In modern industries such as defense, aerospace, and shipbuilding, there is a growing demand for aluminum alloy castings characterized by intricate geometries, high dimensional accuracy, and superior internal quality. Traditional sand casting methods often struggle to meet these stringent requirements. To address these challenges, we explored an integrated approach combining anti-gravity low-pressure casting, 3D printing (3DP) for sand mold fabrication, and computational simulation for process optimization. This article details our first-person experience in designing, simulating, optimizing, and manufacturing a complex bridge bracket casting, a representative example of high-performance sand casting parts. The journey underscores the synergy between digital design and physical manufacturing in producing demanding sand casting parts.
The bridge bracket, a critical structural component, exemplifies the complexities inherent in modern sand casting parts. Its geometry, featuring thin and thick sections, internal cavities, and high-integrity bearing bores, necessitates a meticulous casting strategy. The primary technical specifications for this sand casting part were as follows. The material was specified as ZL114A aluminum alloy per GB/T1173-2013, subjected to T5 heat treatment. The overall envelope dimensions were 500 mm in length, 374 mm in width, and 388 mm in height, with a nominal weight of 27 kg. Wall thickness varied from 9 mm to 34 mm. Dimensional tolerance was required to meet DCTG8 level as per GB/T6414-2017. Most critically, the bearing bore areas demanded a defect-free internal structure, prohibiting shrinkage porosity, cavities, or sand inclusions. Furthermore, the entire casting required X-ray inspection, with internal quality conforming to Class II of GB/T9438-2013. The part’s design included hollow sections beneath the base and two large internal weight-reduction cavities, accessible only through small process holes, adding significant difficulty to mold design and core stability.
We selected the low-pressure casting process for its inherent advantages in producing high-quality sand casting parts: tranquil mold filling under controlled pressure and directional solidification under sustained pressure to enhance soundness. The initial gating system was designed as a bottom-feeding system combined with vertical slot gates. This design aimed to achieve smooth metal ascent and establish favorable temperature gradients. To prevent distortion, two technological ribs were added at the top of the casting. The system comprised four slot gates (35 mm x 20 mm each) feeding into four slag traps (ø80 mm x 400 mm). At the bottom, twelve tapered rectangular inlets were placed on the internal ribs of the cavity, each with a height of 45 mm and an 8° draft. The main runners at the bottom were 90 mm and 45 mm wide, with a height of 50 mm. Thirty chill plates, each 20 mm thick, were strategically placed on thick sections of the casting pattern to promote rapid cooling.
We employed ProCAST simulation software to virtually validate the casting process for this sand casting part. The parameters were set with a pouring temperature of 690 °C and a filling time of 45 seconds. The simulation of the filling sequence confirmed a calm, wave-like metal advance from the bottom inlets upward, with no indications of turbulent flow or jetting, which is crucial for avoiding oxide film entrapment in sand casting parts. The solidification analysis revealed that the areas with chills (the top and the bearing bores) solidified first, while the lower sections, particularly the gating inlets, remained liquid longest, indicating a generally favorable progression towards directional solidification. However, the initial simulation predicted minor shrinkage porosity in the bottom region near the inlets and at the top technological ribs. While the overall process was deemed feasible, this highlighted areas for potential improvement in the production of these sand casting parts.
The mold for this complex sand casting part was designed as a three-part assembly (cope, drag, and intermediate section) to accommodate the complex core geometry. The intermediate section contained the integrated core for the internal cavities and weight-reduction pockets. To ensure strength and prevent distortion during handling and pouring, reinforcing ribs were incorporated into the intermediate section print. The mold was fabricated using binder jetting 3D printing (3DP) technology. The printing material was 100-mesh silica sand, with a layer thickness of 0.3 mm. This digital, tooling-less method allowed for the rapid creation of the intricate mold geometry, including complex cores that would be extremely difficult or expensive to produce with traditional pattern-making. The mold parts were printed, assembled, and then coated with refractory paint and dried at 120-140°C for 4 hours before casting.
For the first trial production, the ZL114A alloy was melted and treated. Modification was carried out at 730°C using a ternary modifier (62.5% NaCl, 25% NaF, 12.5% KCl). Degassing was performed at 720°C using argon, with the melt cleanliness assessed by the density index (DI) method. The density index is calculated by comparing the density of a solidified sample under vacuum ($\rho_1$) to one solidified at atmospheric pressure ($\rho_2$):
$$ DI = \left(1 – \frac{\rho_2}{\rho_1}\right) \times 100\% $$
A lower DI indicates lower hydrogen content and higher melt quality. After treatment, a DI of 0.3% was achieved, which is acceptable for high-integrity sand casting parts. The melt was then adjusted to 680-690°C for pouring. The low-pressure casting cycle was executed with a fill speed of 45 mm/s, an initial pressurization of 50 kPa, and a solidification time under pressure of 600 seconds. After shakeout, the castings underwent T5 heat treatment: solution treatment at 535°C for 12 hours, quenching in 80°C water, and aging at 160°C for 6 hours.
The initial batch of three sand casting parts revealed several issues upon inspection. Visually, one casting exhibited a missing wall section in the bottom cavity. Dimensional measurement showed that the internal spherical radius (SR217 mm) was out of tolerance. X-ray inspection confirmed soundness in the critical bearing bores but revealed scattered shrinkage porosity in the thick bottom plate. Analysis traced the wall defect to the fracture and floating of the slender core segments forming the weight-reduction cavities during pouring. The core sections were only connected to the main body through small process holes, leading to insufficient strength. The dimensional error on the internal radius was also a consequence of this core movement. The shrinkage in the bottom was attributed to localized overheating due to the confluence of multiple gates.
The key measurements from the first trial are summarized in Table 1, highlighting the dimensional deviation. The chemical composition and mechanical properties from accompanying test bars, however, met the specified standards, as shown in Tables 2 and 3 respectively.
| Theoretical Dim. (mm) | Measured Dim. (mm) | Tolerance DCTG8 (mm) | Conformance |
|---|---|---|---|
| 465 | 465.8, 466.0, 465.6 | ±1.3 | Yes |
| 270 | 270.6, 270.4, 270.7 | ±1.1 | Yes |
| 374 | 374.5, 374.3, 374.6 | ±1.1 | Yes |
| 9.5 | 9.3, 9.6, 9.4 | ±0.5 | Yes |
| 14.5 | 14.8, 14.9, 14.8 | ±0.55 | Yes |
| SR217 | SR215, SR216, SR216.5 | ±1.0 | No |
| Element | Si | Mg | Ti | Fe | Al |
|---|---|---|---|---|---|
| Standard (ZL114A) | 6.5-7.5 | 0.45-0.75 | 0.10-0.20 | ≤0.20 | Bal. |
| Measured | 6.87 | 0.53 | 0.15 | 0.16 | Bal. |
| Property | Specimen 1 | Specimen 2 | Specimen 3 | Standard Requirement | Result |
|---|---|---|---|---|---|
| Tensile Strength (MPa) | 316 | 303 | 312 | ≥ 290 | Pass |
| Elongation (%) | 3.0 | 3.0 | 3.5 | ≥ 2 | Pass |
Based on this analysis, we implemented a comprehensive optimization plan targeting the mold design, the casting process, and the gating/feeding system to perfect the manufacture of these sand casting parts.
1. Core Strength Enhancement: The diameter of the process holes connecting the fragile core segments was increased. Four original ø10 mm holes were changed to two ø20 mm and two ø14 mm holes, resulting in a final configuration of six ø20 mm and two ø14 mm holes. This significantly improved the mechanical interlocking and strength of the printed sand core.
2. Improved Cooling: To address the shrinkage in the thick bottom plate, ten additional chill plates (15 mm thick) were incorporated into the mold design at the bottom cavity surface.
3. Mold Structure Optimization: The parting line and the mating interfaces between the drag and intermediate mold sections were revised. Clearances were adjusted: a 1 mm gap was set at the primary load-bearing interface, while other interfaces retained a 0.5 mm gap. This redesign helped distribute the buoyancy forces experienced by the drag during metal pouring more evenly across the intermediate section, preventing stress concentration and core lift.
A new simulation was run with the modified design, including the added bottom chills. The results showed a complete elimination of the predicted shrinkage defects in the bottom region. Following this digital validation, new 3DP sand molds were fabricated incorporating all optimizations. A subsequent low-pressure casting trial was conducted using identical melting and process parameters.
The results from the optimized process were markedly superior. The castings were visually complete with no missing sections. Dimensional measurement confirmed all critical features were within the specified DCTG8 tolerance, with most even meeting the tighter DCTG7 grade, as detailed in Table 4. X-ray inspection revealed a sound internal structure throughout the casting, including the previously problematic bottom area, with no shrinkage porosity detected. The successful casting met all requirements for Class II internal quality.
| Theoretical Dimension (mm) | Measured Dimension (mm) | Tolerance DCTG8 (mm) | Conformance |
|---|---|---|---|
| 465 | 465.8 | ±1.3 | Yes |
| 270 | 270.7 | ±1.1 | Yes |
| 374 | 374.8 | ±1.1 | Yes |
| 9.5 | 9.2 | ±0.5 | Yes |
| 14.5 | 14.7 | ±0.55 | Yes |
| SR217 | SR217.5 | ±1.0 | Yes |
The optimized process was then locked in and used for small-batch production. All subsequent sand casting parts produced consistently met the quality standards, demonstrating the robustness and repeatability of the developed method. The integration of simulation-driven design with the flexibility of 3DP sand mold fabrication proved to be a powerful strategy for conquering the challenges associated with complex sand casting parts. It enabled rapid iteration—from identifying issues in the virtual prototype to implementing and testing physical solutions with minimal lead time and cost compared to traditional tooling modifications.
In conclusion, this project successfully demonstrated a holistic digital-physical workflow for manufacturing high-integrity, complex sand casting parts. We effectively combined low-pressure casting for superior metallurgical quality, 3D sand printing for unparalleled geometric freedom and rapid prototyping, and computational simulation for predictive optimization. The key takeaways are manifold. Firstly, the approach enables the production of intricate sand casting parts with internal quality reaching Class II per GB/T9438-2013 and dimensional accuracy exceeding DCTG7 per GB/T6414-2017. Secondly, the iterative cycle of simulation, physical trial, analysis, and optimization is highly effective for first-time-right development of challenging castings, significantly improving first-article yield. Thirdly, for low-volume or prototype production of complex sand casting parts, 3DP sand molds offer an indispensable advantage in speed, flexibility, and cost-effectiveness over conventional pattern making, while ensuring consistent mold quality. This case study on the bridge bracket casting underscores a transformative pathway in foundry engineering, where digital tools are not just for planning but are integral to the agile and precise creation of advanced sand casting parts.