New insights into the formation mechanisms of channel segregation in lateral solidification of alloys: A numerical study using an improved macrosegregation model

Abstract

Channel segregation has been extensively studied due to its detrimental impact on alloy castings and the complex fluid-mechanical phenomena underlying its formation. While the mechanisms of channel formation in upward solidification are well understood, those in lateral solidification remain elusive due to a lack of consensus on mushy zone destabilization. To address this gap, this study develops a comprehensive macrosegregation model incorporating several novel approaches: a rate equation describing solute accumulation in the solid phase, a concentration equation system ensuring global solute conservation, and an extended liquid fraction iteration method coupling macro- and micro-segregation within a thermodynamic equilibrium framework. The model is validated against a benchmark experiment and subsequently applied to investigate channel initiation. It is found that mushy zone destabilization is not primarily caused by remelting or flow instability, as suggested in previous studies, but rather by thermal gradient that induces protrusions on the solid/liquid (S/L) interface, forming a step-like morphology. The interdendritic flow is hindered at the protrusions, promoting localized solute accumulation and hence initiating channel segregation. The effects of microstructure and thermal conditions on channel development are further explored. Channel formation is significantly suppressed when the dendrite arm spacing becomes finer, and the morphology varies with the permeability laws used. Additionally, the inclination angle of the channels relative to gravity increases with increasing temperature gradient. Channel discontinuity is associated with disturbances in the bulk liquid flow pattern. Under high temperature gradients, thermally driven circulations remain stable over long time scales, leading to well-aligned and developed channels. In contrast, increasing the cooling rate effectively suppresses channel formation. Finally, Rayleigh-number-based criteria are employed to evaluate channel formation. A critical Rayleigh number of 0.55 is identified for channel initiation, and when it exceeds 1.0, highly developed channels are more likely to form.