【作者】 武文斌；

【导师】 周锦进； 徐文骥；

【作者基本信息】 大连理工大学 ， 机械制造及其自动化， 2008， 博士

【摘要】 金属板件等离子体弧柔性成形是近几年发展起来的金属板材成形技术,是一种新兴的、有广阔发展前景的板材柔性成形技术,其核心是:以等离子体弧为热源,通过合理控制弧柱功率、扫描轨迹和速率,使材料内部形成可控的温度梯度进而产生非均匀热应力场,使板材成形为预期要求的形状。该方法不需要花费大量的时间和资金来制造工模具,对降低新产品的开发成本、缩短开发周期具有积极意义,尤其在新车开发等大型板件多品种小批量成形生产中,具有广阔的应用前景。本研究针对金属薄板等离子体弧柔性成形研究中存在的问题,采用理论分析、数值计算和实验测试相结合的方法,以建立等离子体弧柔性成形数值计算模型、技术参数关系、实验手段等关键技术为重点,以复杂三维曲面成形和成形精度控制为目标,研究了等离子体弧板材柔性成形的应变场、热参数、扫描间距、路径规划等重要问题,实现了复杂曲面和目标圆弧面的加工成形。论文的主要研究工作如下:分析了等离子体弧柔性成形技术的研究现状,论述了等离子体弧弯曲成形的两种基本形式——正向弯曲和反向弯曲及其成形机理和控制方法,分析了温度梯度机理、屈曲机理和增厚机理产生的机制及作用过程,结果表明在温度梯度机理、屈曲机理与耦合机理条件下,垂直于扫描线方向的横向应力和板材上下表面塑性应变的不均匀分布是板材弯曲变形的主要因素;板材既产生绕扫描线弯曲也产生垂直扫描线弯曲变形,弯曲成形的变化大小主要取决于塑性区的大小。为定量描述等离子体弧作用下金属板材的传热规律和应力应变状态,在重点研究了模型选取、移动热源模拟、材料特性和载荷施加等问题的基础上,建立了等离子体弧柔性成形过程的瞬态温度场有限元模型和应力应变场热弹塑性有限元模型;对温度场分布、应力应变状态以及形变规律进行了计算和分析,得出了在各种参数组合条件下的薄板温度场和应力应变场。为解决金属板复杂曲面三维成形问题,根据成形目标形状,基于大变形弹塑性有限元理论,计算出双曲率曲面及冠面成形应变场,根据主应变矢量方向与扫描路线的垂直关系,完成了球冠面和马鞍形曲面的等离子体弧扫描路径规划;在建立了加热区应变值与等离子体弧功率和扫描速度之间关系数据库和等离子体弧柔性成形实验装置的基础上,将数控技术与等离子体弧柔性成形技术相结合,实现等离子体弧扫描速度以及弧柱热能加载的调整,成形出球冠面和马鞍形面的目标形状,为复杂曲面的成形奠定了设计制造基础。为保证光滑连续圆弧面的成形质量,在数值计算和实验的基础上,确定了扫描间距、扫描顺序的选取原则,证明较小的扫描间距会得到连续性较好的塑性应变场,可以有效提高成形件的成形质量;给出了成形光滑连续圆弧面主要参数γ与扫描间距、成形圆弧半径的关系,数值模拟和实验采用渐进式等离子体弧扫描成形的方法,成形出了符合预期半径要求的连续圆弧面。对成形中出现的棱角效应进行了机理分析和数值计算,结果表明棱角效应是板材沿扫描线方向受几何约束产生了较小的压应变及在扫描线上、下表面的收缩量不同所造成;提出了通过分段扫描解决棱角效应的方法,明显提高了成形质量。研究了等离子体弧柔性成形试件的金相组织,结果表明较低功率的等离子体弧柔性成形会使板材晶粒细化,有利于改善材料的显微组织结构,采用合适的参数加工能在一定程度上改善成形件的机械性能。利用BP神经网络对板材成形进行了预测。通过实验数据的训练,建立了等离子体弧金属板材柔性成形效果与加工工艺参量的神经网络预测模型,并利用此模型进行了成形弯曲角度和扫描次数的预测,弯曲角度的预测误差小于5%,而扫描次数的预测误差小于2%。应用灰色系统理论与时序分析,对金属板的动态角度变形进行预测,当参与模型预测的观测序列为最新序列时,可保持较高精度的短期预测,预测误差小于3%。上述工作为成形精度的控制奠定了基础。更多还原

【Abstract】 Bending forming of sheet metal is an advanced processing technique for sheet metal developed in recent three years, which is newly emerging and promising. The core of the technology is that it takes plasma arc as heat source to form a controllable temperature gradient and an inhomogeneous thermal stress field through controlling arc-power, scanning path and scanning speed reasonably. Thus, the expected goal can be satisfied. The flexible forming without mould and external force is one of the advantages. The traditional technology does not only cost a lot of time and money, but also take up considerable human resource. So, the new technology will do goods to reduction of production costs and shortening developing time of new product, especially the production of multi-type and small batch of the large-scale sheets such as cars.The research status domestic and overseas of flexible forming of sheet metal with plasma arc and the mechanism and features of plasma arc were introduced in the paper. Two basic forms of the FFUPA, bending towards and away from plasma arc, were stressed. Temperature gradient mechanism, buckling mechanism and upsetting mechanism and causes induced were also analyzed. Further more, technology problem of the flexible forming using plasma arc was brought forward.In order to show the inner relations among part accuracy, arc characteristics and scanning path, and solve the technology problems such as power, scanning path and scanning speed, finite element method analytical model of thermo-elasto-plastic stress during plasma arc scanning was established. The plasma arc was treated as a jumping heat source with small steps, which was loaded on the small metal, according to its process of floating scanning. Transient responses of stress and elasto-plastic large deformation were solved. Distribution of temperature field, stress state and result of deformation were calculated. The thesis developed a 3-D FEM simulation model of plasma arc forming which included a nonlinear thermal-structural analysis and the calculates of the temperature field and stress- strain field of metal plate under different series of process parameter. It had also analyzed the influence of process parameters on temperature field and stress- strain field.The stress- strain field induced by the forming of doubly curved surface was calculated using large-deformation elastic FEM to make sheet-metal form better comply with the object shape. Scanning paths of heat source were designed, including spherical-crown and saddle-shaped surfaces. The mechanisms of edge effects induced in the experiments had been discussed and the solutions were brought forward to form the proper sheet metals through forming experiments and numerical analysis.The principle of selection of scanning interval is determined to improve the forming quality of smooth continuous arc surface on the base of simulation computation and experiment. The minimized scanning space can obtain strain-plastic fields with good continuity. The geometrical relationship between circular arc bending part and V-shape was investigated. In this paper, the author advanced the relationship between a parameterγand the degree of circular arc smoothness firstly. A simulation program of circular arc flexible forming of sheet metal using plasma arc was programmed using the ANSYS Parametric Design Language (APDL). The numerical simulation and experiments adopted increased forming method using plasma arc. The results gained were consistent with the required shapes after several V-shape bends.For angular effects in the experiment the mechanism analysis and numerical calculations are performed. The warp is mainly induced by the smaller compression strain caused by geometric constraint and different shrinkage between top and bottom surface. The solution of angular effects is brought forward and the satisfied sheets are formed.Based on the above analysis, the experimental facility of bending forming using plasma arc was built up. In order to form complex 3D surfaces, flexible forming with plasma arc and CNC were combined to realize the adjustment of plasma arc heat loading under the conditions of speed variation. Thus, the spherical-crown and saddle-shaped surfaces were demonstrated. Consequently, the basic rules and key technology of bending forming using plasma arc in practical work were obtained and the application level of the flexible forming using plasma arc had been further improved. The effects of the main parameters including power, scanning speed, scanning distance and scanning sequence had been investigated.Metallurgical analysis of the formed sheet metal was executed on the basis of mass of experimental data, and the change of microstructure of sheet metal after hot working could be mastered to make sure that the grain refining were induced by bending forming. Accordingly, the material microstructure was improving and the mechanical properties were superior to original materials.BP neural network was applied to flexible forming with plasma arc on the base of MATLAB. Inputs and outputs to the neural network were properly chosen. The mathematical model between the forming effect and experimental parameter was established, which was used of the forecast of bending angle and scanning times. The prediction error of bending angle is less than 5% and that of scanning times is less than 2%. The grey system theory and time series analysis was applied to forecast the dynamic deformability of angle. When observation sequence is the latest, it can keep the short-term prediction with a high accuracy. The prediction accuracy is.less than 3%.更多还原