Numerical study of melt hydrodynamics in pulsed laser processing of nickel-based superalloy

Pulsed laser processing (PLP) of materials has wide-spread applications in the industry, yet the underlying complex material response to the incident laser pulse especially the melt flow mechanism is not fully understood. Knowledge of melt hydrodynamics can provide systematic theoretical guidance for the improvement of PLP quality. In this paper, a two-dimensional numerical model was built to simulate melt flow during PLP of nickel-based superalloy. The driving forces for melt hydrodynamics at different stages of laser pulse irradiation were studied comprehensively. Numerical results reveal that thermo-capillary stress drives radially-outward melt flow at the initial stage of laser pulse irradiation. At the middle stage, evaporation-induced recoil pressure is the dominant driving force for melt pool deformation, creating a depression at the center and a hump at the periphery. Thermo-capillary force has a minor effect on melt pool deformation but a large contribution to the flow velocity field. At the post-irradiation stage, the peripheral molten hump flows back rapidly and the melt pool experiences some gradually damping oscillations under the action of Young-Laplace stress. Moreover, it was demonstrated that laser power density plays an important role in deciding the melt flow characteristics and the relative contributions of different driving forces. The results in this study provide an unambiguous physical picture of the melt hydrodynamics in pulsed laser processing.