Abstract
This study aims to optimize the casting process of rare-earth magnesium alloy frames using the ProCAST software and experimental validation methods. The internal quality, microstructure, and mechanical properties of the castings were characterized through X-ray non-destructive testing, optical microscopy, and mechanical testing. The results indicate that after optimization, the casting defects were significantly reduced, and the pass rate of the castings increased from 68.2% to 93.5%. The grain size of the castings decreased to an average of 35.99 μm, and the average tensile strength and elongation at break in the T6 state increased by 23.03% and 26.46%, respectively.
Introduction
Magnesium alloys are attractive materials for structural applications due to their low density, high specific strength, and excellent corrosion resistance. Rare-earth elements can further enhance the mechanical properties and corrosion resistance of magnesium alloys, making them suitable for high-performance applications such as aerospace, automotive, and electronic devices. However, the casting process of rare-earth magnesium alloys is complex and prone to defects, including porosity, shrinkage, and hot tearing. Therefore, optimizing the casting process is crucial to improve the quality and performance of the castings.
This study focuses on the optimization of the casting process for a rare-earth magnesium alloy frame, which is a critical load-bearing structure in aerospace and automotive applications. The optimization was achieved using numerical simulation and experimental validation methods, and the results were analyzed in terms of internal quality, microstructure, and mechanical properties.
Materials and Methods
1. Material Preparation
The rare-earth magnesium alloy used in this study was prepared from pure Mg, pure Zn, Mg-30Nd master alloy, Mg-20Y master alloy, and Mg-30Zr master alloy. The melting process involved the following steps:
- Melting of Pure Mg: 250 kg of pure Mg was melted in a Φ500 mm × 1000 mm iron crucible using a flux-covered melting method to prevent oxidation.
- Addition of Master Alloys: The melt temperature was raised to 730-740°C, and the master alloys were added sequentially. After complete melting, the melt was stirred for 2 minutes and allowed to stand for 12-15 minutes.
- Refining: The melt was refined at 750-760°C using a JDMJ flux in an amount equivalent to 2.5% of the alloy weight. The refined melt was allowed to stand for 15 minutes before pouring.
- Gravity Pouring: The melt was poured at 735°C into clay-bonded sand molds prepared specifically for the frame geometry.
Table 1 summarizes the theoretical and actual chemical compositions of the alloy.
| Element | Theoretical wt. % | Actual wt. % |
|---|---|---|
| Mg | Balance | Balance |
| Nd | 2.6 | 2.53 |
| Y | 0.15 | 0.18 |
| Zn | 0.5 | 0.52 |
| Zr | 0.9 | 0.78 |
2. Numerical Simulation
The ProCAST software was used to simulate the filling and solidification processes of the magnesium alloy frame castings. The material properties used in the simulation are summarized in Table 2.
| Property | Value |
|---|---|
| Density | 1.82 g/cm³ |
| Thermal Diffusivity | 2.8560 × 10⁻⁵ m²/s |
| Coefficient of Thermal Expansion | 25.3 × 10⁻⁶ /°C |
| Elastic Modulus | 45 GPa |
| Poisson’s Ratio | 0.35 |
| Shear Modulus | 16.20 GPa |
| Specific Heat Capacity | 0.98 J/(g·K) |
| Thermal Conductivity | 9.476 W/(m·K) |
3. Casting Process Optimization
The initial casting process resulted in numerous defects, such as porosity, shrinkage, and slag inclusion. To address these issues, the following optimizations were implemented:
- Gating System Modification: The height of the缝隙浇道 was adjusted to be the same as the riser to improve the feeding efficiency.
- Riser Placement: A riser was placed at the top of the casting corresponding to the缝隙浇道 to collect gases and slags during pouring.
- Cold Iron Insertion: Cold irons were placed between the large side walls to regulate the solidification sequence and enhance feeding.
4. Post-processing and Testing
The castings were subjected to T6 heat treatment (490°C × 24 h + 200°C × 18 h, both followed by air cooling) to improve their mechanical properties. Microstructural analysis was performed using an optical microscope, and mechanical properties were evaluated through tensile testing according to ASTM E8 standards.
Results and Discussion
1. Casting Process Optimization Results
The optimized casting process significantly improved the quality of the castings. The filling and solidification processes were simulated using ProCAST, and the results are presented, respectively.
The simulated filling time was approximately 9 seconds, and the controlled pouring rate (13-15 seconds) minimized the risk of gas entrapment and oxidation. The solidification sequence showed an orderly progression from the riser to the casting body, ensuring effective feeding and minimizing defects.
2. Internal Quality Analysis
X-ray non-destructive testing was performed on both the optimized and unoptimized castings to assess their internal quality. The results are ssummarized in Table 3.
| Casting Type | Defect Observation | Casting Yield |
|---|---|---|
| Unoptimized | Large areas of porosity and slag inclusion | 68.2% |
| Optimized | Minimal defects; mainly slag inclusion at the bottom | 93.5% |
Table 3: Comparison of internal quality between unoptimized and optimized castings
3. Microstructure Analysis
The microstructures of the unoptimized and optimized castings were analyzed using optical microscopy. the representative micrographs, and Table 4 summarizes the average grain sizes.
| Casting Type | Average Grain Size (μm) |
|---|---|
| Unoptimized | 47.34 |
| Optimized | 35.99 |
Table 4: Average grain sizes of unoptimized and optimized castings
The optimized casting exhibited a finer grain structure due to the improved feeding efficiency and faster solidification rates facilitated by the cold irons.
4. Mechanical Properties
The mechanical properties of the optimized castings were evaluated through tensile testing, and the results are presented in Table 5.
| Casting Type | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation at Break (%) |
|---|---|---|---|
| Unoptimized | 215.67 | 130.33 | 5.67 |
| Optimized | 265.33 | 169.33 | 7.17 |
Table 5: Mechanical properties of unoptimized and optimized castings
The optimized castings exhibited significant improvements in tensile strength, yield strength, and elongation at break. The enhanced mechanical properties can be attributed to the reduced casting defects and refined grain structure.
5. Fracture Surface Analysis
The fracture surfaces of the optimized and unoptimized castings were analyzed using scanning electron microscopy (SEM). representative SEM images.
The unoptimized casting exhibited a brittle fracture surface with numerous porosity regions, indicating poor internal quality. In contrast, the optimized casting showed a more ductile fracture surface with reduced porosity areas, confirming the effectiveness of the optimization measures.
Conclusion
This study successfully optimized the casting process for rare-earth magnesium alloy frames using numerical simulation and experimental validation methods. The key findings are as follows:
- Optimized Gating and Feeding System: Adjusting the height of the缝隙浇道 and placing a riser at the top significantly improved the feeding efficiency and reduced casting defects.
- Internal Quality Improvement: The optimized casting process reduced defects such as porosity and slag inclusion, resulting in a casting yield of 93.5%.
- Refined Microstructure: The use of cold irons refined the grain structure, with an average grain size of 35.99 μm.
- Enhanced Mechanical Properties: The optimized castings exhibited improved mechanical properties, with an increase of 23.03% in tensile strength, 29.92% in yield strength, and 26.46% in elongation at break.
In conclusion, the casting process optimization significantly improved the quality and performance of rare-earth magnesium alloy frames, making them more suitable for high-performance applications. Future work could focus on further refining the casting process to achieve even higher casting yields and mechanical properties.