Crystal Growth in Laser Surface Melting and Cladding of Ni-Base Single Crystal Superalloy

Abstract

Crystal growth behaviours in the laser surface-melted region and overlay weld metal of/on Ni-base single crystal superalloy CMSX-4 has been investigated. The Ni-base superalloy Rene 142 was used as the filler metal for overlay welding. Surfaces of the specimens were prepared as to be coincident with the (001) plane. The surface melting and overlay welding were conducted using a 2 kW diode laser along the [100] direction. Electron back scattering analysis for the crystal orientation revealed that, under low heat input conditions, the surface-melted region solidified as a single crystal with unidirectional dendrites which grew from the base metal epitaxially along the [001] direction. For intermediate heat input conditions, the surface-melted region was also found to be single-crystalline; however, it consisted of orthogonal dendrites along the [001], [010] and [100] directions. In contrast, under high heat input conditions, stray crystals were observed in some parts of the surface-melted region resulting in the melted region becoming polycrystalline. A numerical analysis of the dendritic growth direction and the constitutional supercooling at solidification front indicated that the overlay weld metal grew epitaxially on the substrate and stray crystals tended to form under conditions of higher laser power and (V f / V w) ratio (V f: wire feeding speed, V w: laser scanning speed), which agreed with the metallographic observation. A numerical calculation combining CET (columnar to equiaxed transition) theory with an inverse heat conduction analysis allowed us prediction of the conditions needed to suppress the formation of stray crystals and to promote single-crystal production in the overlay welded region with epitaxial dendritic growth of [001] orientation and/or dendritic growth of [100] and [010] orientations. Using these predictions, a single crystalline layer of 3 mm thickness on a single crystal substrate could be achieved by laser overlay welding with ten passes. Furthermore, creep rupture properties of single-crystalline surface-melted region were comparable or slightly better than that of the CMSX-4 base metal.