【作者】 李恒；

【导师】 杨合； 詹梅；

【作者基本信息】 西北工业大学 ， 材料加工工程， 2004， 硕士

【摘要】 薄壁管数控弯曲精密成形技术是管弯曲成形技术向先进塑性加工技术发展的必然趋势。然而这是一个多因素耦合交互作用下易发生失稳起皱的复杂非稳态物理过程。特别对于大口径、薄壁管小半径弯曲成形过程，失稳起皱成为稳定成形的瓶颈问题。因此失稳起皱及成形极限是这一先进加工技术研究与发展中迫切需要解决的关键问题。为此，本文采用有限元数值模拟和理论解析及实验研究相结合的方法，对该过程中的失稳起皱及成形极限进行了深入系统的研究，取得了如下成果： (1)改进了薄壁管数控弯曲起皱数值预测系统TBWS-3D：修正了薄壁管数控弯曲成形失稳起皱预测模型，使预测结果更加准确：改善了绝对-相对自由度壳单元边界条件的处理方法，使有限元模拟结果更符合实际；针对不同情况，丰富了弯曲过程中摩擦和压块侧推速度等边界条件的处理方法，使得系统深入地研究成形参数对失稳起皱的影响成为可能；将面向对象技术引入有限元程序的编制，基于VC++平台开发了有限元起皱预测系统TBWS-3D，从而可以方便地嵌入起皱预测等成形质量控制模块，并易于实现上述提出的改进算法。 (2)采用改进的起皱数值预测系统TBWS-3D，系统深入地研究并揭示了薄壁管数控弯曲成形过程中成形参数对失稳起皱的影响规律。从而为成形过程参数优化及成形极限的确定奠定坚实的理论基础。 (3)摩擦和模具间隙对失稳起皱的影响规律表明：随着管坯与防皱块及管坯与芯棒间摩擦系数的增大，起皱因子有减小，有利于防止起皱的发生；随着管坯与弯曲模及管坯与压块间摩擦系数的提高，起皱因子增大，但幅度并不大，对管坯起皱的影响较小；随着管坯与防皱块间隙的增大，起皱因子逐渐增大；当模具间隙大于0.5mm时，失稳起皱趋势急剧增大。即管坯与防皱块间隙对管坯失稳起皱的影响很大。 (4)影响失稳起皱的主要因素有：相对管径、弯曲半径、硬化指数、芯棒的伸出长度、管坯与防皱块间摩擦、管坯与芯棒间摩擦、管坯与防皱块间隙等。基于虚拟正交试验，获得成形参数对失稳起皱影响的显著性大小依次为：相对管径、弯曲半径、硬化指数、管坯与防皱块间隙、管坯与防皱块间摩擦、管坯与西北工业大学硕士学位论文芯棒间摩擦、管材与弯曲模摩擦等。 (5)结合薄壁管数控弯曲过程的成形特点，定义了基于失稳起皱的薄壁管数控弯曲成形极限的概念;基于起皱预测系统TBWS一3D，提出了成形极限的搜索算法，得到了不同工艺参数下的成形极限，建立了初步的成形极限库:通过成形参数对失稳起皱影响规律的研究，获得了提高成形极限的方法;在此基础上提出了基于成形极限与数值模拟的参数确定与优化方案，使薄壁管数控弯曲过程成形的参数快速确定与优化成为可能。最后对今后的进一步研究方向进行了探讨。 本研究对提高薄壁弯管制品的质量、缩短产品开发周期、降低生产成本、充分发挥数控弯管设备的优势，从而实现薄壁管数控弯曲精密成形具有重要意义，同时也丰富了薄壁件塑性成形失稳起皱理论。更多还原

【Abstract】 NC precision bending process of thin-walled tube is showing an inevitable tendency to improve and develop tube bending process into advanced plastic forming technology. However, it is a complex astable process with coupling interactive multi-factor effects and wrinkling phenomenon as a compress instability often occurring , when the process parameters are inappropriate, especially for tubes with large diameter and thin wall-thickness. Thus, the wrinkling and forming limit have become one difficult and key problem urgently to be resolved in the R&D of the process. In this dissertation, a systematical and thorough investigation on the wrinkling and the forming limit of the process has been carried out by using rigid-plastic FEM simulation combining with analytical and experiment methods. The main achievements of the project made are as following:(1) Some of the key techniques related to the 3D rigid-plastic FEM simulation and wrinkling prediction of thin-walled tube NC bending process have been studied and improved methods and algorithms have been proposed: The instability predicting model has been improved which make prediction results more precise. Shell element with absolute-relative degree of freedom and corresponding setting method of velocity boundary conditions make the simulation results more efficiently and more accurately. Frictions on four areas in the forming process and side pushing velocity have been added in to the finite element equations, which makes FEM model more correspond the reality. Based on OOP ( Object-Oriented Programming ) theory, one numerical wrinkling prediction system TBWS-3D for visual NC Tube Bending have been reconstructed successfully with VC++. The validity of system has been verified by experimentation and comparison with data in the literature.(2) By using improved wrinkling system TBWS-3D, a systematical and thorough investigation into influence of forming parameters on wrinkling onset has been carried out and influence laws are obtained, which may be helpful to the parameters design and determination of forming limit.(3) Influence law of friction factor and clearance between tube and die on wrinkling initiation is obtained. The results show: with the friction factors between tube and mandrel and the one between tube and wiper die turn bigger, the wrinkling factor may become lower to some extent. With the other friction factors between tubeand pushing die, as well as between tube and bend die increase, the wrinkling factor rises a little. Namely the friction factor between these dies have little influence on the wrinkling onset, when clearance between tube and wiper die rises, the wrinkling factor increases; the wrinkling factor increases sharply, when clearance is larger than 0.5mm.(4) The main parameters are gained: relative diameter, bending radius, hardening exponent, mandrel extension length, friction factor between tube and wiper die, friction factor between tube and mandrel, clearance between tube and wiper die. Based on a virtual orthogonal experiment, information regarding the relative importance of every processing variables for wrinkling onset has been investigated. The results are listed according to relative importance for wrinkling: relative diameter, relative bending radius, hardening exponent, clearance between tube and wiper die, friction factor between tube and wiper die, friction factor between tube and mandrel, friction factor between tube and bend die.(5) According to analysis of forming characteristics of thin-walled tube bending, concept of forming limit for the process has been put forward. A searching algorithm of forming limit by using developed FEM wrinkling system TBWS-3D as virtual experiment has been brought forward. Employing the algorithm, wrinkling limit under various process conditions can be obtained conveniently and quickly, and one primary forming limit data base has been established. Influence law of main forming parameters on wrinkling limit has been investigated thoroughly. Thus measures to improve the wrinkling limi更多还原