Sintering of Titanium with Yttrium Oxide Additions for the Scavenging of Chlorine Impurities

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POWDER metallurgy (PM) is a promising avenue to low-cost titanium products. The appeal of PM methods is associated with potential cost and energy savings compared to traditional methods of manufacture based on repeated ingot metallurgy.[1–4] Such potential savings are derived principally from more efficient processing conditions, reduced material wastage, and the benefits of avoiding bulk melting of titanium. Chlorine impurities have been a significant technical challenge in titanium production ever since the inception of the Kroll[5] and Hunter[6] processes themselves. Immediately after thermochemical reduction, the titanium product exists as a porous mass of metal (termed ‘‘sponge’’), intermingled with chloride byproducts (MgCl2 or NaCl) and excess reactants. The removal of the residual chlorides is one of the most difficult and expensive processes in the production of titanium sponge.[7] Purification, principally by vacuum distillation, is an energy-intensive batch process operated at elevated temperatures (107 K to 1233 K [800 C to 960 C]) over several days.[8] Since complete removal of chloride impurities from titanium sponge is either not possible or not feasible, powders derived directly from sponge retain substantial R.J. LOW, formerly Ph.D. Candidate, with The University of Queensland, School of Mechanical and Mining Engineering, ARC Centre of Excellence for Design in Light Metals, Brisbane, QLD 4072, Australia, is now Engineer with the Materials Technology Group, GHD Pty. Ltd., Brisbane, QLD 4000, Australia. M. QIAN, Reader in Materials Engineering and G.B. SCHAFFER, Professor, are with The University of Queensland, School of Mechanical and Mining Engineering, ARC Centre of Excellence for Design in Light Metals. Contact e-mail: [email protected] Manuscript submitted February 6, 2012. Article published online August 1, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

levels of chloride. This includes sponge fines containing up to 1800 ppm of Cl[9,10] and hydride-dehydride (HDH) powder derived from the milling of sponge containing up to 600 ppm of Cl. Powders made from bar stock or ingots are regarded as essentially chloride free.[11–13] This includes PREP powder, gas atomized powder, and milled ingot metallurgy material (via the HDH process). However, these powders are significantly more expensive. The high volatility associated with residual chlorides causes a variety of detrimental effects during subsequent processing.[4,14,15] Even at levels as low as 50 ppm, chloride impurities in PM products cause porosity,[16–18] microinclusions,[16,19,20] grain boundary embrittlement,[13,16,20] and poor weldability.[4,12,17] The effects of chlorides on sintering are most significant in the presence of liquid.[21–23] For example, chloride volatilization during liquid phase sintering leads to the formation of very large pores (sometimes millimeters in size) that are few in number.[21,23] Chlorine concentrations greater than 50 ppm are reported to cause severe sputtering and a highly porous weld zone during fusion welding. M

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