In our company’s development of a new generation of high-performance diesel engine cylinder blocks using compacted graphite iron (CGI), specifically RuT450 material, we encountered significant challenges related to metal casting defects. This project aimed to produce cylinder blocks for heavy-duty applications, such as trucks, buses, and construction machinery, with stringent requirements for weight reduction, durability, and performance. The cylinder block has a complex geometry with multiple cavities, including six cylinder bores, a crankshaft chamber, and oil passages, making it prone to issues like leakage and shrinkage during casting. This article details our first-person experience in identifying, analyzing, and resolving these metal casting defects through process optimizations, with a focus on practical solutions that can be applied in similar industrial settings.
The cylinder block casting, with a maximum轮廓尺寸 of 880 mm × 560 mm × 430 mm and a weight of 203 kg, features thin walls of 4.5 mm and thick sections up to 61 mm. Such variations in wall thickness increase the susceptibility to metal casting defects, particularly in CGI, which exhibits different solidification behavior compared to gray iron. Our production process involved cold-box core making with triethylamine, medium-frequency electric furnace melting, OCC wire feeding for vermicularization, and high-pressure molding on a HWS static pressure line. The initial casting工艺 employed a horizontal pouring system with side gating, as illustrated in the process diagrams, where molten metal flowed through a sprue, runner, and ingates into the mold cavity. Despite these setups, we faced recurring metal casting defects that impacted product quality and yield.
The material specifications for RuT450 required precise control over chemical composition and mechanical properties. Table 1 summarizes the target chemical composition ranges, which include elements like carbon, silicon, manganese, phosphorus, sulfur, copper, and tin. These elements influence fluidity, shrinkage tendency, and graphite morphology, all critical in minimizing metal casting defects. For instance, carbon and silicon levels affect the carbon equivalent (CE), a key parameter in cast iron quality, calculated as: $$CE = C + \frac{1}{3}Si$$ where C and Si are the weight percentages of carbon and silicon, respectively. A higher CE generally improves castability but must be balanced against strength requirements. The mechanical properties, as shown in Table 2, demanded a tensile strength of at least 450 MPa after stress relief annealing and a hardness range of 190–250 HB. Achieving these while avoiding defects like leakage and shrinkage required meticulous process adjustments.
| Element | Range |
|---|---|
| C | 3.65–3.85 |
| Si | 1.8–2.2 |
| Mn | 0.3–0.5 |
| P | ≤0.02 |
| S | 0.010–0.018 |
| Cu | 0.8–0.9 |
| Sn | 0.08–0.09 |
| Property | Value |
|---|---|
| Tensile Strength | ≥450 MPa |
| Surface Hardness | 190–250 HB |
| Heat Treatment | Stress Relief Annealing |
During the initial trial production, we observed two primary types of metal casting defects: leakage defects and shrinkage porosity. Leakage defects, also known as run-out or bleeders, occurred at the gear chamber flange on the rear face of the cylinder block. This area, being the highest point during pouring and distant from the pouring cup, was vulnerable to incomplete filling. The defect manifested as hollow shells or missing sections, compromising the structural integrity. Our analysis revealed that core misalignment during the setting process created gaps between the sand core and mold, allowing molten metal to escape. This type of metal casting defect is common in complex castings where core positioning is critical, and it underscored the need for better process control.
Shrinkage defects, on the other hand, appeared as concentrated small cavities in the thick sections between cylinder bores. These metal casting defects resulted from the “mushy solidification” characteristic of CGI, where the material solidifies in a pasty state, making it prone to shrinkage if adequate feeding is not provided. The initial pouring temperature of (1430 ± 10)°C was relatively high, exacerbating the shrinkage tendency by increasing the thermal gradient and solidification time. Additionally, the carbon equivalent was on the lower side, ranging from approximately 4.25 to 4.58, which reduced the fluidity and feeding capacity during solidification. The relationship between shrinkage tendency and process parameters can be expressed using a simplified solidification model: $$V_s = k \cdot \Delta T \cdot (1 – f_s)$$ where \(V_s\) is the shrinkage volume, \(k\) is a material constant, \(\Delta T\) is the temperature drop, and \(f_s\) is the solid fraction. Higher pouring temperatures and lower CE values increase \(V_s\), leading to more pronounced metal casting defects.
To address the leakage defects, we implemented two key measures. First, we calibrated the core-setting fixtures by adjusting equipment parameters and repositioning定位 points to ensure precise core placement. This reduced misalignment and minimized gaps between cores and the mold. Second, we identified and sealed potential leakage points by inserting refractory pads at critical interfaces, such as between the main core and auxiliary cores in the gear chamber area. These pads acted as barriers, preventing molten metal from penetrating unintended paths. The effectiveness of these interventions was evident in the drastic reduction of leakage incidents, as summarized in Table 3. By combining these approaches, we eliminated this type of metal casting defect, highlighting the importance of robust tooling and proactive sealing in complex casting assemblies.
| Action Taken | Number of Trials | Leakage Defects | Defect Rate (%) |
|---|---|---|---|
| No action | 24 | 10 | 41.67 |
| Fixture calibration only | 24 | 6 | 25.00 |
| Leakage point sealing only | 24 | 1 | 4.17 |
| Both measures implemented | 24 | 0 | 0.00 |
For shrinkage defects, our strategy focused on optimizing the melting and pouring parameters. We reduced the pouring temperature from (1430 ± 10)°C to (1410 ± 10)°C, which lowered the thermal energy and promoted a more controlled solidification, reducing the risk of shrinkage cavities. This adjustment aligns with the general principle that lower pouring temperatures decrease the solidification time and shrinkage volume, as captured by the equation: $$Q = m \cdot c \cdot \Delta T$$ where \(Q\) is the heat content, \(m\) is the mass, \(c\) is the specific heat, and \(\Delta T\) is the temperature change. By reducing \(Q\), we minimized the thermal contraction effects that contribute to metal casting defects. Simultaneously, we adjusted the chemical composition to increase the carbon equivalent. Specifically, we raised the carbon content from 3.60–3.80% to 3.78–3.85% and the silicon content from 1.85–1.90% to 2.10–2.20%, while lowering manganese from 0.40–0.50% to 0.30–0.40%. These changes improved fluidity and feeding during solidification, as higher CE values enhance the graphitization potential, which compensates for shrinkage. The modified composition ranges are compared in Table 4, and the impact on defect rates is shown in Table 5.
| Parameter | Before Optimization | After Optimization |
|---|---|---|
| Pouring Temperature (°C) | 1430 ± 10 | 1410 ± 10 |
| C (%) | 3.65–3.80 | 3.78–3.85 |
| Si (%) | 1.85–1.90 | 2.10–2.20 |
| Mn (%) | 0.40–0.50 | 0.30–0.40 |
| Action Taken | Number of Trials | Sampled for Analysis | Shrinkage Defects | Defect Rate (%) |
|---|---|---|---|---|
| No action | 24 | 8 | 3 | 37.50 |
| Lower pouring temperature only | 24 | 8 | 1 | 12.50 |
| Composition adjustment only | 24 | 8 | 1 | 12.50 |
| Both measures implemented | 24 | 8 | 0 | 0.00 |
The implementation of these measures yielded significant improvements in product quality. For leakage defects, the defect rate dropped from 41.67% to 0% after applying both fixture calibration and leakage point sealing. Similarly, for shrinkage defects, the rate decreased from 37.50% to 0% when both pouring temperature reduction and composition adjustments were combined. These results demonstrate that a holistic approach, addressing multiple facets of the casting process, is essential for mitigating metal casting defects. The visual inspection of castings confirmed the absence of defects, with complete fill and sound internal structure, as seen in the improved samples. This success underscores the value of systematic problem-solving in overcoming the challenges associated with CGI casting, particularly for high-integrity components like engine blocks.
In conclusion, our experience with the 8Y cylinder block project highlights effective strategies for resolving common metal casting defects in compacted graphite iron castings. For leakage issues, calibrating core-setting fixtures and sealing potential leakage points proved highly effective, and these methods can be referenced for similar problems in other casting applications. For shrinkage defects, adjusting pouring temperatures and optimizing chemical composition provided a reliable solution, especially in scenarios where traditional methods like chills or risers are impractical. Compacted graphite iron offers a compelling combination of strength, ductility, and machinability, but its casting requires careful consideration of all process variables to avoid defects. By sharing these insights, we aim to contribute to the broader foundry industry’s efforts in producing high-quality castings with minimal metal casting defects, ultimately enhancing product performance and reliability in demanding applications.