【作者】 张庆芳；

【导师】 蔡中义；

【作者基本信息】 吉林大学 ， 材料加工工程， 2014， 博士

【摘要】 目前，航空航天、汽车及造船等多个领域对金属三维曲面件都有大量需求。传统的三维曲面件生产主要通过模具来实现，模具特别适用于大批量的零件生产模式。随着工业技术发展及人们对个性化产品的需求，新型产品、试制样品不断被研制开发。每一件产品的外形及尺寸均不相同，若采用模具加工，需要花费大量的模具设计、制造、维护等费用，成本高昂，无法获得效益。因此，亟需一种新型技术满足这种单件、小批量加工方式的要求。多点成形正是针对这种需求创建的快速、高效的成形技术，其柔性可调整的多点模具型面，能够快速响应产品个性化要求，加快产品更新换代速度。与传统模具相比，多点成形技术能够实现无模成形，具有快速柔性成形的优势，但该技术也存在回弹、起皱等成形缺陷，尤其是回弹问题，会严重影响多点成形件的加工精度，无法得到准确的三维曲面零件。多点模具可以根据需要快速调整模具型面，采用补偿方法控制多点成形的回弹是一种有效地、切实可行的手段。本文给出了补偿回弹的曲率计算方法，构建复杂曲面的回弹补偿面，建立了多点成形回弹补偿方法，并通过数值模拟与多点成形实验对该方法进行了验证。研究的主要内容如下：（1）基于三种材料模型，推导了简单的单曲率曲面补偿回弹的曲率计算公式及残余应力公式。给出了复杂单曲率曲面的回弹补偿方法，采用结构离散方式将复杂曲面处理成若干微小的简单曲面，基于简单曲面的回弹补偿计算公式，采用曲线插值法或有限差分法得到连续的补偿回弹的曲面。再采用三次B样条法对得到的补偿曲面进一步拟合，得到光滑的曲率连续的曲面。对插值法与差分法这两种计算回弹修正曲面的方法进行了实例计算，并进行了数值模拟和实验验证，发现插值法的成形精度更高。（2）推导了简单的双曲度曲面回弹前曲率（即补偿回弹的曲率）计算公式，给出了复杂双曲度曲面的回弹补偿方法，该方法对以简单曲面补偿回弹的曲率计算公式为基础，引入离散化思想，根据每块微板元的曲率信息反算板元的特征点，采用插值法得到补偿回弹的曲面上一系列特征点矩阵。再采用贝赛尔曲面拼接法，得到光滑连续的回弹补偿面。研究了多点模具调形成准确的回弹补偿面的方法，多点模具调形中要考虑基本体球冠及公切点位置，依据回弹补偿面反求基本体冲头球心坐标和切点坐标，确定基本体行程，即可构建修正回弹的多点模具型面。（3）基于ABAQUS软件建立了多点成形有限元模型，描述了模拟过程中单元选择、接触的定义、摩擦系数、约束、位移等边界条件的处理，以及显-隐式结合算法进行多点成形回弹模拟的分析过程，阐述了回弹模拟过程中约束的处理问题。研究了多点成形中弹性垫的使用以及板料厚度、材料、曲率半径等对回弹的影响。指出选用适当厚度、压缩率和弹性模量的弹性垫可以达到良好的表面成形质量，有效降低回弹量；随着板厚的增加，回弹量下降的趋势逐渐变缓；材料的弹性模量越大，工件卸载后回弹量就越小；另外，随着曲率半径减小，回弹量也减小。（4）模拟了多种典型的复杂曲面件多点成形回弹过程，根据修正的模具型面进行数值模拟，得到了补偿回弹的曲面件，根据目标形状进行数值模拟，得到了未补偿回弹的曲面件。通过回弹前、后的应力云图及Z向位移云图对比，发现回弹后的曲面件应力和曲率减小，曲面形状发生了改变。取模拟结果沿x,y方向的轮廓和目标形状对比，发现补偿回弹的模拟结果与目标轮廓接近，误差很小；未补偿回弹的模拟结果与目标形状存在偏差，误差较大。（5）采用多点模具成形了多种典型曲面件，并进行了精度测量。发现补偿回弹的曲面件，其测量结果与目标轮廓吻合良好，未补偿回弹的测量结果误差较大。通过模拟、实验结果的误差对比，可以发现采用补偿回弹的模具型面成形的曲面误差范围小，和目标形状吻合，这也验证了文章中所述的回弹补偿计算方法具有良好的预测效果和应用价值。更多还原

【Abstract】 At present, Sheet metal forming processes are widely used to produce three-dimensionalsurface in many fields, such as aerospace, automotive, aircraft manufacturing, vehicle body,shipbuilding and pressure vessel forming and so on. Stamping is one of the most commonsheet metal forming methods. For conventional stamping process, which involves a matchedsolid die set, its advantages are a short production time and high productivity. Nevertheless,large initial investments and long setup time make its processes inflexible and onlyprofitable for mass production and economically unsuitable for single or small batchproducts. Multi-point forming (MPF), a novel flexible forming technology develops wellrecent years. In MPF, conventional solid stamping dies are replaced by a pair of opposedreconfigurable dies comprised by punch matrices. Based on this technology, dieless, rapidand digital manufacturing of sheet metal parts can be realized. Comparing with conventionalstamping, MPF technology is more suitable for small-lot and individualized production, andresponding to customers’ requirements quickly, meanwhile, accelerating the productrenovation.Comparing with conventional stamping, MPF technology is with advantages of rapid andflexible forming. However, whether it is conventional stamping or MPF, springback is aninevitable phenomenon in sheet metal forming, and it greatly affects the geometricalaccuracy of products. Springback caused by elastic recovery and release of residual stressafter forming, and the final shape of part depends on the value of springback. Once the valueof springback exceeds the allowable tolerance, it becomes defects and affects the wholeassembly of other parts. Moreover, it is an accumulated effect of the entire processing history.Hence the prediction of springback is difficult and remains an important problem in themanufacturing industry. The die surface of MPF can be adjusted quickly, realizing thereconfiguration of various curve surfaces; therefore, springback compensation method isvery suitable for MPF. Compensation method is an effective, practical method for controlling MPF springback. An algorithm was established for springback compensation inMPF by combination methods of theoretical analysis, numerical simulation and formingexperiment. Calculation formula for curvature before springback was obtained basing onthree material models, the method describing the shape of die-face after springbackmodification was proposed, Simulation and experimental results shows that thecompensation algorithm can effectively control errors caused by springback in MPF.The main contents and conclusions are as follows:(1)Calculation formulas for curvature before springback for single-curvature surface wasobtained basing on three material models, For irregular single curvature surface,interpolation and difference method are used to obtain continuous surface with first-orderderivative, then a cubic B-spline fitting method are used to obtain the smooth and continuoussurface. Comparison of the interpolation and difference method, it is found that theinterpolation method is with excellent accuracy.(2) For irregular doubly curved surface, the calculation formula for curvature beforespringback was obtained basing on three material models. Then, with discrete method, thewhole surface was divided into tiny pieces, interpolation processing and Bezier surfaceblending methods are used to describe the surface after springback compensation. Due to thedie-face of MPF is composed by a series of discrete punch elements, the center position andthe points of tangency of punch elements are obtained basing on the surface after springbackcompensation, the adjusting heights of punch elements can be achieved for MPF diecompensation surface.(3)Based on the ABAQUS software, the finite element models were established, and theunite selection, contact definition, friction coefficient, constraint and displacement boundarywere described in simulation. Simulating the process of MPF and springback, and theSpringback process is simulated combining explicit and implicit algorithm. The influencingfactors on the springback, such as, using cushion, material, thickness, radius of curvature,were researched. The results show: the greater the thickness is, the springback decreasesgradually; the greater elastic modulus is, smaller the springback after unloading is; with theradius of curvature decreases, the springback is reduced; in addition, with the increase ofcompensation coefficient, the error value is decreasing.(4)Simulating the springback process of MPF on single-curvature surface and doubly curvedsurface. Effective stress distribution and Z-displacement distribution are achieved.Comparing the simulation result with the target shapes, it is found that the single-curvature surface is easily to generate large springback; for doubly curved surface, if the bendingdirection is coincident, the springback is smaller, otherwise, the springback is bigger.Comparing the simulation result with and without springback compensation, we found thatthe errors is very small with springback compensation, otherwise, the errors is relativelylarge.(5) A series of experiments were carried out by means of MPF equipment, comparing theresults with compensation and without compensation, it is observed that simulated andmeasured results with compensation matched well with target shapes, and overall errors arerelatively small and satisfied processing precision. The compensation method presented inthis paper provides an effective calculation method to springback compensation ofsingle-curvature and doubly curved surface, and it is of good springback predition effect andapplication value.更多还原