Si modification in hypereutectic Al-Si: Combining rapid solidification and alloying

Abstract

Four alloys were studied to observe the effects of Rapid Solidification and alloying on the morphology of a hypereutectic Al-40Si. Impulse Atomization was used to produce the samples studied. This processing technique was used to attain high undercoolings and liquid cooling rates ranging 103-105 K/s, thus producing rapidly solidified microstructures. Chemically modified alloys include Al-40Si-1.5Ce, Al-40Si-9.2Mg, and Al-40Si-2.75Fe-2.75Mn-1.5Sc. The resulting microstructures were qualitatively analyzed by comparing the changes in primary Si morphology and distribution across different powder sizes for each alloy and across all alloys for specific powder sizes. Quantitative results include measured phase fractions of primary Si for all alloys, and halo and eutectic for Al-40Si and Al-40Si-1.5Ce. The results presented suggest Ce and Mg additions are suitable for modifying primary Si shape and distribution in rapidly solidified Al-40Si. Both alloying approaches led to an improvement in primary Si distribution when produced at high cooling rates as compared to Al-40Si. Al-40Si-1.5Ce presents a similar primary Si morphology evolution to that seen in Al-40Si as cooling rate increases. This alloy also contains a large Al-halo vol%, which may be conducive to improved toughness in this alloy compared to Al-40Si. Primary Si morphology in Al-40Si-9.2Mg does not respond to changes in cooling rate. However, primary Si in this alloy consistently presents a small aspect ratio, which leads to a more homogeneous primary Si distribution in this alloy than in Al-40Si (particularly at slow cooling rates).