Microstructural Evolution in the Friction Stir Processed AA2024

Friction stir processing (FSP) has been performed on 2024 aluminum alloy sheets. The process was carried out using two different cooling rate i.e., Air Cooling (FSP-AC) and cooling using the liquid nitrogen (FSP-NC). In the current research, the effect of different cooling rates on microstructures and relating precipitates has been investigated. The results have been shown that the average grain size in the stirred zone (SZ) has been decreased significantly in the FSP-NC (1.75 mu m) condition, with respect to the FSP-AC (3.3 mu m). The misorientation angle of grain boundaries created the special structure. The grain boundaries were facet in FSP-AC and FSP-NC and the rough boundaries was observed in FSP-AC specimen.TEM and EBSD analysis have been indicated the dissolution of Omega-Al2Cu precipitates in the SZ in the FSP-NC specimen, while FSP-AC caused formation of supersaturated copper solid solution with Omega-Al2Cu precipitates in the grain boundaries. Furthermore, in the both FS-processed specimens (FSP-AC and FSP-NC) fine and dispersed Al20Cu2Mn3 precipitates have been observed.