Surface roughness and finishing techniques in selective laser melted GRCop-84 copper for an additive manufactured lower hybrid current drive launcher

Abstract

Recent advances in selective laser melting (SLM) 3D printing technology allows additive manufacture of lower hybrid current drive (LHCD) RF launchers from a new material, Glenn Research Copper 84 (GRCop-84), a Cr2Nb (8 at. % Cr, 4 at. % Nb) precipitation hardened alloy, in configurations unachievable with conventional machining. Rough surfaces in additive manufactured components are a limiting factor in RF structures. Surface roughness increases RF losses and impedes conditioning by trapping gas and contaminants that induce arcing by evolving from the surface at high power. SLM printed GRCop-84 is post-processed with mass finishing to reduce surface roughness. Wet blasting, vibratory finishing, and chemical etching remove adhered powder grains from SLM printed GRCop-84. Profilometry and scanning electron microscopy were used to quantify resulting surface quality. Chemical etching is examined to remove zinc contamination from wire electrical discharge machining slag prior to vacuum use. Etch rates in SLM printed GRCop-84 are anisotropic, and etch times should be limited to prevent pitting corresponding to the laser hatch pattern. Vibratory mass finishing and mechanical polishing produced surfaces suitable for low RF loss. Use of chemo-mechanical finishing is recommended for the interior of SLM printed structures.