Exploration of powder synthesis and additive manufacturing of refractory metal alloys for space nuclear propulsion

Space nuclear propulsion (SNP) systems have been identified to efficiently provide sustained high power for deep-space and sustained human exploration missions, but technological advancements in materials and manufacturing of refractory (i.e., high-temperature) materials is necessary to enable them. The objective of this work is to develop a fundamental understanding of the processing of tungsten (W) and W alloys via ultrasonic atomization (UA) for powder feedstock production and additive manufacturing (AM) for near-net-shape fabrication of SNP components. Nuclear electric propulsion (NEP) and nuclear thermal propulsion (NTP) operations rely on the superior properties of W (e.g., high-temperature strength, creep resistance, compatibility with lithium and hydrogen fuels, thermionic emission, etc.). Magnetoplasmadynamic (MPD) thrusters are a specific type of NEP system that use pure W for complex multi-channel/rod cathodes which are currently assembled manually and exhibit geometric inhomogeneity. Their performance efficiency is thus intricately linked to processing capabilities of W and its alloys. Processing of refractory metals has historically been carried out under vacuum, but novel powder production techniques with rapid alloy development capabilities like UA and modern AM techniques with geometric design freedom like laser-powder bed fusion (L-PBF) make use of an inert atmosphere. This is a marked departure from conventional processing, and the effect of impurity interstitial (e.g., carbon, oxygen) evolution during these processes needs to be understood to further resolve issues like cracking during AM of refractory metals. The exploration of AM for SNP is limited by a lack of fundamental process understanding and contaminant mitigation during powder synthesis and AM processing of high-purity refractories. The overall objective of this research is to understand the influence of UA powder synthesis and L-PBF AM on composition, processability, and mechanical performance of W and W-based alloys. The following three thrusts will be pursued in order to achieve this overarching objective: (1) probe the evolution of W purity during UA, (2) explore the parameter space and compositional evolution of W in L-PBF, and (3) correlate alloying influence to processing and properties of UA and L- PBF of W. This research probes the fundamentals of powder synthesis and L-PBF of W and W-based alloys. It will advance the concept of powder synthesis via UA and processing via L-PBF of high-purity refractories from technology readiness level (TRL) 1 to TRL 3. The proposed work achieves this by developing a processing framework which connects the influence of UA and L-PBF processing parameters and alloying additions to interstitial contaminant evolution, microstructure, and SNP-relevant properties (e.g., strength, density, thermionic emission). This work is associated with the Advanced Manufacturing Capability Area within the EXPLORE Thrust of the Space Technology Mission Directorate's Strategic Framework and specifically with the 3D Printing/Additive Manufacturing Priority. It also extends into the Space Nuclear Propulsion Capability Area within the GO Thrust and NEP and NTP sub-areas. It extends the groundwork for novel SNP manufacturing capabilities to enable the production of complex refractory components to meet the extreme environmental requirements for long-duration exploration missions.

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