Characterization of the microstructure in friction stir welds of EN AW-2219 using coincident Doppler-broadening spectroscopy

Friction stir welding is an innovative welding technology that is often employed to weld aluminum alloys of the 2xxx-series. These alloys are primarily used in the aerospace industry to join structural parts or tanks. For these applications, high quality requirements have to be met. In a previous study, it was shown that controlling the welding temperature is one key to ensuring the quality of the welds. It was observed that the tensile strength varies depending on the welding temperature and the welding speed. In this paper, Positron Annihilation Spectroscopy (PAS) with a scanning positron beam was applied to detect crystallographic defects and changes of the microstructure of friction stir welds. The results were compared to optical microscopy and to hardness maps. Hereby, vacancy formation in the cross section of friction stir welds could be observed. Furthermore, it could be demonstrated that the state of the precipitates is the primary determinant for the mechanical properties of the welds. Finally, the observations made through PAS were employed to explain the trends in the mechanical properties, which were observed in the previous study. In particular, the varying ultimate tensile strength (UTS) could be clearly attributed to dissolution and re-precipitation phenomena, which occur during and after welding.