In the pursuit of advanced aerospace propulsion systems, materials capable of withstanding extreme temperatures while minimizing weight are paramount. TiAl-based intermetallic alloys, particularly the Ti-48Al-2Cr-2Nb composition, have emerged as a leading candidate due to their high specific strength and modulus at elevated temperatures (600–900 °C). However, their inherent brittleness and complex solidification behavior present significant challenges for net-shape manufacturing. Among various forming techniques, investment casting stands out as a critical and promising method for producing complex, thin-walled components with the required dimensional accuracy. The central obstacle to its widespread adoption is the effective control of metal casting defects, such as shrinkage porosity, gas porosity, and misruns, which are exacerbated by the alloy’s poor fluidity and significant solidification contraction. This study systematically investigates the influence of component geometry—represented by eight characteristic structural features—and a key shell process parameter, the solid-liquid ratio of the face coat, on the formability and defect propensity of TiAl alloy castings. Our goal is to establish a foundational understanding that supports the structural design optimization and precision casting process development for complex TiAl components.
The experimental methodology involved the gravity casting of Ti-48Al-2Cr-2Nb alloy into preheated ceramic molds. The alloy was prepared from high-purity raw materials and melted in a vacuum induction skull furnace, followed by pouring at 1600°C into shells preheated to 500°C. Crucially, we designed a test coupon incorporating eight fundamental structural elements prevalent in aerospace components: Plate, Variable Section, Sharp Corner, Rib, Transition Arc, Hole, Ring, and Curved Surface. Each element was fabricated with varying dimensional parameters to assess their individual effects. Furthermore, to isolate the influence of the shell’s thermophysical properties, molds were prepared with three different face coat solid-liquid ratios (by volume): 1.8:1, 2:1, and 2.4:1. This ratio directly affects the shell’s permeability and thermal conductivity, which are vital for gas escape and solidification rate control.
The characterization of the castings focused on quantifiable metrics related to metal casting defect formation. We measured the fill height or fill ratio to assess fluidity limitations. Linear shrinkage was measured in multiple directions. The distribution and severity of internal defects were evaluated using X-ray radiography to determine the projected area ratio of shrinkage and gas porosity, and via metallographic examination of cross-sections to calculate the precise sectional shrinkage porosity ratio. The following sections detail our findings on how geometric factors and shell properties independently and interactively influence these critical outcomes.
Influence of Characteristic Structure on Fill Ability and Shrinkage
The ability of the molten TiAl alloy to completely fill the mold cavity is the first prerequisite for a sound casting. Our results demonstrate a clear correlation between geometric parameters and fill capability. For plate structures, fill height increased significantly with wall thickness. Complete filling was essentially achieved only when the plate thickness was 6 mm or greater. This is attributed to the slower cooling rate in thicker sections, which provides a longer time window for liquid metal flow and feeding before solidification seals the flow path. Similarly, for variable section structures (wedges), the fill height increased with the thickness at the base, or more precisely, with the thickness change rate. A base thickness of 8 mm (corresponding to a change rate ≥ 0.075 for a 2 mm top) was necessary for complete fill. This geometry promotes directional solidification from the thin top to the thick base, aiding metal flow. For sharp corners, the fill ratio was highly sensitive to the included angle. While a 90° corner filled completely, smaller angles (60°, 30°) exhibited progressively lower fill ratios due to the rapid cooling and large thermal gradient at the acute apex, which caused premature solidification and blocked the liquid metal.
| Structure Type | Designation | Key Dimension (mm or °) | Fill Height (mm) or Fill Ratio (%) |
|---|---|---|---|
| Plate (Pl) | Pl2 | Thickness = 2 | 35.8 mm (55.3%) |
| Pl4 | Thickness = 4 | 112.9 mm (94.3%) | |
| Pl6 | Thickness = 6 | 118.1 mm (98.5%) | |
| Variable Section (Va) | Va6 | Base Thickness = 6 | 72.6 mm (90.8%) |
| Va8 | Base Thickness = 8 | 78.6 mm (98.4%) | |
| Va10 | Base Thickness = 10 | 78.6 mm (98.4%) | |
| Sharp Corner (Co) | Co30 | Angle = 30° | 83.7% |
| Co60 | Angle = 60° | 96.2% | |
| Co90 | Angle = 90° | 99.9% |
The linear shrinkage of the Ti-48Al-2Cr-2Nb alloy was found to be in the range of 1.8% to 3.3%, which is notably higher than that of conventional titanium alloys like Ti-6Al-4V. This underscores the greater challenge in managing volumetric contraction during the casting of TiAl alloys. Interestingly, the type of characteristic structure and its specific dimensions had a relatively minor influence on the magnitude of linear shrinkage. Measurements in the length, height, and thickness directions were generally consistent for a given casting. A subtle trend indicated that shrinkage in the thickness direction tended to increase slightly with greater wall thickness, likely due to reduced restraint against contraction in the bulkier section. For hole features, the shrinkage of the hole diameter itself was around 1%, decreasing marginally with larger hole sizes, as the increased rigidity of the surrounding material imposes more restraint.
| Structure Type | Designation | Shrinkage in Length (%) | Shrinkage in Thickness (%) | Shrinkage in Hole Diameter (%) |
|---|---|---|---|---|
| Plate | Pl4 | 1.93 | 3.16 | – |
| Pl6 | 1.87 | 3.68 | – | |
| Sharp Corner | Co90 | 3.28 | 4.60 | – |
| Hole | Ho1005 | 2.28 | 1.72 | 1.01 |
| Ho1505 | 2.33 | 1.92 | 0.74 |
Influence of Characteristic Structure on Shrinkage Porosity Defects
The formation of shrinkage porosity, a critical metal casting defect, was profoundly affected by the geometry of the characteristic structure. We evaluated this using both the projected area ratio from X-rays and the precise cross-sectional area ratio from microscopy.
1. Wall Thickness Effects (Plate, Ring, Hole): For plate, ring, and hole structures, a consistent trend was observed: the projected area of porosity decreased with increasing wall thickness, while the local cross-sectional shrinkage porosity ratio increased. Thinner walls solidify rapidly, leaving distributed micro-porosity along their height. Thicker walls solidify slowly, creating larger thermal masses and hot spots at their centers where feeding is most difficult, leading to more concentrated but severe porosity. In hole structures, porosity predominantly accumulated in the region above the hole, as the narrow ligament at the hole’s equator solidified quickly, isolating and preventing adequate feeding to the upper bulk section.
2. Thickness Transition Effects (Variable Section): The variable section geometry proved highly beneficial in mitigating metal casting defects. Its tapered form promotes directional solidification. The cross-sectional shrinkage porosity ratio for variable sections was only 5-10% of that found in a plain plate of equivalent cross-sectional area at the same height. The porosity ratio decreased with an increasing thickness change rate (steeper taper), which enhances the thermal gradient for feeding.
3. Angle and Junction Effects (Sharp Corner, Rib, Transition Arc): These features often act as stress concentrators and hot spots. For sharp corners, both the porosity distribution area and ratio increased with the included angle. Larger angles contain a greater volume of material that behaves like a thick plate, prone to internal porosity. Smaller angles, functioning more like a tapered section, showed better feeding. Ribs exhibited a complex relationship: an optimal rib thickness (4 mm in this study) minimized porosity. Thinner ribs cooled too quickly, creating distributed defects, while thicker ribs created a severe hot spot at the junction with the base plate, leading to a high localized porosity ratio. For transition arcs (fillets), a very small radius (2 mm) resulted in the lowest porosity, as it minimized the hot spot. Both intermediate (4 mm) and large (6 mm) radii created significant hot spots that were difficult to feed, with the intermediate radius performing worst, likely due to an unfavorable balance between hot spot size and feeding path restriction.
4. Feature Size Effects (Hole Diameter, Surface Curvature): For holes, a larger diameter worsened both the porosity area and ratio. A larger hole creates a longer, more restrictive feeding path around its perimeter, exacerbating the isolation of the upper section. For curved surfaces, a smaller curvature (larger radius, flatter surface) led to a lower projected porosity area, as it provided a more direct thermal gradient aligned with the filling direction, aiding feeding to the top of the casting.
The following table summarizes the average sectional shrinkage porosity ratio ranges for the different characteristic structures, highlighting which geometries are most and least prone to this type of metal casting defect.
| Characteristic Structure | Average Sectional Shrinkage Porosity Ratio Range | Relative Tendency for Shrinkage Porosity |
|---|---|---|
| Variable Section | 0.012% – 0.018% | Lowest |
| Hole | 0.04% – 0.20% | Low |
| Transition Arc | 0.03% – 0.27% | Medium |
| Rib | 0.03% – 0.28% | Medium |
| Plate | 0.17% – 0.27% | Medium-High |
| Sharp Corner | 0.21% – 0.46% | Highest |
Influence of Shell Solid-Liquid Ratio on Gas Porosity and Shrinkage Defects
The properties of the investment shell, governed by the face coat solid-liquid ratio, significantly impacted the formation of both gas-related and shrinkage-related metal casting defects.
Effect on Gas Porosity: The shell’s permeability, which affects its ability to allow gases to escape during pouring and solidification, is strongly influenced by this ratio. Our results were clear: castings produced in shells with a solid-liquid ratio of 2.4:1 consistently exhibited the highest gas porosity distribution area. The area of gas porosity in castings from 1.8:1 and 2:1 shells was only 10% to 80% of that in the 2.4:1 shells. The low-permeability 2.4:1 shell trapped evolved and entrapped gases within the mold cavity. Furthermore, gas porosity tended to increase with greater casting wall thickness and with flatter surface curvatures, as both conditions lead to slower cooling, allowing more time for gas nucleation and growth, but in a less favorable geometry for buoyant escape.
The gas porosity distribution area ratio $$ \Psi_{gas} $$ for a casting can be conceptually related to the shell’s permeability $$ K $$ and the local solidification time $$ t_f $$. While a full model is complex, the trend suggests an inverse relationship with permeability and a direct relationship with solidification time for gas nucleation sites:
$$\Psi_{gas} \propto \frac{f(t_f)}{K}$$
Where a lower $$ K $$ (2.4:1 shell) leads to a higher $$ \Psi_{gas} $$.
| Shell Solid-Liquid Ratio | Typical Gas Porosity Distribution Area Ratio Relative to 2.4:1 Shell | Primary Cause |
|---|---|---|
| 1.8 : 1 | 10% – 80% (Often lower than 2:1) | High permeability allows gas escape. Potentially rapid cooling limits gas growth time. |
| 2.0 : 1 | 10% – 80% (Often the lowest) | Good balance of permeability and thermal properties minimizes gas entrapment. |
| 2.4 : 1 | 100% (Reference – Highest) | Low permeability traps gases within the mold cavity. |
Effect on Shrinkage Porosity: The shell’s thermal conductivity, also affected by the solid-liquid ratio, controls the cooling rate and thus the solidification morphology and feeding efficiency. The impact on shrinkage porosity was structure-dependent. For simple geometries like plates, variable sections, sharp corners, and ribs, the cross-sectional shrinkage porosity ratio generally increased with a higher solid-liquid ratio (i.e., 2.4:1 > 2:1 > 1.8:1). The less conductive 2.4:1 shell caused slower cooling, resulting in longer solidification times and larger, more poorly fed mushy zones, culminating in greater shrinkage porosity—a key metal casting defect in these sections. Conversely, for more complex geometries like transition arcs and holes, the relationship was non-monotonic. The 2:1 shell consistently yielded the lowest shrinkage porosity, while the highly conductive 1.8:1 shell often resulted in the highest. We hypothesize that in these features, the very rapid cooling induced by the 1.8:1 shell may cause the thin sections or restrictive feeding paths to solidify prematurely, isolating and exacerbating shrinkage in adjacent thicker sections or hot spots.
The local solidification time and thermal gradient $$ G $$ are critical. For a plate-like structure, the shrinkage porosity volume fraction $$ f_{sh} $$ often relates to the Niyama criterion-type function, where a lower $$ G/\sqrt{t_f} $$ promotes porosity. A shell with lower thermal conductivity (higher solid-liquid ratio) reduces $$ G $$ and increases $$ t_f $$, worsening this ratio:
$$ f_{sh} \propto g\left(\frac{G}{\sqrt{t_f}}\right) $$
This aligns with our observations for simple shapes.
Conclusion
This comprehensive investigation into the gravity investment casting of Ti-48Al-2Cr-2Nb alloy provides crucial insights for mitigating metal casting defects through synergistic design and process control. The key conclusions are as follows:
- Fill Ability is Geometry-Limited: Complete filling requires minimum geometric thresholds: plate thickness ≥ 6 mm, variable section thickness change rate ≥ 0.075, and sharp corner angles ≥ 90°. Designs violating these limits are highly prone to misruns.
- Uniform but High Shrinkage: The alloy exhibits a consistently high linear shrinkage (1.8–3.3%), largely independent of feature type, necessitating generous pattern allowances and effective feeding system design across all components.
- Geometry Dictates Shrinkage Porosity Severity: The propensity for shrinkage porosity, a major internal metal casting defect, varies dramatically with feature geometry. Variable section designs are most resistant, while sharp corners and thick ribs are most susceptible. Wall thickness, transition rates, and hole diameters directly influence the location and concentration of porosity.
- Shell Properties Target Specific Defects: The shell’s solid-liquid ratio is a powerful tool for defect control. A lower ratio (1.8:1, 2:1) dramatically reduces gas porosity by improving shell permeability. For shrinkage control, the optimal ratio is feature-dependent: a 2:1 ratio generally performed well, but a 1.8:1 ratio minimized shrinkage in simple bulk sections, while sometimes exacerbating it in complex junctions.
These findings establish a foundational framework for the design of TiAl alloy cast components. Engineers should prioritize geometries that promote directional solidification (e.g., tapered sections over uniform thick plates) and avoid acute angles and disproportionate rib junctions. Furthermore, selecting an appropriate shell solid-liquid ratio—likely in the 1.8:1 to 2:1 range—based on the dominant defect concern (gas vs. shrinkage) in the critical features of a component is essential. This work thereby provides actionable guidelines to advance the reliable precision casting of complex TiAl-based intermetallic components, directly contributing to the reduction of costly metal casting defects in high-performance aerospace applications.