Epitaxial laser deposition of single crystal Ni-based superalloy ІI: Variation of stray grains

A successful repair of single crystal Ni-based superalloy components requires maintaining the stability of the laser cladding process. So, it is critical to understand the relationship between stray grains and the actual repair condition including cooling condition, laser beam scanning pattern and unstable powder feeding. In this study, the variation of stray grains was investigated in terms of volume, crystal orientation, and distribution through experimental approaches in the laser cladding of single crystal superalloy. The results indicated that the formation of stray grains, low angle boundaries, and dendritic deflections or rotations could be suppressed on the condition of continuous substrate cooling and unidirectional laser beam scanning. In particular, a single crystal deposit with no stray grains was fabricated in a liquid nitrogen cooling condition. Besides, the dendritic deflection and rotation, which were harmful to epitaxial growth, were affected by the laser scanning pattern. Additionally, it was proved that stray grains preferably appeared at the entrance of powder into the molten pool. The offset of powder relative to the laser beam reshaped the distribution of stray grains in the molten pool, which would provide a new strategy to repair single crystal with laser cladding.