Enhanced cooling, coupled with novel designs and packaging of semiconductors, has revolutionized communications, computing, lighting, and electric power conversion. It is time for a similar revolution that will unleash the potential of electrified propulsion technologies to drive improvements in fuel-to-propulsion efficiency, emission reduction, and increased power and torque densities for aviation and beyond. High efficiency and high specific power (kW/kg) electric motors are a key enabler for future electrification of aviation. To improve cooling of emerging synchronous machines, and to realize performance and cost metrics of next-generation electric motors, electromagnetic and thermomechanical co-design can be enabled by innovative design topologies, materials, and manufacturing techniques. This paper focuses on the most recent progress in thermal management of electric motors with particular focus on electric motors of significance to aviation propulsion.
Laser powder bed fusion (LPBF) is an additive manufacturing technology that uses a laser to selectively melt powder feedstock to build parts in a layer-by-layer process. For metals-based LBPF additive manufacturing, the interaction of the laser and powder feedstock creates byproducts such as a plume, spatter, and powder ejecta. Directional gas flow, typically nitrogen or argon, is used to remove or mitigate the negative effects of these byproducts. This report documents and presents gas flow measurements using hot-wire anemometers (HWA) for two different nozzles on a commercial LPBF machine and two different gasses at the NIST Additive Manufacturing Metrology Testbed (AMMT). The AMMT gas flow system generates comparable volumetric flow rates with argon and with nitrogen, which result in comparable flow speed profiles for both gasses. There are significant differences in the gas speed profiles along the gas flow direction (Y-position) and minimal differences perpendicular to the gas flow direction (X-position) with both machines. The speed differences with Y-position are in part due to the elevated inlet and outlet nozzles from the build platform. The average speed only decreases slightly from the inlet to the outlet despite these differences in the speed profiles. The grid nozzle on the commercial machine with a downward facing row of channels at the base of the nozzle increases the gas speed close to the build platform; however, non-uniform speed profiles remain. Gas speed and therefore gas speed measurements with HWAs are highly dependent on Z and Y position. This should be considered when prescribing machine performance protocols. Additional suggestions for measuring and reporting gas flow are made as well as recommended future experiments and simulations to assist in machine performance standards.
In laser powder bed fusion metal additive manufacturing, insufficient shield gas flow allows accumulation of condensate and ejecta above the build plane and in the beam path. These process byproducts are associated with beam obstruction, attenuation, and thermal lensing, which then lead to lack of fusion and other defects. Furthermore, lack of gas flow can allow excessive amounts of ejecta to redeposit onto the build surface or powder bed, causing further part defects. The current investigation was a preliminary study on how gas flow velocity and direction affect laser delivery to a bare substrate of Nickel Alloy 625 (IN625) in the National Institute of Standards and Technology (NIST) Additive Manufacturing Metrology Testbed (AMMT). Melt tracks were formed under several gas flow speeds, gas flow directions, and energy densities. The tracks were then cross-sectioned and measured. The melt track aspect ratio and aspect ratio coefficient of variation (CV) were reported as a function of gas flow speed and direction. It was found that a mean gas flow velocity of 6.7 m/s from a nozzle 6.35 mm in diameter was sufficient to reduce meltpool aspect ratio CV to less than 15 %. Real-time inline hotspot area and its CV were evaluated as a process monitoring signature for identifying poor laser delivery due to inadequate gas flow. It was found that inline hotspot size could be used to distinguish between conduction mode and transition mode processes, but became diminishingly sensitive as applied laser energy density increased toward keyhole mode. Increased hotspot size CV (associated with inadequate gas flow) was associated with an increased meltpool aspect ratio CV. Finally, it was found that use of the inline hotspot CV showed a bias toward higher CV values when the laser was scanned nominally toward the gas flow, which indicates that this bias must be considered in order to use hotspot area CV as a process monitoring signature. This study concludes that gas flow speed and direction have important ramifications for both laser delivery and process monitoring.
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