In this investigation, I focused on a large, complex, thin-walled casing made from ZTi60 high-temperature titanium alloy and developed a practical route for investment precision casting. High-temperature titanium alloys have been developed primarily by the demand of aero engines, because they can significantly reduce structural mass and increase thrust-to-weight ratio. ZTi60 is a near-alpha high-temperature titanium alloy of the Ti-Al-Sn-Zr-Mo-Nb-Ta-Si system. It was designed for service temperatures near 600 °C and combines high thermal strength, oxidation resistance, and acceptable thermal stability. However, because it contains refractory elements such as Nb and Ta, its melt fluidity is poor, its susceptibility to casting defects is high, and its cracking tendency is severe. For this reason, early applications were mostly forged parts, while castings were limited to small, simple structures. Titanium alloys are also difficult to machine, so investment precision casting is attractive as a near-net-shape process. Nevertheless, investment precision casting has a complex process chain. For large, complex, thin-walled casings, the external dimensions are large, the metal flow path is long, and the superheat of titanium alloy is low. As a result, the melt may solidify before filling thin sections, producing misruns. Complex geometry also creates more hot spots, makes feeding difficult, and complicates shell building and drying, which can degrade metallurgical quality after pouring. Therefore, a casting process must be designed according to both the alloy characteristics and the structural features of the part.
Earlier work on large complex thin-walled titanium castings has used numerical simulation, centrifugal pouring, and optimized runner designs. Simulation has been used to study filling and solidification of large intermediate casings, and centrifugal casting has produced acceptable components with high yield. Spiral and diverging runner systems have been proposed to stabilize melt motion during filling. Numerical tools such as ProCAST have been used to analyze cold shuts and shrinkage porosity, and centrifugal pouring has produced casings with good metallurgical quality. Although a general foundation exists for large complex titanium alloy casing investment precision casting, public reports on ZTi60 high-temperature titanium alloy in large investment precision castings remain limited. The available reference cases and data for large, complex, thin-walled ZTi60 structures are insufficient. In this work, I therefore selected a large complex thin-walled casing as the research object and carried out process exploration for ZTi60. Through casting process design, computer simulation, actual pouring verification, and iterative optimization, I effectively addressed misrun, shrinkage cavity, and thin-wall porosity in the ZTi60 casing. The results provide technical guidance for batch production of similar large complex thin-walled high-temperature titanium alloy casings produced by investment precision casting.
The selected casing has a circular rotary structure. Its outer envelope is approximately 909 mm in diameter and 226 mm in height. It contains inner and outer flow passages and six struts. The large-area wall thickness is about 2.5 mm, and the narrow edge of the split-ring overlap section is only about 1.0 mm. These features make the part highly sensitive to filling behavior, cooling rate, and feeding distance. The chemical composition specification of ZTi60 is summarized in Table 1. The alloying system provides high-temperature strength and oxidation resistance, but the refractory elements also increase melt viscosity and reduce fluidity. In investment precision casting, this combination is challenging because thin walls require rapid filling while the alloy has limited superheat and short fluid life.
| Element | Content, wt.% | Element | Content, wt.% |
|---|---|---|---|
| Al | 5.5-6.5 | Ta | 0.1-1.0 |
| Sn | 3.0-4.5 | Fe | ≤0.1 |
| Zr | 2.5-4.0 | C | ≤0.08 |
| Mo | 0.2-1.2 | O | ≤0.15 |
| Si | 0.2-0.5 | N | ≤0.05 |
| Nb | 0.2-0.8 | H | ≤0.01 |
For the initial process design, I selected centrifugal investment precision casting. Centrifugal motion promotes radial filling, helps remove gas and inclusions from the liquid metal, and generally improves surface quality, dimensional accuracy, mechanical properties, and material utilization compared with conventional gravity casting. The original material for the casing was ZTC4, so I first reused the mature ZTC4 gating system as a baseline for ZTi60 exploration. The pouring method was bottom-pour centrifugal casting. This configuration was intended to promote directional solidification and enhance riser feeding. The sprue was placed at the center of the casting. The runner was a multi-branch spiral diverging disk that could deliver metal rapidly and smoothly toward the outermost regions. Inner gates were distributed uniformly along the outer mounting flange. A complete ring-shaped inner gate was placed on the inner-ring mounting flange, and an overlap gate connected this ring to the farthest end of the runner. The geometric intention was to fill the outer ring first, then the struts and inner ring, with the inner-ring upper flange filling last.
I used ProCAST for simulation. The casting mesh size was set to 5, the gating system mesh size was set to 10, and the final mesh contained about 530,000 surface elements and 4.64 million volume elements. The shell thickness was 20 mm. The pouring temperature was 1,700 °C, the pouring time was 7 s, and the mullite ceramic shell preheat temperature was 200 °C. The heat transfer coefficient between the casting and the shell was 500 W/(m²·K). The rotation speed was 140 r/min, and pouring was performed in vacuum. I compared ZTC4 and ZTi60 under the same conditions to identify differences in filling and solidification. The centrifugal pressure available for filling can be estimated as
$$ \Delta P_c = \frac{1}{2}\rho \omega^2 (r_2^2-r_1^2) $$
where \(\rho\) is melt density, \(\omega\) is angular velocity, and \(r_1\) and \(r_2\) are the inner and outer radial positions. The filling time can be approximated by
$$ t_f = \frac{V_c}{Q} $$
where \(V_c\) is the cavity volume and \(Q\) is the volumetric flow rate. Solidification time can be related to the volume-to-area ratio by Chvorinov’s rule:
$$ t_s = B \left(\frac{V}{A}\right)^n $$
where \(B\) is a mold constant and \(n\) is commonly around 2 for many castings. These relations help explain why thin walls in investment precision casting are difficult: they have small \(V/A\), so \(t_s\) is short, while the flow path is long, so \(t_f\) is relatively long.
| Simulation Parameter | Value |
|---|---|
| Casting mesh size | 5 |
| Gating system mesh size | 10 |
| Surface mesh count | About 530,000 |
| Volume mesh count | About 4.64 million |
| Shell thickness | 20 mm |
| Pouring temperature | 1,700 °C |
| Pouring time | 7 s |
| Shell preheat temperature | 200 °C |
| Heat transfer coefficient | 500 W/(m²·K) |
| Rotation speed | 140 r/min |
| Environment | Vacuum |
The filling simulation showed that metal entered the sprue, passed into the spiral runner disk, and moved toward the farthest end under centrifugal action. It entered the casting cavity through the outer-ring inner gates. The outer ring filled first. After the outer ring was full, the struts and the overlap gate filled the inner ring simultaneously. The inner-ring upper flange was the last region to fill. ZTC4 and ZTi60 followed the same filling sequence, but the filling rates at the same time differed. ZTi60 had worse filling ability than ZTC4. At 98% filling, the ZTC4 filling time was 6.898 s, while the ZTi60 filling time was 7.2548 s. According to the simulation, both materials could fill the cavity, and no misrun was predicted. This later proved to be an important limitation of the simulation for thin-walled ZTi60 investment precision casting.
The solidification simulation also revealed differences between ZTC4 and ZTi60. Before the cavity was completely filled, at the same time, ZTC4 had a lower solid fraction than ZTi60, meaning that ZTi60 solidified faster in the early stage. After complete filling, the trend reversed: ZTC4 had a higher solid fraction than ZTi60. The solid fraction can be written as
$$ f_s(t)=\frac{V_s(t)}{V_0} $$
where \(V_s(t)\) is the solid volume at time \(t\) and \(V_0\) is the total volume. The shrinkage porosity fraction was calculated as
$$ P = \frac{V_{\text{shrink}}}{V_{\text{casting}}} \times 100\% $$
The predicted porosity of ZTC4 was 5.964%, while the predicted porosity of ZTi60 was 6.988%. Thus, ZTi60 had a stronger shrinkage tendency. New shrinkage locations appeared in the ZTi60 simulation, and some shrinkage volumes were larger than those in ZTC4. These results indicated that the baseline gating and feeding design, although mature for ZTC4, would require adjustment for ZTi60 investment precision casting.
| Response | ZTC4 | ZTi60 |
|---|---|---|
| Filling sequence | Outer ring, struts, inner ring, upper flange last | Outer ring, struts, inner ring, upper flange last |
| Time at 98% filling | 6.898 s | 7.2548 s |
| Early solidification behavior | Lower solid fraction at equal time | Higher solid fraction at equal time |
| Post-filling solidification | Higher solid fraction | Lower solid fraction |
| Predicted porosity | 5.964% | 6.988% |
| Shrinkage tendency | Baseline | Larger |
To verify the simulation and the process scheme, I poured the casing with ZTi60 using the same parameters. The sequence included wax pattern injection, assembly welding, shell preparation, dewaxing, high-temperature shell roasting, pouring, and sprue removal. After cleaning, I inspected the casting visually, by fluorescent penetrant inspection, and by X-ray. The ZTi60 casing showed three major defects. First, the 1 mm thin split-ring overlap section had a misrun. Second, the mounting flange riser root had a large shrinkage cavity. Third, the approximately 3 mm flow passages and struts had large-area porosity. Metallographic analysis of the thin-wall porosity showed intergranular porosity, and the porosity depth essentially penetrated the 3 mm wall. These results were broadly consistent with the simulation, except that the simulation did not reveal the misrun. Compared with a ZTC4 casing poured using the same scheme, the ZTC4 casting had no misrun, no riser-root shrinkage cavity, and no large-area thin-wall porosity. This confirmed that ZTi60 has lower fluidity and a greater shrinkage and porosity tendency than ZTC4. For investment precision casting of large thin-walled ZTi60 casings, the conventional ZTC4 process window is therefore insufficient.
| Location | Observed Defect | Severity | Likely Cause |
|---|---|---|---|
| Split-ring overlap, about 1 mm wall | Misrun | Local but critical | Wall thickness below critical filling thickness; low superheat; rapid heat loss |
| Mounting flange riser root | Shrinkage cavity | Large | Insufficient riser volume and feeding ratio for ZTi60 |
| Flow passages and struts, about 3 mm wall | Large-area porosity | Extensive | Short feeding distance; rapid solidification; insufficient local feeding |
To solve the misrun in the split-ring overlap, the riser-root shrinkage, and the thin-wall porosity, I revised the initial design. I added an overlap gate at the split-ring overlap section to supplement filling. I increased the riser dimensions to improve feeding. I also added gap gates to the struts and thin flow-passage walls to assist filling and feeding. The revised scheme was poured with the same process parameters. The overlap gate improved the 1 mm thin-wall misrun, but local misrun still remained. This showed that for a large complex ZTi60 casing, a 1 mm wall is below the critical wall thickness for reliable filling. The only robust solution was to increase the local wall thickness and later remove the excess material by another method. Increasing the riser height and size effectively solved the riser-root shrinkage cavity. When designing risers for ZTi60 investment precision casting, the riser proportion should be larger than that used for ZTC4. The gap gates on the flow passages and struts did not produce a satisfactory improvement. Only the small regions immediately adjacent to the bonded gap gates showed no porosity under fluorescent inspection; the remaining thin-wall areas still had extensive porosity. This demonstrated that gap gates have a limited feeding range in thin-walled investment precision casting.
| Initial Defect | Optimization Action | Observed Result | Interpretation |
|---|---|---|---|
| 1 mm split-ring overlap misrun | Add overlap gate | Improved but local misrun remained | 1 mm is below the reliable filling limit for large ZTi60 investment precision casting |
| Riser-root shrinkage cavity | Increase riser height and size | Defect effectively eliminated | ZTi60 requires a larger riser feeding ratio than ZTC4 |
| Thin-wall porosity in flow passages and struts | Add gap gates | Only local improvement near gates | Feeding range of gap gates is limited in thin sections |
Because gap gates did not eliminate thin-wall porosity, I changed the approach and added process allowance to increase the wall thickness in the thin regions. I then tested how different wall thicknesses affected porosity. The results are summarized in Table 5 and can be explained by the thermal mass per unit area:
$$ E_A = \rho c_p \delta (T_p-T_s) + \rho H_f \delta $$
where \(\delta\) is wall thickness, \(c_p\) is specific heat, \(T_p\) is pouring temperature, \(T_s\) is solidus temperature, and \(H_f\) is latent heat. A thicker wall stores more heat per unit area and loses heat more slowly, so solidification is delayed and local feeding is improved. A simple fluidity-length relation can be written as
$$ L_f \propto \delta (T_p-T_m) $$
where \(T_m\) is the melting range representative temperature. This relation explains why increasing wall thickness improves both filling and feeding. In the casing trials, increasing the thin-wall thickness effectively reduced or eliminated porosity. The improvement was especially clear for the annular inner and outer flow passages. For the struts, the fluorescent indication did not improve as strongly, but the porosity depth became shallower after thickening. After acid pickling with a removal amount of about 0.2 mm, the surface porosity could be effectively removed.
| Region | Wall Thickness, mm | Porosity Condition |
|---|---|---|
| Outer ring | 3.0-3.5 | Large-area porosity |
| Outer ring | 4.0-4.5 | Essentially none |
| Outer ring | 5.0-5.5 | Essentially none |
| Inner ring | 3.0-3.5 | Large-area porosity |
| Inner ring | 4.0-4.5 | Partial porosity, partial sound |
| Inner ring | 5.0-5.5 | None |
| Struts | 3.0-3.5 | Large-area porosity |
| Struts | 4.0-4.5 | Partial porosity, partial sound |
| Struts | 5.0-5.5 | Less porosity |
The wall-thickening strategy also helped solve the misrun caused by the poor filling ability of ZTi60. When excess wall thickness is removed later, shallow surface defects can be removed at the same time. This reduces the need for repair welding. Because ZTi60 has poor weldability, avoiding repeated welding is important. For a large thin-walled complex ZTi60 casing produced by investment precision casting, increasing wall thickness is therefore an effective quality measure. After pouring, three-dimensional scanning can be used to inspect the dimensions of each region and fit the results to the three-dimensional model. The excess dimension in each region can then be determined, and local acid pickling can be used to reduce the wall thickness to meet design requirements.
From a process-physics perspective, the results can be interpreted through several competing mechanisms. The first is fluid life. In thin sections, the melt loses heat rapidly, so the local fluid life is short. If the local fluid life is shorter than the filling time required to reach the end of the thin section, a misrun occurs. The second is feeding resistance. Even if the cavity fills completely, solidification shrinkage must be fed through a mushy or partially solid network. In thin walls, the feeding path is narrow and the solidification time is short, so shrinkage porosity cannot be avoided by distant risers alone. The third is local hot-spot formation. Complex intersections and strut-to-ring connections create hot spots that require local feeding. The fourth is shell thermal resistance. Shell thickness and preheat temperature affect heat extraction and therefore the local cooling rate. The fifth is centrifugal pressure. Centrifugal pressure assists filling, but it cannot compensate for very thin walls if the melt has already lost fluidity.
The local cooling rate can be expressed as
$$ \dot{T} = \frac{dT}{dt} $$
and the volumetric shrinkage can be expressed as
$$ \epsilon_v = \frac{V_s – V_l}{V_s} $$
where \(V_s\) and \(V_l\) are solid and liquid volumes. When \(\epsilon_v\) cannot be compensated by feeding, porosity forms. A feeding-distance relation can be written as
$$ L_f = k \sqrt{\delta} $$
where \(k\) depends on alloy and mold conditions. This relation indicates that reducing wall thickness sharply reduces the permissible feeding distance. That is why 3 mm walls showed large-area porosity, while 4 mm and 5 mm walls showed much less porosity. In the same way, the critical wall thickness for filling can be approximated as
$$ \delta_c \approx \sqrt{\frac{\lambda (T_p-T_0) t_f}{\rho H_f}} $$
where \(\lambda\) is thermal conductivity and \(T_0\) is mold temperature. Although this is a simplified expression, it captures the observed behavior: a 1 mm wall was below the critical thickness for reliable ZTi60 investment precision casting, even with an auxiliary overlap gate.
| Mechanism | Physical Meaning | Observed Consequence | Practical Response |
|---|---|---|---|
| Short fluid life | Thin walls cool rapidly and lose fluidity | Misrun at 1 mm split-ring overlap | Increase wall thickness, add local gates where possible |
| Short feeding distance | Feeding cannot reach thin-wall extremities | Porosity in 3 mm flow passages and struts | Increase wall thickness to 4 mm or more |
| Hot-spot formation | Complex intersections solidify late | Shrinkage at riser root and junctions | Enlarge risers and adjust feeding ratio |
| Limited gap-gate feeding | Local gate affects only a small volume | Porosity remained away from gates | Do not rely only on gap gates for thin walls |
| Centrifugal filling | Pressure assists radial flow and gas removal | Improved general fill and inclusion control | Use bottom-pour centrifugal investment precision casting |
The process window I established for this ZTi60 casing can be summarized as follows. Pouring temperature should be high enough to maintain fluidity but not so high that shell reaction and grain growth become excessive. Shell preheat should be controlled to reduce thermal shock and avoid premature freezing. Rotation speed should provide sufficient centrifugal pressure without causing turbulence or mold damage. Riser size should be increased relative to ZTC4. Thin sections below about 4 mm should be treated as high-risk regions for porosity. When design allows, process allowance should be added and later removed by acid pickling. Three-dimensional scanning should be used after pouring to determine the local excess thickness. Local acid pickling can then remove the excess and also reduce shallow surface defects. This integrated approach improves the success rate of large complex thin-walled ZTi60 investment precision casting.
| Process Variable | Recommended Direction for ZTi60 | Reason |
|---|---|---|
| Pouring temperature | Maintain controlled superheat near the proven range | Balance fluidity against shell reaction and grain growth |
| Shell preheat temperature | Use controlled preheat, around the validated level | Reduce premature freezing and thermal shock |
| Rotation speed | Use validated centrifugal speed | Assist radial filling and gas removal |
| Riser design | Increase riser proportion relative to ZTC4 | Compensate for larger shrinkage tendency |
| Thin-wall design | Avoid walls below about 4 mm when possible | Reduce porosity and misrun risk |
| Local feeding | Use gap gates only as supplements | Their feeding range is limited |
| Dimensional control | Use process allowance plus acid pickling and 3D scanning | Remove excess thickness and shallow defects |
I also compared the observed behavior with the initial simulation assumptions. The simulation correctly predicted that ZTi60 would fill more slowly than ZTC4 and that it would have a larger shrinkage tendency. It did not predict the 1 mm misrun, which means that for large thin-walled investment precision casting, simulation must be complemented by experimental validation. The simulation can guide gating and riser design, but the critical wall thickness for a specific alloy and shell system must be determined empirically. In this case, the 1 mm section was below the critical thickness for ZTi60. The 3 mm sections were also below the thickness needed to avoid extensive porosity. The 4 mm and 5 mm sections showed much better behavior. These findings are directly useful for future investment precision casting designs using ZTi60 and similar high-temperature titanium alloys.
The use of acid pickling after casting deserves further emphasis. When process allowance is added to thin walls, the excess material not only improves filling and feeding but also provides a sacrificial layer. Shallow surface porosity and other near-surface defects can be removed during pickling. This reduces fluorescent indications and repair welding. Since ZTi60 has poor welding performance, reducing repair welding improves production yield and reliability. The combination of wall thickening, controlled solidification, enlarged risers, and acid pickling creates a practical route for large complex thin-walled ZTi60 casings made by investment precision casting. The final dimensions can be adjusted by local pickling after three-dimensional scanning, so the design intent can still be met even when the as-cast walls are intentionally thickened.
In terms of production workflow, I found that the following sequence is effective for this type of investment precision casting. First, evaluate the casting structure and identify thin walls, hot spots, and long filling paths. Second, reuse a mature gating system only as a starting point, not as a final design. Third, perform filling and solidification simulation with actual alloy properties. Fourth, pour a trial casting and inspect it visually, by fluorescent penetrant, and by X-ray. Fifth, classify defects by location and mechanism. Sixth, adjust local gates, riser size, and wall thickness. Seventh, repeat pouring and inspection. Eighth, apply acid pickling and three-dimensional scanning to control final wall thickness and remove shallow defects. This iterative method was necessary because the initial ZTC4-based design was insufficient for ZTi60.
| Step | Action | Purpose |
|---|---|---|
| 1 | Review structure for thin walls and hot spots | Identify high-risk regions |
| 2 | Use mature gating as baseline | Provide initial filling and feeding layout |
| 3 | Run filling and solidification simulation | Predict flow, solid fraction, and shrinkage |
| 4 | Pour trial casting | Validate simulation and reveal unpredicted defects |
| 5 | Inspect by visual, fluorescent, and X-ray methods | Locate and classify defects |
| 6 | Adjust gates, risers, and wall thickness | Improve local feeding and filling |
| 7 | Repeat pouring and inspection | Confirm improvements |
| 8 | Apply acid pickling and 3D scanning | Control final dimensions and remove shallow defects |
The conclusions of this work can be summarized as follows. Computer simulation of the large complex thin-walled ZTi60 casing filling and solidification was useful, but it did not reveal the misrun defect. The 1 mm wall of the ZTi60 large complex thin-walled casing exceeded the filling capability of the alloy; even with an auxiliary overlap gate, misrun risk remained. The shrinkage of the ZTi60 large casing was larger than that of ZTC4, and the riser feeding ratio had to be increased. For thin-walled structures below about 4 mm, large-area porosity formed easily. Adding gap gates provided only limited local feeding. A more effective solution was to increase the thin-wall thickness to 4 mm or more. As wall thickness increased, porosity decreased or disappeared, and the remaining porosity depth became shallower. Acid pickling could remove the surface porosity. Excess wall thickness could also be removed by acid pickling, which simultaneously reduced shallow surface defects and repair welding. For large complex thin-walled ZTi60 casings produced by investment precision casting, the combination of thickened process allowance, enlarged risers, centrifugal filling, simulation, and acid pickling is a practical and effective process strategy.
| Conclusion | Practical Meaning for Precision Casting |
|---|---|
| Simulation alone did not predict the 1 mm misrun | Experimental pouring remains necessary for critical thin walls |
| 1 mm wall was below the reliable filling limit | Avoid 1 mm ZTi60 walls or thicken them before casting |
| ZTi60 had greater shrinkage than ZTC4 | Increase riser size and feeding ratio |
| 3 mm thin walls formed extensive porosity | Treat walls below 4 mm as high-risk regions |
| Gap gates had limited feeding range | Use them as supplements, not as the main solution |
| Thickening to 4 mm or more reduced porosity | Add process allowance and remove excess later |
| Acid pickling removed shallow porosity | Combine pickling with 3D scanning for final dimensions |
| Reduced repair welding is beneficial | Avoid repeated welding of ZTi60 castings |
Overall, this investigation demonstrates that large complex thin-walled ZTi60 casings can be produced by centrifugal investment precision casting when the process is designed around the alloy’s low fluidity and high shrinkage tendency. The key is not simply to copy a ZTC4 process, but to adjust the thermal and feeding design for ZTi60. The most important practical measures are to enlarge risers, avoid or thicken thin walls below about 4 mm, use local gates only as supplements, and apply acid pickling after three-dimensional scanning to remove excess thickness and shallow defects. These measures reduce misrun, shrinkage cavity, and thin-wall porosity, and they improve the overall quality of ZTi60 investment precision castings. The results provide a basis for further development and batch production of similar high-temperature titanium alloy casings using investment precision casting.