Effect of Pulse Frequency on the Transport Phenomena and Crystal Growth Behavior in Quasi-Continuous-Wave Laser Powder Deposition of Single-Crystal Superalloy

The effect of pulse frequency on the transport phenomena and crystal growth behavior in the quasi-continuous-wave laser powder deposition process of single-crystal superalloy were studied through an improved three-dimensional mathematical model and experiments. Laser powder deposition experiments with single-crystal superalloy were conducted to verify the computational results and analyze the crystal growth behavior. Results showed that the laser mode has a predominant effect on the transport phenomena and the associated crystal growth behavior. The increase of pulse frequency weakens the fluctuation motion of the molten pool and shallows the solidification interface. Under the given processing parameters, the molten pool size decreases quickly with the increase of pulse frequency from 0 to 15 Hz, and then gradually tends to a steady state in the range of 15 to 35 Hz. When the pulse frequency exceeds 35 Hz, the molten pool does not disappear completely during the laser-off period of a pulse cycle. With the increase of pulse frequency from 0 to 35 Hz, the peak temperature of molten pool decreases from 2567.5 K to 2324.7 K, while the peak normal thermal gradient at the solidification interface increases from 2.24 × 106 K/m to 1.01 × 107 K/m. The fast solidification speed driven by the contraction speed of molten pool during the laser-off period of quasi-continuous-wave laser powder deposition process refines the trunk size of columnar dendrites and facilitates the formation of shrinkage cavities and chainlike carbides. Due to the significant increase of thermal gradient and solidification speed, the quasi-continuous-wave laser powder deposition process of single-crystal superalloy features the columnar dendrites with a better epitaxial growth ability, and acts as an effective method to expand the repair-processing window of single-crystal components.