Effect of Structural Refinement and Modification on the Mechanical Properties of Al-7Si Alloy

In this present research, the effects of grain refiner (Ti-B) and eutectic modifier (Sr) on the microstructural
feature, casting defects, and mechanical properties of Al-7Si alloys are studied. Addition of grain refiner
(0.1 wt.% Ti) into Al-7Si melt can significantly change the morphology of dendritic a-Al phase to finer equiaxed grains and eutectic modifier (0.05 wt.% Sr) changes the plate-type eutectic Si particles into fine fibrous-/spherical-type particles leads to improve mechanical properties. AFM analysis reveals that combined effect of grain refiner and eutectic modifier on Al-7Si alloy can easily reduce the surface roughness and casting defects such as porosity and shrinkage. The formation of TiAl3 and Ti7Al5Si14 precipitates refines the a-Al grains, and SrSi2Al2 precipitate modifies eutectic Si particles during solidification. Both the precipitates are identified by XRD and TEM analysis. The ultimate tensile strength (UTS, MPa), yield strength (YS, MPa), and elongation to fracture (El, %) of developed AS4 alloy are 164, 91 MPa, and 30%, respectively, which are significantly higher than those of developed as-cast AS1 alloy (UTS: 117 MPa, YS: 78 MPa, and El: 16%). The developed microstructural features are correlated with mechanical properties of the alloy by using a liner equation. This correlation suggests that SDAS and morphology of eutectic Si particles play a vital role in the enhancement of mechanical properties (UTS, MPa) of the developed alloys. The grain-refined and eutectic modified alloy exhibit more dimple formation on the fracture surface
indicating more ductility.