Near-Net-Shape Production of Hollow Titanium Alloy Components via Electrochemical Reduction of Metal Oxide Precursors in
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INTRODUCTION
TITANIUM metal and its alloys exhibit high specific strength, light weight, outstanding biocompatibility, and excellent corrosion resistance.[1,2] However, due to the inherent costliness, they are seriously restricted to critical and demanding niche applications,[3] such as the aeroplanes,[4] medical implants,[5] and performanceimproving sports equipment.[6] Broadly speaking, the high cost of titanium and its alloys arises from two factors: the energy- and labor-intensive extraction of titanium metal (sponge) via the Kroll Process[7] (6 to 29 $/kg depending on demand) and the complex and expensive downstream processes for alloying and manufacturing of the final components (10 to 30 times of the sponge value).[8] The cost of the post-extraction alloying processes is significant, because the wide differences in the melting points and densities of the alloying elements necessitate multiple remelting steps to insure homogeneity of the final alloys. Furthermore, due to the high affinity to oxygen and the poor workability of titanium and its
DI HU, Postdoctoral Research Fellow, and GEORGE ZHENG CHEN, Professor of Electrochemical Technologies, are with the Department of Chemical and Environmental Engineering, and Energy and Sustainability Research Division, Faculty of Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD UK, Contact e-mail: [email protected] WEI XIAO, formerly Postdoctoral Research Fellow, University of Nottingham, is now Associate Professor in School of Resource and Environmental Science, Wuhan University, Wuhan 430072, P.R. China. Manuscript submitted April 23, 2012. Article published online February 12, 2013. 272—VOLUME 44B, APRIL 2013
alloys, the downstream shaping processes are also beset with difficulties, which drastically increase the cost of the final components.[2] For instance, the cost of downstream processes for the production of a 2.54-cm-thick titanium alloy plate accounts for 62 pct of the total cost;[8] fabrication of more complex shapes is of course more expensive, constituting a commensurately greater proportion of the bottom-line figure. Consequently, in recent years, the primary focus in titanium technology development has been the production of cost-affordable titanium and titanium alloy components rather than the development of new alloys with enhanced properties.[9] The results are two research routes: devising novel methods for the extraction of titanium metal and developing creative techniques for the fabrication of titanium and its alloy components. Over the last two decades, there have been approximately 20 newly proposed processes, some of which are being commercially developed, directed toward the lowcost extraction of titanium coupled with optimization of existing processes.[8,10] A majority of these developments focus on the production of commercially pure titanium, with few having the capability to produce titanium alloys directly and fewer still having done so. Notable among these, the FFC-Cambridge Process[11] has been successfully de
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