The origin of nuclei and the refinement mechanism for solidified superalloy IN718 under pulsed magnetic field
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The grain refinement effect of a pulsed magnetic field on superalloy IN718 was investigated. The experimental results show that fine equiaxed grains are acquired under the pulsed magnetic field. The refinement effect of the pulsed magnetic field is affected by the mold temperature. And the mixed grains are found in the solidified microstructures under the pulsed magnetic field. The origin of nuclei under the pulsed magnetic field is from the mold wall because of the rough mold surface, the undercooling, and the melt vibration, which all contribute to the refinement of solidified grains.
I. INTRODUCTION
B. Experimental apparatus and procedure
The electromagnetic processing of materials as a novel grain refinement approach was developed recently, in which a pulsed magnetic field arouses much interest because of its distinctive refinement effect. Researches show that a strong pulsed magnetic field can significantly refine the microstructures of aluminum alloy1 and columnar grains of stainless steel2,3 due to magnetic pressure and subsequent melt vibration. More recent research shows that a weak pulsed magnetic field can refine ascast grains of magnesium alloy remarkably well.4,5 The advantage of this method is the low voltage of the pulsed magnetic field, which can be easily used in practice. Our recent work6,7 shows that the weak pulsed magnetic field can refine the solidified grains of superalloy. In our articles, we suggested that the refinement effect of the pulsed magnetic field is attributed to the dissociation of nuclei from the mold wall by melt vibration and the subsequent dispersion of nuclei by melt convection. However, more detailed refinement mechanism needs to be proposed and more evidence should be provided. In this article, some new results are revealed and discussed.
The experimental apparatus for solidification under a pulsed magnetic field is sketchily shown in Fig. 1. The apparatus consists of a generator of the pulsed magnetic field, a cooling water system, heat insulation refractory, and an alumina mold with an inner diameter of 15 mm and height of 80 mm. In experiments, the superalloy IN718 was melted to 1550 C in an induction furnace, and then the melt was poured at 1410 C into the mold that was preheated in a resistance furnace together with the heat insulation refractory. The pulsed magnetic field was applied to the melt immediately after pouring. The frequency of excitation current is 5 Hz in all experiments discussed in this article. The melt was solidified with and without the pulsed magnetic field, respectively. And the effects of mold temperature on microstructure refinement under the pulsed magnetic field were investigated by altering the preheating temperature of the mold from 800 to 900 C. The ingots were cut into samples along the longitudinal axial. The samples were grinded, polished and then were etched by a CuSO4–HCl–H2SO4–H2O solution, and then the solidification microstructure of the samples was observed with an optical microscope.
II. EXPERIMENTAL PROCEDURE A. Material
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