This study investigates the impact of mechanical vibration parameters on the microstructure evolution and mechanical performance of QT400-18 ductile iron produced through lost foam casting. The research focuses on optimizing vibration frequency (0–50 Hz) and amplitude (0–2 mm) to enhance graphite morphology, ferrite refinement, and pearlite distribution.
1. Vibration Effects on Graphite Characteristics
Mechanical vibration significantly modifies graphite nucleation and growth dynamics in lost foam casting. At constant amplitude (1.5 mm), increasing frequency from 0 Hz to 40 Hz improves spheroidization rate from 78.8% to 85.9% while reducing average graphite diameter from 33.86 μm to 27.09 μm. Excessive vibration (50 Hz) causes turbulence that degrades spheroidization efficiency.
| Frequency (Hz) | Spheroidization (%) | Graphite Size (μm) | Graphite Content (%) |
|---|---|---|---|
| 0 | 78.8 | 33.86 | 13.58 |
| 30 | 83.1 | 29.61 | 15.21 |
| 40 | 85.9 | 27.09 | 16.26 |
| 50 | 82.3 | 26.44 | 16.41 |
The vibration-induced shear stress promotes dendrite fragmentation through the relationship:
$$ \tau = \eta \cdot \gamma \cdot \omega $$
where τ represents shear stress (Pa), η dynamic viscosity (Pa·s), γ strain rate (s⁻¹), and ω angular frequency (rad/s). This mechanism increases nucleation sites and restricts graphite growth.
2. Ferrite-Pearlite Matrix Modification
Vibration parameters critically influence matrix microstructure in lost foam casting. Ferrite grain refinement follows the Hall-Petch relationship:
$$ \sigma_y = \sigma_0 + k_y \cdot d^{-1/2} $$
where σy is yield strength (MPa), σ0 lattice friction (120 MPa), ky locking coefficient (0.6 MPa·m1/2), and d grain diameter (μm). Optimal vibration (40 Hz, 2 mm) reduces ferrite grain size by 16.7% compared to non-vibrated specimens.
| Amplitude (mm) | Ferrite Size (μm) | Pearlite Spacing (μm) |
|---|---|---|
| 0 | 33.76 | 0.34 |
| 1 | 32.24 | 0.27 |
| 1.5 | 29.93 | 0.22 |
| 2 | 29.14 | 0.19 |
3. Mechanical Performance Enhancement
Mechanical vibration in lost foam casting improves tensile properties through combined matrix strengthening and graphite optimization. The tensile strength improvement follows:
$$ \Delta\sigma = \Delta\sigma_{\text{graphite}} + \Delta\sigma_{\text{grain}} + \Delta\sigma_{\text{dislocation}} $$
where Δσgraphite (15–22 MPa) comes from improved spheroidization, Δσgrain (12–18 MPa) from ferrite refinement, and Δσdislocation (5–8 MPa) from vibration-induced dislocation density.
| Parameter | 0 Hz/0 mm | 40 Hz/2 mm | Improvement |
|---|---|---|---|
| Tensile Strength (MPa) | 419.5 | 462.8 | +10.3% |
| Elongation (%) | 18.3 | 22.8 | +24.6% |
| Impact Energy @20°C (J) | 18.23 | 19.86 | +8.9% |
| Wear Loss (g) | 0.04369 | 0.04191 | -4.1% |
4. Low-Temperature Toughness
Vibration-modified lost foam casting specimens demonstrate superior cryogenic performance with ductile-brittle transition temperature lowered by 10°C. The impact energy-temperature relationship follows:
$$ E_{\text{impact}} = E_0 \cdot \exp\left(-\frac{T-T_0}{\alpha}\right) $$
where E0 = 20.5 J, T0 = -45°C, and α = 22.3°C for vibrated specimens versus α = 18.7°C for conventional casting.
5. Tribological Behavior
The wear resistance improvement in vibrated lost foam casting components correlates with surface hardening:
$$ H_v = H_0 + k_h \cdot \varepsilon^{n} $$
where Hv represents hardened surface hardness (158.5 HB), H0 initial hardness (144.4 HB), ε plastic strain, and n work-hardening exponent (0.15).
This comprehensive study confirms that optimized mechanical vibration parameters in lost foam casting significantly enhance the service performance of QT400-18 ductile iron components through microstructural refinement and defect minimization.