Sand casting remains a cornerstone in manufacturing due to its cost-effectiveness and adaptability for complex geometries. This study investigates the influence of antimony (Sb) on the microstructure evolution of hypoeutectic Al-7Si alloys under gravity sand casting conditions, focusing on primary α-Al grains and eutectic Si morphology. By analyzing thermal profiles, grain characteristics, and modification mechanisms, we establish a comprehensive framework for understanding Sb’s role in sand-cast Al-Si systems.
1. Solidification Behavior and Thermal Analysis
The temperature-time curves during sand casting reveal critical insights into Sb’s impact on nucleation undercooling. For the unmodified Al-7Si alloy, primary α-Al nucleation occurs at 619.5°C, while Sb addition lowers this temperature to 611.8°C (Table 1). This increased undercooling (ΔT) follows:
$$
\Delta T = T_{\text{equilibrium}} – T_{\text{nucleation}}
$$
where decreased \( T_{\text{nucleation}} \) indicates suppressed nucleation kinetics. The eutectic reaction temperature similarly decreases from 578.1°C to 572.6°C, demonstrating Sb’s dual influence on both primary and eutectic phases.
| Alloy | \( T_{\text{Al-N}} \) (°C) | \( T_{\text{Si-N}} \) (°C) | Undercooling ΔT (°C) |
|---|---|---|---|
| Al-7Si | 619.5 | 578.1 | 15.2 |
| Al-7Si-0.09Sb | 611.8 | 572.6 | 23.4 |
2. Primary α-Al Grain Evolution
Polarized light microscopy demonstrates that Sb increases primary α-Al grain size from 319 μm to 353 μm in sand-cast specimens (Fig. 1). This coarsening correlates with reduced nucleation efficiency, quantified by the classical nucleation theory:
$$
N = N_0 \exp\left(-\frac{\Delta G^*}{kT}\right)
$$
where \( \Delta G^* \) represents the critical nucleation energy barrier. Sb elevates \( \Delta G^* \) through interfacial poisoning, necessitating greater undercooling to initiate nucleation. The slower heat extraction inherent to sand casting amplifies this effect, allowing fewer nuclei to dominate microstructural development.
3. Eutectic Silicon Modification Mechanism
Sb induces significant eutectic Si refinement, reducing average length from 19.7 μm to 13.3 μm and width from 8.1 μm to 5.4 μm. This modification stems from altered solute partitioning rather than twinning or nucleation site changes. The chemical potential gradient driving Si rejection from α-Al follows:
$$
\mu_{\text{Si}} = \mu_{\text{Si}}^0 + RT\ln(a_{\text{Si}})
$$
where Sb decreases Si solubility in α-Al (\( a_{\text{Si}} \)), increasing \( \mu_{\text{Si}} \) and accelerating Si expulsion. EBSD and TEM analyses confirm no Sb segregation at eutectic Si twins or nucleation cores, negating traditional modification models.
4. Sand Casting-Specific Considerations
The inherent characteristics of sand casting profoundly influence Sb’s effectiveness:
- Cooling Rate: Typical sand casting cooling rates (0.1–1°C/s) permit extended solute interaction times, enabling Sb to maximize its solubility effects.
- Thermal Gradients: Lower gradients compared to die casting reduce constitutional undercooling, favoring Sb’s chemical potential-driven modification over mechanical fragmentation.
Comparative analysis with Sr-modified systems shows Sb’s superior stability in sand casting environments, particularly for components requiring extended melt holding times.
5. Industrial Implications for Sand Casting
The experimental findings directly translate to production-scale sand casting applications:
| Property | Unmodified | 0.09% Sb | Improvement |
|---|---|---|---|
| UTS (MPa) | 185 | 218 | 17.8% |
| Elongation (%) | 2.1 | 3.9 | 85.7% |
| Hardness (HB) | 72 | 81 | 12.5% |
These enhancements derive from Sb’s unique modification mechanism synergizing with sand casting’s thermal characteristics. The technology proves particularly advantageous for engine blocks and structural components requiring balanced strength and ductility.
6. Conclusion
This study establishes that Sb modifies hypoeutectic Al-Si alloys in sand casting through thermodynamic rather than morphological mechanisms. By depressing nucleation temperatures and altering solute redistribution, Sb enables production of components with refined eutectic structures without requiring rapid cooling rates. The findings provide a scientific basis for optimizing Sb-containing alloys in sand casting applications, particularly where thermal management challenges limit conventional modifier effectiveness.