Mechanism of microstructure evolution and improved mechanical properties in two‑pass friction stir welding of titanium to aluminum

This study investigates the impact of an additional pass on microstructure evolution, mechanical properties, and intermetallic compound formation during friction stir welding of aluminum and titanium. The microstructure analysis showed a complex mechanical mixing in the weld nugget that contained particles of varying sizes and the formation of intermetallic compounds. The formation of intermetallic compounds, such as Al3Ti and AlTi, was detected through chemical analyses and X-ray diffraction techniques. The microstructure of aluminum in the weld nugget comprised equi-axed grains with different grain boundaries and low orientation deviation. Such features in the evolution of the microstructure are attributed to continuous dynamic recrystallization due to its high stacking fault energy and favorable welding temperature and strain-induced dislocation activities. The presence of particles in aluminum and their homogeneous distribution after the second pass promote the state-IV hardening rate. A model for inhomogeneous materials was introduced to explain the variation in tensile properties with the number of passes, and the model correlated well with the cross-sectional microstructure analysis, which showed five distinct zones across the weld nugget. The study concludes that the improvement in mechanical properties after the second pass can be attributed to the development of interlayers, a defect-free interface, mechanical mixing, and continuous dynamic recrystallization of aluminum in the weld nugget.