In the advancement of modern aviation engines, the demand for complex shell components that can withstand high temperatures and pressures has intensified. These aerospace casting parts require materials with exceptional strength, heat resistance, and superior castability to ensure reliability in critical applications. Traditional aluminum alloys like ZL101A and ZL205A have been widely used, but they face limitations in high-temperature performance and casting suitability for intricate geometries. This study focuses on a novel Al-Si-Cu-Mg-Sc alloy, evaluating its casting properties, mechanical behavior, and applicability in producing complex aerospace castings. We compare it with established alloys to demonstrate its advantages in terms of fluidity, hot tearing resistance, and elevated temperature strength, which are crucial for aerospace casting parts in engines.
The development of high-performance cast aluminum alloys is driven by the need for lightweight and durable components in aerospace applications. Aerospace casting parts, such as oil pump casings, must exhibit high integrity under severe operational conditions, including thermal cycling and mechanical loads. In this work, we investigate the Al-Si-Cu-Mg-Sc alloy, which incorporates scandium to enhance thermal stability, and assess its potential for replacing conventional alloys in castings aerospace. Our approach includes experimental analysis of casting performance, microstructural characterization, and mechanical testing, supported by thermodynamic modeling to predict solidification behavior.
Experimental Materials and Methods
We prepared the Al-Si-Cu-Mg-Sc alloy with a nominal composition of Al-7Si-4Cu-0.35Mg-0.15Sc (in wt%), using high-purity raw materials and master alloys. The actual chemical composition was verified via spectroscopy, as summarized in Table 1. For comparison, ZL101A and ZL205A alloys were also examined, as they represent common choices for aerospace casting parts. The melting process involved resistance heating to 720°C, followed by the addition of alloying elements and degassing with argon. Casting was performed using metal molds preheated to 200°C for fluidity tests and 300°C for complex shell castings. We employed a tilting pouring technique to minimize defects in the castings aerospace, with a pouring time of 11 seconds for the oil pump壳体.
| Element | Si | Cu | Mg | Sc | Al |
|---|---|---|---|---|---|
| Content | 7.1 | 3.8 | 0.32 | 0.13 | Bal. |
Casting performance was assessed through fluidity tests using bar-shaped specimens and hot tearing susceptibility with ring-shaped samples. The alloys were subjected to T6 heat treatment: solution treatment at 495°C for 24 hours and aging at 180°C for 8 hours. Microstructural analysis involved optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Mechanical properties were evaluated at room and elevated temperatures (up to 250°C) using tensile tests on both separately cast specimens and samples extracted from the castings aerospace. Quality inspections included X-ray radiography, penetrant testing, and pressure tests to ensure compliance with aerospace standards for castings aerospace.
Casting Performance Analysis
The casting fluidity of the Al-Si-Cu-Mg-Sc alloy was measured at 400 mm, which is significantly higher than that of ZL205A (245 mm) but lower than ZL101A (>420 mm). This indicates improved flow characteristics for producing thin-walled aerospace casting parts. Hot tearing tests revealed that the new alloy exhibited cracking at a ring width of 5.0 mm, whereas ZL205A showed cracks at 15.5 mm, and ZL101A had no cracks. This demonstrates a lower hot tearing tendency compared to ZL205A, making it more suitable for complex geometries in castings aerospace.
To understand these results, we simulated the solidification process using the Scheil model. The solid fraction as a function of temperature is described by the equation:
$$ f_s(T) = 1 – \left( \frac{T_L – T}{T_L – T_S} \right)^{1/(1-k)} $$
where \( T_L \) is the liquidus temperature, \( T_S \) is the solidus temperature, and \( k \) is the partition coefficient. For the Al-Si-Cu-Mg-Sc alloy, \( T_L = 600^\circ \text{C} \) and \( T_S = 522^\circ \text{C} \), resulting in a freezing range of 78°C. In contrast, ZL205A has a wider range of 92°C, which contributes to its higher hot tearing susceptibility. The hot tearing criterion based on the derivative \( |dT/df_s^{1/2}| \) was calculated, yielding values of 39 for ZL101A, 207 for the new alloy, and 436 for ZL205A, confirming the intermediate performance of the Al-Si-Cu-Mg-Sc alloy for aerospace casting parts.
The improved castability of the Al-Si-Cu-Mg-Sc alloy is attributed to its finer microstructure and the presence of scandium, which refines grains and reduces shrinkage defects. This is particularly beneficial for aerospace casting parts with intricate internal passages, as it enhances fillability and reduces the risk of cold shuts or misruns. In metal mold casting, the alloy achieved a qualification rate of 80% for complex shells, comparable to ZL101A, highlighting its practicality for mass production of castings aerospace.
Mechanical Properties and Microstructural Characterization
The tensile properties of the Al-Si-Cu-Mg-Sc alloy were evaluated under various conditions. At room temperature, the T6-treated alloy exhibited an average tensile strength (\( R_m \)) of 425 MPa for separately cast specimens and 448 MPa for specimens from the casting itself, surpassing ZL101A (310 MPa) but lower than ZL205A (484 MPa). The elongation (\( A \)) was around 1.2-1.5%, which is lower than that of ZL101A and ZL205A, primarily due to microporosity observed in the fracture surfaces. High-temperature tests showed that the alloy maintains superior strength up to 250°C, with \( R_m = 242 \) MPa, outperforming ZL205A (204 MPa) and ZL101A (140 MPa). This makes it ideal for aerospace casting parts operating in elevated temperature environments.
| Alloy | \( R_m \) (MPa) | \( A \) (%) | Remarks |
|---|---|---|---|
| Al-Si-Cu-Mg-Sc | 425 ± 7 | 1.2 ± 0.3 | Separately cast |
| Al-Si-Cu-Mg-Sc | 448 ± 23 | 1.3 ± 0.3 | From casting |
| ZL101A | 310 ± 5 | 3.3 ± 0.7 | Separately cast |
| ZL205A | 484 ± 7 | 7.2 ± 7.0 | Separately cast |
Microstructural analysis revealed that the as-cast Al-Si-Cu-Mg-Sc alloy consists of dendritic α-Al with eutectic Si phases and intermetallic compounds such as Al2Cu and Al(Cu,Sc). After T6 treatment, the dissolution and spheroidization of Si phases occurred, along with the precipitation of θ’-Al2Cu and Q’-Al5Cu2Mg8Si6 phases. The presence of Q’ phase, which has high thermal stability, contributes to the alloy’s strength retention at high temperatures. The precipitation kinetics can be described by the Avrami equation:
$$ f = 1 – \exp(-kt^n) $$
where \( f \) is the phase fraction, \( k \) is the rate constant, and \( n \) is the time exponent. For θ’ and Q’ phases, the slow coarsening at elevated temperatures ensures sustained strengthening in aerospace casting parts.
Fractography indicated that microporosity with diameters of 0.5-1 mm was present in some regions, leading to reduced ductility. However, in defect-free areas, dimpled fractures suggested good plasticity. The combination of high strength and moderate ductility makes this alloy suitable for critical castings aerospace, where both performance and reliability are paramount.
Complex Shell Casting Process for Aerospace Applications
The production of complex oil pump shells for aviation engines requires precise control over casting parameters to avoid defects such as shrinkage, hot tears, and gas porosity. We designed a metal mold casting process using tilting pouring to ensure sequential solidification and minimize turbulence. The shell geometry, with wall thicknesses ranging from 4 mm to 25 mm and intricate internal oil passages, was simulated using AnyCasting software to optimize the gating and riser system. The pouring scheme included top and side risers to feed thick sections, as illustrated in the methodology.
For the Al-Si-Cu-Mg-Sc alloy, the casting process yielded a high integrity product with a surface quality meeting HB 963-2005 standards. X-ray inspection showed dense internal structures, and penetrant testing detected no linear or penetrating defects. The castings aerospace underwent pressure testing at 0.5 MPa for air tightness and 33 MPa for hydraulic strength, with no leaks or failures, confirming their suitability for aerospace casting parts. The qualification rate of 80% is comparable to that achieved with ZL101A, demonstrating the alloy’s robustness in industrial applications.
The success of this process hinges on the alloy’s balanced solidification characteristics. The fraction solid evolution during cooling can be modeled as:
$$ \frac{df_s}{dT} = -\frac{1}{T_L – T_S} \left( \frac{1-k}{1-f_s} \right) $$
This equation helps predict shrinkage behavior, allowing for better riser design in castings aerospace. The Al-Si-Cu-Mg-Sc alloy’s narrower freezing range compared to ZL205A reduces the risk of microporosity, enhancing the pressure tightness of the final components.
Quality Evaluation and Performance in Service Conditions
Comprehensive quality assessments were conducted on the Al-Si-Cu-Mg-Sc alloy castings to ensure they meet the stringent requirements for aerospace casting parts. Low-power examination revealed a pin-hole rating of Grade 1, equivalent to ZL101A castings, indicating excellent gas tightness. Mechanical tests on samples from the castings showed consistent tensile strengths above 420 MPa at room temperature and 242 MPa at 250°C, with elongations around 1.3%. These properties ensure that the castings aerospace can withstand operational stresses without failure.
| Temperature (°C) | Al-Si-Cu-Mg-Sc \( R_m \) (MPa) | ZL101A \( R_m \) (MPa) | ZL205A \( R_m \) (MPa) |
|---|---|---|---|
| 150 | 346 | 220 | 390 |
| 180 | 319 | 199 | – |
| 220 | 288 | 175 | 204 |
| 250 | 242 | 140 | 204 |
The microstructure-property relationships were further analyzed using TEM, which confirmed the presence of nano-scale precipitates that impede dislocation motion at high temperatures. The strengthening contribution from these precipitates can be estimated using the Orowan equation:
$$ \Delta \sigma = \frac{MGb}{\lambda} $$
where \( M \) is the Taylor factor, \( G \) is the shear modulus, \( b \) is the Burgers vector, and \( \lambda \) is the inter-precipitate spacing. For the Al-Si-Cu-Mg-Sc alloy, the fine dispersion of θ’ and Q’ phases provides sustained strengthening, making it ideal for castings aerospace exposed to thermal cycles.
In summary, the Al-Si-Cu-Mg-Sc alloy offers a compelling combination of castability and mechanical performance, positioning it as a superior material for aerospace casting parts. Its ability to maintain strength at elevated temperatures, coupled with good fluidity and low hot tearing tendency, enables the production of reliable complex components for aviation engines. Future work could focus on optimizing the scandium content to further enhance ductility without compromising strength, expanding its applications in castings aerospace.
Conclusion
This study demonstrates that the Al-Si-Cu-Mg-Sc high-strength heat-resistant cast aluminum alloy exhibits excellent casting properties, including improved fluidity and reduced hot tearing compared to ZL205A, making it suitable for metal mold processes in producing aerospace casting parts. The alloy’s mechanical performance, with room-temperature tensile strengths exceeding 420 MPa and superior retention at 250°C, outperforms conventional alloys like ZL101A and ZL205A in high-temperature applications. Complex shell castings manufactured using this alloy show high quality, with no defects in surface or internal integrity, and they pass rigorous pressure tests. These findings underscore the potential of the Al-Si-Cu-Mg-Sc alloy as a next-generation material for critical castings aerospace, contributing to the advancement of aviation engine technology. Further investigations into microstructure optimization and large-scale production are recommended to fully exploit its capabilities.