Introduction
Die-casting is a highly efficient manufacturing process widely used in various industries such as automotive, communications, and home appliances. This paper examines the process improvements in the die-casting of a gearbox rear cover shell for heavy-duty trucks. The study utilizes simulation software to optimize mold structure and process parameters to enhance the product’s quality by addressing cold shut and porosity defects.
Material and Design Specifications
The gearbox rear cover shell is made from aluminum alloy ADC12, adhering to JIS H5302:2000 standards for chemical composition, as shown in Table 1 below:
| Element | Si | Fe | Cu | Mn | Mg | Ni | Zn | Sn | Ti | Cr | Pb | Al |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ADC12 | 9.6-12 | ≤1.3 | 1.5-3.5 | ≤0.5 | ≤0.3 | ≤0.5 | ≤1.0 | ≤0.2 | – | – | – | Balance |
The rear cover shell has a complex structure with external dimensions of 578 mm × 417 mm × 98 mm, and a weight of 9.5 kg.
Problem Identification
The primary issues faced during the die-casting process include cold shut and porosity defects, particularly around the oil hole area of the rear cover shell. These defects significantly affect the yield rate of the product, as indicated in Table 2.
| Batch | Product No. | Qualified Parts | Cold Shut Defects | Porosity Defects | Yield Rate (%) |
|---|---|---|---|---|---|
| Batch 1 | 10647017-3 | 126 | 15 | 0 | 89.4 |
| Batch 2 | 10647017-3 | 224 | 155 | 15 | 56.9 |
| Batch 3 | 10647017-3 | 582 | 180 | 49 | 71.8 |
The defects were linked to the cooling of the material and air entrapment during the casting process, especially at the far end of the casting, which is the last area to fill during the die-casting process.
Defect Analysis
- Cold Shut and Porosity Defects
Cold shut defects occur when the metal cools and solidifies before fully filling the mold. Porosity defects are typically caused by trapped air or gas within the material. Using MAGMA software, simulations identified that these defects primarily occurred at the oil hole area due to the material being the last to fill and having reduced mold temperature in this region. - Temperature and Flow Analysis
Simulations of the mold temperature revealed that the significant temperature loss in the far end of the casting was a major cause of the cold shut defects. The distance from the injection gate to the far end of the mold contributed to the cooling of the aluminum, leading to incomplete filling and defect formation.
Process Improvement Strategies
To address these issues, several improvements were implemented in the die-casting process:
- Slag Bag Design
A slag bag was added near the oil hole area to capture cold material, air, and impurities, preventing them from entering the main body of the casting . - Mold Temperature Control
The mold temperature was increased by adding oil channels to maintain the mold temperature around 180°C during continuous production. - Increased Pouring Temperature
The pouring temperature of the aluminum alloy was raised from 660°C to 670°C to compensate for heat loss during the long flow path, thus reducing the likelihood of cold shut defects. - Mold Core Temperature Increase
The core temperature of the dynamic mold was raised from 110°C to 180°C to improve material flow and reduce thermal losses. - Auxiliary Runner
An auxiliary runner was designed to shift the filling endpoint to a non-critical area of the casting. However, simulations showed that this change did not significantly affect the reduction of defects, suggesting that the original mold design needed further optimization.
Results and Validation
The optimized process was tested in a small production run, achieving a significant increase in product yield. As shown in Table 3, the yield rate increased to 98.2%.
| Batch | Product No. | Qualified Parts | Cold Shut Defects | Porosity Defects | Yield Rate (%) |
|---|---|---|---|---|---|
| Batch 4 | 10647017-3 | 862 | 16 | 0 | 98.2 |
The cold shut and porosity defects around the oil hole area were effectively reduced, confirming that the process improvements successfully addressed the issues.
Conclusion
The improvements in the die-casting process for the gearbox rear cover shell, including the addition of a slag bag, enhanced mold temperature control, and optimized pouring conditions, resulted in a significant reduction of cold shut and porosity defects. These process improvements can be applied to similar products to improve yield and product quality.
Key Takeaways
- The primary cause of cold shut and porosity defects was the improper filling and cooling at the far end of the mold.
- Implementing mold temperature control and material flow improvements significantly enhanced product yield.
- The strategies discussed can serve as a reference for future die-casting process optimizations.
References
- Lu, Hongyuan. Die Casting Technology and Production, Beijing: Machinery Industry Press, 2008.
- Ma, Hongbing & Chuan, Haijun. Effect of Rear Cover Shell Structure on Casting Defects and Performance, Foundry Technology, 2015.
- Huang, Yong et al. Numerical Simulation and Die-Casting Process Study on Aluminum Alloy Brackets, Special Casting and Nonferrous Metals, 2011.