Effect of Strontium and Magnesium Additions on the Microstructure and Mechanical Properties of Al–12Si Alloys

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In this study, a binary Al–12Si, eight ternary Al–12Si–Sr, and six quaternary Al–12Si–0.1Sr–(0.2–1)Mg alloys were produced by permanent mold casting. It was observed that microstructure of the binary alloy consisted of the phases of aluminum rich α grains (dendrites), primary silicon, plate like β and eutectic Al–Si containing needle like silicon particles. The ternary alloys have fine and globular (modified) eutectic silicon particles and higher volume fraction of α (Al) dendrites than binary alloys. They also contained Al4Sr phase after 0.02 wt% Sr, in addition to the phases in the binary alloy. This phase got coarse when the strontium ratio exceeded 0.1%. It was observed that the plate like β phase seen in the binary alloys transformed into the fibrous form δ phase in the ternary alloys. Magnesium addition resulted in transformation of δ phase into script like π phase, and the formation of lamellar like Mg2Si phase when the ratio of it in the quaternary alloys reached the 0.6 wt%. The lamellar like form of Mg2Si phase changed to Chinese-script type after the 0.6 wt% Mg. The results showed that hardness, yield and tensile strength of the Al–12Si–Sr alloys increased with increasing strontium content up to 0.1 wt%. The results also showed that hardness of the quaternary alloys increased with increasing magnesium content, while yield and tensile strength increased only up to 0.6 wt% Mg.