Macrosegregation Evolution in Eutectic Al-Si Alloy under the Influence of a Rotational Magnetic Field

Macrosegregation is a phenomenon that affects the quality of industrial castings and therefore attracted many research groups for several decades. Using magnetic stirring during solidification provides a good opportunity to control the microstructure of the alloys, thus controlling their physical properties. However, magnetic stirring is often accompanied by a change in local concentrations, and new structures form, which could harm the physical properties This chapter highlights the effect of forced melt flow by a rotating magnetic field (RMF) on the macrostructure of an Al-Si eutectic alloy. To serve this purpose, Al-12.6wt% Si alloy samples were solidified in a vertical Bridgman-type furnace equipped with a rotating magnetic inductor to induce the flow in the melt. The diameter and length of the sample are 8 mm and 120 mm, respectively. The solidification parameters are; The temperature gradient (G) of 6 K/m, and the solid/liquid front velocity (v) of 0.1 mm/s. These samples were divided into parts during the solidification process, where some of these parts were solidified under the effect of RMF stirring while others were solidified without stirring.

The structure obtained after solidification showed a distinct impact of stirring by RMF; new phases have been solidified which are not originally present in the structure before stirring. Besides the eutectic structure, the new phases are the primary aluminum and the primary silicon. The Si concentration and the volume fraction of each phase were measured using EDS and new image processing techniques. The experimental results reveal that applying the RMF during the solidification has a distinct effect on the macrostructure of Al-Si eutectic alloys.

Indeed, the RMF provokes macro-segregation, reduces the amount of eutectic structure, and changes the sample’s Si concentration distribution.

Finally, it is concluded that The RMF destroys the coupled growth of the
-Al and Si eutectic phases at the edges and alters the leading phase from the Si to the -Al phase.