We are developing a microwave investment casting process as a means of fabricating magnetron parts using lunar resources. Since a magnetron is the core part of a microwave oven, this work is the first step towards a self-replicating fabricator. Resources available on the moon are used wherever possible at each stage of the process, so this work also paves the way for development of an entire lunar infrastructure from a single seed fabrication unit. Magnetrons also have potential for other components of lunar and space infrastructure: they demonstrate the potential for vacuum tube electronics for rad-hard systems and could be used as microwave generators for solar power satellites. Our microwave investment casting process comprises four steps: 1) 3D printing of a 'positive' of the part, 2) encapsulation of the positive in a silicon carbide and calcium-aluminate based hydraulic cement mould, 3) removal of the positive, and 4) microwave casting, using either metal powders/solid charges, metal oxide with carbon as a reducing agent, or precursors to ferrite magnetic materials. 3D printing of the positives allows for flexibility in part shaping. The positive material is not of great consequence so long as it is water resistant - we have used positives of polylactic acid (PLA) and polyvinyl alcohol (PVA) (in spite of its water solubility), while spirulina positives were also used with early cement mixes. Positives are vibration set into the cement. After the cement cures, they are either burned out (spirulina, PLA) or dissolved in water (PVA). Powdered metal (or other material) is then poured into the cavity (which includes additional 'crucible' volume for shrinkage as required), and the filled mould is fired in a conventional home microwave oven. To date ingots of copper, iron, nickel, cobalt, and copper-nickel-iron have been successfully cast, with no apparent porosity, while attempts with aluminium, barium ferrite, and strontium ferrite have been unsuccessful. This has demonstrated the principle of casting metal magnetron parts in 3D print-formed moulds. This technology looks promising for application on the Moon in extending 3D printing beyond regolith processing to a wide range of in-situ sourced metals.

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70th International Astronautical Congress, IAC 2019
Department of Mechanical and Aerospace Engineering

Ellery, A, & Schmidtke, N. (Nicholas). (2019). In situ resource utilization - Analogues for a lunar constructed magnetron via 3D printing and microwave casting. In Proceedings of the International Astronautical Congress, IAC.