Grain Refinement of Inconel 718 Superalloy-The Effect of Rotating Magnetic Field

The effect of the application of a rotating magnetic field on the average grain size of IN718 castings was experimentally studied. For the purpose, four parts were produced by investment casting and characterized. The first casting was produced without application of RMF for comparison. The remaining ones were submitted to different RMF frequencies for 15 min and subsequently to the pouring of the nickel-based superalloy. In these three castings, the RMF frequencies applied were, respectively, 15 Hz, 75 Hz and 150 Hz. All the other process parameters were kept constant during the execution of the experimental procedure. The average grain size of the samples was determined according to the ASTM E112-13 standard, using intercept methods. Macro hardness measurements, tensile testing and SEM-EDS analysis were conducted in order to evaluate the casting's mechanical properties and microstructures. The results demonstrate a noticeable grain size reduction in the samples submitted to rotating magnetic field. An average grain area reduction, greater than 96%, was achieved in the castings where RMF frequencies of 75 Hz and 150 Hz were applied. The application of RMF also caused a morphological change in the casting's dendrites from cellular to almost equiaxed. Additionally, it originated the decrease of the size and amount of needle-like δ phase. Regarding mechanical properties of the cast parts, no major differences were verified.