【作者】 李卓；

【导师】 胡志清； 李立恒；

【作者基本信息】 吉林大学 ， 材料工程（专业学位）， 2019， 硕士

【摘要】 为了满足市场对大型化、大曲率半径曲面件需求,且传统的拉伸成形工艺仍存在技术不足,近些年提出了柔性拉伸成形工艺。柔性拉伸成形工艺特点是夹板机构柔性化,使板材贴模更容易,提高材料利用率,减少起皱、拉裂等成形缺陷。本文以柔性拉伸成形技术为基础,提出了推拉式拉伸曲面成形工艺,该工艺最大特点是将液压无杆腔的推力转化为促使板材拉伸变形的拉伸力。本文采用数值模拟的方法对该成形工艺进行分析,为推拉式拉形机在板材拉伸曲面件领域的应用提供技术支撑和理论基础。本文研究的主要内容和结论如下:(1)对推拉式拉形机的结构及关键部件进行研究;分析拉缸式板材拉伸机构与推拉式板材拉伸机构在曲面成形过程中的运动特性和成形力;对推拉式板材拉伸曲面成形工艺的特点进行概括总结。(2)确定了有限元模型中材料属性、接触、摩擦及边界条件等模型参数,建立了推拉式拉伸成形工艺的有限元模型,对成形曲面的形面特征、塑性应变、等效应力、厚度分布以及成形轨迹进行分析;对推拉式拉形机的可行性进行验证;并对成形曲面过程中常见的问题:起皱、应力集中、板材撕裂的产生机理进行了探讨。(3)研究在推拉式拉伸曲面成形工艺中夹钳离散化对曲面成形结果的影响,结果表明:随着夹钳离散化的增加,在推拉曲面成形的过程中,材料的每一部分都能单独地实现被拉伸和随着夹钳转动,使板材可以更好地随着模具的形面进行贴合。在成形曲面的有效成形区内,变形更加均匀;在过渡区内,变形过大且集中现象明显改善,曲面成形质量得到提高。(4)在推拉式拉伸曲面成形工艺中,引入了典型的加载模式。以不同加载模式对成形曲面结果的影响进行了数值模拟,结果表明:在有效成形区内,最小伸长增量加载模式下的成形曲面应力应变集中基本消失;在过渡区内,夹持区前沿的应力应变集中改善明显。从成形曲面的整体分布来讲,整体加载模式下的应力应变最小、厚度最大,但成形均匀性最差;最小伸长增量加载模式下的应力应变最大、厚度最小,但成形均匀性最好。(5)研究了夹钳离散化对不同加载模式下曲面成形结果的影响,实验结果表明:对比于整体加载模式以及等伸长增量加载模式,最小伸长增量加载模式下的成形曲面的平均应力应变以及厚度差值最小,并且综合两个方向上的平均差值可见,整体加载模式下曲面成形分布受到夹钳离散化影响最大,最小伸长增量加载模式下成形分布受到夹钳离散化影响最小。验证了在推拉式拉伸曲面成形工艺中,选取适当的加载模式,即使在低夹钳离散化的条件下仍然可以获得较好质量的曲面。更多还原

【Abstract】 In order to meet the market requirement of three-dimensional surface parts with large-size and large curvature radius,and the traditional stretch forming process still has technical problems,a flexible stretch forming process was proposed in recent years.The flexible stretch forming process is characterized by the flexibility of clamps,a sheet metal can more easily conform to the contour of the die,material utilization is increased,and the forming defects such as wrinkles,stress concentration and cracks and so on are decreased.Based on the flexible stretch surface forming process,a push-pull stretch surface forming process is proposed in this paper.Push force of the rodless cavity of the hydraulic cylinder converts to pull force for stretching sheet metal,which is the most important feature of this process.In this paper,the numerical simulation method is applied to analyze the forming process,which provides technical support and theoretical basis for the application of push-pull stretch equipment in the field of curvature surface stretch forming.The main contents and conclusions of this paper are as follows:(1)The structural and key components of the push-pull stretch equipment were studied;the kinetic characteristic and the forming force in forming process are compared between flexible stretch forming process and push-pull stretch forming process.The characteristics of the push-pull stretch forming process were summarized.(2)The model parameters such as material properties,contact,friction and boundary conditions were determined,the finite element model of push-pull stretch equipment was built.The shape characteristics,plastic strain,equivalent stress,thickness distribution and forming track of the forming surface parts were analyzed;the feasibility of the push-pull stretch equipment was verified;The wrinkling,stress concentration of forming surface parts were discussed.(3)The effect of clamps discretization on the forming result of forming surface parts in the push-pull stretch forming process were studied.The results show that: as the clamps discretization increase,more parts of material can be independently stretched and rotate with the clamps,so the sheet metal can preferably conform to the contour of the die.In the effective forming zone of the forming surface part,the deformation is more uniform with the clamps discretization increase;in the transition zone,The concentration phenomenon of the deformation is obviously improved with the clamps discretization increase,and the quality of forming surface part is improved.(4)The typical loading modes were introduced in the push-pull stretch forming process;By numerical simulation,the effect of different loading modes on the forming surface parts results were studied,the results show that: under the minimum stretch elongation loading mode,the stress and strain concentration of the forming surface part basically disappear in the effective forming zone;In the transition zone,the concentration of stress and strain were significantly improved in the front of outside clamps.In terms of the overall distribution of the forming surface part,the stress and strain of forming surface part in the whole loading mode are the minimum and the thickness is the maximum,but the forming uniformity is the worst.In the minimum stretch elongation loading mode,the stress and strain of forming surface part are the maximum and the thickness is the minimum,but the forming uniformity is the best.(5)To study the effect of clamps discretization on the forming surface parts results in different loading mode,the experimental results show that: compared with the whole load mode and the equivalent stretch elongation load mode,the average stress,strain and thickness difference of forming surface part are minimum in the minimum stretch elongation load mode.By analyzing the average deformation difference in the two directions,the effect of clamps discretization on the forming distribution in the whole loading mode is maximum;the effect of clamps discretization on the forming distribution in the minimum stretch elongation loading mode is minimum.It is verified that: in the push-pull stretch forming process,the appropriate loading mode is selected,a surface part with better quality can also be obtained under the condition of low clamps discretization.更多还原