Real-Time Observation of Melt Pool in Selective Laser Melting: Spatial, Temporal, and Wavelength Resolution Criteria

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 the molten pool's fast dynamics and small spatial extent, which require high-performance monitoring systems in terms of temporal and spatial resolutions, 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 paper 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 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 frames per second, 4.3 mm imes 4.3 mm field of view, and 14~mu extm /pixel spatial resolution. The process emission in visible (640 nm) and near-infrared regions (850-1000 nm) was also acquired and the band choice discussed. The proposed solution captured successfully the melting conditions from both spatial and temporal viewpoints. The monitoring system depicted the melt pool shape variations when processing different geometries using both modulated and continuous wave laser emissions.