Microwave Reflectometry for Online Monitoring of Metal Powder Used in Laser Powder Bed Fusion Additive Manufacturing

This study presents the results of using a millimeter-wave reflectometer system, operating at 150 GHz, for demonstrating the basic efficacy of measuring electromagnetic scattering of metal powder used in laser powder bed fusion (LPBF) additive manufacturing (AM). Metal spatter (spatial) properties—particles ejected during laser interaction with metal powder—are potential indicators of process deviations (from a prescribed manner) or defect formation in a printed part. Electromagnetic modeling of scattering properties of metal powder has shown to be a potentially viable tool for assessing metal powder cloud spatial distribution and other properties. This work takes the next natural step by measuring the scattering properties of a cloud of metal powder. This investigation begins with samples of stationary powder, demonstrating a strong correlation between packing density and the measured output voltage of the reflectometer. The study progresses into detecting the flow of relatively large metal particles (i.e., solder balls) in air and measuring responses of flowing metal powder blown inside a nitrogen-filled chamber. Results crucially confirm that this method can distinguish a cloud of metal powder from the baseline condition where no powder is present. While promising, this investigation represents an initial step in the long journey toward optimizing millimeter-wave methods for integration into real-world LPBF AM systems.