In modern industries such as defense, aerospace, and marine engineering, the demand for high-integrity aluminum alloy casting parts has surged, driven by requirements for complex geometries, tight dimensional tolerances, and superior internal quality. Traditional sand casting methods often fall short in meeting these stringent standards, especially for intricate casting parts that pose challenges in mold making, dimensional accuracy, and cost-effectiveness. To address these issues, I have explored an integrated approach combining advanced simulation software, 3D printing (3DP) of sand molds, and low-pressure casting techniques. This article details my first-person experience in designing, simulating, optimizing, and manufacturing a bridge bracket casting part, emphasizing the iterative process that led to a successful outcome. Throughout this work, the focus remains on enhancing the quality and precision of the casting part, leveraging innovative technologies to overcome traditional limitations.
The casting part in question is a bridge bracket, which serves as a structural component in mechanical assemblies. This casting part features a complex design with a contour size of approximately 500 mm × 374 mm × 388 mm and a wall thickness ranging from 9 mm to 34 mm. The material specified is ZL114A aluminum alloy per GB/T1173-2013, subjected to T5 heat treatment to achieve optimal mechanical properties. Key technical requirements for this casting part include a dimensional tolerance grade of DCTG8 according to GB/T6414-2017, a mass of 27 kg, and high internal quality in critical areas such as bearing holes, where defects like shrinkage porosity, cavities, and sand inclusions are unacceptable. Furthermore, the entire casting part must undergo X-ray inspection to ensure compliance with Class II standards per GB/T9438-2013. The complexity arises from hollow sections in the base, which are connected to the external environment only through small process holes, making conventional molding difficult and necessitating a novel approach.
To tackle these challenges, I opted for a low-pressure casting process combined with 3DP sand molds. Low-pressure casting offers advantages such as smooth filling, reduced turbulence, and directional solidification under pressure, which enhance the density and integrity of the casting part. Meanwhile, 3DP sand molds enable rapid, mold-less fabrication of complex geometries, bypassing the need for expensive patterns and allowing for quick iterations. The initial step involved designing a gating system tailored for this casting part. I chose a bottom-gating system with slit feeders to ensure controlled metal flow and minimize defects. The design included four slit gates measuring 35 mm × 20 mm, four slag traps of ϕ80 mm × 400 mm, and twelve rectangular ingates at the bottom, each with a height of 45 mm and an 8° taper to match the internal rib thickness. Additionally, chill blocks were strategically placed in thick sections to promote rapid cooling and reduce shrinkage. The overall gating system was modeled in CAD software, with a casting shrinkage allowance of 1% applied to account for solidification contraction.
Next, I utilized ProCAST simulation software to virtually analyze the casting process. The simulation parameters were set based on typical low-pressure casting conditions: a pouring temperature of 690°C, a filling time of 45 seconds, and a solidification time of 600 seconds. The temperature field during filling showed that molten metal entered the mold smoothly through the bottom ingates, with no signs of splashing or turbulent flow, which is crucial for preventing oxide inclusions and gas entrapment in the casting part. The solidification simulation revealed a progressive cooling pattern, starting from the top sections with chill blocks and moving toward the bottom ingates, aligning with the desired directional solidification. However, the initial simulation predicted minor shrinkage defects in the top risers and bottom regions, indicating a need for optimization. The density index (DI), used to assess hydrogen content in the aluminum melt, was calculated using the formula:
$$ DI = \left(1 – \frac{\rho_2}{\rho_1}\right) \times 100\% $$
where $\rho_1$ and $\rho_2$ are the densities of reference and test samples, respectively. A lower DI value corresponds to higher melt quality, and after refining with argon gas, the DI was reduced to 0.3%, ensuring a sound melt for the casting part.
| Dimension No. | Theoretical Size (mm) | Measured Size (mm) | Tolerance (DCTG8) (mm) | Out-of-Tolerance? |
|---|---|---|---|---|
| 1 | 465 | 465.8, 466.0, 465.6 | ±1.3 | No |
| 2 | 270 | 270.6, 270.4, 270.7 | ±1.1 | No |
| 3 | 374 | 374.5, 374.3, 374.6 | ±1.1 | No |
| 4 | 9.5 | 9.3, 9.6, 9.4 | ±0.5 | No |
| 5 | 14.5 | 14.8, 14.9, 14.8 | ±0.55 | No |
| 6 | SR217 | SR215, SR216, SR216.5 | ±1.0 | Yes |
With the simulation results indicating feasibility, I proceeded to design the 3DP sand molds. The mold was structured as a three-part system: an upper cope, a middle section containing the core, and a lower drag. The cope included预留 slots for chill blocks, which were later fixed using foundry adhesives. The core was designed as a single piece to maintain dimensional accuracy and strength, with reinforcing ribs added to prevent deformation during drying. The drag featured定位子扣 for precise alignment, with a draft angle of 8° and a fit clearance of 0.5 mm. The mold was printed using 100-mesh silica sand with a layer thickness of 0.3 mm, resulting in a robust and detailed sand mold capable of producing the intricate features of the casting part. After printing, the mold surfaces were coated twice with refractory paint and dried at 120–140°C for 4 hours to enhance surface finish and prevent metal penetration.
The first production trial was conducted using low-pressure casting. The aluminum alloy was melted, treated with a modifier (62.5% NaCl, 25% NaF, 12.5% KCl by weight) at 730°C, and refined with argon at 720°C to achieve the desired melt quality. The low-pressure casting machine was set to a filling speed of 45 mm/s, a filling pressure of 50 kPa, and a crystallization pressure maintained for 600 seconds. After casting, the casting part was subjected to T5 heat treatment: solution treatment at 535°C for 12 hours followed by water quenching at 80°C, and aging at 160°C for 6 hours. However, initial inspection revealed several issues with the casting part. Visually, there were localized areas lacking wall thickness in the base, and the internal cavity bottom was not flat, with a height deviation of up to 2 mm. X-ray inspection showed shrinkage porosity in the bottom region, and dimensional measurements indicated that the spherical surface radius (SR217) was out of tolerance, as summarized in Table 1.
Analysis identified the root causes: the core in the middle section had insufficient strength, leading to fracture and upward movement during metal injection due to fluid forces. The砂芯, connected only through small process holes, failed under the冲击力 of the molten metal. Additionally, the bottom gating caused局部过热, resulting in shrinkage defects. To address these, I optimized the process in three key areas. First, the process holes were enlarged from the original four ϕ10 mm holes to a combination of six ϕ20 mm and two ϕ14 mm holes, increasing the core’s structural integrity. Second, ten additional chill blocks, each 15 mm thick, were placed on the bottom surface to accelerate cooling and reduce shrinkage. The effect of chill blocks can be approximated using Chvorinov’s rule for solidification time:
$$ t = C \left( \frac{V}{A} \right)^2 $$
where $t$ is the solidification time, $C$ is a constant dependent on mold material and casting conditions, $V$ is the volume of the casting part section, and $A$ is its surface area. By increasing the effective surface area via chill blocks, $t$ decreases, promoting faster solidification and minimizing shrinkage. Third, the sand mold structure was revised to better distribute forces: the middle section’s contact area with the drag was adjusted to a 1 mm clearance, while other parting surfaces retained a 0.5 mm clearance, ensuring that浮力 from the molten metal was分散 across multiple surfaces rather than concentrating on a single point.
Re-simulating the optimized process with ProCAST showed a significant improvement: the predicted shrinkage defects in the bottom region were eliminated, confirming the effectiveness of the added chill blocks. I then printed new 3DP sand molds based on the updated design and conducted another production run. The results were markedly better. The casting part exhibited complete filling with no visual defects, and dimensional measurements met the required tolerances, as shown in Table 2. Notably, the spherical surface radius was within the specified range, and the internal cavity bottom was flat, indicating successful core stabilization.
| Dimension No. | Theoretical Size (mm) | Measured Size (mm) | Tolerance (DCTG8) (mm) | Out-of-Tolerance? |
|---|---|---|---|---|
| 1 | 465 | 465.8 | ±1.3 | No |
| 2 | 270 | 270.7 | ±1.1 | No |
| 3 | 374 | 374.8 | ±1.1 | No |
| 4 | 9.5 | 9.2 | ±0.5 | No |
| 5 | 14.5 | 14.7 | ±0.55 | No |
| 6 | SR217 | SR217.5 | ±1.0 | No |
Chemical analysis and mechanical testing of samples from the optimized batch confirmed compliance with standards. The chemical composition, presented in Table 3, aligns with ZL114A specifications, and the mechanical properties, summarized in Table 4, exceed the minimum requirements of GB/T1173-2013. X-ray inspection of the optimized casting part revealed no shrinkage porosity or cavities, even in the critical bottom areas, demonstrating that the process adjustments effectively enhanced internal quality. This casting part now fully meets the Class II criteria for internal soundness.
| Element | Standard Range (ZL114A) | Measured Value |
|---|---|---|
| Si | 6.5–7.5 | 6.87 |
| Mg | 0.45–0.75 | 0.53 |
| Ti | 0.10–0.20 | 0.15 |
| Fe | ≤0.2 | 0.16 |
| Al | Balance | Balance |
| Sample | Tensile Strength (MPa) | Elongation (%) | Standard Requirement |
|---|---|---|---|
| 1 | 316 | 3.0 | ≥290 MPa, ≥2% |
| 2 | 303 | 3.0 | ≥290 MPa, ≥2% |
| 3 | 312 | 3.5 | ≥290 MPa, ≥2% |
The success of this project underscores the synergy between digital simulation, additive manufacturing, and advanced casting techniques. By iteratively refining the process based on simulation predictions and real-world trials, I achieved a high-quality casting part that satisfies stringent industrial standards. The use of 3DP sand molds eliminated the need for complex patterns, reduced lead time, and allowed for rapid modifications, while low-pressure casting ensured dense microstructure and minimal defects in the final casting part. Moreover, the integration of chill blocks and optimized gating design demonstrated how targeted cooling can control solidification behavior, a principle encapsulated in the following heat transfer equation relevant to casting:
$$ \frac{\partial T}{\partial t} = \alpha \nabla^2 T $$
where $T$ is temperature, $t$ is time, and $\alpha$ is thermal diffusivity. By manipulating boundary conditions through chill blocks, the temperature gradient $\nabla T$ is enhanced, promoting directional solidification and reducing shrinkage in the casting part.
In conclusion, this case study highlights a robust methodology for producing complex aluminum alloy casting parts. The bridge bracket casting part, initially plagued by defects, was transformed into a compliant component through systematic optimization of the gating system, sand mold design, and cooling strategy. The final casting part exhibits excellent dimensional accuracy (achieving DCTG7 per GB/T6414-2017, surpassing the required DCTG8), superior internal quality, and consistent mechanical properties. This approach is particularly valuable for low-volume, high-complexity casting parts, where traditional methods are cost-prohibitive or technically inadequate. Future work could explore further refinements, such as integrating real-time monitoring during casting or using machine learning to optimize simulation parameters, but the current results affirm the viability of combining 3DP sand molds with low-pressure casting for premium-grade casting parts. The journey from design to production reinforced the importance of adaptability and precision in modern foundry practices, ensuring that every casting part meets the evolving demands of advanced engineering applications.