Multi-Length Scale Characterization of Microstructure Evolution and Its Consequence on Mechanical Properties in Dissimilar Friction Stir Welding of Titanium to Aluminum

In dissimilar friction stir welding (FSW), the weld nugget is composed of two elements, which are mechanically mixed. This microstructure helps enhance the mechanical properties when they are homogeneously mixed, and the particles are sub-micron in size. Therefore, it is important to understand the mechanism of the particle formation and their distribution for engineering the mechanical properties of the weld. In the present investigation, dissimilar FSW between commercial purity Al and Ti has been carried out. The weld nugget consisted of distributed Ti particles in an Al matrix. The distribution of the Ti particles in the weld nugget was characterized using X-ray micro-computed tomography (XCT). Microstructural evolution in Ti and Al was examined using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS), X-ray diffraction and electron back-scattered diffraction (EBSD). Hardness and tensile tests were carried out to determine the integrity of the welds. The XCT result shows that the weld nugget contains Ti particles of variable size. The finer particles of Ti are distributed homogeneously in the weld nugget, unlike large particles. The deformation mechanisms and microstructural evolution of the Ti interface and Al matrix are investigated using EBSD. It is observed that the microstructure of both Ti and Al is substantially refined. However, for a given grain in Al, the boundary is of mixed character, namely low- and high-angle boundaries. Hardness data of the weld indicate large variation within the nugget region. The tensile test sample revealed that the failure of the sample occurs on the Al side of the weld. The fractograph indicates ductile and brittle modes of fracture with a bimodal distribution of dimples at the surface. Lack of twining and fine grains (40 to 5 µm) at the Ti interface indicates high-temperature deformation. Deformation of Ti at low temperature and high strain rate is caused by adiabatic shear banding (ASB). In these shear bands, a high level of grain refinement is observed and is a path for easy crack propagation. It is proposed that the ASB-controlled deformation of Ti leads to a recrystallized microstructure at the interface and fragmentation of Ti. These Ti particles undergo further fragmentation to form smaller particles. On the other hand, microstructural evolution in Al is gradual because of the high stacking fault energy, which leads to continuous dynamic recrystallization (CDRX) through the dynamic recovery (DRV) mechanism. The mechanical properties of the weld depend on the characteristics of particles in the Al matrix. Proper control of the fragmentation and distribution of Ti particles and interface property can lead to superior mechanical properties of the weld.