Abstract
Crystal growth behavior in the overlay welded regions of Ni-base single crystal superalloy CMSX-4 has been investigated. The Ni-base superalloy René142 was used as the filler metal for overlay welding. The surface of the specimens was prepared as to be coincident with the (001) plane. The overlay welding was conducted using a 2kW diode laser along the [100] direction. Microstructural observation revealed that the overlay weld metal grew epitaxially on the substrate and stray crystal tended to form in the conditions of higher power and (Vf /Vw) ratio (Vf : wire feeding speed, Vw : laser scanning speed). Furthermore, the dendritic growth direction and the single-crystallized condition were analyzed by the theoretical model with combining the geometric model, CET (columnar to equiaxed transition) model and CFD (computational fluid dynamics) analysis. The predicted results of the dendritic growth direction and the stray crystal formation in the overlaid regions corresponded approximately with the experimental ones, which indicated that the current theoretical calculation can be used to predict the conditions 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 the dedritic growth of [100] and [010] orientations. On the basis of the condition by the theoretical prediction, a single crystalline layer of 3 mm thickness on a single crystal substrate could be achieved by laser overlay welding with ten passes.
