The impact of oxidised powder particles on the microstructure and mechanical properties of Ti-6Al-4 V processed by laser powder bed fusion

In selective laser melting (SLM) heat diffusing from the melt pool promotes the growth of surface oxide layers on powder particles surrounding the built part, and material ejected from the melt pool oxidises rapidly before landing on the powder bed, creating local variability in the oxygen content of any used powder. Although large particles are removed when recycling, smaller oxidised particles (the size of the specified powder for the machine) and oxide residue (<10 mu m) are not removed and become incorporated into subsequent builds on powder reuse. This paper considers the effect these oxidised particles may have on part integrity and how they affect mechanical failure. In this research, grade 23 Ti-6Al-4 V metal powder was artificially oxidised to produce a range of interference colours that correspond to specific oxide thicknesses. Powder characterisation established the oxygen wt% of each coloured powder. Yellow and blue powder were chosen for further investigation as in the context of this study, they represent low (0.4 wt%) and high (0.7 wt%) oxygen levels respectively. Tensile builds were produced using SLM with a known feedstock layer, part way up the build, formed of a blend of oxidised and virgin particles. Tensile tests were performed for each build to evaluate the failure modes. Microscopy techniques were used to examine the material near the fractured region, including chemical composition and semiquantification of the oxygen levels, allowing any microstructural and chemical changes to be investigated. It was shown that the region doped with oxidised particles negatively affected the mechanical properties of the final build, as they produce mechanical (oxide films/residue) and chemical flaws (interstitial elements). Unless these particles can be removed from recycled feedstock their effects will limit the reuse of powder, especially in safety critical industries, significantly increasing the costs of components produced by this route.