The role of impurities in bubble formation during directed light processing of tantalum

Abstract

The utility of directed-light fabrication (DLF) as a method for the manufacture of refractory metal parts has been hampered by the formation of bubbles in the finished product. This study examines the connection between these bubbles and impurities found in several Ta feedstock powders. Bulk and surface impurities associated with the powders were determined using glow-discharge mass spectroscopy (GDMS), thermal desorption and X-ray photoelectron spectroscopy (XPS). A cylindrical part with a high bubble density was fabricated from the Ta powder using DLF and was subsequently fractured in vacuum. The exposed bubble surfaces were examined with Auger electron spectroscopy (AES) and secondary electron microscopy (SEM). Unlike the surrounding region, the bubble surfaces were coated with a K-rich layer. Potassium was an impurity found in the feedstock powder by GDMS. Due to incompletely understood process dynamics, a simple equilibrium model was used to examine the likelihood that gaseous K was trapped in the molten Ta to produce the bubble growth. The results suggest that another impurity, such as hydrogen, may have a primary role in the bubble formation. Analysis of recycled powder that had not been fused during DLF processing showed a decreased concentration of K, Na, F, H and water impurities, implying that some high-temperature purification of the feedstock powder might improve the quality of parts fabricated in this manner.