For the existence of bending deformation during low pressure tube hydroforming(LPTH), springback is unavoidable and deteriorate the forming accuracy. In this paper, an analytical model of springback for LPTH was proposed. The simulation and experiment were carried out to investigate the effects of the main factors on the springback, such as internal pressure, corner radius and thickness. The results indicate that, different from expansion deformation, bending forming is the main and basic deformation for tube forced into the required shape during the LPTH. Consequently, springback appears. However, the deformation type has great dependence on the internal pressure, which ensures the internal pressure of the function to reduce the springback. There is a critical pressure which would induce pure bending deformation and result in the largest springback. Consequently, it need try to avoid the critical pressure for the LPTH. On the contrary, whatever the internal pressure is less or bigger than the critical pressure, springback are all decreased. In contrast, it is more effective to decrease the springback when pressure is bigger than the critical pressure. In addition, bigger corner radius can also contribute to decrease the springback because of a much larger tensile force introduced under the same internal pressure. For a definite component, improving internal pressure is the only method to decrease the springback.

The deformation behavior of a 2024 aluminum alloy sheet at elevated temperatures was studied by uniaxial hot tensile tests over the nominal initial strain rate range of 0.001–0.1 s⁻¹ and temperature range of 375–450 °C. In order to analyze the deformation behavior with higher accuracy, a digital image correlation (DIC) system was applied to determine the strain distribution during hot tensile tests. Local stress-strain curves for different local points on the specimens were calculated. The strain rate evolution of each point during the tensile tests was investigated under different deformation conditions. Then, an improved Fields–Backofen (FB) model, taking into account the local strain rate evolution instead of the fixed strain rate, was proposed to describe the constitutive behaviors. It has been found that obvious non-uniform strain distribution occurred when the true strain was larger than 0.3 during hot tensile tests. The strain rate distribution during deformation was also non-uniform. It showed increasing, steady, and decreasing variation tendencies for different points with the increasing of strain, which led to the local flow stress being different at different local points. The flow stresses predicted by the improved FB model showed good agreement with experimental results when the strain rate evolutions of local points during tensile tests were considered. The prediction accuracy was higher than that of traditional FB models.

In sheet metal forming, the material is usually subjected to a complex nonlinear loading process, and the anisotropic hardening behavior of the material must be considered in order to accurately predict the deformation of the sheet. In recent years, the homogeneous anisotropic hardening (HAH) model has been applied in the simulation of sheet metal forming. However, the existing HAH model is established in the second-order stress deviator space, which makes the calculation complicated and costly, especially for a plane stress problem such as sheet metal forming. In an attempt to reduce the computational cost, an HAH model in plane stress state is proposed, and called the HAH-2d model in this paper. In the HAH-2d model, both the stress vector and microstructure vector contain only three in-plane components, so the calculation is significantly simplified. The characteristics of the model under typical nonlinear loading paths are analyzed. Additionally, the feasibility of the model is verified by the stress–strain responses of DP780 and EDDQ steel sheets under different two-step uniaxial tension tests. The results show that the HAH-2d model can reasonably reflect the Bauschinger effect and the permanent softening effect in reverse loading, and the latent hardening effect in cross loading, while the predictive accuracy for cross-loading softening remains to be improved. In the future, the HAH-2d model can be further modified to describe more anisotropic hardening behaviors and applied to numerical simulations.