial for the quality of wax molds. Four gate location schemes were designed and analyzed using Moldflow software. The optimal gate scheme was selected based on mold-filling simulations.
Table 3.1 Gate Location Schemes
| Scheme | Description |
|---|---|
| Scheme 1 | Gates located on one side of the door body |
| Scheme 2 | Gates located on both sides of the door body |
| Scheme 3 | Gates located at the top and bottom of the door body |
| Scheme 4 | Gates located at multiple positions on the door body |
3.2 Ventilation and Cooling System Design
A ventilation system was designed to ensure smooth mold filling and prevent air entrapment. A cooling system was also designed to control the solidification process and reduce defects.
3.3 Process Parameter Optimization
The effects of melt temperature, mold temperature, wax injection time, and holding pressure on wax mold quality were studied using single-factor experiments. Orthogonal experiments were designed to optimize process parameters.
4. Design and Optimization of Low-Pressure Investment Casting Process for Car Door Body
4.1 Door Body Structure Analysis
The door body has a complex structure with a window frame, outer streamline shape, and various installation positions for components such as door locks and speakers. The minimum wall thickness is 1.5 mm, and the maximum thickness of reinforcing ribs is 3 mm.
4.2 Low-Pressure Investment Casting Process Introduction
Low-pressure casting involves applying pressure to the metal liquid surface to achieve smooth mold filling and avoid defects such as splashing and turbulence. Investment casting involves creating a precise mold shell using a wax pattern, which is then melted out and filled with metal liquid.
4.3 Casting Process Parameter Selection
4.3.1 Determination of Casting Taper
Appropriate casting tapers were set to facilitate mold release. The casting taper for the outer surface with a height greater than 100 mm and a non-machined surface with a thickness less than 5 mm was set to 0°10′, and the machining taper for the inner surface was set to 0°15′.
Table 4.1 Casting Tapers
| Type | Application Scope | Casting Taper (°) |
|---|---|---|
| Increasing wall thickness | Non-machined surface, wall thickness < 5 mm | 0°10′ (outer), 0°15′ (inner) |
4.3.2 Dimensional Tolerance
The dimensional tolerance of the door body was determined to be 1.02 mm based on the maximum size of 1248 mm and the material used (ZL114A alloy).
Table 4.2 Dimensional Tolerances of Molten Mold Casting
| Basic Casting Dimension (mm) | General Tolerance (mm) | Special Tolerance (mm) |
|---|---|---|
| >203.6 to 228.6 | ±0.86 | ±0.46 |
| >228.6 to 254.0 | ±0.94 | ±0.48 |
| >254.0 | ±1.02 | – |
4.3.3 Shrinkage Rate
The shrinkage rate of the casting was determined to be 1% based on factors such as alloy type, mold material, and shell selection.
4.4 Gating System Design
The gating system, consisting of a pouring cup, sprue, runner, and ingate, was designed to ensure smooth mold filling and high process yield. The ingate was designed to have a cross-sectional area of 48 cm² based on the calculated filling time and metal liquid velocity.
Table 4.3 Cross-Sectional Area Ratios of Gating System Components
| Gating System Component | Cross-Sectional Area Ratio |
|---|---|
| Sprue | – (dependent on design) |
| Runner | – (dependent on design) |
| Ingate | Determined by filling time and metal liquid velocity |
4.5 Numerical Simulation and Analysis
The filling and solidification processes of the metal liquid were simulated using ProCAST software. Based on the simulation results, the gating system was optimized, and the effects of pouring temperature, mold preheating temperature, and filling time on casting quality were studied using single factor variable experiments. Furthermore, a 3-factor, 3-level orthogonal experiment was designed to determine the optimal process parameters. The results indicated that the optimal pouring temperature was 720°C, the mold preheating temperature was 480°C, and the filling time was 16 seconds. These parameters ensured stable filling and effective reduction of casting defects.
During the numerical simulation process, various defects such as porosity and shrinkage were predicted based on the solidification behavior of the metal liquid. Subsequently, measures for feeding and shrinkage optimization were implemented. The selection of cooling media and the determination of cooling locations were crucial steps in this process. Liquid nitrogen was utilized to enhance the cooling effect, and the amount of liquid nitrogen used was calculated based on specific criteria. After optimization, the volume of porosity and shrinkage within the casting was significantly reduced to 49 cm³, demonstrating the effectiveness of the proposed measures.
In addition to optimizing the pouring system and process parameters, attention was also paid to the design of feeding systems to ensure adequate feeding during the solidification process. This involved the strategic placement of feeding channels to provide metallic liquid to areas prone to shrinkage, thereby minimizing the formation of defects.
Overall, the numerical simulation and analysis conducted in this study provided valuable insights into the filling and solidification processes of the metal liquid during the low-pressure investment casting of car door bodies. The optimized process parameters and design measures significantly improved the quality of the castings, making them more suitable for practical applications.