In the manufacturing of locomotive bogie components, the axle box body plays a critical role in ensuring structural integrity and performance. This study focuses on optimizing the lost wax investment casting process for producing high-quality axle box bodies, which are subjected to stringent technical requirements, including radiographic and magnetic particle inspections. The lost wax investment casting method was chosen due to its ability to achieve complex geometries and excellent surface finish, but initial trials revealed challenges such as dimensional inaccuracies, shrinkage defects, and sand inclusions. Through iterative improvements in riser design, pouring position, and process shrinkage rates, we successfully enhanced the internal quality and dimensional precision of the castings. This article details our methodology, experimental results, and key insights, emphasizing the importance of systematic process adjustments in lost wax investment casting.
The axle box body, fabricated from Grade B steel per TB/T 2942.1-2020 standards, has a maximum dimension of 700 mm and significant variations in wall thickness, ranging from 12 mm to 55 mm. The component weighs 64.1 kg as a finished part, with a casting weight of 92 kg. Key technical specifications mandate radiographic inspection levels of Grade 2 or higher in critical areas and magnetic particle inspection at Grade 1 quality. These requirements necessitate a robust lost wax investment casting process to minimize defects and ensure reliability. The structural complexity, particularly around bolt holes and thick sections, poses challenges for feeding and solidification, often leading to shrinkage porosity and sand inclusions if not properly addressed.
Initial process design for the lost wax investment casting involved a linear shrinkage rate of 2.5%, supplemented with large risers and chills to facilitate feeding. The wax pattern assembly, as illustrated in the initial trials, included triangular gates and rectangular risers. The shell-building process comprised one primary coat, one transition coat, and seven reinforcement coats, with additional wire wrapping after the third reinforcement layer to enhance mold strength. However, post-casting evaluations revealed several issues: dimensional deviations in critical center distances, radiographic inspection failures in specific zones, and sand inclusions on large planar surfaces after rough machining.
To quantify the dimensional inaccuracies, we measured the center distance between key features, which was specified as 460 ± 0.1 mm. The initial casting measured 463.7 mm, indicating an excessive shrinkage allowance. This was attributed to the restrictive cross-rib structure hindering contraction during solidification. The shrinkage rate in lost wax investment casting can be modeled using the formula: $$ \text{Actual Shrinkage} = \frac{L_{\text{mold}} – L_{\text{casting}}}{L_{\text{mold}}} \times 100\% $$ where \( L_{\text{mold}} \) is the mold dimension and \( L_{\text{casting}} \) is the final casting dimension. For the axle box body, the initial 2.5% rate overestimated shrinkage, leading to oversizing.
Radiographic inspection results from the first trial are summarized in Table 1. Defects such as shrinkage cavities (denoted as ‘Cc’) and gas porosity (‘B’) were observed in zones 3, 7, and 8, exceeding the allowable Grade 2 threshold. This indicated inadequate feeding from the rectangular risers, which failed to compensate for solidification shrinkage in thick sections. Additionally, sand inclusions on large planar surfaces, as shown in Figure 4 of the reference, were traced to high-temperature metal flow冲刷 the mold surface during pouring, causing shell erosion and inclusion defects.
| Film No. | Defect Analysis | Grade | Film No. | Defect Analysis | Grade |
|---|---|---|---|---|---|
| 1 | None | 1 | 8 | Cc | 4 |
| 2 | None | 1 | 9 | B | 1 |
| 3 | Cc | 4 | 10 | B | 2 |
| 4 | None | 1 | 11 | B | 1 |
| 5 | B | 2 | 12 | None | 1 |
| 6 | None | 1 | 13 | None | 1 |
| 7 | Cc | 4 | 14 | None | 1 |
Based on these findings, we implemented three key improvements in the lost wax investment casting process. First, the linear shrinkage rate was adjusted to 1.5% to account for the hindering effect of the cross-rib structure on contraction. This was calculated using the modified formula: $$ \text{Optimal Shrinkage Rate} = k \times \text{Theoretical Shrinkage} $$ where \( k \) is a correction factor (empirically determined as 0.6 for this geometry) to address structural restraints. Second, the rectangular risers were replaced with waist-shaped (looped) risers to enhance feeding efficiency. The feeding capacity of a riser in lost wax investment casting can be estimated using Chvorinov’s rule: $$ t = C \left( \frac{V}{A} \right)^2 $$ where \( t \) is solidification time, \( V \) is volume, \( A \) is surface area, and \( C \) is a mold constant. Waist-shaped risers provide a higher \( V/A \) ratio, prolonging solidification and improving feeding. Third, the pouring position was altered to avoid direct metal flow over large planar surfaces, reducing the risk of sand inclusions.
The revised lost wax investment casting process involved assembling wax patterns with a spherical gate for pouring and waist-shaped risers at critical sections. The gating system was designed to ensure uniform filling and minimize turbulence. Mathematical modeling of fluid flow during pouring was considered to optimize gate placement, using the Bernoulli equation for incompressible flow: $$ P + \frac{1}{2} \rho v^2 + \rho g h = \text{constant} $$ where \( P \) is pressure, \( \rho \) is density, \( v \) is velocity, and \( h \) is height. This helped in positioning the gate to reduce velocity gradients and shell erosion.
Post-improvement trials demonstrated significant enhancements. Dimensional inspections confirmed that the center distance met the 460 ± 0.1 mm specification, with measurements averaging 460.05 mm. Radiographic inspections showed all zones achieving Grade 1 or 2, as summarized in Table 2. Magnetic particle inspections passed Grade 1 requirements, and no sand inclusions were detected after machining. The successful outcomes underscore the effectiveness of the tailored lost wax investment casting approach for complex components like the axle box body.
| Film No. | Defect Analysis | Grade | Film No. | Defect Analysis | Grade |
|---|---|---|---|---|---|
| 1 | None | 1 | 8 | None | 1 |
| 2 | None | 1 | 9 | B | 2 |
| 3 | None | 1 | 10 | None | 1 |
| 4 | None | 1 | 11 | None | 1 |
| 5 | B | 2 | 12 | None | 1 |
| 6 | None | 1 | 13 | None | 1 |
| 7 | None | 1 | 14 | None | 1 |
Further analysis involved evaluating the thermal gradients during solidification. Using Fourier’s law of heat conduction, we modeled the temperature distribution: $$ \frac{\partial T}{\partial t} = \alpha \nabla^2 T $$ where \( T \) is temperature, \( t \) is time, and \( \alpha \) is thermal diffusivity. This highlighted the importance of riser placement in promoting directional solidification, reducing shrinkage defects in the lost wax investment casting process. The waist-shaped risers, with their optimized geometry, provided better thermal profiles compared to rectangular ones.
In conclusion, the lost wax investment casting process for the axle box body was successfully refined through systematic adjustments. Key lessons include the necessity of customizing shrinkage rates based on structural constraints, avoiding pouring over large planar surfaces to prevent inclusions, and utilizing waist-shaped or spherical risers for superior feeding. The lost wax investment casting technique, when meticulously engineered, can meet high-quality standards for critical automotive components. Future work could explore advanced simulation tools to further optimize riser designs and minimize trial iterations in lost wax investment casting applications.
The integration of these improvements has led to a reliable production process, with castings passing all quality checks and being approved for use in locomotive bogies. This case study highlights the iterative nature of process optimization in lost wax investment casting and serves as a reference for similar applications in heavy machinery and transportation industries. Continuous monitoring and adaptation are essential to address the unique challenges posed by complex geometries in lost wax investment casting.