超薄壁管材颗粒介质辅助推弯力学特性及成形极限研究

Ultra-thin walled tube bends with small bending radius are one of the key components used in airspace and aerospace launch vehicles and various electrical equipment. Traditional forming technology for such bends is welding of two stamped half bends, which can not meet the high reliability demand anymore, so forming technology for the seamless bends becomes an imperative requirement. Previous study of the applicant team showed that the granule medium aided push bending process can be a feasible choice. However, the forming mechanism and forming limits of the process are still not clear, which becomes key scientific issues limiting the practical application of the process. Thus, this project will firstly build a statics calculative model for micro contact forces of granule fillers for quantitative analysis of the inner tube pressure distribution by granule medium; then establish a coupled modeling method combined statics model for discrete media and finite element method for continuum media to study effect of granule fillers on forming defects such as thinning, wrinkling,spring back and cross section distortion of tube bends, disclose the operation mechanism of granule fillers during push bending process, and achieve optimal design and control strategy for inner pressure of granule fillers; On the above base, forming limits of granule medium aided push bending process will be studied for typical bend materials; and finally forming experiments will be carried out for validation of the above theoretical studies.The research will offer theoretical support and practical base for spread application of granule medium aided push bending technology

超薄壁小弯曲半径弯管是航空航天等运载装备及化工和各种电力装备中的关键构件。传统的半管冲压再焊接的成形技术，越来越无法满足对高可靠性的使用要求，迫切需求无缝整体弯曲成形技术。在颗粒介质压力成形技术基础上发展的颗粒填料推弯为可行的成形工艺，但该工艺的力学特性及成形机理尚不明确，对成形极限还缺乏清楚的认识，成为限制该技术实际应用亟待解决的科学问题。为此，本项目拟建立颗粒填料微观接触力的静力学计算模型，定量分析颗粒填料的内压分布特性及影响因素；构建离散体静力学模型与连续体有限元法的多场耦合模拟方法，研究颗粒填料对弯管减薄、起皱、回弹和截面畸变等缺陷的影响规律，揭示颗粒填料内压特性在推弯成形中的作用机制，获得颗粒填料内压的优化设计及调控方法；以此为基础研究获得典型材料薄壁管颗粒填料推弯的成形极限；最后通过管材推弯实验验证上述研究的可靠性。本研究可为颗粒辅助推弯技术应用提供理论依据和实践基础。

针对颗粒填料辅助推弯成形中颗粒填料非均布内压特性的影响因素、调控方法及其在推弯过程中具体的作用机制和规律还不清楚、弯管的成形极限还不明确的问题，本项目探讨了颗粒填料的力学特性及其在弯管中起的作用，并提出了弯管起皱预测方法，从起皱机理出发提出了控制弯管起皱策略，最后使用响应面法研究了弯管成形极限。研究发现颗粒填料在弯曲过程中存在弯曲变形压力增强现象，而在弯管过程中，局部起皱起始点恰好位于初始弯曲段，颗粒填料的弯曲变形压力增强现象可以防止管材成形缺陷，尤其是起皱缺陷。这是颗粒介质填料辅助推弯薄壁管的主要优点之一。分析了不同颗粒参数和不同弯曲半径对颗粒填料力学特性的影响规律，得到调控弯曲颗粒时内压力分布的参考依据。基于能量法建立了内压支撑下弯管起皱模型，建立颗粒填料辅助推弯工艺的FEM-DEM耦合模型，结合起皱模型和耦合模拟，获得颗粒介质辅助推弯过程中临界起皱压力。通过对比颗粒填料提供的内压力和弯管过程中的临界起皱压力来预测弯管过程中的起皱缺陷，当颗粒内压力大于临界起皱压力时不会产生起皱，而颗粒内压力小于临界起皱压力时，弯管产生起皱的风险较大。提出两类控制弯管起皱策略，一种是提高填料的等效压力，另一种是降低弯管临界起皱压力。通过调整颗粒参数可以调整颗粒压力以实现对弯管起皱缺陷的控制；采用差异润滑可以降低弯管环向临界起皱压力，降低侧边起皱的风险。基于响应面法获取了弯管质量参数和工艺参数之间经验函数关系，通过函数关系转换获取分别以临界最大减薄率和临界最大横截面椭圆度为弯曲极限指标的管材弯曲成形极限图。最终在提出的起皱预测和成形极限图指导下，使用颗粒介质填料辅助推弯工艺成形了不同尺寸的薄壁弯管，成形件均满足行业标准。本项目完成了预定的研究目标，研究结果丰富了管材弯曲成形理论，实现了大口径小半径薄壁弯管的成形，为推动薄壁管颗粒填料辅助推弯技术在航空航天装备等领域的应用提供了理论基础和技术支撑。