Effect of high oxygen content on unalloyed copper produced by electron beam powder bed fusion

Pure copper with low oxygen content is known to produce wide columnar epitaxial grains when processed by raster scans with high scan rotation angles via electron beam powder bed fusion. However, oxygen, commonly found as non-passivating surface copper oxide in otherwise unalloyed precursor copper powder, can restrict the grain growth because of high grain growth restriction factor, and hence, can lead to a refined microstructure along build direction. Previous studies, although limited to LPBF, showed insignificant effect of the oxides on the electrical properties, while improved strength properties are often reported for copper with high oxygen content. In this study, copper powder was furnace treated to increase its oxygen content and processed under similar conditions to the processing of pure copper, contrary to the previous LPBF studies that used different process settings. Oxidized copper shows refined grain structure with cellular solidification sub-grains evolved with ordered copper oxide particles at the sub-grain boundaries. Thermal stability of these oxide particles is tested by TG and conventional heat treatment (HT) at 400oC in argon, commonly used for copper. While HT led to a more preserved cellular sub-grain structure similar to as-fabricated condition, the TG experiment at 950oC under vacuum led to loss of mass due to the disassociation of copper oxide and more thermally activated agglomeration of the particles. Microstructural evolution is explained by lattice mismatch between oxide particles and the copper matrix, aided by simple thermal model data to approximate the temperature fields. With high oxygen content, a good combination of strength and electrical conductivity is found, caused by the interface properties and the strengthening effect from the harder phase of copper oxide. This approach demonstrates the feasibility of using oxidized copper powder in a favorable way instead of undergoing the recycling process for the need of high purity virgin powder.