In modern manufacturing, the demand for thin-walled complex components has surged, particularly in aerospace, automotive, and defense sectors. These components must possess excellent dimensional accuracy, surface finish, and mechanical properties. Among the available casting techniques, high precision investment casting combined with gypsum molds offers superior capability for producing intricate thin-walled shell structures. This work presents a systematic study on the design and optimization of a gypsum mold investment casting process for a thin-walled shell component, utilizing numerical simulation to predict and eliminate internal defects. The entire investigation is conducted from a first‑person perspective, encompassing initial process design, numerical modeling, defect analysis, optimization, and experimental validation.
The target component is a protective shell with an arched profile, featuring a complex internal concave surface. The nominal wall thickness is 4 mm, while the radii range from 239 mm to 259.5 mm. Such geometry imposes strict requirements on mold filling and solidification control. Conventional casting methods often fail to meet the required precision and soundness, hence the adoption of high precision investment casting using a gypsum mold is justified. The gypsum mold provides excellent thermal insulation, low thermal conductivity, and the ability to replicate fine details, making it ideal for thin-walled aluminum alloy castings.
Initial Process Design
Based on the structural characteristics and performance requirements, a bottom‑gating system was initially designed. Bottom filling ensures smooth mold filling and reduces turbulence, thereby minimizing oxide inclusion and gas entrapment. The pouring position was set at one side of the shell, with multiple ingates distributing the melt uniformly. The main dimensions of the gating system are summarized in Table 1.
| Component | Shape | Dimensions (mm) |
|---|---|---|
| Pouring cup | Conical | d1=18, d2=36, h=18 |
| Sprue | Conical | d1=18, d2=16, h=250 |
| Runner | Trapezoidal | a=40, b=35, h=33, L₁=45 |
| Ingate | Cylindrical | d=9.6, L=4 |
The alloy selected was ZL101A (Al‑Si‑Mg), which offers good castability and mechanical properties after heat treatment. Pouring temperature was set at 705 °C, and the gypsum mold preheat temperature at 220 °C. The thermal properties of the alloy and gypsum mold are listed in Table 2.
| Material | Thermal conductivity λ (W·m⁻¹·K⁻¹) | Specific heat c (J·kg⁻¹·K⁻¹) | ||||||
|---|---|---|---|---|---|---|---|---|
| 100 °C | 200 °C | 300 °C | 400 °C | 100 °C | 200 °C | 300 °C | 400 °C | |
| ZL101A | 154.9 | 163.3 | 167.5 | 167.5 | 879 | 921 | 1005 | 1100 |
| Gypsum | 0.72 | 0.6 | 0.5 | 0.5 | 1100 | 1000 | 900 | 1000 |
The solidification process was analyzed using the Chvorinov rule, where the local solidification time is proportional to the square of the modulus (volume/surface area). For a thin-walled shell, the modulus varies significantly, leading to potential hot spots. The fundamental heat conduction equation in the mold is given by:
$$ \frac{\partial T}{\partial t} = \alpha \nabla^2 T $$
where α is the thermal diffusivity. In gypsum molds, low thermal conductivity slows heat extraction, promoting directional solidification. However, improper gating design can still cause isolated hot spots.
Numerical Simulation of the Initial Design
The commercial software View Cast was employed to simulate filling and solidification. The STL model of the casting with gating system was imported and meshed into 2 million elements. Material properties from Table 2 were assigned. The simulation revealed smooth filling without splashing. The filling sequence is described as follows: at 1.20 s the sprue was filled and melt entered the ingates; at 1.52 s about 30 % of the cavity was filled; complete filling occurred at 2.55 s. Temperature distribution remained relatively uniform during filling.
Solidification simulation, however, indicated potential problems. By 245 s, thin sections solidified completely, while thicker regions near the top‑right corner remained liquid. At 545 s, most of the casting solidified except for a localized area corresponding to the top‑right thick section. The defect prediction map showed shrinkage porosity concentrated in that area, as clearly identified by the software’s Niyama criterion:
$$ N_y = \frac{G}{\sqrt{R}} $$
where G is the temperature gradient and R is the cooling rate. Low Niyama values indicate shrinkage tendency. The initial design yielded low Ny values in the top‑right region, confirming insufficient feeding. The primary cause was the single‑side pouring location, which caused the thick section to be isolated from the sprue and runner, preventing adequate liquid metal supply during final solidification.
Process Optimization
Based on the defect location and magnitude, the gating system was redesigned. The pouring position was shifted from one side to the center of the shell, and the ingates were distributed in a multi‑point pattern around the central axis. This configuration ensures that the thick sections are closer to the feeding source and that solidification proceeds directionally from the thin walls toward the gating system. The optimized design is shown schematically (not included due to figure constraints) but its dimensions remain similar to the initial one except for the relocation.
The rationale behind the optimization involves modifying the solidification sequence to promote progressive solidification. The governing equation for solidification shrinkage is:
$$ V_s = V_l \beta $$
where Vₛ is the volume shrinkage, Vₗ the liquid volume, and β the solidification shrinkage factor (approximately 3.7 % for Al‑Si alloys). Adequate feeding requires that the liquid path remains open until the last liquid region solidifies. By centralizing the sprue, the feeding distance to any part of the casting is reduced, and the multi‑point ingates provide multiple feed paths.
Simulation Results of the Optimized Design
The optimized design was simulated under identical conditions. Filling time was similar (2.6 s), and the flow remained stable. The solidification sequence changed markedly: at 172 s the thin walls began to solidify; at 272 s the previously problematic thick area was already partially solidifying simultaneously with the thin regions. By 422 s, the gating system started to solidify, and the entire casting solidified at 1127 s. The defect prediction map showed no shrinkage porosity within the casting; all remaining porosity was confined to the sprue and runner, which are removed after casting.
Table 3 compares the defect characteristics of the initial and optimized designs.
| Design | Defect location | Defect type | Extent |
|---|---|---|---|
| Initial | Top‑right thick section | Shrinkage porosity | Moderate |
| Optimized | Only in gating system | Shrinkage | Negligible in casting |
The simulation conclusively demonstrated that the optimized gating system eliminates internal defects in the thin‑walled shell, validating the effectiveness of the central pouring strategy.
Experimental Validation and Mechanical Properties
To confirm the simulation predictions, the optimized process was implemented for trial production. The gypsum mold mixture was prepared according to the proportions listed in Table 4.
| Component | Particle size (mm) | Mass fraction (%) |
|---|---|---|
| Gypsum | 0.075–0.053 | 28–32 |
| Quartz powder | 0.053–≤0.053 | 9–11 |
| Quartz sand | 0.43–0.20 | 5–8 |
| Bauxite | <0.053 | 31–35 |
| Bauxite sand | 0.43–0.20 | 11–16 |
| Coal gangue | 0.21–0.11 | 4–6 |
| Diatomaceous earth | 0.43–0.20 | 2–4 |
| Water | – | 28–32 |
The castings were subjected to T6 heat treatment (solution treatment at 535 °C for 8 h, water quench, then artificial aging at 175 °C for 6 h). X‑ray inspection confirmed that the castings were free from shrinkage defects. Mechanical specimens were taken from different locations of the shell; results are presented in Table 5.
| Specimen | Tensile strength (MPa) | Elongation (%) | Hardness (HBW) |
|---|---|---|---|
| 1 | 326 | 5.5 | 99.5 |
| 2 | 324 | 4.0 | 89.2 |
| 3 | 305 | 4.5 | 104.0 |
All measured values exceeded the required minimum (tensile strength ≥275 MPa, elongation ≥2 %, hardness ≥80 HBW), demonstrating that the optimized high precision investment casting process yields sound, high‑performance thin‑walled shells. The combination of gypsum mold technology and numerical simulation optimization proves to be a robust methodology for producing complex aluminum castings with minimal trial‑and‑error.
Conclusion
This study successfully demonstrates the design and optimization of a high precision investment casting process for a thin‑walled aluminum shell using gypsum molds. Numerical simulation identified shrinkage porosity in the initial design, located at the top‑right thick section due to inadequate feeding. By shifting the pouring position to the center and adopting multi‑point ingates, the feeding path was shortened and directional solidification achieved. The optimized process eliminated internal defects, as confirmed by both simulation and X‑ray inspection. Mechanical properties of the T6‑treated castings fully satisfied the specifications. The work highlights the importance of integrating computational fluid dynamics and solidification modeling into high precision investment casting development, enabling rapid, cost‑effective optimization of complex thin‑wall components.