Role of plastic deformation mechanisms during the microstructural evolution and intermetallics formation in dissimilar friction stir weld

During dissimilar Friction Stir Welding (FSW) of titanium (Ti) to aluminum (Al), particles of titanium are inhomogeneously distributed in the aluminum matrix, in the weld nugget. The Al/Ti interfaces are the potential zones for intermetallic formation. The inhomogeneous distribution of Ti particles and intermetallic particles can have a negative impact on the mechanical properties of the weld. The objective of the present investigation is to highlight the mechanism of such inhomogeneity in the distribution and formation of intermetallics within the weld. In the present investigation, FSW of Al to Ti was carried out in butt-weld configuration and microstructure across the weld was characterized using Scanning Electron Microscopy (SEM) and X-ray Tomography (XCT). Results from the weld nugget showed three regions of Ti particle distribution, variation in their fragmentation and chemical composition across the Al/Ti interfaces when investigated from top to bottom of the weld. Such variations were correlated with conventional strain rate and temperature variation in the weld nugget. Detailed analysis of microstructure in the weld nugget illustrated that both titanium and aluminum undergo dynamic recrystallization. However, the mechanisms leading to such microstructural evolution were different. Recrystallization in titanium occurred due to low-temperature deformation and evolution of adiabatic shear bands whereas dynamic recrystallization in aluminum was attributed to its high stacking fault energy. The present results and mechanism can be useful in further development of the weld nugget in dissimilar FSW and development of composite by friction stir processing with homogeneous microstructure and superior mechanical properties.