Effects of melt-pool geometry on the oriented to misoriented transition in directed energy deposition of a single-crystal superalloy

The successful repair of single-crystal (SX) gas turbine blades using the directed energy deposition (DED) technique requires unidirectional dendrite growth without an oriented to misoriented transition (OMT). In this work, the effects of melt-pool geometrical parameters on OMT behavior during single-track deposition of SX superalloys were thoroughly studied. The variations in melt-pool length, width, height and depth on the solid-ified structure characteristics (e.g., the number of anisotropic dendrites and the area fraction of [001]-oriented dendrites) were quantitatively examined through experimental observations. A simple mathematical model was established to elucidate the correlations between these parameters and OMT behavior, and the errors in this model were analyzed in detail. The DED parameters were linked with the melt-pool geometrical parameters to find out the suitable processing conditions for SX repair. It is concluded that a melt pool with a relatively low depth-to-height ratio and a high length-to-height ratio is generally beneficial for unidirectional dendrite growth, decreasing the heat and mass input is conducive to obtain such melt-pool shape. The presented findings afford an in-depth understanding of the effects of melt-pool geometry on competitive dendrite growth and are instrumental for the repair of SX gas turbine blades.