Characterization of plastic flow and resulting microtextures in a friction stir weld
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1/1/04
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Characterization of Plastic Flow and Resulting Microtextures in a Friction Stir Weld J.A. SCHNEIDER and A.C. NUNES, Jr. In friction stir welding (FSW), a rotating threaded pin tool is inserted into a weld seam and literally stirs the edges of the seam together. The dynamically recrystallized zone (DXZ) of a polished and etched FSW cross section exhibits contrasting bands (the “onion-ring” structure), the origins of which are unclear. An orientation image mapping (OIM) study suggests that the bands may correspond, respectively, to a “straight-through” current of metal bypassing the pin tool in a single rotation or less and a “maelstrom” current rotating a number of times around the pin tool.
I. INTRODUCTION
THIS solid-state friction stir welding (FSW) technique is a relatively new process useful for the joining of metals that are difficult to fusion weld. During joining, a rotating threaded pin tool is inserted into a weld seam and translated along the seam to stir the edges of the seam together. The metal is subjected to thermomechanical processing in which the temperature, strain, and strain rates are not completely understood. This study is intended to cast light on the nature of the flow process occurring around the pin tool with an eye to the ultimate goal of controlling and optimizing the process. The cross section of an FS weld is shown in Figure 1 with a white line marking the location of the initial weld seam. Using standard welding convention, this cross section can be divided into “zones.” Proceeding inward from the parent metal toward the centerline of the weld, the first zone to be encountered is the heat-affected zone (HAZ). It is distinguished by alterations in hardness and in etch contrast caused by heating of the metal, but no significant mechanical distortion of the metal is present. After the HAZ, the thermomechanical zone (TMAZ) is encountered. The microstructure in this region exhibits both thermal effects and the effects of a mechanical distortion, apparent in the curvature of the formerly flat pancake-shaped grains of the parent metal. Beyond the HAZ and the TMAZ is a complex region at the center of the weld. Sometimes this region is called the “stir zone” (SZ). It is a zone of residue from the metal passing closest to and most affected by the pin tool. The SZ is not generally homogeneous. The bulk of the SZ is a zone of small equiaxed grains, the dynamically recrystallized zone (DXZ). The DXZ is very distinct from the parent metal. It can be seen in Figures 1 and 2. Some authors call it the “weld nugget” or “nugget zone” (NZ) because of its shape and central location. This is the zone exhibiting the banding of concern here. The DXZ occupies the lower part of the weld and extends all the way up to the J.A. SCHNEIDER, Assistant Professor, is with the Department of Mechanical Engineering, Mississippi State University, Mississippi State, MS 39762. Contact e-mail: [email protected] A.C. NUNES, Jr., Engineer, is with the Materials, Processes and Manufacturing De
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