Chemistry Modification of High Oxygen-carbon Powder by Plasma Melting: Follow up to Complete the Story

State-of-the-art melting of tantalum and tantalum alloys has relied on electron beam (EB) or vacuum arc remelting (VAR) for commercial ingot production. Plasma arc melting (PAM) provides an alternative for melting tantalum that contains very high levels of interstitials where other melting techniques can not be applied. Previous work in this area centered on plasma arc melt quality and final interstitial content of tantalum feedstock containing excessive levels of interstitial impurities as a function of melt rate and plasma gas.( 1) This report is an expansion of this prior study and provides the findings from the analysis of second-phase components observed in the microstructure of the PAM tantalum. In addition, results from subsequent EB melting trials of PAM tantalum are included. INTRODUCTION Because of the high melting point and reactivity of tantalum, production of mill forms rely on EB or EB+VAR melting to achieve purity levels greater than 99.95%. High vacuum levels inherent in EB melting, combined with the high melting point of tantalum, acts to purifL the product by volatilizing interstitial and low melting metallic impurities(2). High-quality Ta products exhibiting consistent mechanical and metallurgical properties are produced from large diameter (300mm) ingots which were triple EB melted(3). Here, tantalum powder billets are side fed into the furnace and melted into an ingot which is then inverted and hung in the furnace and remelted; this process is repeated to produce a 3EB ingot. The purpose of the first EB melt is to consolidate the powder into a homogeneous form; purification is achieved primarilly from the final two EB melts. An alternative to the 3EB process involves double EB melting of a tantalum electrode followed by a final VAR melt. The VAR process affords a greater melt rate, and operates at a vacuum pressure which is 2-3 orders of magnitute higher than EB melting. Tantalum plate stock produced from 2EB+VAR ingots has been characterized as having a finer grain size and a greater strength compared to 3EB products. Early speculation was that the difference in DroDerties between 3EB and 2EB+VAR tantalum produck was due to the finer grain size of t i e VAR differences are a reflection of product purity.(3,4)