Influence of Welding Temperature on Microstructure and Crystallographic Texture Evolution in the Different Weld Zones of Underwater Friction Stir Welding of Dissimilar CuZn40 and AA1100-O Alloys

Underwater friction stir welding (UwFSW) of dissimilar brass (CuZn40) and aluminum (AA1100-O) joints have a more pronounced effect on the microstructure and crystallographic texture evolution than classical open-air friction stir welding (C-AFSW). In this research, the microstructure and texture evolution mechanism across the weld thickness and different FSW zones are studied. Cross-section of the weld joints developed by UwFSW and C-AFSW were investigated via transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). EBSD data of C-AFSW joints reveal that significant grain refinement occurs in the stirred zone (SZ) due to continuous dynamic recrystallization. As compared to C-AFSW, the microstructural evolution mechanism in UwFSW was found to be very complex in the different parts of the SZ. For UwFSW, discontinuous dynamic recrystallization and geometric dynamic recrystallization were found to be the main microstructural evolution mechanism in the SZ. In addition, the enhanced cooling rates of UwFSW produce a fine grain structure and a large number of high angle boundaries (HABs). Both C-AFSW and UwFSW showed mixed grain structure in the thermomechanically affected zone. TEM results showed dislocation accumulation and annihilation were predominant in UwFSW with fine and denser rod-shaped (θ́-Al₂Cu) precipitates. The shear textures \(A/\overline{A }\), A₁*/A₂* and \(B/\overline{B }\) are formed in the SZ of both C-AFSW and UwFSW. However, the shear components \(B/\overline{B }\) dominates in the C-AFSW as compared to\(A/\overline{A }\). The result and findings of this research help to understand the microstructure evolution mechanism of CuZn40/AA1100-O FSW joints and further optimize the welding process for application.