In modern aerospace, automotive, and other advanced industries, the demand for lightweight components has significantly increased. Aluminum alloys, due to their high strength-to-weight ratio, low density, excellent corrosion resistance, machinability, and good castability, are widely used. After cold/hot working and heat treatment, they exhibit superior mechanical and physical properties. Therefore, thin-walled aluminum shell casting parts are extensively applied in aerospace, defense electronics, shipbuilding, nuclear power, and other fields. With the rapid upgrading of advanced equipment in recent years, requirements for internal quality, mechanical properties, and dimensional accuracy of aluminum alloy casting parts have become increasingly stringent. This study focuses on a thin-walled conical shell casting part, which serves as a critical front-end device component in an aircraft. It is a high-strength aluminum alloy casting part made of ZL114A. The casting part has a semi-enclosed structure, making it prone to internal quality issues during casting. To ensure all indicators meet specifications, we conducted research on the casting process using counter-pressure casting combined with computer simulation analysis. During the study, we identified shrinkage porosity risks in the triangular apex region during solidification. Through simulation-guided adjustments to the gating system and addition of chills, we eliminated the shrinkage tendency and completed trial production. The internal quality of the casting part met Class I requirements, and the attached test bars showed average tensile strength, yield strength, and elongation of 364.67 MPa, 306 MPa, and 7.5%, respectively, satisfying all technical specifications.
The casting part is a large thin-walled shell component with high performance requirements. Traditional investment casting might face challenges in shell-making and pouring due to the size, and its mechanical properties are relatively lower. Sand casting, however, offers better process flexibility, allowing optimization of gating and chilling systems to improve internal quality, ensure density, and enhance mechanical properties. Since the casting part is thin-walled, metal fluidity in the mold cavity is poor, requiring higher pouring temperatures. Gravity pouring has poor feeding capacity and is prone to defects like pinholes. Thus, counter-gravity pouring was chosen. As a Class I casting part, it has strict requirements against shrinkage porosity, shrinkage cavities, pinholes, and slag inclusions. Research indicates that the feeding capacity of counter-pressure casting is 4–5 times that of low-pressure casting. To ensure first-time success, counter-pressure pouring was selected, allowing crystallization and solidification under pressure, improving reliability, feeding capability, and resulting in denser casting parts with better internal quality and mechanical properties. The trial production used sand molding combined with counter-pressure pouring.
The casting part features a conical shell geometry with a maximum outer dimension of Φ500 mm × 1000 mm. The main wall thickness is 8 mm, with an upper triangular apex structure and a lower spherical base. The entire outer surface is machined, and the port inner flange is machined, while the inner cavity remains unmachined. To prevent insufficient wall thickness after machining due to casting deviations, a 1 mm unilateral allowance was added to non-machined inner surfaces, ensuring wall thickness requirements. A 6 mm machining allowance was set for the outer profile and bottom. For machining purposes, a process boss was added at the cone apex, and a process ring was added at the bottom. The single casting part weight is approximately 60 kg. Based on experience, a linear shrinkage allowance of 1% was applied. The structure of this thin-walled conical shell casting part requires careful design to avoid defects.
For gating system design, a uniformly distributed slit-type gating system was adopted to ensure smooth filling and effective feeding during solidification. Using empirical formulas for aluminum alloy casting, the slit gate dimensions and number were calculated. The formulas are as follows:
Number of vertical gates: $$n = (0.016 \sim 0.028) \frac{S}{\delta}$$
Slit gate thickness: $$\delta = (0.8 \sim 1.5) \delta_{\text{casting part}}$$
Slit gate width: $$b = 15 \sim 35 \text{ mm}$$
Vertical gate diameter: $$d = (4 \sim 6) \delta$$
Where \(S\) is the outer perimeter of the casting part, \(\delta\) is the slit gate thickness, \(\delta_{\text{casting part}}\) is the wall thickness of the casting part at the gate connection, \(b\) is the slit gate width, and \(d\) is the vertical gate diameter. Given the thin walls and limited feeding distance, multiple slit gates were used. Based on calculations and experience, we set \(b = 20 \text{ mm}\), \(d = 60 \text{ mm}\), and \(n = 8\). The cross-sectional area of the horizontal runner is 50 mm × 60 mm, and the sprue diameter is 120 mm. The process boss at the top was designed as an inverted cone to serve as a top riser and slag collector. To enhance cooling and establish directional solidification, chills were placed between each pair of vertical gates, expanding the feeding zone. At the bottom flange thick section, internal and external ring chills were used to promote solidification from the casting part toward the gating system. The apex region, due to space constraints, initially had no chills, which later caused issues.
For counter-pressure casting parameters, the pressure difference \(\Delta p\) required to fill the mold was calculated using the formula:
$$\Delta p = \mu \rho g H$$
Where \(\Delta p\) is the pressure difference in Pa, \(H\) is the vertical distance from the crucible liquid surface to the top of the casting part in meters, \(\rho\) is the liquid metal density in kg/m³, \(g\) is gravitational acceleration in m/s², and \(\mu\) is the filling resistance coefficient, typically 1.0–1.5. For this casting part with uniform structure, \(\mu\) was set to 1.2. After calculation, \(\Delta p\) was determined as 62 kPa. To minimize air entrapment, slag inclusion, and sand washing, the liquid rise and filling speeds were controlled between 20–80 mm/s. The pressure stages and times are summarized in the table below:
| Stage | Pressure (kPa) | Time (s) |
|---|---|---|
| Liquid Rise | 19 | 24 |
| Filling | 62 | 72 |
| Pressure Increase | 118 | 100 |
| Pressure Holding | 118 | 460 |
| Pressure Release | 0 | – |
The pouring temperature was set at \(720 \pm 10\)°C to balance fluidity and minimize shrinkage and pinhole tendencies.
Computer CAE simulation was employed to verify the process. Initial simulations revealed shrinkage porosity risks in the apex region and excessive shrinkage in the top riser. The simulation results indicated that during filling, the process was smooth without splashing. Temperature field analysis showed that the gating system generally had a temperature gradient from bottom to top, with the casting part cooler than the vertical gates. However, the cone apex, without chills, remained significantly hotter, creating a risk zone. Solidification process simulation confirmed that the apex region stayed liquid too long, lacking a directional solidification gradient toward the riser. This was attributed to the semi-enclosed apex being surrounded by metal, poor heat dissipation, reduced cross-sectional area leading to heat concentration, and closely spaced vertical gates exacerbating overheating. Thus, the initial design posed shrinkage risks for this casting part.
To address this, we optimized the process. Four of the eight vertical gates in the upper region were removed symmetrically, and chills were added in their places. Additionally, conformal chills were placed on the top of the sand core at the apex to enhance cooling. This aimed to achieve “simultaneous solidification overall, with localized directional solidification in zones” for the casting part. After optimization, simulation showed improved temperature distribution: the apex region cooled faster, forming a clear temperature gradient toward the riser. Solidification proceeded more uniformly, and shrinkage risks were eliminated. The riser shrinkage was controlled within a safe distance from the apex.
For trial production, qualified aluminum ingots were melted, modified, refined, and degassed. Hydrogen content and fracture samples met requirements. Chemical composition analysis confirmed conformity to standards, as shown below:
| Element | Required (wt%) | Result (wt%) |
|---|---|---|
| Si | 6.5–7.5 | 6.72 |
| Ti | 0.08–0.25 | 0.12 |
| Mg | 0.45–0.75 | 0.56 |
| Fe | ≤0.20 | 0.07 |
| Cu | ≤0.10 | 0.01 |
| Zn | ≤0.10 | 0.03 |
| Be | ≤0.07 | 0.04 |
| Mn | ≤0.10 | 0.01 |
| Others | ≤0.15 | 0.01 |
Pouring was conducted at 721°C. After cleaning and gating removal, X-ray inspection revealed no defects exceeding standards, confirming internal quality. The casting part underwent T6 heat treatment (solution treatment and artificial aging) along with attached test bars. Mechanical properties were tested, with results meeting the highest grade requirements for ZL114A-T6:
| Property | Standard Requirement | Sample 1 | Sample 2 | Sample 3 | Average |
|---|---|---|---|---|---|
| Tensile Strength (MPa) | ≥300 | 366 | 367 | 361 | 364.67 |
| Yield Strength (MPa) | ≥240 | 307 | 308 | 303 | 306 |
| Elongation (%) | ≥3 | 8 | 8 | 6.5 | 7.5 |
Dimensional inspections after initial, final, and machining stages showed all dimensions met specifications, and the finished casting part was satisfactory.
In conclusion, using counter-pressure casting for thin-walled shell casting parts ensures stable filling and solidification under pressure, reducing gas evolution, slag inclusion, and pinholes, while improving density and mechanical properties. Computer CAE simulation is an effective tool for analyzing and optimizing casting processes, mitigating shrinkage risks. For thin-walled conical casting parts with enclosed apexes, heat dissipation is challenging; reducing vertical gates and adding chills at the apex can establish directional solidification and prevent defects. This research demonstrates a successful approach for producing high-quality ZL114A thin-walled conical shell casting parts, meeting stringent aerospace requirements. The integration of simulation and practical adjustments proved crucial for optimizing the casting part’s integrity and performance.