The Influence of Fluid Flow on the Microstructure of Directionally Solidified AlSi-Base Alloys

To obtain a quantitative understanding of the effect of fluid flow on the microstructure of cast alloys, a technical Al-7 wt pct Si-0.6 wt pct Mg alloy (A357) has been directionally solidified with a medium temperature gradient under well-defined thermal and fluid-flow conditions. The solidification was studied in an aerogel-based furnace, which established flat isotherms and allowed the direct optical observation of the solidification process. A coil system around the sample induces a homogeneous rotating magnetic field (RMF) and, hence, a well-defined flow field close to the growing solid-liquid interface. The application of RMFs during directional solidification results in pronounced segregation effects: a change to pure eutectic solidification at the axis of the sample at high magnetic field strengths is observed. The investigations show that with increasing magnetic induction and, therefore, fluid flow, the primary dendrite spacing decreases, whereas the secondary dendrite arm spacing increases. An apparent flow effect on the eutectic spacing is observed. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: in Honor of Prof. John Hunt,” which occurred March 13–15, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.