Manufacture of Ultra-Thin Large Titanium Alloy Tube Using of the Novel Hot Gas Pressure-Bending Process

Abstract

Bending of ultra-thin large tubular shell is extremely difficult due to the coexistence of wrinkling at the intrados, cross-section deformation and over thinning at the extrados. Such difficulty is further exaggerated for TC4 titanium alloy with large deformation resistance and low ductility at room temperature. In this paper, a novel hot gas pressure-bending method is developed to form TC4 bent tube with ultra-large and thin features (D 203 ×  1.5 × R 495, mm) with a thickness-to-diameter ratio as small as 7‰. Fundamentally, a mathematical model is established to analyse bending behaviour of the bent tube under internal gas pressure. The mechanic analysis proved that the adjustment of stress state from compressive to tensile by internal gas pressure is the key advantage of this new process to prevent wrinkling and cross-section distortion. Secondly, the positive effect of internal gas pressure on the dimensional accuracy is investigated by experiment, i.e. the shape accuracy of cross-section is improved and the possibility of wrinkling is reduced with increasing the internal gas pressure. However, the increased internal gas pressure induces over thinning at the extrados due to increased axial stress. Finally, for a thorough understanding of process parameter effects on dimensional accuracy and local thinning, a thermal-mechanical coupling finite element model is established and validated by experimental data. The obtained results show that an ideal combination of internal gas pressure and heating strategy can effectively solve the problems during bending of ultra-thin large titanium alloy tube, i.e. wrinkling, cross-section distortion and over thinning. The novel hot gas pressure-bending enables to deliver a new method for bending ultra-thin large titanium alloy bent tubes.

Keywords: Ultra-thin large tubular components, Tc4, Hot gas pressure-bending, Stress-state change method, Strain & strain rate hardening