Ductile iron casting is a pivotal manufacturing process in the automotive industry, particularly for components like camshafts that demand high strength, wear resistance, and economic viability. The QT700-3 grade of ductile iron, with a minimum tensile strength of 700 MPa and elongation of 3%, presents significant challenges in casting due to its tendency to form shrinkage defects along the central axis—a consequence of its “mushy” solidification behavior. While iron mold coated sand casting is often considered optimal, it involves high initial costs and mold deformation risks. This research explores shell mold casting as an alternative, leveraging computational simulation and process refinements to achieve defect-free camshafts with superior mechanical and microstructural properties. The study emphasizes the integration of CAE analysis, shell design, and metallurgical controls to enhance the ductile iron casting process for QT700-3 applications.
In ductile iron casting, the solidification morphology critically influences defect formation. The QT700-3 alloy, characterized by a pearlitic matrix with controlled ferrite, requires precise cooling rates to avoid shrinkage porosity and ensure uniform graphite distribution. Shell mold casting, with its resin-bonded sand molds, offers flexibility in mold thickness and cooling modulation, but it necessitates careful design to mitigate issues like gas entrapment and poor feeding. We conducted this study using a systematic approach, encompassing numerical simulation, mold preparation, melting, and characterization, to establish a robust shell mold casting methodology for QT700-3 camshafts.
The experimental framework began with the design of a shell mold casting layout for a camshaft measuring 376 mm in length, with a large-end diameter of 46 mm and a base shaft diameter of 22 mm. To ensure consistency, we adopted a two-cavity mold arrangement, producing two camshafts per shell. The gating system was designed as semi-closed, with the choke at the ingate positioned at the large end of the camshaft, and a riser on the runner to facilitate feeding. Venting and slag traps were incorporated at the ends of each cavity to reduce turbulence and inclusion accumulation. This configuration aims to optimize fluid flow and solidification patterns in ductile iron casting.
We employed FLOW-3D CAST software for CAE numerical simulation to predict defect formation and guide process adjustments. The model utilized hexahedral mesh cells with a size of 1.5 mm, totaling approximately 4.3 million elements, divided into mold and pouring gate blocks. Key parameters included: ductile iron QT700 as the casting material with a density of 6.459 g/cm³ (liquid) and 7.240 g/cm³ (solid), specific heat of 780 J/(kg·K), thermal conductivity of 38 W/(m·K), and a pouring temperature of 1,350°C. The shell mold was modeled with precoated sand properties: density × specific heat of 1.7×10⁶ J/(m³·°C), thermal conductivity of 0.6 W/(m·K), and an initial temperature of 20°C. Boundary conditions set convective heat transfer coefficients at 1,000 W/(m²·K) between metal and mold, and 300 W/(m²·K) between metal and air.
The simulation results revealed critical insights into the ductile iron casting process. At the end of filling, minor gas entrapment and oxide inclusions were observed at the distal surfaces of the camshafts, but the core regions remained sound. Solidification analysis showed sequential cooling from surface to center; however, isolated liquid pockets formed in the thick sections of the base shaft during late stages, leading to shrinkage porosity due to interrupted feeding channels. This is mathematically represented by the solidification time equation, where the local cooling rate affects defect formation:
$$ t_s = \frac{V}{A} \cdot \frac{\rho L}{h(T_p – T_m)} $$
Here, \( t_s \) is solidification time, \( V \) is volume, \( A \) is surface area, \( \rho \) is density, \( L \) is latent heat, \( h \) is heat transfer coefficient, \( T_p \) is pouring temperature, and \( T_m \) is mold temperature. For ductile iron casting, reducing \( t_s \) in thick zones minimizes shrinkage. Thus, we proposed varying shell thickness: 14 mm at the base shaft thick sections to accelerate cooling, and 18 mm elsewhere to maintain mold integrity. This tailored approach in shell mold casting effectively eliminated predicted shrinkage defects.
Shell mold preparation is crucial in ductile iron casting for achieving dimensional accuracy and minimizing defects. We selected precoated sand with properties outlined in Table 1, ensuring high strength and low gas evolution to withstand the thermal stresses of ductile iron pouring. The shell production parameters were optimized through trials, as summarized in Table 2, focusing on shooting pressure, time, and curing conditions to produce robust molds.
| Grain Size | Tensile Strength at Room Temperature (MPa) | Hot Tensile Strength (MPa) | Flexural Strength at Room Temperature (MPa) | Gas Evolution at 850°C (mL/g) |
|---|---|---|---|---|
| 70/140 | 5.0 | ≥2.5 | ≥10 | ≤16.5 |
| Parameter | Value |
|---|---|
| Shooting Pressure (MPa) | 0.06 |
| Shooting Time (s) | 4 |
| Exhaust Time (s) | 6 |
| Curing Temperature (°C) | 190–210 |
| Curing Time (s) | 100–120 |
The melting and treatment processes are integral to achieving the desired QT700-3 properties in ductile iron casting. We used low-manganese carbon steel scrap as the primary charge, supplemented with low-sulfur, low-nitrogen graphite-based carburizers to adjust carbon content. The chemical composition was controlled within ranges specified in Table 3, balancing carbon equivalent and alloying elements to promote pearlite formation while retaining adequate ferrite for ductility. Copper addition was emphasized to enhance hardenability and stabilize pearlite, given the slower cooling rates in shell mold casting compared to iron mold methods.
| Element | Range |
|---|---|
| C | 3.7–3.9 |
| Si | 2.2–2.6 |
| P | ≤0.06 |
| S | ≤0.05 |
| Mn | 0.5–0.7 |
| Cu | 0.8–0.9 |
Ductile iron casting requires meticulous spheroidization and inoculation to ensure graphite nodularity and matrix uniformity. For a 500 kg batch, we employed a conventional sandwich method with rare-earth magnesium ferrosilicon as the spheroidizer (1.3–1.5% addition) at a treatment temperature of 1,520–1,560°C. Inoculation involved silicon-calcium-barium alloy (1.0–1.3% addition) through three stages: ladle inoculation (40%), stream inoculation during tapping (50%), and pouring stream inoculation (10%). This multi-stage approach enhances nucleation sites, crucial for fine graphite formation in ductile iron casting. Pouring temperatures were maintained at 1,380–1,420°C to ensure fluidity while minimizing gas absorption.
Microstructural evaluation of the shell-mold-cast QT700-3 camshafts revealed excellent graphite characteristics. In the cam lobes, graphite nodularity rated 2-1 grade with a count of approximately 656 nodules/mm² and size classification of 6-7; in the base shafts, nodularity was similar, but counts averaged 300 nodules/mm² with sizes of 5-7. This disparity arises from faster cooling in thinner sections, which increases undercooling and nucleation density—a key aspect of ductile iron casting. The matrix structure comprised 85–95% pearlite and 5–15% ferrite, with no free carbides or phosphide eutectics, meeting QT700-3 specifications. The relationship between cooling rate and graphite count can be expressed as:
$$ N = N_0 \exp\left(-\frac{Q}{RT}\right) $$
where \( N \) is graphite nodule count, \( N_0 \) is a pre-exponential factor, \( Q \) is activation energy, \( R \) is gas constant, and \( T \) is temperature. Higher cooling rates reduce \( T \), boosting \( N \), which explains the finer graphite in cam lobes. This microstructural refinement directly impacts mechanical performance in ductile iron casting.
Mechanical properties were assessed via tensile testing of separately cast Y-block samples, with results complying with GB/T 1348-2009 standards. As shown in Table 4, all specimens exceeded the minimum requirements for QT700-3, with tensile strengths ranging from 715 to 813 MPa and elongations of 3.1–3.4%. The consistency underscores the efficacy of the optimized shell mold casting process for ductile iron components. Notably, the enhanced copper content and robust inoculation mitigated the slower cooling effects of shell molds, ensuring high pearlite fractions and strength.
| Sample ID | Tensile Strength (MPa) | Elongation (%) | Ferrite (%) | Pearlite (%) | Graphite Nodularity (%) | Graphite Size Grade | Graphite Count (nodules/mm²) |
|---|---|---|---|---|---|---|---|
| 1 | 750 | 3.1 | 10 | 90 | 95 | 6–7 | 528 |
| 2 | 715 | 3.2 | 10 | 90 | 95 | 6–7 | 687 |
| 3 | 720 | 3.4 | 10 | 85 | 95 | 6–7 | 752 |
| 4 | 735 | 3.3 | 10 | 85 | 95 | 6–7 | 670 |
| 5 | 795 | 3.2 | 5 | 95 | 95 | 5–7 | 425 |
| 6 | 813 | 3.15 | 5 | 95 | 95 | 5–7 | 455 |
Defect analysis in ductile iron casting identified shrinkage porosity, sand inclusions, gas holes, and surface folds as primary concerns. Shrinkage occurred in thick base shaft regions due to inadequate feeding, addressed by local shell thinning to increase cooling rates, as per the solidification modulus principle:
$$ M = \frac{V}{A} $$
where \( M \) is modulus; reducing \( M \) by thinning the mold wall shortens solidification time. Sand and gas defects stemmed from high gas evolution or incomplete mold curing. Countermeasures included using low-gas precoated sand, ensuring full curing (indicated by a brown-red surface), diligent mold cleaning, and improved slag filtration. Surface folds resulted from high viscosity or low pouring pressure, mitigated by higher carbon equivalents, elevated pouring temperatures, and venting designs to reduce back-pressure. These adjustments are vital for defect-free ductile iron casting in shell molds.
The comparative analysis with iron mold coated sand casting highlights distinct advantages of shell mold casting for ductile iron. While iron molds offer faster cooling, they entail higher costs and durability issues. Shell molds provide flexibility in thickness control and lower initial investment, but require compositional tweaks—such as increased copper—to compensate for slower solidification. The success of this ductile iron casting process hinges on integrating simulation-driven design with metallurgical precision, enabling scalable production of high-integrity camshafts.
In conclusion, this study demonstrates that shell mold casting is a viable and efficient method for producing QT700-3 ductile iron camshafts. Key findings include: a two-cavity layout with semi-closed gating and varied shell thickness (14 mm at thick sections, 18 mm elsewhere) effectively prevents shrinkage defects; enhanced copper addition and multi-stage inoculation ensure target microstructure and mechanical properties; and process controls like low-gas sands and optimized curing minimize other defects. The ductile iron casting approach outlined here balances performance and economics, offering a robust alternative to traditional methods. Future work could explore automated shell molding or alloy modifications to further enhance ductile iron casting for high-stress applications.
Overall, the integration of CAE simulation, tailored shell design, and rigorous metallurgical practices has advanced the ductile iron casting process for QT700-3 camshafts. This research underscores the importance of holistic process optimization in ductile iron casting, where every parameter—from mold thickness to pouring temperature—plays a critical role in achieving defect-free, high-performance components. As the automotive industry continues to demand lighter and stronger parts, innovations in ductile iron casting will remain pivotal, and shell mold techniques offer a promising pathway for meeting these challenges efficiently.