Channel segregation during columnar solidification: Relation between mushy zone instability and mush permeability

Abstract

A model of mushy zone instability is developed for characterization and prediction of channel segregation in castings. The model highlights the connection of the mushy zone instability with the mush permeability. A new criterion for amplification of the mush instability is derived, which depends on the interdendritic velocity, the isotherm velocity, the temperature gradient, and explicitly on the mush permeability. The capability of the instability criterion in the characterization of channel segregates is illustrated by comparing the estimated possible channel locations with that of numerically simulated channel segregations in a benchmark test case of solidification of Sn-Pb alloy. An Opensource CFD software OpenFOAM is used to simulate the solidification of Sn-Pb alloy in a side-cooled rectangular cavity. The new instability criterion very well characterizes the channel segregates shown in the numerical simulations. The near-liquidus part of the mushy region is more prone to the initiation of instability, which can initiate the formation of channel segregates in castings. Permeability and its derivative with respect to the liquid fraction in the outer part of mushy zone (liquid fraction more than 0.9) plays a key role in the amplification of this instability that can be responsible for the development and the formation of channel segregates. The locations of channel segregates estimated by the new instability model are also compared with those with a remelting criterion, and it has been noticed that the remelting criterion severely under characterizes these defects (e.g., their number and length). We noticed that local remelting might not be necessary for the initiation of a channel; some destabilization of the mushy zone due to local instability could be sufficient.

Three-Dimensional Modeling of Segregation Behavior during Solidification of a Sn-6 wt.% Pb Alloy

“…In the last five years, the research on numerical simulations of the segregation of Sn-Pb alloys is mainly based on two-dimensional (2D) models. Kumar et al [35] developed a 2D model of mushy zone instability for the characterization and prediction of channel segregation in Sn-5 wt.% Pb alloy casting under horizontally solidified conditions. Wu et al [36,37] established a 2D model to simulate the complexity of transient flow and macrosegregation distribution in a Sn-Pb alloy under vertically solidified conditions.…”

Numerical simulation of the EM forced flow during Sn-Pb alloy directional horizontal solidification

“…Moreover, the literature analysis showed that several computational codes were also validated on Hebditch and Hunt data. We also processed the data from these works [38,39,35,21] and plotted the results on this graph.…”

“…It should be noted that this research formulation and the chosen type of experimental setup were recognized as successful and later identical experimental setups were created in different laboratories [15,13,16]. Based on the experimental data obtained in this experiment, a bench of numerical models are successfully validated and tested (see for example [17,18,19,20,21,22,23]).…”

Numerical investigations on solute transport and freckle formation during directional solidification of nickle-based superalloy ingot

“…The casting produced by this technology has longer fatigue life and higher creep resistance, thus becoming the main process in the production of alloy components for advanced manufacturing industries, such as aerospace, automobile and medical care [1]. However, the change of liquid density caused by thermal and solute concentration gradient usually cause the thermosolutal convection, which may lead to elongated solute enrichment channels [2]. These channels usually cannot be eliminated by heat treatment or rolling process after solidification, showing as spot defects, also known as "freckles" [3].…”