This study focuses on optimizing the casting process of a rare-earth magnesium alloy frame to enhance internal quality, refine microstructure, and improve mechanical properties. Through numerical simulations, experimental validations, and comprehensive analyses, we demonstrate how strategic adjustments to gating, feeding systems, and cooling strategies significantly elevate casting performance. Below, we present the methodology, results, and insights derived from this investigation.
1. Introduction
Magnesium alloys, particularly rare-earth variants, are pivotal in aerospace and automotive industries due to their low density, high specific strength, and excellent machinability. However, defects such as porosity, shrinkage cavities, and slag inclusions often plague traditional casting processes, limiting component reliability. In this work, we address these challenges by redesigning the gating system, optimizing riser placement, and introducing chill plates to regulate solidification dynamics. Our goal is to leverage ProCAST simulations and empirical testing to achieve defect-free castings with superior mechanical properties.
2. Materials and Methods
2.1 Alloy Composition and Melting
The rare-earth magnesium alloy was prepared using pure Mg, Zn, and Mg-Nd/Y/Zr master alloys. Table 1 compares theoretical and actual chemical compositions.
Table 1: Chemical Composition (wt.%)
| Element | Theoretical | Actual |
|---|---|---|
| Nd | 2.6 | 2.53 |
| Zn | 0.5 | 0.52 |
| Y | 0.15 | 0.18 |
| Zr | 0.9 | 0.78 |
Melting involved:
- Pure Mg melted in a Φ500 mm × 1000 mm crucible under flux protection (JDMF).
- Addition of master alloys at 730–780°C, followed by stirring and refining (JDMJ flux, 2.5% alloy mass).
- Gravity casting at 735°C.
2.2 Numerical Simulation (ProCAST)
ProCAST simulations modeled mold filling and solidification. Key parameters included:
- Density: ρ=1.82 g/cm3ρ=1.82g/cm3
- Thermal diffusivity: α=2.856×10−5 m2/sα=2.856×10−5m2/s
- Thermal conductivity:
- Specific heat:
The casting process was optimized by adjusting gating height, riser placement, and chill plate thickness (15 mm).
2.3 Experimental Validation
- X-ray inspection: Detected defects per NBT 47013.2–2015.
- Microstructure analysis: Samples polished and etched (4% HNO₃ ethanol) for optical microscopy.
- Mechanical testing: Tensile tests (GB/T 228.1–2010) at 1 mm/min strain rate.
3. Results and Discussion
3.1 Defect Reduction via Casting Process Optimization
Initial trials revealed severe porosity and slag inclusions (68.2% pass rate). Key issues included inadequate riser spacing and suboptimal gating height. Post-optimization (Figure 1), defects were confined to risers, and pass rate surged to 93.5%.
Table 2: Defect Distribution Before/After Optimization
| Defect Type | Before (%) | After (%) |
|---|---|---|
| Slag inclusions | 22.4 | 3.1 |
| Porosity | 18.7 | 1.8 |
| Shrinkage cavities | 15.3 | 0.9 |
3.2 Microstructural Refinement
The as-cast microstructure comprised α-Mg grains and intermetallic phases. Post-optimization, grain size decreased from 47.34 μm47.34μm to 35.99 μm35.99μm, attributed to faster cooling via chill plates. The Hall-Petch relationship explains strength enhancement:σy=σ0+ky⋅d−1/2σy=σ0+ky⋅d−1/2
where σyσy = yield strength, σ0σ0 = friction stress, kyky = strengthening coefficient, and dd = grain size.
3.3 Mechanical Performance
T6 heat treatment (490°C × 24 h + 200°C × 18 h) enhanced properties:
Table 3: Mechanical Properties Before/After Optimization
| Property | Before | After | Improvement (%) |
|---|---|---|---|
| Tensile strength (MPa) | 215.67 | 265.33 | 23.03 |
| Yield strength (MPa) | 130.33 | 169.33 | 29.92 |
| Elongation (%) | 15.67 | 7.17 | 26.46 |
Fractography revealed brittle failure with reduced porosity, aligning with mechanical gains.
4. Process Optimization Strategy
4.1 Gating and Feeding System
- Increased gating height to match risers.
- Added top risers for slag collection and shrinkage compensation.
4.2 Solidification Control
Chill plates accelerated cooling in thick sections, mitigating thermal hotspots. Solidification time (tt) followed:t=L2α⋅π2t=α⋅π2L2
where LL = characteristic length, αα = thermal diffusivity. Faster cooling suppressed grain growth.
5. Economic Impact
The optimized casting process reduced scrap rates, saving ~25% in production costs. For 1,000 castings:
Table 4: Cost Savings Analysis
| Metric | Before | After |
|---|---|---|
| Defective units | 318 | 65 |
| Material cost saved | $15,900 | $3,250 |
6. Conclusion
By integrating ProCAST simulations with empirical refinements, we achieved a robust casting process for rare-earth magnesium alloys. Key outcomes include:
- 93.5% casting pass rate.
- Grain refinement (35.99 μm35.99μm).
- 23–30% improvement in mechanical properties.
This work underscores the value of systematic casting process optimization in advancing lightweight alloy applications.
Formulas Used
- Hall-Petch Relationship:
σy=σ0+ky⋅d−1/2σy=σ0+ky⋅d−1/2
- Solidification Time:
t=L2α⋅π2t=α⋅π2L2
Key Parameters
- Thermal diffusivity (αα): 2.856×10−5 m2/s2.856×10−5m2/s
- Poisson’s ratio: 0.35
- Elastic modulus: 45 GPa