Ultimate strength evaluation of multi-stage cold forming technique for manufacture of thin-walled vessels

The multi-stage process of cold forging of thin-walled steel vessels was evaluated taking into account technological heredity. The quality of the product is evaluated by the degree of deviation of its stress- strain state from the ultimate states of the forming limit diagram. The process is calculated based on the model of large plastic deformations of the anisotropic shell, which takes into account the dynamics and contact interactions of the shell with the tool. The model was numerically implemented in the LS-DYNA® package. Numerical simulation was performed using the simulation tools of the package library, such as the models of plastic flow of an anisotropic sheet with the power law isotropic strain hardening, associated with the Barlat criterion Yld 2000-2d, the Peng-Landel potential of nonlinear elastic behavior of a polyurethane die, and the Coulomb friction law of contact interaction of the product with the tool. The model constants for low-carbon sheet steel DC04EK 0.7 mm and polyurethane SKU-PFL were determined from the experimental data. The forming limit curve was plotted using as the basic data the distortions of the coordinate grid near the zones of strain localization and failure of the vessel during the technological process without intermediate annealing and the results of failure tests under uniaxial tension. The features of the strain paths are studied at thе control points of the vessel at each stage of the technological process, including the sequence of drawing, bulging and reducing operations. The calculated strain paths were verified by the experiment, in which pressing equipment was used as the test facility. It was found that the operation of work piece bulging after its rapid drawing leads to the limit state and therefore requires a preliminary recovery of the plasticity resource by annealing. The obtained results demonstrate the advantages of forming the relief of the vessel by smaller degrees of bulging and greater degrees of reducing for eliminating the limit states and intermediate annealing.