Solidification behavior and porosity in electron-beam powder bed fusion of Co–Cr–Mo alloys: Effect of carbon concentrations

An increased carbon content strengthens Co–Cr–Mo alloys for use in a broad range of industrial applications. In this study, we investigated the influence of the carbon content (0.04–2.5 mass%) on the porosity and microstructure of Co–27Cr–6Mo (mass%) alloys during atomization and electron-beam powder bed fusion (EB-PBF). Quantitative X-ray computed tomography clarified that the volume fraction of pores in the raw powders monotonically increased with the carbon content, as a potential effect of the significant reduction in the liquidus temperature. In contrast, the porosity evolution in the investigated alloys during EB-PBF under identical building conditions suggested an influence of carbon concentration that was distinct from that in the powder. These alloys exhibited negligible porosity fraction for 0.04 and 0.22 mass% and a maximum volume fraction (∼0.3 vol.%) at 2.0 mass%, followed by a remarkable reduction caused by further carbon addition. The porosity of the as-built alloys could be correlated to the solidification behavior varying with carbon concentration. The smoother and more flat solidification front during the cellular (0.04 and 0.22 mass%) and eutectic (2.5 mass%) solidification could effectively eliminate the gas bubbles from the melt pool, whereas the complicated morphology at the solid–liquid interfaces during the dendritic growth (1.5 and 2.0 mass%) hindered the pore elimination in the melt pool. Adding carbon significantly increased the Rockwell hardness of the as-built specimens, reaching a significantly high value of HRC59 at 2.5 mass% of carbon, primarily due to the formation of hard carbide precipitates. The obtained findings could be beneficial to reduce entrapped gas pores thereby contributing to the development of highly durable metal components.