Friction stir welding of 2195 Al[sbnd]Li alloy: Effect of microstructure on joint failure

This study presents a comprehensive analysis of the failure mechanisms in friction stir welded (FSW) joints of 2195 aluminum‑lithium alloy, aiming to uncover the microstructural evolution, texture changes, precipitate phase behavior, and their impact on mechanical properties in regions near failure locations. FSW of 2195 Al[sbnd]Li alloy was performed at room temperature with welding parameters of 425 rpm rotational speed and 100 mm/min welding speed. The analysis, combining tensile testing and digital image correlation (DIC) techniques, revealed that local strain migrated from the heat-affected zone (HAZ) on the retreating side towards the HAZ on the advancing side, with fracture ultimately occurring at the interface between the HAZ and the thermo-mechanically affected zone on the advancing side (TMAZ-AS). A systematic investigation of the microstructural evolution in the fracture region was conducted using transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), differential scanning calorimetry (DSC), microhardness testing, and finite element modeling (FEM). The results indicated an increase in deformation texture strength in the HAZ, while the TMAZ-AS exhibited shear texture. The HAZ showed lower average kernel average misorientation values and dislocation density, indicating less localized plastic deformation in this region. The decrease in microhardness from the base material (BM) to the HAZ was attributed to the coarsening of T₁ and δ’/β’ phases, while the increase in hardness from the HAZ to the TMAZ-AS was due to solid solution and dislocation strengthening. FEM simulations demonstrated that the HAZ experienced temperatures between 220 and 300 °C, while the TMAZ-AS reached 300–500 °C. Micro-cracks were observed along grain boundary precipitates in the HAZ, and the presence of a wide precipitate-free zone (PFZ) was identified as the primary cause of tensile fracture, which exhibited a quasi-cleavage fracture mechanism.