Four-Phase Dendritic Model for the Prediction of Macrosegregation, Shrinkage Cavity, and Porosity in a 55-Ton Ingot
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LARGE steel ingot has been widely used to produce heavy steel components to various industries, such as next generation nuclear power plant, ship building, and industrial machinery. Defect controlling, including macrosegregation, shrinkage cavity, and porosity, is one of the key factors ensuring the quality of large steel ingot. However, the formation mechanisms of these defects are extremely complicated, which may be due to not only multiphasic formation process but also the complex interactions among different phases. Elucidating the dominant factors from each other during solidification experiment may not be a practical attempt. Developing simulation and modeling is, therefore, of great necessity. In terms of macrosegregation, it is generally believed to be a compositional heterogeneity defect in most large-scale or complex structure ingots/castings. The formation of this defect has been the subject of extensive study in the past half centuries. In 1970s, Hultgren[1] suggested that the macrosegregation appears when relative flow between the liquid and solid phases occurs during solidification. This relative flow is normally caused by many factors, including thermal buoyancy, solutal buoyancy, sedimentation or floatation of free moving grains[2] or inclusion,[3] solidification HONGHAO GE, FENGLI REN, JUN LI, XIUJUN HAN, MINGXU XIA, and JIANGUO LI are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240 P.R. China. Contact e-mail: [email protected] Manuscript submitted May 24, 2016. Article published online January 3, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
contraction induced fluid flow, stirring, deformation.[4] A general approach for predicting macrosegregation is modeling these flows with a continuum formulation. Several numerical models were developed to predict macrosegregation since the first mushy zone model was introduced by Fujii et al.[5] in 1970s. After that, the grains sedimentation and melt convection were taken into account by Wang et al.[6] in 1990s. Combeau et al.[7] studied the effects of the morphology and motion of equiaxed grains on the final macrosegregation but the effects of the columnar phase were omitted . Wu and Ludwig[8] provided a mixed three-phase model, which takes the interaction between liquid, equiaxed, and columnar phases into consideration during solidification, in 2000s. This mixed three-phase model was later extended to a five-phases model[9] with the consideration of both columnar and equiaxed dendritic structures. Due to the complexity of the model and limited computational capabilities, however, up to now, this model is being mainly applied to laboratorial scale ingots. Furthermore, the solidification contraction is omitted. In addition, the comprehensive reviewer work of macrosegregation modeling has been made by Pickering[10] and Ludwig.[11] On the other hand, some microstructure models coupled macrosegregation were developed in recent decades. A coupled Cellular Automaton (CA)-Finite Element (FE) model is developed
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