Freckle Defect Formation near the Casting Interfaces of Directionally Solidified Superalloys

Abstract

Freckle defects usually appear on the surface of castings and industrial ingots during the directional solidification process and most of them are located near the interface between the shell mold and superalloys. Ceramic cores create more interfaces in the directionally solidified (DS) and single crystal (SX) hollow turbine blades. In order to investigate the location of freckle occurrence in superalloys, superalloy CM247 LC was directionally solidified in an industrial-sized Bridgman furnace. Instead of ceramic cores, Alumina tubes were used inside of the casting specimens. It was found that freckles occur not only on the casting external surfaces, but also appear near the internal interfaces between the ceramic core and superalloys. Meanwhile, the size, initial position, and area of freckle were investigated in various diameters of the specimens. The initial position of the freckle chain reduces when the diameter of the rods increase. Freckle area follows a linear relationship in various diameters and the average freckle fraction is 1.1% of cross sectional area of casting specimens. The flow of liquid metal near the interfaces was stronger than that in the interdendritic region in the mushy zone, and explained why freckle tends to occur on the outer or inner surfaces of castings. This new phenomenon suggests that freckles are more likely to occur on the outer or inner surfaces of the hollow turbine blades.

Defect Creation in the Root of Single-Crystalline Turbine Blades Made of Ni-Based Superalloy

“…It is known that many types of defects in single-crystalline (SX) turbine blades are created during directional crystallization by the Bridgman technique, while the crystallization front passes through areas with abrupt changes in a geometry of the cast [1,2,3,4,5,6,7,8]. The layers of the blade root located near the plane of connection with a selector (selector-root (S–R) connection), where branching of the primary dendrite arms and lateral growth occur, are especially important for defects creation [9,10,11,12,13,14]. In these layers, the low-angle boundaries (LABs), related to the local crystal misorientation, are created [15].…”

Influence of Geometric Structure and Feeding Behavior of Casting on Freckle Formation during Directional Solidification of a Ni‐Based Single‐Crystal Superalloy

“…Furthermore, it was found that the casting size was influential in the formation of freckle defects. Giamei et al [6] and Hong et al [17] showed that the freckle chains in the larger diameter single-crystal bars were more than those in the smaller diameter bars during the same solidification process. In addition, casting geometry can also influence freckle formation.…”

Polymer Binders of Ceramic Nanoparticles for Precision Casting of Nickel-Based Superalloys

“…The process of making subsequent layers in a casting mould using the lost-wax process requires the use of appropriate binding agents to ensure the production of ceramic moulds with adequate parameters, i.e., high heat resistance, mechanical strength, gas permeability, dimensional stability, and required surface smoothness of the first layer [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. The basic materials for the production of sand and ceramic casting moulds are binders, ceramic powders, and auxiliary materials, i.e., agents that limit foaming during mixing, wetting agents, pH stabilizers, and modifiers of rheological properties of the ceramic slurry [ 9 , 10 , 11 , 12 , 13 , 14 ].…”