Accelerating pore nucleation and eutectic Si growth kinetics by increasing Cu and Sc for Al-Si-Mg alloys: In-situ observation

The mechanical performance of Al-Si-Mg cast alloys is highly dependent on the pore size and morphology of eutectic Si, which is especially critical for high fatigue life components. Many studies have shown the linear relationship between them. However, it is unknown how the beneficial alloying elements for grain refinements and precipitation strengthening will have an impact on those critical properties. In this study, we used synchrotron X-ray to quantify the kinetics of eutectic Si growth for three alloys. These eutectic Si nucleate and grow rapidly at the initial stage of growth, and then the morphology tends to remain unchanged. Surprisingly, the Sc addition which refines the grains and results in fibrous morphology instead of highly faceted Si particles. The addition of precipitation strengthening element Cu can significantly increase the hydrogen content and increase the number of porosity by an order of magnitude. Therefore, in situ observation and X-ray tomography in this study enabled us to understand the key roles of Cu and Sc and control the kinetic process of both eutectic Si and defects for better mechanical performance.