Axial thrust induced neutral layer evolution mechanism in free bending

In 3D free bending forming process, the additional axial thrust can cause neutral layer shifting (NLS), which significantly impacts forming quality and springback behavior. However, most of the existing springback prediction models ignore the axial thrust. Consequently, the influence mechanism of neutral layer (NL) on springback has not been deeply studied. This paper proposes a novel and comprehensive analytical springback model to describe the relationship between NLS and springback. By incorporating axial thrust and considering the influence of elastic modulus degradation, wall thickness variation and cross-sectional elliptization, the limitations of existing models are addressed. Furthermore, based on this model, the relationships between geometric parameters, material parameters and process parameters and NLS displacement is further quantitatively determined, as well as the sensitivity of springback to these parameters. Taking free bending of AA6061 tubes as the cases, the established analytical model is comprehensively evaluated through finite element (FE) simulations and experiments, and the phenomenon of NLS outward during free bending is proved through texture evolution. The results indicate that the model has higher accuracy in predicting springback. Especially, under small curvature conditions (R=4D), the prediction error of springback is less than 5%. Moreover, the wall thickness is identified as the main factor affecting springback, significantly exceeding the influence of the change in moment of inertia and elastic modulus degradation.