【作者】 周林；

【导师】 薛克敏；

【作者基本信息】 合肥工业大学 ， 材料加工工程， 2008， 博士

【摘要】 管材内高压成形作为一种制造空心轻体构件的先进制造技术,近年来发展迅速,目前已成为塑性加工领域中一个热点研究方向。但内高压成形还是一门相对年轻的技术,其成形机理十分复杂,目前国际上尚无太多的理论知识和经验可以借鉴,而国内的研究起步较晚,与国外的差距较大。内高压成形技术的迅速发展得益于其在汽车制造业中的广泛应用,而汽车零部件多为复杂空心构件,截面及轴线形状各异,因此开展异形截面构件内高压成形技术的研究,具有重要理论及工程实际意义。本文采用理论分析、数值模拟和试验研究相结合的方法,对带轴向进给直管胀形类及不带轴向进给的预弯管胀形两大类管材内高压成形工艺进行深入分析,探索内高压成形内在规律及工艺控制策略。基于金属塑性理论,通过理论解析的方式揭示了两类胀形工艺中管坯自由胀形区应力应变特点及其工艺控制方法。建立了典型直管胀形类零件一非对称三通管有限元模型,通过数值模拟揭示了管坯材料流动、应力应变及壁厚分布特点,分析了左右轴向进给量、最大内压、加载路径形式、摩擦系数及背压等主要工艺参数对成形结果的影响规律,并给出了这类零件内高压成形的工艺设计准则及工艺控制策略。随后,对典型预弯管胀形类异形截面构件—汽车前梁内高压成形过程进行了有限元分析,针对复杂异形截面构件成形工序多、预弯及预成形工序对下道工序影响大等工艺特点,建立了前梁预弯—胀形多道次成形有限元模型,对其成形全过程进行了分析,研究了预弯工序对内高压成形结果的影响,并揭示了其内高压成形过程中管坯材料流动、应力应变及壁厚分布特点及管坯尺寸、内压加载路径形式等主要工艺参数对成形结果的影响规律。针对异形截面构件内高压成形特点,提出一种带预应力的内压加载路径形式,以较低的内压载荷成形小截面圆角并获得了均匀的壁厚分布,最终给出了这类零件内高压成形的工艺控制策略。管材内高压成形是一个多重非线性过程,应变路径复杂,生产中常出现起皱、缩颈破裂、屈曲等缺陷。由于易受应变路径影响,传统的基于应变的成形极限图(Forming Limit Diagram,FLD)在预测内高压成形等复杂应变路径下成形极限时误差较大。为此,本文提出将独立于应变路径的成形应力极限图(Forming Limit Stress Diagram,FLSD)引入管材内高压成形研究中,以极限应力作为分析管材成形过程中破裂缺陷产生的判据。文中通过力学性能试验及成形极限试验建立了LF21的FLD并通过塑性应力应变关系及Hill’79屈服准则转换得到LF21的FLSD,分别对三通管及变截面结构件内高压成形过程进行数值模拟,在模拟结果分析中引入FLSD作为破裂极限判据,结果表明FLSD预测结果与传统FLD预测结果相比更接近于理论计算及试验结果,取得了较好的效果,证明FLSD作为管材内高压成形极限判据是准确可行的。内高压成形是一个非常复杂的动态过程,内部压力和轴向进给量之间的关系很难用显函数形式表达,目前研究中广泛使用的传统数值模拟及多目标优化方法本质上仍是一种试错寻优法,需要大量仿真工作,耗费人力物力,并且由于路径中控制点的数量限制,传统优化方法并不能实现对加载路径的全程控制,往往只是对极限值的优化。针对这一现状,本文提出一种结合模糊控制与自适应模拟的实时反馈优化方法,建立缺陷控制规则,通过模糊控制器在有限元模拟过程中实时侦测缺陷的发展趋势并反馈至模拟程序以调整工艺参数,避免起皱及破裂缺陷的发生,最终获得优化的成形加载路径。通过对三通管及汽车前梁零件的研究表明:优化加载路径后零件成形质量有了明显改善。模糊控制实现了预期的控制目标。最后,在数值模拟及工艺参数优化基础上,设计内高压成形试验装置及模具工装,进行典型异形截面构件一汽车前粱工艺试验,通过物理试验分析了内压加载路径形式、摩擦条件等主要工艺参数对成形结果的影响规律,并对比验证模拟结果。结果显示,模拟结果与试验实测结果吻合度较好,两者最大相对误差不超过10%,从而验证了所建立有限元分析模型及模拟结果的准确性。更多还原

【Abstract】 Tube hydroforming (THF) is an advanced process of forming closed-section, hollow parts with different cross sections by applying an internal hydraulic pressure and additional axial compressive loads. In the past few years, it has been rapidly developed as a remarkable research area. But it is still not fully implemented in the domestic industry due to its complexity of process and short history. In particular, tubular hydroforming, which makes onestep forming of complex closed hollow parts possible, is effectively used for high volume application in automotive industry because various closed sections resulting in the highest bending and torsional stiffness sectional shape can be easily formed by use of this technology. In this paper, the hydroforming of straight tube with axial feeding and hydroforming of bended tube without axial feeding are studied by plasticity theory, numerical simulation and experiment, the forming rules and process control strategy are discussed.Based on metal plasticity theory, the stress strain state and process control principles of two types of hydroforming are proposed. The non-symmetry T-shaped tube is simulated, and the material flow, stress strain state and thickness distribution of non-symmetry T-shaped tube are analysised, the influence of process parameters such as axial feeding, limit pressure, loading path, friction condition and counter force are discussed.Then the process control strategy of this kind of hydroforming parts is delivered. The typical bended hydroforming part—front transom is also investigated, the FEA model of whole process including bending and hydroforming is set up and the influence of bending process is discussed. Aim at the characteristic of profiled cross-section tube hydroforming, this paper present a per-loaded pressure loading path. By this loading path, the forming result is better than that of conventional loading path. The forming part can obtain smaller corner radius and uniform thickness distribution.The strain-based forming limit diagram (FLD) as an effective criterion of sheet forming quality has been extensively applied in THF research. But the traditional strain-based FLD is a function of strain history. Strain path effects undermine the utility of the traditional FLD for formability assessment of processes that are inherently non-linear, such as hydroforming of tubes. In this paper, the stress-based forming limit diagram (FLSD) is introduced in the forming limit investigate of tube hydroforming. The forming limit of LF21 Aluminum alloy sheet was tested and its forming limit diagram (FLD) was determined. Then the FLSD of LF21 was constituted by transformation formulas between limit strain and limit stress and Hill’79 yield model. This FLSD was used in conjunction with finite element simulations to predict the onset of fracture and limit forming pressure in tube hydroforming. Results indicate that the simulation result compares well with the theoretics analysis and experimental result and the FLSD is able to predict the forming limit of tube hydroforming with remarkable accuracy.In order to overcome the disadvantages of conventional optimization methods, a feedback optimization methods consist of fuzzy logic control algorithm and adaptive simulation is proposed. By failure control rule constituted, failure indicators obtained from the simulation results are used as the input of the fuzzy logical control, and the output sets of the fuzzy logical control are used for adjusting the loading path. In this way, a reasonable loading path for hydroforming parts can be obtained. Its validity is verified by the optimization results of T-shaped tube and front transom.At last, on the basis of simulation and optimization results, the experiment equipment and forming die are produced and a series of experiments of front transom are performed. From the experimental results, the influence of pressure loading path pattern and friction condition are investigated. Then, the results from experiments are compared with the results from the FEA simulation and optimization, the percentage error between experiment and simulation results is not exceeding 10%, validated the veracity of FEA results.更多还原