【作者】 刘启骞；

【导师】 付文智；

【作者基本信息】 吉林大学 ， 材料加工工程， 2011， 博士

【摘要】 多点成形是将传统模具离散为规则排列的、高度可调的基本体群,并通过控制基本体的相对位置,使基本体冲头构造出所需的包络面对坯料进行曲面成形。与传统模具成形相比,多点成形节约了大量制造模具的时间和费用,实现了一机多用的设想。多点成形作为一种新颖的柔性成形工艺已被广泛应用于工业生产领域。随着当今社会现代化的不断发展,在各行各业中,节约能源已被公认为是现代企业生存的基础。所以,在飞机、汽车和轮船等制造业中,人们越来越重视轻量化结构设计。在轻量化结构设计中,往往采用一种框架式结构,而型材弯曲类零件不仅具有轻量化的结构,同时还能满足较高的刚度要求,具有广泛的应用前景。因此,型材弯曲工艺成为当今材料加工领域的研究热点之一。型材多点弯曲成形工艺将多点成形与型材成形相结合,利用多点成形压力机的柔性特点,实现型材弯曲类零件的柔性化和高效性成形,特别适用于生产多品种、中小批量的产品以及研制开发新产品,同时也拓宽了多点成形技术的应用范围。随着计算机技术和有限元数值模拟技术的不断发展,数值模拟技术已成为型材弯曲研究的重要方法之一。本文利用有限元数值模拟技术全面系统地分析型材多点弯曲成形中工艺参数对成形件的影响规律,探讨成形过程中的变形特点,采用三维测量仪对制件进行检测,对比分析模拟结果和实验结果,并探寻抑制及消除成形缺陷的有效方法。本文研究的主要内容和结论总结如下:1.研究建立型材多点弯曲成形工艺的有限元模型在板材多点成形数值模拟的基础上,提出了型材多点弯曲成形工艺,探讨了冲头与型材的相对位置,对冲头结构进行了改进,编制了型材多点弯曲成形的基本体调形程序,探讨了型材多点弯曲成形方式,建立了型材多点弯曲成形过程的有限元模型,实现了型材多点弯曲成形过程的数值模拟,得到了压痕、截面畸变和回弹等缺陷的有限元模拟结果。2.基于有限元方法研究板管多点成形中的压痕压痕是多点成形所特有的成形缺陷。本文分析了多点成形中压痕的产生机理以及压痕的类型。以一个新的材料本构关系为基础,对微型板多点成形进行了模拟分析,探讨了尺寸效应对成形件表面质量的影响,得到了板材厚度、晶粒尺寸和冲头尺寸对表面质量的影响规律。对管材多点弯曲成形过程进行了数值模拟,通过分析模拟结果,探讨了变形程度、管壁厚度和管材直径等工艺参数对压痕的影响。分析结果表明:变形程度越小,管壁厚度越大,管材直径越小时成形件的压痕越不明显;反之,则成形件的压痕越明显。利用实验室现有成形设备对管材多点弯曲成形中的压痕进行了验证,实验结果与模拟结果基本吻合。为了抑制压痕的产生,提高成形件的表面质量,提出了采用添加弹性介质的方法增大接触面积,分析了弹性介质的材质、厚度和成形过程中其压缩量对压痕的影响规律。3.采用数值模拟方法研究管材多点弯曲成形中的截面畸变截面畸变是型材弯曲成形中特有的成形缺陷。本文分析了截面畸变的形成机理,并定义了截面畸变率作为衡量截面变形程度的标准,对无弹性介质条件下管材多点弯曲成形的截面变形进行了数值模拟研究,分析了几种工艺参数对截面畸变的影响规律。获得了管壁厚度、管材直径和变形程度与截面畸变率的变化规律。研究结果表明:随着管壁厚度和目标曲率半径的减小,截面畸变率增大,而管材直径越大,截面畸变率也越大。同时,提出了填充液体对管壁施加一个支撑力的方法,抑制管材截面畸变;在模拟时,用均布压强代替填充液体对管壁的支撑,分析了不同均布压强和不同弹性介质压缩量条件下端部截面畸变率的情况;根据端部截面变形的程度,给出了工艺改进前后管材直径与最小弯曲半径之间的关系。4.角型材多点弯曲成形过程的研究简述了角型材多点弯曲方式,分析了不同弯曲方式下角型材的变形过程和受力情况。模拟了不同截面尺寸、不同材料参数和不同变形程度的角型材多点弯曲成形过程,通过分析模拟结果,得到了不同工艺参数对弦板侧向移动量和扭转角的影响规律。总结了不同截面尺寸、不同材料参数和不同目标曲率半径对成形件精度影响的变化规律。根据这些曲线,可以确定弦板侧向移动量和扭转角随不同工艺参数变化而改变的趋势。为了提高角型材多点弯曲成形件的精度,提出了采用两个角型材同时成形的对称成形法来抑制弦板的侧向移动和扭转,并进行了数值模拟和成形实验,证明了对称成形法的成形效果和可行性。同时,基于平截面和单向应力等假设条件,分析了角型材多点弯曲成形中的成形力,从弹塑性力学和有限元接触分析角度推导了成形力的计算方法,并通过分析模拟结果获得了几种因素对成形力的影响规律,即随着弦板与腹板的比值和成形件变形程度的增大,所需成形力增大;收边弯曲时所需成形力大于放边弯曲时所需成形力;对称成形时的成形力大于单独成形时成形力的2倍。5.研究型材的回弹及其抑制方法研究了型材弯曲后产生回弹的原因,分析了回弹后成形件中应力的分布规律;根据型材回弹前后的平均曲率,得到了成形件回弹量的计算公式;探讨了采用显式-隐式算法进行回弹分析的可行性。对管材和角型材的弯曲回弹进行了系统的数值模拟研究,分析了型材厚度、截面尺寸和变形程度对成形件回弹量的影响规律。归纳了不同工艺参数与回弹量的变化规律。研究结果表明:型材厚度越大、截面尺寸越大,目标曲率半径越小时,回弹量越小。并进行了管材多点弯曲成形和回弹实验,通过对比实验结果和模拟结果,验证了管材弯曲回弹分析模型的可靠性。提出了采用反复成形法抑制回弹,简述了反复成形法的基本思路和具体的实现步骤。通过数值模拟获得了补偿量对成形曲率的影响规律,对不同补偿量下成形件的精度进行了分析,研究结果表明:补偿量与回弹后的曲率半径成反比;随着补偿量的增加,成形误差减小。实现了动态显式算法与静态隐式算法的相互转化;完成了型材反复弯曲成形过程的数值模拟。在补偿量一定的条件下,分析了反复成形次数对成形件精度的影响,研究结果表明:随着反复成形次数的增加,成形误差逐渐减小,成形件形状越来越接近目标形状。更多还原

【Abstract】 Multi-point forming (MPF) is a flexible forming technology for 3-D surface parts. Its basic idea is to make the traditional die discrete, using the ordered and height adjustable elements instead of the traditional die, which make punches formed the required envelope surface by controlling the relative displacements of elements. Compared with the traditional die forming, MPF can save a lot of time and cost in manufacturing die and achieve a plan that is one machine can have several usages. As a new-style flexible forming process, MPF has been widely applied to the industrial production. With the development of modernization in current society, energy conservation is publicly known as the survival foundation for modern enterprises. Therefore, in the manufacture, such as aircraft, automobile and shipbuilding, people pay more and more attention to the structural design of lightweight in which a kind of frame style structure is usually adopted. While the bending parts of profile not only have the lightweight structure, but also can satisfy the requirement of higher stiffness, therefore, they will have an extensive future. For this reason, the bending process of profile has become one of hot topics in the material processing field.The multi-point bending process of profile combines MPF with profile forming, using the flexible features of MPF press to realize the flexible and efficient forming for the bending parts of profile. It is particularly suitable for producing the multi-item and small-lot products and developing new products, meanwhile, it also widens the application of MPF technology. With the further development of computer technology and finite element method, the technology of numerical simulation has become one of the important methods for studying the profile bending. In this paper, finite element method was utilized to analyze the influence of the technological parameter on formed parts in profile multi-point bending, discussing the deformation characters during the forming process, detecting formed parts by a 3-D measuring instrument, comparing simulation results with experimental results and seeking the effectiveness method to suppress and remove the forming defects.The main contents and conclusions are as follows:1. Study on building the finite element model of multi-point bending process for profileBased on the numerical simulation of MPF for sheet, the multi-point bending process of profile was put forward, and the relative positions between the punch and the profile were discussed. The punch was carried out the optimal selection. The element adjustment program of the multi-point bending process for profile was compiled. The forming mode of profile multi-point bending was discussed. The finite element model of multi-point bending for profile was built. The numerical simulation of the process of multi-point bending for profile was achieved, obtaining the defects from the results of finite element simulation, such as dimple, cross-section distortion and springback.2. Based on the finite element method, study the dimple in sheet and tube multi-point formingDimple is a particular forming defect in the process of MPF. In this paper, the reasons for dimples on formed parts and the types of dimples were analyzed in MPF. Based on a new material constitutive relation, simulation analysis of the micro multi-point sheet forming was carried out, discussing the influence of size effect on the surface quality of formed parts and obtaining the influence rules of sheet thickness, grain size and punch size on surface quality. Meanwhile, the numerical simulation of the process of tube multi-point bending was carried out. By analyzing the results of numerical simulation, the influence of the technological parameters, including deformation degree, the thickness of tube wall and tube diameter, on dimple was discussed. The results show that when the deformation degree is smaller, the tube wall is thicker and the tube diameter is smaller, then the dimple of formed parts is not obvious; on the contrary, the dimple of formed parts is obvious. With the forming device in my lab, the simulation results of tube multi-point bending, which are nearly the same as the experimental results, were verified. In order to suppress dimpling and improve the surface quality of formed parts, a method that is to use an elastic medium to increase the contact area was put forward. The influence rule of the material and thickness of elastic medium and its decrement in forming process on dimple was analyzed.3. Research on the cross-section distortion in tube multi-point bending by using numerical simulation methodCross-section distortion is a special forming defect in profile bending forming. In this paper, the reasons for cross-section distortion were analyzed and the cross-section distortion rate was defined as the criterion to measure the deformation degree of cross-section. Without elastic medium, the cross-section deformation of tube multi-point bending was simulated, which analyzed the influence rule of several forming parameters on cross-section distortion. The change rule between the cross-section distortion rate and the parameters which include the thickness of tube wall, tube diameter and deformation degree was obtained. The results show that the cross-section distortion rate increases as the thickness of tube wall and the target curvature radius decreases, while the cross-section distortion rate increases as the tube diameter increases. Meanwhile, a method, which is to fill the tube with liquid to support tube wall, was put forward to suppress the cross-section distortion of tube, but in the simulation, the liquid was replaced by the uniform pressure, and the edge cross-section distortion rate was analyzed under different uniform pressure and different elastic medium decrements. According to the deformation degree of the edge cross-section, the relationship between the tube diameter and the minimum bending radius was given before and after improving technology.4. Research on the process of multi-point bending for angle profileRecounted the multi-point bending modes of angle profile simply, the deformed process and the forced situation were analyzed under different bending modes. The process of multi-point bending for angle profile was simulated under different cross-section dimensions, material parameters and deformation degrees and the influence of different technological parameters on the lateral movement value and torsional angle of boom plate was obtained by analyzing simulation results. The change rules which describes the influence of different cross-section dimensions, material parameters and target curvature radii on the precision of formed parts was concluded. According to these curves, the variation tendency of the lateral movement value and torsional angle of boom plate can be determined as the technological parameter changed. In order to improve the precision of formed parts in multi-point bending for angle profile, the symmetrical forming method which made two angle profiles formed simultaneously was put forward to suppress the lateral movement value and torsion of boom plate. According to the numerical simulation and forming experiment, the effectiveness and feasibility of the symmetrical forming method were verified. At the same time, based on the section assumption and the uniaxial stress assumption, the forming force in multi-point bending for angle profile was analyzed and the computing method of forming force was deduced from the view of the elastic-plastic mechanics and the finite element contact analysis. The influence rules of several factors on forming force was obtained by analyzing simulation results, that is, the required forming force increases as the rate of boom plate to web plate and the deformation degree of formed parts increases; the required forming force in“flange in”bending is larger than that in“flange out”bending; the forming force in symmetrical forming is more than twice as large as that in separate forming.5. Study on springback and its suppression method for profile The causes for generating springback were studied after bending profile and the distribution rule of stress in the formed parts of profile was analyzed after springback. According to the average curvature of profile before and after springback, the computing formula of springback amount of formed parts was deduced. The feasibility, with the explicit-standard algorithm to carry out the analysis of springback, was discussed. The process of the bending springback for tube and angle profile was simulated systematically and the influence rule of the thickness of profile, cross-section dimension and deformation degree on springback amount of formed parts was analyzed. The change rule between different technological parameters and springback amount was concluded. It results that when the thickness of profile and the cross-section dimension are larger and the target curvature radius is smaller, then the springback amount is smaller. The forming and springback experiments of tube multi-point bending were carried out. The precision of springback analysis model for tube bending parts was verified by comparing the experimental results with the simulation results. The reiterative forming method was put forward to reduce springback, and the basic ideas and concrete step of reiterative forming process were given. The effect of the compensation quantity on the forming curvature was obtained by numerical simulation, and the precision of formed parts was analyzed under different compensation quantities. It results that the compensation quantity is inversely proportional to the curvature radius after springback; the forming error decreases gradually as the compensation quantity increases. The mutual transformation between the explicit results and the standard results was realized, and the numerical simulation of reiterative bending process for profile was achieved. With a fixed compensation quantity, the influence of reiterative forming time on the precision of formed parts was analyzed. It results that the forming error gradually decreases as the reiterative forming time increases, and the shape of formed parts is more and more close to the target shape.更多还原