Microstructure and thermal stability of oxidized copper produced by electron beam powder bed fusion

Oxygen in copper can potentially minimize the grain width because of the high grain growth restriction factor, and hence can modify the overall microstructure. In this study, bulk copper samples were produced via electron beam powder bed fusion with the same process parameters using different powder types: one virgin powder type with low oxygen content and two furnace-treated powder types with progressively higher oxygen contents. The oxidized copper samples showed refined grain structures with cellular sub-grains that evolved with cuprous oxide nanoparticles at the sub-grain boundaries. However, despite a high grain growth restriction factor, oxide nanoparticles do not serve as active sites for heterogeneous nucleation because of the high lattice mismatch. The thermal stability of the oxide nanoparticles and cellular sub-grain structure was tested by thermogravimetric analysis (TGA) and annealing heat treatment (AHT). While AHT preserved the sub-grain structure, the TGA experiment led to a concomitant dissolution of the same followed by thermally activated agglomeration of the cuprous oxide nanoparticles. Nevertheless, even with high oxygen content, a good combination of microhardness and electrical conductivity was observed. This approach demonstrates the feasibility of using oxidized copper powder for certain applications instead of recycling due to the need for high-purity virgin powder.