Effect of Grain Refiner on Microstructural Feature Influence Hardness and Tensile Properties of Al-7Si Alloy

The effect of grain refiner on microstructural features, casting defects, formation of precipitates and eutectic Si particles, which influence the hardness and tensile properties of Al-Si alloys, is investigated in this study. The A-7Si alloy was prepared through melting and casting route with varying wt.% of Ti up to 0.2%. XRD, SEM and HRTEM analyses characterize the phases and aluminide (TiAl₃ and Ti₇Al₅Si₁₂) precipitates. The thermodynamic analysis, carried out by using FactSage software, confirms the presence of aluminides particle in the microstructure, which is formed at the liquid stage. The microstructure of as-cast Al-7Si alloy contains primary α-Al phase with dendritic morphology and eutectic phase having plate-like eutectic Si particle which is distributed at interdendritic regions. The morphology of primary α-Al grains is altered from dendrite network to fine equiaxed rosette type structure. Secondary dendrite arm spacing (SDAS) is reduced after adding grain refiner (Al-5Ti-1B) to the Al-7Si alloy. The eutectic Si is also refined to fine fibrous type particles. The shrinkage porosity and microcracks are also reduced after the addition of a grain refiner. The yield strength, ultimate tensile strength and elongation to fracture increase from 84, 117 MPa and 16% to 112 MPa, 148 MPa and 22%, respectively, after the addition of 0.1% Ti due to significant reduction in SDAS. Elongation decreases when the percentage of Ti increases more than 0.1%. The fracture mechanism of grain-refined alloy is changed from brittle to ductile fracture.