Optical Methods for In Situ Monitoring of Powder Feed Stock in Blown Powder Directed Energy Deposition

Abstract

Inconsistencies in the flow characteristics of powder feedstock in laser-directed energy deposition are detrimental to final build quality, impacting both metallurgical and geometrical accuracies. To this extent, an in-line monitoring system to assess the powder flow characteristics by minimally invasive means is necessary. The present work investigates powder flow characteristics, namely, mass flowrate, particle velocity, and particle size distribution, utilizing a minimally intrusive in-line measurement system. A high-intensity backlight coupled with an imaging system was used to capture powder particles flowing through an optically clear tube at 1000 fps with 30μs exposure time. The mass flowrate was found to fluctuate at a rate relative to the powder metering disk rotational rate. A subsequent experiment validated this observation by directly measuring the powder metering disk rotational rate, with the two experiments aligning to within 2%. The particle size distribution was closely aligned with a baseline comparison, differing no more than one pixel (12μm). The time to determine the size distribution was less than 0.1 s, thus enabling the tracking of time-varying particle sizes arising from the powder segmentation in the hopper. Particle location relative to the tube wall was also determined to mitigate the concerns of premature measurement system failure. This work has established the feasibility of implementing an optical measurement system to monitor the intrinsic characteristics of powder flow variations that can have adverse effects on directed energy deposition-laser powder.