In the production of ductile iron castings, particularly for critical components like planetary carriers in gear transmission systems, achieving high integrity without defects such as shrinkage porosity is paramount. As a key part bearing maximum external torque, the planetary carrier must be lightweight, rigid, and free from internal flaws. This article details the casting process design for a bilateral-plate integral planetary carrier produced using a DISA vertical molding line, focusing on structural analysis, simulation, and practical implementation to ensure quality in ductile iron castings.
The planetary carrier discussed here features a spatial frame structure composed of two annular side plates and four evenly distributed support columns. With a maximum outer dimension of ϕ245 mm × 98 mm, a wall thickness ranging from 13 mm to 22 mm, and a rough weight of 9.8 kg, the component is made of QT500-7 ductile iron. The complex geometry, with approximately 60 mm between the side plates, necessitates meticulous process design to address thermal hotspots and potential shrinkage. Our production utilizes green sand molding, 20L cold box core making, medium-frequency electric furnace melting, and DISA vertical molding, emphasizing the importance of optimizing ductile iron castings for high-performance applications.
Product Structure and Hot Spot Analysis
When designing the casting process for ductile iron castings, the primary concern is mitigating shrinkage porosity, which arises from thermal hotspots. These hotspots are classified into structural hotspots, caused by geometric concentrations of metal, and process hotspots, induced by elements like risers and gating systems. In this planetary carrier, structural hotspots are concentrated at the junctions between the side plates and support columns, totaling eight locations. These areas, due to their higher metal volume, release more heat during solidification, leading to prolonged solidification times and increased risk of defects if not properly fed.
To analyze these hotspots, we employed AnyCasting simulation software, setting the initial casting temperature at 1,385°C and using default parameters. The solidification sequence revealed that the smaller end plate solidifies first, fragmenting into isolated liquid phases around 53 seconds, while the larger end plate retains larger isolated liquid phases up to 183 seconds. This differential solidification is critical for designing feeding systems in ductile iron castings. The CAE analysis indicated over 30% of potential defects located at these structural hotspots, necessitating targeted solutions such as self-feeding via graphite expansion for the smaller end and riser feeding for the larger end.
The solidification process can be modeled using Chvorinov’s rule, where the solidification time \( t \) is proportional to the square of the volume-to-surface area ratio (modulus \( m \)):
$$ t = k \cdot m^2 $$
where \( k \) is a constant dependent on the mold material and casting conditions. For ductile iron castings, the modulus \( m \) for hotspots was calculated to be approximately 1.2 cm, guiding the riser design. The simulation results underscored the importance of sequential solidification, where the larger end, with a higher modulus, requires external feeding, while the smaller end can leverage the graphite expansion characteristic of ductile iron for self-compensation.
Casting Process Design
The design of the casting process for ductile iron castings involves multiple aspects, including parting plane selection, core design, gating system, and riser configuration. Given the use of a DISA vertical molding line, which operates with automatic core setting and high-pressure squeezing, the process must align with the machine’s capabilities to produce sound ductile iron castings.
Parting Plane Selection
Two parting plane options were evaluated. Option 1 offered simpler cores and uniform machining allowances but posed challenges in feeding the hotspots due to the distance from the risers. Option 2, though resulting in more complex cores and uneven machining, allowed direct gating near the larger end hotspots, facilitating better feeding. After comparative analysis, Option 2 was selected for its superiority in achieving directional solidification, a key requirement for high-quality ductile iron castings. This choice leverages the DISA line’s characteristics, where the smaller end faces outward, benefiting from enhanced cooling, while the larger end is oriented inward for effective riser placement.
Core Design
The core was designed using 20L cold box technology, adhering to DISA manual guidelines for automatic setting. The core features locating and clamping surfaces on the positive plate with a clearance of -0.1 to -0.3 mm for secure positioning, while other areas have a 0–0.5 mm gap. This design ensures stability during molding and minimizes defects in the internal cavities of ductile iron castings. The core’s complexity, due to the parting plane choice, is managed through precise dimensions to maintain casting integrity.
Gating System and Riser Design
The gating system is crucial for controlling mold filling and temperature distribution in ductile iron castings. In this design, the ingates are positioned between two support columns on the larger end plate, enabling one riser to feed two adjacent hotspots. The mold plate dimensions of 600 mm × 480 mm accommodate two castings per mold, with a symmetrical layout to balance metal flow and solidification.
The gating system includes a 60 mm × 12 mm wide and thin ingate to proximity-feed the hotspot regions, promoting efficient feeding. A foam ceramic filter measuring 82 mm × 82 mm × 12.5 mm with 10 pores per inch (PPI) is incorporated to purify the molten iron and reduce turbulence. Additionally, venting and overflow pieces of 18 mm × 3 mm are designed to release mold gases and optimize feeding, preventing defects like sand adhesion in ductile iron castings.
For riser design, two cylindrical risers of ϕ70 mm are used to feed the four hotspots on the larger end. A 30 mm × 8 mm thin connector between risers prevents inter-riser feeding, enhancing the upper riser’s efficiency. The modulus method confirms the adequacy of these risers for the ductile iron castings, with the solidification feeding requirements calculated as:
$$ V_r = V_c \cdot \beta \cdot \alpha $$
where \( V_r \) is the riser volume, \( V_c \) is the casting volume to be fed, \( \beta \) is the feeding efficiency factor, and \( \alpha \) is the shrinkage factor for ductile iron (typically 1–2% due to graphite expansion). This approach ensures that the larger end achieves directional solidification, while the smaller end relies on self-feeding, leveraging the inherent properties of ductile iron.
Table 1 summarizes the key parameters of the gating and riser system for the ductile iron castings:
| Parameter | Value | Description |
|---|---|---|
| Ingate Dimensions | 60 mm × 12 mm | Wide, thin shape for hotspot proximity |
| Riser Dimensions | ϕ70 mm cylindrical | Feeds two hotspots each |
| Filter Size | 82 mm × 82 mm × 12.5 mm | 10 PPI foam ceramic for purification |
| Venting Pieces | 18 mm × 3 mm | Reduces mold pressure and improves feeding |
Solidification Simulation and CAE Analysis
Using AnyCasting software, the initial process design was modeled and simulated to predict solidification behavior and defect formation in the ductile iron castings. The results, as shown in the solidification sequence, indicate that the smaller end plate begins solidifying first, with isolated liquid phases forming and dissipating by approximately 125 seconds. In contrast, the larger end plate maintains liquid phases up to 335 seconds, confirming the need for riser feeding. The simulation highlights that isolated liquid phases do not necessarily lead to shrinkage if properly managed through design, such as using graphite expansion in ductile iron for self-feeding.
The defect probability analysis, based on temperature gradient and solidification time, can be expressed as:
$$ P_d = f(\Delta T, t_s) $$
where \( P_d \) is the probability of defects, \( \Delta T \) is the temperature gradient, and \( t_s \) is the local solidification time. For the ductile iron castings, the CAE results showed minimal risk at the larger end due to riser design, while the smaller end required monitoring; if shrinkage occurred, chills could be added. This iterative simulation approach is essential for refining the process of ductile iron castings, ensuring high yield and quality.
Melting and Metallurgical Process
The melting process for ductile iron castings involves a medium-frequency electric furnace, with wire feeding for spheroidization and inoculation treatments. Post-inoculation is performed during pouring to enhance the microstructure. The pouring temperature is maintained between 1,370°C and 1,390°C to optimize fluidity and feeding in ductile iron castings. The chemical composition is tightly controlled, as outlined in Table 2, to achieve the desired mechanical properties and graphite morphology in the final ductile iron castings.
| Element | Content (wt.%) |
|---|---|
| C | 3.6–3.8 |
| Si | 2.5–2.8 |
| Mn | 0.1–0.4 |
| P | < 0.06 |
| S | < 0.02 |
| Mg | 0.03–0.05 |
| Sn | 0.02–0.04 |
The metallurgical quality of ductile iron castings depends on factors like nodule count and matrix structure. The inoculation efficiency \( \eta \) can be approximated by:
$$ \eta = \frac{N_n}{N_t} $$
where \( N_n \) is the number of graphite nodules and \( N_t \) is the total potential nucleation sites. In our process, this resulted in a spheroidization rate of 85%, maximum graphite size of 6 μm, pearlite content of 45%, tensile strength of 527 MPa, elongation of 11%, and hardness of 187 HBW, meeting the standards for ductile iron castings.
Trial Production and Validation
In trial production, the designed process was implemented for the ductile iron castings. The castings were examined for defects, with anatomical sections revealing no shrinkage porosity. Metallographic analysis confirmed a predominantly pearlitic-ferritic matrix with well-dispersed graphite nodules, as expected for high-quality ductile iron castings. The mechanical properties aligned with QT500-7 specifications, demonstrating the effectiveness of the process in producing reliable ductile iron castings for demanding applications.
The success of these ductile iron castings underscores the importance of integrating simulation with practical design. For instance, the smaller end’s self-feeding capability, driven by graphite expansion, eliminated the need for additional risers, reducing material waste and improving yield in ductile iron castings. This approach can be generalized to other complex ductile iron castings, where hotspot management is critical.
Conclusion
Through the development of the planetary carrier on a DISA vertical molding line, we have established a robust process for producing high-integrity ductile iron castings. Key insights include the necessity of detailed structural analysis, strategic placement of gating and risers to control solidification, and the use of simulation to validate designs. The ductile iron castings produced exhibit excellent mechanical and metallurgical properties, free from defects, proving the process’s viability. Future work could focus on optimizing riser sizes and incorporating real-time monitoring to further enhance the quality and efficiency of ductile iron castings in mass production.