Process Optimization for Friction-Stir-Welded Martensitic Steel
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y focus for the current alloy development is to achieve a high strength-to-weight ratio keeping cost of processing as low as possible. In this context, ultrahigh-strength steel came in the limelight for automobile industries because of ease of fabrication in thin gauges, reduction in weight of vehicle and capability of providing better safety condition.[1] The fabrication of components with new alloys necessarily involves joining. One of the widely accepted joining methods in auto industries is electric resistance spot welding.[2] Undesirable results of such fusion welding include low ductility, high hardness, and segregation of alloying elements in weld assembly.[3] In most cases, these heterogeneities are mitigated by postweld heat treatment involving additional cost and time. Friction-stir welding (FSW) is a remarkable breakthrough for joining materials with poor fusion weldability. Since its invention, attempts have been made to use the technique for different grade of ferrous alloys like ultralow, hypoeutectoid, and hypereutectoid carbon steels. In this class, 99.9 pct C steel is simplest with a single-phase structure at room temperature. The microstructure was studied in details for this alloy by Mironov M. GHOSH, Scientist, is with the Materials Science & Technology Division, CSIR - National Metallurgical Laboratory, Jamshedpur 831 007, India. Contact e-mail: [email protected] K. KUMAR, formerly Research Associate, with the the Centre for Friction Stir Processing, Department of Materials Science & Engineering, Missouri University of Science & Technology, Rolla, MO 65401, is now Research Associate, with the Department of Materials Science & Engineering, University of Northern Texas, Denton, TX 76203- 5017. R.S. MISHRA, formerly Professor & Centre Director, with the Centre for Friction Stir Processing, Department of Materials Science & Engineering, Missouri University of Science & Technology, is now Professor, Department of Materials Science & Engineering, University of Northern Texas. Manuscript submitted September 23, 2011. Article published online February 11, 2012 1966—VOLUME 43A, JUNE 2012
et al.[4] after friction-stir processing at 200 rpm and 1.0 mm/s tool rotational and traversing speed, respectively. They discussed two phenomena, first grain structure at weld nugget as a result of grain subdivision and state of strain and grain boundary migration, and second, the development of (112)[111] texture at stirring zone. Friction-stir welding was executed for low-carbon steel, i.e., steel with a relatively higher carbon content with respect to the previous one.[5,6] Ozekcin et al.[5] carried out FSW for 0.13 pct C steel under different tool rotational speeds while keeping the other parameters same. They reported the development of a bainitemartensite microstructure in the thermomechanically affected zone (TMAZ) and ferrite-bainite-martensite phase constituents in the heat-affected zone (HAZ). In the case of DH36 steel (£0.18 pct C), the tool traversing speed was varied at a constant weld pitch during friction-st
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