Two-pass friction stir welding of aluminum alloy to titanium alloy: A simultaneous improvement in mechanical properties

During Friction Stir Welding (FSW) of aluminum (Al 2024) to titanium (Ti-6Al-4V), it is observed that titanium fragments at the interface get distributed in the weld nugget. These particles are both coarse and fine in size. Such a particle distribution, particularly due to presence of coarse particles, is expected to negatively impact the mechanical properties of the welds. In an effort to further fragment the coarse Ti particles, FSW was performed with an additional pass in the weld nugget region. Characterization was done using X-ray Micro-Computed Tomography (XCT), Scanning Electron Microscope (SEM) equipped with an Energy Dispersive Spectrometer (EDS), X-ray Diffraction and Electron Back-scattered Diffraction (EBSD) method. Tensile tests were performed to determine the mechanical properties of the weld. The Ti particles of various shapes and sizes were seen to be inhomogeneously distributed in the weld nugget even after the second pass. A detailed observation revealed that the larger particles (as flakes) were inhomogeneously distributed but the finer particles (more spherical) were homogeneously distributed in the weld nugget. It is observed that the weld after second pass contains a higher number of finer Ti particles. The variation in particles size and number fraction is due to the continuous fragmentation that occurs during the processing. Phase analysis reveals the formation of intermetallic compounds such as Al₃Ti and AlTi in the welds. A detailed analysis discloses that the fraction of intermetallics in the weld nugget increases in the second pass when compared to the first pass. Aluminum in the weld nugget was substantially refined and recrystallized with grains consisting of mixed types of grain boundaries, which points to the mechanism of Continuous Dynamic Recrystallization (CDRX) through Dynamic Recovery (DRV). The EBSD analysis also reveals that the weld after second pass promotes the development of a high fraction of recrystallized grains (87%) when compared to that of first pass (78%). The second pass resulted in a significant improvement in the ultimate tensile strength (from 231 ± 8 MPa to 271 ± 6 MPa) and ductility (from 7.4 ± 0.3% to 9 ± 1.0%) of the weld. Such improvement in joint properties is analyzed considering the relative mechanical properties of the different zones across the weld nugget. A spring model has been employed to characterize the fracture behavior of the welds. This method and mechanism may be used to produce composite and dissimilar welds with unique mechanical properties by employing multi-pass processing and welding methodology.