基于体积平均法的NH_4Cl水溶液凝固行为研究

Study on the solidification process of NH_4Cl aqueous solution by volume-averaged method

根据合金凝固理论和体积平均多相模型,对NH4Cl-70%H2O凝固过程进行了数值模拟和实验验证.虽然研究者已研究过NH4Cl-70%H2O凝固过程,但是只针对单个现象进行分析,比如通道偏析的形成、对流形式以及晶粒的形成.在前人研究的基础上,本文首次通过数值模拟和实验对比两种手段相结合的方式全面地研究了氯化铵水溶液凝固整个计算域的全部现象,尤其再现了等轴晶在铸锭中的下落漂移现象以及由此引起的对流,并且更深入地探究了偏析的形成原因.通过计算发现等轴晶从型壁处沉降并逐渐向铸型底部积聚,直到体积分数达到一临界值后,柱状晶停止生长,完成柱状晶向等轴晶转化过程.由于溶质再分配,底部晶粒集中的区域形成了负偏析,在尚未凝固的上部区域形成较大范围的正偏析.通过实验验证发现,等轴晶在铸锭中的下落漂移现象和对流形式的预测值与实验值较为一致,从而全面揭示出凝固过程中固相的移动和对流是产生宏观偏析的关键因素.

The solidification process of NH4Cl-70% H2 O 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% H2 O 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.