Millimeter-Wave Reflectometry for Distinction Among Critical Metal Powder Properties Used in Additive Manufacturing (AM)

additive manufacturing (AM) or 3-D printing is the process of rapidly manufacturing complex parts that are used in a wide range of applications encompassing nearly unlimited types of critical and noncritical components. When considering metal AM, one of the more prominent processes involves layer-by-layer melting of fine metal powder into the desired part geometry, using an electron or a laser beam. The latter is referred to as the laser powder bed fusion (LPBF). The quality of the final printed part is directly impacted by the properties of the feedstock powder. This includes but is not limited to the metal powder size distribution, surface condition (i.e., oxidation), new or recycled powder, powder distribution surface nonuniformities, and streaks. The ability to determine metal powder properties prior to melting provides significant manufacturing quality control capability. Millimeter-wave nondestructive evaluation (NDE) techniques, spanning a frequency range of 30–300 GHz, offer several advantageous features for this purpose. These methods are noncontact, provide a high degree of measurement sensitivity to the metal powder properties of interest, and can provide real-time information. In addition, the reflection properties of the powder are the result of complex electromagnetic interactions among the powder particles and the irradiating wave. This article provides the results of a comprehensive investigation into the millimeter-wave reflection properties of several different types of metal powder at 32–40 GHz. The results demonstrate the ability to distinguish among metal powder types as a function of size distribution, powder stratification, alloy composition, recycled versus new and compacted powder using an open-ended circular waveguide probe, operating in its $TE_{01}$ mode.