Effects of melt convection on stray grain formation in single crystal superalloys during directed energy deposition

Abstract During laser repairing of Ni-based single crystal (SX) superalloys, stray grain (SGs) formation leads to a deterioration of high-temperature properties and hot cracking. The main goal is to prevent SGs formation from forming, which controls the local solidification front thermal gradient (G) and solidification velocity (V). Meanwhile, the melt pool geometry, which is influenced by melt convection, has a significant impact on G and V. Here, experiments and theoretical calculations were used to study the evolution of the melt pool geometry during melt convection and its effects on the formation of SGs. Melt convection, i.e., Marangoni convection, outperformed thermal conduction in the SX superalloys melt pool during directed energy deposition, as evidenced by the estimated Peclet number and Prandtl number. The integrative impact of laser power, scan velocity, and beam diameters exacerbated Marangoni convection as the laser energy density distribution increased. Marangoni convection caused the melt pool shape to change from a near-semicircle to an undulating morphology, promoting the oriented-to-misoriented crystal growth transition and resulting in high stray grain sensitivity. The heat and mass transport in front of the dendrite tips was altered by Marangoni convection, which aggravated the deviation of the epitaxial dendrite orientation with respect to the substrate. This work provides an in-depth insight into controlling SG formation during additive manufacturing of SX superalloys.