# Characteristics and Processing of Hydrogen-Treated Copper Powders for EB-PBF Additive Manufacturing
## 4 Discussion  
The oxygen content of each of the three starting powder conditions results from surface oxide films and oxides along the grain boundaries. The copper oxide will continue to grow when exposed to oxygen, as it does not passivate [42]. The surface oxide thickness as a function of time at room temperature was empirically derived by [PERSON] and [PERSON] [43]. A typical oxide thickness would fall between 3-5 nm, but could be much thicker. The overall contribution of this oxide layer becomes a significant portion of the chemistry for a powder with a high specific surface area [44]. Figure 19 shows the calculated wt. ppm of oxygen as a function of the particle diameter for cuprous oxide surface films of various thicknesses. In the case of a 5 nm coating, and a typical Gaussian powder size distribution centered at a 35 \(\upmu\)m diameter, the oxygen content is estimated to be roughly 75 wt. ppm oxygen, assuming all oxygen neglecting the 2 wt. ppm in solid solution at room temperature [39]. The experimental data gathered in this study suggest that the oxide thickness on the LO-Cu particles used in this study is \(\sim\)15-20 nm, consistent with our STEM observations. Volumetrically, this accounts for the measured 200 wt. ppm oxygen, which was mainly surface oxides in the LO-Cu material for particles in the range of 15-53 \(\upmu\)m.  
Figure 17: Oxygen and hydrogen content of precursor powders and solid copper cylinders produced with the EB-PBF of untreated and hydrogen-treated MO-Cu powder, and (inset) in-situ residual gas analysis of both EB-PBF runs, showing the layerwise outgassing of H\({}_{2}\)O.  
Figure 18: Optical micrographs showing polished and etched microstructures of copper samples fabricated with EB-PBF using hydrogen heat-treated MO-Cu powder (**A**) and untreated MO-Cu powder (**B**). The build direction for both samples are indicated by the arrow in B. The MO-Cu and HO-Cu particles greatly exceed this oxygen content and show a large number of submicron oxides along the grain boundaries, as seen in Figure 7B,C. Naturally, the HO-Cu particles show the largest number of submicron oxides along the grain boundaries, with several larger oxides at triple points of the microstructure because of its exposure to air at elevated temperatures. To remove the oxide particles, a hydrogen atmosphere heat treatment is employed to reduce the copper oxide to copper and H\({}_{2}\)O. This was first observed in the MO-Cu powder in Figure 7C,D, where the surface oxides were removed by hydrogen treatment. As hydrogen diffuses rapidly in copper (900 \(\upmu\)m\({}^{2}\)/s at 400 \({}^{\circ}\)C [34]), it allows for the reduction of oxides present inside the particles in a relatively short amount of time. The hydrogen treatment temperature and time was chosen to allow for sufficient hydrogen diffusion through the particles, while not reaching sintering temperatures for the fine powder size distribution. Although not directly reported herein, it is worth noting that, not since the completion of this study, we have subsequently demonstrated the hydrogen reduction of copper powder in several furnace types (including batch and continuous) in both pure hydrogen and in forming gas (5% h, 95% Ar), and we have observed consistent results in all scenarios for the equivalent treatment times. After hydrogen treatment, the MO-Cu and HO-Cu powders exhibit a micron scale porosity at the triple points of the grain boundaries, and a smaller submicron porosity along the grain boundaries. These pores have previously been shown in the literature to be trapped H\({}_{2}\)O vapor [28; 30; 35]. According to [PERSON] and [PERSON] [35], the formation of H\({}_{2}\)O vapor in closed pores is as a result of the reduction of copper oxide during hydrogen heat treatment. The H\({}_{2}\)O vapor pressure inside a closed pore is higher than that of the surface stress of the pore, which inhibits the pore from collapsing.