Experimental and numerical investigation of dendrite fragmentation, solute concentration, and channel formation in a transparent NH4Cl-70.43%H2O solution

Understanding and predicting the effect of hot melt flow on solidification structure is critical for enhancing the homogeneity of both the structure and elemental distribution. This study developed a mixed columnar-equiaxed solidification model that incorporates a mechanism for columnar dendrite fragmentation. This model was applied, in conjunction with NH 4 Cl-70.43%H 2 O experiments, to investigate the evolution of the mushy zone and melt flow during solidification. Analysis of the simulation results has yielded new knowledge into the flow-solidification interactions: a satisfactory experiment-simulation agreement in the solidification structure, columnar dendrite fragmentation, solute (H 2 O) concentration, and defect formation, was obtained. The water-rich solution rejected by columnar dendrites growing from the sidewall is transported upward in the mushy zone under the effect of solutal buoyancy forces. Part of the colder water-rich solution, during its concentrated transport, forms a channel. At a height of 74 mm, this channel is located about 1 mm from the sidewall and has a width of about 3 mm. Upon discharge from the top corner, the temporal trend of the simulated fragmentation-induced grain density increase shows good consistency with the experimentally observed evolution of the grain cloud area. Both trends reach their respective peaks at 1080 s and 960 s, with peak values of 25.84 mm 2 and 1.25 × 10 7 m −3 s −1 , respectively. The low-temperature, water-rich solution accumulates in the upper region of the melt pool, resulting in melt stratification and lower upper temperature. These findings are relevant to metallurgy, materials science, chemical engineering, and environmental science. They contribute to controlling crystal formation and growth, and provide a theoretical foundation for enhancing industrial process design.