Abstract: This paper focuses on the design of an investment casting process for an impeller casting with complex curved blades. By implementing segmented model scaling and anti-deformation corrections, the mold shell size is ensured to match the process design dimensions. The top injection and dispersed inner gate introduction methods are adopted to achieve stable mold filling. The technology successfully yields castings with high internal quality, surface quality, and dimensional accuracy.
1. Introduction
Titanium alloy, characterized by its low density, high specific strength, excellent corrosion resistance, low thermal conductivity, small coefficient of linear expansion, non-toxic and non-magnetic properties, is widely utilized in aviation, aerospace, shipbuilding, and petrochemical industries. However, high production costs hinder its broader application. Investment casting, as a near-net-shape technology, is particularly suitable for components with complex shapes that are difficult to directly machine or weld, or economically impractical to do so.
2. Casting Overview
The titanium alloy impeller casting with complex curved blades has the following specifications:
| Dimension | Value |
|---|---|
| Outline Dimensions | φ490mm × 290mm |
| Mass | 39kg |
| Hub Dimensions (Small End/Large End) | φ116mm / φ186mm × 290mm |
| Blade Quantity | 4 open blades |
| Blade Characteristics | Complex curved surface, irregular variable cross-section, minimum wall thickness of 3.8mm, minimum edge radius of R1.5mm |
| Material | ZTC4 titanium alloy |
| Casting Category & Quality Grade | Category III, Grade C |
| Inspection Requirements | X-ray inspection, dye penetrant inspection, surface roughness of 6.3μm, dimensional tolerance according to GB/T6414-1999 CT9 |
3. Investment Casting Process Analysis
3.1 Wax Pattern Design
The wax pattern is produced using 3D printing with a linear contraction ratio of 1.0% to 2.0%. Due to the varying distances from the blade tips to the casting’s contraction center, the contraction ratios differ between the blade tips and the base. Therefore, segmented scaling is applied, with a contraction ratio of 1.0% at the base gradually increasing to 1.0% to 2.0% towards the tips.
Titanium alloy has a linear contraction ratio of 0.5% to 0.8%. Considering the structural characteristics and wall thickness variations, anti-deformation corrections are set based on casting zones, with a correction of 0.8% at the base gradually decreasing to 0.8% to 0.5% towards the tips, and up to 1.0% in some thicker areas.
3.2 Gating and Risering System
Given the unstable filling process of curved blades, which can lead to defects such as misruns, cold shuts, porosity, and shrinkage cavities, the design of the gating and risering system is crucial. A vertical top-injection method is adopted to ensure stable bottom-up filling and sequential solidification from the inside out. A ring-shaped gating system is placed at the top center, with individual vents at the blade tips and five inner gates to enhance the floating speed of alloy liquid, facilitating the collection of impurities and gases in the risers.
4. Mold Shell Production
Titanium alloy reacts chemically with almost all oxide refractory materials in the molten state. Hence, zirconia with high chemical stability is chosen as the facing layer material for the mold shell, with zirconium diacetate as the binder. The back-up layer consists of aluminum oxide and silica sol slurry, coated with mullite sand. After drying, the mold shell is dewaxed in hot water and fired in a box-type resistance furnace at 900 to 950°C in a reducing atmosphere for 2 to 2.5 hours.
5. Casting Trial Production and Performance Inspection
5.1 Melting and Pouring
Melting is performed using a ZHK400 vacuum consumable electrode arc skull melting furnace. The melting parameters are summarized in Table 1.
| Parameter | Range/Value |
|---|---|
| Arcing Current | 1000 to 1500 A |
| Preheating Current | 3000 to 5000 A |
| Preheating Time | 60 to 80 s |
| Melting Current | 28000 to 36000 A |
| Melting Voltage | 34 to 41 V |
| Melting Vacuum | 3.0 to 5.0 Pa |
| Melting Time | 8 min (initial), 60 to 45 min (subsequent) |
5.2 Casting Performance Inspection
5.2.1 Surface Quality and Dimensional Tolerance
After cleaning, sandblasting, and visual and dye penetrant inspections, the casting surface is free of microcracks, cold shuts, and flow marks, with a roughness of 3.2μm. Dimensional measurements confirm compliance with the specified tolerances.
5.2.2 Internal Quality
The casting undergoes hot isostatic pressing at 920±10°C, 100 to 140 MPa for 2 to 2.5 hours, followed by X-ray inspection. The results (Table 2) indicate no internal defects.
| Quality Grade | Casting Wall Thickness (mm) | Internal Defects |
|---|---|---|
| C | 3.8 to 31 | None (porosity, shrinkage, spongy, dendritic, low-density inclusions) |
5.3 Chemical Composition
Samples from the impeller tips, base, and hub are analyzed. The results (Table 3) show compliance with standard values.
| Sampling Location | Measured Values (wt%) | Standard Range (wt%) |
|---|---|---|
| Al, V, Fe, Si, C, N, H, O, Ti (Impeller Tips) | 5.98, 4.08, 0.17, 0.08, 0.069, 0.022, 0.0078, 0.11, Balance | 5.5 to 6.8, 3.5 to 4.5, ≤0.30, ≤0.15, ≤0.10, ≤0.05, ≤0.015, ≤0.20, Balance |
| (Similar ranges for Impeller Base and Hub) |
6. Conclusion
- The segmented model scaling and anti-deformation corrections ensure precise dimensions and correct flow line shapes of the complex impeller.
- Vertical top-injection with a ring-shaped gating system achieves stable filling and sequential solidification.
- Investment casting of the complex impeller yields high internal and surface quality, dimensional accuracy, and surface finish.
- Melting in a vacuum consumable electrode arc skull melting furnace is optimized with current maintained at 28 to 36 kA, voltage at 34 to 41 V, vacuum at 3.0 to 5.0 Pa, and crucible return water temperature not exceeding 45°C.