Control of crystal orientation and continuous growth through inclination of coaxial nozzle in laser powder deposition of single-crystal superalloy

The effects of inclining angle of the coaxial nozzle in the longitudinal section of deposited bead on the molten pool geometry and the corresponding crystal growth in laser powder deposition of single-crystal superalloy are studied through the coupling of a numerical FLUENT program and a three-dimensional transient transport phenomena mathematical model. Systematical experiments with single-crystal nickel-based superalloy were conducted to verify the computational results. The results show that the inclination angle of the coaxial nozzle in the longitudinal section of deposited bead has a predominant effect on the molten pool geometry and the solidified microstructure. The inclination of coaxial nozzle reduces the height of the molten pool while increases the melting depth compared to the normal-direction deposition. The epitaxial grain growth in the deposited bead is enhanced when the coaxial nozzle inclines toward the laser scanning direction, while it is restrained as the nozzle inclines toward the opposite direction. When the coaxial nozzle inclines to a +45°, the ratio of melting depth to the height of equiaxied stray grain on the top of the previous layer of deposited bead exceeds 1.0, which implies that the deposited layer can completely remelt the stray grains in the previous layer. The capacity of continuous-epitaxial grain growth can be therefore achieved through the nozzle inclination effectively. This method can be used to optimize and control the processing parameters and broaden the processing window for a single-crystal turbine blade tip repair.