the workpiece/weld tool interface are smaller than in the FSW wake, which has been attributed to grain growth during the slow cooling of the workpiece (Ref. 55). As grain sizes have been reported to increase with increasing tool rotation due to postweld grain growth, use of the Zener-Holloman method results in an underestimation of the strain rate. The highest shear strain rate has been estimated based on a kinematic approach that does not rely on an assumption of material properties at the FSW conditions (Refs. 11, 49). Using this approach, an estimate of the mean shearing strain rate (γ) across the shear surface of thickness (δ) at the workpiece/weld tool interface has been made using Equation 4 (Ref. 11). (4) ≅(r * ) In Equation 4, r is the radius of the shear surface approximated by the pin tool radius and ω is the angular velocity of the metal inside the shear surface taken to be approximately the same as that of the tool. The shear zone thickness, δ, is estimated to be on the order of 0.1 times the pin diameter (Refs. 1, 3, 46, 49). The estimated shear strain rates are summarized in Table 1 showing increasing rates as the tool rotation increases. As the travel speed was constant in this study at 114 mm/min, the higher strain rate corresponded to a faster heating rate at the shear surface surrounding the SZ. Note that this was an instantaneous shear strain rate that the material experienced as it crossed the shear zone. Neighboring material adjacent to the shear zone experienced less shearing, and hence, lower temperatures. The intertwining of these two flow paths in the SZ region was reported to result in the shear textures or onion ring pattern observed in the FSW SZ (Refs. 56, 57) Macrographs of the etched transverse sections of the three welds are shown in Fig. 6. They were repolished to obtain the SPM surface profiles shown in Fig. 7. Preferential polishing around the harder Curich particles reveal an increasing number as the rotation is increased from 150 to 200 rev/min. However, at 300 rev/min, a decrease in the average size and the volume fraction of hard Cu-rich particles assumed to be the θ phase was observed. This would correspond to an increased dissolution rate of the θ phase as the rev/min, and hence the strain rate, increased above a critical level. The representative grain size measurements for the three FSWs in this study were obtained using electron backscattered diffraction (EBSD)/orientation image mapping (OIM). Table 2 summarizes the variation in grain size observed between the AS and RS of the FSWs. The larger, more uniform grain size in the 300 rev/min FSW specimen was consistent with exposure to higher temperatures or longer cooling times for the workpiece, similar to other reports (Refs. 39, 55). Thus, the higher SZ strength at the higher revs/min cannot be attributed to Hall-Petch strengthening, but rather to the precipitate state. The horizontal dashed line, shown on the macrographs in Fig. 6, indicate the location of the nanoindentations summarized in Fig. 8. While a reduction in hardness was observed for the welds made at 150 and 200 rev/min, the 300 rev/min FSW had a higher value. Table 3 lists a comparison of the FSW strengths to the base metal. Although all FSWs had a lower strength than the base metal, a trend toward increased tensile strength was noted for the SZ as the tool rotation increased. Estimating a weld temperature based on conversion of power to heat, assuming a 100% efficiency and constant contact conditions, predicted a higher temperature at the higher tool rotation. For natural aging to occur, Cu-rich phases in the 2219-T87 material would have to dissolve and increase the amount of solute in the α-matrix in the wake of the FSW. TEM images indicated that the θ phase was dissolved, thus replenishing the solute in the α phase for postweld natural aging. On the basis of the hardness data and corresponding SPM images, there was a significant change in either the localized temperature or the heating rate between the FSWs made at 300 rev/min and the 150 and 200 rev/min. There was no evidence of exceeding the eutectic temperature, either by γ ω δ WELDING JOURNAL 15-s WELDING RESEARCH Fig. 7 — SPM images show higher amounts of precipitates on the surfaces of the FSWs made at A — 150 rev/min; B — 200 rev/min; than on the C — 300 rev/min sample surface. Fig. 8 — Nanohardness measurements on FSW samples showing higher hardness at 300 rev/min due to natural aging. .
Welding Journal | January 2013
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