Springback behaviors of bi-layered non-homogeneous bellows in hydroforming

Due to the complicated material and structural characteristics of bi-layered non-homogeneous bellows (BNBs) in hydroforming, the bellow deviates easily from its designed profile and this inevitable phenomenon results in a low forming precision. Therefore, it is important to study the springback behavior of bellows for precision forming. Based on finite element (FE) analysis, comparative studies on profiles of single-layered bellows, bi-layered homogeneous bellows (BHBs), and BNBs with two expansion ratios (k, the ratio of outer-to-inner diameter) k = 1.2 and k = 1.6 as well as two materials 304SS and Inconel 718 are implemented. The springback behaviors of different simulation settings are investigated, and several conclusions are drawn: (1) after springback, the U-shaped convolution profile is changed to tongue shape accompanied by a 2.5 similar to 38.5% axial elongation and a 0.1 similar to 0.6% radial shrinkage; (2) the springback tendency grows with the increase of number of layers, the improvement of mechanical properties of material, and the decrease of expansion ratio.