Development of Microstructure and Crystallographic Texture in a Double-Sided Friction Stir Welded Microalloyed Steel
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FRICTION stir welding (FSW), a solid-state welding technique originally developed for joining Al-alloys,[1,2] was invented by The Welding Institute (TWI) in the early 1990s, and is now a mature process for welding Mg, Cu and Ti alloys, and Al-metal matrix composites. It is claimed that the advantages of FSW include rapid joining speeds, elimination of weld cracking and porosity, and no need for shielding gas of filler metal. All lead to improved mechanical and metallurgical weld properties, and normally, a lower residual stress than fusion welding.[1–5] The restriction of welding only in lower melting temperature alloys such as aluminum, due to the
S. RAHIMI, Principal Materials Fellow, is with the Advanced Forming Research Centre (AFRC), University of Strathclyde, 85 Inchinnan Drive, Inchinnan, Glasgow, PA4 9LJ, UK. Contact e-mails: [email protected] and [email protected] B.P. WYNNE, Culham Chair in Materials Technology, is with the Department of Materials Science and Engineering, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, UK. T.N. BAKER, Emeritus Professor of Metallurgy, is with the Department of Mechanical and Aerospace Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street, Glasgow, G1 1XJ, UK. Manuscript submitted May 8, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
severe conditions encountered by the rotating tool, has over the past decade, to some extent, been overcome, and FSW has been developed to process some steels. FSW has been developed to process steels, which has been achieved through improvements in the tools which are capable of withstanding higher flow stresses at higher temperatures, in the range 1273 K to 1493 K (1000 C to 1200 C).[3,5–9] Austenitic stainless steels were among the first to be studied,[5,6] and there is now a growing body of literature demonstrating successful welds in ferritic steels, together with the potential for joining dissimilar steels.[7–9] Also, the lower heat inputs, producing fewer metallurgical changes in the heat-affected zone (HAZ) compared to fusion welding, are regarded as an additional advantage for FSW of steels.[10] Generally, the ferritic steel joined by the FSW process has been limited to a thickness of £10 mm. This, for the most part, is a consequence of the tool strength and toughness placing a size constraint on the FSW tool. An approach developed for thicker plate involves passing a tool of slightly greater length than half the thickness of the plate on both sides of the abutted plates in sequence.[11–13] This gives rise to an interaction zone (IZ) where the root of the first pass weld is reprocessed by the second pass on the reverse side of the plate, producing a complex deformation zone. The HAZs are also affected by the plate reverse side FSW pass. In the first published account of
double-sided FSW,[12] a tungsten tool was used to weld up to 12.7 mm thick HSLA-65 (ASTM A945) steel plate. It was found that following the procedure laid down by a British standard involving bending to 120 de
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