Effect of Withdrawal Rate on Non-uniform Distribution of Eutectic in Ni-based Single Crystal Superalloy Castings

Abstract:

The microstructure of single crystal superalloy is relatively simple, consisting primarily of γ dendrites and γ/γ′ eutectics. During the directional solidification process of Ni-based single crystal superalloys, withdrawal rate is a critical parameter affecting the spatial distribution of γ/γ′ eutectic along gravity direction. The results show that the γ/γ′ eutectic fraction of the upper platform surface is always higher than that of the lower one, regardless of withdrawal rate. As the withdrawal rate decreases, there is a significant increase in γ/γ′ eutectic fraction on the upper surface, while it decreases on the lower surface. The upward accumulation of γ/γ′ eutectic becomes more severe as the withdrawal rate decreases. It is also found that the percentage of Al+Ta is positively correlated with the γ/γ′ eutectic fraction. Thermo-solute convection of Al and Ta solutes in the solidification front is the prime reason for the non-uniform distribution of eutectic. The non-uniform distribution of γ/γ′ eutectic cannot be eliminated even after subsequent solution heat treatment, resulting in excess eutectic on the upper surface and thus leading to the scrapping of the blade.