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Melt pool monitoring in selective laser melting (SLM) is a key challenge to enhance the understanding of the basic process physics as well as the in-situ identification of drifts. The main issues are related to its fast dynamics and small spatial extent, which require high performance monitoring systems in terms of temporal and spatial resolution, often resulting in unmanageable data rates. Furthermore, the broadband thermal emission from the process can be viewed in different spectral ranges depending on the sensor and optical system employed. Accordingly, a wide range of sensing configurations is possible. The choice of monitoring parameters is crucial to achieve a solution capable of describing process dynamics with industrial applicability in terms of data transport and analysis. This work discusses the design of a coaxial monitoring module to assess melt pool dynamics in SLM using temporal and spatial cues related to the process physics. Restrictions regarding data management were considered to ensure a future inline process control. The system was tested with an external illuminator to reveal the actual melt pool geometry employing an acquisition rate of 1200 fps, 4.3 mm x 4.3 mm field of view and 14 µm/pixel spatial resolution. The process emission in visible (640 nm) and nearinfrared regions (850-1000 nm) was also acquired and the band choice discussed. The proposed solution captured successfully the melting conditions from both a spatial and temporal viewpoint. The monitoring system depicted variations of the melt pool shape when processing different geometries using modulated and continuous wave laser emission. Index Terms-Selective laser melting, process monitoring, thermal emission, infrared imaging, melt pool geometry.

Coaxial monitoring in SLM can be applied using different configurations in terms of sensor choice and observed bandwidth. The use of external illumination to observe the melt pool geometry by suppressing the process emission is an option, where the melt pool geometry can be visualized independently from the changes in the emission behavior. However, the correct choice of the illuminator and the configuration in which it is implemented is an issue that requires further attention. This paper is aimed at obtaining a direct observation of the molten pool geometry using an external illumination source to suppress process emission. A coaxial imaging system was devised for this purpose and two different setups for light launching were designed and tested, namely a diode laser beam coaxial to the working laser and a lateral low-coherence laser illuminating the whole build platform. The advantages and criticalities of each experimental setup are extensively discussed. External illumination was found to be useful for interpreting directly the SLM melting conditions. Furthermore, the real scan position and velocity could be measured through an image processing algorithm on the captured frames.