Evolution of Microstructure During Double-Sided Friction Stir Welding of Microalloyed Steel
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TRODUCTION
FRICTION stir welding (FSW) is now a mature procedure for joining the light metals and alloys of aluminum, magnesium, and titanium.[1,2] More recently, attention has been paid to steels, through work initially on stainless steels[1,3–5] and on mild and microalloyed steels.[5–9] This has been achieved through improvements in tools, in particular polycrystalline boron nitride (PCBN) pins for use at higher temperatures, in the range 1273 K to 1473 K (1000 C to 1200 C). The majority of this research has been concentrated on alloys with plate thicknesses £ 10 mm, where the weld can be accomplished in a single pass. The plate thickness which can be welded is limited by the length of the tool, which for PCBN pins, often the pin of choice for FSW of steels, is ~ 8 mm. To weld thicker alloys, the plates must be turned after the first pass has cooled to ambient, and a second pass made to complete the weld.[10] The
T.N. BAKER and N.A. McPHERSON are with the Department of Mechanical and Aerospace Engineering, University of Strathclyde, 75 Montrose Street, Glasgow, G1 1XJ, UK. S. RAHIMI is with the Advanced Forming Research Centre (AFRC), University of Strathclyde, 85 Inchinnan Drive, Inchinnan, Glasgow, PA4 9LJ, UK. Contact e-mails: [email protected], [email protected] B. WEI and K. HE are with the Electron Microscopy Laboratory, Advanced Research Centre, Central South University, Changsha, 410083, Hunan, China. Manuscript submitted November 8, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
material forming the weld undergoes a complex thermomechanical treatment as a result of friction generated between the tool and the work-piece. In this process the plastically deformed material under the shoulder is rearranged by the tool’s rotation and sideways movement. This results in improved mechanical and metallurgical weld properties, and normally, a lower level of residual stress compared to other methods of welding.[11–13] Several publications describe the results of this double-sided FSW technique for steels.[10,14,15] The study reported in Reference 10 investigated the dominant deformation mechanisms by looking at the evolution of microstructure and crystallographic texture in different regions of the weld, but has not considered in detail the evolution of microstructure at submicron scales. A variety of terms are used in the literature to describe different microstructural zones observed during FSW of different materials.[16] For a single pass FSW, the postweld microstructural zones are classified as (i) unaffected base material (BM), (ii) the heat affected zone (HAZ), (iii) the thermomechanically affected zone (TMAZ). The TMAZ extends across the weld from the HAZ on the advancing side (AS) to the HAZ on the retreating side (RS), which has frequently been subdivided to subzones depending on microstructure. The central part of the TMAZ is generally known as the stirred zone (SZ), or nugget (NG). An area immediately below the upper face of the weld, known as the shoulder affected zone (SAZ), in which the m
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