Friction stir welding of aluminum to titanium: quest for optimum tool-offset, deformation of titanium, and mechanism of joint formation

In dissimilar friction stir welding (FSW), the influence of tool position with respect to the weld interface (tool-offset) on the microstructural evolution and its effect on joint formation, particularly considering the difference in deformation behavior of the two joining materials, has not been fully explored. This investigation highlights the influence of tool position in friction stir welding of titanium to aluminum and the mechanisms of microstructural evolution that leads to joint formation. It was observed that the tool position controls the deformation behavior and the mixing mechanism of the two metals in the weld nugget. In titanium, the deformation mechanism changed from the formation of "continuous flakes" to "finely distributed particles" when the tool offset was more toward the aluminum side. The deformation and fracture mechanism in titanium is mediated through the formation of adiabatic shear bands (ASBs). Furthermore, the amount of intermetallic phases formed is affected by tool offset. At the optimum tool offset the presence of Ti particles, intercalated particles, interfaces without defects, and controlled diffusion gives rise to improved tensile properties. The mechanisms involved in the joint formation process, such as deformation and distribution, are also discussed in this paper.