Multi-faceted monitoring of powder flow rate variability in directed energy deposition

•Powder delivery systems for directed energy deposition processes can exhibit oscillatory variation in mass flow rates, matching the characteristic speed of individual hopper components.•Flow rate variation can be observed at the nozzle exit, both by offline weight measurement and by imaging the powder flow.•Flow rate variation is observed to have a measurable effect on melt track height, with the potential to cause build quality issues.•Melt pool imaging shows a correlating pattern, indicating that flow rate variation can affect melt pool image brightness, and may influence the behaviour of image based control approaches. Powder flow rate is a key parameter in Directed Energy Deposition (DED) processes. During a typical build, if powder flow rate is reduced for just 1 second, 30 mm of melt track is affected. Consequently, even a small variation in powder flow rate can have significant implications on build quality. In this work, the powder flow stability for different types of 316 L steel powders was quantified using a combination of methodologies including offline weight measurements, flow imaging, in-situ build data and coaxial melt pool imaging. Flow rate oscillation was observed, correlated with the periodicity of powder hopper turntable rotation, at sufficient magnitude to cause build quality effects and be identifiable in coaxial melt pool imaging. The implications of flow rate variation on the use of melt pool imaging for closed-loop control are discussed. [Display omitted]