Manufacturing of ultra-thin large titanium alloy tube using the novel hot gas pressure-bending process

The bending of ultra-thin large tubular shells is extremely difficult owing to the coexistence of wrinkling at the intrados, cross-sectional deformation, and overthinning at the extrados. This difficulty is further exaggerated for TC4 titanium alloys, which have high deformation resistance and low ductility at room temperature. In this study, a novel hot gas pressure-bending method is developed to form TC4 bent tube with ultra-large and thin features (D 203 xti 1.5 x R 495, mm) with a thickness-to-diameter ratio as small as 7 parts per thousand, and that cooperatively solves the problems of wrinkling at the intrados and cross-sectional distortion by the fundamental stress-state change method. First, a mathematical model was established to analyse the bending behaviour of the bent tube under internal gas pressure, and it was confirmed that the adjustment of the stress state from compressive to tensile by the internal gas pressure is the key advantage of this new process to prevent wrinkling and crosssectional distortion. Second, the positive effect of the internal gas pressure on the dimensional accuracy was investigated experimentally, that is, the shape accuracy of the cross section was improved, and the possibility of wrinkling was reduced with increasing internal gas pressure. However, the increased internal gas pressure induces overthinning at the extrados owing to the increased axial stress, which can be solved by enhancing the hardening behaviour of TC4 titanium during hot bending. Finally, for a thorough understanding of the effects of the process parameters on the dimensional accuracy and local thinning, a thermal-mechanical coupling finite element model was established and validated using experimental data. Using the computed optimum internal gas pressure 4.5 MPa, the maximum cross-sectional deformation rate is only 0.72% and the maximum thinning ratios is 7.03%. Novel hot gas pressure bending enables the delivery of a new method for forming ultra-thin large titanium alloy bent tubes using the regulations of the external stress state and intrinsic hardening behaviour of the material.