Combined effect of Sr-addition and pressure induced solidification on eutectic-Si morphology and mechanical properties of squeeze cast Al-Si binary alloy

The effect of casting method and alloy modification on secondary dendrite arm spacing (SDAS) and eutectic Si size of Al-Si binary alloy was investigated. For this, the Al-5Si binary alloy was modified with Sr (0.04 wt. %) and prepared using squeeze casting (SC) and gravity die casting (GDC) methods. The response of Sr modified and SC alloy to heat treatment was analysed by solutionising the alloy at 540 degrees C for 30-120 min. The results highlighted that the Sr addition and/or changing the method from GDC to SC technique reduces the SDAS and eutectic Si size. Solutionising of the alloy revealed a contrasting mechanism of modifying the eutectic Si morphology in case of SC or Sr added alloys, as compared to GDC-unmodified alloy; the first one revealed a quick response to heat treatment due to the presence of finer eutectic Si and it was evaluated through hardness measurement. Irre-spective of casting methods, Sr-modified alloy promotes larger undercooling with reduced recalescence which causes a finer eutectic Si with higher percentage fraction of Si in the Al matrix. The strength of the Sr added-GDC alloy was improved by 9.6 % compared to unmodified-GDC alloy, whereas Sr added-SC alloy enhanced with both strength (32 %) and ductility (21 %) when compared to unmodified-GDC alloy. It is concluded that the combined effect of Sr addition and adopting SC method is beneficial in terms of improving the mechanical properties of the alloy by forming a fine and fibrous eutectic Si.