I treated the production of a large, complex, thin-walled ZTi60 high-temperature titanium alloy casing as a combined alloy-fluid-flow, heat-transfer, shell-making, and centrifugal lost wax casting problem. The alloy is a near-alpha titanium alloy of the Ti-Al-Sn-Zr-Mo-Nb-Ta-Si family, designed for service near 600 °C. I recognized that its combination of high-temperature strength, oxidation resistance, and thermal stability makes it attractive for aero-engine casings, but the same chemistry creates severe process difficulty. Niobium and tantalum raise the melting range and reduce fluidity. The alloy has a narrow freezing range in practical casting terms, low superheat, and a strong tendency to form cold shuts, underfill, shrinkage porosity, and cracks. For a large thin-walled casing, these are not isolated defects; they interact with shell preheating, centrifugal pressure, gating geometry, and local wall thickness.
I selected lost wax casting because the geometry is difficult to machine and because near-net-shape production is the most credible route for a large thin-walled titanium casing. In lost wax casting, the wax pattern, ceramic shell, dewaxing, high-temperature burnout, vacuum or controlled-atmosphere melting, and centrifugal pouring are all coupled. A small change in wax assembly, shell thickness, or gate placement changes the thermal history of the entire casing. I therefore did not treat the process as a simple copy of a conventional ZTC4 casing route. I used the mature ZTC4 gating architecture only as an initial reference, then redesigned the lost wax casting process around the measured behaviour of ZTi60.
The casing I studied is an intermediate casing with a circular, axisymmetric body. Its outer envelope is approximately 909 mm in diameter and 226 mm in height. It contains inner and outer flow passages and six support struts. The dominant wall thickness is about 2.5 mm, and the narrow edge of the split-ring overlap is only about 1.0 mm. This is a severe combination for lost wax casting: the thin sections cool quickly, the flow path is long, and the struts and mounting flanges create hot spots that are difficult to feed. I recorded the nominal composition range of ZTi60 as the basis for all subsequent melting, pouring, and simulation assumptions.
| Element | Mass fraction range, % | Primary process relevance |
|---|---|---|
| Al | 5.5 to 6.5 | Alpha stabilizer, strength, oxidation resistance |
| Sn | 3.0 to 4.5 | Solid-solution strengthening, thermal stability |
| Zr | 2.5 to 4.0 | Solid-solution strengthening, creep resistance |
| Mo | 0.2 to 1.2 | Beta stabilizer, high-temperature strength |
| Si | 0.2 to 0.5 | Creep resistance, silicide formation |
| Nb | 0.2 to 0.8 | High-temperature strengthening, reduced fluidity |
| Ta | 0.1 to 1.0 | Refractory strengthening, reduced fluidity |
| Fe | 0.1 maximum | Impurity control, beta stabilizer |
| C | 0.08 maximum | Impurity control, carbide formation risk |
| O | 0.15 maximum | Alpha stabilization, embrittlement risk |
| N | 0.05 maximum | Impurity control, hardness risk |
| H | 0.01 maximum | Porosity and hydride risk |
I quantified the geometric severity before designing gates. The ratio of outer diameter to nominal wall thickness is above 300. For a conventional alloy with high superheat, that ratio alone is challenging. For ZTi60, the effective fluidity is further reduced by the refractory elements. I expressed the fill fraction during simulation as a simple volume ratio:
$$\phi(t)=\frac{V_f(t)}{V_c}\times 100\%$$
Here, the symbol for fill fraction is the volume already filled divided by the cavity volume. I used the same definition for both ZTC4 and ZTi60 so that I could compare the two alloys at equal time points. I also tracked solid fraction during solidification:
$$f_s(t)=\frac{V_s(t)}{V_c}\times 100\%$$
where the numerator is the volume of solid metal at time t. In a lost wax casting process, the competition between fill fraction and solid fraction determines whether the last liquid reaches the thin edge before it freezes. The split-ring overlap at 1.0 mm is the most dangerous location because it combines the smallest local volume-to-area ratio with the longest local flow distance.
| Feature | Approximate dimension | Lost wax casting risk |
|---|---|---|
| Overall outer diameter | 909 mm | Long radial fill path, large shell volume |
| Overall height | 226 mm | Vertical thermal gradients, shell handling risk |
| Main wall thickness | 2.5 mm | Rapid cooling, limited feeding, shell strength sensitivity |
| Split-ring overlap narrow edge | 1.0 mm | Underfill and cold shut risk |
| Flow passages | Inner and outer rings | Thin annular sections, porosity risk |
| Support struts | Six struts | Hot spots and isolated thin walls |
| Mounting flanges | Circular arrays | Riser feeding and shrinkage cavity risk |
I chose centrifugal lost wax casting as the core route. Centrifugal motion improves radial filling, helps expel gas and inclusions, and can increase the effective feeding pressure in thin sections. The benefit is not automatic. If the rotation speed is too low, the centrifugal pressure is insufficient to push ZTi60 into the 1 mm edge. If the rotation speed is too high, shell cracking, turbulence, and segregation risks increase. I expressed the centrifugal acceleration at radius r as:
$$a_c = r\omega^2 = r\left(\frac{2\pi n}{60}\right)^2$$
where n is the rotation speed in revolutions per minute. For n equal to 140 r/min, the angular velocity is about 14.66 rad/s. I also estimated the radial pressure available for feeding:
$$\Delta p_c = \frac{\rho \omega^2}{2}(r_o^2-r_i^2)$$
This pressure acts outward from the central sprue toward the outer rim. In principle, it helps the lost wax casting fill the outer flow passage and the outer mounting flange. In practice, the pressure must be transmitted through a partially solidified network. Once a thin section develops a continuous solid skin, additional centrifugal pressure cannot feed the isolated liquid pocket. This is why I did not rely on rotation alone; I combined centrifugal pressure with gating and thermal design.
For the initial process, I used a bottom-poured centrifugal lost wax casting arrangement. The central sprue was placed at the centre of the casing. The runner was a multi-strand spiral diverging runner plate intended to distribute metal quickly and smoothly to the outermost radius. On the outer mounting flange, I placed evenly distributed ingates. On the inner mounting flange, I used a full circular ingate. A lap runner connected the farthest end of the spiral runner to the inner ring. This architecture was inherited from a mature ZTC4 casing route, but I re-evaluated it for ZTi60 because the alloy has lower fluidity and greater shrinkage tendency.
| Design element | Initial choice | Intended function |
|---|---|---|
| Pouring mode | Bottom-poured centrifugal | Orderly fill, reduced turbulence, improved feeding |
| Central sprue | Located at casing centre | Radial distribution to all sectors |
| Runner | Multi-strand spiral diverging runner plate | Fast and stable transport to outer rim |
| Outer ingates | Uniformly distributed on outer flange | Fill outer flow passage and flange |
| Inner ingate | Full circular ring | Fill inner mounting flange |
| Lap runner | Connects outer runner end to inner ring | Balance inner and outer filling |
| Rotation speed | 140 r/min | Centrifugal pressure and gas expulsion |
I simulated the initial design with a finite-volume casting model. I set the mesh size to 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 used a shell thickness of 20 mm, a pouring temperature of 1,700 °C, a pouring time of 7 s, a preheat temperature of 200 °C for the mullite ceramic shell, and an interfacial heat transfer coefficient of 500 W/(m²·K). The rotation speed was 140 r/min. I assumed vacuum pouring. I then compared ZTC4 and ZTi60 under identical conditions to isolate the effect of alloy chemistry on fill and solidification.
| Parameter | Value |
|---|---|
| Casting mesh size | 5 |
| Gating mesh size | 10 |
| Surface mesh count | Approximately 530,000 |
| Volume mesh count | Approximately 4,640,000 |
| 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 |
| Atmosphere | Vacuum |
| Materials compared | ZTC4 and ZTi60 |
The simulation showed that the global fill sequence was similar for both alloys. Metal entered the central sprue, moved through the spiral runner plate, and then flowed under centrifugal action toward the farthest radius. It entered the casing cavity through the outer ingates, filled the outer ring first, and then advanced inward through the struts and the lap runner toward the inner ring. The inner mounting flange was the last region to fill. This sequence was logical for the lost wax casting design, but the timing differed. ZTi60 reached a given fill fraction later than ZTC4. At 98% fill, ZTC4 required 6.898 s, while ZTi60 required 7.2548 s. I interpreted this delay as direct evidence of lower fluidity and greater resistance to thin-section filling.
| Response | ZTC4 | ZTi60 | Process implication |
|---|---|---|---|
| Fill fraction at about 2.0 s | 28.7% | 27.4% | ZTi60 starts slightly slower |
| Fill fraction at about 4.0 s | 57.2% | 54.3% | ZTi60 lags during mid-fill |
| Fill time to 98% | 6.898 s | 7.2548 s | ZTi60 needs longer to approach complete fill |
| Solid fraction at about 4.0 s | 0.8% | 1.0% | ZTi60 begins to freeze slightly earlier before full fill |
| Solid fraction at 98% fill | 4.3% | 5.9% | ZTi60 has less remaining liquid for final fill |
| Solid fraction at about 9.8 to 9.9 s | 20.8% | 20.6% | Similar global freezing after fill |
| Solid fraction at about 18.0 to 18.8 s | 42.6% | 40.2% | ZTi60 retains slightly more liquid later |
| Predicted porosity | 5.964% | 6.988% | ZTi60 has a stronger shrinkage tendency |
I also compared the predicted shrinkage distribution. ZTC4 had a porosity value of 5.964%, while ZTi60 had 6.988%. The difference is not merely numerical. ZTi60 produced new shrinkage locations and, in some regions, larger individual cavities. This told me that a gating system that was acceptable for ZTC4 would not automatically be acceptable for ZTi60 lost wax casting. The alloy required a larger feeding margin, especially at the riser roots and in the thicker mounting flange regions. I expressed porosity as:
$$P=\frac{V_p}{V_t}\times 100\%$$
where the numerator is predicted pore volume and the denominator is total volume. I used this value as a comparative index rather than as an absolute production prediction. The more important conclusion was directional: ZTi60 shifts the process window toward higher feeding requirement and shorter allowable fill time.
I then conducted an actual pouring trial using the same process conditions. The trial followed the full lost wax casting chain: wax injection, pattern assembly, shell building, dewaxing, high-temperature shell burnout, vacuum centrifugal pouring, and knockout. After pouring, I performed visual inspection, fluorescent penetrant inspection, and X-ray inspection. The results confirmed part of the simulation and revealed one critical limitation. The simulation did not predict underfill, but the real casting showed underfill at the 1 mm split-ring lap edge. The riser roots showed large shrinkage cavities. The flow passages and struts, which were about 3 mm thick, showed extensive porosity. Metallography identified the porosity as intergranular or interdendritic in character, and in some locations it extended through the full 3 mm wall.
| Location | Feature | Observed defect | Primary interpretation |
|---|---|---|---|
| Split-ring lap edge | 1.0 mm thin edge | Underfill | Below practical critical wall thickness for ZTi60 |
| Riser roots | Mounting flange connection | Large shrinkage cavity | Insufficient riser volume and feeding path |
| Flow passages | About 3 mm wall | Extensive porosity | Rapid freezing, limited interdendritic feeding |
| Support struts | About 3 mm wall | Extensive porosity | Isolated thin section, low thermal modulus |
| Thin-wall cross sections | 3 mm | Intergranular porosity through thickness | Poor local feeding and high cooling rate |
The same gating architecture produced no underfill, no root shrinkage, and no large-area thin-wall porosity in ZTC4. This comparison was decisive. ZTi60 has lower castability in the lost wax casting process. I did not attribute the result to a single cause. The alloy contains Nb and Ta, which increase viscosity and reduce fluidity. Its superheat at 1,700 °C is limited. The thin walls lose heat rapidly to the ceramic shell. The 1 mm edge has almost no thermal mass. The riser roots have enough thermal mass to form a hot spot, but the feeding path from the riser to the last liquid region is too restrictive. These effects combine to produce underfill, shrinkage, and porosity in the same casting.
I used a Niyama-type criterion to interpret the thin-wall porosity. In simplified form:
$$N_y=\frac{G}{\sqrt{\dot{T}}}$$
where G is the local temperature gradient and the dot over T represents cooling rate. A low Niyama value indicates a region where feeding is difficult and shrinkage porosity is likely. Thin walls have high cooling rate and low thermal gradient, so they naturally produce low Niyama values. In ZTi60 lost wax casting, the problem is amplified because the alloy has a short feeding window. I also considered the Chvorinov relation for solidification time:
$$t_s=B\left(\frac{V}{A}\right)^n$$
Here, V/A is the volume-to-surface-area ratio, or thermal modulus. The 1 mm edge has a very small modulus, so it solidifies quickly. The 3 mm flow passage still has a small modulus compared with the flange. It freezes before the riser can feed it. This explains why simply adding a small slit gate near the thin wall produced only local improvement.
For the first optimization, I modified the lost wax casting layout in three ways. I added a lap runner at the split-ring lap section to supplement filling. I increased the riser size to improve feeding at the flange roots. I added slit gates to the struts and flow passages to provide auxiliary filling and feeding. The intention was to bring liquid metal closer to the thin sections and to reduce the distance over which feeding must occur.
| Modification | Target defect | Reason | Observed result |
|---|---|---|---|
| Add lap runner at split-ring lap | Underfill at 1 mm edge | Supplement local filling | Improved, but local underfill remained |
| Increase riser size and height | Shrinkage cavity at riser root | Increase feeding volume and pressure | Root shrinkage effectively eliminated |
| Add slit gates on struts and passages | Thin-wall porosity | Shorten feeding distance | Only small local areas improved |
The first optimization confirmed two important process rules for ZTi60 lost wax casting. First, the riser must be enlarged relative to a ZTC4 design. The shrinkage tendency of ZTi60 is higher, so the feeding ratio must be increased. Second, auxiliary slit gates are not a complete solution for thin-wall porosity. They act only in the immediate vicinity of the gate. Away from the gate, the wall still solidifies too quickly and remains isolated from the feeding path. The 1 mm lap edge remained especially difficult. I concluded that 1 mm is below the practical critical wall thickness for ZTi60 in this large lost wax casting geometry. Even with a local lap runner, the alloy cannot reliably fill that edge.
I therefore changed the approach. Instead of trying to force ZTi60 to fill an unrealistically thin section, I added process allowance to the thin walls. The casting was intentionally thickened, and the excess material was later removed by acid pickling or machining. This is a deliberate use of lost wax casting as a near-net-shape process: the casting is not made to final wall thickness directly, but to a process-compatible thickness that can be reduced after inspection. The process allowance can be expressed as:
$$t_{cast}=t_{design}+t_{allow}+t_{pickle}+t_{machine}$$
where the cast thickness is the design thickness plus allowance for filling, pickling, and machining. For the thin flow passages and struts, I tested several thickness ranges. The improvement was clear. For the outer ring, 3.0 to 3.5 mm produced large-area porosity, 4.0 to 4.5 mm was essentially free of porosity, and 5.0 to 5.5 mm was also essentially free. For the inner ring, 3.0 to 3.5 mm produced large-area porosity, 4.0 to 4.5 mm produced partial porosity, and 5.0 to 5.5 mm produced no porosity. For the struts, 3.0 to 3.5 mm produced large-area porosity, 4.0 to 4.5 mm produced partial porosity, and 5.0 to 5.5 mm produced less porosity.
| Region | Cast wall thickness, mm | Porosity result | Process conclusion |
|---|---|---|---|
| Outer ring | 3.0 to 3.5 | Large-area porosity | Below reliable feeding threshold |
| Outer ring | 4.0 to 4.5 | Essentially none | Reliable window |
| Outer ring | 5.0 to 5.5 | Essentially none | Reliable but more excess material |
| Inner ring | 3.0 to 3.5 | Large-area porosity | Below reliable feeding threshold |
| Inner ring | 4.0 to 4.5 | Partial porosity | Transitional window |
| Inner ring | 5.0 to 5.5 | None | Reliable window |
| Struts | 3.0 to 3.5 | Large-area porosity | Severe isolated thin-wall condition |
| Struts | 4.0 to 4.5 | Partial porosity | Improved but not fully robust |
| Struts | 5.0 to 5.5 | Less porosity | Improved, still geometry-sensitive |
The results showed that thickening to at least 4 mm greatly reduces or eliminates porosity in the annular flow passages. The struts remained more difficult because they are isolated and have complex local heat flow. However, after thickening the struts, the porosity depth became shallower. I then used acid pickling with a removal of about 0.2 mm. The pickling removed the surface-connected porosity and produced a clean fluorescent penetrant inspection result. This is important because ZTi60 is not easy to weld. Repeated weld repair can introduce new risks, including embrittlement, distortion, and property loss. If a shallow surface defect can be removed by controlled pickling instead of welding, the overall lost wax casting yield and reliability improve.
I also considered the dimensional consequence of adding process allowance. After pouring, the casting can be scanned in three dimensions and compared with the design model. The excess wall thickness is mapped by region. Then local acid pickling or machining can remove the excess. I expressed the dimensional correction as:
$$\Delta t_i = t_{meas,i}-t_{design,i}$$
where the measured thickness at location i is compared with the design thickness. The required removal depth is the positive part of that difference. This allows the thickened lost wax casting to be converted into a conforming thin-wall casing without relying on welding. The pickling step also removes the shallow, surface-connected porosity that would otherwise appear as fluorescent indications.
| Step | Method | Purpose | Outcome |
|---|---|---|---|
| Cast thickening | Add process allowance to thin walls | Improve filling and feeding | Reduced underfill and porosity |
| Three-dimensional scan | Compare cast surface with design model | Map excess thickness | Identified local removal zones |
| Acid pickling | Remove about 0.2 mm | Remove surface porosity and excess wall | Improved fluorescent inspection results |
| Local machining | Remove remaining excess if required | Meet final wall tolerance | Reduced weld repair |
| Final inspection | Visual, fluorescent, X-ray, dimensional | Verify lost wax casting quality | Confirmed process capability |
I also evaluated the feeding mechanics more explicitly. In a thin-wall lost wax casting, the feed distance from a riser or gate depends on the thermal gradient, the freezing range, and the local cooling rate. I used a simplified feed-distance relation:
$$L_f \approx C\left(\frac{\Delta T}{\dot{T}}\right)^{m}$$
where L_f is the feeding distance, the temperature difference drives flow, and the cooling rate limits the available time. For ZTi60, the effective L_f in 3 mm walls is short. Adding a slit gate reduces the local L_f to zero only near the gate. The rest of the wall remains at a distance greater than the effective L_f. Increasing wall thickness reduces the cooling rate and increases the thermal modulus, so L_f increases. This is why thickening worked better than adding many small gates. The lost wax casting design must respect the alloy-specific feed distance rather than simply increasing gate count.
I also expressed the shrinkage driving force as a volume deficit. During solidification, the density difference between liquid and solid creates a volume contraction:
$$\epsilon_s=\frac{V_l-V_s}{V_l}\times 100\%$$
If this contraction cannot be compensated by liquid feeding, it appears as dispersed porosity or concentrated shrinkage. ZTi60 has a higher effective shrinkage tendency than ZTC4 under the same lost wax casting conditions. The porosity comparison from simulation, 5.964% for ZTC4 and 6.988% for ZTi60, is consistent with this statement. The practical consequence is that riser design for ZTi60 must be more generous. I found that increasing riser height and size solved the root shrinkage cavity, but it did not solve thin-wall porosity. That is because the root is a localized hot spot with a direct feeding path, while the thin wall is a distributed network of small channels with high flow resistance.
I further examined the role of the ceramic shell in lost wax casting. The shell thickness was 20 mm, and the preheat temperature was 200 °C. A thicker shell stores more heat, but it also changes the cooling rate. A higher preheat can improve fill, but it can also reduce the thermal gradient and delay solidification in the gating system. For ZTi60, I did not find a simple shell preheat change that solved the 1 mm underfill. The alloy lacks sufficient superheat and fluidity. The dominant issue is the local modulus and the alloy freezing behaviour. I therefore prioritized wall thickness allowance and feeding design over shell preheat alone.
The rotation speed also required judgment. Centrifugal lost wax casting can drive metal outward, but it cannot push metal into a region that has already formed a solid network. I used 140 r/min in the initial trial. Higher speed might improve radial filling, but it also increases the risk of shell cracking and turbulent entrapment. Since the underfill was at a thin edge and the porosity was in thin walls, I did not increase speed as the main corrective action. Instead, I used local process allowance and improved feeding. In a production setting, I would optimize rotation speed together with pouring temperature and shell preheat, but only after the wall thickness and riser design are physically capable of feeding the section.
I summarized the alloy-specific process window for ZTi60 lost wax casting in a design table. This table is the practical core of the study. It translates the observed defects into rules for future large complex thin-walled casings.
| Design variable | ZTC4 reference practice | ZTi60 adjusted practice | Reason |
|---|---|---|---|
| Minimum reliable wall thickness | Thin sections can fill with mature gating | Do not rely on 1 mm; thicken to 4 mm or more for critical sections | ZTi60 has low fluidity and short feeding distance |
| Riser size | Standard riser ratio | Increase riser volume and height | Higher shrinkage tendency than ZTC4 |
| Slit gates | Local feeding aid | Useful only near the gate; not a global fix | Feeding distance in thin walls is short |
| Process allowance | Limited | Add intentional thickening, then pickle or machine | Improves fill and feeding; permits defect removal |
| Weld repair | Possible fallback | Minimize; use pickling for shallow defects | ZTi60 welding is difficult and risky |
| Simulation use | Predicts shrinkage reasonably | Use for shrinkage and solidification; do not trust it alone for underfill | The trial showed underfill not captured by simulation |
| Inspection | Visual and X-ray | Add fluorescent inspection, metallography, and three-dimensional scanning | Thin-wall porosity and dimensional excess must be controlled |
The statement that simulation did not capture the underfill deserves emphasis. My simulation predicted complete filling for both alloys in the initial design, but the actual ZTi60 lost wax casting showed underfill at the 1 mm edge. This means the simulation model, mesh, boundary conditions, or material data were not sufficient to resolve the last-stage thin-edge flow. In large thin-wall lost wax casting, the final fill is controlled by local capillary, viscous, and freezing effects that are difficult to capture with a coarse mesh and a simplified heat transfer coefficient. I therefore treat simulation as a powerful tool for shrinkage, solidification sequence, and gating balance, but not as a guarantee of fill for sub-millimetre or 1 mm features. Production validation remains necessary.
I also noted that the thin-wall porosity was not just a feeding problem; it was also a local thermal problem. The 3 mm wall has a high surface-to-volume ratio. Heat is extracted quickly by the shell. The centreline remains liquid for a short time and then cannot be fed. The result is intergranular or interdendritic porosity. When the wall is thickened to 4 mm or more, the thermal modulus increases, the cooling rate decreases, and the feeding distance increases. The porosity becomes shallower or disappears. This explains why the same slit gate that worked locally did not work globally. The gate changed the boundary condition at one point, but the wall as a whole remained too thin.
For the annular flow passages, the outer ring responded well to thickening. At 4.0 to 4.5 mm, porosity was essentially absent. The inner ring needed 5.0 to 5.5 mm for a completely clean result, with 4.0 to 4.5 mm showing partial porosity. The struts were the most stubborn. Even at 5.0 to 5.5 mm, some porosity remained, though it was less severe. This is likely because the struts are isolated between the inner and outer rings and have complex thermal boundaries. In future lost wax casting designs, I would treat struts as a separate feeding problem. They may require dedicated local risers, thicker sections, or a revised orientation that allows gravity or centrifugal pressure to assist feeding.
The acid pickling result was valuable. After thickening and pickling about 0.2 mm, the surface porosity in the struts was effectively removed. This suggests that the remaining porosity after thickening was shallow and surface-connected. If the porosity had been deep and isolated, pickling would not remove it. Therefore, the combination of thickening and controlled pickling is a practical route for ZTi60 lost wax casting. It reduces the need for weld repair and improves the fluorescent inspection result. The excess wall thickness from thickening can also be removed by pickling, so the same operation serves two purposes: dimensional correction and surface defect removal.
I considered the economic and production implications. Adding process allowance increases the amount of metal poured and the amount of acid pickling required. However, it reduces scrap, rework, and weld repair. For a large complex thin-walled casing, the cost of a failed pour and the cost of repairing a difficult titanium weld are high. A robust process that intentionally thickens thin walls may be more economical than a process that tries to cast to final thickness and repeatedly repairs defects. In lost wax casting, near-net shape does not always mean net shape at the first solidification step. It can mean a controlled excess that is removed after inspection.
I also evaluated the risk of introducing new defects through pickling. If the pickling is uneven, wall thickness may fall below design tolerance. If the pickling time is too long, surface quality may degrade. I therefore recommended three-dimensional scanning before and after pickling, local masking where removal is not needed, and a controlled acid concentration and temperature. The removal depth should be based on the measured excess thickness, not on a fixed time. I expressed the target removal as:
$$h_i=\max\left(0,\ t_{meas,i}-t_{design,i}-m_i\right)$$
where the removal at location i is the positive excess after accounting for a minimum machining margin. This gives a location-specific pickling plan. It also ensures that the final wall meets design requirements while removing the shallow porosity layer.
For future scale-up, I would integrate the following control points into the lost wax casting route. First, wax pattern assembly must include the thickened process allowance in the thin-wall regions. Second, the shell must be built with sufficient strength to survive centrifugal pouring but without excessive thickness that changes cooling. Third, the gating system must include enlarged risers and a logical centrifugal runner layout. Fourth, simulation must be calibrated with actual alloy properties for ZTi60, especially viscosity, freezing range, and shell heat transfer. Fifth, pouring trials must include fluorescent inspection, X-ray, metallography, and dimensional scanning. Sixth, acid pickling must be controlled by measurement, not by estimate. These steps are not independent; they form a single process chain in lost wax casting.
| Stage | Control point | Key variable | Quality target |
|---|---|---|---|
| Wax pattern | Thin-wall allowance | Local thickness increase | 4 mm or more in critical passages |
| Shell building | Shell thickness and preheat | 20 mm shell, 200 °C preheat | Stable fill and no shell failure |
| Gating design | Riser and runner sizing | Enlarged risers, spiral runner, slit gates | Reduced root shrinkage and local feeding |
| Simulation | Fill and solidification prediction | Mesh, heat transfer, rotation speed | Shrinkage trend and sequence |
| Pouring | Centrifugal casting | 1,700 °C, 7 s, 140 r/min | Complete fill and controlled freezing |
| Inspection | Defect detection | Visual, fluorescent, X-ray, metallography | No underfill, no large porosity |
| Dimensional correction | Scanning and pickling | 0.2 mm removal range | Design wall thickness and clean surface |
I also explored the effect of metal temperature and pouring time through the lens of fluidity. The initial pouring temperature was 1,700 °C, and the pouring time was 7 s. At 98% fill, ZTi60 required 7.2548 s, which is longer than the nominal 7 s pour. This is a subtle but important point. The nominal pour time is not the same as the time required for the last thin section to fill. The last liquid may still be moving after the main pour is complete, driven by centrifugal pressure and gravity. However, if the thin section is already freezing, that additional time does not help. I therefore used the fill-time difference as a warning: ZTi60 needs either a shorter flow path, a thicker section, or a higher effective pressure. Increasing pouring temperature alone may improve fluidity, but it also increases shell reaction and gas entrapment risk. I preferred geometry-based corrections.
The alloy also has a crack sensitivity risk. In this study, the primary defects were underfill, shrinkage, and porosity, but crack sensitivity remains a background concern for large thin-wall lost wax castings. Thermal gradients during cooling, shell restraint, and knockout stresses can all initiate cracks. Thickening thin walls and using controlled cooling can reduce thermal gradients. A well-designed gating system that promotes directional solidification can also reduce hot tearing. I did not observe a dominant cracking problem in the trials, but I would include crack inspection in any production protocol. For ZTi60, the combination of refractory elements and high strength makes crack avoidance an important process design objective.
I also reflected on the role of the spiral runner. The spiral diverging runner plate provides multiple radial paths. It helps distribute metal around the large circumference and reduces the chance of a single dead zone. In the initial simulation, the fill sequence was consistent, and the outer ring filled before the inner ring. This is desirable because the outer ring has a larger circumference and more surface area to cool. The inner ring is filled last through the struts and lap runner. For ZTi60, the inner ring was more prone to porosity unless thickened to 5.0 to 5.5 mm. I therefore see the spiral runner as necessary but not sufficient. It solves distribution, not local feeding.
The full circular inner ingate is also important. It feeds the inner mounting flange uniformly and reduces circumferential temperature differences. However, it can create a thermal hot spot if it is too large. In future designs, I would balance the inner ring ingate area against the local modulus. If the ingate is too small, the inner ring will not fill; if it is too large, it will form a hot spot and shrink. The optimal design is not simply larger or smaller; it is matched to the local wall thickness and the centrifugal pressure field.
For the outer mounting flange, the evenly distributed ingates performed well in terms of distribution, but the riser roots still required enlargement. This shows that distribution and feeding are different functions. A gate can deliver liquid to a region, but it may not provide enough volume to compensate for shrinkage. The riser must remain liquid long enough and contain enough volume to feed the hot spot. In ZTi60 lost wax casting, the riser must be designed with a higher feeding ratio than for ZTC4.
I used the following inequality as a conceptual check for riser design:
$$M_r \geq k M_c$$
where M_r is the riser modulus, M_c is the casting modulus at the hot spot, and k is a safety factor. For ZTi60, I found that k must be larger than the value used for ZTC4. This is consistent with the larger predicted porosity and the observed root shrinkage. The exact value depends on the local geometry, but the principle is clear: the riser must freeze later than the section it feeds. If the riser freezes too early, it cannot compensate shrinkage. If the riser is too large, it may reduce yield and create new hot spots. I therefore recommend a calibrated k value for ZTi60 based on trial data.
Similarly, the slit gates can be assessed by a local feeding distance criterion. If the distance from the slit gate to the last liquid region exceeds the effective feeding distance, the slit gate will not prevent porosity. This is why the slit gates only improved a small area around their attachment points. The correct design question is not how many slit gates are added, but whether the wall thickness and thermal gradient allow the gate to feed the entire region. In most 3 mm thin walls, the answer is no. In 4 mm or thicker walls, the answer improves.
I also considered the effect of section transitions. The split-ring lap edge is a transition from a thinner edge to a thicker ring. Such transitions concentrate stress and heat flow. In lost wax casting, they can also create flow hesitation. When metal reaches a thin edge, it loses heat rapidly and may freeze before the edge is filled. A smooth transition and a local process allowance can reduce this risk. In the optimized design, I would not leave a 1 mm edge in a large ZTi60 casing. I would cast it thicker and then machine or pickle it to the final geometry. This is a direct application of the process window identified in the trials.
I further note that the shell material and preheat interact with the alloy. A mullite ceramic shell at 200 °C is relatively cold compared with the 1,700 °C melt. The shell extracts heat rapidly. A higher shell preheat can improve fill, but it also reduces the thermal gradient needed for directional solidification. For a large lost wax casting, the shell temperature is not uniform; it varies with position and time. The outer regions may cool faster than the central regions. This non-uniformity can shift the last liquid location. In future work, I would use a shell preheat gradient or local insulation to balance cooling. However, the first-order solution for ZTi60 was to increase wall thickness and riser size, because those changes are more robust than small thermal adjustments.
The rotation speed also creates a pressure gradient that varies with radius. The pressure is lowest near the centre and highest at the outer rim. This helps the outer ring fill, but it does not necessarily help the inner ring. The inner ring is closer to the centre, so it receives less centrifugal pressure. That may explain why the inner ring required a larger thickness increase than the outer ring to eliminate porosity. The inner ring relies more on the full circular ingate and the struts. If the struts are thin, they freeze early and isolate the inner ring. Thickening the struts and adding a lap runner can improve the pressure path to the inner ring. In the trials, the inner ring needed 5.0 to 5.5 mm for a clean result, while the outer ring was clean at 4.0 to 4.5 mm. This radial difference is consistent with the centrifugal pressure field.
I also examined the implications for yield. A larger riser and thicker walls increase the poured weight. The initial trial used the mature ZTC4 gating system, but the optimized ZTi60 design requires more metal. The yield may be lower, but the quality is higher. In lost wax casting of high-value titanium casings, yield is not the only economic metric. Scrap rate, repair rate, and inspection cost matter. If the thickened process allowance reduces the scrap rate from a high level to a low level, the overall cost can be lower even with a lower yield. I therefore recommend evaluating the process by total cost of ownership rather than by yield alone.
I would also include a statistical process control plan. The critical variables are pouring temperature, rotation speed, shell preheat, shell thickness, wax pattern thickness, and pickling removal. If these vary, the thin-wall fill and porosity will vary. I would record the fill time, the visual underfill result, the fluorescent inspection result, the X-ray porosity level, and the final wall thickness. A simple capability index can be used:
$$C_{pk}=\min\left(\frac{USL-\mu}{3\sigma},\frac{\mu-LSL}{3\sigma}\right)$$
where USL and LSL are the upper and lower specification limits for wall thickness or porosity. A high C_pk indicates a stable process. For a large lost wax casting, the wall thickness distribution is especially important. If the process allowance is too small, underfill and porosity return. If it is too large, pickling time increases and dimensional control becomes difficult. The target is a robust window, not a single set point.
| Variable | Initial value | Recommended direction | Reason |
|---|---|---|---|
| Pouring temperature | 1,700 °C | Maintain or optimize slightly | Higher temperature improves fluidity but increases reaction risk |
| Pouring time | 7 s | Ensure final fill before freezing | ZTi60 reached 98% fill at 7.2548 s in simulation |
| Rotation speed | 140 r/min | Optimize with shell strength | Centrifugal pressure aids radial fill but can crack shell |
| Shell preheat | 200 °C | Consider controlled increase | Improves fill but reduces thermal gradient |
| Shell thickness | 20 mm | Keep sufficient for strength | Excessive thickness changes cooling |
| Critical wall thickness | 3 mm or less | Increase to 4 mm or more | Reduces porosity and underfill |
| Riser size | ZTC4-based | Increase | ZTi60 shrinkage tendency is higher |
| Pickling removal | 0.2 mm | Control by measurement | Removes surface porosity and excess wall |
I also considered the metallurgical quality of the thin-wall regions. The intergranular porosity observed in the 3 mm walls is a sign of incomplete feeding at the end of solidification. It is not the same as gas porosity, although gas can contribute. In ZTi60, the freezing range and the low fluidity mean that the interdendritic liquid channels close quickly. Once they close, any remaining shrinkage becomes porosity. A thicker wall keeps the channels open longer and allows feeding from the gate or riser. This is why the wall thickness effect was so strong. It is a direct consequence of the local thermal modulus. The 4 mm threshold is not arbitrary; it reflects the balance between cooling rate and feeding distance for this alloy and this shell condition.
The acid pickling step also has a metallurgical benefit. It removes the surface layer that may contain alpha case, contamination, or shallow porosity. In titanium lost wax casting, surface contamination can affect fatigue life and corrosion resistance. Controlled pickling can restore a clean surface. However, pickling must not remove too much material or create a dimpled surface. I therefore recommend a two-stage pickling process with intermediate dimensional inspection. The first stage removes the bulk excess, and the second stage removes the shallow defect layer. This is more controllable than a single long pickle.
For future large complex thin-walled casings, I would also consider additive manufacturing of the wax pattern or shell components. Lost wax casting still relies on a wax pattern, but the pattern can be produced with high dimensional accuracy. If the thin-wall allowance is designed into the pattern, the casting can be made thicker and then reduced. This is a practical way to combine the design freedom of additive manufacturing with the metallurgical quality of lost wax casting. I would not replace lost wax casting, but I would use digital design and simulation to define the allowance map.
I also note that the simulation grid size may need refinement near the 1 mm edge. A mesh size of 5 for the casting may not resolve the last-stage flow in a 1 mm feature. If I were to repeat the simulation, I would use a local mesh refinement of 0.2 to 0.5 mm at the split-ring lap and the thin passage edges. I would also calibrate the heat transfer coefficient with thermocouple data from the shell. Without calibration, the simulation may predict fill too optimistically. The actual underfill observed in the trial is a reminder that simulation is a guide, not a replacement for physical trials.
I also recommend using a fillability test in the same shell and alloy. A spiral or finger test can measure the actual fluidity of ZTi60 under the same pouring conditions. The fluidity length can be expressed as:
$$L_{fluid}=\frac{\rho v D^2}{32\eta}\left(\frac{\Delta P}{L}\right)$$
where the fluidity length depends on density, velocity, channel diameter, viscosity, and pressure gradient. Although this is a simplified expression, it captures the key idea: higher viscosity and lower pressure gradient reduce fluidity. ZTi60 has higher viscosity and lower superheat than ZTC4, so its fluidity length is shorter. A fillability test would help set the minimum wall thickness and the maximum flow distance for future designs.
I also note that the thin-wall porosity was not uniformly distributed. It was more severe in the flow passages and struts, and less severe near the ingates. This spatial pattern confirms that feeding distance is the controlling factor. Near the ingate, the local liquid supply is adequate. Away from the ingate, the wall freezes before feeding can occur. The slit gates improved only the immediate vicinity because they acted as local liquid reservoirs. To improve the entire wall, the wall itself must have a larger thermal modulus. This is why thickening is more effective than adding many small gates.
I would also consider the orientation of the casing during pouring. In a bottom-poured centrifugal lost wax casting, the casing axis is vertical and the sprue is central. The outer rim is at a larger radius, so it receives more centrifugal pressure. If the casing were oriented differently, the pressure field would change. However, the circular geometry is well suited to centrifugal casting. The main limitation is the inner ring, which is closer to the centre. A larger inner ingate or a local riser on the inner ring could help. In the trials, thickening the inner ring to 5.0 to 5.5 mm was necessary for a clean result. This is a direct design rule.
I also considered the effect of the six struts. They connect the inner and outer rings and create local thermal bridges. During solidification, the struts can act as feed paths or as hot spots, depending on their thickness and cooling. When they are too thin, they freeze early and isolate the inner ring. When they are thicker, they remain open longer and help feed the inner ring. This is why the strut thickness affected inner ring porosity. In future designs, I would treat the struts as part of the feeding system, not just as structural features. Their thickness should be chosen to balance structural requirements with feeding requirements.
The process window I identified can be summarized by three inequalities. First, the local wall thickness should be greater than or equal to the alloy-specific critical thickness:
$$t_{local}\geq t_{cr}(ZTi60)$$
Second, the riser modulus should exceed the casting hot-spot modulus by a sufficient factor:
$$M_r\geq k_{ZTi60}M_c$$
Third, the fill time should be shorter than the local freezing time:
$$t_{fill,local}\leq t_{freeze,local}$$
These inequalities are simple, but they capture the main lessons. For ZTi60, t_cr is above 1 mm and may be around 4 mm for robust thin-wall production. The factor k is larger than for ZTC4. The local fill time is longer than for ZTC4. The lost wax casting process must be designed around these realities.
| Inequality | Meaning | ZTi60 action |
|---|---|---|
| Local thickness greater than critical thickness | Thin walls must be thick enough to fill and feed | Increase thin walls to 4 mm or more |
| Riser modulus greater than hot-spot modulus | Riser must freeze after the hot spot | Increase riser volume and height |
| Local fill time shorter than local freeze time | Liquid must arrive before the section closes | Shorten flow path, add process allowance, optimize gating |
I also reviewed the inspection strategy. Visual inspection can detect gross underfill, but it cannot detect internal porosity. X-ray inspection can detect larger shrinkage cavities, but it may miss fine dispersed porosity. Fluorescent penetrant inspection can detect surface-connected porosity, especially after pickling. Metallography is necessary to confirm the nature and depth of porosity. Three-dimensional scanning is necessary to verify wall thickness after pickling. I used all of these methods in the trials, and the combination provided a clear picture. For production, I would use the same combination, with sampling at critical locations.
The thin-wall porosity was especially challenging because it was distributed over a large area. A single X-ray image may not reveal it clearly if the porosity is fine and diffuse. Fluorescent inspection after pickling was more effective for surface-connected porosity. If the porosity is deep and not surface-connected, pickling will not remove it. Therefore, the process must reduce deep porosity by thickening and feeding, and then remove the shallow residual layer by pickling. This two-step strategy is more robust than relying on inspection alone.
I also considered the possibility of hot isostatic pressing. Hot isostatic pressing can close internal porosity in titanium castings, but it is expensive and may not be suitable for large thin-walled casings with complex internal passages. It also adds a process step and can cause dimensional changes. In this study, I focused on preventing porosity through casting design rather than closing it after casting. The thickening and feeding approach is more direct and more economical for large lost wax castings. If porosity remains after process optimization, hot isostatic pressing could be considered as a final safeguard, but it should not be the primary solution.
I also considered the effect of oxygen and nitrogen pickup. ZTi60 is an alpha-stabilized alloy, and interstitial elements can affect ductility. The composition limits for O, N, and H are tight. In lost wax casting, the melt is usually vacuum arc melted or induction skull melted in a controlled atmosphere. The shell must be sufficiently inert. In this study, I did not observe an interstitial problem, but it remains a control point. A high oxygen content can reduce fluidity and increase crack sensitivity. Therefore, the melting and pouring practice must maintain clean conditions.
I also note that the shell composition can react with titanium. Mullite is relatively stable, but the binder and impurities can still react. A face coat with low reactivity is important. For large thin-wall castings, the shell must also have enough strength to resist centrifugal force. There is a trade-off between shell strength and thermal conductivity. A thicker shell is stronger but changes cooling. I used a 20 mm shell and 200 °C preheat as a baseline. Future work could optimize the shell system to improve fill without sacrificing strength.
In summary, the main findings from my ZTi60 lost wax casting study are clear. Computer simulation is useful for shrinkage and solidification sequence, but it did not capture the underfill at the 1 mm edge. ZTi60 has lower fluidity and a stronger shrinkage tendency than ZTC4. A 1 mm wall is below the practical critical thickness for this alloy in a large lost wax casting. Riser design must be enlarged relative to ZTC4. Thin walls below about 4 mm are prone to large-area porosity. Slit gates provide only local feeding. Thickening the wall to 4 mm or more reduces or eliminates porosity. Acid pickling removes shallow surface porosity and corrects excess wall thickness. The optimized lost wax casting route therefore combines enlarged risers, local feeding aids, process wall thickening, controlled pickling, and multiple inspection methods.
I would apply these findings to future large complex thin-walled ZTi60 casings by starting with a process allowance map. The thinnest design sections should not be cast directly. They should be thickened to a level that supports filling and feeding. The gating system should be designed for ZTi60, not copied from ZTC4. The risers should be larger. The inner ring and struts should receive special attention because of the centrifugal pressure gradient and isolated thermal mass. Simulation should be used with refined mesh at critical edges. Physical trials should include fluorescent inspection and metallography. Pickling should be controlled by dimensional measurement. This approach turns the difficult castability of ZTi60 into a manageable lost wax casting process.
I also conclude that the most robust solution for the 1 mm split-ring lap edge is not a gating change. It is a geometry change. The edge should be cast thicker and then reduced. This avoids the risk of underfill and cold shut. The same logic applies to other sub-4 mm thin walls. In a large thin-walled casing, every thin section competes for liquid metal. The alloy has a limited fluidity and a limited feeding window. The process must be designed with a margin. The margin is best provided by thickness, not by additional gates alone.
For the riser roots, the enlarged riser solved the shrinkage cavity. This confirms that the root defect was feeding-limited. For the thin-wall porosity, the solution was thickness-limited. For the underfill, the solution was also thickness-limited. The three defects had different local causes, but they shared a common theme: ZTi60 requires a larger process window than ZTC4. The lost wax casting process must be adjusted at the design stage, not only at the pouring stage.
I would also recommend a feedback loop between casting trials and design. If a thin wall is found to be below the critical thickness, the design should be revised to add a process allowance. If a riser root shrinks, the riser should be enlarged. If a strut shows porosity, its thickness should be increased or a local riser should be added. This feedback loop is essential for large complex thin-walled casings. It is not enough to optimize a single trial; the design rules must be captured and reused. The tables in this study are a first step toward such rules.
Finally, I consider the use of lost wax casting for ZTi60 to be feasible, but only with a disciplined process. The alloy is not a drop-in replacement for ZTC4 in a mature gating system. It requires a higher casting modulus, a larger feeding ratio, and a thicker minimum wall. When these requirements are met, the lost wax casting route can produce large complex thin-walled casings with acceptable metallurgical quality. The combination of process allowance, acid pickling, and inspection is particularly effective. It reduces weld repair, removes shallow porosity, and allows the final wall thickness to meet design requirements. This is the practical path I would follow for future production.
| Aspect | Initial strategy | Optimized strategy | Result |
|---|---|---|---|
| Split-ring lap edge | Cast 1 mm directly | Thicken, then machine or pickle | Avoids underfill |
| Riser | ZTC4-based size | Enlarged height and volume | Eliminates root shrinkage |
| Thin flow passages | 3 mm with slit gates | 4 mm or more with process allowance | Reduces or eliminates porosity |
| Struts | 3 mm, isolated | Thicker and locally fed | Shallower porosity |
| Inner ring | 3 mm | 5.0 to 5.5 mm for clean result | No porosity in trial |
| Outer ring | 3 mm | 4.0 to 4.5 mm | Essentially no porosity |
| Surface defects | Weld repair considered | Acid pickling about 0.2 mm | Reduced weld repair |
| Simulation | Predicted fill | Used mainly for shrinkage and sequence | Physical trial still required |
I therefore treat the optimized ZTi60 lost wax casting process as a validated starting point for large complex thin-walled casings. The process is not simply a modified ZTC4 route. It is a ZTi60-specific route built around measured fluidity, shrinkage, and feeding behaviour. The key numbers are 1 mm as an unreliable edge, 4 mm as a practical minimum for many thin walls, 5.0 to 5.5 mm for the most isolated inner ring sections, and about 0.2 mm of controlled pickling removal. The key qualitative rules are larger risers, local feeding only where effective, process allowance, and surface defect removal by pickling rather than welding. These rules can be transferred to similar high-temperature titanium casings and can reduce the risk of underfill, shrinkage, and thin-wall porosity in future lost wax casting production.