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Acta Metall Sin  2015, Vol. 51 Issue (7): 844-852    DOI: 10.11900/0412.1961.2014.00692
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Yuefei TENG,Yingju LI,Xiaohui FENG,Yuansheng YANG()
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016
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The researches on the grain refinement by applied pulsed magnetic field (PMF) during solidification have received much attention in recent years and lots of positive experimental results indicate that it is a potential method for controlling solidification process. Various grain refinement mechanisms under PMF are proposed and most of them are considered to be relevant to the convection of melt driven by the electromagnetic force. An obvious fact is that the forced convection caused by PMF is strongly limited by the shape of the melt. However, most of previous studies were focused on the cylindrical samples rather than rectangular ones, and actually the later one was widely used in industry. The aim of this work is to investigate the influence of PMF on the grain refinement of K4169 superalloy rectangular samples with various aspect ratios. Grain refinement of K4169 superalloy under PMF was experimentally investigated in the rectangular samples with the aspect ratios of 1.0, 2.0, 4.5 and 5.5 on the transverse section. In order to study the influence of aspect ratio on the forced convection, the distributions of the electromagnetic field, electromagnetic force and melt flow caused by PMF were numerically simulated by finite element software ANSYS. The experimental results show that the grains of the K4169 rectangular samples are coarse equaxied grains without PMF and the grain size slightly decreases with the increase of aspect ratio . Under the PMF with same excitation voltage and frequency, the grains are refined remarkably in the sample with the aspect ratio of 1.0. As the aspect ratio is increased, the grain refinement effect can still be observed but not such obvious. The numerical simulation results indicate that the periodic pushing-pulling electromagnetic force is induced by the PMF, which drives the melt to vibrate and flow circularly. Under the same PMF, the electromagnetic force and fluid rate decreases with the increase of aspect ratio. When the aspect ratio increases from 1.0 to 5.5, the average electromagnetic force and fluid rate in the melt is reduced to 40% and 60%, respectively. The strongest fluid flow and vibration occur in the sample with section aspect ratio 1.0 in the present experiment, which is beneficial for grain refinement due to detachment of the solidified nuclei from mould wall and the break of dendrite arms from dendrite trunks.

Key words:  superalloy      grain refinement      pulsed magnetic field      aspect ratio      numerical simulation     
Fund: Supported by National Natural Science Foundation of China (No.51034012) and National Basic Research Program of China (No.2010CB631205)

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Fig.1  Excitation current of pulsed magnetic field in one period (ta—ascending stage, td—descending stage, tp— pause stage)
Fig.2  3D FEM model and mesh of rectangular sample with aspect ratio of 5.5
Fig.3  Solidification microstructures of superalloy K4169 rectangular samples without (a, c, e, g) and with (b, d, f, h) pulsed magnetic field under aspect ratios of 1.0 (a, b), 2.0 (c, d), 4.5 (e, f) and 5.5 (g, h)
Fig.4  Average grain size of K4169 rectangle samples with different aspect ratios without and with pulsed magnetic field (PMF)
Fig.5  Distributions of magnetic flux density at peak value of excitation current (a) and descending stage of excitation current (b)
Fig.6  Distributions of induced current density (a, b) and electromagnetic force (c, d) at peak value of excitation current (a, c) and descending stage of excitation current (b, d) (Detailed figures in the upper right corner show the corresponding distributions on the middle transverse section)
Position Calculated Measured
I 0.056 0.052
II 0.051 0.048
III 0.134 0.128
Fig.7  Maximum and average electromagnetic force densities in rectangular samples with different aspect ratios
Fig.8  Evolution of fluid rate at 75 mm height on central axis of the sample with aspect ratio of 1.0 (Inset shows the fluid rate in 5 pulse periods between 25 s and 26 s)
Fig.9  Fluid patterns under pulsed magnetic field in rectangular samples with aspect ratio of 1.0 (a), 2.0 (b), 4.5 (c) and 5.5 (d) at 25 s
Fig.10  Maximum and average fluid velocities in rectangular samples with different aspect ratios
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