Dissertation
Modeling of macrosegregation and shrinkage cavity during solidification of a multi-components steel ingot
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2019
DOI: 10.17077/etd.005194
Abstract
One of the most important solidification defects which is the subject of the present study is macrosegregation, addressed to chemical inhomogeneities in castings after solidification. In fact, not only the as cast product but also subsequent processes after the casting, like welding, rolling, heat treatment and machining are significantly influenced by the chemical composition of the material. For example, electrode selection for welding process, design of thermo-mechanical process for the rolling, cooling and heating regime of heat treatment and cutting plan are carried out based on the composition of materials. For instance, segregated regions exhibit different microstructure and consequently different mechanical and physical properties in comparison with the bulk after the heat treatment process. It’s important to note that binary alloys have limited applications in industries. Because of this, most of the casting productions are categorized as multicomponent alloys which have three or more elements in their chemical composition. The purpose of the present study is to predict the macrosegregation pattern of a multicomponent 13-tons steel ingot.
Prediction of macrosegregation in the ingot casting process is highly depending on the thermal boundary conditions, properties and microstructure of material. Having access to the precise experimental data improve the accuracy of the prediction. Hopefully, a comprehensive experimental analysis, including thermocouple data, X-ray fluorescence (XRF) of chemical composition and microstructure of ingot after the solidification is available for this 13-tons steel ingot which makes it possible to have a precise prediction. In order to predict the macrosegregation pattern we need to solve the conservation equations of mass, momentum, energy and solutes. For this purpose, a previously developed multi-phase model for binary alloys was extended to be applicable for prediction of macrosegregation in a multicomponent system. This new developed model can be applicable for any multicomponent alloy not only steel. Subsequently, ingot solidification, by considering all casting components, was simulated in MAGMASOFT and acceptable agreement obtained with thermocouple data. After that, the thermal boundary conditions were extracted from MAGMASOFT simulation and used as boundary conditions in multi-phase model, which is implemented in openFOAM platform, for prediction of macrosegregation. Consequently, macrosegregation was predicted by considering infinite diffusion in solid (lever rule), and using constant phase diagram properties. This macrosegregation pattern was compared with the experiments.
In order to improve the accuracy of the simulations, new phase diagram properties varying with temperature and solute were developed using JmatPro (a CALPHAD-based tool for prediction of phase diagram properties). It was interesting that, using these new developed properties the accuracy of the simulation results improved significantly.
Due to the fact that using Lever and Scheil models results in predefined solidification paths, a model by considering back-diffusion in the solid, which is more realistic, developed and implemented. In this model, depends on the cooling rates and materials properties, the solidification path varies between the Lever and Scheil models. This implemented model verified by comparing the solidification paths obtained from simulations of binary and multi-components alloys with the one from the JmatPro. Finally, solidification of multi-component steel ingot with back-diffusion in solid and using new developed phase diagram properties was simulated and compared with the XRF elemental map. In the last chapter, a simplified model for prediction of shrinkage cavity on the top of the ingot is presented. This model calculates the total shrinkage volume of the ingot at each time step and then distribute it over the cells which have enough liquid for feeding process. The next level of this study would be redistribution of the solute elements based on the shrinkage pattern on the top of the ingot. In addition, prediction of columnar to equiaxed transition (CET) may change the solute distribution in the castings and need to add to the model.
Details
- Title: Subtitle
- Modeling of macrosegregation and shrinkage cavity during solidification of a multi-components steel ingot
- Creators
- Amir Baghani
- Contributors
- Christoph Beckermann (Advisor)H.S. Udaykumar (Committee Member)Albert Ratner (Committee Member)James H.J. Buchholz (Committee Member)Hongtao Ding (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Mechanical Engineering
- Date degree season
- Autumn 2019
- Publisher
- University of Iowa
- DOI
- 10.17077/etd.005194
- Number of pages
- xiv, 91 pages
- Copyright
- Copyright 2019 Amir Baghani
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 88-91).
- Public Abstract (ETD)
Many metallic materials are produced by the aim of metal casting process. Ingot is a piece of metal which has a known chemical composition and frequently used for a second operation such as cold or hot working, machining and remelting. One of the most important defects in ingot casting is macrosegregation which is referred to the non-uniform distribution of the chemical elements in the casting products. Solidification in ingots starts from the walls which are colder than the hot liquid metal and continues to the middle and top portion of the ingot which solidified last. During solidification, solute elements are rejected from the solidified portion to the middle of the ingot which still contains liquid metal. These rejected elements are redistributed by the melt convection and other buoyancy forces. For instance, the lighter liquid metal which contains lighter elements and hotter liquid tends to move upwards. In this research a numerical model was developed to solve mass, momentum, energy and solute conservation equations to predict macrosegregation during the solidification of a 13-tons multicomponent steel ingot. The final simulation results were compared to the experiments which showed acceptable agreement.
- Academic Unit
- Mechanical Engineering
- Record Identifier
- 9983779799202771
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