Microstructural Evolution During Friction Stir Welding of Mild Steel and Ni-Based Alloy 625
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NI-BASED alloys are often welded to dissimilar alloys, particularly steels, to achieve certain combined properties for specific applications. In general, these applications take advantage of several inherent properties of Ni-based alloys. Ni-based alloys 625 are used in the construction of a large number of power plant parts that require creep resistance at elevated temperature, thermal-fatigue and corrosion resistance. Steel A516 is used primarily for the construction of equipment components in the oil industry and has the highest tensile strength and ductility among the available pressure vessel steels.[1] Conventional processes are frequently used to join these materials. However, welding defects associated with some metallurgical issues during solidification remain. Solid-state joining processes are alternatives for dissimilar welding due to lower peak temperatures during welding. Friction stir welding (FSW) is a solid-state joining process, where the heat is generated by friction and deformation on the surface of a rotation tool.[2–5] As the tool is translated along the materials being welded, severe plastic deformation and flow of material occurs.[3] FSW has been successfully applied to non-dissimilar systems, such as Al and, Mg alloys, and to high melting
JOHNNATAN RODRIGUEZ FERNANDEZ is with the Brazilian Nanotechnology National Laboratory, Rua Giuseppe M´aximo Scolfaro 10000, Campinas, SP, 13083970, Brazil, and also with School of Mechanical Engineering, University of Campinas, Rua Mendeleyev 200, Campinas, SP, 13083860, Brazil. ANTONIO J. RAMIREZ formerly with the Brazilian Nanotechnology National Laboratory, and also with the School of Mechanical Engineering, University of Campinas, is now with the Department of Materials Science and Engineering, The Ohio State University, 1248 Arthur E. Adams Drive, Columbus, OH, 43221. Contact e-mail: [email protected] Manuscript submitted March 29, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
temperatures alloys, such as steel, titanium and Ni-based alloys.[6] FSW of dissimilar alloys has a number of potential applications, and understanding the associated microstructural evolution plays a key role in process optimization and quality improvements. Microstructural evolution has been extensively studied in single-phase alloys, such as Al, Mg, Cu and Fe. It has been conclusively demonstrated that severe grain refinement during the FSW is a very complex process. The formation of the grain structure in the regions affected by the thermo-mechanical effect of the welding tool has been attributed to dynamic recrystallization (DRX).[7] In face-centred-cubic (FCC) metals with low stacking fault energy (SFE), the nucleation mechanism of DRX during hot deformation has been shown to be a discontinuous dynamic recrystallization (DDRX) that occurs due to bulging of the grain boundaries.[8] However, some characteristics of continuous dynamic recrystallization (CDRX), such as the formation of subgrains with low-angle boundaries (LABs) in the interior of the original grains
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