In the automotive industry, the bridge box serves as a critical component in the rear axle system of heavy-duty trucks, subjected to significant impact forces and vibrations during operation. This necessitates exceptional mechanical properties and denseness in the final product, with strict requirements to eliminate casting defects such as shrinkage cavities and inclusions. The material of choice, QT450-10 ductile iron, must exhibit a nodularity exceeding 80%, a ferritic matrix, and a Brinell hardness between 160 and 210 to meet performance standards. This article details a comprehensive study on addressing shrinkage-related issues in ductile iron castings through process optimization, leveraging numerical simulation and practical adjustments.
The bridge box casting features a complex thin-walled structure, with overall dimensions of 553 mm × 370 mm × 159 mm and a mass of 40 kg. Wall thickness varies significantly, ranging from 6 mm at the thinnest sections to 46.5 mm at the thickest areas, such as around bearing housings and flange protrusions. Key structural elements include two primary bearing bores, multiple small bosses surrounding the larger bore, and raised sections on the top flange. Internal passages must be free from sand adhesion and fins, verified through 100% endoscopic inspection, while the entire casting undergoes X-ray examination to ensure integrity.
Initial production utilized a 3-ton medium-frequency induction furnace for melting, with the chemical composition outlined in Table 1. The process involved a primary inoculation addition of 0.6% and was conducted on a SAVELLI high-pressure molding line using green sand, with two castings per mold. The molding sand exhibited a wet compressive strength of 175 kPa, compactability of 33%, moisture content of 3.75%, and permeability of 105. Cores were produced using shell sand technology, and the gating system was designed as a bottom-pouring semi-closed symmetrical type to minimize turbulence and erosion during filling. A filter block was incorporated at the junction of the runner, and the gating ratio was set as ΣFinner : ΣFrunner : ΣFsprue = 1 : 1.4 : 1.2. Three ingates were positioned on the flange, avoiding raised sections, to ensure smooth filling. Top risers, specifically BKS61 exothermic risers, were employed to feed the thick sections around bearing bores and the base, also serving to vent gases. The pouring temperature was initially maintained between 1,380°C and 1,420°C.
| Element | C | Si | Mn | P | S | Cu |
|---|---|---|---|---|---|---|
| Content | 3.6-3.8 | 2.2-2.45 | 0.35-0.45 | <0.05 | <0.02 | 0.1-0.15 |
Despite these measures, severe shrinkage porosity and cavities were consistently observed in the flange boss regions, as illustrated in the defect analysis. The rejection rate reached an unacceptable 98%, primarily due to subsurface holes and lack of denseness revealed during machining. This issue was widespread among suppliers, indicating a fundamental challenge in producing sound ductile iron castings for this component.
A detailed analysis identified several contributing factors to the shrinkage defects. Inadequate mold wall thickness resulted in uneven cooling rates. The low rigidity of the green sand mold insufficiently resisted the metallostatic pressure and graphite expansion during solidification, leading to mold wall movement and inadequate feeding. Consequently, the isolated hot spots, particularly in the flange bosses, failed to receive sufficient liquid metal compensation, and the secondary shrinkage could not be mitigated by the graphite expansion.
The optimization strategy focused on enhancing the inherent feeding characteristics of the ductile iron and modifying the local cooling conditions. Ductile iron exhibits a unique solidification behavior characterized by graphite precipitation, which causes a volume expansion that can counteract the shrinkage of the austenite. The effectiveness of this self-feeding mechanism depends on the solidification morphology, which is governed by the eutectic saturation degree and the recalescence behavior. The eutectic saturation (Sc) is a key parameter, defined as:
$$S_c = \frac{C}{4.26 – 0.31 \times Si}$$
where C and Si are the carbon and silicon contents in weight percent. A value of Sc closer to 1 promotes a more eutectic structure with less dendrite formation, which is favorable for reducing shrinkage tendency. Recalescence (ΔTr), the temperature rise during eutectic solidification, should be minimized to ensure a controlled and uniform graphite precipitation. A lower recalescence indicates a higher nodule count and finer microstructure, enhancing the self-feeding capability.
The melt treatment process was meticulously revised. The charge composition was set to 50% pig iron, 20% steel scrap, and 30% returns. Wire injection inoculation was employed for nodularization, with a wire addition of 1.0%. Post-inoculation, 0.4% barium-silicon inoculant was added to the ladle. A stream inoculation with 0.1-0.13% sulfur-oxygen inoculant was implemented during pouring to enhance nucleation. The target was to adjust the base iron composition such that after treatment, the final ductile iron melt achieved a eutectic saturation very close to 1.0 and a recalescence below 3°C. The chemical compositions before and after treatment are compared in Table 2.
| Stage | C | Si | Mn | P | S | Cu | Mg | Sc | ΔTr (°C) |
|---|---|---|---|---|---|---|---|---|---|
| Before Treatment | 3.825 | 1.529 | 0.427 | 0.0319 | 0.0113 | 0.12 | – | 0.943 | – |
| After Treatment | 3.72 | 2.156 | 0.424 | 0.033 | 0.0095 | 0.12 | 0.049 | 0.998 | 2.5 |
To address the localized shrinkage in the flange bosses, the standard sand cores in these specific regions were replaced with chromite sand cores. Chromite sand possesses a high thermal conductivity and chilling power, significantly increasing the local cooling rate. This alteration promotes directional solidification away from these hot spots towards the risers, preventing the formation of isolated liquid pools and encouraging the use of graphite expansion for feeding. The strategic placement of these chromite sand cores is a critical aspect of the optimized ductile iron casting process.
Numerical simulation using Procast software was employed to validate the proposed modifications. The simulation parameters are detailed in Table 3. The casting material was defined as ductile iron with average chemical composition, the mold as green sand, and the specific chromite cores were assigned their respective material properties with a higher heat transfer coefficient.
| Parameter | Value |
|---|---|
| Pouring Conditions | Gravity Pouring |
| Pouring Temperature | 1,420 °C |
| Pouring Time | 169.8 s |
| Pouring Velocity | 5,500 mm/s |
| Gravity Parameter | 3,000 mm/s² |
| Heat Transfer Coefficient (Casting-Mold) | 500 W/m²·°C |
| Heat Transfer Coefficient (Casting-Chromite Core) | 1,000 W/m²·°C |
The simulation results of the solidification process are depicted through the liquid fraction distribution. The sequence clearly shows that solidification initiates from the regions adjacent to the chromite sand cores at the bottom and sides of the casting. A progressive, directional solidification pattern is established, moving upwards, with the exothermic risers being the last to solidify. Crucially, no isolated liquid regions are formed within the main casting body. The shrinkage prediction module confirmed a substantial reduction in the risk of shrinkage porosity and cavities in the critical flange boss areas, validating the design approach for this ductile cast iron component.
Production trials were conducted implementing the optimized process: controlling the melt to achieve Sc ≈ 1 and ΔTr < 3°C, and incorporating chromite sand cores in the designated locations. Subsequent inspection and sectioning of the previously problematic boss regions confirmed the complete elimination of shrinkage defects. The internal passages were clear, and the microstructure met the required specifications. The rejection rate for the ductile iron castings was dramatically reduced to below 2%, demonstrating the effectiveness of the optimization.
In conclusion, the successful production of heavy-duty truck bridge boxes from QT450-10 ductile iron hinges on a holistic approach to process control. Firstly, precise regulation of the molten metal’s eutectic saturation and recalescence is paramount to maximize the self-feeding potential inherent in ductile iron castings. Secondly, for complex geometries where thermal gradients are difficult to control, the strategic use of high-chill materials like chromite sand can effectively manipulate the solidification sequence, suppress isolated hot spots, and leverage graphite expansion to achieve soundness. This methodology provides a valuable framework for optimizing similar challenging ductile cast iron components, ensuring high quality and reliability in demanding applications. The journey of perfecting this ductile iron casting process underscores the importance of integrating fundamental metallurgical principles with advanced simulation tools and practical foundry techniques.