Additive manufacturing titanium powder oxygen variation within a single powder bed due to differences in powder size and oxygen content

Abstract

Thermogravimetric analysis of Ti-6Al-4V powders (virgin Grade 5 and a once used Grade 23) was conducted up to 800 ∘C in an industrial grade Ar environment. A standard starting particle size distribution (PSD) for electron beam powder-bed fusion (PBF-EB) additive manufacturing (AM) was divided into two smaller ranges based on the D50 to observe the effects of oxidation (e.g. mass gain) resulting from the differences in available, reactive surface area during testing. Significant mass gain was still recorded throughout the testing owing to titanium’s extreme reactivity at elevated temperatures even with the Ar cover gas. The mass gain can be attributed to impurities in the purge gas and absorbed water in the powder. The smaller particle size distribution powders accumulated more % mass gain than the full range and larger PSD, likely due to the increased surface area per unit volume of the smaller PSD powder. The virgin Grade 5 powder generally accumulated more oxygen/mass gain than the once used Grade 23 powder, likely resulting from a previously existing thicker TiO2 shell on the once used Grade 23 powder. The results suggest that within a single AM PBF powder-bed, there may be varying levels of oxidation (both interstitial and oxide) depending on the particle size distribution and resulting surface area exposed to high temperatures for a given volume, and level of reuse (thickness of the oxide layer) on a per-particle basis if the batch had been blended.11 This work is an official contribution of the National Institute of Standards an Technology and is not subject to copyright in the United States.

Keywords

• Additive manufacturing
• Ti-6Al-4V
• Oxidation
• Thermogravimetric analysis
• Powder reuse