Microstructural Characterization of a Polycrystalline Nickel-Based Superalloy Processed via Tungsten-Intert-Gas-Shaped M
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AS part of ongoing trials assessing various techniques for their suitability to manufacture shaped metal deposited (SMD) structures, our latest efforts have concentrated on tungsten inert gas (TIG) deposition. SMD typically involves increasing the thickness of a material via localized, additive melting and solidification. The superposition of weld deposition beads is one technique that has received interest, largely because it can employ standard welding equipment. This latest research has benefited from a previous study using metal inert gas (MIG) because many processing techniques were shared.[1] One major conclusion from the previous work was that a smaller volumetric weld pool would be preferred to provide a faster freezing rate and ultimately better microstructural control. This is a fundamental benefit of TIG because the improved control of heat input at lower currents than those used for MIG can add to process stability for complex substrate forms. TIG also has a benefit in terms of the precision of build. However, it is recognized that TIG offers a much slower deposition rate, which will limit the economic benefits of this technique. It is possible to preheat the wire electrically as employed in ‘‘hot wire TIG’’ to produce DANIEL CLARK, Specialist Welding Engineer, is with the RollsRoyce plc, Derby, DE24 8BJ, England. MARTIN R. BACHE, Professor, and MARK T. WHITTAKER, RCUK Research Fellow, are with the School of Engineering, Swansea University, Swansea, SA2 8PP Wales, UK. Contact e-mail: [email protected] Manuscript submitted March 24, 2010. Article published online July 21, 2010. 1346—VOLUME 41B, DECEMBER 2010
faster melt rates, although this process has not been considered in this investigation. For simplicity, a round wire consumable was used, not flattened, to change lateral stiffness. In contrast, the lower deposition rate offers an advantage in terms of the rate of heat dissipation, allowing for a more consistent substrate temperature during the build-up of the product form. Alternative economic factors are in favor of TIG, including low-capital equipment cost, especially when compared with competing wire-based deposition techniques using wide spot lasers[2] or electron beam guns. This technique is in addition to the efficient use of consumable material, which is usually the main driver for additive techniques.[3] In contrast to powder consumables, the use of wire as the feed supply for TIG provides a simple route to consistency and cleanliness of the source material. TIG lends itself to the use of finer feed wires that give a benefit in terms of current flux, again encouraging the relatively smaller melt pool, which in turn allows finer feature resolution. Any risk associated with a lack of fusion can be countered by the consistent delivery of the wire, as opposed to powder fed into the hot zone of a weld pool, ultimately providing a more consistent microstructure within the deposit beads. In comparison with other arc-based techniques including the use of plasma, TIG is tolerant in terms of standoff
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