Modelling of the microstructure formation during solidification of binary alloy Al–Si under stirring

Abstract

The growth of primary crystals from a supercooled binary melt is modeled on the basis of the phase-field method with an approximate consideration for melt stirring. Changes in the concentration of the solute in the melt near the solidification region due to stirring are considered as the main reason for modifying the dendritic morphology of the crystals. The stirring effect results in a partial removal of the melt with an increased solute concentration from the region near the interface, which is called washout in the work. This effect is approximately modeled as a forced periodic replacement of the current high solute concentration in this region either at the initial concentration or at the averaged concentration in the melt. In this paper, we propose a new algorithm for choosing such a correction region, which can be used for a small intensity of stirring. A value is introduced to describe the washout intensity-the washout parameter. It is shown that when washed away dendritic morphology passes into a rosette morphology, depending on the intensity of stirring. A numerical analysis of the growth of perturbations on the surface of the initial embryo of a circular crystal is carried out. The main differences in the development of such perturbations are shown, which leads to a non-dendritic morphology of the crystal. The growth of additional branches is demonstrated due to a decrease in the solute concentration near the center of the crystal. Calculations have been performed for the case of crystal growth with a decrease in washout intensity with growth time. For this case, the secondary arms grow on additional branches growing from the center of the crystal. It is shown that at a constant value of the washout intensity near the surface of a growing crystal, only two types of morphology are possible from a small round embryo, dendritic and rosette. At the same time, if the crystal already has a rosette morphology, it is retained and with its further growth with a decrease in the washout intensity.