Influence of Forced/Natural Convection on Segregation During the Directional Solidification of Al-Based Binary Alloys

Abstract

We analyzed the columnar solidification of a binary alloy under the influence of an electromagnetic forced convection of various types and investigated the influence of a rotating magnetic field on segregation during directional solidification of Al-Si alloy as well as the influence of a travelling magnetic field on segregation during solidification of Al-Ni alloy through directional solidification experiments and numerical modeling of macrosegregation. The numerical model is capable of predicting fluid flow, heat transfer, solute concentration field, and columnar solidification and takes into account the existence of a mushy zone. Fluid flows are created by both natural convection as well as electromagnetic body forces. Both the experiments and the numerical modeling, which were achieved in axisymmetric geometry, show that the forced-flow configuration changes the segregation pattern. The change is a result of the coupling between the liquid flow and the top of the mushy zone via the pressure distribution along the solidification front. In a forced flow, the pressure difference along the front drives a mush flow that transports the solute within the mushy region. The channel forms at the junction of two meridional vortices in the liquid zone where the fluid leaves the front. The latter phenomenon is observed for both the rotating magnetic field (RMF) and traveling magnetic field (TMF) cases. The liquid enrichment in the segregated channel is strong enough that the local solute concentration may reach the eutectic composition.

Intermittent Behavior Caused by Surface Oxidation in a Liquid Metal Flow Driven by a Rotating Magnetic Field

“…[3,7,8,17,[34][35][36] However, the attainment of an optimal microstructure requires a precise and comprehensive knowledge of the flow structure in the vicinity of the solidification front. In particular, beneficial effects like a distinct grain refinement or the promotion of a transition from a columnar to an equiaxed dendritic growth have been achieved; however, melt convection might also produce macrosegregation effects especially in solute-rich alloys showing different equilibrium concentrations of solute in the mixed crystal and the surrounding melt.…”

“…Equations [5][6][7] were solved using a pseudospectral method. [29] The solutions were obtained in the collocation Gauss-Lobatto mesh points r(i), z(k):…”

“…In the preliminary calculations, Eqs. [7] and [15] with zero terms at the right-hand side were solved with special boundary conditions for the vorticity, where the vorticity is taken as zero in all boundary collocation points except one. On each time step, the governing equations are solved in a first step with vanishing vorticity on the borders.…”

“…[1][2][3][4][5][6][7][8] Predictions with respect to the resulting heat and mass transfer in such electromagnetically stirred liquid metals are of key interest for an optimization of the respective industrial technologies. RMF-driven liquid metal flows inside infinitely long as well as finite cylindrical containers have been investigated frequently by numerical simulations.…”

Unidirectional solidification of Sn- Pb alloys under forced melt flow

“…After switching on the RMF the columnar structure decomposed and changed into an equiaxed dendritic one. A "Christmas tree"-like macro-segregated structure (see figure 2) was formed in the centerline of the samples [10,11]. When the RMF was switched on the Pb-concentration (figure 6) suddenly dropped and the SDAS increased ( figure 5).…”