Mode I fracture and microstructure for 2024-T3 friction stir welds

Detailed microstructural studies and mode I fracture experiments have been performed on both base material and three families of friction stir welds (FSWs) in 7 mm thick, 2024-T3 aluminum plate, designated hot, medium and cold due to the level of nominal heat input during the joining process. Microstructural studies indicate that the FSW nugget grain structure is relatively uniform in all welds, with a banded microstructure existing on horizontal cross-sections traversing the weld region; the spatial wavelength of the bands corresponds to the tool advance per revolution. The microstructural bands have elevated particle concentrations, with the particles having the same elemental content as base metal impurities, implying that the FSW process is responsible for the observed particle redistribution and microstructural banding. Furthermore, particle redistribution in all welds resulted in (a-1) particle size and volume fraction reduction on the advancing side of the weld nugget and (a-2) an attendant increase in particle volume fraction on the retreating side of the weld nugget. Finally, results indicate that hardness minima are present in the heat affected zone (HAZ) outside of the weld nugget on both the advancing side and retreating side for all welds, with the hot weld having the lowest overall weld hardness. Results from mode I fracture tests indicate that the measured critical COD at a fixed distance behind the crack tip is a viable fracture parameter for FSW joints that is capable of correlating the observed load-crack extension response for both the base metal and all FSWs. In addition, critical COD measurements indicate that FSW joints have a through-thickness variation in fracture resistance. Finally, the observed ductile crack growth path (which remained in the FSW region for all FSW joints) can be correlated with the locations of hardness minima, with microstructure variations affecting local fracture processes and the corresponding crack path.