Formation of as-solidified structure and macrosegregation

Motivation and goal: Most metal alloys are (or have to be) produced by means of a solidi- fication process. How to optimize the structural and compositional homogeneity of the as-solidified metal materials still represents a major challenge for materials scientists. The main reason is that the role of the flow in the formation of as-solidified structure is not yet fully understood. For example, a casting sample, as solidified in the materials science laboratory of the International Space Station, where the flow is suppressed, can obtain an ideal columnar structure (directionally-oriented crystals); while a casting as solidified under normal gravity conditions, where the flow is not avoidable, may have an equiaxed structure or a mixed columnar-equiaxed structure. The hypothesis is that the flow causes fracturing of columnar crystals and forms many crystal fragments. These fragments are further transported by the flow into other regions of the casting, and they continue to grow, leading to the structural transition from a columnar crystal structure into an equiaxed crystal structure. These phenomena are known as fragmentation and columnar-to-equiaxed transition. This Austrian-Hungarian joint project is going to es- tablish and refine a multiphase solidification model to calculate and predict the as- solidified structure and macrosegregation by considering fragmentation and its subse- quent influence on the formation of the assolidified structure under different flow con- ditions. In order to verify the numerical model, a precisely controlled laboratory exper- iment has been designed: an alloy sample solidifies unidirectionally under the controlled flow conditions by means of implementing a rotational magnetic field (RMF). State-of-the-art: Although studies on fragmentation phenomena during solidification have been carried out in last decades, no numerical model exists to calculate the for- mation of as-solidified structure by considering the fragmentation as the origin of the equiaxed crystals. There is no model which can couple fragmentation with the flow and crystal transport. Methods: An advanced solidification model was recently proposed by the applicant (Aus- tria). It is based on the volume-average approach, and has the capacity to deal with the mixed columnar-equiaxed dendritic solidification. This model will be extended for the new functionality of fragmentation. The RMF-induced flow and crystal transport will also be integrated into the solidification model. The experimental side of the project will be performed by the Hungary applicant. A setup of the unidirectional solidification (Bridgman-type) under RMF stirring is available. This device is subject to corresponding modifications for the purpose of current project. Expected results and novelties: (1) A novel model with the capability for calculation of the fragmentation, the flow and crystal transport, and their influence on the formation of as- solidified structure and macrosegregation during solidification of alloy castings will be developed and experimentally verified. (2) By reproducing the laboratory experiment numerically, new knowledge about the formation of as-solidified structure and mac- rosegregation during alloy solidification will be obtained. (3) The experimentally- verified model has an application potential to optimize the structural and compositional homogeneity of as-solidified metal materials.

Controlling the microstructure and macrosegregation (compositional heterogeneity) of cast-ings has been since long a goal for metallurgists and materials scientists. The development of modern production techniques has led to greater possibilities to influence the solidification process, hence to improve the microstructure and reduce the macrosegregation in the as-solidified products. The as-solidified structure of different alloys can be completely modified by applying a rotational magnetic field (RMF). This promising result has motivated the industry to develop new production processes. Unfortunately, the mechanisms behind the structural and compositional modification by RMF, due to the complexity of the involving solidification and coupled multiphase transport phenomena, are not fully understood. Implementing this technique into practice is not possible without a series of costly experiments, ranging from laboratory experiments and pilot-plant trials to final production. The motivation of this Austrian-Hungarian joint project is to establish and refine a multiphase solidification model. The purpose is to calculate and predict the as-solidified structure and macrosegregation by considering fragmentation, the equiaxed-to-columnar transition (ECT), and columnar-to-equiaxed transition (CET) under different flow conditions. The main out-comes include two parts: (I) model development and validation, and (II) exploring new solidification knowledge behind the solidification experiment. Regarding the model development and validation, several sub-models for solidification have been developed. They include (I) a capil-lary-driven fragmentation model, (II) a grain remelting and destruction model, and (III) a diffusion-governed solidification model for intermetallics. All the developed sub-models were vali-dated by solidification experiments of different alloys. The most interesting knowledge ob-tained from this project includes: (I) the origin of fragments via capillary-driven and flow-driven fragmentation mechanisms, (II) CET and ECT under natural convection and forced flow conditions, (III) simultaneous solidification/melting phenomenon during the casting process; and (IV) the formation mechanism of intermetallics during the solidification of AlSi7Fe1 alloy. The most recent findings have been published in world-renowned scientific journals and presented at international conferences.