Visualization of the material flow in AA2195 friction-stir welds using a marker insert technique
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I. INTRODUCTION
FRICTION Stir Welding (FSW), developed at The Welding Institute (Cambridge, UK) in 1991,[1] is especially well suited for joining high-strength aluminum alloys. The FSW process is a solid-state joining process combining friction, deformation heating, and mechanical work to obtain high-quality, defect-free joints. The required heat is produced during transport of material from the two plates to be joined around a nonconsumable, rotating tool. The shape of the tool promotes a high hydrostatic pressure along the joint line, causing consolidation of the material plasticized due to heat generation. Although significant effort has been expended in putting FSW to use in the full-scale production of such products as ferry boats, rocket fuel, and oxidizer tanks, the operating mechanisms and, in particular, the material flow during FSW are not fully characterized. The microstructure of a typical FSW has been described in numerous previous publications[2–6] but will be briefly reviewed here. Figure 1 shows the microstructure resulting from the FSW of AA2195-T8. The weld microstructure features can be separated into two broad categories: the thermomechanically affected zone (TMAZ), and the heataffected zone (HAZ). This HAZ is similar to HAZs resulting from conventional, fusion welding processes. Depending on the alloy, its initial heat treatment, and the proximity to the weld centerline, processes occurring in the FSW HAZ might include precipitate coarsening, precipitate dissolution, recovery, recrystallization, and grain growth. The TMAZ of a friction stir weld might be considered analogous to the fusion zone of a conventional weld except that, instead of being melted, the material in the TMAZ has been mechanically worked. Within the TMAZ, there are three somewhat distinct regions. The first and most obvious is the “weld nugget.” T.U. SEIDEL, Graduate Research Assistant, and A.P. REYNOLDS, Associate Professor, are with the Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208. Manuscript submitted February 1, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
The weld nugget is the region that has undergone the most severe plastic deformation and is characterized by a fine, relatively equiaxed, recrystallized grain structure. The width of the nugget is normally similar to, but slightly greater than, the diameter of the pin. Outside of the nugget on either side is a second region, one that has been deformed to a lesser extent and that, depending on the alloy, may or may not show signs of recrystallization. In Figure 1, the delineation between the nugget and the rest of the TMAZ is quite sharp because of the recrystallization resistance of the AA2195T8 base metal. The deformation of the base metal grains manifests itself as bending in the plane of the metallographic section (the grains are also bent in the horizontal plane perpendicular to the section). The third region of the TMAZ is the “flow arm.” This is the region of material above the nugget. The flow arm is formed when the
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