sulted in this material staying at temperature longer. Thus, the high heating rate increase combined with a longer time at temperature at 300 rev/min resulted in the θ dissolution that replenished the solute in the supersaturated α phase for postweld natural aging. Based on the macro and micro scale data, the SZ of the FSW had a mixed state of stable and metastable Cu-rich phases. To obtain both dissolved larger particles and coarsened small particles implied that the material was subjected to two different temperature fields (Refs. 56, 57). This was explained using the kinematic model for FSW in which some material flow lines near the weld tool crossed a severe high shear rate region while other material flow lines further from the weld tool were subjected to lower shear rates and hence lower temperature (Refs. 11, 49, 56, 57). Other researchers have observed decreased second phase particle size in the FSW microstructure corresponding with increased tool rev/min, which was attributed to fragmentation resulting from the shearing action of the material flow in the FSW process (Refs. 60, 61). Although agglomeration of θ particles have also been reported in a study at higher tool revs/min (Ref. 40), in addition to a study on second pass repair FSWs (Ref. 38). The results in this study were consistent with another study on the microstructural evolution in AA2219-T87 (Ref. 15). Although that study (Ref. 15) only reported one set of unknown FSW parameters, similar FSW strengths and precipitate state B were reported that align with the results of the 200 rev/min specimen in this study. Correlation of microstructural evolution with the FSW temperature relied on the use of thermocouples mounted away from the SZ (Ref. 15). The measured temperature was extrapolated to the SZ resulting in a estimated value of 475°C or 0.8 Tm, which is lower than the 532°C or 0.86 Tm temperature calculated from the FSW data in this study for the shear zone. Conclusions In all the FSWs, a coarsening of the θ′ phase was observed that resulted in the decreased SZ hardness and tensile strength. The solute lost from the α-matrix due to the coarsening of the θ′ phases was eventually replaced by the dissolution of the θ phase at the higher tool rotation, which promoted postweld natural aging. Occurrence of coexisting coarsened θ′ and θ phases in the SZ result from the combined effect of two flow streams of metal, which were subjected to different thermomechanical processing conditions. Thus, only the metal flow stream that crossed the severe shear zone experienced either higher temperatures or more severe shear as influenced by the tool rotation. At higher revs/min, the material also remains around the tool for a longer time, which suggests time at temperature was also critical to the final precipitate state. Using the alternative heat indexing method, the calculated temperature at 300 rev/min was estimated to be 542°C, which was close to the 548°C eutectic temperature shown on the Al-Cu phase diagram in Fig. 3. This provided a temperature rate sufficient for up-quenching to dissolve the θ phase in the FSW nugget region, but insufficient temperature to cause spontaneous melting of the θ phase. The resulting microstructure was similar to the base metal in conductivity as shown in Fig. 10 and hardness as shown in Fig. 8. While the calculated temperatures for the shear zone were not extreme over the range of FSW parameters investigated, they did highlight a region where critical changes in the microstructure in the SZ occurred. It was speculated that further increases in FSW rev/min may result in liquation as evidenced by a drop in weld power or torque. These FSWs were not performed as higher rev/min conditions in combination with the tool used in this study have resulted in voids. The results of these experiments showed that processing parameters of FSW have a strong impact on precipitate position and dispersion, affecting localized mechanical and electrical properties. Due to the nonhomogeneity of the resulting FSW SZ, microscale hardness and conductivity measurements were useful in understanding the effect of precipitate state on the resulting electrical properties. References 1. Schmidt, H. B., and Hattel, J. H. 2005. A local model for the thermomechanical conditions in friction stir welding. Modelling Simul. Mater. Sci. Engr. 13: 77–93. 2. Mishra, R. S., and Ma, Z. Y. 2005. Friction stir welding and processing. Mat. Sci. & Engr. R50: 1–78. 3. Mendez, P. F., Tello, K. E., and Lienert, T. J. 2010. Scaling of coupled heat transfer and plastic deformation around the pin in friction stir welding. Acta Mater. 58: 6012—6026. 4. Record, J. H., Covington, J. L., Nelson, T. W., Sorensen, , C. D., and Webb, B. W. 2007. A look at statistical identification of critical process parameters in friction stir welding. Welding Journal 86(4): 97-s to 103-s. 5. Lakshminarayanan, A. K., and Balasubramanian, V. 2008. Process parameters optimization for friction stir welding of RDE-40 aluminum alloy using Taguchi technique. Trans. Nonferrous Met. Soc. China 18: 548–554. 6. Arora, K. S., Pandey, S., Schaper, M., and Kumar, R. 2010. Effect of process parameters on friction stir welding of aluminum alloy 2219- T87. Int. J. Adv. Manuf. Technol. 50: 941–952. 7. Xu, W., Liu, J., Guohong, L., and Dong, C. 2009. Temperature evolution, microstructure and mechanical properties of friction stir welded thick 2219-O aluminum alloy joints. Mat. & Design 30: 1886–1893. 8. Askari, A., Silling, S., London, B., and Mahoney, M. 2001. Modeling and analysis of friction stir welding process. Friction Stir Welding & Processing, ed. K. V. Jata, M. W. Mahoney, R. S. Mishra, S. L. Semiatin, and D. P. Field, pp. 43–54, TMS Pub. JANUARY 2013, VOL. 92 18-s WELDING RESEARCH Fig. 14 — XRD results showing increasing presence of stable CuAl2 precipitates from the base metal (A); to the 150 rev/min FSW (B). Intensity of the stable CuAl2 decreases slightly in the 200 rev/min FSW (C); reverting to similar intensity as the base metal in the 300 rev/min FSW (D). A C D
Welding Journal | January 2013
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