Effect of Electromagnetic Oscillation on Grain Structure of K477 Superalloy Turbine Guide Castings

The electromagnetic oscillation treatment was introduced to enhance the fine-grain quality of the Cr–Co–Mo K477 superalloy turbine guide castings, and the effect of electromagnetic oscillation on the microstructure was investigated. The results indicate that the electromagnetic oscillation induced the large columnar grains into fine grains, with a reduction of 59.9% and 30.3% in grain sizes at the blade concave and inlet edge, respectively, and the Micro-Vickers hardness increased by 4.6%. The application of an alternating magnetic field in the melt induces Lorentz force, leading to short-range forced oscillations that break dendrites and inhibit the growth of columnar grains. On the one hand, the magnetic field penetrates the melt and decays due to the skin effect, and the decay rate of the magnetic field after penetrating the melt is related to the conductivity of the melt and the frequency of the magnetic. On the other hand, the electromagnetic oscillation device is positioned beneath the investment casting mold, causing a distribution of large magnetic field at the lower part and small at the upper end. Thus, along the height direction, the grain size gradually increases and tends to be irregular. The magnetic field cannot cover the blades entirely, exhibiting a fine grain—short columnar grain—coarse grain distribution. The refinement effects of the grain are more prominent when the height of the blades is lower and the blade structure is shorter.