A B C A A B C the base metal shown in Fig. 5. To obtain a bulk characterization of the precipitate state in the FSW nugget, corresponding XRD analysis was also conducted. The XRD data are summarized in Fig. 14 with the minor peaks identified as the stable θ phase (Ref. 59). The θ phase peaks increased in intensity for the SZ of the 150 rev/min weld shown in Fig. 14B, decreased in intensity in Fig. 14C of the 200 rev/min weld, with further reductions in the 300 rev/min weld in Fig. 14D, which were similar in intensity to the base metal in Fig. 14A. This bulk XRD analysis was in agreement with the B TEM images in Figs. 11–13. Using the torque data from the FSW panels, an alternative heat index (Ref. 27) was used to calculate the FSW temperatures as summarized in Table 5. The calculated temperatures ranged from 0.86–0.88 Tm, corresponding to the empirically published range of 0.80–0.90 Tm for FSW of AA2xxx alloys (Refs. 1, 2, 14–20). The phase diagram in Fig. 4 for Al-Cu binary system showed the nominal 6.30% Cu alloy was slightly above the maximum solid solubility composition. The α and θ phases can form a eutectic at a composition of 33.2 wt-% Cu with a eutectic temperature of 548°C. The C calculated temperatures were in agreement with experimental studies that showed an increase in peak temperature as the tool rotation increased. However, whether the small amount of temperature difference was responsible for the variations observed in the microstructure is questionable. Rather than a critical temperature threshold being crossed, it was proposed that only the material flow that crosses the severe shear zone into the SZ experiences heating rates that drive the stable θ phase into dissolution. Since a constant tool travel was maintained, the corresponding higher shear strain rate in addition to the higher tool rotation re- WELDING JOURNAL 17-s WELDING RESEARCH Fig. 11 — A and B — TEM images of the 150 rev/min FSW specimen; C — corresponding SAD pattern for the 100Al zone axis of the aluminum matrix. Fig. 12 — A and B — TEM images of the 200 rev/min FSW specimen; C — corresponding SAD pattern for the 100Al zone axis of the aluminum matrix. Fig. 13 — A and B — TEM images of the 300 rev/minFSW specimen; C — corresponding SAD pattern for the 100 zone axis of the aluminum matrix. Note the superlattice reflections in the SAD pattern corresponding to the Θ' phase.
Welding Journal | January 2013
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