In the production of large and complex components for railway systems, the lost wax investment casting process is widely employed due to its ability to achieve high dimensional accuracy and excellent surface finish. However, challenges such as cracking, shrinkage porosity, and low yield rates often arise, particularly in heavy-duty parts like freight couplers. This study focuses on optimizing the gating system for a locomotive coupler using numerical simulation to address these issues. We designed two gating system schemes, A and B, and utilized ProCAST software to simulate the filling and solidification processes, analyzing temperature fields, flow fields, and defects like shrinkage cavities. Our goal was to identify a feasible design that minimizes defects and enhances product quality in lost wax investment casting.
The coupler, a critical connection component in trains, experiences complex loads including impact, tension, compression, and torsion during operation. Fatigue failure is a common issue, especially in areas like the hook head and hook tongue connection, which bear significant alternating stresses. The coupler studied here has a large structure with dimensions of approximately 594 mm × 370 mm × 350 mm, a volume of about 9,534,593 mm³, and a weight of 70 kg. It features varying wall thicknesses, from 12 mm at the thinnest sections to 43 mm at the thickest, leading to potential hotspots and stress concentrations. The material used is ZG25MnCrNiMo cast steel, with chemical composition as shown in Table 1. This alloy is chosen for its high strength and toughness, but its solidification behavior in lost wax investment casting can result in defects if not properly managed.
| Element | Content (%) |
|---|---|
| C | 0.26 |
| Si | 0.45 |
| Mn | 1.40 |
| Cr | 0.55 |
| Ni | 0.45 |
| Mo | 0.25 |
| P | ≤0.35 |
| S | ≤0.35 |
| Fe | Balance |
Initial trials with a traditional gating system, referred to as Scheme A, revealed superficial cracks and shrinkage cavities at the junction of the hook head and hook tongue, as detected by fluorescent magnetic particle inspection. These defects are primarily attributed to non-uniform solidification, where thinner sections solidify faster, isolating liquid regions and causing shrinkage porosity. Additionally, thermal stresses from uneven cooling can lead to cracking. In lost wax investment casting, the ceramic shell’s high strength restricts contraction, exacerbating these issues. To overcome this, we proposed Scheme B, which incorporates a symmetric vertical side gating system with multiple gates and larger cross-sections to promote directional solidification and reduce defect formation.
For numerical simulation, we input the material properties of ZG25MnCrNiMo into ProCAST, including temperature-dependent thermal conductivity and density, as illustrated in Figure 1. The mold shell was modeled with a thickness of 8 mm, and key process parameters are summarized in Table 2. Scheme A used a pouring temperature of 1,550°C and a filling time of 15 s, while Scheme B employed a higher pouring temperature of 1,580°C and a longer filling time of 30 s to account for the increased gating system mass. The heat transfer coefficients between the mold and casting, and the mold and environment, were set to 500 W/m²·K and 10 W/m²·K, respectively, with an ambient temperature of 20–25°C.
| Parameter | Value |
|---|---|
| Pouring Temperature for Scheme A (°C) | 1,550 |
| Pouring Temperature for Scheme B (°C) | 1,580 |
| Mold Preheating Temperature (°C) | 400 |
| Ambient Temperature (°C) | 20–25 |
| Filling Time for Scheme A (s) | 15 |
| Filling Time for Scheme B (s) | 30 |
| Heat Transfer Coefficient (Mold-Casting) (W/m²·K) | 500 |
| Heat Transfer Coefficient (Mold-Air) (W/m²·K) | 10 |
| Mesh Size for Gating System (mm) | 8 |
| Mesh Size for Casting (mm) | 5 |
The simulation results for Scheme A indicated a turbulent filling process with a maximum velocity of 0.71 m/s, causing minor gas entrapment. Although the mold cavity was fully filled in 15 s, the solidification analysis revealed isolated liquid zones in critical areas, such as the hook head and tongue junction, leading to shrinkage defects. The temperature distribution showed that thin walls solidified first, while thicker sections remained liquid, resulting in poor feeding and stress concentrations. The solidification time and stress fields were analyzed using nodal points, and the stress at critical locations reached up to 470 MPa, increasing the risk of hot tearing. The shrinkage porosity volume was significant in key regions, rendering the casting unacceptable.
In contrast, Scheme B demonstrated a smoother filling process with a maximum velocity of 0.48 m/s, reducing turbulence and gas entrapment. The symmetric gating design ensured uniform metal distribution, and the larger gates facilitated better feeding during solidification. The temperature field analysis confirmed directional solidification, with the casting solidifying before the gating system, allowing effective compensation for shrinkage. The stress levels were lower, with peak values around 400 MPa, and shrinkage defects were minimized to non-critical areas, with a total porosity volume of only 1.13 cm³. This improvement is attributed to the optimized lost wax investment casting gating system, which enhances thermal management and reduces defect propensity.
To quantify the solidification behavior, we applied the Chvorinov’s rule for solidification time, which can be expressed as:
$$ t = C \left( \frac{V}{A} \right)^2 $$
where \( t \) is the solidification time, \( C \) is a constant dependent on mold material and casting conditions, \( V \) is the volume of the casting, and \( A \) is the surface area. For Scheme B, the higher \( V/A \) ratio in the gating system delayed solidification, enabling better feeding. Additionally, the thermal stress during cooling can be modeled using the equation for thermal strain:
$$ \epsilon = \alpha \Delta T $$
where \( \epsilon \) is the strain, \( \alpha \) is the coefficient of thermal expansion, and \( \Delta T \) is the temperature difference. In lost wax investment casting, controlling \( \Delta T \) through gating design is crucial to minimize cracking.
Experimental validation was conducted by producing castings using Scheme B. The resulting components exhibited a smooth surface without macroscopic defects like gas pores or inclusions. X-ray and fluorescent magnetic particle inspections confirmed the absence of cracks and shrinkage cavities in critical areas, aligning with simulation predictions. Microstructural analysis of samples from the shoulder region showed uniform grain size of grades 1–3, consisting of fine ferrite at austenite grain boundaries and a mixture of acicular ferrite and pearlite within grains. Mechanical testing revealed a tensile strength of up to 675 MPa and an average elongation of 2.14% in the as-cast state, meeting industry standards for railway applications.
The success of Scheme B underscores the importance of gating system optimization in lost wax investment casting. By employing numerical simulation, we can predict and mitigate defects, leading to higher yield and improved performance. The symmetric vertical gating with adequate gate size and placement ensures controlled solidification, reducing thermal stresses and shrinkage. This approach is applicable to other large complex castings in the lost wax investment casting process, offering a pathway to enhance manufacturing efficiency and product reliability.
In conclusion, the lost wax investment casting process for heavy haul freight couplers requires careful gating design to avoid defects. Scheme A, with its horizontal single-side gating, led to unacceptable shrinkage and cracking, while Scheme B’s symmetric vertical gating provided a robust solution. Through ProCAST simulations and experimental trials, we demonstrated that Scheme B achieves stable filling, directional solidification, and minimal defects, resulting in high-quality castings that meet operational demands. Future work could explore further refinements, such as variable pouring temperatures or advanced alloy modifications, to push the boundaries of lost wax investment casting for even more demanding applications.