Transient Laser Energy Absorption, Co-axial Melt Pool Monitoring, and Relationship to Melt Pool Morphology.

Melt pool monitoring (MPM) is a technique used in laser powder bed fusion (LPBF) to extract features from insitu sensor signals that correlate to defect formation or general part fabrication quality. Various melt pool phenomena have been shown to relate to measured transient absorption of the laser energy, which in turn, can be relatable to the melt pool emission measured in MPM systems. This paper describes use of a 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, often found in commercial LPBF monitoring systems, synchronously measured of the incandescent emission from the melt pool 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.