# Characteristics and Processing of Hydrogen-Treated Copper Powders for EB-PBF Additive Manufacturing
## 1 Introduction
This poses a unique obstacle for the copper powder used in AM, which is typically produced by nitrogen gas atomization, and quickly oxidizes if exposed to ambient conditions at any point in the powder fabrication and handling process. Because of the very high surface area of these small diameter particles, the relative contribution of oxygen from the surface oxide film typically causes the feedstock to exceed 400-600 wt. ppm oxygen. For example, in their study on the EB-PBF fabrication of copper, [PERSON] et al. [7], [PERSON] et al. [8], and [PERSON] et al. [9] utilize copper powder feedstocks with reported purities of 99.90 wt%, 99.94 wt%, and 99.95 wt%, respectively, while [PERSON] et al. [1] and [PERSON] et al. [10] used powders with 99.99 wt% and 99.80 wt% purity, respectively. In our preliminary studies, a chemical analysis conducted directly after the powder atomization process, and after the screening, indicates that much of the oxygen pickup in the copper powder occurs during the handling, screening, and packaging processes, which are typically carried out in ambient atmosphere. Preserving even these oxygen contents during the AM melting and solidification process is extremely challenging, and requires near perfect conditions, as imperfect inert or vacuum atmospheres, moisture absorption, complex thermal cycles, powder recycling, and storage exacerbate oxygen pickup. [PERSON] et al. [1] showed a significant oxidation of feedstock powders after several EB-PBF reuse cycles. Once the oxygen contamination is present in the powder feedstock as oxide films or grain boundary particles, the contamination is directly transferred to the fabricated AM components [11]. This combination of factors hinders the fabrication of quality copper components by most powder bed AM processes. Reduction processes may be employed in order to improve the copper feedstock purity for AM processing. Hydrogen, carbon monoxide, and ethanol are commonly employed reducing agents of Cu\({}_{2}\)O and CuO in thin films [31; 32; 33]. [PERSON] et al. [6] treated copper powders for L-PBF in a heated forming gas atmosphere (4% H\({}_{2}\) + Ar bal. ), theorizing that the hydrogen would reduce the surface Cu\({}_{2}\)O films into Cu + H\({}_{2}\)O, however, no quantitative data on the composition of the powders or solid samples produced were provided [6]. In addition, this study only considered the reduction of oxides from the surface of the Cu powders, and did not address the possibility that hydrogen also reduces the internal oxides. Previous work has shown that the diffusivity of hydrogen in copper is high at elevated temperatures (e.g., \(\sim\)9.58 m/s\({}^{2}\) at 400 \({}^{\circ}\)C) [34], and that at temperatures above about 400 \({}^{\circ}\)C, hydrogen reduces the grain boundary oxides, forming H\({}_{2}\)O gas. The larger H\({}_{2}\)O molecules do not diffuse, and the resulting pressure forms high pressure steam pores along the grain boundaries [28; 35; 36]. This well-known embrittlement mechanism is considered detrimental for most traditional copper powder processing routes because of their significant impacts on downstream processing, such as swelling, porosity, and embrittlement, through plastic deformation, work hardening, the reduction of grain boundary area, and material failure at elevated temperatures [28; 36; 37; 38], as [PERSON] and [PERSON] demonstrated with the swelling and cracking of copper powders during the sintering of copper heat pipes [36]. On the other hand, PBF processes utilize thin layers of powder, which are spread over a substrate and selectively melted by a focused energy source. In the present work, we hypothesize that this localized processing can be leveraged to liberate the retained H\({}_{2}\)O from the hydrogen heat-treated copper powder during melting on a layer-by-layer basis. This results in a significantly higher copper purity, without the detrimental effects of embrittlement observed in traditional powder metal processes.