Shrinkage porosity and shrinkage cavities remain critical challenges in precision investment casting. These defects typically manifest as irregular surface depressions or internal voids with dendritic structures, primarily occurring at thermal junctions and thick sections of castings. This paper systematically analyzes the formation mechanisms, influencing factors, and practical solutions for shrinkage-related defects through theoretical models and industrial case studies.
1. Formation Mechanisms
The shrinkage process in precision investment casting involves three distinct phases:
- Liquid Contraction: Occurs during cooling from pouring temperature ($T_p$) to liquidus temperature ($T_L$):
$$\varepsilon_{V}^{\text{liq}} = \alpha_{V}^{\text{liq}}(T_p – T_L) \times 100\%$$
where $\alpha_{V}^{\text{liq}}$ = 1.6×10-4°C-1 for carbon steels. - Solidification Contraction: Volume reduction during phase transition:
$$\varepsilon_{V}^{\text{sol}} = 3\% \text{ (for medium-carbon steels)}$$ - Solid-State Contraction: Thermal contraction from solidus temperature ($T_S$) to ambient:
$$\varepsilon_{V}^{\text{solid}} = \alpha_{V}^{\text{solid}}(T_S – T_{\text{amb}}) \times 100\%$$
Typical linear contraction rates vary with carbon content:
| C (%) | Linear Contraction (%) |
|---|---|
| 0.10 | 2.47 |
| 0.35 | 2.40 |
| 0.90 | 2.18 |
2. Critical Influencing Factors
Key parameters affecting shrinkage defects in precision investment casting include:
| Factor | Impact | Optimization Range |
|---|---|---|
| Gating Design | Determines feeding efficiency | Runner:Gate = 1.4:1 |
| Pouring Temperature | $\Delta T$ = 100°C (optimal) | 1,500–1,550°C |
| Shell Conductivity | $K_{\text{shell}} \geq 0.8$ W/m·K | Zircon-based systems |
3. Prevention Strategies
Effective shrinkage control in precision investment casting requires:
3.1 Sequential Solidification Design
Implement directional solidification using chills and risers:
$$D_{\text{riser}} = K \cdot D_{\text{hotspot}}$$
where $K$ = 0.7–0.9 for cylindrical risers.
3.2 Alloy Modification
Rare earth (RE) additions significantly improve feeding characteristics:
| RE Addition (%) | Shrinkage Reduction (%) | Hardness (HRC) |
|---|---|---|
| 0 | – | 59–61 |
| 0.15 | 40 | 60–62 |
3.3 Process Optimization
Key parameters for precision investment casting:
- Maintain shell preheat temperature ≥ 800°C
- Use stepped pouring: rapid initial fill (0.8–1.2 kg/s) followed by slow feeding
- Apply exothermic riser covers ($\Delta T \approx 150$°C)
4. Industrial Validation
A case study on ZG20CrMnMoRE components demonstrated:
- Service life increased from 4,000 m to >10,000 m
- Shrinkage defects reduced by 85%
- Mechanical property improvements:
$$\sigma_b: 198 \text{ MPa (vs. 187 MPa)}$$
$$\alpha_k: 6.4 \text{ J/cm}^2 \text{ (vs. 2.2 J/cm}^2\text{)}$$
5. Conclusion
Through systematic control of gating design, alloy composition, and solidification parameters, shrinkage defects in precision investment casting can be reduced by 70–90%. The integration of computational simulation and rare earth modification provides a robust framework for producing high-integrity cast components across aerospace and energy applications.