Keyhole formation driven by absorption of first-reflected laser light in laser processing based on <i>in situ</i> synchrotron and high-speed imaging

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.