Casting superalloy is a crucial material for aerospace vehicle. In order to improve the mechanical properties of nickel based casting superalloy, especially its anti-fatigue property, fine equiaxed casting grains are expected. But the casting processing for superalloy tends to produce coarse microstructure. Thus, efficient fine grain casting processing for superalloy has aroused much research. Recent years, the electromagnetic processing of materials provides a new method for grain refinement. In this dissertation, fine grain casting processing for nickel based superalloy is researched by applying a pulsed magnetic field with controlling the heat transfer in solidification. The coupled physical field, electromagnetic force, melt flow and Joule heat, are simulated by FEM software ANSYS for nickel melt under the pulsed magnetic field. In experiments, the effect of frequency, exciting voltage and applying time of the pulsed magnetic field on the refinement is investigated, and the influence of pouring and mold temperature on refinement effect of pulsed magnetic field is researched. Finally, the refinement mechanism of the coupled pulsed magnetic field and thermal control is studied, and the pulsed magnetic field is applied to thin-wall casting of superalloy.
The finite element model is established for the nickel melt under the pulsed magnetic field, the magnetic field and flow field are simulated in different stages of the pulse period by using ANSYS software. The results show that the pulsed magnetic field produces a periodic magnetic field which is stronger in the pulse stage compared with the pulse absent stage. In the pulse stage, the magnetic field concentrates near the surface of the melt. The variable magnetic field produces excited current which then reacts with the magnetic field and finally produces the electromagnetic force. The electromagnetic force is an alternate push and pull force which is stronger in the pulse stage compared with the pulse absent stage. The decrease of pulse stage time and the increase of exciting current can intensify the magnetic field and the magnetic force.
The simulation results show that the component of the electromagnetic force in the axial direction produces convection in the melt. The velocity of the convection increases at the pulse stage but decreases at the pulse absent stage. The radial component of the electromagnetic force produces vibration in the melt. The excited current caused by the pulsed magnetic field produces Joule heat in the melt. Because of the skin effect of the excited current, the Joule heat concentrates in the melt near the surface of the mold.
The experiment results show that the pulsed magnetic field can refine the solidified microstructures of superalloy K417 and IN718 greatly. The fine equiaxed grains about 60μm are obtained. More fine solidified grains are produced with the increase of exciting voltage and frequency of the pulsed magnetic field, but the grains are coarsed when the pulse frequency increases to 10Hz. The applying time of the pulsed magnetic field significantly influence the refinement effect. The refinement effect of the pulsed magnetic field becomes weak if the applying time is delayed.
Thermal control and the pulsed magnetic field have interaction on grain refinement of the superalloys, which means that pouring temperature and mold temperature should been controlled. The solidified microstructure is refined by the pulsed magnetic field with low pouring temperature and high mold temperature. As the increase of pouring temperature and decrease of mold temperature, the refinement effect of the pulsed magnetic field is impaired. The fine grain casting is achieved for a thin-wall casting with different thickness by coupled the pulsed magnetic field and thermal control, and the size of refined equiaxed grains is about 50μm.
The experiments of solidification with a stainless sieve and different mold are carried out to explain the refinement mechanism of coupled the pulsed magnetic field and thermal control. The detachment mechanism of heterogeneous nuclei from the mold wall is proposed for the refinement effect of pulsed magnetic field. Because of large undercooling and much nucleation position, abundant nuclei are produced on the mold wall. The melt vibration produced by the pulsed magnetic field will promote the nuclei to detach from the mold wall, and then the nuclei can be dispersed uniformly in the whole melt by the melt convection produced by the pulsed magnetic field. The Joule heat and the skin effect by the pulsed magnetic field will prolong the formation time of the solidified shell near the mold wall, which promotes the detachment of nuclei from the mold wall. Higher mold temperature can defer the formation of the solidified shell, which is beneficial to the detachment of nuclei, and lower pouring temperature can increase the survival probability of the detached nuclei in the melt. Finally, the copious nuclei in the melt grow to the fine equiaxed grains.
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