Monitoring Laser Material Processing by Simultaneous Inline Coherent Imaging and Integrating Sphere Radiometry
Understanding the underlying physics of laser welding and metal additive manufacturing (AM) is crucial to the advancement of laser-based manufacturing. One aspect that especially requires careful attention and control is the formation and evolution of vapour depressions, or keyholes, within the melt pool. The dynamic geometric behaviour of these depressions is intrinsically linked to the instantaneous energy coupled into the system, and therefore has a dramatic effect on the final properties of the material. In this work, we combine integrating sphere radiometry (ISR) and inline coherent imaging (ICI) to simultaneously measure time-resolved absorptance and vapour depression depth during stationary, high-irradiance laser illumination of a metal (AISI 316 stainless steel). This allows, for the first time, the exploration of the complex interdependence between energy coupling and geometry that underpins important industrial processes such as laser welding and additive manufacturing. Vapour depression depth measurements are acquired at a rate of 200 kHz and the time resolution of absorptance measurements is 4.4 µs. Time-resolved data reveals a general correlation between vapour depression depth and absorptance at high laser irradiance, showing that greater depth leads to an increase in absorptance from about 0.4 up to 0.9. There is also specific correlation with temporal features such as vapour depression initiation and oscillation frequency during periods of instability. An understanding of traditional welding regimes (conduction and keyhole mode) is enhanced by revealing distinguishing time-resolved features, and a transition mode is shown that exhibits behaviour of both regimes. Finally, the experimental results are compared with ray-tracing simulations that provide validation and explanation of the results, most notably revealing the first experimental observation of an incremental increase in number of laser beam reflections due to increasing vapour depression depth. The work presented here provides new insight into the underlying physics of laser-based manufacturing that can be used to advance progress towards deterministic control of the process.
URI for this recordhttp://hdl.handle.net/1974/26497
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