Effect of Temperature Profile Curvature on the Formation of Atypical Inhomogeneity of Dendritic Microstructure Across the Width of a Single Crystal Blade

Abstract The effect of temperature profile curvature on the formation of atypical inhomogeneity of the dendritic microstructure over the width of a single crystal blade was investigated. Blade castings were produced by the Bridgman directional solidification method with a mold withdrawal velocity of 5 mm/min. The inner radiation baffle technique was used to flatten the temperature profile. It was found that an increase in temperature profile curvature resulted in greater inhomogeneity of the dendritic microstructure. When the inclination angle of the temperature profile reached 20 deg, difference in PDAS values between the edge and middle regions of the cross section increased to 206 µ m. This significant inhomogeneity was mainly due to the unexpected formation of the most refined microstructure in the region close to the shadow zone of the casting cross section, in both the root and airfoil of the blade, although the temperature gradient value in this area did not allow for such a low PDAS. However, flattening the temperature profile resulted in a reduction in microstructure inhomogeneity, obtaining similar PDAS values over the width of the casting. The mechanism of formation of dendritic microstructure inhomogeneity was proposed, in which it was assumed that apart from the temperature gradient, the convective melt flow and the direction of the circular flow pattern above the solidification front, driven by the curvature of the temperature profile, also influenced the growing dendrites. Finally, it was concluded that the temperature profile inclination should also be considered when designing the dendritic microstructure of single crystal blades.