A Multiscale Transient Modeling Approach for Predicting the Solidification Structure in VAR-Processed Alloy 718 Ingots
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THE purpose of this work was to develop a fully transient multiscale modeling approach to get an insight into the effect of processing parameters on the cast micro- and macro-structure during VAR processing. The developed model assists in achieving better control of the ingot solidification structure and to determine trends in the macrosegregation in these ingots. Another objective of this work was to analyze the transient effects of production rate and ingot size on the resultant ingot microstructures. This objective was met by first understanding the effects of the full range of processing parameters typically used in the VAR process. Processing parameters such as power input and heat extraction rate significantly influence the ingot liquid pool size and shape as well as the ingot structure, which in turn will greatly impact the final ingot quality. The model capabilities include the effects of process parameters (casting rate, power input, mold cooling conditions and ingot size) on the ingot structure at both the mesoscopic and the microscopic levels. At the mesoscopic level, grain size, grain morphologies (dendritic columnar or equiaxed), grain direction and columnar-to-equiaxed transition (CET) can be predicted. At the microscopic level, the size and amount of secondary phases, such as carbides and Laves phases can be simulated. Other capabilities of the developed model include the capability to analyze the effects of melt transients, typically observed in the VAR process. The overall multiscale modeling methodology is shown in Figure 1. Certainly, all length scales must eventually be coupled to accurately predict the material properties
LAURENTIU NASTAC, Professor, is with Department of Metallurgical and Materials Engineering, The University of Alabama, Box 870202, Tuscaloosa, AL 35487. Contact e-mail: [email protected] Manuscript submitted May 19, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
and as well as the specific product performance during exploitation. The previous published VAR models,[1–8] though some of them are also time-dependent, do not provide the most effective framework for fully transient computations of the solidification structure evolution by using the stochastic mesoscopic modeling approach. Also, some of them do not even have the capability of modeling the solidification structure evolution, which is a desirable feature of a VAR model. Another grain structure model was developed and coupled with another code[1,2] as described in Reference 9. However, as it can be seen in this paper and in the previous author’s work,[3–5] the developed ingot structure model is more comprehensive and compares more favorably with the experimental data[10] than the grain model presented in Reference 9. This paper describes the development effort and the validation of a comprehensive multiscale transient modeling approach capable of predicting the ingot solidification structure and solidification-related defects commonly occurring during the vacuum arc remelting (VAR) process.
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VAR PROCESS AND MULTISCALE
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