In the field of aerospace engineering, the demand for lightweight and high-performance materials has driven extensive research into magnesium alloys, particularly ZM5, due to their excellent properties such as low density, good rigidity, and superior damping capacity. As a key material for aerospace casting parts, ZM5 magnesium alloy offers significant advantages over aluminum alloys, reducing weight by approximately 35%, which enhances aircraft maneuverability and supports the lightweight design requirements of modern cockpit components. However, producing large and complex castings aerospace components, such as shell structures, presents challenges like hot spot formation, shrinkage, and cracking due to uneven solidification. Traditional gravity casting methods often result in defects such as porosity and inclusions, leading to low qualification rates. To address these issues, we employed differential pressure casting technology, which provides controlled filling and solidification under pressure, improving metallurgical quality and mechanical properties. This study focuses on the development of a large ZM5 magnesium alloy shell casting for aerospace applications, utilizing differential pressure casting to achieve superior performance compared to conventional methods.
The ZM5 magnesium alloy, with its composition primarily including aluminum, zinc, and manganese, is widely used in aerospace casting parts due to its balance of strength and lightness. However, its poor oxidation resistance and susceptibility to hot tearing during solidification necessitate advanced casting techniques. Differential pressure casting involves applying a pressure differential between the upper and lower chambers of a mold, enabling precise control over metal flow and solidification. This method reduces turbulence and oxidation, minimizes defects, and enhances density through effective feeding. In this research, we designed a complex shell casting with dimensions of approximately 617 mm × 645 mm × 412 mm, featuring intricate geometries and varying wall thicknesses. The casting process was optimized to handle the challenges of solidification in thick-thin junctions and rib areas, which are prone to defects in castings aerospace components.
To implement differential pressure casting for this aerospace casting part, we developed a detailed gating system and sand core design. The mold consisted of five boxes: upper, middle 1, middle 2, middle 3, and lower, with heights of 200 mm or 400 mm, ensuring accurate alignment and core positioning. The gating system was designed to facilitate smooth metal flow and minimize turbulence, incorporating features like filters to reduce oxide inclusions. Sand cores were assembled and treated to remove moisture and enhance surface smoothness, promoting better filling. The key to success in producing high-quality castings aerospace components lies in the precise control of process parameters, which we optimized through iterative testing. The pressure differential, filling speed, and solidification time were critical factors in achieving defect-free castings.
The differential pressure casting process relies on fundamental principles of fluid dynamics and heat transfer. The pressure difference, ΔP, between the upper and lower chambers can be expressed as:
$$ \Delta P = P_{\text{upper}} – P_{\text{lower}} $$
where ΔP drives the metal upward into the mold cavity. The filling velocity, v, is proportional to the pressure difference and inversely related to the viscosity η of the melt, as given by:
$$ v = k \frac{\Delta P}{\eta} $$
Here, k is a constant dependent on the gating system geometry. For ZM5 magnesium alloy, with a typical viscosity range, we calibrated v to ensure laminar flow and avoid defects. The solidification time, t, for a casting can be estimated using Chvorinov’s rule:
$$ t = C \left( \frac{V}{A} \right)^2 $$
where V is the volume, A is the surface area, and C is a constant based on the material and mold properties. By controlling these parameters, we achieved sequential solidification, reducing hot spots and improving feeding efficiency in aerospace casting parts.
We conducted stabilization production in three batches, each comprising three castings, to evaluate the consistency of the differential pressure casting process. The chemical composition and mechanical properties of the ZM5 magnesium alloy were analyzed according to aerospace standards. Table 1 summarizes the key process parameters used in the differential pressure casting, which were critical for producing high-integrity castings aerospace components.
| Parameter | Value |
|---|---|
| Synchronized Pressure (kPa) | 500 |
| Lifting Speed (mm/s) | 50 |
| Lifting Pressure (kPa) | 28 |
| Filling Speed (mm/s) | 50 |
| Filling Pressure (kPa) | 28 |
| Shell Formation Time (s) | 10 |
| Shell Pressure Increase (kPa) | 6 |
| Shell Pressure Rate (kPa/s) | 3 |
| Crystallization Time (s) | 500 |
| Crystallization Pressure (kPa) | 2 |
| Crystallization Pressure Rate (kPa/s) | 1 |
| Pouring Temperature (°C) | 670–680 |
The mechanical performance of the castings was evaluated through tensile tests on single-cast specimens and dissected samples from the castings. The results demonstrated that the differential pressure casting method significantly enhanced the properties compared to technical requirements. For instance, the ultimate tensile strength, yield strength, and elongation all exceeded the minimum standards, highlighting the suitability of this approach for aerospace casting parts. The relationship between stress and strain in the ZM5 alloy can be described by the Hollomon equation:
$$ \sigma = K \varepsilon^n $$
where σ is the true stress, ε is the true strain, K is the strength coefficient, and n is the strain-hardening exponent. Our tests showed improved K and n values, indicating better ductility and strength in the castings aerospace components produced via differential pressure casting.
Table 2 presents the chemical composition and mechanical properties of the ZM5 magnesium alloy from the stabilization production batches. All values met the aerospace specifications, confirming the reliability of the process for manufacturing high-quality aerospace casting parts.
| Batch | Zn (%) | Al (%) | Fe (%) | Cu (%) | Mn (%) | Si (%) | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|---|---|---|---|---|---|
| First | 0.55 | 8.40 | 0.01 | 0.01 | 0.38 | 0.06 | 264/237 | 11.3/11.0 |
| Second | 0.59 | 8.82 | 0.01 | 0.01 | 0.39 | 0.05 | 275/241 | 12.5/10.3 |
| Third | 0.55 | 8.05 | 0.015 | 0.01 | 0.40 | 0.04 | 261/271 | 10.8/11.7 |
To further assess the performance, we conducted dissection tests on samples from each batch, measuring tensile properties and corrosion resistance. The tensile strength, yield strength, and elongation were evaluated, with results showing averages and minima well above the technical requirements for castings aerospace applications. The corrosion resistance was compared between differential pressure cast and gravity cast specimens through salt spray testing, following standard protocols. The corrosion rate, R, can be calculated as:
$$ R = \frac{W}{A \cdot t} $$
where W is the weight loss, A is the surface area, and t is the exposure time. The differential pressure cast specimens exhibited lower corrosion rates, indicating superior performance for aerospace casting parts in harsh environments.
Table 3 outlines the test methods and conditions used for evaluating the mechanical and corrosion properties of the ZM5 magnesium alloy aerospace casting parts.
| Test Item | Test Content | Test Standard | Test Conditions | Sample Quantity |
|---|---|---|---|---|
| Tensile Properties | Ultimate tensile strength, yield strength, elongation | HB 5143-1996 | Room temperature | At least 3 valid data points |
| Salt Spray Corrosion | Corrosion weight loss, morphology analysis, corrosion rate | GB/T 10125-2012 | Neutral salt spray, 24 hours | 12 specimens |
The dissection tests revealed that the average tensile strength of the differential pressure cast specimens was 214 MPa, exceeding the standard requirement by 29.7%, with a minimum value of 188 MPa, 44.6% above the minimum standard. The yield strength averaged 123 MPa, surpassing the standard by 36.7%, and the elongation averaged 9.1%, which is 264% higher than the standard. These results underscore the effectiveness of differential pressure casting in enhancing the mechanical integrity of castings aerospace components. The improvement in properties can be attributed to the reduced porosity and better grain structure achieved under pressure, as described by the equation for density improvement:
$$ \rho = \rho_0 (1 – \beta \Delta P) $$
where ρ is the final density, ρ₀ is the initial density, β is a compressibility factor, and ΔP is the applied pressure. This relationship highlights how pressure during solidification minimizes voids and increases density in aerospace casting parts.
In terms of corrosion resistance, the salt spray tests showed that differential pressure cast specimens had an average corrosion rate of 0.06 mg/cm²·h, compared to 0.099 mg/cm²·h for gravity cast specimens. This demonstrates the superior durability of differential pressure cast aerospace casting parts, which is critical for long-term performance in aerospace applications. The corrosion mechanism in magnesium alloys often involves galvanic reactions, and the improved microstructure from differential pressure casting reduces susceptibility to such degradation.
Table 4 provides detailed mechanical properties from the dissection tests of ZM5 magnesium alloy aerospace casting parts produced via differential pressure casting.
| Item | Standard Average | Standard Minimum | Batch | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|---|---|---|
| Tensile Strength | ≥165 | ≥130 | 14YGM016 | 202 | 129 | 11.6 |
| 14YGM025 | 204 | 120 | 6.6 | |||
| 14YGM026 | 236 | 120 | 9.2 | |||
| Yield Strength | ≥90 | ≥80 | 14YGM016 | – | 125 | – |
| 14YGM025 | – | 118 | – | |||
| 14YGM026 | – | 118 | – | |||
| Elongation | ≥2.5 | ≥1.5 | 14YGM016 | – | – | 10.7 |
| 14YGM025 | – | – | 5.0 | |||
| 14YGM026 | – | – | 8.0 |
Despite the overall success, we identified defects such as shrinkage and cracks in initial trials, primarily at junctions between thick and thin sections and near the gating system. Analysis indicated that these issues arose from improper solidification sequences and inadequate feeding. To address this, we modified the gating system by adding auxiliary runners at critical junctions, incorporating filters to reduce oxide inclusions, and applying insulation to enhance feeding. The filling speed was increased to 50 mm/s to promote sequential solidification. The effectiveness of these improvements can be modeled using thermal analysis, where the temperature gradient G and solidification rate R influence defect formation:
$$ G \cdot R = \text{constant} $$
By optimizing G and R through parameter adjustments, we achieved a more uniform solidification, eliminating defects in the aerospace casting parts. The qualification rate for differential pressure cast components reached 44.4%, significantly higher than the 11.9% for gravity cast parts, demonstrating the superiority of this method for complex castings aerospace applications.
Table 5 compares the corrosion rates from salt spray tests between differential pressure cast and gravity cast ZM5 magnesium alloy aerospace casting parts, highlighting the enhanced corrosion resistance.
| Corrosion Time | Casting Method | Sample ID | Corrosion Rate (mg/cm²·h) |
|---|---|---|---|
| 24 hours | Differential Pressure | 4 | 0.081 |
| 5 | 0.063 | ||
| 6 | 0.035 | ||
| Gravity | 4 | 0.130 | |
| 5 | 0.038 | ||
| 6 | 0.133 |
In conclusion, the differential pressure casting process has proven highly effective for producing large, complex ZM5 magnesium alloy aerospace casting parts. The method ensures controlled solidification, reduces defects, and enhances mechanical and corrosion properties. The average tensile strength, yield strength, and elongation exceeded technical standards by significant margins, and the corrosion performance was superior to gravity casting. By refining process parameters and the gating system, we resolved issues like shrinkage and cracking, achieving a high qualification rate. This research underscores the potential of differential pressure casting for advanced castings aerospace applications, contributing to lighter and more reliable aircraft components. Future work could focus on optimizing pressure profiles and integrating real-time monitoring to further improve the quality of aerospace casting parts.