Study of Microstructure in Surface-Melted Region of Ni-Base Single Crystal Superalloy CMSX-4
This work investigated microstructure in the surface-melted region of Ni-base single crystal superalloy CMSX-4 by using a diode laser beam as a heating source. Such processing parameters as laser power and scanning speed in laser surface melting were varied while defocusing distance and shielding gas (Ar) flow rates were fixed. Specimen surfaces were arranged parallel to the (001) of base alloy. The microstructure in the melted region was analyzed by optical microscopy and SEM. Crystal orientation of the melted region was analyzed using electron backscattered pattern analysis. The microstructure was remarkably changed when the heat input of surface melting was varied. The surface-melted region was found to solidify into a single crystal with directional dendrites that grew along the  directions under low heat input conditions. The surface-melted region was also a single crystal with disoriented dendrites that grew along the  or  directions under medium heat input conditions. In contrast, the melted region consisted of poly crystals with stray crystals under high heat input conditions. Such tendencies were also observed in the melted region with gas tungsten arc. These results demonstrate that the surface-melted region can solidify into a single crystal under conditions in which larger temperature gradient and higher solidification rates can be achieved.
Yukichi Umakoshi and Shinji Fujimoto
Y. Fujita et al., "Study of Microstructure in Surface-Melted Region of Ni-Base Single Crystal Superalloy CMSX-4", Materials Science Forum, Vol. 512, pp. 313-318, 2006