Sintered powder oxidation variation as a function of build height for titanium alloy produced by electron beam powder-bed fusion
Nicholas Derimow, Alejandro Romero, Aldo Rubio, Cesar Terrazas, Francisco Medina, Ryan Wicker, Nik Hrabe
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. In order to study the powder bed, thin-walled cylindrical powder capsules were printed along side of solid parts in order to capture the sintered powder for controlled chemical sampling. Powder collected from the bottom 3 mm and top 3 mm of the powder capsules show no morphological differences from the virgin powder. The largest oxidation is observed at the bottom of the powder capsules, and that oxidation decreases as a function of build height to approximately zero by 35 mm build height. This magnitude of oxidation and the 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). Because oxidation changes as a function of build height, the predominant source of oxygen must be transient. This helps prioritize some potential sources of oxygen (e.g. powder moisture) over others (e.g. chamber leaks, helium cover gas impurities) when developing oxidation mitigation techniques. Heterogeneous oxidation within a single powder bed in PBF-EB Ti-6Al-4V has implications on powder handling and mixing procedures, possibly leading to undesirable mechanical behavior variation within single builds. All of these observations from this work motivate scrutiny of powder handling and mixing procedures as well as development of oxidation mitigation techniques.
, Romero, A.
, Rubio, A.
, Terrazas, C.
, Medina, F.
, Wicker, R.
and Hrabe, N.
Sintered powder oxidation variation as a function of build height for titanium alloy produced by electron beam powder-bed fusion, Additive Manufacturing Letters, [online], https://doi.org/10.1016/j.addlet.2021.100023, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=933557
(Accessed January 16, 2022)