In the field of precision casting, investment casting plays a critical role in producing complex and high-integrity components for demanding applications. This study focuses on the investment casting process for an axle box body used in locomotive bogies, which requires exceptional dimensional accuracy and internal quality. The axle box body, made of Grade B steel per TB/T 2942.1-2020, is a key structural element with a maximum dimension of 700 mm, varying wall thicknesses from 12 mm to 55 mm, and a casting weight of 92 kg. The stringent technical specifications include Level 2 or higher radiographic inspection in critical areas and Level 1 magnetic particle inspection, necessitating a refined precision casting approach to mitigate defects such as shrinkage, sand inclusion, and dimensional inaccuracies.
The initial investment casting process was designed with a linear shrinkage rate of 2.5%, incorporating large risers and chills to address the significant wall thickness variations. The wax pattern assembly, as illustrated in the initial trials, featured triangular gates and square risers to facilitate feeding and solidification. However, this setup led to several issues upon testing. Radiographic inspection revealed Level 4 defects in specific areas, indicating inadequate feeding and potential shrinkage porosity. Additionally, dimensional measurements showed deviations, such as a center distance of 463.7 mm against a required 460 ± 0.1 mm, and post-machining examination identified sand inclusion defects on large planar surfaces, attributed to shell erosion during pouring.
To analyze these problems, we considered the fundamental principles of precision casting. The excessive dimensional error was primarily due to the hindered contraction caused by the cross-rib structure of the axle box body, which resisted the anticipated shrinkage. The linear shrinkage rate in investment casting can be expressed as: $$ \text{Shrinkage Rate} = \frac{L_{\text{mold}} – L_{\text{casting}}}{L_{\text{mold}}} \times 100\% $$ where \( L_{\text{mold}} \) is the mold dimension and \( L_{\text{casting}} \) is the final casting dimension. In this case, the initial 2.5% rate overestimated the actual contraction, leading to oversizing. For the radiographic defects, the square risers provided insufficient feeding, resulting in shrinkage porosity in areas like the bolt holes. The feeding efficiency in investment casting can be modeled using Chvorinov’s rule for solidification time: $$ t = k \left( \frac{V}{A} \right)^2 $$ where \( t \) is solidification time, \( V \) is volume, \( A \) is surface area, and \( k \) is a constant. Thicker sections with higher \( V/A \) ratios solidified slower, exacerbating shrinkage without adequate riser design. Sand inclusion occurred because the molten metal entered through the triangular gate directly impinging on large planar surfaces, causing thermal shock and shell degradation.
Based on this analysis, we implemented several improvements to the investment casting process. First, the linear shrinkage rate was adjusted to 1.5% to account for the restrictive geometry, recalculating mold dimensions accordingly. This change aimed to enhance dimensional precision in precision casting by aligning the process with actual material behavior. Second, the square risers were replaced with waist-shaped (elliptical) risers to improve feeding efficiency. The volume of a waist-shaped riser can be approximated as: $$ V_{\text{riser}} = \pi \cdot a \cdot b \cdot h $$ where \( a \) and \( b \) are the semi-axes, and \( h \) is the height, providing a larger feeding volume and better thermal characteristics than square risers. Third, the gating system was modified to avoid direct metal flow over large planes; instead, a spherical gate was introduced to distribute the pour more evenly, reducing the risk of sand inclusion in investment casting.
The revised wax pattern assembly for precision casting incorporated these changes: waist-shaped risers at the end covers and a spherical gate for pouring, while the triangular gate was repositioned to minimize turbulence. The shell-building process remained consistent with one primary layer, one transition layer, and seven reinforcement layers, reinforced with 3 mm steel wires after the third layer to bolster strength. Pouring was conducted using two 50 kg ladles through the spherical gate to ensure uniform filling. This investment casting setup aimed to optimize solidification and reduce defects.
Post-improvement trials demonstrated significant enhancements. Dimensional inspection confirmed that the center distance met the 460 ± 0.1 mm requirement, validating the adjusted shrinkage rate. Radiographic and magnetic particle inspections showed full compliance with Level 2 and Level 1 standards, respectively, indicating improved internal quality in precision casting. The table below summarizes the radiographic results after improvements, highlighting the elimination of Level 4 defects:
| Inspection Area | Defect Analysis | Level |
|---|---|---|
| 1 | None | 1 |
| 2 | None | 1 |
| 3 | Minor porosity | 2 |
| 4 | None | 1 |
| 5 | Small inclusion | 2 |
| 6 | None | 1 |
| 7 | Minor porosity | 2 |
| 8 | None | 1 |
| 9 | None | 1 |
| 10 | Small inclusion | 2 |
| 11 | None | 1 |
| 12 | None | 1 |
| 13 | None | 1 |
| 14 | None | 1 |
Furthermore, mechanical testing and chemical analysis confirmed that the material properties adhered to Grade B steel standards, and post-machining examinations revealed no sand inclusion defects, underscoring the effectiveness of the gating modification in investment casting. The success of these adjustments highlights the importance of iterative design in precision casting, particularly for components with complex geometries and high-quality demands.
In conclusion, this research on investment casting for the axle box body demonstrates that optimizing process parameters is essential for achieving precision casting standards. Key lessons include the need to tailor shrinkage rates to geometric constraints, employ waist-shaped risers for better feeding, and avoid gating configurations that promote sand inclusion. The formula for calculating the required riser volume in investment casting can be refined as: $$ V_{\text{riser}} \geq \frac{V_{\text{casting}} \cdot \Delta T \cdot c}{\rho \cdot L} $$ where \( \Delta T \) is the temperature drop, \( c \) is the specific heat, \( \rho \) is density, and \( L \) is latent heat, ensuring adequate compensation for shrinkage. Future work in precision casting could explore advanced simulation techniques to predict fluid flow and solidification, further enhancing the reliability of investment casting for critical applications. By continuously refining these aspects, investment casting can meet the evolving demands of high-performance industries, ensuring durability and safety in locomotive components.