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Phosphorus in iron-based lunar in-situ resource utilization (ISRU) alloys
Corwin, Peter E.
Corwin, Peter E.
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2024
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With a growing global space industry and the plan to return humans to the Moon within the decade, countries are examining in-Situ Resource Utilization (ISRU) as a means to reduce costs and expand capability. For the past 50 years, ISRU research has primarily focused on propellant production, only recently growing to seriously consider the production of metals. Through additive manufacturing (AM), these ISRU metals could be used for surface construction and repairs, opening new opportunities in addition to substantial improvements to mission costs and capability. This dissertation examines iron-based alloys (steels) produced by hydrogen reduction of lunar regolith simulants, including testing of the first publicly demonstrated consolidated metal produced from regolith or simulants. Phosphorus was found to be a major contaminant within the hydrogen reduction steel at a concentration between 0.86-0.91wt% due to phosphorus naturally found in regolith, transferred to the steel during the melt refining operation. Phosphorus leads to brittle behavior in steel; testing of analog alloys from 0.18-1.09 wt% phosphorus focused on maximizing ductility through heat treatment while considering lunar restrictions. Boron was tested as a potential addition for mitigating the embrittling effects of phosphorus but was unsuccessful at the levels tested.
In addition to finding the high phosphorus content of lunar steels, this research contributed by developing a heat treatment procedure to induce room temperature ductility in samples with 0.36wt% phosphorus, with an estimated maximum phosphorus content of 0.45-0.50wt%. Four methods are proposed for reducing phosphorus below this maximum. The mechanisms of phosphorus transfer mean that phosphorus contamination in a steel product will occur in all four leading ISRU technologies (hydrogen reduction, carbothermal reduction, molten regolith electrolysis (MRE), and the FFC process) making this finding important across the field. This research additionally examined the processing required to convert ISRU steels into a feedstock for additive manufacturing, demonstrating wire drawing and wire arc additive manufacturing (WAAM) using a commercial low carbon steel. A 6-axis robot arm was used to produce walls and components in non-vertical orientations as well as example components to highlight potential applications for these steels.
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