Effect of Pulse Frequency on the Columnar-to-Equiaxed Transition and Microstructure Formation in Quasi-Continuous-Wave Laser Powder Deposition of Single-Crystal Superalloy

The columnar-to-equiaxed transition (CET) plays a vital role in keeping the monocrystalline nature in the repaired single-crystal superalloy by the laser powder deposition process. In this study, the effect of pulse frequency on the CET and microstructure formation in a novel quasi-continuous-wave laser powder deposition process of single-crystal superalloy was studied through an improved three-dimensional mathematical model and corresponding experiments. Results showed that the pulse frequency has a predominant effect on the CET conditions and associated microstructure formation in the deposited bead. In each pulse cycle, the CET position at the solidification interface moves upward synchronously with the contraction of the molten pool, tending to induce the epitaxial growth of columnar dendrites with a tapered distribution. The overlaps of adjacent pulsed molten pools contribute to the undulated distribution of columnar dendrites in the deposited bead. The increase of pulse frequency weakens the oblique fluctuation of CET positions and resultantly narrows the undulated distribution of epitaxial columnar dendrites. Under the given conditions, the height ratio of epitaxial columnar dendrites obtains a minimum value of 71 pct with pulse frequency = 50 Hz and has a minimum value of 50 pct with pulse frequency = 0 Hz (continuous-wave mode), indicating that the quasi-continuous-wave laser mode can effectively enhance the epitaxial growth ability of columnar dendrites with the optimized pulse frequency. The quasi-continuous-wave laser powder deposition process exhibits an attractive capability to balance deposition efficiency and epitaxial growth continuity in the deposited beads. Our results provide a potential method to tailor the CET conditions and optimize the laser repair processing window for the single-crystal superalloy via adjusting the pulse frequency of the quasi-continuous-wave laser.