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.
Highlights
Superalloys offer excellent high temperature tensile strength, stress rupture and creep properties, fatigue strength, oxidation and corrosion resistance, and micro-structural stability at elevated temperatures 600 ◦ C or above [1,2,3,4,5,6]
The freckles occurred on the external surface of all of the were parallel to the solidification direction
The results indicated an interesting phenomenon, even though in the invariable casting condition and superalloy components, most of the freckle chains were observed on the shadow side and near the interface of the superalloys and ceramic materials
Summary
Superalloys offer excellent high temperature tensile strength, stress rupture and creep properties, fatigue strength, oxidation and corrosion resistance, and micro-structural stability at elevated temperatures 600 ◦ C or above [1,2,3,4,5,6]. Nickel-based superalloys are the most complex and the most widely used in high temperature applications. These solidification defects have a negative influence on high temperature mechanical properties, and reduce the life of aero engine and gas turbine land-based power generation [7,8,9]. Materials 2016, 9, 929 industrial castings, such as vacuum arc remelting (VAR) and electro-slag remelting (ESR) superalloy billets, nickel-based superalloys, or specialty steel, that occur during solidification. They are presently one of the main defects encountered in the advanced casting technology of superalloys [10]
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