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Defect-free friction stir welds of 5-mm-thick pure copper plates were produced in relatively low heat input conditions. The characteristics of the microstructure and mechanical properties of the welded joints were investigated. The stir zone (SZ) exhibited equiaxed recrystallized grains, whose size decreased as the heat input was decreased. The percentage of high-angle grain boundaries (grain boundary misorientation angle >15 deg) in the SZ was quite high (90.2 to 94.5 pct) and increased as the heat input was increased. When the heat input was decreased, the percentage of the twin boundaries (TBs) dropped, and the number of the twin lamellas was reduced. Under a very low heat input condition, the typical characteristics of thermomechanically affected zone (TMAZ) were discernible; however, the TMAZ was characterized by a recrystallized grain structure at higher heat input conditions. The grains in the heat-affected zone (HAZ) were slightly coarsened compared to those in the parent material (PM), but the grain size varied a little under different parameters. The hardness of the SZ increased as the heat input was increased, and the lowest hardness appeared at the HAZs where the welds failed. The ultimate tensile strength (UTS) was similar to that of the PM under various heat input conditions, but the yield strength (YS) and elongation were lower. The YS increased as the lowest hardness value increased, and the elongation decreased due to the enhanced strain localization.

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Three rotation rates were selected for friction stir welded (FSW) rolled AZ31 plates, at a constant welding speed of 100 mm/minutes. As the rotation rate was increased from 800 and 2000 to 3500 rpm, the nugget shape varied from a basin- or ellipse-shaped homogeneous structure to a two-layer structure. The grain characteristic was similar in the nugget zones (NZs) at 800 and 2000 rpm, with the column-rotated basal plane. However, at 3500 rpm, the equiaxed fine grains and column-rotated basal plane were found in the lower zone of the two-layer structure, while elongated coarse grains and the basal plane with little inclination comparable to that in the parent material (PM)were detected in the upper zone. Tensile tests indicated that the fracture behaviors of the FSW AZ31 joints at 800 and 2000 rpm were similar with the weak zones being located at the NZ middle and along the boundary between the NZ and the thermo-mechanically affected zone, while contrasting fracture features were found at 3500 rpm. The ultimate tensile strength (UTS) of the joints increased as the rotation rate increased, with the highest UTS being about 95 pct of the PMat 3500 rpm.The variations in the strength and fracture behavior of the jointswith the rotation rate were accounted for by the variation in the texture. DOI: 10.1007/s11661-012-1373-4 The Minerals, Metals & Materials Society and ASM International 201

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