The fabrication of components featuring deep, slender internal channels or cavities represents a significant challenge in the field of titanium alloy casting. Conventional core-making techniques, including the use of ceramic or soluble salt cores, often reach their limits when dealing with high aspect-ratio holes due to difficulties in core removal, potential core fracture during metal pouring, and insufficient dimensional accuracy. In our research, we explored and developed an integrated manufacturing strategy that combines the principles of inlay casting with the intricate capabilities of precision investment casting. This report details our first-person investigation into this hybrid process, focusing on the formation mechanism, interfacial characteristics, and practical feasibility of creating slender holes in ZTC4 titanium alloy castings.
The core concept of our approach involves pre-placing a consumable titanium tube (the inlay) within the wax assembly prior to shell building. During subsequent precision investment casting, the molten titanium alloy is poured around this tube. Under ideal conditions, the outer surface of the inlay tube metallurgically bonds with the casting body. After solidification, the embedded tube is chemically leached out, leaving behind a precise, hollow channel. This method circumvents the core removal problem entirely for certain geometries. Our study systematically examined the influence of the casting-to-inlay wall thickness ratio (inlay ratio) on the quality of the interfacial bond and the integrity of the final channel.
Experimental Methodology and Material Selection
The success of the inlay process within a precision investment casting framework hinges on several critical factors related to the inlay material itself. We selected commercially pure TA2 titanium tubes as the sacrificial inlay material based on the following rationale:
- Metallurgical Compatibility: TA2 offers excellent compatibility with the ZTC4 alloy, promoting the potential for diffusion and metallurgical bonding rather than just mechanical interlocking.
- Thermal Stability: Its high melting point ensures it retains structural integrity and does not collapse or melt completely during the brief exposure to the superheated ZTC4 melt.
- Chemical Selectivity: The TA2 tube can be subsequently removed via acid pickling, a standard post-casting treatment for titanium, without damaging the ZTC4 casting body.
Our experimental design focused on cylindrical specimens with a nominal length of 160 mm. The primary variable was the “inlay ratio,” defined as the ratio of the casting body wall thickness to the thickness of the TA2 inlay tube. We prepared tubes with different wall thicknesses via chemical thinning to explore the limits of this ratio. The target internal diameter of the channel was fixed at Φ6 mm. The specific experimental matrix is summarized in the table below.
| Specimen ID | Casting Body Wall Thickness (mm) | TA2 Inlay Tube Wall Thickness (mm) | Inlay Ratio (Body:Inlay) | Outer Diameter of Casting (mm) |
|---|---|---|---|---|
| XZTA-2 | 5 | 1.01 | 5:1 | 16 |
| XZTA-4 | 5 | 0.64 | 8:1 | 16 |
| XZTA-6 | 10 | 1.02 | 10:1 | 26 |
| XZTA-9 | 10 | 0.51 | 20:1 | 26 |
The integrated precision investment casting process was executed as follows. First, the chemically thinned TA2 tubes were 3D-scanned. Their digital models were then incorporated into the main casting CAD model with a deliberate gap allowance (approximately 0.1 mm) for thermal expansion. The tubes were physically prepared by sealing both ends to prevent shell material intrusion. These prepared inlays were then carefully assembled and fixed within the wax pattern cluster. The standard shell-building process for titanium precision investment casting was followed, employing refractory stucco and binders. The shell was de-waxed and fired at high temperature. Melting and pouring of ZTC4 alloy were conducted in a vacuum arc skull furnace, with a pouring current maintained between 19-22 kA. After casting and shell removal, the specimens were subjected to radiographic inspection (X-ray), macroscopic examination, and microstructural analysis using optical microscopy (OM) and scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS).
Results: Macroscopic Integrity and Interfacial Analysis
Macroscopic inspection confirmed that all cast specimens successfully formed a continuous, slender internal channel corresponding to the position of the TA2 inlay. However, the quality of the interface between the casting body and the inlay varied dramatically with the inlay ratio. Radiographic images clearly revealed a dark, distinct line tracing the inlay contour for specimens with lower ratios (XZTA-2, 5:1). This line indicated a lack of fusion or a gap at the interface. As the inlay ratio increased, this line became progressively finer and more blurred. For specimen XZTA-9 with a ratio of 20:1, the distinct linear indication vanished entirely in the radiograph, suggesting a much more intimate contact or fusion.
Cross-sectional analysis provided definitive evidence. For inlay ratios of 5:1 and 8:1, a clear physical gap was visible between the TA2 tube and the ZTC4 matrix under low magnification. The tube remained largely distinct. In contrast, the cross-section of the XZTA-9 (20:1) specimen showed no visible tube remnant; instead, the channel wall was smooth, with only localized areas showing evidence of the tube having been partially melted through. This established that an inlay ratio of 20:1, corresponding to a tube wall thickness of 0.51 mm, approached a critical threshold where the inlay could be partially consumed by the melt while still maintaining the channel definition.
Microstructural examination of the interface provided profound insights. For the low-ratio specimen (XZTA-2, 5:1), the interface was predominantly characterized by mechanical interlocking. A visible, often discontinuous gap or a series of “saw-tooth” like voids separated the two materials. The ZTC4 matrix exhibited its typical lamellar α+β structure, while the TA2 inlay showed an equiaxed structure. No significant diffusion zone was observed.
The scenario changed remarkably for the high-ratio specimen (XZTA-9, 20:1). At the interface, a distinct, fine, and continuous network-like layer, approximately 10-20 µm wide, was formed. This layer represented a true metallurgical fusion zone where diffusion of elements between the TA2 and ZTC4 had occurred. This can be conceptually related to Fick’s law of diffusion. The flux of atoms across the interface is proportional to the concentration gradient and the diffusion coefficient, which is highly temperature-dependent:
$$ J = -D \frac{\partial C}{\partial x} $$
where \( J \) is the diffusion flux, \( D \) is the diffusion coefficient, \( C \) is the concentration, and \( x \) is the position. The thinner inlay wall in the 20:1 specimen heats up much more rapidly, achieving a temperature closer to that of the melt at the interface for a longer effective time, thereby significantly increasing \( D \) and promoting atomic interdiffusion.
EDS analysis on the low-ratio samples revealed a critical issue: the presence of oxygen enrichment on the TA2 tube surface adjacent to the gap. This indicated that during the high-temperature shell firing stage prior to pouring, the surface of the TA2 inlay had oxidized. This oxide layer acted as a barrier, preventing wetting and diffusion between the molten ZTC4 and the underlying titanium, leading to the non-fused interfaces observed. The formation of this layer can be described by a parabolic growth law common for oxide scales:
$$ x^2 = k_p t $$
where \( x \) is the oxide thickness, \( k_p \) is the parabolic rate constant, and \( t \) is the time at temperature. This underscores the importance of process control during shell firing for inlay components.
Process Optimization: Leaching and Theoretical Considerations
The final, crucial step in this integrated precision investment casting process is the removal of the bonded inlay to reveal the free-standing channel. For specimens with a metallurgical fusion layer (like XZTA-9), this involves chemically leaching the remnant TA2 material. We employed a standard pickling solution for titanium: a mixture of HNO₃ (150-550 mL/L) and HF (20-100 mL/L). The objective was to selectively remove the inlay material and the fusion zone without attacking the ZTC4 casting body excessively.
Post-leaching analysis of the XZTA-9 channel interior showed a uniform, smooth surface. Microscopic examination confirmed the complete removal of the TA2-based fusion layer, with no residual gaps or embedded foreign phases. The ZTC4 substrate was intact, demonstrating that the pickling process could effectively resolve the inlay without compromising the casting. The leaching process kinetics can be influenced by the thickness and composition of the interface. The removal rate \( R \) in a well-agitated acid bath can be approximated by a relationship considering acid concentration and temperature:
$$ R = A e^{(-E_a / RT)} [HF]^m [HNO_3]^n $$
where \( A \) is a pre-exponential factor, \( E_a \) is the activation energy, \( R \) is the gas constant, \( T \) is temperature, and \( m, n \) are reaction orders.
The thermal dynamics during pouring are paramount. The thinner inlay in the high-ratio specimen presents less thermal mass. We can model the heat transfer using the concept of the Biot number \( Bi \), which compares internal conductive resistance to external convective resistance:
$$ Bi = \frac{h L_c}{k} $$
where \( h \) is the heat transfer coefficient from the melt, \( L_c \) is the characteristic length (tube wall thickness), and \( k \) is the thermal conductivity of TA2. For a thin wall (small \( L_c \)), \( Bi \) is small, indicating the tube quickly approaches the temperature of the surrounding melt, facilitating surface melting and diffusion rather than acting as a strong heat sink that promotes rapid solidification and gap formation at the interface.
The following table summarizes the key findings and the governing mechanisms for the different inlay conditions:
| Inlay Ratio | Interfacial Character | Primary Bonding Mechanism | Governing Physical Principle | Post-Casting Result |
|---|---|---|---|---|
| Low (5:1, 8:1) | Discontinuous gap/oxide layer | Mechanical interlocking (poor) | High Biot number, oxide barrier kinetics | Distinct inlay, removable but leaves imperfect channel. |
| High (20:1) | Continuous metallurgical fusion zone | Atomic diffusion & melting | Low Biot number, enhanced Fickian diffusion | Inlay integrated, fully leachable to form smooth channel. |
Conclusion and Perspective
Our investigation demonstrates that the integration of inlay casting techniques with advanced precision investment casting presents a viable and innovative pathway for manufacturing titanium alloy components with complex, slender internal passages. The inlay ratio is a critical design and process parameter. We found that a high ratio (approximately 20:1 in this geometry) is essential to promote sufficient heat transfer from the molten metal to the inlay tube, enabling surface melting and the formation of a thin, continuous metallurgical fusion zone. This zone, while providing initial integrity, remains selectively dissolvable using standard titanium pickling chemistry, resulting in a precise final channel.
The challenges identified, particularly the oxidation of the inlay during shell firing, highlight areas for further process refinement. Potential solutions include the application of inert or vacuum firing atmospheres, or the use of protective coatings on the inlay prior to shell building. This hybrid precision investment casting approach expands the design space for titanium castings, allowing engineers to incorporate internal features that were previously considered too difficult or expensive to achieve with conventional core technology. It stands as a testament to the potential of combining traditional foundry concepts with the meticulous control offered by modern precision investment casting to solve specific manufacturing challenges in high-performance alloys.