Efficient Melt Stirring Using Pulse Sequences of a Rotating Magnetic Field: Part II. Application to Solidification of Al
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NETIC fields are a powerful tool to provide expedient flow pattern in the melt with prospects for tailoring the microstructure of the solidifying alloy. There are two main reasons electromagnetic stirring is attractive for industrial applications: (a) this method allows for a completely contactless influence on the liquid metal; and (b) a direct and simple control of the flow intensity is possible through electrical control parameters, namely, electric current and frequency. In addition, various kinds of magnetic fields can in principle be combined at random, providing the possibility to create different flow pattern as required. Previous studies particularly concerned with applications exclusively of a rotating magnetic field (RMF)[1–4] or a traveling magnetic field (TMF)[5–7] to solidification processes in order to achieve a purposeful alteration of the microstructure of casting ingots. Generally, the electromagnetically-driven convection promotes a transition from a columnar to an equiaxed, dendritic growth (CET) and provokes a distinct grain refining effect. The production of equiaxed, fine grain structures is often preferred in solidification processes, because such morphologies promise to improve the homogeneity of the B. WILLERS, Senior Research Scientist, S. ECKERT, Group Leader, D. RA¨BIGER, Graduate Student, and G. GERBETH, Department Head, are with the MHD Department, Forschungszentrum Dresden-Rossendorf, FZD, 01314 Dresden, Germany. Contact e-mail: [email protected]; J. DONG, Postdoctoral Fellow, formerly with the MHD Department, Forschungszentrum Dresden-Rossendorf, is with School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, P.R. China. P.A. NIKRITYUK, Postdoctoral Fellow, and K. ECKERT, Group Leader, are with the Institute of Aerospace Engineering, Dresden University of Technology, 01062 Dresden, Germany. Manuscript submitted August 13, 2007. Article published online March 18, 2008. 304—VOLUME 39B, APRIL 2008
alloy properties as well as the mechanical properties of the castings. On the other hand, forced convection in the melt may enhance a relative motion of alloy species, which has been identified to be responsible for macrosegregation especially occurring in solute-rich alloys. Usually, the equilibrium concentration of solute in the primary crystal is lower than in the melt, which leads to the formation of a thin, diffusion-controlled boundary layer ahead of the solidification front. A sufficiently strong convection toward the solidification front reduces the thickness of the boundary layer. Redistribution of the excessive solute by convective transport might result in the formation of pronounced segregated zones in the solidified ingot. For instance, channel segregates develop because of thermosolutal convection in the mushy zone.[8,9] Additional melt stirring using electromagnetic forces modifies the melt flow and hence, influences the development of such segregation defects. Medina et al.[7] studied the impact of different configurations of an AC traveling magnetic field on the segreg
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