# Characteristics and Processing of Hydrogen-Treated Copper Powders for EB-PBF Additive Manufacturing
## 3 Results
### Electron Beam Fabrication  
EBM involves the selective melting of an elevated temperature powder bed for part fabrication. In this study, the powder bed temperature for part fabrication in copper is below the critical temperature for H\({}_{2}\)O release in hydrogen-treated powder. The reported powder bed temperatures for the EB-PBF of copper range from 380 \({}^{\circ}\)C ([PERSON] et al. [8]) to 530 \({}^{\circ}\)C ([PERSON] et al. [9]), with oxygen contents comparable to the MO-Cu used in this study (although with a larger powder size distribution). It was observed early in the development of this study that the removal of surface oxides with hydrogen heat treatment resulted in difficulty in spreading the powder at elevated powder-bed temperatures (\(>\)350 \({}^{\circ}\)C). Without the presence of surface oxides, the activation energy for sintering is greatly reduced. In some cases, sintering was observed during spreading, resulting in homogenous and uneven layering. A lower powder bed temperature alleviates this problem to some extent, and, from a practical standpoint, facilitates the removal of powder from closed cavities. Unlike other common materials utilized in EB-PBF (e.g., Ti6 Al4V, TiAl, and 718), copper powder is less susceptible to charge-induced scattering, because of its high conductivity, which also enables EB-PBF processing at lower temperatures, with minimal sintering. The single melt track width experiments, described in Section 2.5, were carried out to determine whether the powder adjacent to the melt-pool front reaches a sufficiently high temperature for a sufficiently long duration, in order to induce the powder cracking H\({}_{2}\)O vapor release mechanism  
Figure 11: Scanning electron micrographs of a HO-Cu particle after hydrogen treatment, before (**A**) and after (**B–D**) in-situ SEM heating on a SiC chip to 400 \({}^{\circ}\)C at 100 \({}^{\circ}\)C/s under a vacuum. observed in the heated stage experiments. The thermal input conditions were modeled using the 3D transient heat transfer model developed by [PERSON] et al. (2019) at the Oak Ridge National Laboratory, to guide the experimental design. A 2D contour plot of the temperature profile during melting with a 2000 \(\upmu\)m hatch spacing is shown in Figure 12. The heat affected zone in the powder bed surrounding the melt pool extends several hundred microns below and to the sides of the melt pool, ranging from 500 \({}^{\circ}\)C at 150 \(\upmu\)m from the center of the melt pool, to 350 \({}^{\circ}\)C at a distance of 250 \(\upmu\)m.  
The powder samples were harvested from between each of the EB-PBF wafers from the edge/wall wafers and in \(\sim\)0.1 mm increments so as to determine the influence of the heat affected zone of the melt pool on the surrounding powder bed particles. Figure 12A-C are the SEM images of the sintered powder adhered to the fabricated part (Figure 12A), \(\sim\)0.1 mm distance (Figure 12B) and \(\sim\)0.3 mm distance (Figure 12C) from the part where the surface characteristics of the particles correlate to their respective associated temperature. The sintered powder at the edge of the melt pool was estimated to reach temperatures of 700 \({}^{\circ}\)C, where the subsurface H\({}_{2}\)O is expected to be expelled from the grain boundaries, and the particle surface is smoothed due to diffusion at these relatively high temperatures. Powder images at 0.1 mm from the edge of the wall experienced temperatures of approximately 400-500 \({}^{\circ}\)C, according to the thermal model, and had features similar to the in-situ SEM heating (Figure 11), where obvious grain boundary cracking occurred, but no surface smoothing was visible. The collected powder imaged at 0.3-0.5 mm from the melt pool showed no obvious surface changes because of the relatively low temperature (\(<\)350 \({}^{\circ}\)C) estimated by the thermal model, and still contained the characteristic H\({}_{2}\)O vapor pores. These data support the hypothesis that H\({}_{2}\)O is released in the heat affected zone ahead of the melt pool in EB-PBF.