Abstract
Understanding the Si segregation behavior in hypereutectic Al-Si alloys is important for controlling the micro- and macrostructures of ingots. The macrosegregation mechanism and morphological evolution of the primary Si phase were investigated during electromagnetic directional solidification (EMDS). Both numerical simulations and experimental results strongly suggested that the severe macrosegregation of the primary Si phase was caused by fluid flow and temperature distribution. Microscopic analysis showed that the morphological evolution of the Si crystal occurred as follows: planar → cellular → columnar → dendritic stages during EMDS. Based on constitutional supercooling theory, a predominance area diagram of Si morphology was established, indicating that the morphology could be precisely controlled by adjusting the values of temperature gradient (G), crystal growth rate (R), and solute concentration (C0). The results provide novel insight into controlling the morphologies of primary Si phases in hypereutectic Al-Si alloys and, simultaneously, strengthen our understanding of the macrosegregation mechanism in metallic alloys.
Highlights
Macrosegregation often occurs during the solidification of metallic materials and can significantly deteriorate the mechanical properties of the final ingots [1]
A previous study [22], which focused on Si segregation, showed that the macrosegregation behavior of the primary Si phases in hypereutectic Al-Si alloys involves a complex coupling of solute behavior of the primary Si phases in hypereutectic Al‐Si alloys involves a complex coupling of solute distribution, fluid flow, heat transfer, and phase transformation
This study investigated the Si segregation behaviors and morphological evolution of primary Si phases in hypereutectic Al‐Si alloys
Summary
Macrosegregation often occurs during the solidification of metallic materials and can significantly deteriorate the mechanical properties of the final ingots [1]. It is recognized that fluid flow, arising from either natural or forced convection, can profoundly affect macrosegregation [3]. Natural convection (i.e., thermosolutal convection) is driven by differences in the densities and temperatures of liquid melts. Forced convection, on another hand, often arises from mechanical, electromagnetic, or other types of stirring. An intense convection can always result in solute redistribution and the relative movement of solid and liquid phases, leading to a nonuniform structure of the alloy. Microstructures are the results of the complex coupling of fluid flow, heat transfer, Materials 2019, 12, 10; doi:10.3390/ma12010010 www.mdpi.com/journal/materials
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