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

The utility of directed-light fabrication (DLF) as a method for the manufac ture of refractory metal parts has been hampered by the formation of bubble s in the finished product. This study examines the connection between these bubbles and impurities found in several Ta feedstock powders. Bulk and sur face impurities associated with the powders were determined using glow-disc harge mass spectroscopy (GDMS), thermal desorption and X-ray photoelectron spectroscopy (XPS). A cylindrical part with a high bubble density was fabri cated from the Ta powder using DLF and was subsequently fractured in vacuum . The exposed bubble surfaces were examined with Auger electron spectroscop y (AES) and secondary electron microscopy (SEM). Unlike the surrounding reg ion, the bubble surfaces were coated with a K-rich layer. Potassium was an impurity found in the feedstock powder by GDMS. Due to incompletely underst ood process dynamics, a simple equilibrium model was used to examine the li kelihood that gaseous K was trapped in the molten Ta to produce the bubble growth. The results suggest that another impurity, such as hydrogen, may ha ve 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 pu rification of the feedstock powder might improve the quality of parts fabri cated in this manner. (C) 2001 Elsevier Science B.V All rights reserved.