Friction Stir Welding in HSLA-65 Steel: Part I. Influence of Weld Speed and Tool Material on Microstructural Development
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INTRODUCTION
THERE has been considerable interest in the friction stir welding (FSW) process since it was developed by TWI in 1991.[1] FSW is a solid-state joining process derived from conventional friction welding, in which a nonconsumable rotating cylindrical tool is plunged into the material at the interface between the plates to be joined and is then moved along the interface. Frictional heat generated primarily from the tool shoulder, and to some extent also from the pin, softens the materials to be joined, which are then plastically deformed around the FSW tool in a constrained extrusion process to combine at the rear of the tool.[2] A high-quality solid-state welded joint can be produced, since any surface oxides are broken up and dispersed through local plastic deformation. When the process parameters are controlled correctly, no parent metal melting occurs.[3] The FSW S.J. BARNES, NDT Manager, and P.J. WITHERS, Professor of Materials Science, are with the School of Materials, Materials Science Centre, University of Manchester, Manchester M13 9PL, United Kingdom. Contact e-mail: [email protected] A.R. BHATTI, formerly Research Assistant, with the School of Materials, Materials Science Centre, University of Manchester, is now Postdoctoral Research Assistant, with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. A. STEUWER, Visiting Professor for Materials Engineering, is with ESS Scandinavia, University of Lund, 22350 Lund, Sweden, and is also with NMMU, Port Elizabeth 6031, South Africa. R. JOHNSON, formerly Project Leader, with TWI Yorkshire, Wallis Way, Catcliffe, Rotherham S60 5TZ, United Kingdom, is now retired. J. ALTENKIRCH, formerly Researcher, with the School of Materials, Materials Science Centre, University of Manchester, is now Project Manager, with Siemens AG, M¨lheim an der Ruhr D-45473, Germany. Manuscript submitted July 14, 2010. Article published online April 11, 2012 2342—VOLUME 43A, JULY 2012
process has been researched and developed extensively for joining aluminum alloys.[4] Many aluminum alloys, particularly the high-strength ones used for aerospace applications, are difficult to join by conventional fusion welding techniques. FSW can be used to join aluminum alloys without problems such as porosity or solidification cracking and with lower distortion when compared to standard fusion welding processes. Additionally, FSW offers the possibility of joining dissimilar materials.[5] Friction stir welds often exhibit improved mechanical properties in comparison to fusion welds of the same materials[6] and have been successfully produced for most classes of commercial aluminum alloys.[6–12] Although joining of aluminum-based alloys has been the main focus of FSW development, the process can be used for other nonferrous and ferrous alloys. Until recently, relatively little research had been published on the FSW of steels,[13–30] but the literature on the subject is expanding rapidly. There have been two main obstacles to
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