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Microstructure and mechanical properties of friction stir welded Sc-modified Al-Zn-Mg-Cu alloy

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Conventional Al-Zn-Mg-Cu (7000 series) alloys are widely used in aerospace applications due to their favorable mechanical properties. Their strength, the highest among all aluminum alloys, arises from a high density of minute precipitates produced upon aging. However, the mechanical properties degrade above 150°C due to the coarsening and/or dissolution of the strengthening phases within the microstructure. There is currently strong interest in adding zirconium and scandium to Al alloys in order to improve the overall performance of these alloys [1, 2]. Additions of scandium and zirconium to these alloys can stabilize the microstructure at elevated temperatures and can augment the mechanical properties through the formation of fine, secondary strengthening phases such as Al3(Sc, Zr) [3, 4]. Addition of Zr together with Sc improves the effectiveness of Sc as an inhibitor of recrystallization and increases the stability of the alloy during prolonged exposure to elevated temperatures [5]. What is more, the addition of Zr reduces the susceptibility of the Al3Sc precipitates to coarsening. Because the nanometer-sized Al3(Sc, Zr) particles stabilize the microstructure in elevated temperatures, the potential for enhanced properties following joining operations, such as friction stir welding (FSW), arises. Friction stir welded joints display excellent mechanical properties when compared to conventional fusion welds, and as such, the aerospace industry is embracing the FSW technology and implementing its capabilities into their manufacturing sectors [6]. Over the last fifteen years, numerous investigations have sought to characterize the principles of FSW and to model the microstructural evolution [7÷12]. The current status of FSW research pertaining to aluminum alloys has been recently well summarized by Threadgill et al. [13]. The following paper contains introductory results on the characterization of microstructure and mechanical pr[...]

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