Backscatter absorption spectroscopy for process monitoring in powder bed fusion

Abstract

This paper presents an optical sensor that employs backscatter tunable laser absorption spectroscopy (BTLAS) for in-situ monitoring of laser powder bed fusion (L-PBF). The measured signal depends on the conditions within the gas vapor cavity, whose dynamic interaction with the melt pool surface is known to be a major cause of defects in the final part. The sensor spectrally resolves the shape of the absorption lines of metal vapors, which is influenced by gas pressure, temperature, concentration, and velocity. This pilot study demonstrates that the absorption line strength and line shape of Ti varies significantly with changes in the process chamber pressure and laser power when processing a Ti-6Al-4V, potentially allowing the technique to be applied for process monitoring and closed-loop control. Additionally, the technique provided a signal under near-vacuum conditions, suggesting its utility for fundamental research on electron beam powder bed fusion (EB-PBF). The authors outline future steps for integrating this sensor into existing L-PBF systems for real-time operando process monitoring. Laser-induced fluorescence (LIF) was also observed during the experiments, which may provide further opportunities for in situ process monitoring.