The relentless pursuit of performance and efficiency in aerospace, defense, and high-tech industries has consistently driven the demand for lightweight, high-strength components. Titanium alloys, renowned for their exceptional specific strength, superior corrosion resistance, and excellent high-temperature performance, have become indispensable in these fields. Consequently, the requirements for titanium alloy casting parts have evolved significantly, pushing towards more complex, integrated, and thin-walled geometries to achieve weight reduction and functional integration. The production of such sophisticated thin-walled titanium casting parts presents formidable technical challenges, primarily concerning dimensional accuracy, surface integrity, and the complete filling of intricate features.
Traditionally, investment casting and graphite molds with ceramic cores have been the go-to processes for producing precision titanium casting parts. While capable of delivering good surface finish, these methods are often hampered by extended production cycles, high core manufacturing costs, and significant difficulties in core removal post-casting, especially for parts with complex internal cavities. These limitations can adversely affect the overall cost-effectiveness and production scalability for thin-walled components. This research investigates an alternative approach: a compound mold casting process utilizing a machined graphite mold combined with a precision, coated sand core. This methodology aims to retain the dimensional stability of a rigid mold while leveraging the advantages of sand cores, such as lower cost, excellent collapsibility, and reduced thermal conductivity, to address the challenges associated with producing high-quality thin-walled titanium alloy shell casting parts.
Structural Analysis and Technical Specifications of the Target Casting Part
The subject of this study is a critical thin-walled shell component. As depicted in the engineering drawing, the casting part features a cylindrical geometry with an overall envelope dimension of approximately Ø216 mm by 183 mm in height. A defining characteristic of this component is its minimal wall thickness. The main body wall and several annular features are designed with a nominal thickness of only 4.3 mm. Furthermore, the internal contour is reinforced with an array of radial and axial ribs, each merely 3 mm thick, creating a complex, lightweight internal structure. The presence of these thin ribs and an irregular internal cavity classifies this component as a structurally complex, thin-walled casting part.
The alloy specified for this component is ZTA15, a near-alpha titanium alloy commonly used in cast aerospace components. Its chemical composition and post-HIP (Hot Isostatic Pressing) mechanical property requirements are strictly defined, as summarized in the tables below.
| Al | V | Mo | Zr | Fe ≤ | Si ≤ | C ≤ | N ≤ | H ≤ | O ≤ | Ti |
|---|---|---|---|---|---|---|---|---|---|---|
| 5.5-6.8 | 0.8-2.5 | 0.5-2.0 | 1.5-2.5 | 0.30 | 0.15 | 0.13 | 0.05 | 0.01 | 0.16 | Bal. |
| Condition | Tensile Strength (Rm), MPa ≥ | Yield Strength (Rp0.2), MPa ≥ | Elongation (A), % ≥ | Reduction of Area (Z), % ≥ |
|---|---|---|---|---|
| HIPed | 885 | 785 | 5 | 12 |
The technical specifications impose stringent demands on the final casting parts. The dimensional tolerances for all non-machined surfaces must conform to CT6 level as per GB/T 6414-1999. Most critically, the entire internal contour, which houses the thin ribs and complex geometry, is designated as an “as-cast” surface. This means its final dimensional profile and surface roughness must be achieved directly through the casting process, eliminating the possibility of remedial machining. This requirement directly challenges the capability of the molding and core-making process to replicate fine features with high fidelity and surface quality.
Process Selection and Foundry Challenge Analysis
The primary challenge in manufacturing these thin-walled titanium casting parts lies in simultaneously achieving three objectives: complete mold filling of the 3-4.3 mm sections, maintaining tight dimensional control on the internal “as-cast” surfaces, and ensuring a sound internal microstructure. Traditional graphite molding offers good thermal conductivity, promoting directional solidification, but machining complex internal cavities directly into graphite is difficult and expensive. Investment casting with ceramic cores can produce complex internals, but the core removal from the thin-walled titanium casting part is problematic and risks damaging the fragile ribs.
The proposed solution is a hybrid or compound mold approach. The external mold (cope and drag) is machined from high-purity, high-density graphite. This provides a rigid, dimensionally stable, and thermally conductive environment for the external surfaces of the casting part. For the complex internal cavity, a precision sand core is employed. The core is fabricated using a high-alumina sand aggregate bonded with a colloidal silica binder system. The key innovation lies in the post-processing of this sand core. After drying, it is coated with a refractory slurry based on yttria (Y2O3), a material known for its high thermodynamic stability and low reactivity with molten titanium. This coating serves multiple critical functions:
- It creates a smooth, inert barrier between the molten titanium and the sand core, preventing chemical reaction and sand burn-on, which is essential for achieving the required surface finish on the internal contours of the casting part.
- It effectively seals the porous sand surface, preventing metal penetration.
- It moderates the chilling effect of the graphite mold. The lower thermal conductivity of the coated sand core, relative to graphite, helps prevent premature freezing of the metal in thin sections, thereby aiding complete filling of the ribs and thin walls.
This graphite-sand composite mold strategy is designed to combine the strengths of both materials, optimizing the conditions for producing sound, precise thin-walled titanium casting parts.
For melting and pouring, the vacuum arc skull melting process coupled with centrifugal casting was selected. Centrifugal force is crucial for enhancing the feeding pressure and ensuring complete filling of the intricate thin sections in these casting parts. The process parameters for this stage must be meticulously controlled.
Engineering of the Casting Process for Thin-Walled Parts
Pattern Allowance and Dimensional Planning
Determining the correct shrinkage allowance is fundamental for achieving the final dimensional accuracy of the casting parts. The total observed shrinkage in a casting is a superposition of the alloy’s liquid-to-solid contraction, solid-state thermal contraction, and constraints imposed by the mold. For the ZTA15 alloy in a graphite-sand composite mold, the empirical shrinkage allowance was determined to be between 0.5% and 1.5%, varying based on local geometry and restraint. This can be conceptually represented by a simplified model accounting for major factors:
$$S_{\text{total}} = S_{\text{metal}} + S_{\text{mold}} + S_{\text{thermal}}$$
Where \(S_{\text{metal}}\) is the alloy’s intrinsic solidification shrinkage, \(S_{\text{mold}}\) is an empirical factor for mold wall movement/restraint (positive for expansion, negative for restraint), and \(S_{\text{thermal}}\) is the contraction from solidus temperature to room temperature. In practice, additional local “pattern allowances” or “purposeful oversizing” of 0.3 to 1.5 mm were applied to specific non-machined surfaces to ensure final dimensions after shot blasting and cleaning would meet the CT6 tolerance band.
Gating System Design and Numerical Simulation
A bottom-gating system integrated with the centrifugal process was designed. The system features a central downgate that splits into two horizontal runners on the centrifugal plane, which then connect to four ingates placed at the base of the thickest sections (bosses) of the casting part. This design aims to utilize centrifugal force to fill the mold cavity progressively from the bottom (thick sections) upwards (towards thin ribs), promoting favorable thermal gradients for feeding.
To validate this design and predict potential defects, a full-scale numerical simulation was conducted using ProCAST software. The simulation model incorporated the exact geometries of the casting part, the graphite mold, and the coated sand core. The material properties and mesh parameters were defined as follows:
| Component | Mesh Size (mm) | Material Assigned | Element Type | Thermal Conductivity (W/m·K) |
|---|---|---|---|---|
| Casting Part (ZTA15) | 1.0 (3 layers min in thickness) | ZTA15 Database | Deformable | Variable with Temperature |
| Graphite Mold | 10.0 | Graphite | Rigid | 100.0 |
| Coated Sand Core | 3.0 | Y2O3 / Sand Composite | Rigid | 2.5 |
The initial simulation of the solidification process for the casting part alone (without gating) identified isolated hot spots at the internal bosses, indicating a high risk of macro-porosity. The governing heat transfer during solidification is described by the Fourier equation:
$$\frac{\partial T}{\partial t} = \alpha \nabla^2 T$$
where \(T\) is temperature, \(t\) is time, and \(\alpha\) is the thermal diffusivity. The simulation solved this equation numerically across the entire system.
The final simulation of the complete process, including the designed gating system and centrifugal parameters (Tpour = 1750°C, Tmold = 300°C, centrifugal speed = 150 rpm, pour time = 4 s), yielded promising results. The filling sequence showed a controlled, bottom-up fill without excessive turbulence. The solidification analysis confirmed that the ingates positioned at the bosses effectively acted as feeders, eliminating the isolated hot spots and significantly reducing the predicted shrinkage porosity in these critical areas of the casting part. Any remaining micro-porosity was deemed acceptable and within the closure capability of the subsequent HIP treatment.
Mold and Core Manufacturing
Graphite Mold Fabrication: The graphite molds were CNC machined from high-purity, fine-grained graphite blocks. To ensure dimensional accuracy, the machined molds were 3D-scanned and compared against the digital CAD model, with iterative corrections made as necessary. A critical preparatory step was vacuum degassing of the graphite molds. Graphite is porous and can absorb moisture and gases, which may lead to gas defects in the reactive titanium casting parts. The molds underwent a thermal cycle under vacuum, heating to 800°C, holding, and slowly cooling to desorb contaminants.
Precision Sand Core Production: The core-making process was a sequence of precise steps:
- Mixing: High-alumina sand, colloidal silica binder, and a catalyst were mixed in a controlled ratio using an intensive mixer.
- Molding: The sand mixture was manually packed into a corebox (with a 2° draft angle) and compacted to ensure high density and sharp reproduction of the thin ribs.
- Curing & Drying: The cores were allowed to harden at room temperature for 4-6 hours before demolding, followed by 12-16 hours of air drying.
- Coating: The dried cores were dipped in a Y2O3-based slurry to form a uniform, smooth coating layer.
- Firing: The coated cores were subjected to a carefully controlled firing cycle, slowly ramping to 1050°C to burn out the organic binder from the core and sinter the Y2O3 coating, developing its strength and inertness. Slow cooling prevented cracking.
The finished cores were inspected for surface finish, cracks, and dimensional accuracy via 3D scanning before assembly into the graphite mold. The assembly used pins and square registers to ensure precise and rigid core location within the mold cavity.
Melting, Centrifugal Casting, and Post-Processing
The casting trials were conducted in a vacuum arc skull melting furnace with an integrated centrifugal casting system. Two molds were placed symmetrically on the centrifugal turntable for each melt to improve yield. The key process parameters are summarized below:
| Process Parameter | Value or Setting |
|---|---|
| Casting Method | Centrifugal Casting |
| Centrifugal Rotation Speed | 150 rpm |
| Melting Current | ~15 kA |
| Arc Voltage | 35-50 V |
| Chamber Pressure During Melt | ≤ 6 x 10-2 Pa |
| In-Mold Cooling Time | 180-240 min |
| Mold Preheating Temperature | 250-300 °C |
| Titanium Pouring Temperature | ~1750 °C |
After casting and cooling under vacuum, the molds were broken out. The sand cores demonstrated excellent collapsibility; the Y2O3 coating shell fractured easily, and the un-bonded sand readily fell away from the complex internal passages of the casting part without damaging the thin ribs. The raw casting parts then underwent standard post-processing: abrasive blasting to remove any surface oxide scale, followed by Hot Isostatic Pressing (HIP) at appropriate parameters (typically 920°C / 100 MPa / 2 hours) to close any residual micro-porosity and enhance mechanical properties. Finally, non-destructive testing (radiography) was performed to qualify the internal soundness of the casting parts.
Results and Validation of the Composite Mold Approach
The application of the graphite-sand composite mold process successfully produced thin-walled titanium alloy shell casting parts. Visual and tactile inspection confirmed that the thin walls (4.3 mm) and intricate internal ribs (3 mm) were completely filled and formed with sharp definition. No visible cold shuts, mistuns, or gross surface defects were present on the critical internal “as-cast” surfaces.
A comprehensive dimensional inspection was conducted using high-precision 3D scanning. The scan data of the actual casting part was compared against the nominal CAD model. The results demonstrated that the dimensional profile of all non-machined surfaces, particularly the challenging internal contours, was consistently maintained within a deviation band of ±0.2 mm. This comfortably meets the CT6 tolerance requirement for the overall dimensions of the casting part.
| Quality Attribute | Target Requirement | Achieved Result | Assessment |
|---|---|---|---|
| Dimensional Accuracy (Non-machined surfaces) | CT6 per GB/T 6414 | ±0.2 mm profile deviation | Conforms |
| Surface Roughness (Internal contour) | As-cast, no machining | 3.2 – 6.3 μm (Ra) | Acceptable for as-cast surface |
| Wall Thickness Integrity | Complete fill of 3-4.3 mm sections | All thin sections fully formed | Conforms |
| Internal Soundness (Post-HIP) | No unacceptable porosity per RT standards | Radiographic inspection passed | Conforms |
The surface roughness of the internal as-cast surfaces was evaluated using visual comparators and measured to be in the range of Ra 3.2 to 6.3 micrometers. This level of finish is considered excellent for an “as-cast” titanium surface, directly attributable to the smooth, inert Y2O3 coating on the sand core. Mechanical testing of coupons from the same heat confirmed that the properties of the casting parts met the specified requirements for ZTA15 after HIP, validating the soundness of the metallurgical structure.
Conclusion
This study successfully demonstrates the viability and advantages of a graphite mold combined with a precision coated sand core for manufacturing complex, thin-walled titanium alloy casting parts. The composite mold strategy effectively addresses the key limitations of traditional methods:
- It achieves the high dimensional accuracy and stability required for precision casting parts through the use of a rigid, machined graphite outer mold.
- It enables the production of complex internal geometries with excellent collapsibility via the sacrificial sand core, simplifying post-casting core removal significantly compared to ceramic cores.
- The application of an yttria-based refractory coating on the sand core is crucial. It prevents metal-core reaction, ensures a good surface finish on the casting part, and modulates cooling to aid in filling thin sections.
- The process proves to be cost-effective with a shorter lead time for core production compared to investment casting or complex ceramic core fabrication.
The final casting parts produced met all critical specifications: dimensional tolerances within CT6, acceptable as-cast surface roughness, complete filling of sub-5mm features, and satisfactory mechanical properties. This graphite-sand composite mold technology presents a robust and economical manufacturing route for high-integrity, thin-walled titanium casting parts, offering a compelling alternative for applications where cost, cycle time, and geometric complexity are driving factors. The principles established here can be extended to the development of other challenging titanium alloy casting parts with intricate internal features and stringent quality requirements.