Analytical springback modelling for thermal-mechanical bending of TA18 tube under non-isothermal loading

Abstract

The thermal-mechanical bending process is a promising forming method for high-strength hollow tubular structures, enabling the forming limit's extension, particularly for difficult-to-bend metal tubes such as TA18 tubes. However, this forming process complicates the springback characteristics due to the thermal-mechanical coupling effect under multi-die heating and constraint strategies. To further reveal the elastic release mechanism of TA18 thin-walled tubes post-unloading, we propose an analytical springback modeling method for the warm rotary draw bending (WRDB) process. A continuous steady-state heat transfer theoretical model across the tube cross-section is developed to model the thermal field and exactly capture the material flow behavior. Based on the experimental TA18 temperature softening behavior, the distribution of asymmetric strain and the nonlinear thermal field is introduced into the springback modeling, revealing the behavior of neutral layer shifting (NLS) and the evolution of yield surface across the cross-section under non-isothermal loading. Compared to finite element (FE) modeling, quantitative results show that the theoretical model can accurately predict tube springback behavior in experiments, achieving an average absolute relative error (AARE) below 1.4 % with low computational cost. The temperature effect on springback under different heating strategies is attributed to NLS and stress-strain redistribution induced by material temperature softening. The temperature difference of the local thermal field aggravates the NLS and contributes to the non-uniform tensile and compressive deformation, which is significant in the bending case with large diameter tubes and induces pronounced springback behavior.