External Illumination Strategies for Melt Pool Geometry Monitoring in SLM

Coaxial monitoring in selective laser melting (SLM) can be applied using different configurations in terms of sensor choice and observed bandwidth. Use of external illumination to observe the melt pool geometry by dominating the emission from the process itself is one option, where the melt pool geometry can be visualized independently of changes in the emission behavior. However, the correct choice of the illumination source and the configuration in which it is implemented is an issue that requires further attention. The aim of this work is to investigate methods for direct observation of the molten pool geometry using an external illumination source to dominate the emission from the process. A coaxial imaging system was devised for this purpose, and two different setups for illumination 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 flaws of each experimental setup are extensively discussed. External illumination was found to be useful for direct interpretation of the SLM melting conditions. Furthermore, the actual scan position and velocity could be determined by applying an image processing algorithm to the captured frames.