Abstract:
In this paper, we explore the significant challenges posed by controlling the dimensional accuracy of large die-casting in the manufacture of electric vehicles (EVs). Specifically, we focus on the SUV model Wenjie M9, launched by Seres Automobile, which adopts an advanced integrated die-cast aluminum chassis. This innovation effectively reduces unsprung mass, thereby enhancing handling performance. However, achieving precise dimensional control in die-casting remains a critical industry challenge. We propose an optimization strategy targeting various aspects, including the die-casting process, exemption from heat treatment materials, fixture equipment for shaping, process control, and press parameter adjustment. This strategy not only contributes to advancing EV manufacturing standards but also offers valuable insights to the die-casting industry.
1. Introduction
As the global automotive industry increasingly prioritizes energy conservation and environmental protection in line with “dual carbon” goals, the development of EVs has garnered significant support from various countries. Leading EV manufacturers such as Tesla, NIO, and XPeng have actively invested in integrated die-casting technology, anticipating that die-casting machines will gradually replace welding robots as the core equipment in EV manufacturing. The essence of this technology lies in the remarkable performance of large die-casting machines, innovative material formulas exempt from heat treatment, precise die designs, and optimized die-casting process parameters. Notably, aluminum alloy integrated die-casting technology stands out due to its lightweight and efficient production characteristics, becoming a standard technology for EV manufacturers. This paper delves into the application of integrated die-casting technology in controlling the dimensional accuracy of EV rear body structures.
2. Overview of Die-Casting Technology
2.1 Definition of Die-Casting
Die-casting, or High Pressure Die Casting (HPDC), is a molding process where molten liquid or semi-solid metal is pressed into a mold cavity under high pressure (20–120 MPa) and speed (20–100 m/s) and solidified under external pressure. In die-casting, the molten metal rapidly fills the mold cavity. Under external pressure, it maintains consistency with the mold cavity in size and shape. Furthermore, the external pressure causes the molten metal to adhere tightly to the mold, enhancing the heat dissipation of the mold-casting assembly. This results in rapid solidification of the molten metal within a short period, producing castings with fine grains and uniform structures.
2.2 Integrated Die-Casting
Integrated die-casting involves redesigning traditional separate and dispersed small components for high integration. Utilizing large die-casting machines for one-time molding eliminates the welding process, resulting in a complete large-scale component.
2.3 Differences Between Traditional and Integrated Die-Casting
Table 1: Comparison Between Traditional and Integrated Die-Casting
| Aspects | Traditional Die-Casting | Integrated Die-Casting |
|---|---|---|
| Material Selection | Mature material properties, can undergo T6/T7 heat treatment | Larger size, complex shape; requires exemption from heat treatment material |
| Mold Design | Smaller size, simpler structure; less demanding design requirements | Larger size, complex structure; higher requirements for pouring, mold temperature, ejection, and vacuum settings |
| Equipment | Smaller projection area, lower locking force requirements for die-casting machines | Larger integrated components, higher locking force requirements, typically ≥6000 tons |
| Process | Relies on experience and empirical formulas | Limited similar products,突破traditional process parameters; higher purity, low gas-emitting coating materials, and pre-mold vacuum requirements |
| Application | Motor shells, transmission cases, electronic control housing | Torsion beams, front compartment assemblies, rear floors, and side pillar inner panels |
3. Manufacturing Process of Integrated Die-Cast EV Rear Bodies
The manufacturing process of integrated die-cast EV rear bodies is precise and complex, involving multiple key steps from die casting the blanks to assembling standard parts and delivering qualified products.
4. Dimensional Accuracy Control Scheme
4.1 Material Selection
To minimize deformation in large castings after heat treatment, the primary approach is to develop materials exempt from heat treatment. Research in automotive structural components focuses on Al-Si and Al-Mg series aluminum alloys. These materials achieve comparable mechanical properties to traditional materials through elemental addition and optimized die-casting conditions.
Table 2: Chemical Composition and Mechanical Properties of C611 and SF36
| Alloy Grade | Si (%) | Fe (%) | Cu (%) | Mn (%) | Mg (%) | Zn (%) | Ti (%) | Sr (%) | Al (%) | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation A50 (%) | Average Bend Angle (°) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C611 | 2.0–7.0 | 0.15–0.2 | 0.4–0.8 | 0.15–0.25 | 0.1 | 0.01–0.015 | Balance | – | – | 220 | 105 | ≥45 | – |
| SF36 | 9.5–11.5 | ≤0.15 | ≤0.03 | 0.5–0.8 | 0.1–0.5 | ≤0.07 | ≤0.15 | 0.01–0.03 | Balance | 180 | 120 | ≥60 | – |
4.2 Fixture Equipment for Shaping
4.3 Process Control
4.4 Press Parameter Adjustment
Table 3: Press Parameter Settings
| Parameter | Setting Range |
|---|---|
| Pressure (Injection) | 30–90 MPa |
| Pressure (Intensified) | 50–300 MPa |
| Speed (Low Stage) | 0.1–0.5 m/s |
| Speed (High Stage) | 3–6 m/s |
| Speed (Gate) | 30–60 m/s |
| Filling Time | Determined by casting volume, average thickness, and filling speed |
| Dwell Time (Thin Wall) | 1–2 s |
| Dwell Time (Thick Wall) | 3–5 s |
| Mold Retention Time | Determined by product size, structure, and process temperature |
| Pouring Temperature | 700–740 °C |
| Preheated Mold Temperature | 150–180 °C |
| Continuous Working Temperature | 180–220 °C |
5. Comparison Before and After Improvement
Through optimization measures in four aspects, including material selection, fixture equipment for shaping, process control, and press parameter adjustment, the overall dimensional accuracy of die-castings has been significantly improved. To quantify this improvement, sampling measurements and detailed analyses were conducted on the integrated rear bodies before and after the improvement.
Table 4: Descriptive Statistics
| Sample | N | Mean | Standard Deviation | Error |
|---|---|---|---|---|
| Before Rectification | 9 | 9.52 | 1.83 | 0.61 |
| After Rectification | 9 | 8.30 | 0.13 | 0.04 |
From the table, it can be seen that the mean value before rectification was 9.52, with a standard deviation of 1.83, indicating a relatively dispersed size distribution. After rectification, the mean value dropped to 8.30, and the standard deviation decreased to 0.13, suggesting a significant improvement in dimensional accuracy and a more concentrated size distribution.
Table 5: Difference Estimation and Hypothesis Testing
Although the complete content of Table 5 is not directly given in the original text, based on the descriptive statistics and common hypothesis testing methods, we can infer the possible contents of Table 5.
Difference Estimation: The mean difference before and after rectification is 1.22 (9.52 – 8.30), which is an important indicator for measuring the effectiveness of the improvement.
Pooled Standard Deviation: The pooled standard deviation is commonly used in hypothesis testing to calculate the T-value. Although the specific value is not given in the original text, it can be estimated based on the difference and standard deviation calculation formulas. However, here we mainly focus on the results of the hypothesis testing.
95% Lower Limit of the Difference: This is the lower limit of the confidence interval for the difference estimate, used to determine whether the improvement effect is significant. The original text gives a value of 0.15, indicating that we have 95% confidence that the dimensional accuracy after rectification is at least 0.15 higher than before.
Hypothesis Testing:
Null Hypothesis (H0): There is no significant difference in dimensional accuracy before and after rectification (i.e., μ1 – μ2 = 0).
Alternative Hypothesis (H1): The dimensional accuracy after rectification is significantly improved (i.e., μ1 – μ2 > 0).
Hypothesis testing is conducted based on the T-value, degrees of freedom, and P-value:
T-value: The T-value measures the relative size of the difference compared to the pooled standard deviation, used to determine whether the difference is significant. The original text gives a value of 2.0016.
Degrees of Freedom: The degrees of freedom are usually related to the sample size and testing method. Although not explicitly given in the original text, they can be calculated using T-distribution tables or statistical software. However, here we mainly focus on the P-value.
P-value: The P-value is used to determine the probability of rejecting the null hypothesis. The original text gives a value of 0.032, which is less than the commonly used significance level of 0.05. Therefore, we can reject the null hypothesis and accept the alternative hypothesis.
Conclusion
Through the analysis of Table 4 and Table 5 (and hypothesis testing), we can draw the following conclusions:
The dimensional accuracy of die-castings after rectification has been significantly improved, with a mean difference of 1.22, and a 95% confidence interval lower limit of 0.15, indicating a significant improvement effect.
The P-value is 0.032, which is less than the significance level of 0.05. Therefore, we can reject the null hypothesis and conclude that the dimensional accuracy after rectification is indeed significantly higher than before.
In summary, through optimization measures such as material selection, fixture equipment for shaping, process control, and press parameter adjustment, we have successfully improved the dimensional accuracy of large die-castings for new energy vehicles.