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 ($\ge 0.49{\mathrm{MW}/\mathrm{cm}}^2$), we observe excellent correlation (0.86) between the instantaneous depth (down to 800 $\mu \mathrm{m}$) 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.

中文翻译：

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激光金属加工过程中同时进行的小孔深度和吸收率测量揭示了能量耦合机制

高辐射光与熔融金属之间的相互作用是复杂的多物理场现象，它是基于激光的增材制造，焊接和切割等工业过程的基础。需要仔细注意的一个方面是熔融金属内蒸汽凹陷或键孔的形成和发展。这些凹陷的动态行为会极大地改变激光束反射的次数，因此与耦合到系统中的瞬时能量有内在联系。尽管它很重要，但仍然严重缺乏直接就地，这是这种关系的实验证据，这对那些旨在建模或控制基于激光的制造过程的人提出了挑战。在这项工作中，我们结合了两种同时进行的最新实时测量技术（在线相干成像和积分球辐射法）来确认和探索高动态蒸汽压降几何形状与激光能量吸收率之间的正相关关系。对于产生蒸气压降低的辐射（$\ge 0.49{\mathrm{兆瓦}/\mathrm{厘米}}^2$），我们观察到瞬时深度（低至800）之间的良好相关性（0.86） $\mu \mathrm{米}$）和吸收率（最高0.92），直接证明了它们的相互依赖性。在过渡模式下，这是增材制造的重要机制，我们观察到暂时的蒸汽压降低形成，吸收率从0.34到0.53随之变化。在较高的辐照度下，我们检测到随着键孔深度的平稳增加，吸收的激光功率逐步增加，这是对激光金属加工过程中多次反射的影响的实时实验观察。使用光线跟踪模拟显示了同时进行的深度和吸收测量值对预测模型的验证，该值还可以通过反射计数的增量增加来确认吸收增强。