Optical-thermal energy conversion and coupling characteristics in the keyhole induced by high-power fiber laser

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.