The increasing demand for lightweight military equipment has driven the development of high-strength, wear-resistant, and corrosion-resistant cast aluminum alloys. In this context, thin-walled spherical shells, as critical structural components for precision equipment such as naval gun systems and missile launchers, require stringent airtightness and defect-free properties. This study focuses on optimizing the casting process of a thin-walled spherical shell made of ZL104 aluminum alloy, addressing shrinkage porosity and gas entrapment defects through systematic design improvements and numerical simulations.
1. Technical Requirements and Structural Challenges
1.1 Structural Characteristics
The spherical shell features an irregular geometry with non-uniform wall thickness (4 mm at critical regions), left-side protrusions (“wings”), a large right-side through-hole, and internal discontinuous convex platforms. Key dimensions are 960 mm × 510 mm × 430 mm, with a mass of approximately 16 kg. The structural complexity complicates mold design, core placement, and feeding system optimization.
1.2 Technical Specifications
- Airtightness: Pressure drop ≤ 1 kPa under 1.35 atm for 10 minutes.
- Chemical Composition: Compliant with GB/T 1173-2013 (Table 1).
- Mechanical Properties: T6 heat-treated state with tensile strength ≥ 225 MPa, elongation ≥ 2%, and hardness ≥ 70 HB (Table 2).
Table 1: Chemical Composition of ZL104 Alloy (wt%)
| Si | Mg | Mn | Fe (max) | Al (balance) |
|---|---|---|---|---|
| 8–10.5 | 0.17–0.35 | 0.2–0.5 | 0.6 | Remaining |
Table 2: Mechanical Properties of ZL104 Alloy
| Heat Treatment | Tensile Strength (MPa) | Elongation (%) | Hardness (HB) |
|---|---|---|---|
| T6 | ≥225 | ≥2 | ≥70 |
2. Initial Casting Process and Defect Analysis
The original process employed resin sand gravity casting with an open gating system. Key parameters included:
- Gating Ratio: ∑Asprue:∑Arunner:∑Agate=3.3:3.5:1∑Asprue:∑Arunner:∑Agate=3.3:3.5:1.
- Feeding System: Eight risers (six open, two blind) and chill placements at thick sections.
2.1 Defect Identification
Post-machining inspections revealed shrinkage porosity at sidewall corners (Figure 4 in the original text), attributed to:
- Poor riser placement due to the curved upper surface.
- Insufficient venting in “dead zones” near sidewall corners.
- Inadequate gating design, leading to turbulent flow and gas entrapment.
3. Process Optimization Strategies
3.1 Parting Line Redesign
To enhance riser efficiency, the parting line was shifted to the bottom outer surface (non-critical machining area). This allowed:
- Simplified riser placement.
- Reduced slag accumulation on critical surfaces.
3.2 Gating System Optimization
A hybrid gating system combining slot and fan gates was adopted (Figure 5 in the original text). Key modifications included:
- Semi-restricted Gating Ratio: ∑Asprue:∑Arunner:∑Agate=1.3:1.6:1∑Asprue:∑Arunner:∑Agate=1.3:1.6:1.
- Flow Stabilization: Ceramic filters at sprue bases minimized turbulence and impurity entrainment.
3.3 Venting System Enhancement
Total venting area (AventAvent) was increased to meet the criterion Avent≥1.5×AgateAvent≥1.5×Agate.
- Original venting ratio: AventAgate=1.13AgateAvent=1.13.
- Optimized venting ratio: AventAgate=2.1AgateAvent=2.1, achieved by adding 16-mm-diameter vent channels at sidewall corners.
4. Numerical Simulation Using ProCAST
4.1 Model Setup
- Material: ZL104 alloy.
- Boundary Conditions:
- Pouring temperature: 690°C.
- Mold temperature: 20°C.
- Cooling method: Air cooling.
4.2 Temperature Field Analysis
The temperature gradient during solidification (Figure 6 in the original text) showed:
- Highest temperatures at risers (Tmax≈650°CTmax≈650°C), ensuring effective feeding.
- Uniform cooling from bottom to top, minimizing thermal stresses.
4.3 Defect Prediction
Shrinkage porosity was localized near risers and gates (Figure 7 in the original text), with minimal defects at sidewalls. The probability of porosity (PP) followed:P=k⋅(ΔTtsolidification)0.5P=k⋅(tsolidificationΔT)0.5
where kk is a material constant, ΔTΔT is the temperature gradient, and tsolidificationtsolidification is the local solidification time.
5. Production Validation
Post-optimization trials demonstrated:
- Defect Elimination: No shrinkage porosity at sidewalls (Figure 9 in the original text).
- Airtightness Compliance: Pressure drop ≤ 0.8 kPa under 1.35 atm.
- Yield Improvement: First-pass acceptance rate increased from 54.5% to 89%.
6. Conclusion
- Optimized parting and gating systems significantly improved feeding efficiency and reduced turbulence in the thin-walled spherical shell.
- Venting area expansion to Avent=2.1×AgateAvent=2.1×Agate effectively eliminated gas-related defects.
- Numerical simulations using ProCAST provided critical insights into temperature fields and defect formation, guiding iterative process improvements.
This study underscores the importance of integrated design and simulation in achieving high-quality castings for mission-critical spherical shell applications.