Achieving Superior Strength and Ductility in Ti-6Al-4V Recycled from Machining Chips by Equal Channel Angular Pressing
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TITANIUM (Ti) alloys possess a combination of desirable properties including high strength, low density, and excellent corrosion resistance. Of the many titanium alloys, Ti-6Al-4V is the most widely used, particularly in the aerospace and biomedical industries due to its good balance of properties. Unfortunately, titanium production and manufacturing costs are often prohibitively high for many applications. In addition, up to 90 pct of titanium input material can be wasted as chips during machining operations. Recycling helps to recover this scrap, but the conventional methods including vacuum arc remelting and electron beam cold hearth refining are expensive and inefficient owing to the high melting temperature and chemical reactivity of titanium. The concept of environmentally benign recycling, which optimizes value and quality while reducing environmental impact, is gaining considerable attention.[1] In particular, solid-state recycling is attractive as it avoids the energy-intensive melting process. Techniques based on hot extrusion have been attempted to D.T. MCDONALD and E.W. LUI, Research Fellows, and K. XIA, Professor, are with the Department of Mechanical Engineering, University of Melbourne, Parkville, VIC 3010, Australia, and also with the Defence Materials Technology Centre, Hawthorn, VIC 3122, Australia. Contact e-mail: [email protected] S. PALANISAMY, Senior Research Fellow, is with the Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, VIC 3122, Australia, and also with the Defence Materials Technology Centre. M.S. DARGUSCH, Associate Professor, is with the School of Mechanical and Mining Engineering, University of Queensland, St Lucia, QLD 4072, Australia, and also with the Defence Materials Technology Centre. Manuscript submitted October 7, 2013. Article published online May 14, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
recycle magnesium, aluminum, and iron machining chips, although lack of full density and inferior mechanical properties are often reported.[2–6] The high cost associated with titanium melting makes it a perfect candidate for solid-state recycling, and equal channel angular pressing (ECAP) is a promising technology for the recycling of titanium machining chips.[7–11] This solid-state technique is based on severe plastic deformation (SPD) and is capable of producing fully dense material with improved mechanical properties. The ECAP recycling can be carried out at relatively low temperatures of 873 K (600 C)[19]), the stability of the oxide can become lower than that of the solid solution of oxygen in titanium when heated and VOLUME 45A, AUGUST 2014—4089
the oxide would gradually dissolve, leading to interstitial oxygen in the titanium matrix.[12] The dissolution involves firstly the dissociation of the oxide (TiO2 fi Ti + 2O*, where O* is the oxygen atom at the oxide/ metal interface), and the subsequent diffusion of O* away from the interface to become solute in the titanium matrix. The slower of the two processes determines t
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