I selected a large, complex, thin-walled casing made from ZTi60 high-temperature titanium alloy as the focus of my casting development work. The alloy belongs to the Ti-Al-Sn-Zr-Mo-Nb-Ta-Si family and is a near-alpha titanium alloy designed for service near 600 °C. In my assessment, its high specific strength, oxidation resistance, and thermal stability make it attractive for aero-engine casings, but its casting behavior is far less forgiving than conventional ZTC4 titanium alloy. The presence of Nb and Ta increases high-temperature capability, yet it also reduces fluidity and raises the risk of shrinkage, porosity, and cold shuts. I therefore treated the component as a coupled problem involving alloy fluidity, shell making, centrifugal filling, directional solidification, riser design, and final dimensional control. Although my selected production route was investment casting rather than evaporative pattern casting, I continuously benchmarked the process against evaporative pattern casting because both near-net-shape routes share challenges in thin-wall filling, pattern removal, shell integrity, and defect control. In several early reviews, I asked whether evaporative pattern casting could offer a simpler tooling path, but the required ceramic shell precision, internal passages, and titanium reactivity made investment casting the more realistic route. Even so, evaporative pattern casting remained a useful comparator for understanding filling limits and defect formation.
The casing geometry was circular and revolved, with an outer envelope of approximately ϕ909 mm × 226 mm. It contained inner and outer flow passages, six support struts, large thin regions near 2.5 mm, and a split-ring overlap zone with a local wall thickness near 1.0 mm. I recognized that such a geometry would stretch the capabilities of ZTi60. In my experience, evaporative pattern casting can sometimes accommodate complex external shapes because the foam pattern is expendable, but internal ceramic cores and thin titanium sections still demand high dimensional stability. For this casing, the internal flow path and the large diameter meant that any filling or solidification error would be amplified over a long flow distance. I therefore established a first-principles approach: estimate fill time, evaluate centrifugal pressure, calculate solidification modulus, screen hot spots, and compare simulation against trial castings. That approach allowed me to move from a baseline ZTC4-derived gating design to a ZTi60-specific process window.
| Item | Value or range |
|---|---|
| Alloy | ZTi60 near-alpha titanium alloy |
| Service temperature target | Approximately 600 °C |
| Outer diameter | ϕ909 mm |
| Height | 226 mm |
| General wall thickness | 2.5 mm |
| Minimum split-ring overlap thickness | 1.0 mm |
| Major features | Inner and outer flow passages, six struts, mounting flanges |
| Casting route selected | Centrifugal investment casting |
| Alternative route considered | Evaporative pattern casting |
I used the chemical specification of ZTi60 to guide melt superheat and shell interaction control. The alloy contains Al, Sn, Zr, Mo, Si, Nb, Ta, Fe, C, O, N, and H. I paid particular attention to oxygen, nitrogen, and hydrogen because they affect alpha-case formation, ductility, and defect sensitivity. In my process design, I treated oxygen and nitrogen as interstitial strengtheners that can also embrittle thin sections if pickup is excessive. I also recognized that Mo, Nb, and Ta raise melting range and reduce fluidity. This is one reason why evaporative pattern casting, which often relies on gravity or low-pressure filling, would be difficult for a 1 mm titanium section. In contrast, centrifugal investment casting provides additional pressure that can help overcome capillary resistance and viscous drag, but it cannot eliminate the fundamental solidification problem in a cold thin wall. I therefore regarded alloy composition, shell preheat, pouring temperature, and rotation speed as a single coupled system.
| Element | Mass fraction range (%) |
|---|---|
| Al | 5.5–6.5 |
| Sn | 3.0–4.5 |
| Zr | 2.5–4.0 |
| Mo | 0.2–1.2 |
| Si | 0.2–0.5 |
| Nb | 0.2–0.8 |
| Ta | 0.1–1.0 |
| Fe | ≤0.1 |
| C | ≤0.08 |
| O | ≤0.15 |
| N | ≤0.05 |
| H | ≤0.01 |
Alloy and Component Definition
I defined the baseline process by adapting a mature ZTC4 casing gating system. The original route used bottom-fill centrifugal pouring with a central sprue, a multi-branch spiral runner disk, outer ring ingates, an inner ring ingate, and a bridge runner connecting the far end of the spiral to the inner ring. My intention was to preserve the proven filling pattern while changing the alloy from ZTC4 to ZTi60. At first glance, this seemed reasonable because the geometric flow path was unchanged. However, my simulations and trials showed that ZTi60 has lower filling capacity and a stronger shrinkage tendency than ZTC4. The difference was not large in a single number, but it was decisive in 1 mm and 3 mm sections. I therefore treated the baseline not as a final design but as a diagnostic platform. I also reviewed evaporative pattern casting literature indirectly through my internal comparisons, because evaporative pattern casting often uses a full-mold pattern and gravity pouring, which would be even less capable of feeding a 1 mm titanium wall. That comparison reinforced my decision to keep centrifugal investment casting as the core process.
For the baseline simulation, I used ProCAST with a mesh size of 5 for the casting and 10 for the gating system. The final mesh contained approximately 530,000 surface elements and 4.64 million volume elements. I set the shell thickness to 20 mm, the pouring temperature to 1,700 °C, the pouring time to 7 s, and the shell preheat temperature to 200 °C. The casting-to-shell heat transfer coefficient was 500 W/(m²·K), the rotation speed was 140 r/min, and the environment was vacuum. I compared ZTC4 and ZTi60 under identical conditions. This comparison was essential because it isolated alloy effects from geometry effects. I also calculated the theoretical fill time using the simple relation
$$t_f = \frac{V}{Q} = \frac{V}{A v}$$
where \(t_f\) is fill time, \(V\) is casting volume, \(Q\) is volumetric flow rate, \(A\) is gate area, and \(v\) is melt velocity. In practice, the effective \(v\) is not constant because the melt cools, viscosity rises, and solid fraction increases. I therefore used simulation to capture the transient behavior. I also estimated centrifugal pressure using
$$P_c = \frac{1}{2}\rho \omega^2 \left(r_2^2 – r_1^2\right)$$
where \(P_c\) is centrifugal pressure, \(\rho\) is melt density, \(\omega\) is angular velocity, and \(r_1\) and \(r_2\) are inner and outer radii. This relation helped me understand why the outer ring filled first and why the inner ring and upper flange depended on bridge runners and struts. It also explained why evaporative pattern casting without centrifugal assistance would have a much smaller pressure head in the same geometry. I repeatedly used this comparison in design reviews: evaporative pattern casting can provide a low-cost pattern, but for ZTi60 the pressure head and shell preheat must be far more carefully controlled than in a conventional evaporative pattern casting cell.
| Simulation parameter | Value |
|---|---|
| Software | ProCAST |
| Casting mesh size | 5 |
| Gating mesh size | 10 |
| Surface mesh count | ≈530,000 |
| Volume mesh count | ≈4,640,000 |
| Shell thickness | 20 mm |
| Pouring temperature | 1,700 °C |
| Pouring time | 7 s |
| Shell preheat | 200 °C |
| Heat transfer coefficient | 500 W/(m²·K) |
| Rotation speed | 140 r/min |
| Environment | Vacuum |
| Alloys compared | ZTC4 and ZTi60 |
The filling sequence I observed was consistent for both alloys. Metal entered the central sprue, moved into the spiral runner disk, and was driven outward by centrifugal force. It first filled the outer ring through the outer ingates. After the outer ring was full, the melt advanced through the struts and bridge runners toward the inner ring. The inner upper flange was the last region to fill. ZTC4 and ZTi60 followed the same order, but the filling rate differed. At the same time points, ZTi60 had a lower fill fraction. When the model reached 98% fill, ZTC4 required 6.898 s, while ZTi60 required 7.2548 s. This may seem like a small difference, but in a 1 mm section the local solidification can begin before the final fill is complete. I therefore concluded that the simulation predicted complete macroscopic filling, but it did not capture the local underfill that would appear in the real casting. This was an important lesson: a global fill fraction above 98% does not guarantee local filling in a thin titanium wall. I also noted that evaporative pattern casting models often show similar limitations because the pattern decomposition and gas evolution add additional local transients that are difficult to predict.
| Alloy | Fill stage | Time (s) | Fill fraction (%) |
|---|---|---|---|
| ZTC4 | Early | 2.009 | 28.7 |
| ZTC4 | Middle | 4.0045 | 57.2 |
| ZTC4 | Late | 6.898 | 98.0 |
| ZTi60 | Early | 2.0217 | 27.4 |
| ZTi60 | Middle | 4.0084 | 54.3 |
| ZTi60 | Late | 7.2548 | 98.0 |
From the solidification simulation, I found another important difference. Before the casting was fully filled, ZTi60 had a higher solid fraction than ZTC4 at the same time. For example, at about 4.008 s, ZTC4 had a solid fraction near 0.8%, while ZTi60 had about 1.0%. At about 7.25 s, ZTC4 had about 4.3% solid fraction, while ZTi60 had about 5.9%. After complete filling, the trend reversed, and ZTC4 showed a slightly higher solid fraction than ZTi60. I interpreted this as a sign that ZTi60 begins to lose fluidity earlier during filling, which increases the risk of local underfill, but after filling it retains heat somewhat differently because of its thermal properties. Nevertheless, the shrinkage tendency was clearly stronger for ZTi60. The predicted porosity was 5.964% for ZTC4 and 6.988% for ZTi60. More importantly, new shrinkage locations appeared in ZTi60, and some existing pores were larger. I therefore treated the simulation as a warning rather than a validation. It correctly predicted a higher shrinkage risk, but it did not predict the actual 1 mm underfill. This gap between simulation and reality is common in thin-wall titanium castings, and I have also seen analogous gaps in evaporative pattern casting when gas pressure and pattern collapse are not fully represented.
| Response | ZTC4 | ZTi60 | Interpretation |
|---|---|---|---|
| Time to 98% fill | 6.898 s | 7.2548 s | ZTi60 fills more slowly |
| Solid fraction at 4.0 s | ≈0.8% | ≈1.0% | Earlier stiffening in ZTi60 |
| Solid fraction at 7.25 s | ≈4.3% | ≈5.9% | Lower fluidity near end of fill |
| Solid fraction at 9.8 s | ≈20.8% | ≈20.6% | Similar overall cooling after fill |
| Solid fraction at 18.0 s | ≈42.6% | ≈40.2% | ZTi60 retains slightly more liquid |
| Predicted porosity | 5.964% | 6.988% | Higher shrinkage tendency in ZTi60 |
Baseline Trial and Defect Signatures
I then cast the baseline ZTi60 casing using the same parameters as the simulation. After wax pattern assembly, shell building, dewaxing, high-temperature shell firing, pouring, and knockout, I inspected the casting visually, by fluorescence, and by X-ray. The results were consistent with the simulation in terms of shrinkage risk but revealed additional defects that the simulation had not predicted. The split-ring overlap zone at 1 mm showed underfill. The riser root on the mounting flange showed a large shrinkage cavity. The flow passages and strut thin walls near 3 mm showed extensive porosity. Metallographic examination showed that the porosity was intergranular and extended through most of the 3 mm wall. Compared with a ZTC4 casing made with the same gating design, the ZTi60 casing had more underfill, more shrinkage at the riser root, and more thin-wall porosity. I therefore concluded that ZTi60 requires a different process window, not merely a parameter adjustment. I also considered whether evaporative pattern casting could avoid some of these defects by changing the filling mechanism, but the low fluidity of ZTi60 and the high reactivity of titanium made that route less attractive. In evaporative pattern casting, the decomposition of the foam pattern can create gas pressure that opposes filling, and in a 1 mm titanium section this effect would be severe. I kept evaporative pattern casting as a reference concept but did not select it for production.
| Inspection method | Observed condition in baseline ZTi60 casing | Severity |
|---|---|---|
| Visual | Underfill at split-ring overlap | Critical |
| Visual and X-ray | Shrinkage cavity at riser root | Major |
| Fluorescence | Extensive porosity in 3 mm flow passage and strut walls | Major |
| Metallography | Intergranular porosity through most of wall thickness | Major |
| Comparison with ZTC4 | ZTC4 showed no underfill, no riser-root shrink, and no large thin-wall porosity | Alloy-driven |
To understand the shrinkage behavior, I used the Niyama criterion. The Niyama value is defined as
$$N = \frac{G}{\sqrt{\dot{T}}}$$
where \(G\) is the temperature gradient and \(\dot{T}\) is the cooling rate. A low Niyama value indicates a high probability of shrinkage porosity. In my thin-wall regions, the temperature gradient was low and the cooling rate was high, which reduced \(N\) and promoted dispersed porosity. I also used the Darcy flow relation for mushy-zone feeding:
$$v = -\frac{K}{\mu}\left(\nabla P – \rho g\right)$$
Here \(v\) is superficial velocity, \(K\) is permeability, \(\mu\) is dynamic viscosity, \(P\) is pressure, \(\rho\) is density, and \(g\) is gravity. In a thin wall, the permeability \(K\) falls rapidly as solid fraction increases, and the pressure gradient available for feeding is limited. This explains why adding a gap gate only locally improved the porosity: the gate can supply liquid near its attachment, but it cannot maintain a pressure gradient across a long, thin, rapidly solidifying wall. I also considered the Chvorinov rule,
$$t_s = B \left(\frac{V}{A}\right)^n$$
where \(t_s\) is solidification time, \(V\) is volume, \(A\) is surface area, \(B\) is a mold constant, and \(n\) is an exponent near 2. This rule shows that a thinner wall has a smaller modulus \(V/A\) and therefore solidifies faster. For a 1 mm wall, the modulus is so small that the local solidification time can approach the fill time. In that case, the melt front can freeze before it reaches the end of the cavity. This is exactly what I observed. For a 3 mm wall, the modulus is larger, but still small enough that feeding is difficult. The same reasoning applies to evaporative pattern casting, where the mold is typically not preheated to the same degree and the pattern decomposition can further cool the advancing front. I therefore concluded that wall thickness is the dominant geometric variable for ZTi60 thin-wall castings, more important than small changes in gate area or runner length.
Optimization Strategy and First Iteration
For the first optimization, I added a bridge gate at the split-ring overlap, enlarged the riser, and added gap gates along the struts and flow passages. The bridge gate was intended to supply additional melt to the 1 mm overlap. The enlarged riser was intended to increase the thermal mass and feeding reservoir at the mounting flange. The gap gates were intended to provide local feeding to the 3 mm walls. After casting, the enlarged riser solved the riser-root shrinkage cavity. The bridge gate improved the split-ring underfill, but a local underfill remained. This confirmed that 1 mm is below the critical filling thickness for ZTi60 under the selected centrifugal conditions. The gap gates produced only a small porosity-free zone around each attachment; the rest of the thin wall still showed extensive porosity. I therefore concluded that gap gates are useful for local hot spots but not for large-area thin-wall porosity. I also compared this outcome with evaporative pattern casting experience, where local gates can sometimes reduce shrinkage in isolated thick sections but cannot compensate for a globally thin wall. The result strengthened my decision to change the wall thickness rather than relying only on gating.
| Optimization action | Targeted defect | Trial result | Decision |
|---|---|---|---|
| Add bridge gate at split-ring overlap | Underfill at 1 mm overlap | Improved but local underfill remained | Not sufficient alone |
| Enlarge riser at mounting flange | Riser-root shrinkage cavity | Defect eliminated | Adopt |
| Add gap gates on struts and flow passages | Large-area thin-wall porosity | Only local improvement near gates | Limited benefit |
| Increase shell preheat or rotation speed | Fluidity and fill length | Not sufficient for 1 mm section | Secondary lever |
I also derived a simple critical thickness estimate to guide the next iteration. If I approximate the thin wall as a one-dimensional moving front, the available fill time is
$$t_f = \frac{L}{v_f}$$
where \(L\) is flow length and \(v_f\) is front velocity. The local solidification time is approximately
$$t_s = \frac{\rho c_p}{h} \frac{w}{2} \ln\left(\frac{T_p – T_0}{T_s – T_0}\right)$$
where \(\rho\) is density, \(c_p\) is specific heat, \(h\) is heat transfer coefficient, \(w\) is wall thickness, \(T_p\) is pouring temperature, \(T_0\) is mold initial temperature, and \(T_s\) is solidus temperature. A necessary condition for complete filling is \(t_f < t_s\). For a 1 mm wall, \(t_s\) is very small, so the inequality fails unless \(v_f\) is very high. Centrifugal force can increase \(v_f\), but it also increases turbulence and can create splashing and gas entrapment. In evaporative pattern casting, the same inequality is even harder to satisfy because the foam pattern must decompose and the gas must escape through the coating. I therefore used this inequality to justify increasing the wall thickness in the thin regions. The goal was not to change the final design arbitrarily, but to add a temporary process allowance that could later be removed by acid etching or machining. This is a practical approach in both investment casting and evaporative pattern casting: cast thicker, then remove excess material to expose sound metal.
Wall Thickness and Porosity Control
I tested different wall thicknesses for the outer ring, inner ring, and struts. The results were clear. For the outer ring, 3.0–3.5 mm produced large-area porosity, 4.0–4.5 mm produced essentially no porosity, and 5.0–5.5 mm also produced essentially no porosity. For the inner ring, 3.0–3.5 mm produced large-area porosity, 4.0–4.5 mm produced partial porosity, and 5.0–5.5 mm produced no porosity. For the struts, 3.0–3.5 mm produced large-area porosity, 4.0–4.5 mm produced partial porosity, and 5.0–5.5 mm produced less porosity. These results showed that increasing wall thickness above approximately 4 mm is an effective way to suppress thin-wall porosity. The improvement was most pronounced in the annular flow passages. In the struts, the improvement was less dramatic, but the porosity depth became shallower. After acid etching with a removal of about 0.2 mm, the surface porosity could be removed effectively. I therefore adopted a strategy of local thickening followed by controlled acid etching. This strategy also helps remove shallow surface defects and reduces the need for weld repair. Since ZTi60 has relatively poor weldability, reducing repair welding is a major quality benefit. I also noted that evaporative pattern casting can use a similar “cast thick and etch back” strategy, but the dimensional tolerance and shell strength must be controlled more tightly because the foam pattern is less rigid than a wax pattern.
| Region | Wall thickness (mm) | Porosity condition | Action |
|---|---|---|---|
| Outer ring | 3.0–3.5 | Large-area porosity | Thicken |
| Outer ring | 4.0–4.5 | Essentially none | Acceptable |
| Outer ring | 5.0–5.5 | Essentially none | Acceptable |
| Inner ring | 3.0–3.5 | Large-area porosity | Thicken |
| Inner ring | 4.0–4.5 | Partial porosity | Evaluate locally |
| Inner ring | 5.0–5.5 | None | Acceptable |
| Struts | 3.0–3.5 | Large-area porosity | Thicken |
| Struts | 4.0–4.5 | Partial porosity | Evaluate locally |
| Struts | 5.0–5.5 | Less porosity | Acceptable with etch |
| Struts after acid etch | 0.2 mm removal | Surface porosity removed | Adopt |
I further quantified the feeding resistance in the mushy zone. For a dendritic network, permeability can be approximated as
$$K = K_0 \left(1 – f_s\right)^m$$
where \(f_s\) is solid fraction, \(K_0\) is a reference permeability, and \(m\) is an exponent typically between 2 and 5. As \(f_s\) increases, \(K\) falls rapidly. The pressure drop over a thin wall is then
$$\Delta P = \frac{\mu L v}{K}$$
For a 3 mm wall, the available pressure from the riser and centrifugal field may be insufficient to drive liquid through the mushy zone over a long distance. For a 4–5 mm wall, the permeability and thermal modulus are both larger, so \(\Delta P\) is lower and feeding is more effective. This explains why the porosity decreased sharply above 4 mm. I also considered the cooling rate relation
$$\dot{T} = \frac{h \left(T – T_0\right)}{\rho c_p w}$$
which shows that a thicker wall cools more slowly. A slower cooling rate allows more time for feeding and gas escape. In evaporative pattern casting, the effective \(h\) can be different because the decomposing pattern produces gas and may alter the local heat transfer. That is another reason why evaporative pattern casting and investment casting must be modeled separately, even when the alloy and geometry are the same.
Gating, Centrifugal Filling, and Riser Design
I retained the bottom-fill centrifugal configuration because it promotes stable filling and helps exclude gas and inclusions. The central sprue feeds a spiral runner disk, which distributes metal to the outer ring. The outer ring ingates fill the outer flow passage first. The inner ring is fed through bridge runners and struts. In the optimized design, I increased the riser size and adjusted the feeder modulus. The riser design can be evaluated with the modulus ratio
$$M_r = \frac{V_r}{A_r}$$
and
$$M_c = \frac{V_c}{A_c}$$
where \(M_r\) is the riser modulus and \(M_c\) is the casting modulus at the hot spot. A practical feeding criterion is
$$M_r \ge k M_c$$
where \(k\) is a safety factor. For ZTC4, \(k\) can often be smaller because the alloy has better fluidity and feeds more readily. For ZTi60, I found that \(k\) must be larger. In my trial, enlarging the riser eliminated the riser-root shrinkage cavity. I therefore recommend increasing the riser modulus by at least 15–30% relative to a comparable ZTC4 design. I also considered the centrifugal pressure contribution to feeding:
$$P_{\text{feed}} = P_c + P_h – \Delta P_{\text{loss}}$$
where \(P_h = \rho g h\) is the gravity head, and \(\Delta P_{\text{loss}}\) includes friction and mushy-zone resistance. In a thin wall, \(\Delta P_{\text{loss}}\) dominates, so the riser must remain liquid long enough to maintain pressure. This is why riser insulation and shell preheat are important. In evaporative pattern casting, the riser is often surrounded by a refractory coating, and the same modulus ratio principle applies, but the foam pattern and gas pressure can complicate the pressure balance. I therefore used the investment casting trials to define a robust riser window for ZTi60 and treated evaporative pattern casting as a separate future option.
| Gating or feeding feature | Baseline design | Optimized design | Effect |
|---|---|---|---|
| Pouring position | Bottom fill | Bottom fill | Stable fill, reduced turbulence |
| Sprue | Central | Central | Uniform radial feed |
| Runner | Spiral multi-branch disk | Spiral multi-branch disk | Rapid outer-ring fill |
| Outer ingates | Distributed on outer flange | Distributed on outer flange | Outer ring fills first |
| Inner ingate | Full ring plus bridge | Full ring plus bridge and local gap gates | Improved inner-ring fill |
| Riser | ZTC4-scale | Enlarged ZTi60-specific | Eliminated riser-root shrink |
| Thin-wall strategy | Direct 1–3 mm | Thicken to 4–5 mm and etch back | Reduced porosity and underfill |
| Alternative route | Evaporative pattern casting considered | Investment casting retained | Better shell precision and titanium compatibility |
Thermal and Fluid Modeling Details
I used the heat conduction equation as the basis for the thermal model:
$$\rho c_p \frac{\partial T}{\partial t} = \nabla \cdot \left(k \nabla T\right) + Q$$
where \(Q\) is the latent heat source. The latent heat release during solidification was modeled with an effective specific heat or a solid fraction curve. The solid fraction curve controls the mushy-zone permeability and the feeding resistance. I also used the continuity and momentum equations for the melt:
$$\frac{\partial \rho}{\partial t} + \nabla \cdot \left(\rho \mathbf{v}\right) = 0$$
$$\rho \left(\frac{\partial \mathbf{v}}{\partial t} + \mathbf{v} \cdot \nabla \mathbf{v}\right) = -\nabla P + \mu \nabla^2 \mathbf{v} + \rho \mathbf{g} + \mathbf{F}_c$$
where \(\mathbf{F}_c\) is the centrifugal body force. For a rotating frame,
$$\mathbf{F}_c = \rho \omega^2 \mathbf{r}$$
and the Coriolis term can be neglected in a quasi-steady filling approximation. These equations explain why the outer ring fills first and why the inner ring depends on bridge feeding. They also show why an increase in rotation speed can improve fill length but may also increase turbulence. I used a Reynolds number estimate,
$$Re = \frac{\rho v D}{\mu}$$
to check whether the flow remained in a reasonable regime. In thin sections, the hydraulic diameter \(D\) is small, so \(Re\) is moderate even at high velocity. However, local transitions and splashing can still occur at gate entrances. I therefore kept the spiral runner design and avoided abrupt area changes. In evaporative pattern casting, the foam pattern changes the effective channel geometry as it decomposes, so the Reynolds number and pressure loss become transient and harder to predict. That is another reason why I preferred investment casting for this ZTi60 casing.
Defect Formation and Microstructural Interpretation
The intergranular porosity I observed in the 3 mm walls was consistent with insufficient feeding during the final stages of solidification. The pores formed along grain boundaries because the remaining liquid was isolated and could not be replenished. The depth of the porosity extended through most of the wall, which means that surface inspection alone would not have been sufficient. Fluorescence inspection detected the surface-connected porosity, while X-ray and metallography revealed the subsurface extent. I also observed that the porosity was more severe in regions with lower thermal gradients. These regions included the mid-span of the flow passages and the central portions of the struts. The outer ring, which had a slightly larger modulus and better access to the outer ingates, showed less porosity when thickened. The inner ring, which filled later, was more sensitive. The struts, which acted as feeding channels to the inner ring, were also sensitive because they solidified while trying to feed other regions. I therefore treated the struts as both structural features and feeding paths. In the optimized design, I increased their thickness and added local gap gates where possible. The combination reduced but did not eliminate strut porosity at 3 mm. Only when the strut thickness approached 5 mm did the porosity become acceptable, and even then acid etching was beneficial.
| Defect | Location | Mechanism | Process response |
|---|---|---|---|
| Underfill | Split-ring overlap, 1 mm | Fill time exceeds local solidification time | Increase thickness; local bridge gate; evaluate preheat |
| Shrinkage cavity | Riser root at mounting flange | Insufficient riser modulus | Enlarge riser; improve insulation |
| Intergranular porosity | 3 mm flow passages and struts | Low Niyama value; poor mushy-zone feeding | Thicken to 4–5 mm; gap gates; acid etch |
| Surface porosity | Thin-wall surfaces | Gas entrapment and shrinkage near skin | Acid etch 0.2 mm; improve shell permeability |
| Local cold shut | Thin-wall junctions | Low superheat and rapid heat loss | Increase preheat; shorten fill path |
Process Window and Optimization Matrix
I built an optimization matrix to compare the impact of each variable on filling, shrinkage, porosity, and dimensional tolerance. The matrix helped me prioritize actions. The most influential variable was wall thickness. The second was riser modulus. The third was shell preheat and pouring temperature. Rotation speed had a positive effect on fill length but a limited effect on thin-wall porosity once the wall was below the critical thickness. Gap gates were useful locally but not globally. Acid etching was beneficial for surface defects and dimensional adjustment. I also included evaporative pattern casting in the matrix as a reference route. In that route, pattern decomposition, coating permeability, and sand compaction would become dominant variables. For ZTi60, I judged that the investment casting route had a better chance of meeting the internal quality requirements, especially for the 1 mm overlap and 3 mm flow passages. The matrix also showed that no single variable could solve all defects. The successful process required a combination of local thickening, enlarged risers, controlled preheat, and post-cast etching.
| Variable | Effect on fill | Effect on shrinkage | Effect on thin-wall porosity | Practical recommendation |
|---|---|---|---|---|
| Wall thickness | Strong positive | Strong positive | Strong positive | Increase to 4–5 mm, then etch back |
| Riser modulus | Weak positive | Strong positive | Moderate positive | Increase by 15–30% over ZTC4 |
| Shell preheat | Moderate positive | Moderate positive | Moderate positive | Use higher controlled preheat |
| Pouring temperature | Moderate positive | Negative if excessive | Moderate positive | Balance fluidity and grain growth |
| Rotation speed | Strong positive | Weak | Weak to moderate | Optimize without turbulence |
| Gap gates | Local positive | Local positive | Local only | Use at hot spots, not whole wall |
| Acid etching | None | None | Removes surface porosity | Remove 0.2 mm or as needed |
| Evaporative pattern casting | Potentially positive for simple shapes | Variable | Difficult for 1–3 mm titanium | Reference route only |
Dimensional Control and Post-Cast Processing
After casting, I used three-dimensional scanning to compare the actual casing with the digital model. This allowed me to map the excess thickness at each region. The goal was to remove the process allowance by local acid etching or machining while maintaining the final wall thickness. I calculated the removal amount as
$$r = w_{\text{cast}} – w_{\text{design}}$$
where \(w_{\text{cast}}\) is the as-cast thickness and \(w_{\text{design}}\) is the design thickness. I then adjusted the etching time using an approximate relation
$$r = k_e t_e$$
where \(k_e\) is an effective etching rate and \(t_e\) is etching time. In practice, \(k_e\) varies with local flow, surface condition, and alloy microstructure, so I used iterative measurements rather than a fixed time. This approach is similar in principle to dimensional control in evaporative pattern casting, where pattern shrinkage and coating thickness must be compensated. In both routes, the final dimensions depend on a chain of allowances: pattern or wax shrinkage, shell expansion, alloy contraction, and post-processing removal. For ZTi60, the alloy contraction and thin-wall solidification make the allowance chain more sensitive. I therefore recommend that any future evaporative pattern casting trial for ZTi60 include a full dimensional compensation study before production.
| Stage | Dimensional allowance or control | Risk |
|---|---|---|
| Pattern or wax | Shrinkage compensation | Thin-wall distortion |
| Shell | Expansion and firing stability | Shell cracking, dimensional drift |
| Casting | Alloy contraction | Hot tears, warpage |
| Thin-wall allowance | Local thickening to 4–5 mm | Excess removal required |
| Acid etching | 0.2 mm or more | Nonuniform removal |
| Final inspection | 3D scan and wall mapping | Local thin spots |
| Evaporative pattern casting reference | Pattern and coating allowances | Gas pressure and coating defects |
Centrifugal Parameter Calculations
For the centrifugal process, I calculated the angular velocity from rotation speed:
$$\omega = \frac{2 \pi N}{60}$$
where \(N\) is in r/min. At 140 r/min,
$$\omega = \frac{2 \pi \times 140}{60} \approx 14.66 \text{ rad/s}$$
The centrifugal pressure at the outer ring can then be estimated from
$$P_c = \frac{1}{2}\rho \omega^2 \left(r_o^2 – r_i^2\right)$$
For a titanium melt density near 4,100 kg/m³ and radial distances on the order of 0.45 m, the pressure is significant but not unlimited. I also estimated the centrifugal acceleration:
$$a_c = \omega^2 r$$
At the outer ring, \(a_c\) can be many times gravity, which helps fill the outer passages. However, the inner ring is closer to the axis, so the centrifugal pressure is lower. This is why the inner ring filled last. The bridge runner and struts must therefore carry sufficient superheat and momentum to fill the inner ring before local solidification blocks the path. I also considered the Froude number,
$$Fr = \frac{v}{\sqrt{g L}}$$
to characterize the relative importance of inertial and gravity forces. In centrifugal casting, the effective gravity is replaced by \(a_c\), so the Froude number should be interpreted in the rotating frame. These calculations helped me explain why increasing rotation speed improved outer-ring fill but did not solve the 1 mm inner overlap. The thin wall simply solidified too quickly. In evaporative pattern casting, the absence of a strong centrifugal field means the effective pressure head is much lower, which would make the same thin wall even more difficult to fill.
Solidification Path and Alloy-Specific Behavior
I attributed the higher shrinkage tendency of ZTi60 to its wider solidification interval and lower fluidity compared with ZTC4. The solidification path can be represented by the solid fraction curve \(f_s(T)\). The mushy-zone width is
$$\Delta T_m = T_l – T_s$$
where \(T_l\) is liquidus and \(T_s\) is solidus. A wider \(\Delta T_m\) generally increases the risk of dispersed porosity because the mushy zone is longer and feeding is more difficult. I did not have a complete experimental \(f_s(T)\) curve for every local condition, but the simulation and trial results were consistent with a wider mushy zone in ZTi60. I also considered the thermal gradient:
$$G = \frac{\partial T}{\partial x}$$
and the cooling rate:
$$\dot{T} = \frac{\partial T}{\partial t}$$
The Niyama criterion combines them. In thin walls, \(G\) is low and \(\dot{T}\) is high, so \(N\) is low. In thicker walls, \(G\) is higher and \(\dot{T}\) is lower, so \(N\) is higher. This is why thickening the wall improved the porosity. I also used the solidification time ratio
$$\frac{t_{s,1}}{t_{s,2}} = \left(\frac{w_1}{w_2}\right)^n$$
to estimate the effect of wall thickness. If \(n \approx 2\), doubling the wall thickness increases solidification time by a factor of about 4. This gives a large process window. In my trials, the improvement from 3 mm to 4–5 mm was consistent with this strong thickness dependence. I therefore recommend that ZTi60 casing designs avoid long unsupported 3 mm walls unless they can be locally thickened and later etched. For evaporative pattern casting, the same thickness dependence applies, but the mold constant \(B\) may be different because of coating and sand properties. I would expect evaporative pattern casting to require even thicker walls for the same alloy and geometry.
Quality Control and Inspection Logic
I used a multi-stage inspection logic for the optimized casing. Visual inspection checked for gross underfill and surface defects. Fluorescence inspection checked for surface-connected porosity and cracks. X-ray inspection checked for internal shrinkage and inclusions. Metallography checked the depth and morphology of porosity. Three-dimensional scanning checked dimensions and wall thickness after etching. I also compared the inspected results with the simulation predictions. The simulation was useful for ranking hot spots and alloy differences, but it was not sufficient to certify the process. The 1 mm underfill was not predicted, and the exact porosity distribution in the 3 mm walls differed from the simulation. I therefore treated simulation as a design aid and trial casting as the final validation. This is a common experience in both investment casting and evaporative pattern casting: the model captures the major physics, but local phenomena such as shell permeability, gas evolution, and thin-wall heat transfer require experimental calibration.
| Inspection stage | Purpose | Finding in optimized casing |
|---|---|---|
| Visual | Gross geometry and underfill | No major underfill after thickening |
| Fluorescence | Surface porosity and cracks | Reduced after thickening and etching |
| X-ray | Internal shrinkage and inclusions | Riser-root shrink eliminated |
| Metallography | Porosity depth and morphology | Shallower porosity after thickening |
| 3D scanning | Dimensional and wall-thickness control | Excess thickness mapped for etching |
| Comparison with simulation | Model validation and process learning | Good for shrinkage trend; limited for local underfill |
Comparison of Investment Casting and Evaporative Pattern Casting for ZTi60
I frequently compared investment casting and evaporative pattern casting because both are near-net-shape processes, but their physical mechanisms differ. In investment casting, a wax pattern is coated with ceramic shell, dewaxed, fired, and filled with molten metal. In evaporative pattern casting, a foam pattern is coated, embedded in sand, and replaced by molten metal as the foam decomposes. For ZTi60, the investment casting route offers better shell rigidity, better internal core support, and a more controlled mold atmosphere. The evaporative pattern casting route offers simpler pattern production and fewer steps, but the foam decomposition produces gas that must escape through the coating and sand. In a 1 mm titanium section, that gas pressure can oppose filling and cause cold shuts or gas porosity. I therefore did not select evaporative pattern casting for this casing. However, I still consider evaporative pattern casting a valuable alternative for larger, less thin-walled titanium components if the coating permeability and pour temperature can be optimized. My trials with ZTi60 showed that wall thickness above 4 mm is much more forgiving, and that is the regime where evaporative pattern casting might become more competitive. For thin walls below 3 mm, investment casting with centrifugal assistance remains the stronger route.
| Factor | Investment casting | Evaporative pattern casting | Implication for ZTi60 |
|---|---|---|---|
| Pattern material | Wax or plastic | Foam | Wax gives better dimensional precision |
| Mold configuration | Ceramic shell | Refractory-coated foam in sand | Ceramic shell is more rigid for thin walls |
| Pattern removal | Dewaxing and firing | Thermal decomposition during pour | Foam gas can hinder titanium filling |
| Filling assistance | Centrifugal or gravity | Usually gravity or low pressure | Centrifugal investment casting fills thin sections better |
| Thin-wall capability | Good to moderate | Moderate to poor for titanium | Investment casting preferred for 1–3 mm |
| Surface finish | High | Moderate | Investment casting reduces machining |
| Tooling cost | Higher | Lower | Evaporative pattern casting is attractive for simple shapes |
| Defect risk | Shrinkage, underfill, shell defects | Gas porosity, coating defects, incomplete fill | Both require process control; ZTi60 is sensitive |
Optimized Process and Final Observations
The optimized process I established combined several changes. I retained bottom-fill centrifugal investment casting. I kept the central sprue and spiral runner disk. I enlarged the riser at the mounting flange. I added local bridge and gap gates at critical locations. I thickened the 1 mm split-ring overlap and the 3 mm flow passages and struts to at least 4 mm, with 5 mm preferred in the most sensitive regions. I used a controlled shell preheat and a pouring temperature that balanced fluidity with grain control. After casting, I used three-dimensional scanning and local acid etching to remove the process allowance and surface porosity. The result was a significant improvement in filling and a major reduction in shrinkage and thin-wall porosity. The riser-root cavity was eliminated, the split-ring underfill was avoided by thickening, and the thin-wall porosity was either eliminated or reduced to a shallow surface layer that could be removed by etching. I also reduced the need for weld repair, which is important for ZTi60 because of its welding sensitivity. In my assessment, the most important lesson is that ZTi60 cannot be treated as a drop-in replacement for ZTC4 in a large thin-wall casing. The alloy requires a larger feeding modulus, a thicker wall allowance, and a more forgiving filling strategy. The second lesson is that simulation must be calibrated with trial data; it predicted the alloy difference but not the 1 mm underfill. The third lesson is that evaporative pattern casting, while attractive for some near-net-shape applications, is not a simple substitute for investment casting in this thin-wall ZTi60 casing. The gas evolution and lower filling pressure of evaporative pattern casting would make the 1 mm and 3 mm sections even more difficult. I would consider evaporative pattern casting only after the wall thickness is increased and the coating system is specifically developed for titanium.
| Final process element | Setting or design | Reason |
|---|---|---|
| Casting route | Centrifugal investment casting | Best filling and shell control for thin walls |
| Pouring position | Bottom fill | Stable front and reduced turbulence |
| Sprue and runner | Central sprue plus spiral runner disk | Uniform radial distribution |
| Riser | Enlarged ZTi60-specific modulus | Eliminate riser-root shrinkage |
| Thin-wall allowance | Thicken 1 mm and 3 mm regions to 4–5 mm | Improve fill and feeding |
| Local gates | Bridge and gap gates at critical zones | Local feeding and fill assistance |
| Post-cast etching | Remove approximately 0.2 mm or as measured | Remove surface porosity and excess wall |
| Dimensional control | 3D scan and local etching | Meet final wall thickness and profile |
| Alternative route | Evaporative pattern casting deferred | Thin-wall titanium filling risk remains high |
Key Equations Used in My Process Development
I summarize below the main equations that guided my decisions. These equations are not a substitute for simulation and trial casting, but they helped me organize the physics and justify process changes.
$$t_f = \frac{V}{Q} = \frac{V}{A v}$$
$$P_c = \frac{1}{2}\rho \omega^2 \left(r_2^2 – r_1^2\right)$$
$$\omega = \frac{2 \pi N}{60}$$
$$N = \frac{G}{\sqrt{\dot{T}}}$$
$$t_s = B \left(\frac{V}{A}\right)^n$$
$$v = -\frac{K}{\mu}\left(\nabla P – \rho g\right)$$
$$K = K_0 \left(1 – f_s\right)^m$$
$$\Delta P = \frac{\mu L v}{K}$$
$$\rho c_p \frac{\partial T}{\partial t} = \nabla \cdot \left(k \nabla T\right) + Q$$
$$\dot{T} = \frac{h \left(T – T_0\right)}{\rho c_p w}$$
$$M_r = \frac{V_r}{A_r}$$
$$M_c = \frac{V_c}{A_c}$$
$$M_r \ge k M_c$$
$$Re = \frac{\rho v D}{\mu}$$
$$Fr = \frac{v}{\sqrt{g L}}$$
$$r = w_{\text{cast}} – w_{\text{design}}$$
$$r = k_e t_e$$
These relationships explain the observed behavior. The fill time and centrifugal pressure control how far the melt can travel. The Niyama criterion and Darcy flow describe why thin walls form porosity. The Chvorinov rule and modulus ratio explain why thicker walls and larger risers improve feeding. The heat transfer and cooling rate equations show why shell preheat and wall thickness are so important. The dimensional equations support the etch-back strategy. I used these equations in combination with ProCAST simulations and actual trials. The final process was not derived from a single equation but from an iterative loop: design, simulate, cast, inspect, and revise. That loop is essential for ZTi60 because the alloy is sensitive to small changes in wall thickness, filling rate, and thermal history. The same loop would be necessary for evaporative pattern casting if that route were developed for a similar component.
Practical Recommendations
Based on my work, I recommend the following for large complex thin-wall ZTi60 casings. First, avoid designing long unsupported walls below 4 mm unless there is a clear plan to thicken and etch back. Second, increase the riser modulus relative to a ZTC4 baseline. Third, use centrifugal investment casting with bottom fill and a spiral runner disk for uniform radial filling. Fourth, add local gap gates only where they can feed a hot spot; do not expect them to solve global thin-wall porosity. Fifth, use controlled shell preheat and pouring temperature to extend the filling window without causing excessive grain growth. Sixth, use three-dimensional scanning and local acid etching to remove process allowance and surface porosity. Seventh, validate every simulation with trial castings because local underfill and thin-wall porosity are not always predicted. Eighth, treat evaporative pattern casting as a separate development route rather than a direct replacement. In evaporative pattern casting, the foam pattern, coating permeability, sand compaction, and gas evolution must be optimized specifically for titanium. For ZTi60, I would only consider evaporative pattern casting after the wall thickness is increased to a range where filling is robust. For the 1 mm and 3 mm sections in this casing, investment casting remains the more reliable process.
| Recommendation | Technical basis | Expected benefit |
|---|---|---|
| Avoid long 1–3 mm ZTi60 walls | Critical fill and feeding limit | Reduced underfill and porosity |
| Increase riser modulus | Feeding criterion \(M_r \ge k M_c\) | Eliminate riser-root shrink |
| Use centrifugal bottom fill | Stable filling and gas exclusion | Better fill and cleaner metal |
| Use local gap gates selectively | Darcy feeding is local | Reduce local hot-spot porosity |
| Control shell preheat | Extends local solidification time | Improved thin-wall fill |
| Thicken and etch back | Increases modulus and removes defects | Sound final wall with less weld repair |
| Validate simulation with trials | Local phenomena are not fully captured | More reliable process window |
| Defer evaporative pattern casting for thin walls | Gas evolution and low filling pressure | Avoids high-risk development path |
Conclusion of My Development Work
I successfully moved the ZTi60 large complex thin-wall casing from a baseline concept to an optimized investment casting process. The baseline design, inherited from ZTC4, produced underfill at the 1 mm split-ring overlap, a large shrinkage cavity at the riser root, and extensive intergranular porosity in the 3 mm flow passages and struts. Simulation showed that ZTi60 fills more slowly and has a higher shrinkage tendency than ZTC4, but it did not predict the local underfill. Trial casting confirmed the simulation trend and revealed the actual defects. I then optimized the process by enlarging the riser, adding local bridge and gap gates, and, most importantly, increasing the thin-wall thickness to at least 4 mm and preferably 5 mm in sensitive regions. This changed the thermal modulus, slowed local cooling, improved feeding, and reduced porosity. Acid etching removed the process allowance and shallow surface defects. Three-dimensional scanning supported dimensional control. The final process significantly improved filling, eliminated the riser-root shrinkage cavity, and reduced thin-wall porosity to an acceptable level. I also concluded that evaporative pattern casting is not a straightforward alternative for this ZTi60 casing because the 1–3 mm walls require strong filling and feeding assistance. Although evaporative pattern casting has advantages in tooling simplicity for some components, its gas evolution and lower filling pressure make it less suitable for the thin-wall titanium sections studied here. For future work, I would extend the wall-thickness window, refine the shell preheat and rotation speed, and develop a calibrated thin-wall database for ZTi60. I would also keep evaporative pattern casting in the option space for thicker, less demanding titanium castings where its cost and tooling advantages can be realized without compromising internal quality.