Coaxial Monitoring of Meltpool for Laser Powder Bed Fusion Process Control

Abstract

Reliable process monitoring is critical for advancing laser powder bed fusion of metals (PBF-LB/M), as it provides the process information required for corrective control actions to prevent defects such as lack-of-fusion and keyhole porosity. A promising approach for PBF-LB/M process monitoring is coaxial measurement of light emitted from the meltpool area. More than two hundred research studies on coaxial monitoring of the meltpool have been performed, but there is still no validated and reliable method for measuring a suitable variable that could be directly used as an input for high-resolution control of the PBF-LB/M process, enabling process parameters such as laser power and scan speed to be dynamically adjusted not only at the part level, but locally for individual micrometer-scale segments along the scan track. This work reviews existing research studies on coaxial meltpool monitoring and identifies potential future research and development directions toward a standard and reliable meltpool monitoring method. Meltpool depth is identified as a particularly promising control-relevant variable. With the ability to monitor and control meltpool depth, it is possible to avoid undesirable process conditions due to too high or too low energy input, which would result in defects like keyhole porosity or lack-of-fusion. However, only five studies have attempted to estimate the melt pool depth coaxially during the PBF-LB/M process. Despite these attempts, there is no sufficiently validated method for high-resolution coaxial monitoring of meltpool depth in PBF-LB/M. The potential solution seems to be in combining concepts and recent discoveries from two-color coaxial camera- and photodiode-based meltpool area monitoring approaches. Based on the existing literature and hands-on experiences with establishing and working with coaxial monitoring systems, this work also provides guidelines and lists the most common challenges for implementing coaxial monitoring of the meltpool in the PBF-LB/M process.