Considerations for interpreting in-situ photodiode sensor data in pulsed mode laser powder bed fusion

Meltpool emissions across the visible and infrared spectrum are typically used to monitor the quality of laser welds. Laser powder bed fusion (L-PBF) is essentially laser welding on a powder bed. Hence, several commercially available L-PBF systems are equipped with photo sensors installed in co-axial and off-axial orientations. In-situ sensor data is often treated as a proxy for part quality to be used for certification. However, several considerations like design of sensor setup, laser source, material, thermal lensing, etc. can largely vary the quality metrics for anomaly detection. Compared to continuous wave (CW), pulsed mode L-PBF has advantages in manufacturing thin and complex shaped features. In this study, an in-situ photodiode based co-axial and off-axial sensor setup was designed and installed on an open-architecture pulsed mode L-PBF system. The sensor module acquires the real encoder position of the galvo mirrors thus allowing the conversion of time series data into spatial coordinates. The co-axial sensor was found to be very sensitive to alignment, positional dependence, thermal lensing, filters and area of focus. The off-axial sensor is more robust in defect identification. A set of single-track samples were made at varying duty cycles, power levels and speeds. The analysis of the photodiode data showed that anomaly detection in pulsed L-PBF is different than in CW mode of operation. The current study lays the groundwork for a certification of pulsed mode L-PBF components.