# Characteristics and Processing of Hydrogen-Treated Copper Powders for EB-PBF Additive Manufacturing
## 3 Results
### Powder Characterization
LO-Cu powder lacks significant grain boundary oxides, and therefore does not exhibit H\({}_{2}\)O vapor induced porosity. Note that scale bars vary in the images. Figure 10 shows the results of the thermogravimetric analysis (TGA) of untreated and hydrogen-treated powders for the three initial oxygen contents. A TGA was carried out to ascertain the temperature at which the internal pressure of the H\({}_{2}\)O vapor pores exceeds the grain boundary strength of the copper powder particles. All of the untreated powders demonstrate no significant change in weight loss after heating to 600 \({}^{\circ}\)C at 5 \({}^{\circ}\)C/s. However, both hydrogen-treated MO-Cu and HO-Cu powders experience weight loss beginning at 375 \({}^{\circ}\)C, which then stabilizes at approximately 525 \({}^{\circ}\)C. Hydrogen-treated LO-Cu powder also shows minimal mass change while heating when compared to the hydrogen treated MO-Cu and HO-Cu particles. The weight loss can be correlated to the initial oxygen content of the powder before hydrogen treatment, indicating an increased presence of H\({}_{2}\)O in the hydrogen-treated particles at higher oxygen contents. To directly observe the mechanism for the H\({}_{2}\)O release during the heating of the treated powders, treated and untreated particles were heated to 400 \({}^{\circ}\)C, and then to melting at 1200 \({}^{\circ}\)C, using an in-situ SEM heating stage (Protochips Fusion). It is important to note that the reported temperature is that of the surface of the heating elements, not necessarily the equilibrium temperature of the particles. The temperature of the stage is calibrated with a pyrometer and correlated to the input current. Here, it is assumed that the mass of the particle is small compared with the input power, and that the equilibrium temperature is reached quickly. This assumption is corroborated by the data acquired from multiple sources in this study (TGA, in-situ EB-PBF, etc.). A temperature of 400 \({}^{\circ}\)C was chosen for imaging, based on the onset temperature of weight loss observed by TGA. Figure 11A shows a typical hydrogen-treated HO-Cu particle at room temperature. Additional treated and untreated particles are included as Supplemental video files. This particle was placed on the SiC chip in the SEM, and  
Figure 10: thermogravimetric analysis of untreated and hydrogen-treated LO-Cu, MO-Cu, and HO-Cu powder under flowing nitrogen. Figure 9: HAADF STEM micrograph and energy dispersive spectroscopy (EDS) of hydrogen-treated MO-Cu powder grain boundary triple point showing submicron scale H\({}_{2}\)O vapor induced porosity. heated in-situ to 400 \({}^{\circ}\)C at 100 \({}^{\circ}\)C/s. Figure 11B is an SEM image of the particle surface at 400 \({}^{\circ}\)C immediately after fracture lines are observed, as H\({}_{2}\)O vapor escapes from the particle, illustrated by the arrows. Subsequently, as shown in the Supplemental videos, the fracture lines close and heal through diffusion mechanisms. Figure 11C shows images of another particle surface at 400 \({}^{\circ}\)C immediately after the onset of the grain boundary fracture. In this case, the particle was quenched from this condition, and subsequently cross sectioned with a FIB, which reveals interconnected porosity along the grain boundaries inside the particle (Figure 11D). Here, the release of H\({}_{2}\)O vapor at 400 \({}^{\circ}\)C causes the interconnected porosity along the grain boundaries underneath the particle surface, with an additional porosity towards the center of the particle.