Eliminating Equiaxed Grain Defects in a Ni‐Based Single‐Crystal Blade Platform by Flattening the Liquidus Isotherm

Abstract

The article presents a mechanism for the formation of equiaxed grain defects and a new approach to eliminate them during the directional solidification of single‐crystal CMSX‐4 nickel‐based superalloy blades. The standard Bridgman method produces blade castings with clusters of equiaxed grains in platform areas above the root corners containing long freckle chains. The mechanism proposed considers the curvature of the liquidus isotherm and suggests that isolated dendrite fragments move upward from the freckle channel to the melt of the platform area. The floating surviving grains settle at the columnar dendrite tips, where they continue to grow into large equiaxed grains that block the development of the single‐crystal structure in the platform area located above the root corners. To eliminate these defects from the platform, a new approach is proposed, which involves introducing inner radiation baffles along the mold height to control the liquidus isotherm curvature in the blade. This stops the movement of small dendritic fragments from the root corner to the platform area, resulting in a significant reduction of freckles at the root and obtaining a typical single‐crystal structure free of equiaxed grains in the platform.