In the development of metro vehicle transmission systems, gearbox housings play a critical role as key components subjected to complex dynamic loads during operation. These ductile iron castings must exhibit high strength and toughness to ensure safety, with stringent requirements prohibiting defects such as shrinkage porosity, cold shuts, or cracks. Internal integrity is verified through X-ray inspection, while surface quality is assessed via wet fluorescent magnetic particle testing. Traditionally, metro gearbox housings are manufactured from ductile iron with a primary wall thickness of approximately 10–12 mm. However, lightweight design has emerged as a pivotal strategy to reduce vehicle energy consumption and align with carbon neutrality policies. The most direct approach to lightweighting involves structural optimization to minimize wall thickness without compromising safety. Nevertheless, reducing wall thickness in ductile iron castings poses challenges, including diminished fluidity of molten metal, which can lead to defects like misruns and cold shuts, and narrowed feeding channels that increase the risk of shrinkage defects. This study focuses on optimizing the housing structure through simulation, adjusting chemical composition, enhancing mold conditions, refining casting processes, and implementing advanced spheroidization methods to achieve a 20% weight reduction while ensuring production stability for small batches.
The conventional production of metro gearbox housings utilizes ductile iron grade QT450-10, with a typical weight of 170 kg and wall thickness of 10–12 mm. These ductile iron castings are produced using furan resin sand molding and medium-frequency furnace melting, with melting temperatures around 1500°C and pouring temperatures of approximately 1400°C. The inherent糊状凝固 (mushy solidification) behavior of ductile iron exacerbates fluidity issues in thin-walled sections, necessitating comprehensive process adjustments. Through finite element analysis, the optimized housing structure reduces the main wall thickness to 6–7 mm, achieving a target weight of 135 kg. Simulations confirm that the maximum stress under static loads is 171.1 MPa, well below the material’s yield strength of 310 MPa, and fatigue assessments indicate that all surface nodes fall within the Goodman fatigue limit diagram, validating structural integrity.
Casting process design for lightweight ductile iron castings requires meticulous attention to fluidity and feeding. The chemical composition is tailored to enhance流动性 (fluidity), with increased carbon and silicon content promoting better mold filling. Experiments using spiral test specimens with an 8 mm × 8 mm cross-section demonstrate the impact of carbon equivalent on fluidity, as summarized in Table 1. Based on empirical data and production experience, the optimized composition for ductile iron castings is set as follows: C: 3.5–3.8%, Si: 2.5–2.8%, Mn: <0.5%. This adjustment significantly improves the flow characteristics of the molten metal, reducing the likelihood of casting defects in thin-walled sections.
| C + Si Mass Fraction (%) | Molding Material | Pouring Temperature (°C) | Spiral Length (mm) |
|---|---|---|---|
| 6.2 | Furan Resin Sand | 1300 | 1800 |
| 5.9 | Furan Resin Sand | 1300 | 1300 |
| 5.2 | Furan Resin Sand | 1300 | 1000 |
| 4.2 | Furan Resin Sand | 1300 | 600 |
Mold conditions are critical for achieving high-quality ductile iron castings. Preheating the mold reduces the temperature gradient between the metal and mold, enhancing fluidity. In winter conditions, where ambient temperatures drop to 0°C, industrial heaters are employed to bake molds at 120°C for 2 hours, raising the internal mold temperature to approximately 80°C. This practice mitigates misruns and cold shuts, ensuring consistent production of thin-walled ductile iron castings. The relationship between mold temperature and fluidity can be expressed using the following empirical formula for fluidity length \( L_f \):
$$ L_f = k \cdot \Delta T + L_0 $$
where \( \Delta T \) is the temperature difference between the molten metal and mold, \( k \) is a material constant, and \( L_0 \) is the base fluidity length. For ductile iron castings, higher mold temperatures correlate with improved filling capacity.
The gating system design is simplified to minimize flow resistance and enhance filling. Increasing the metallostatic pressure head improves fluidity, but excessive pressure can cause turbulence, oxidation, and gas entrapment. Simulation of flow fields for both upper and lower housings, as depicted in the figures below, shows uniform temperature distribution during 25%, 50%, 75%, and 100% filling stages, with no isolated cold zones or risks of misruns. The process ensures stable filling without splashing or air inclusion. The gating ratio is optimized to balance flow velocity and pressure, with the cross-sectional areas of the sprue, runner, and ingate designed to maintain laminar flow. The fluidity \( F \) in thin-section ductile iron castings can be modeled as:
$$ F = \frac{\rho \cdot g \cdot h \cdot A}{\mu} $$
where \( \rho \) is density, \( g \) is gravitational acceleration, \( h \) is the pressure head, \( A \) is the cross-sectional area, and \( \mu \) is dynamic viscosity. This equation underscores the importance of controlled pressure and viscosity in achieving defect-free ductile iron castings.
Melting and spheroidization processes are refined to further enhance the quality of ductile iron castings. Charge materials, including pig iron, scrap steel, and returns, are meticulously cleaned to remove rust, sand, and oil, while alloys like spheroidizing and inoculating agents are dried to minimize inclusions. The melting temperature is set at 1520°C, and the盖包球化法 (covered ladle spheroidization) is adopted over conventional冲入法 (pour-over method). This approach reduces temperature loss by approximately 40°C—from 120°C to 80°C in a 1-ton ladle—thereby conserving energy. Additionally, it minimizes splash and fume emissions, improving workplace safety and allowing for quicker slag removal and pouring, which mitigates spheroidization衰退 (recession) and enhances nodule formation. The efficiency of spheroidization can be quantified using the nodule count per unit area \( N_n \):
$$ N_n = \frac{C_{Mg} \cdot e^{-k \cdot t}}{A} $$
where \( C_{Mg} \) is the magnesium content, \( t \) is time, \( k \) is a decay constant, and \( A \) is the area. Covered ladle processing maintains higher \( C_{Mg} \) levels over time, leading to superior graphite nodularity in ductile iron castings.
Small-batch trials involving 24 sets of gearbox housings were conducted using the optimized parameters. All ductile iron castings exhibited no defects such as cold shuts, shrinkage, or slag inclusions. Mechanical properties, X-ray inspections, and fluorescent magnetic particle tests met technical specifications, achieving a 100% pass rate. The successful implementation demonstrates the viability of lightweight ductile iron castings for mass production, contributing to energy efficiency and emission reduction goals. Further analysis of the solidification process reveals that the feeding efficiency \( \eta_f \) for thin-walled sections can be expressed as:
$$ \eta_f = \frac{V_f}{V_c} \cdot \frac{\Delta T_s}{\Delta T_p} $$
where \( V_f \) is the volume of feed metal, \( V_c \) is the volume of the casting, \( \Delta T_s \) is the solidification temperature range, and \( \Delta T_p \) is the pouring temperature range. Optimized compositions and mold preheating increase \( \eta_f \), reducing shrinkage in ductile iron castings.
In conclusion, the lightweight design of metro gearbox housings through structural optimization and advanced casting techniques achieves a significant weight reduction of approximately 20% while maintaining safety standards. The integration of adjusted chemical compositions, mold preheating, streamlined gating systems, and covered ladle spheroidization ensures stable small-batch production. This research provides a reliable foundation for the design of energy-efficient ductile iron castings, supporting broader environmental sustainability initiatives. Future work could explore the application of these methods to other complex thin-walled ductile iron castings, further advancing lightweighting in the transportation industry.