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 ( 203 × 1.5 × 495, mm) with a thickness-to-diameter ratio as small as 7‰, 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 cross-sectional 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.

中文翻译：

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采用新型热气体压力弯曲工艺制造超薄大型钛合金管

超薄大管壳的弯曲加工难度极大，其内弧线起皱、截面变形、外弧线过薄等现象并存。对于室温下具有高变形抗力和低延展性的TC4钛合金来说，这一困难被进一步放大。本研究开发了一种新型热气体压力弯曲方法，可成形具有超薄特征（203×1.5×495，mm）的TC4弯管，厚径比小至7‰，并且通过基本的应力状态变化法，协同解决了内弧面起皱和截面变形的问题。首先，建立了数学模型来分析弯管在内部气体压力下的弯曲行为，并证实了通过内部气体压力将应力状态从压缩状态调整为拉伸状态是该新工艺的关键优势。防止起皱和横截面变形。其次，通过实验研究了内部气体压力对尺寸精度的积极影响，即随着内部气体压力的增加，截面形状精度提高，起皱的可能性降低。然而，由于轴向应力增加，内部气压增加会导致外弧处过薄，这可以通过增强 TC4 钛在热弯过程中的硬化行为来解决。最后，为了全面了解工艺参数对尺寸精度和局部减薄的影响，建立了热力耦合有限元模型并使用实验数据进行验证。当计算出的最佳内部气体压力为4.5 MPa时，最大截面变形率仅为0.72%，最大减薄率为7.03%。新型热气体压力弯曲技术为利用材料的外部应力状态和内在硬化行为的调节形成超薄大型钛合金弯管提供了一种新方法。