Backscatter absorption spectroscopy for process monitoring in powder bed fusion

7 This paper presents an optical sensor that employs backscatter tunable laser absorption 8 spectroscopy (BTLAS) for in-situ monitoring of laser powder bed fusion (L-PBF). The measured 9 signal depends on the conditions within the gas vapor cavity, whose dynamic interaction with 10 the melt pool surface is known to be a major cause of defects in the final part. The sensor 11 spectrally resolves the shape of the absorption lines of metal vapors, which is influenced by gas 12 pressure, temperature, concentration, and velocity. This pilot study demonstrates that the 13 absorption line strength and line shape of Ti varies significantly with changes in the process 14 chamber pressure and laser power when processing a Ti-6Al-4V, potentially allowing the 15 technique to be applied for process monitoring and closed-loop control. Additionally, the 16 technique provided a signal under near-vacuum conditions, suggesting its utility for fundamental 17 research on electron beam powder bed fusion (EB-PBF). The authors outline future steps for 18 integrating this sensor into existing L-PBF systems for real-time operando process monitoring. 19 Laser-induced fluorescence (LIF) was also observed during the experiments, which may provide 20 further opportunities for in situ process monitoring.