Baoqi Zhu

High-power laser manufacturing relies on the optical-thermal effect for material machining, which represents its core working mechanism. In this paper, through in situ optical observation and simulation experiments, the conversion of optical to thermal energy and coupling characteristics inside the keyhole were studied. During the evolution from molten pool formation to keyhole initiation, melting efficiency rises gradually from zero and experiences an obvious jump at the moment of keyhole formation, with its maximum increment reaching 23.5%. As the keyhole continues to deepen, the melting efficiency eventually stabilizes, reaching a balance between the energy dissipated through heat conduction and the energy absorbed by the keyhole. Inside the keyhole, laser beams primarily interact with the front keyhole wall (FKW). Repeated multireflection and energy absorption of laser beams on keyhole inner walls constitute the primary cause of the sharp increase in overall energy absorptivity and melting efficiency. The laser energy reflected from FKW to the rear keyhole wall causes only 17.9% of the melting induced by FKW's primary absorption, having a limited impact on keyhole depth. Consequently, the direct absorption of laser energy on the FKW surface is the key factor determining the keyhole depth.

In laser processing under keyhole mode, multiple reflections within the keyhole are known to enhance energy absorption and influence process outcomes. However, whether multiple reflections initiate keyhole formation, and under what conditions, remains unclear. Here, we demonstrate that the absorption of laser light after its first reflection at the material surface within the vapor depression critically drives keyhole formation. An in situ monitoring system, combining synchrotron radiation with high-speed imaging, captures real-time vapor depression dynamics, gas-liquid interface angles, and plume evolution. Detailed analysis reveals that absorption of first-reflected laser light substantially enhances local energy deposition, enabling the transition to a stable keyhole. Experiments with varied laser power-to-spot diameter ratios, supported by simulations, confirm the essential role of first-reflection absorption. This work uncovers a previously overlooked mechanism underlying keyhole initiation.