A new mandrel design with mandrel ball thickness variation for the bending process of aviation ultra-thin-walled tubes

Due to their unique advantages of high strength, high performance, and light weight, ultra-thin-walled (UTW) tubes have attracted increasing application in the aviation and aerospace industries. However, wrinkling, cracking, cross-section distortion, and even collapse easily occur in UTW tube bending process. In this study, a new mandrel design method with variation in the mandrel ball thickness for bending UTW tubes was proposed to improve the forming quality. First, a basic mandrel design scheme with a uniform mandrel ball thickness was determined according to engineering experiences and preliminary research. Then, three different mandrel ball thicknesses were determined based on the same mandrel support angle of the basic scheme (BS), and combination schemes (CSs) of mandrel balls with different thicknesses were designed. A 3D elastic–plastic finite element (FE) model of the numerical control (NC) bending process of UTW tubes was established and verified, and the influence of different combinations of mandrel ball thickness variation on the bending quality of UTW tubes was investigated. The results showed that compared with the BS, the CSs can effectively improve the von Mises stress distribution and wall thickness uniformity on the extrados, and significantly reduce the wall-thinning ratio, wall-thickening ratios, and ovality of the UTW bent tube. The combination scheme with as many thin mandrel balls as possible at the beginning of the bending and thick mandrel balls at the end of the support was beneficial to reduce the wall-thinning ratio. Increasing the number of medium or thick mandrel balls in combination scheme can increase the mandrel support angle and reduce the ovality of the UTW bent tubes. These results are of great significance for guiding the design of mandrels and improving the forming quality of UTW tubes.