Title : Investigation on forming limits under complex loading paths considering intermediate annealing: Experiments and multi-scale simulations
Abstract:
Multi-stage forming processes typically involve complex loading and annealing sequences that substantially alter the microstructure and macroscopic formability. The process-induced microstructural changes, such as lattice rotation, dislocation accumulation, and recrystallization, make it challenging to accurately predict forming limits using conventional approaches. An integrated multi-scale simulation framework was developed that coupled crystal plasticity finite element simulations (CPFE) with a cellular automaton (CA) model and incorporated the Marciniak–Kuczynski (MK) instability criterion to evaluate forming limits under various multi-stage processing routes. The two-stage loading tests were performed under uniaxial and biaxial prestrain conditions to quantify the influence of loading path history on subsequent instability behaviour. Additional annealing experiments following different prestrain levels were carried out to assess the extent of formability recovery. Based on the experimental results, the CPFE and CA models were employed to simulate the deformation and annealing processes, respectively.
The results indicate that prestrain significantly reduces the forming limit during subsequent loading, and the extent of degradation strongly depends on the strain path. The dislocations accumulated during the prestrain process diminish the material’s ability to undergo subsequent uniform plastic deformation. As the prestrain increases, the internal dislocation density and local defects become more pronounced, further reducing the strain-hardening capacity of the material and making critical instability more prone to occur. Annealing after prestrain effectively restores formability by reducing dislocation density and promoting recrystallization. Particularly under higher prestrain conditions, annealing produces the most pronounced improvement in the forming limit near the plane-strain state, while the enhancement of the uniaxial and equi-biaxial tensile strain points is relatively comparable. Under different prestrains, the forming limit curves after annealing become relatively close to one another, indicating that the annealing treatment mitigates the influence of prestrain level differences on the forming limits. The established CPFE–CA–MK simulation framework effectively captures the combined effects of deformation accumulation and recrystallization on forming limits during the reloading process, successfully reproducing the evolution of forming limits under various prestrain and annealing conditions, and showing good agreement with the experimental results. The proposed multi-scale coupled simulation model provides a reliable basis for process design and formability evaluation in multi-stage forming of complex thin-walled components.
