Abstract
The fabrication of high purity copper using additive manufacturing has proven difficult because of oxidation of the powder feedstock. Here, we present work on the hydrogen heat treatment of copper powders for electron beam powder bed fusion (EB-PBF), in order to enable the fabrication of high purity copper components for applications such as accelerator components and vacuum electronic devices. Copper powder with varying initial oxygen contents were hydrogen heat-treated and characterized for their chemistry, morphology, and microstructure. Higher initial oxygen content powders were found to not only reduce surface oxides, but also reduce oxides along the grain boundaries and form trapped H2O vapor inside the particles. The trapped H2O vapor was verified by thermogravimetric analysis (TGA) and residual gas analysis (RGA) while melting. The mechanism of the H2O vapor escaping the particles was determined by in-situ SEM heated stage experiments, where the particles were observed to crack along the grain boundaries. To determine the effect of the EB-PBF processing on the H2O vapor, the thermal simulation and the validation of single melt track width wafers were conducted along with melting single layer discs for chemistry analysis. A high speed video of the EB-PBF melting was performed in order to determine the effect of the trapped H2O vapor on the melt pool. Finally, solid samples were fabricated from hydrogen-treated copper powder, where the final oxygen content measured ~50 wt. ppm, with a minimal residue hydrogen content, indicating the complete removal of trapped H2O vapor from the solid parts.
Highlights
IntroductionThe copper used in these applications approaches the theoretical maximum achievable quality in terms of the purity, density, and metallurgical properties, such as crystallographic texture and grain size
Applications including particle accelerators and vacuum electronic devices (VEDs) require materials with the highest electrical and thermal conductivity, as well as ultra-high vacuum compatibility.The copper used in these applications approaches the theoretical maximum achievable quality in terms of the purity, density, and metallurgical properties, such as crystallographic texture and grain size
The untreated MO-Cu powder contained ~450 wt. ppm oxygen and ~2 wt. ppm hydrogen, and after hydrogen treatment, the oxygen content was reduced to ~280 wt. ppm, but the hydrogen content increased to ~30 wt
Summary
The copper used in these applications approaches the theoretical maximum achievable quality in terms of the purity, density, and metallurgical properties, such as crystallographic texture and grain size. These applications require intricate designs and extensive metallurgical processing routes, followed by the assembly and brazing or welding of multiple components into a final part. The resulting rapid solidification of the melt pool, coupled with the low viscosity of molten copper, tends to retain defects such as keyhole porosity [15]. The low absorptivity of copper for the lasers used in many commercial L-PBF systems (~1060 nm wavelength) necessitates the use of high-power lasers increasing the recoil pressure, vaporization, spatter, and related defects [16]
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