74 FRICTION STIR WELDING Vol.36,No.I in a flowing pattern around the weld nugget.TEM reveals that these grains have not recrystallized,as occurred in the weld nugget,Figure 10. The larger(50-75 nm)hardening precipitates appear not to have been significantly altered,either in size or morphology,during the joining process,Figure 11.The smaller precipitates,on the other hand, have coarsened during welding.They are disks having diameters on the order of 20 nm and thicknesses of about 6 nm,Figure 11.Precipitate-free zones are present around many of the larger matrix precipi- tates and around all of the grain-boundary precipitates. The dislocation density is quite low,with individual dislocations extending between particles,pri- marily in the PFZ near grain boundaries.There is no evidence of the tangles that existed in the parent metal. Discussion Friction stir welding has been shown to be a viable solid-state joining process for 7075 Al.Although there are a weld nugget and a heat affected zone,the microstructural changes were found to be less drastic than those occuring in a fusion weld. The most significant changes in microstructure in the 7075 Al examined in this study were the re- crystallization of the elongated grains and the solution of the fine(10-20nm)hardening precipitates in the weld nugget.A thermocouple placed in front of the rotating tool recorded a temperature of~425C. However,no thermocouple could be applied to the trailing portion of the weld where the temperature is higher,so an accurate measure of nugget temperature was not made.The subtle changes in micro- structure provide an estimate of the temperature excursion.The solution of all precipitates and re- precipitation of the strengthening precipitates(Mgs2(Al,Zn)49 or Mg(Zn2,AlCu))indicate the tempera- ture has exceeded 400C and was likely closer to 480C in the weld nugget.The sizes and distributions of the two populations of hardening precipitates in the mechanically affected zone indicate that that zone remained below 400C. The concentric flow lines observed in the weld nugget likely have an influence on the recrystalliza- tion of the matrix grains and on the precipitation of the strengthening particles.Details of these flow lines are currently being studied and will be reported in a subsequent paper. Summary and Conclusions 7075 Al plate has been successfully joined by friction stir techniques.Unlike fusion welding,this is a solid-state process with no evidence of melting.The weld is characterized by a recrystallized nugget having a 2-4 um grain size.The dislocation density in the nugget is lowered from that in the parent metal;strengthening precipitates appear to have been solutionized during the welding proce,with the larger ones re-precipitating on cooling.The process,thus,provides a method for joining an unweldable aluminum alloy without introducing a cast microstructure. Acknowledgments We are pleased to acknowledge the assistance of E.Litwinski of Rockwell Space Systems Division for the preparation of samples and weld support.This work was funded by Rockwell International I.R. and D.74 FRICTION STIR WELDING Vol. 36, No. 1 in a flowing pattern around the weld nugget. TEM reveals that these grains have not recrystallized, as occurred in the weld nugget, Figure 10. The larger (50-75 nm) hardening precipitates appear not to have been significantly altered, either in size or morphology, during the joining process, Figure 11. The smaller precipitates, on the other hand, have coarsened during welding. They are disks having diameters on the order of 20 nm and thicknesses of about 6 nm, Figure 11. Precipitate-free zones are present around many of the larger matrix precipi￾tates and around all of the grain-boundary precipitates. The dislocation density is quite low, with individual dislocations extending between particles, pri￾marily in the PFZ near grain boundaries. There is no evidence of the tangles that existed in the parent metal. Discussion Friction stir welding has been shown to be a viable solid-state joining process for 7075 Al. Although there are a weld nugget and a heat affected zone, the microstructural changes were found to be less drastic than those occuring in a fusion weld. The most significant changes in microstructure in the 7075 Al examined in this study were the re￾crystallization of the elongated grains and the solution of the fine (lo-20nm) hardening precipitates in the weld nugget. A thermocouple placed in front of the rotating tool recorded a temperature of -425°C. However, no thermocouple could be applied to the trailing portion of the weld where the temperature is higher, so an accurate measure of nugget temperature was not made. The subtle changes in micro￾structure provide an estimate of the temperature excursion. The solution of all precipitates and re￾precipitation of the strengthening precipitates (Mgz(Al,Zn)49 or Mg(Znn,AICu)) indicate the tempera￾ture has exceeded 400°C and was likely closer to 480°C in the weld nugget. The sizes and distributions of the two populations of hardening precipitates in the mechanically affected zone indicate that that zone remained below 4OO’C. The concentric flow lines observed in the weld nugget likely have an influence on the recrystalliza￾tion of the matrix grains and on the precipitation of the strengthening particles. Details of these flow lines are currently being studied and will be reported in a subsequent paper. Summary and Conclusions 7075 Al plate has been successfully joined by friction stir techniques. Unlike fusion welding, this is a solid-state process with no evidence of melting. The weld is characterized by a recrystallized nugget having a 2-4 pm grain size. The dislocation density in the nugget is lowered from that in the parent metal; strengthening precipitates appear to have been solutionized during the welding proci , with the larger ones re-precipitating on cooling. The process, thus, provides a method for joining an unweldable aluminum alloy without introducing a cast microstructure. Acknowledgments We are pleased to acknowledge the assistance of E. Litwinski of Rockwell Space Systems Division for the preparation of samples and weld support. This work was funded by Rockwell International I.R. and D