In the air braking systems of high-speed rail trains, the piston within the brake cylinder plays a critical role. This casting part converts pressure into braking force, ensuring balanced and reliable braking performance while maintaining system stability through effective sealing. Therefore, to guarantee the safe and efficient operation of the braking system, the overall quality requirements for the piston casting part are exceptionally stringent. This article details the process development for this casting part, focusing on addressing shrinkage porosity issues through numerical simulation and innovative techniques, ultimately leading to a successful production solution.
The casting part, as shown in the image below, is a bowl-shaped component with dimensions of approximately ϕ220 mm × 85 mm and a rough casting mass of 5.01 kg. Over 80% of its surface requires machining, and it features isolated hot spots at the bottom and sides of the bowl, which are prone to defects. The material specification is EN-GJS-500-7, with mechanical properties requiring a tensile strength ≥ 500 MPa, yield strength ≥ 320 MPa, and elongation ≥ 7%. Quality standards mandate that after machining, all processed areas must be free of visible defects. Non-destructive testing includes full-body X-ray inspection to EN 12681 Level 2 and MT/PT inspection to EN 1369 or EN 1371 standards with Level 2 acceptance criteria.
Initial casting process involved a shell molding technique using hot-box core sand with coated sand, producing two casting parts per mold with a yield of 52.37%. The total pouring weight was 19.13 kg. However, after machining, shrinkage porosity defects were consistently observed in the threaded groove at the bottom of the bowl-shaped region of the casting part. This led to a near 100% rejection rate, severely impacting sample delivery. Analysis revealed that this area, an isolated hot spot, solidified last, leading to porosity formation due to inadequate feeding.
To address this, we first attempted a modification using 3D-printed sand shells combined with insulated chills placed at the bottom of the casting part. The hypothesis was that accelerated cooling would reduce the hot spot size. Two sets of 3D-printed shells were fabricated externally, and成型 chills were positioned accordingly. However, production validation showed that defects were not eliminated; instead, shrinkage porosity became more severe and widespread in the bowl bottom region of the casting part. Subsequent numerical simulation using MAGMASOFT software confirmed that the chill merely shifted the hot spot upward without resolving the underlying feeding issue. The porosity prediction model in the simulation aligns with the empirical observation, often described by equations related to solidification time and temperature gradient. For instance, the Niyama criterion, a porosity predictor, can be expressed as: $$ G / \sqrt{T} $$ where \( G \) is the temperature gradient and \( T \) is the local solidification time. A low value indicates a higher risk of shrinkage porosity. In our case, the simulated porosity zone correlated with areas where this criterion fell below a critical threshold.
Given the failure of the chill-based approach, we redesigned the entire process using resin sand molding with one mold producing four casting parts. This allowed greater flexibility in gating and risering. We explored multiple riser configurations atop the casting part, but simulation indicated that shrinkage defects remained in the bowl bottom, as the risers could not effectively feed this isolated region. The following table summarizes two riser schemes simulated:
| Scheme | Riser Configuration | Simulated Porosity in Bowl Bottom |
|---|---|---|
| A | Two Ø60 mm insulated sleeves | High porosity present |
| B | Two Ø80 mm insulated sleeves | High porosity present |
Since risers proved ineffective, we investigated the use of cooling fins (chills) attached directly to the bowl bottom of the casting part. To efficiently evaluate multiple design variables without extensive trial-and-error, we employed the Design of Experiment (DOE) functionality within MAGMA software. The DOE focused on two parameters: cooling fin diameter and their angular distribution around the bowl bottom. The objective was to minimize porosity in the casting part. The experimental matrix and key outcomes are tabulated below:
| DOE Scheme | Cooling Fin Diameter (mm) | Angular Distribution (°) | Simulated Porosity Severity | Effect on Shrinkage |
|---|---|---|---|---|
| 1 | 3 | 30 | High | No improvement |
| 2 | 5 | 30 | High | No improvement |
| 3 | 3 | 45 | High | No improvement |
| 4 | 5 | 45 | High | No improvement |
| 5 | 3 | 60 | High | No improvement |
| 6 | 5 | 60 | High | No improvement |
The DOE results conclusively showed that varying fin diameter and distribution did not reduce porosity in the critical region of the casting part. The underlying reason is that the fins, while enhancing local cooling, could not alter the fundamental thermal geometry that created an isolated hot spot. The solidification process in such regions can be modeled using the heat conduction equation: $$ \rho C_p \frac{\partial T}{\partial t} = \nabla \cdot (k \nabla T) + L \frac{\partial f_s}{\partial t} $$ where \( \rho \) is density, \( C_p \) is specific heat, \( T \) is temperature, \( t \) is time, \( k \) is thermal conductivity, \( L \) is latent heat, and \( f_s \) is solid fraction. In areas with poor feeding, the last liquid to solidify contracts, leading to porosity formation. For this casting part, the bowl bottom acted as a thermal center, making it susceptible to shrinkage despite external chills or fins.
After exhaustive simulation, we devised a final solution: filling the bowl bottom’s internal blind hole area with additional metal, effectively creating a “filled” region that extends into the rough machining allowance. This design modification, analyzed in MAGMA, showed that shrinkage porosity could be driven into this filled zone. Subsequently, during rough machining, this portion of the casting part containing defects is completely removed, leaving a sound surface for final precision machining. The key to this approach is ensuring that the filled region is thick enough to encompass all predicted porosity, which can be estimated from simulation data. The porosity volume \( V_p \) in a localized hot spot can be approximated by: $$ V_p \approx \beta \cdot V_{liquid} \cdot ( \alpha – \epsilon ) $$ where \( \beta \) is the solidification shrinkage factor (typically 4-6% for ductile iron), \( V_{liquid} \) is the volume of liquid metal in the hot spot, \( \alpha \) is the thermal contraction coefficient, and \( \epsilon \) is the feeding efficiency. By designing the filled region with a volume greater than \( V_p \), defects are confined to an area that is later machined away.
The final process utilized resin sand molds with a yield of 52.65% and a total pouring weight of 40.28 kg for four casting parts. The gating system was optimized to ensure smooth filling, and risers were placed on top to feed general shrinkage. Simulation of the final scheme confirmed that porosity was concentrated in the filled bowl bottom area. Production validation involved casting samples, followed by rough machining to remove the filled region, then precision machining and non-destructive testing. Results showed that all processed areas of the casting part were free of visible defects, and X-ray/MT/PT inspections met the required Level 2 standards. The scrap rate dropped to below 3% in batch production, with no recurrence of bowl bottom shrinkage in finished parts.
This development highlights the importance of integrated simulation and creative design in solving persistent casting defects. For complex casting parts like this piston, traditional methods such as chills or risers may be insufficient for isolated hot spots. The “fill and machine” strategy offers a viable alternative, especially when defect relocation to a removable area is feasible. It balances metallurgical soundness with economic machining, enhancing the overall quality of the casting part. Future work could involve optimizing the fill geometry using advanced topology algorithms to minimize added weight while ensuring defect containment. Additionally, real-time monitoring during solidification, coupled with predictive analytics, could further refine the process for this critical casting part in high-speed rail applications.
In summary, the successful development of this piston casting part for high-speed rail braking systems underscores the value of numerical simulation in diagnosing and resolving shrinkage porosity. By leveraging MAGMA software’s DOE and porosity analysis, we identified that conventional cooling methods failed for this geometry. The ultimate solution—filling the bowl bottom and removing it via rough machining—proved effective, achieving a high-quality casting part that meets stringent performance criteria. This approach can be extended to other casting parts with similar challenging geometries, promoting reliability and efficiency in precision casting manufacturing.