Simultaneous in operando monitoring of keyhole depth and absorptance in laser processing of AISI 316 stainless steel at 200 kHz

Abstract

The formation of keyholes during high-irradiance laser-metal interaction is the complex, multiphysics phenomenon that underpins industrial processes such as laser-based additive manufacturing, laser welding, and laser cutting. The complex dynamics of energy coupling in keyhole formation are not well understood, and the energy absorptance in these processes are often assumed to be constant. Therefore, we implement two state-of-the-art measurement techniques in operando to simultaneously measure keyhole depth using inline coherent imaging and laser energy absorptance using integrating sphere radiometry at imaging rates of 200 kHz. Results directly reveal the time evolution of cavity-enhanced absorptance in these keyholes generated by the laser-metal interaction. For stationary irradiance on AISI 316 stainless steel, we find that processing in an argon-rich environment compared to air reduces coupling efficiency by 50 % ± 11 % in conduction, 27 % ± 2 % in transition, and 8 % ± 3 % in keyhole mode. High imaging rates allow clear observation of liquid surface oscillations and corresponding changes to absorptance, declining from 15 kHz to 10 kHz over the first 10 ms of the spot weld.