Numerical Simulation of Solidification Structure of Continuously Cast Billet with Grain Motion
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THE mechanical properties of alloy casts are mainly determined by their solidification structure and the solute distribution. For high-quality casts, it is necessary to fully understand the multiscale transport phenomena (such as nucleation; grain growth and motion; and momentum, solute, and heat transfer) and the interactive effects occurring during the phase transformation process. The motion of free grains in melt is commonly observed during the solidification process under terrestrial environment and has a great influence on the solidification structure. These free grains usually produced from bulk nucleation and detachment of dendrite arms, and their movement is mainly driven by two forces: (1) a gravity-induced force due to the density difference between solid and melt and (2) the force due to the relative motion of solid and liquid. Grain motion obviously influences solidification structure and solute
ZHAOHUI WANG, SEN LUO, WEILING WANG, and MIAOYONG ZHU are with the Key Laboratory for Ecological Metallurgy of Multimetallic Mineral (Ministry of Education), Northeastern University, Shenyang 110819, Liaoning, P.R. China and also with the School of Metallurgy, Northeastern University, Shenyang 110819, Liaoning, P.R. China. Contact e-mail: [email protected]. Manuscript submitted May 21, 2020.
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distribution of casts, especially those that are large sized because of the existence of long-range grain motion. For example, it is well-known that the proportion of equiaxed grains in continuously cast billets can be promoted via mold electromagnetic stirring. This phenomenon is supposed to be related to the enhanced transfer of dendritic fragments from the mushy zone to the liquid zone and the accumulation of free grains before columnar grains because of rotation flow.[1] In addition, some studies report that the sedimentation of low-concentration grains could be responsible for the negative segregation at the bottom of large steel ingots.[2–4] Therefore, investigations on the formation of free grains, the growth of free grains in fluid flow, the rotation and translation of grains, and the effect of grain motion on the microstructure and solute distribution are very important for further improving the quality of casts. In the past, many researchers have studied grain motion during the solidification process and reported interesting results. Initially, experiments on the growth of movable dendritic grains were mainly carried out with transparent materials because of the convenience of observation. Appolaire et al.[5,6] recorded the morphology and positions of settling equiaxed grains in undercooled NH4Cl-H2O solution with a camera and discussed the evolution of grain size and settling velocity with time. Esaka et al.[7] observed the solidification of transparent flowing succinonitrile and they found that
particles from the columnar grain zone increase with flow rate. Later on, with the help of X-ray radiographic techniques, in-situ observation of micro-scale behavior of metal solidificat
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