Introduction
Vermicular graphite cast iron, characterized by its unique graphite morphology, combines the thermal conductivity of flake graphite and the ductility of spheroidal graphite. This material is widely used in automotive components such as clutch pressure plates due to its balanced mechanical properties. However, residual stresses generated during casting significantly affect dimensional stability, fatigue life, and machining accuracy. This study investigates the formation mechanisms, distribution patterns, and mitigation strategies for residual stresses in vermicular graphite cast iron clutch pressure plates through numerical simulations and experimental validations.
Numerical Simulation of Casting Processes
Model Setup and Boundary Conditions
A 3D model of the clutch pressure plate was constructed using UG NX, with dimensions of 215.9 mm outer diameter, 116 mm inner diameter, and an average wall thickness of 10 mm. The casting process was simulated using ProCAST, incorporating thermal-stress coupling to analyze temperature fields, solidification sequences, and residual stress distributions. Key parameters included:
- Material properties: Thermal conductivity (k), density (ρ), specific heat (Cp), and elastic modulus (E) of vermicular graphite cast iron (GGV30) were derived from experimental data (Table 1).
- Boundary conditions:
- Initial pouring temperature: Tpour=1419∘C
- Sand mold temperature: Tmold=25∘C
- Heat transfer coefficient at mold-casting interface: h=500W/m2K
Table 1: Thermophysical properties of GGV30 vermicular graphite cast iron
| Property | Value |
|---|---|
| Density (ρ) | 7200kg/m3 |
| Thermal conductivity (k) | 38W/m\cdotpK |
| Specific heat (Cp) | 620J/kg\cdotpK |
| Elastic modulus (E) | 145GPa |
| Poisson’s ratio (ν) | 0.28 |
Temperature Field Analysis
The temperature evolution during solidification revealed distinct cooling gradients:
- Filling phase: Liquid metal flowed radially from the sprue to the plate edges, achieving full mold filling in 7.35s.
- Solidification phase: Sequential cooling occurred from the outer edges (small lugs) toward the central region. Temperature differences (ΔT) between critical points reached 159∘C.
- Post-solidification cooling: Radial thermal gradients persisted, with slower cooling rates near risers due to latent heat release.
The temperature distribution followed:T(r,t)=Tmold+4πktqcaste−4αtr2
where r = radial distance, α = thermal diffusivity, and qcast = heat flux from casting.
Residual Stress Formation Mechanisms
Residual stresses in graphite cast iron arise from:
- Thermal stresses: Non-uniform cooling rates induce differential contractions.
- Phase transformation stresses: Volume changes during graphite nucleation and austenite-to-pearlite transformations.
- Mechanical constraints: Mold rigidity restricts free contraction, generating tensile/compressive stresses.
Stress Field Simulation Results
ProCAST simulations predicted maximum tensile stresses (σmax) of 218MPa near small lugs and compressive stresses (−165MPa) at the plate center. The von Mises stress distribution confirmed stress concentration zones aligned with geometry-driven cooling asymmetries.
Table 2: Simulated residual stresses at critical locations
| Location | Radial Stress (σr, MPa) | Hoop Stress (σθ, MPa) |
|---|---|---|
| Small lug edge | 218 | 189 |
| Central plate region | -165 | -142 |
| Riser-proximal zone | 98 | 75 |
The stress evolution during cooling obeyed the elastoplastic constitutive model:σ=E(ϵtotal−ϵthermal−ϵplastic)
where ϵthermal=αΔT and ϵplastic was derived from the Prandtl-Reuss flow rule.
Experimental Validation
Blind Hole Drilling Method
Residual stresses were measured using a DH5929N dynamic strain analyzer and RSD1 drilling equipment. Strain relief coefficients (A,B) were calibrated as:A=2E1+ν,B=2E1
Table 3: Measured vs. simulated residual stresses
| Measurement Point | Simulated σr (MPa) | Experimental σr (MPa) | Error (%) |
|---|---|---|---|
| P1 (Small lug) | 218 | 205 | 6.0 |
| P2 (Center) | -165 | -158 | 4.2 |
| P3 (Riser) | 98 | 91 | 7.1 |
Discrepancies (≤7.1%) stemmed from assumptions in material isotropy and boundary conditions.
Strategies for Residual Stress Mitigation
Effect of Process Parameters
- Pouring temperature: Higher temperatures (1420∘C) reduced σmax by 12% due to slower cooling and stress relaxation.
- Shakeout temperature: Lower shakeout thresholds (<300∘C) minimized thermal gradients, lowering σmax by 18%.
Table 4: Impact of carbon equivalent (CE) on residual stress
| CE (%) | Si/C Ratio | σmax (MPa) | Hardness (HB) |
|---|---|---|---|
| 4.42 | 0.65 | 235 | 245 |
| 4.64 | 0.73 | 198 | 228 |
| 4.81 | 0.69 | 212 | 236 |
Optimal CE (4.64%) and Si/C ratio (0.73) enhanced stress relaxation via increased graphitization:Graphitization degree∝%Mn+%Cr/2%C+%Si/3
Gating System Design
A bottom-gating system reduced turbulence and temperature gradients, achieving a 24% lower σmax compared to top-gating.
Conclusion
- Residual stresses in vermicular graphite cast iron clutch plates are dominated by thermal gradients and phase transformations, with maximum tensile stresses localized near small lugs.
- Numerical simulations using ProCAST achieved 93% accuracy in predicting stress distributions, validated by experimental measurements.
- Mitigation strategies—optimized CE (4.64%), high Si/C ratios (0.73), and bottom-gating systems—effectively reduced residual stresses by 18−24%.
This work provides a framework for designing low-stress graphite cast iron components, enhancing performance in high-precision automotive applications.