# Characteristics and Processing of Hydrogen-Treated Copper Powders for EB-PBF Additive Manufacturing
## 1 Introduction  
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. These applications also require intricate designs and extensive metallurgical processing routes, followed by the assembly and brazing or welding of multiple components into a final part. The consolidation of the component assemblies into complex monolithic parts, and the potential for novel designs achievable through additive manufacturing (AM), is desired in order to reduce the cost andimprove the component performance and reliability [1; 2; 3; 4; 5]. The underlying challenges associated with the AM of high-purity copper, particularly for electron beam powder bed fusion (EB-PBF) and laser powder bed fusion (L-PBF), have been discussed in several recent studies [1; 6; 7; 8; 9; 10; 11]. The high thermal conductivity of copper rapidly removes heat from the melt pool, promoting high local and global thermal gradients; residual stress accumulation; and distortion, which is exacerbated by the significant difference in thermal conductivity between the consolidated material and the surrounding powder bed [12; 13; 14]. The resulting rapid solidification of the melt pool, coupled with the low viscosity of molten copper, also tends to retain defects such as keyhole porosity [15]. Additionally, the low absorptivity of copper for the lasers used in many commercial L-PBF systems (\(\sim\)1060 nm wavelength) necessitates the use of high-power lasers increasing the recoil pressure, vaporization, spatter, and related defects [16]. However, even with these challenges, several promising results have been reported with densities ranging above 99.95% for non-electronic grade copper [1; 7; 8; 9] for EB-PBF, and 96.6% for Nd:YAG fiber laser AM processing [17; 18; 19; 20; 21; 22; 23]. Despite the progress, achieving the high purity requirements for copper using AM has remained a significant challenge. For accelerator applications, copper typically needs to meet or exceed that of ASTM F68 for Class 1 oxygen free electronic (OFE) copper, which is 0.15176 ohms g/m\({}^{2}\) or 101.0% of the minimum International Annealed Copper Standard (IACS) at 20 \({}^{\circ}\)C [24]. The maximum oxygen content for OFE copper is 0.0005 wt% (5 wt. ppm). The negative influence of oxygen contamination on the electrical, thermal, and mechanical properties of copper is small, but not insignificant. The incoherent Cu\({}_{2}\)O found along the grain boundaries has an effect similar to porosity, and less of an influence on electron scattering than the solute contamination that strains the copper lattice. For instance, electrolytic tough pitch (ETP) copper has an oxygen content of \(\sim\)0.04% (400 ppm) and typically exceeds 100-101% IACS [24; 25; 26]. Much more significant is the adverse effect of embrittlement in the copper components caused by an excessive oxygen content during the downstream hydrogen brazing processes [27]. It should be noted that the copper AM literature has either resulted in specimens with a high oxygen content (similar to ETP Cu), or has not reported an oxygen content in the fabricated samples. Of chief concern is the powder feedstock purity. The powder feedstock commonly used in AM is subject to oxygen contamination during handling, screening, loading, and transport after atomization. At ambient conditions, the oxygen solubility in pure copper is less than 2 wt. ppm [24; 28], where the excess oxygen reacts to form cuprous oxide, Cu\({}_{2}\)O, as a non-passivating surface film or along the grain boundaries. At higher temperatures (\(>\)300 \({}^{\circ}\)C), cupric oxide (CuO) and other variants may also form [29; 30].