Detailed Study on the Effect of Forced Melt Flow on Al–Si Eutectic-alloy Microstructures

Al–Si eutectic alloys are industrially important; they play a significant role in the casting-manufacturing of most materials. The Al–Si alloys used therein are eutectic; the two phases of Al and Si grow simultaneously from a liquid state at a constant temperature. In 1966, Jackson and Hunt developed the most comprehensive model for eutectic-structure formation. The properties of the materials are governed by their microstructure, which can be tuned by adjusting the solidification process parameters. Herein, the effect of forced melt flow on the microstructure of an Al-Si eutectic alloy during unidirectional solidification was investigated experimentally. Al–12.6-wt%-Si alloy samples were solidified in a vertical Bridgman-type furnace equipped with a rotating magnetic inductor to induce flow in the melt. The samples were subjected to different magnetic induction conditions during the solidification experiments. The diameter of the samples was 8 mm, and their length was 120 mm. The eutectic alloy samples were solidified unidirectionally at a growth rate of v 0.1 mm/s and a temperature gradient of G 6 K/mm. The inter-lamellar distances , lengths, and orientation angles of the Si lamellae were investigated using new measurement methods. The experimental results reveal that applying the rotating magnetic field (RMF) during the solidification has a distinct effect on the microstructure of Al-Si eutectic alloys. Indeed, the RMF re-fines the eutectic structure, reduces the interlamellar distances, and increases the diversity of the Si lamella angle`s orientations. However, the successive stirring process has a negligible effect on the lengths and angles of Si lamellae.

In general, the flow enriches the center area with the alloying element (Si), which results in the solidification of the eutectic structure in the center area and primary Al at the edges of the sample.