In the aerospace industry, the demand for high-performance metal components has driven continuous innovation in manufacturing technologies and materials. Titanium alloys are widely utilized in critical load-bearing structures due to their low density, high specific strength, excellent corrosion resistance, and superior high-temperature performance. However, traditional machining of titanium components faces challenges such as high cutting forces and rapid tool wear, leading to inefficiencies and elevated costs. Investment casting, also known as precision casting, has emerged as a key solution for producing large, thin-walled, and complex titanium alloy parts with integrated structures, significantly reducing manufacturing expenses while maintaining dimensional accuracy. This paper reviews recent advancements in titanium alloy investment casting technology, materials development, and product applications in the aerospace sector, focusing on process optimization, high-temperature alloy systems, and the expansion of aerospace casting parts.
The investment casting process for titanium alloys involves several critical steps: pattern making, shell building, melting and pouring, and post-processing. Each stage requires precise control to ensure the quality of the final castings aerospace components. A typical flow chart of the titanium alloy investment casting process illustrates the sequence from wax pattern creation to final inspection. Key challenges include controlling shape and dimensions, minimizing defects, and managing the high chemical reactivity of titanium at elevated temperatures. Over decades, investment casting has evolved to support the production of over 90% of titanium alloy castings in the aerospace industry, enabling the manufacture of complex geometries such as engine casings, frames, and ducting systems.
Pattern making is the initial step in investment casting, where the quality of wax patterns directly influences subsequent processes and final part integrity. Wax materials, typically based on paraffin or resin systems, are enhanced with additives to improve environmental resistance, strength, and dimensional stability. Traditional methods involve injecting molten wax into metal dies, but this approach is time-consuming and inflexible for rapid prototyping. The integration of additive manufacturing (AM) technologies, such as stereolithography (SLA), selective laser sintering (SLS), three-dimensional printing (3DP), and laminated object manufacturing (LOM), has revolutionized pattern production. These AM techniques enable rapid fabrication of complex wax patterns, reducing lead times and costs for new product development. For instance, SLA offers high surface finish and strength, suitable for large castings, while resin droplet jetting provides fast scanning speeds and precision for small to medium-sized aerospace casting parts. The dimensional variations in the pattern-tooling-casting system can be modeled to predict and control tolerances, ensuring that the final castings aerospace meet stringent specifications.
Shell building is a decisive phase in determining casting quality, as the ceramic shell interacts directly with the molten titanium alloy. Face coat materials must exhibit high refractoriness, good thermal shock resistance, and controlled surface roughness to prevent defects like microcracks and cold shuts. Commonly used refractory oxides include ZrO₂ and Y₂O₃, which offer low thermal expansion and minimal reactivity with titanium melts. Compared to graphite or metal molds, ceramic shells provide lower thermal conductivity, reducing chilling effects and associated defects. However, the shell must also possess sufficient collapsibility to minimize stress on the casting during solidification. Recent developments focus on optimizing shell compositions and processes, such as using yttria-based face coats and composite shells incorporating metal substrates. For example, ceramic-metal composite molds produced via plasma spraying have demonstrated improved surface quality and reduced internal defects in ZTA15 alloy castings. The table below summarizes the advantages and disadvantages of different shell materials for titanium alloy investment casting.
| Mold Type | Face Coat Material | Advantages | Disadvantages |
|---|---|---|---|
| Metal Mold | Tungsten Powder | High melting point, inert, reusable | High thermal conductivity, requires solvent demolding, environmental concerns |
| Graphite Mold | Graphite | High refractoriness, low thermal expansion, low cost | Surface carburization, poor chemical inertness, high shrinkage |
| Ceramic Mold | ZrO₂ | Low thermal expansion, simple process, cost-effective | Thick contamination layer, prone to surface pores |
| Ceramic Mold | Y₂O₃ | High temperature resistance, high strength, minimal interface reaction | Complex process, high material cost, poor thermal shock resistance |
Melting and pouring processes have advanced with the evolution of vacuum melting equipment, including vacuum consumable electrode arc skull furnaces, plasma arc cold hearth furnaces, electron beam cold hearth furnaces, and induction skull melting furnaces. The vacuum consumable arc skull furnace is the most widely used for aerospace casting parts, integrating melting and pouring to minimize contamination. Induction skull melting, in particular, offers advantages such as uniform composition, high purity, and the ability to melt various feedstock forms without electrode preparation. Large-scale induction melting systems with capacities up to 50 kg enable the production of castings with maximum dimensions of Φ600 mm. Pouring techniques, primarily gravity and centrifugal casting, are employed based on alloy characteristics and part geometry. Centrifugal casting enhances mold filling for thin-walled complex structures, while gravity casting is suitable for larger components. Process simulation software, such as ProCAST and MagmaSoft, aids in optimizing gating systems and predicting defects like shrinkage and misruns. However, the accuracy of simulations depends on reliable material property data, which remains a challenge for new titanium alloys. The relationship between pouring parameters and casting quality can be expressed using fluidity models, such as the following equation for mold filling:
$$F = k \cdot \sqrt{\frac{\Delta T \cdot \rho \cdot g \cdot h}{\mu}}$$
where \(F\) is the fluidity length, \(k\) is a constant, \(\Delta T\) is the superheat, \(\rho\) is the density, \(g\) is gravity, \(h\) is the head pressure, and \(\mu\) is the viscosity. Such models help in designing processes for high-integrity aerospace casting parts.
Post-processing treatments, including heat treatment, hot isostatic pressing (HIP), and welding repair, are essential for achieving the desired mechanical properties and defect-free castings aerospace. Heat treatment relieves residual stresses and optimizes microstructure; for α+β titanium alloys like ZTC4, annealing below the β transus temperature prevents grain coarsening. HIP, conducted at temperatures around 30°C below the β transus and pressures of 110–120 MPa, eliminates internal pores and improves density, though it may slightly reduce strength while enhancing ductility. The effect of HIP on pore closure and mechanical properties can be described by the following creep-based equation:
$$\dot{\varepsilon} = A \cdot \sigma^n \cdot \exp\left(-\frac{Q}{RT}\right)$$
where \(\dot{\varepsilon}\) is the strain rate, \(A\) is a constant, \(\sigma\) is the stress, \(n\) is the stress exponent, \(Q\) is the activation energy, \(R\) is the gas constant, and \(T\) is the temperature. Welding repairs, performed using inert gas tungsten arc welding or laser welding, address surface defects but require careful control of atmosphere and composition to avoid embrittlement. Elements like Sn and Zr improve weldability, while Nb and Ta may promote cracking. The table below compares the effects of alloying elements on welding performance.
| Alloying Element | Source | Effect on Welding |
|---|---|---|
| C | Atmosphere, surface contamination, graphite | Cracking in weld zone |
| O | Atmosphere, surface contamination, mold | Reduced ductility in weld zone |
| N | Atmosphere | Porosity and cracking |
| H | Atmosphere | Hydrogen embrittlement |
| Sn/Zr | Alloy component | Improved weldability |
| Nb/Ta | Alloy component | Susceptibility to cracking |
The development of casting titanium alloys has progressed in tandem with process innovations. Conventional alloys, such as ZTC4 (Ti-6Al-4V) and ZTA15 (near-α alloy), dominate aerospace applications, accounting for over 80% of usage. These alloys offer a balance of strength, ductility, and castability. Recent research focuses on optimizing composition to enhance both casting performance and mechanical properties. For instance, controlling interstitial elements like oxygen, nitrogen, and hydrogen within specific ranges (e.g., O: 0.10–0.15 wt%, N: 0.01–0.02 wt%, H: 0.010–0.012 wt%) improves the strength-ductility balance of ZTC4. Additions of Fe (<0.24 wt%) can enhance strength without compromising fracture toughness in low-interstitial variants. High-temperature titanium alloys, such as ZTi55 and ZTi60, have been developed for service temperatures up to 600°C, featuring improved creep resistance and thermal stability. These alloys are based on Ti-Al-Sn-Zr-Mo-Si systems and are used in components like fuel tank frames and engine casings. The table below lists typical high-temperature casting titanium alloys and their mechanical properties.
| Alloy Grade | Alloy Type | Tensile Strength σ_b (MPa) | Yield Strength σ_0.2 (MPa) | Elongation δ (%) | Service Temperature (°C) |
|---|---|---|---|---|---|
| ZTC4 | α+β | 890 | 820 | 5 | 400 |
| ZTA15 | Near α | 882 | 784 | 5 | 500 |
| ZTi55 | Near α | 950 | 930 | 8 | 550 |
| ZTi60 | Near α | 1000 | 905 | 10 | 600 |
| Ti1100 | Near α | 938 | 848 | 11 | 600 |
For applications above 600°C, advanced titanium-based alloys such as TiAl intermetallics and Ti₂AlNb alloys are being explored. TiAl alloys, with densities around 4 g/cm³, exhibit excellent oxidation resistance and creep properties at 600–1000°C, making them potential replacements for nickel-based superalloys in low-pressure turbine blades and nozzle components. Common grades include Ti-48Al-2Cr-2Nb (Ti-4822) and Ti-45Al-2Mn-2Nb-1B (45XD). However, TiAl alloys pose challenges due to high reactivity, poor fluidity, and significant solidification shrinkage, necessitating specialized shell materials like Y₂O₃ and optimized pouring parameters. The linear shrinkage of Ti-4822, for example, ranges from 2.09% to 3.11%, approximately double that of ZTC4, requiring precise dimensional control. Ti₂AlNb alloys offer superior high-temperature strength and creep resistance up to 800°C but suffer from coarse microstructures and poor castability. Hot isostatic pressing can mitigate defects, but large-scale production of Ti₂AlNb castings aerospace remains limited. The high-temperature mechanical properties of these alloys are summarized below.
| High-Temperature Alloy | Tensile Strength at 650°C (MPa) | Elongation at 650°C (%) | Stress Rupture Life at 650°C (h) |
|---|---|---|---|
| ZTi65 | ~600 | ~8 | 100 (at 240 MPa) |
| Cast TiAl Alloy | 700–800 | 2–10 | — |
| Ti₂AlNb | >880 | ~10 | ~100 (at 360 MPa) |
Titanium alloy investment castings have become integral to aerospace vehicles, contributing to weight reduction, structural integrity, and cost efficiency. In aircraft, cast components are used in non-critical static parts like engine covers and ducts, as well as critical load-bearing structures such as fan hubs, mounting frames, and wing attachments. For instance, the F-22 fighter incorporates approximately 76 titanium castings, including倾斜隔板 and rudder hinges. In spacecraft, castings serve in missile fins, rocket casings, and satellite camera mounts. The shift to integral casting reduces assembly complexity and enhances performance. TiAl alloy castings have also gained traction; for example, Ti-4822 low-pressure turbine blades in the GEnx engine reduce weight by 180 kg per engine. Chinese research institutions have developed TiAl components like diffusers and turbochargers, though large-scale applications are still emerging. The expansion of aerospace casting parts underscores the importance of continuous improvement in casting technologies to meet evolving demands.
Despite progress, several challenges hinder the wider adoption of titanium alloy investment casting in aerospace. First, the application of new casting alloys, such as ZTi65 and TiAl alloys, is slow due to lengthy development cycles, high costs, and limited data on casting performance and mechanical properties. Second, production costs remain elevated because of expensive shell materials, energy-intensive vacuum melting, and low yield rates for complex parts. Third, simulation technologies lag behind, with domestic software suffering from inadequate material databases and inaccurate process predictions. Fourth, competition from titanium additive manufacturing is intensifying as powder costs decrease and capacity increases. To address these issues, future efforts should focus on establishing comprehensive databases linking composition, casting properties, and mechanical performance; leveraging digital twin technology for cost reduction and process optimization; and advancing domestic simulation software through collaboration between industry and academia. By overcoming these barriers, titanium alloy investment casting can further solidify its role in producing high-quality aerospace casting parts, supporting the growth of the global aerospace industry.
In conclusion, titanium alloy investment casting is a vital manufacturing route for aerospace components, offering a blend of performance, precision, and cost-effectiveness. Advances in process techniques, material systems, and post-treatment methods have enabled the production of complex, high-integrity castings aerospace. However, sustained innovation is needed to enhance the adoption of new alloys, reduce costs, and improve simulation capabilities. With the aerospace market poised for significant growth, investment in research and development will be crucial to maintaining competitiveness and meeting the escalating demands for advanced titanium castings in next-generation aerospace vehicles.