Study on the solidification process of NH₄Cl aqueous solution by volume-averaged method

The solidification process of NH₄Cl-70% H₂O ingots was simulated and experimentally investigated based on the solidification theory and a volume-averaged multiphase solidification method. Although the solidification process of NH₄Cl-70% H₂O ingots has been investigated previously,but these researches are mainly focused on the single phenomenon of the process such as the formation of channel segregation,convection or the formation of grains. On the basis of fore-researches,combining numerical simulation with experiment,nearly all the phenomena occurring in the solidification process of the ingot were investigated in this paper,especially the equiaxed crystals' floating and settling down on the bottom of the ingot,and the convection induced by such floating of equiaxed crystals which was reproduced by the simulation. Finally,the mechanism of the forming of macro-segregation was deeply discussed.The calculation showed that equiaxed grains floated down from the mold wall and tended to settle down on the bottom of the ingot.When the volume fraction of equiaxed grains accumulated up to a critical value,columnar grains would stop their growth,and so the columnar-to-equiaxed transition(CET) process was to the end. Owing to solute partitioning and the sedimentation of equiaxed grains,there was negative macro-segregation in triangle shape on the bottom of the ingot,while a wide range of positive macro-segregation was constructed on the upper part of the ingot. The calculated results are relatively conformed to the experimental ones in aspects of the sedimentation of equiaxed grains and the induced fluid convection,indicating that the key factors leading to macro-segregation are crystal sedimentation and fluid convection.