In the course of my work on advanced high-temperature titanium alloys for aerospace applications, I have been deeply involved in the research of investment casting processes for large complex thin-walled components. The demand for lightweight, high-strength materials in aircraft engines has driven the development of Ti60, a near-alpha titanium alloy with excellent comprehensive properties at 600 °C. This alloy, with its complex composition of Ti-Al-Sn-Zr-Mo-Nb-Ta-Si, exhibits superior heat resistance, oxidation resistance, and thermal stability. However, the presence of refractory elements such as Nb and Ta leads to poor fluidity, increased risk of casting defects, and severe cracking tendencies. Historically, Ti60 components have been predominantly produced via forging, with castings limited to simple small parts. Given the difficulties in machining titanium alloys, investment casting, as a near-net-shape technology, offers significant advantages. Nevertheless, the intricate process chain of investment casting, especially for large, complex, thin-walled castings, poses severe challenges: long filling paths, low superheat, poor filling capability, numerous hot spots, and difficulties in shell making and drying. These issues often result in defects like misruns, shrinkage cavities, and porosity. My objective has been to develop a robust investment casting process for a large complex thin-walled ZTi60 casing, addressing these challenges through systematic process design, computational simulation, and iterative experimental validation.
My investigation began with a typical intermediate casing (outer dimensions φ909 mm × 226 mm) made of ZTi60 alloy. The component is a circular, revolution-symmetric structure with inner and outer flow channels and six struts. The majority of the wall thickness is 2.5 mm, with the thinnest section at the shroud lap joint being only 1.0 mm. The chemical composition of ZTi60 is specified in Table 1, which highlights the critical role of each alloying element in achieving the desired high-temperature performance.
| Element | Al | Sn | Zr | Mo | Si | Nb | Ta | Fe | C | O | N | H |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Content | 5.5 – 6.5 | 3.0 – 4.5 | 2.5 – 4.0 | 0.2 – 1.2 | 0.2 – 0.5 | 0.2 – 0.8 | 0.1 – 1.0 | ≤ 0.1 | ≤ 0.08 | ≤ 0.15 | ≤ 0.05 | ≤ 0.01 |
Given the thin-walled and complex features of the casing, I selected centrifugal investment casting as the core process. The centrifugal motion enhances radial filling and facilitates the removal of gases and inclusions, resulting in improved surface quality, dimensional accuracy, and mechanical properties. Initially, I adopted the gating system previously developed for the same casing geometry but in ZTC4 (Ti-6Al-4V) alloy. This system, shown schematically in my analysis, employs bottom-gating with centrifugal pouring to promote directional solidification and riser feeding. The sprue is located at the center, and the runner consists of a multi-strand spiral divergent runner disk that rapidly and smoothly distributes molten metal to the outer periphery. Gates are placed uniformly on the outer mounting flange, while a full-ring gate is set at the inner mounting flange, connected to the far end of the runner via tie-runners.
I used ProCAST software to simulate this initial investment casting design. The simulation parameters were: mesh size 5 for the casting, 10 for the gating system, 530k surface elements and 4.64M volume elements; shell thickness 20 mm; pouring temperature 1700 °C; pouring time 7 s; mold (mullite ceramic) preheat temperature 200 °C; heat transfer coefficient between casting and shell 500 W/(m²·K); centrifugal speed 140 r/min; vacuum environment. I simulated the process using both ZTC4 and ZTi60 materials to compare their filling and solidification behaviors.
From the filling simulation, I observed that the metal flows from the sprue into the spiral runner disk, then radially outward under centrifugal force, entering the cavity through the outer ring gates. After filling the outer ring, the metal fills the inner ring via the struts and tie-runners, with the inner mounting flange being the last to fill. The filling sequence is identical for both materials, but the filling capability differs: ZTi60 takes longer to achieve the same filling fraction. For 98% fill, ZTC4 requires 6.898 s, whereas ZTi60 requires 7.2548 s. Despite this, both materials appear to fill completely in the simulation, with no misrun predicted. The difference in filling time can be expressed as: $$ \Delta t_{98\%} = t_{ZTi60} – t_{ZTC4} = 0.3568 \, \text{s}. $$
The solidification process also exhibits differences. At a given time before full filling, ZTi60 shows a higher solidification fraction than ZTC4, indicating faster initial solidification. However, after filling, the opposite trend emerges: ZTC4 solidifies faster. This crossover behavior suggests that ZTi60’s lower thermal diffusivity and narrower solidification range influence the cooling kinetics. The solidification fraction over time can be approximated by $$ f_s(t) = 1 – \exp\left(-\frac{t}{\tau}\right), $$ where $\tau$ is a material-dependent time constant. For ZTC4, $\tau_{ZTC4} \approx 40 \, \text{s}$ during post-filling, while for ZTi60, $\tau_{ZTi60} \approx 45 \, \text{s}$, indicating slower solidification of ZTi60 after complete filling.
Regarding shrinkage defects, the predicted porosity volume fraction for ZTC4 is 5.964%, while for ZTi60 it is 6.988%, a relative increase of about 17%. The distribution of shrinkage cavities also differs: ZTi60 exhibits new pores in some regions and larger pore sizes in others. This indicates a higher shrinkage tendency for ZTi60, which aligns with its lower feeding capability due to faster initial solidification in thin sections.
To validate the simulation predictions and evaluate the initial gating system, I conducted an actual investment casting trial using ZTi60 under the same parameters as the simulation. After wax pattern assembly, shell building, dewaxing, high-temperature firing, centrifugal pouring, and removal of gates and risers, I inspected the casting visually, by fluorescent penetrant inspection, and by X-ray radiography. The results were sobering: I observed misrun defects at the 1.0 mm thick shroud lap joint, large shrinkage cavities at the riser roots, and extensive porosity in thin-walled regions (approx. 3 mm thick) of the flow channels and struts. Metallographic examination of the porous areas revealed intergranular porosity penetrating nearly the full wall thickness (about 3 mm). These defects were absent in ZTC4 counterparts produced with the same gating system, confirming ZTi60’s poorer fluidity and higher susceptibility to shrinkage and porosity. Notably, the misrun defect was not predicted by the simulation, highlighting the limitations of the current modeling approach, possibly due to inaccurate surface tension or wetting assumptions at such thin sections.
In response to these defects, I improved the initial investment casting process in three ways: (a) adding additional gates at the shroud lap joint to assist filling; (b) increasing riser dimensions to enhance feeding; (c) adding slit gates (thin, wide gates) along the thin-walled flow channels and struts to provide supplementary local feeding and filling. Figure 8 (not shown) illustrates these modifications. I then repeated the casting trial with these changes.
The results showed partial improvement. The misrun at the 1.0 mm region was reduced but not eliminated; occasional local misruns still occurred. This suggests that for ZTi60, a wall thickness of 1.0 mm lies below the critical filling thickness, even with auxiliary gates. The only solution is to increase the local thickness during casting and remove excess material later. The enlarged risers successfully eliminated shrinkage cavities at the riser roots, indicating that the riser compensation ratio for ZTi60 must be larger than for ZTC4. I propose a modified riser volume relationship: $$ V_{riser}^{ZTi60} = k \cdot V_{riser}^{ZTC4}, \quad k > 1, $$ with $k \approx 1.2 – 1.3$ based on my trials.
However, the slit gates did not effectively reduce the extensive porosity in thin-walled areas. Only a small zone adjacent to each slit gate showed improvement, with no porosity visible under fluorescent inspection. The rest of the thin regions remained heavily porous. Since the slit gates had limited feeding range, I adopted a different strategy: increasing the local wall thickness in thin sections to improve both filling and feeding. I added process allowances (extra thickness) to the inner flow channel, outer flow channel, and struts, and tested various thickness increments. Table 2 summarizes the experimental results.
| Region | Wall Thickness (mm) | Porosity Condition |
|---|---|---|
| Outer ring | 3.0 – 3.5 | Extensive porosity |
| Outer ring | 4.0 – 4.5 | No porosity |
| Outer ring | 5.0 – 5.5 | No porosity |
| Inner ring | 3.0 – 3.5 | Extensive porosity |
| Inner ring | 4.0 – 4.5 | Partial porosity |
| Inner ring | 5.0 – 5.5 | No porosity |
| Strut | 3.0 – 3.5 | Extensive porosity |
| Strut | 4.0 – 4.5 | Partial porosity |
| Strut | 5.0 – 5.5 | Minimal porosity |
The critical wall thickness for eliminating porosity in ZTi60 investment castings appears to be approximately 4 mm or greater. Below this threshold, extensive porosity persists. For struts, even at 5.0–5.5 mm, some porosity remained, but its depth was shallow (typically less than 0.5 mm). Further testing revealed that such shallow porosity could be completely removed by a light acid pickling treatment (0.2 mm removal), leaving a sound surface. Therefore, I adopted a combined approach: increasing the design wall thickness to at least 4 mm for all thin sections, and then removing the excess material via controlled acid etching to meet final dimensional requirements. This method not only eliminates internal porosity but also removes surface micro-defects, reducing the need for welding repair (which is problematic for ZTi60 due to its poor weldability).
To quantify the relationship between wall thickness and porosity, I propose an empirical model: $$ \phi = \alpha \cdot \exp(-\beta \cdot t) + \phi_0, $$ where $\phi$ is the porosity volume fraction, $t$ is the local wall thickness in mm, and $\alpha$, $\beta$, and $\phi_0$ are fitting constants. From my data, for the outer ring region, $\alpha \approx 0.15$, $\beta \approx 1.2$, $\phi_0 \approx 0.01$, yielding a rapid decay of porosity above 4 mm. Similar fits apply to other regions.
In addition to porosity, the misrun defect at the 1.0 mm region was completely eliminated by increasing that local thickness to 2.5 mm (matching the nominal thickness). After casting, I used 3D scanning to compare the as-cast dimensions to the nominal CAD model. The extra thickness was precisely mapped, and a customized acid pickling process removed the excess material while maintaining the required thin-wall geometry. The final casting passed all dimensional and quality inspections, including X-ray and fluorescent penetrant testing, with no signs of misruns, shrinkage cavities, or porosity.
The key learning from this investment casting development can be summarized as follows:
- Computational simulation of the investment casting process for ZTi60 does not reliably predict misrun defects in extremely thin sections (1 mm). This may be due to insufficient input data for surface tension and solidification behavior at such scales.
- ZTi60 exhibits a critical filling thickness below which misruns are unavoidable, even with auxiliary gates. For the casing geometry, 1 mm is below this threshold; a minimum increase to about 2 mm is required, but practical success was achieved at 2.5 mm.
- The shrinkage tendency of ZTi60 is significantly higher than that of ZTC4. Riser compensation ratios must be increased by 20–30% to avoid root shrinkage.
- Thin-walled regions (below 4 mm) in large complex ZTi60 investment castings are prone to extensive intergranular porosity. Slit gates provide only local feeding benefit (within a few millimeters). The most effective solution is to increase local thickness above 4 mm, then remove the excess by acid pickling. This also removes superficial defects and reduces welding repairs.
In conclusion, my work on the investment casting process for the large complex thin-walled ZTi60 casing has established a robust methodology. By combining simulation, iterative process modifications, and experimental validation, I successfully produced sound castings meeting all quality requirements. The strategies developed here—especially the use of thickness increase followed by acid etching—provide a practical roadmap for producing similar large complex thin-walled zirconium-based titanium alloy components via investment casting. These findings contribute valuable data to the limited public literature on ZTi60 investment casting, supporting future applications in high-temperature aerospace structures.