Molten pool air-liquid interface in high-power laser manufacturing: Evolution law and energy absorption characteristics

During modern high-power laser manufacturing, the laser beam-heat energy conversion mainly occurs in the airliquid interface of laser-induced molten pool evaporation. The molten pool air-liquid interface's evolution law and laser energy coupling characteristics were performed by using in-situ optical observations and numerical simulation in this study. They revealed the four stages of this evolution process: preheating, expansion, oscillation, and contraction. In the expansion stage of the molten pool, the molten metal will flow from the center of the molten pool to the periphery due to the joint action of surface tension and laser-induced evaporation recoil pressure, and the air-liquid interface exhibits a middle depression. When the laser is turned off, the evaporative recoil pressure drop causes the molten pool's oscillation, which amplitude and duration positively correlate with laser power. Due to the effects of the solid state and the air-liquid interface depression after melting, the absorptivity first drops and then grows with the increase of laser power. Eventually, the melting efficiency increases with laser power.