A new strategy to inhibit stray grain formation during laser directed solidification of single crystal superalloys

Although laser surface remelting displays excellent potential for repairing the nickel-based single crystal (SX) superalloy components, controlling the grain structure while avoiding the stray grain (SG) formation is still a challenge in the industry. In order to inhibit SG formation for nickel superalloys, a novel rotated laser irradiation approach is introduced in this paper and the study of the origin of the SG formation in laser directed solidification process. Besides, a numerical model of combined alloy composition with the tendency of equiaxed grain formation during epitaxial growth processing was developed in this paper to reveal the mechanism of SG formation at the bottom of the molten pool. Results indicate that SGs formed at the bottom of the molten pool are mainly due to the remarkably increased possibility of the equiaxed grain transformation with the rough fusion interface. During the laser process, the number of SGs gradually decreases with the increase of the angle (θ) between the laser irradiation and gravity direction. For θ = 90°, well-aligned epitaxial growth of [001] dendrites without any SGs are observed in the pool with the smooth fusion interface. Furthermore, applying the new laser deposition technique based on this phenomenon during laser directional solidification will be an effective method to control the grain-oriented growth in industrial manufacture for the single crystal components.