Investigation of material flow mechanisms and interface bonding in probeless friction stir spot welding of 2198-T8 aluminum-lithium alloy

The probeless friction stir spot welding (P-FSSW) process of 2198-T8 aluminum‑lithium alloy was investigated to elucidate material flow mechanisms and their influence on joint interface bonding. A coupled numerical simulation and texture analysis approach revealed the complex flow behavior. Tracer particle simulations demonstrated a laminar, radially inward spiral flow pattern in the stirring zone, with weaker flow intensity along the thickness direction due to thermomechanical gradients. Electron backscatter diffraction (EBSD) analysis indicated that dynamic recrystallization was the primary mechanism for grain refinement, creating fine equiaxed grains and forming shear-dominated textures ({111} 〈110〉) that reflect localized material flow trajectories. The bonding mechanism at the joint interface was driven by the fracture of the surface oxide layers, atomic diffusion, and dynamic recrystallization, which facilitated void closure and grain coalescence. Grain boundary migration across the interface was observed, resulting in a metallurgical bond. These results provide an in-depth understanding of the coupling between material flow and the evolution of interface bonding. The research emphasizes the critical role of plastic deformation and recrystallization gradients, determined by the extent of downward material flow, in forming defect-free, metallurgically bonded interfaces.