SUMMARYNumerical models of solidification including a mushy zone are notoriously inefficient; most of them are based on formulations that require the coupled solution to the velocity components in the momentum equation greatly restricting the range of applicability of the models. Initial attempts at modeling directional solidification in the presence of a developing mushy zone using a projection formulation encountered difficulties once solidification starts, which were traced to the inability of the method to deal with large local density differences in the vicinity of the fluid-mush interface. A modified formulation of the projection method has been developed that maintains the coupling between the body force and the pressure gradient and is presented in this work. This formulation is shown to be robust and extremely efficient; reducing very significantly the necessary storage and the computational time required for the simulation of problems involving very large meshes when compared with previously published data. This is illustrated in this work through its application to simulations involving a Pb-Sn alloy.

SUMMARYIn the present work, the element-free Galerkin (EFG) method is applied to a continuum solidification model that calculates thermosolutal convection and macrosegregation during dendritic solidification of multicomponent alloys. Simulations for directional solidification of a binary Pb-Sn alloy and a Ni-base quaternary alloy have been performed in a rectangular two-dimensional domain. In both calculations, the alloy melt is cooled from below and the growth of the mushy zone is followed in time. The formation of macrosegregation defects known as 'freckles' has been successfully simulated using the meshless EFG method. A varying degree of sensitivity of results to the number and distribution of meshfree particles was obtained. The potential of the method for a broader range of solidification models is discussed.

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SUMMARYNew algorithms developed for the simulation of macrosegregation in directionally solidified alloys are presented. The work has been motivated by the inefficiency of models that have been used in the past. Three different developments are reported, the use of a fractional step projection finite element method to solve the momentum equations, the use of mesh-less element free Galerkin formulations, and the combination of the Galerkin finite element method with adaptive mesh refinement. The formulation of the different algorithms is given and a two-dimensional example of application in which the efficiency of the methods is compared is presented. The results of a three-dimensional calculation using the Galerkin finite element method show that very significant gains are effected when compared with previously published simulations.

Modeling the formation of macroscopic segregation channels during directional solidification processes has important applications in the casting industry. Computations that consider thermosolutal convection involve different length scales ranging from the small solute boundary layer at the dendrite tips to the characteristic size of the casting. In general, numerical models of solidification in the presence of a developing mushy zone are computationally inefficient because of nonlinear transport in an anisotropic porous medium. In the current work, mesh adaptation with triangular finite elements is used in conjunction with an efficient fractional-step solver of the momentum equations to predict the occurrence of channel-type segregation defects or freckles. The triangulations are created dynamically using an unstructured grid generator and a refinement criterion that tracks the position of the channel segregates. The efficiency of mesh adaptation is illustrated with simulations showing channel formation and macrosegregation in directional solidification of a Pb-Sn alloy.