A new method to elucidate fracture mechanism and microstructure evolution in titanium during dissimilar friction stir welding of aluminum and titanium

Abstract In the friction stir welding (FSW) of dissimilar materials, the weld nugget exhibits composite properties and is composed of hard particles (high-strength material) distributed in a soft matrix material. The distribution of these particles influences the properties of the weld. Therefore, it is useful to characterize the deformation and fragmentation of the high-strength material from which they originate. In the current study, FSW of aluminum (Al) to titanium (Ti) was performed and a new technique was introduced to remove Al from the post-weld sample to characterize the deformation and fragmentation of Ti in the weld nugget. The post-weld sample showed that Ti particles were inhomogeneously distributed. It was understood that the plastic deformation of the Ti depends on its location of the weld. A detailed investigation revealed a number of fracture zones in titanium. A systematic analysis revealed the evolution of twins in Ti in the thermos-mechanically affected zone (TMAZ), whereas no-twins and recrystallized grains were noticed on the Ti flake that was consolidated in the weld nugget. The X-ray computed tomography (XCT) showed that Ti flakes were subjected to deformation and bending. Furthermore, it was noticed that the width of the flakes was not identical along their length and that the flakes were broken at the middle region into two parts. It can be concluded that Ti, at the interface and as flakes, was subjected to severe deformation at a lower homologous temperature that leads to the formation of adiabatic shear bands (ASBs). The local evolution of temperature within the ASBs was much higher than the general weld temperature, leading to the recrystallization of Ti at the interface. Moreover, ASBs are prone to crack nucleation and propagation; hence, particles with different morphology and sizes were noticed in the nugget zone. The distribution of particles was inhomogeneous due to variation in the particle size and complex mechanical mixing. The distribution of particles varied with location across the weld nugget. A homogeneous mechanical mixing was noticed at the center of the weld nugget due to complex material flow and distribution of finer particles. With this understanding of the deformation and fracture mechanism of titanium during dissimilar FSW of Al to Ti, it is possible to engineer the developed composites and/or weld nuggets of dissimilar materials to achieve the required combination of mechanical properties.