Energy-Coupling Mechanisms Revealed through Simultaneous Keyhole Depth and Absorptance Measurements during Laser-Metal Processing

Abstract

The interaction between high-irradiance light and molten metal is the complex multiphysics phenomenon that underpins industrial processes such as laser-based additive manufacturing, welding, and cutting. One aspect that requires careful attention is the formation and evolution of vapor depressions, or keyholes, within the molten metal. The dynamic behavior of these depressions can dramatically change the number of laser-beam reflections and is therefore intrinsically linked to the instantaneous energy coupled into the system. Despite its importance, there is a severe lack of direct in situ, experimental evidence of this relationship, which creates challenges for those who aim to model or control laser-based manufacturing processes. In this work, we combine two simultaneous state-of-the-art real-time measurement techniques (inline coherent imaging and integrating-sphere radiometry) to confirm and explore the definite positive correlation between the highly dynamic vapor-depression geometry and laser energy absorptance. For irradiances resulting in vapor-depression formation ($\ensuremath{\ge}0.49\phantom{\rule{0.1em}{0ex}}{\mathrm{MW}/\mathrm{cm}}^{2}$), we observe excellent correlation (0.86) between the instantaneous depth (down to 800 $\ensuremath{\mu}\mathrm{m}$) and the absorptance (up to 0.92), directly demonstrating their interdependence. In the transition mode, an important regime for additive manufacturing, we observe temporary vapor-depression formation with concomitant changes in absorptance from 0.34 to 0.53. At higher irradiances, we detect stepwise increases in the absorbed laser power with a smoothly increasing keyhole depth, which is a real-time experimental observation of the effect of multiple reflections during laser-metal processing. The value of simultaneous depth and absorption measurements for predictive model validation is presented using ray-tracing simulations, which also confirm the absorption enhancement via incremental increases in the reflection count. This work provides insight into the underlying physics of laser-based metal manufacturing that is useful toward deterministic modeling and real-time process control.