Validation of a capillary-driven fragmentation model during mixed columnar-equiaxed solidification with melt convection and grain transport

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  • Christian-Doppler Lab for Metallurgical Applications of Magnetohydrodynamics


A mixed columnar-equiaxed solidification model was recently extended to capture the capillary-driven fragmentation phenomenon, which was considered the only mechanism for the formation of equiaxed crystals. The purpose of the present study was to validate the model by replicating a laboratory experiment on the solidification of an aqueous ammonium chloride solution (Gao and Wang, 1999). The experiment was performed by cooling the solution in a vertical test cell from the top surface to allow columnar dendrites to grow. Owing to the fragmentation of the downward-growing columnar dendrites, equiaxed fragments appeared, sedimented, and created a bed of crystals at the bottom of the cell. This pile-up of crystals ultimately met the columnar-tip front coming from the top, thereby leading to a structural transition (columnar-to-equiaxed transition). This experiment was successfully reproduced numerically for the first time, which involved coupling between the following phenomena: fragmentation, melt convection, grain transport, a pile-up of equiaxed crystals, and the potential growth of columnar dendrites from a bed of equiaxed crystals (equiaxed-to-columnar transition). A satisfactory agreement was achieved between the simulation and experimental results. Knowledge about capillary-driven fragmentation was strengthened by analyzing the microstructural evolution. Alloy-dependent parameters Ss0, K0, and a that govern dendrite coarsening and fragmentation were proposed for an aqueous ammonium chloride solution. Finally, the limitations of the current version of the fragmentation model were discussed.


Original languageEnglish
Article number101462
Number of pages12
Issue numberJune
Early online date26 May 2022
Publication statusPublished - Jun 2022