In-line process monitoring in laser welding and powder bed fusion of metals using directional reflection measurements

Recent research has investigated in situ process characterization for laser welding and laser-based powder bed fusion of metals (PBF-LB/M) via reflection measurement in laser wavelength. These studies successfully identified the prevailing process regime and the formation of a vapor depression based on in situ integrating sphere data. This approach has great potential to accelerate process development by reducing the required ex situ specimen testing. Unfortunately, the measurement with an integrating sphere does not allow its application as an in-line monitoring system without significantly restricting the process design because the integrating sphere is placed within the gas flow and limits the working area with its small aperture close to the process zone. Therefore, this work investigates directional reflection-based process monitoring in the laser wavelength for laser welding and PBF-LB/M of AISI 316L under insignificant restrictions to the system. For this, the changes in direction and intensity of the reflected laser radiation are monitored for bare plate experiments at various laser powers with photodiodes, which are fiber-coupled to parabolic dish collectors surrounding the working area. These experiments are conducted in a wide working area. It was found that a characteristic change in the reflection direction coincides with the threshold for vapor depression formation, which was previously identified with the integrating sphere. These results show great potential for in-line process monitoring because this novel measuring approach adds directional information to reflection monitoring while decreasing its impact on the process design.