Effect of bismuth and strontium interaction on the microstructure development, mechanical properties and fractography of a secondary Al-Si-Cu-Fe-Zn alloy

The effect of various additions of bismuth and strontium on the microstructure characteristics, impact and tensile properties, as well as fracture behaviour of a secondary Al-Si-Cu-Fe-Zn under as-cast condition are systematically investigated in this study. The as-cast samples were examined using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Microstructural analyses showed the occurrence of interaction between Bi and Sr, which, significantly, led to the change of eutectic Si morphology from fibrous to flake morphology, with an increase of mean length, area, and shape factor, an alteration of crystallographic orientation, and an increase in twin density. The mechanical properties were investigated by hardness, impact, and tensile tests with various concentrations of Bi and Sr. It was found that the highest mechanical properties obtained for Sr/Bi were for a ratio of more than 0.44, while deterioration occurred at a ratio of less than 0.32. The hardness, impact toughness, ultimate tensile strength (UTS), elongation to fracture (El), and quality index (Q) of alloys decreased by 7%, 62%, 19%, 57% and 48% respectively when the Sr/Bi ratio diminished from 0.44 to 0.32. The fracture behaviour of impact samples with an Sr/Bi ratio of more than 0.44 showed clear ductile fracture, while the specimens with an Sr/Bi ratio of less than 0.32 presented brittle fracture that corresponds to impact toughness values. Furthermore, the fracture surfaces of tensile samples revealed that the dimple ductile fracture transformed to cleavage brittle mode with drop of the Sr/Bi ratio to less than 0.32 due to an increase in the length, area, and shape factor of eutectic Si. Additionally, the interaction mechanism between Bi and Sr are also discussed in this study.