Sintered powder oxidation variation as a function of build height for titanium alloy produced by electron beam powder-bed fusion

It is well-established that titanium alloy (Ti-6Al-4V) powder oxidizes during electron beam powder-bed fusion (PBF-EB) due to the high background temperatures resulting from layer preheating and sintering of the powder bed before melting. However, it is not known if oxidation is homogeneous throughout the entire build area. This work investigates the potential for variation in powder oxidation as a function of build height for PBF-EB Ti-6Al-4V, up to build heights of 35 mm. Thin-walled cylindrical powder capsules were printed in proximity to solid parts in order to capture the sintered powder for controlled chemical sampling. Powders collected from the bottom 3 mm and top 3 mm of the powder capsules show no morphological differences from the virgin powder. Higher oxidation is observed at the bottom of the powder capsules, and decreases with build height to approximately zero at 35 mm height. This magnitude of oxidation and change with build height was consistent across multiple locations in multiple builds, suggesting build height is the main factor in oxidation magnitude. An increase in oxygen content of 0.02 wt.% in a single build is significant when considering the maximum allowable oxygen is only 0.13 wt.% in material specifications (ASTM F3001-14). The predominant source of oxygen must be transient. This helps prioritize some potential sources of oxygen (e.g. powder moisture) over others when developing mitigation techniques. All of these observations from this work motivate scrutiny of powder handling and mixing procedures as well as development of oxidation mitigation techniques.