Flow forming has emerged as an effective route for manufacturing thick-walled Cu–Ni alloy tubes, particularly for producing gradient microstructures. To clarify the evolution of microstructure during deformation and to elucidate the mechanism governing ultrafine-grain formation, flow forming experiments were conducted on BFe10-1-1 thick-walled tubes. Finite element (FE) simulations and electron backscatter diffraction (EBSD) characterization were performed in parallel. On this basis, a coupled finite element analysis–cellular automaton (FEA-CA) microstructure evolution model was established, in which the local equivalent plastic strain (PEEQ) gradient was introduced to modify the geometrically necessary dislocation (GND) distribution at grain boundaries. The results reveal that the microstructural transformation during flow forming proceeds through a continuous high-angle boundary development pathway resembling continuous dynamic recrystallization. A marked through-thickness gradient in grain size is observed. Owing to the higher accumulated strain and stronger strain-gradient effects, the outer surface layer undergoes accelerated refinement and forms a stable banded ultrafine-grained structure with an average grain size of approximately 0.39 μm. The governing mechanism of ultrafine-grain formation exhibits a distinct pass-dependent response. During Pass 1, rapid substructure establishment dominates. In Pass 2, substantial grain refinement is driven by progressive grain-boundary misorientation increase and high-angle transformation. By Pass 3, the refinement rate decreases noticeably, and the microstructure approaches a saturated and relatively stable state.
Zhao et al. (Thu,) studied this question.