Ni基単結晶超合金の表面溶融部の組織学的検討

Abstract

Crystal growth behavior in the surface-melted region of Ni-base single crystal superalloy CMSX-4 was investigated. The surface melting was conducted using 2kW diode laser or gas tungsten arc. Laser power and scanning speed in laser surface melting were varied while defocusing distance and shielding gas (Ar) flow rate were fixed. For arc surface melting, arc current and welding speed were varied. Specimen surfaces were arranged parallel to the (001) of the base metal. The microstructure and crystal orientation in the melted region were analyzed by using optical microscopy, scanning electron microscopy(SEM) and electron backscattered pattern analysis(EBSP). The microstructure and crystal orientation in laser surface-melted region were markedly influenced by heat input. The laser surface-melted region under low heat input conditions was found to be solidified as the single-crystalline state with unidirectional dendrites which grew along the [001] directions. The laser surface-melted region under intermediate heat input conditions was also single-crystalline with dendrites which grew along the [001], [010] and [100] directions. In contrast, in the case of high heat input conditions, the laser surface-melted region was consisted of polycrystal with stray crystals. Such tendencies were also observed in the melted region with gas tungsten arc. These results demonstrate that heat input significantly affects single-crystallinity regardless of the type of heat source, such as laser and arc. Furthermore, stray crystal formation in surface-melted region was investigated in detail by using SEM and EBSP. From the results of these experiments, it is revealed that stray crystal formation was affected by constitutional supercooling and dendrite growth direction.