Transient Laser Energy Absorption, Co-axial Melt Pool Monitoring, and Relationship to Melt Pool Morphology
Brandon M. Lane, Ivan Zhirnov, Sergey Mekhontsev, Steven E. Grantham, Richard E. Ricker, Santosh Rauniyar, Kevin Chou
Many recent and ongoing studies into the complex melt pool physics during laser powder bed fusion (LPBF) metal additive manufacturing (AM) process measure various aspects of energy transport surrounding the laser-induced melt pool, with focus on laser energy absorption. In addition, many more less fundamental studies using in-situ melt pool monitoring (MPM) attempt to determine appropriate signal processing to extract features from in-situ sensor signals that correlate to defect formation or general part fabrication quality. This paper builds upon these areas, and describes experiments utilizing a new reflectometer-based instrument to measure the dynamic laser energy absorption during single-line laser scans. Scans are conducted on bare metal and single powder layer of nickel alloy 625 (IN625) at a range of laser powers. In addition, a photodetector aligned co-axially with the laser, similar to those found in commercial LPBF systems, acquired measure of the incandescent emission from the melt pool synchronously with the dynamic laser absorption. Relationships between the dynamic laser absorption, co-axial MPM, and surface features on the tracks are observed, providing illustration of the melt pool dynamics that formed these features. Time-integrated measurements of laser absorption are shown to correlate well with MPM signal, as well as indicate the transition between conduction and keyhole mode. This transition is corroborated by metallographic cross-section measurement, as well as topographic measurements of the solidified tracks. Ultimately, this paper exemplifies the utility of dynamic laser absorption measurements to inform both the physical nature of the melt pool dynamics, as well as interpretation of process monitoring signals.