In the 21st century, the aerospace, defense, and automotive industries have seen an ever-increasing demand for large thin-walled, precision, high-quality, and high-strength castings. The requirements for dimensional accuracy, density, and mechanical properties of castings are becoming more stringent, with a clear trend towards minimal allowance, high precision, high performance, and monolithic structures. Traditional sand casting methods struggle to meet these demands due to long trial production cycles, high costs, and significant challenges in manufacturing complex cavity molds, often resulting in poor quality and low precision in castings. The integration of rapid prototyping with vacuum counter-pressure casting has emerged as a promising and efficient approach for precision forming. The study of sand casting parts is critical as they directly influence the quality of the final product, especially in complex thin-wall applications.
This research focuses on an aeronautical thin-walled electric box casting, a critical component requiring high air tightness. The unique challenges of this part stem from its complex structure: a length of 220 mm, a width of 120 mm, a height of 66 mm, with internal ribs only 1.4 mm wide and 12 mm high, and a uniform wall thickness of just 1.4 mm. The mass is a mere 0.3 kg. This geometry makes it highly susceptible to defects such as misruns, cold shuts, and porosity. The SLS process was chosen to create the coated sand molds, as it allows for direct fabrication of complex mold geometries without the need for tooling. However, the initial strength and surface quality of SLS-produced sand molds are inadequate for direct use in vacuum counter-pressure casting. Therefore, this study explores the reinforcement of SLS molds via silica sol infiltration and optimizes the vacuum counter-pressure casting process parameters to successfully produce this high-integrity thin-wall component.
Materials and Experimental Methods
The base material used in this research was a resin-coated sand, characterized by a sphericity coefficient of ≤1.1, a smooth surface, and a particle size distribution ranging from 70 μm to 140 μm. The enhancing material was an alkaline silica sol, with a SiO₂ content of 29–31%, a Na₂O content of less than 0.5%, a density of 1.2–1.22 g/cm³, a pH of 9–10, a kinematic viscosity of ≤8×10⁻⁴ m²/s, and a colloidal particle size of 9–20 nm. The metal of choice was ZL114A (ZAlSi7Mg1A), an Al-Si hypoeutectic alloy, with its chemical composition and key thermal properties presented in the tables below. The liquidus and solidus temperatures of ZL114A are 616°C and 556°C, respectively.
| Element | Al | Si | Mg | Ti | Zr | Zn | Mn | Cu | Fe |
|---|---|---|---|---|---|---|---|---|---|
| Content (wt%) | Balance | 6.97 | 0.72 | 0.13 | ≤0.20 | ≤0.10 | ≤0.10 | ≤0.10 | ≤0.084 |
The SLS process was carried out on a LaserCore-5300 system, with key parameters of a preheating temperature of 60°C, a laser power of 45-50 W, a scan speed of 1.5 m/s, and a layer thickness of 0.2 mm. The initial SLS specimens underwent a pre-curing process in a drying oven at 180°C for 2 hours to provide initial handling strength. After pre-curing, specimens were subjected to a silica sol infiltration process. The experimental parameters investigated are outlined in the following section, followed by a post-curing step. The infiltration process was designed to fill the voids and reinforce the binder necks between the sand grains, thereby enhancing the overall strength and surface quality of the molds.
Post-Processing Treatment and Its Effects
The post-processing treatment of SLS molds is a critical step to enhance their performance. This research systematically investigated the influence of silica sol infiltration parameters and post-curing parameters on various properties of the molds, including tensile strength, bending strength, surface roughness, and permeability, which are all indispensable for producing high-quality sand casting parts. The initial tensile strength of SLS samples after 180°C, 2h pre-curing was found to be 1.32 MPa. The following sections detail the effects of each parameter on the properties.
Effect of Infiltration Temperature
The infiltration temperature was varied between 20°C and 60°C while keeping the infiltration time at 15 hours, post-curing temperature at 180°C, and post-curing time at 60 minutes. The influence on strength and surface roughness is summarized in the table below. The results show a significant effect, with an optimal infiltration temperature of 30°C yielding the highest strength and lowest surface roughness.
| Infiltration Temperature (°C) | 20 | 30 | 40 | 50 | 60 |
|---|---|---|---|---|---|
| Tensile Strength (MPa) | 2.1 | 3.45 | 2.8 | 2.2 | 1.8 |
| Bending Strength (MPa) | 2.2 | 3.3 | 2.9 | 2.4 | 1.9 |
| Surface Roughness Ra (μm) | 21 | 14.5 | 17.5 | 19.5 | 22.5 |
| Permeability (unit) | 10.3 | 7.2 | 5.1 | 3.4 | 5.8 |
Below 30°C, increasing the temperature enhances the Brownian motion of silica particles, improving their diffusion and infiltration into the SLS mold’s necks, which leads to a stronger reinforcement. However, above 30°C, the increasing concentration of the sol raises its viscosity, which drastically reduces its ability to penetrate the fine gaps between sand grains, thereby diminishing the reinforcing effect. Permeability reaches a minimum at around 50°C, as the sol blocks the pores more effectively at moderate temperatures, while high temperatures reduce this effect due to poor infiltration.
Effect of Infiltration Time
With an infiltration temperature of 30°C, post-curing temperature of 180°C, and post-curing time of 60 minutes, the infiltration time was varied from 0 to 25 hours. The results indicate an optimal infiltration time of 15 hours. Increasing the time up to 15 hours allows more silica sol to enter the mold, forming larger and new bridging necks between sand grains, thus improving strength and surface quality. Beyond 15 hours, the silica sol becomes saturated, and the hydrophilicity of the sol begins to disrupt the resin bonds, weakening the structure.
| Infiltration Time (h) | 0 | 5 | 10 | 15 | 20 | 25 |
|---|---|---|---|---|---|---|
| Tensile Strength (MPa) | 1.32 | 2.25 | 2.95 | 3.45 | 3.2 | 2.9 |
| Surface Roughness Ra (μm) | 26.4 | 21.2 | 17.5 | 14.5 | 19.8 | 24.5 |
| Permeability (unit) | 18 | 12.5 | 9.8 | 7.2 | 5.5 | 4.1 |
Effect of Post-Curing Temperature
The post-curing temperature, varied from 120°C to 200°C while keeping infiltration parameters at their optimum (30°C, 15h) and post-curing time at 60 minutes, significantly affects the final properties. The optimal post-curing temperature was identified as 180°C. At temperatures between 120°C and 180°C, the phenolic resin melts and flows better with the silica sol, improving the wetting and bonding between sand particles, which increases the density and strength. Above 180°C, the resin begins to decompose and carbonate, which reduces the bonding strength and degrades the surface quality.
| Post-Curing Temperature (°C) | 120 | 140 | 160 | 180 | 200 |
|---|---|---|---|---|---|
| Tensile Strength (MPa) | 1.1 | 1.9 | 2.6 | 3.45 | 3.1 |
| Surface Roughness Ra (μm) | 20.2 | 18.5 | 16.8 | 14.5 | 15.9 |
| Permeability (unit) | 6.8 | 6.4 | 6.1 | 5.8 | 6.2 |
Effect of Post-Curing Time
Finally, the post-curing time, from 30 to 150 minutes, was optimized with all other parameters held at their optimal values (Infiltration at 30°C for 15h, Post-curing at 180°C). A post-curing time of 60 minutes provided the best performance. Insufficient time prevents the complete flow and homogenization of the resin-silica mixture, while excessive time leads to resin degradation and a subsequent drop in strength and integrity of the mold.
| Post-Curing Time (min) | 30 | 60 | 90 | 120 | 150 |
|---|---|---|---|---|---|
| Tensile Strength (MPa) | 2.8 | 3.45 | 3.3 | 3.1 | 2.9 |
| Surface Roughness Ra (μm) | 18.9 | 14.5 | 15.2 | 16.1 | 17.8 |
| Permeability (unit) | 7.0 | 5.8 | 6.7 | 6.1 | 5.9 |
Summarizing the parameter influence, the optimal post-processing schedule was identified as: infiltration at 30°C for 15 hours, followed by post-curing at 180°C for 60 minutes. This optimal treatment resulted in a tensile strength of 3.447 MPa, marking a 2.6-fold improvement over the pre-treated SLS sand molds. This enhancement in mechanical properties is critical for the molds to withstand the pressures and thermal stresses of the vacuum counter-pressure casting process, ensuring the integrity of the sand casting parts. The process also improved surface quality, reducing Ra from 22.1 μm to 14.2 μm.
Other Performance Characteristics
Besides the core mechanical and surface properties, other characteristics such as collapsibility and gas evolution were also assessed. The collapsibility, determined by the residual strength after high-temperature firing, was investigated. As the firing temperature increased, the residual strength decreased, with no firing temperatures above 400°C yielding a residual strength of almost 0 MPa, which is excellent for easy casting removal. The gas evolution of the SLS molds was not significantly affected by the infiltration process, with values consistently around 11 ml/g, well below the acceptable industry limit of 20 ml/g. Furthermore, the infiltration process led to a slight, but measurable, increase in the dimensions and mass of the molds, with an average linear expansion on the order of 0.1 to 0.2 mm being observed, which must be accounted for in the initial mold design (e.g., through software compensation).
Microstructural Analysis and Strengthening Mechanism
SEM analysis was conducted to observe the fracture surfaces and binder necks of the treated SLS samples, confirming the formation of new silica-based necks. Energy dispersive spectroscopy (EDS) analysis of the fracture surface showed a high concentration of Si and O in the neck regions, confirming the presence of silica. The strengthening mechanism of the silica sol infiltration process can be understood as a multi-stage process. During the SLS process, the initial neck formation is driven primarily by viscous flow of the phenolic resin. After infiltration with silica sol at the optimum temperature and time, the sol undergoes area and volume diffusion, filling the voids and forming silica bridges, or new “connective necks,” between sand grains. This stage is dominated by area and volume diffusion of the silica. During the post-curing stage, the viscous flow of the resin and the continued diffusion of the silica sol further densify the binder bridges, creating a uniform composite binder phase. This combined mechanism, which I propose to describe as a “melting-area/volume diffusion mechanism,” is responsible for the significant enhancement in the performance of the SLS molds, ultimately producing high-quality sand casting parts.
Vacuum Counter-Pressure Casting Process Optimization
The vacuum counter-pressure casting of the thin-walled electric box was simulated using the ProCAST software. The primary objective was to study the effects of different process parameters on the filling and solidification of the casting, and to identify the process window that minimizes porosity and ensures complete filling. The process parameters investigated were the pressure profile, mold temperature, and pouring temperature. The simulation results were used to predict defects such as misruns and porosity, guiding the experimental casting trials.
Pressure Profile Optimization
For a successful casting, the pressure must overcome gravity and flow resistance. The fundamental pressure equation is given by:
$$ P = \rho g H \mu $$
where $P$ is the pressure required to fill the mold cavity, $\rho$ is the density of the liquid metal, $g$ is the gravitational constant, $H$ is the vertical height of the liquid metal column, and $\mu$ is the resistance coefficient (typically between 1 and 1.5). Considering the vacuum assistance, the total pressure includes the vacuum pressure. Based on calculations and the need for stable filling, three pressure profiles were simulated. Scheme 1 had a filling time of 1.64 s, Scheme 2 had a filling time of 1.86 s, and Scheme 3 allowed a longer, more controlled fill time of 2.0 s. The simulation results, particularly the shrinkage porosity values and fill stability, are summarized in the table below.
| Pressure Scheme | Fill Time (s) | Solidification Time (s) | Flow Stability | Shrinkage Porosity |
|---|---|---|---|---|
| Scheme 1 (Fast) | 1.64 | 25 | Unstable at corners, prone to turbulence | 0.01000 |
| Scheme 2 (Medium) | 1.86 | 25 | Slightly unstable at corners | 0.00990 |
| Scheme 3 (Slow) | 2.00 | 25 | Stable, smooth filling | 0.00983 |
Scheme 3, with a filling time of 2.0 seconds, was found to be optimal. It provided a stable and smooth filling pattern, preventing turbulent flow and the associated gas entrapment, which is crucial for thin-walled castings. The simulation for this profile showed a favorable temperature gradient, promoting directional solidification from the top of the casting downwards, which enhances feeding and reduces shrinkage porosity.
Effect of Mold Temperature
Using the optimal pressure profile (Scheme 3), the mold temperature was varied at 120°C, 135°C, and 150°C while maintaining a pouring temperature of 720°C. The simulation results are shown in the table below. The lowest shrinkage porosity value and which is classified as micro-porosity was observed at a mold temperature of 120°C.
| Mold Temperature (°C) | Fill Time (s) | Solidification Time (s) | Shrinkage Porosity | Defect Location |
|---|---|---|---|---|
| 120 | 2.0 | 25 | 0.00983 | Runner |
| 135 | 2.0 | 25 | 0.01040 | Casting/Runner |
| 150 | 2.0 | 25 | 0.01020 | Casting/Runner |
A lower mold temperature of 120°C was found to be optimal. This temperature helps to achieve a steeper thermal gradient, promoting more efficient directional solidification. Higher mold temperatures slow down the cooling rate, leading to coarser microstructures and a higher risk of shrinkage porosity within the casting itself. The simulation confirmed that at 120°C, the defects were isolated to the runner system, ensuring the integrity of the sand casting parts. The optimal mold temperature proved to be 120°C.
Effect of Pouring Temperature
Finally, with the optimal pressure profile and mold temperature, the pouring temperature was varied at 640°C, 680°C, and 720°C. In vacuum counter-pressure casting, the pouring temperature can be lower than in gravity casting because the pressure assists in filling. The simulations showed that the pouring temperature had a negligible effect on the filling time, solidification time, or shrinkage porosity, as shown in the table below.
| Pouring Temperature (°C) | Fill Time (s) | Solidification Time (s) | Shrinkage Porosity | Defect Location |
|---|---|---|---|---|
| 640 | 2.0 | 25 | 0.00983 | Runner |
| 680 | 2.0 | 25 | 0.00983 | Runner |
| 720 | 2.0 | 25 | 0.00983 | Runner |
The pouring temperature was found to have little effect on the final quality in the simulation, as long as it is sufficient to avoid misruns. However, based on practical experience and to prevent gas entrapment and excessive oxidation, a pouring temperature of 680°C was chosen as the optimal parameter for the experimental trials. This temperature is low enough to minimize gas porosity while still being high enough to fill the thin-walled mold completely, providing a balance between filling capability and Casting quality.
Experimental Casting and Result
Following the successful optimization of the process parameters through ProCAST simulation, the actual casting trial was conducted using the optimal parameters. The final process parameters for the vacuum counter-pressure casting were: a vacuum pressure of 20 KPa, a filling pressure difference ($\Delta P$) of 35 KPa, a holding time of 160 seconds, and a solidification pressure of 300 KPa. The mold temperature was held at 120°C, and the pouring temperature was 680°C.
Utilizing the silica sol-reinforced SLS molds and the optimized casting parameters, a thin-walled electric box was successfully produced. The casting exhibited excellent surface quality with no visible defects such as misruns, cold shuts, or porosity. The part was dimensionally accurate and, upon testing, demonstrated excellent air tightness, meeting the stringent requirements for its application. Notably, the cast electric box proved superior to a welded counterpart, which had issues with weld integrity and poor airtightness. The integration of SLS mold technology with vacuum counter-pressure casting, supported by numerical simulation, proved to be an exceptionally effective method for producing high-quality complex thin-wall sand casting parts.
Conclusion and Outlook
This research successfully demonstrated the potential of combining silica sol-reinforced SLS sand molds with the vacuum counter-pressure casting process for manufacturing complex thin-walled aerospace components, specifically a high-integrity electric box. The key findings can be summarized as follows:
(1) The post-processing of SLS molds via silica sol infiltration and post-curing is essential for improving their performance. The optimal parameters were determined to be: infiltration temperature of 30°C for 15 hours, followed by post-curing at 180°C for 60 minutes. This treatment significantly improved the tensile strength of the molds by a factor of 2.6 (from 1.32 MPa to 3.447 MPa) and greatly enhanced the surface quality.
(2) The strengthening mechanism involves the area and volume diffusion of silica during infiltration and a combined viscous flow-area/volume diffusion mechanism during post-curing, creating a robust composite binder network between sand grains, improving the quality of sand casting parts.
(3) While the silica sol infiltration process enhances strength and surface quality, it somewhat reduces the permeability of the mold. However, the treated molds still maintain acceptable gas evolution levels and excellent collapsibility, which are vital for preventing casting defects and ensuring easy pattern removal.
(4) The use of ProCAST simulation was instrumental in optimizing the vacuum counter-pressure casting process. The pressure profile with a stable filling time of 2.0 seconds was found to be critical for preventing turbulence. A mold temperature of 120°C and a pouring temperature of 680°C were identified as the optimal parameters to ensure directional solidification and minimize shrinkage porosity.
(5) The actual casting trial confirmed the simulations and the effectiveness of the integrated process. The complex thin-walled electric box was successfully cast with sound internal quality, high dimensional accuracy, good surface finish, and excellent airtightness, highlighting the immense potential of this hybrid manufacturing approach for producing high-quality sand casting parts.
Future research should focus on reducing the infiltration time and further optimizing the permeability of the reinforced molds. Additionally, exploring alternative or supplemental strengthening materials and methods could enhance the high-temperature performance of the molds or facilitate their breakdown after casting. The potential for scaling this process to even larger and more complex thin-walled components also presents a significant opportunity for future investigation.