Modelling mixed columnar-equiaxed solidification with melt convection and grain sedimentation - Part II: Illustrative modelling results and parameter studies

A volume-averaging multiphase solidification model was introduced in Part I. In Part II, illustrative simulations are made for two benchmarks, a unidirectional solidification benchmark and a cylindrical ingot casting, using a binary Al-Cu alloy. For the case of unidirectional solidification the competing growth of columnar and equiaxed structures, evolution of different phase regions, solute redistribution, and the influence of grain sedimentation and melt convection are analyzed in detail. The columnar-to-equiaxed transition (CET) is investigated, with important insights derived from the CET prediction. The new features of the model and its applicability to industrial-type castings are demonstrated with simulations of a cylindrical ingot casting. This is done in both a 2D axisymmetric and a full 3D geometric domain to demonstrate the ability of the model to produce consistent results. The main features of the model that are verified include tracking of the columnar primary dendrite tip, nucleation of equiaxed grains ahead of the columnar tip front, hydrodynamic and solutal interactions between the equiaxed and columnar structures, the columnar-to-equiaxed transition (CET), melt convection and grain sedimentation, and macrosegregation and the final macrostructure. With appropriate modelling parameters the typical columnar-equiaxed macrostructure observed in experiments can be reproduced. Uncertainties due to model parameters and assumptions are addressed and discussed.