Epitaxial growth behavior and stray grains formation mechanism during laser surface re-melting of directionally solidified nickel-based superalloys

Abstract

The epitaxial growth behavior and stray grains formation mechanism at substrate during laser surface re-melting of directionally solidified superalloys were investigated in this paper. It is found that the columnar dendritic structures can be obtained by epitaxial growth from the substrate under laser surface re-melting. Meanwhile, the epitaxial dendrites have the same orientation with that of the substrate. The epitaxial behavior of dendrites is controlled by the constitutional supercooling, which is affected by the solidification conditions, i.e. temperature gradient and crystal growth velocity. Phase field method and Rosenthal’s solution were employed to describe the epitaxial behaviors of microstructure under laser surface re-melting. The disoriented stray grains appeared around the carbides and eutectic phases along the fusion line. The occurrence of stray grains can be ascribed to the liquation of carbides and eutectic phases, which results from the elemental segregation. The occurrence of stray grains is controlled by reducing the heat input to 50 J/mm. These findings could provide technical support in achieving successful repair and additive manufacturing of non-weldable directionally solidified superalloys.