基于离散夹钳与多点模具的板材柔性拉形技术研究
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摘要
目前,拉形工艺主要用于生产飞机蒙皮件,由于其材料利用率很低,生产成本相对较高,制约了它在其他工业领域中的应用。然而拉形工艺具有回弹小、成形精度高等优点,使得其能够适应许多领域对多品种、小批量工件生产的需求。为了提高拉形生产中材料的利用率,同时拓宽拉形工艺的应用领域,亟需一种新技术,板材柔性拉形工艺正是在这种需求下孕育而生的。板材柔性拉形技术是通过离散夹钳与多点模具的相互配合来实现的,该技术将离散夹钳柔性自协调的工作特点与多点模具型面的可重构性融为一体,可以实现拉形工艺的高度柔性化,提高了生产效率,降低了生产成本,能够快速响应钣金日益膨胀的产品个性化要求,迅速提升塑性加工领域中板类件的先进制造水平。板材离散夹钳柔性拉形是一种新型的拉形技术,通过全液压驱动技术来实现多个夹钳的柔性自协调工作,取代了传统拉形工艺中夹钳的整体运动模式。即夹料时,多个夹钳平直排列,夹紧板材两端;拉形过程中,多个夹钳可以根据模具形状的变化自动调整钳口的拉伸量和转角,实现板材被夹持边缘与拉形模端面的曲率变化相一致,这样有助于板材更容易贴合模具,减小拉形工艺余量,提高材料利用率。本文采用LS-DYNA软件对板材柔性拉形工艺进行了数值模拟,研究了夹钳结构与过渡区长度对拉形件成形效果的影响,探讨了拉形过程中回弹与接触力等问题,并进行了相关实验验证。
本文研究主要内容与结论如下:
1.板材离散夹钳柔性拉形工艺和成形装置的研究
研究了板材离散夹钳柔性拉形方法与传统板材拉形方法的区别和联系,指明板材离散夹钳柔性拉形的技术特点。采用解析法对板材柔性拉形过程进行了力学分析,推导出板材柔性拉形过程中应力、应变、厚度减薄和拉力的计算公式。详细阐述了板材离散夹钳柔性拉形实验装置的设备组成、工作原理和结构特点等,并指出该设备区别于传统拉形机的技术特点,不仅可以降低设备造价,还可以提高材料利用率。
2.板材柔性拉形有限元模型的建立
基于金属板材塑性成形的相关理论,建立了板材柔性拉形的有限元模型,采用动态显式算法模拟板材柔性拉形过程,静力隐式算法进行拉形件回弹过程分析。详细阐述了板材柔性拉形有限元模型建立过程中单元选择、网格化分、约束条件、接触与摩擦处理等若干问题,并对不同材料的本构关系进行了分析,板材采用双线性等向强化弹塑性模型,聚氨酯弹性垫采用超弹性材料模型,模具和夹钳定义为刚体模型,为板材柔性拉形数值模拟研究奠定了基础。
3.夹钳离散数量与夹钳形状的数值模拟研究
以球形件为研究对象,采用有限元法对板材柔性拉形工艺过程进行了数值模拟,探讨了夹钳离散数量对工件应力与应变分布、板厚变化、工件夹持面形状以及板料流动趋势的影响,结果表明:整体平夹钳拉形的工件应力与应变分布不均匀,板厚减薄严重,而离散夹钳拉形的工件应力与应变分布相对均匀,并且夹钳的离散数量越多,工件应力与应变分布愈加均匀化,工件夹持面的弯曲弧度越与模具曲率相接近,工件的成形质量也越好。分析了夹钳形状对工件应力与应变分布、板厚变化以及工件夹持面形状的影响,结果显示:圆形夹钳拉形时可以明显改善工件过渡区的应力、应变与板厚的分布状态,能够防止工件产生拉裂现象;圆形夹钳拉形时工件夹持面的弯曲弧度与模具的曲率更加接近,工件成形质量比较好,并对相关的数值模拟结果进行了实验验证。
4.不同过渡区长度的数值模拟研究
利用有限元软件对不同过渡区长度的球形件与鞍形件拉形进行了数值模拟,研究了整体平夹钳与离散夹钳两种拉形方式对工件贴模效果、成形缺陷及材料利用率的影响,结果显示:整体平夹钳需要较长的过渡区才能成形,导致球形件在过渡区产生多条皱纹,鞍形件会在成形区产生单一皱纹;而离散夹钳拉形只需较短的过渡区即可成形,工件表面光滑,没有皱纹,可以减小拉形工艺余量,提高材料利用率。并且离散夹钳拉形件的压痕深度明显小于整体平夹钳拉形件的压痕,只需较薄的弹性垫就能够抑制工件的表面缺陷,减小了工件的成形误差。研究了变形量、板材厚度与过渡区长度对球形件和鞍形件厚度变化的影响,结果显示:其他条件相同时,曲率半径越大,工件减薄率越小;板材厚度越大,工件减薄率越小;过渡区长度越大,工件减薄率越小。采用整体平夹钳拉形机和离散夹钳拉形机对球形件进行了拉形实验,实验结果与模拟结果基本吻合。
5.回弹的数值模拟及模具型面修正回弹方法的研究
采用显式-隐式算法对球形件在整体平夹钳与离散夹钳两种拉形方式下的回弹现象进行了研究,分析了夹持方式、过渡区长度、拉伸量、板材厚度、材料性能等因素对工件回弹量与形状误差的影响。结果表明:整体平夹钳拉形的工件比离散夹钳拉形的工件回弹大,成形误差也大;工件的回弹量和平均形状误差随着过渡区长度的增加而逐渐增大;回弹量随着预拉伸量的等量增加而逐渐减小,相应的平均形状误差也逐渐减小,当预拉伸量超过1%以后,回弹量和形状误差几乎不受影响,优先选用预拉伸量为1%;随着板材厚度的增加,回弹量下降的趋势逐渐变缓,形状误差逐渐减小;材料的弹性模量越大,工件卸载后回弹量就越小,平均形状误差也越小。提出了修正回弹误差的多点模具型面补偿计算方法,并通过数值模拟和实验对模具型面补偿方法进行了应用,验证了多点模具型面修正补偿方法的可行性和实用性。
6.板材与夹钳以及板材与冲头之间接触力的数值模拟研究
通过有限元方法对不同材质、不同板厚的柱形件、球形件以及鞍形件拉形过程进行了模拟,探讨了板材与夹钳之间接触力的分布规律,结果表明:材料的弹性模量和屈服强度越大,板材的变形抗力就越大,拉形时板材与夹钳之间的接触力就越大。板材的厚度对接触力的影响比较大,板材越厚,变形越不容易,拉形时板材与夹钳之间的接触力就越大。还探讨了拉形过程中冲头与板材的接触受力状态,通过模拟不同材料、不同板厚和不同变形量的球形件成形过程,研究了板材与冲头之间接触力的变化情况,结果显示:冲头接触力随着板材弹性模量和屈服强度的增大而增大;对于相同尺寸、相同材料的板材,板材越厚,冲头接触力就越大;目标曲率半径越大,冲头接触力则越小。
Stretch forming technology is mainly used in manufacturing of aircrafts skin and due to its low material utilization and relatively high cost,application of stretch forming technology is limited in other industry field. However, stretch forming technology has the advantages of small springback and high precision of formed parts,which make it still applicable in many other fields that have small demands and variant products.To solve the problem of low material utilization and to broaden the application range for stretch forming technology,a new technology is needed. To meet this need, a flexible stretch forming technology is developed for fabricating sheet metals. Flexible stretch forming technology is realized by interaction of the discrete-gripper with multi-point die. This technology combines self-coordination of the discrete-gripper with reconfigurability of the multi-point die, which can meet the demand in high flexibility, high productivity and low cost, respond swiftly to the increasingly demands of individualized products, quickly improve the ability level of fabricating sheet metals in plastic processing field.
The discrete-gripper flexible stretch forming technology is a new technology for forming sheet metals. With the hydraulic drive, the multiple grippers can work in a self-coordination mode, instead of the mode that gripper moving as a whole in traditional stretch forming process. In clamping process, the multiple grippers are arranged in a line and hold tightly at both sides of the sheet metal; during the stretching forming, the multiple grippers, based on the variation of the die configuration, can automatically adjust displacements and rotations of each gripper jaws correspondingly to make the clamped edges curvatures in accordance with the die-face. So this process can make the sheet metal easily attach the die with less stretching margin and have high material utilization. In this study, numerical simulations on sheet metal flexible stretch forming process were performed by LS-DYNA software, the influences of gripper structure and transition zone length to the quality of formed parts were investigated, springback and contact force were discussed and relative verification experiments were carried out. The main contents and conclusions of this study are as follows:
1. Research on sheet metal discrete-gripper flexible stretch forming process and its forming device
The difference and connection between discrete-gripper flexible stretch forming process and current sheet metal stretch forming process was studied, and the technical features of discrete-gripper flexible stretch forming process were pointed out. Mechanics analysis of flexible stretch forming process was done by using analytical method with derived equations for computing the stress, strain, tensile force and thickness reduction in stretching process. The composition, working principle and structural characteristics of discrete-gripper flexible stretch forming device were elaborated and the technical advantages compare with traditional stretch forming equipment were given, for example, lower equipment building cost and higher material utilization.
2. Building finite element model of flexible stretch forming process
Based on relative theories of sheet metal plastic forming, the finite element model of flexible stretch forming process was established, and dynamic explicit algorithm was chosen for numerically simulating its stretching process and static implicit algorithm for analyzing the springback of formed parts. Problems in establishing the finite element model of flexible stretch forming process such as element selection, meshing, constraint conditions,contact and friction were elaborately discussed. Constitutive relationships of different materials were analyzed, in which sheet metal is taken as bilinear isotropic hardening elastoplastic model, polyurethane elastic cushion is taken as hyper-elastic material model and the die along with grippers is taken as rigid model, these results are foundation of numerical simulations in flexible stretch forming process.
3. Numerical simulations on the quantity and configuration of discrete grippers
Taken spherical parts as investigating objects, numerical simulations on flexible stretching process by finite element method were done. And the influences of discrete gripper’s quantity to the stress, strain distributions, thickness reduction, shape of clamped parts sides, and material flow in sheet metal were studied. The results show: with the flat integral-gripper, the stress and strain distributions of formed parts are uneven, and accompany with serious thickness reduction; while, with the discrete-gripper, the stress and strain distributions are relatively even, moreover, the more the grippers, the more even the stress and strain distributions are, the closer the curvatures of the parts clamped sides approach the die and the better quality the formed parts has. The influences to the configuration of grippers on the parts stress and strain distributions, thickness reduction and the shape of clamped sides of the parts were analyzed, and the results show that the circular grippers can not only improve the distributions of stress, strain and thickness within the transition zones, but also prevent the parts crack. With the circular grippers, the curvatures of clamped sides of parts are closer to the die and the forming quality of the parts is better. Relative experiments were done to verify the simulation results.
4. Numerical simulations on different transition zone length
By means of finite element method, numerical simulations were done for flexible stretching process of spherical parts and saddle parts with different transition lengths, and the influences of both flat integral-gripper and discrete-gripper on sheet metal attaching die, forming defects and material utilization were studied. The results show: the flat integral-gripper need a long transition length for forming, which can cause multiple wrinkles in the transition zones of spherical parts and single wrinkle in the forming areas of saddle parts; the discrete -gripper need a relatively shorter transition length and the formed parts have smooth surface with no wrinkle, so the process margin is reduced and the material utilization is improved. Moreover, the dimpling of the discrete-gripper on the formed parts are less deep than that of the flat integral-gripper, and the surface defects of formed parts can be prevented by using a thinner elastic cushion accompany with decreased forming errors. The impacts of deformation quantity, thickness of sheet metal, and the influence of transition zone length on the thicknesses of the spherical and saddle parts were investigated. Obviously, when other conditions are same, the results are: the greater the curvature, the smaller the maximum thinning rate of the parts thickness is; the thicker the sheet metal, the smaller the maximum thinning rate; the longer the transition zone, the smaller the maximum thinning rate. Verification experiments were done by using both the flat integral-gripper and discrete-gripper forming machines to form spherical parts, and the experimental results are basically agree with the simulated results.
5. Research on springback simulation and the method of correcting springback by die surface compensation
The springback of spherical parts in both flat integral-gripper and discrete-gripper stretch forming processes were investigated by using explicit-implicit algorithm, and the impacts to springback quantity and shape errors caused by clamping mode, length of transition zone, stretching increment and materials were studied. The results show: springback and forming errors in the flat integral-gripper stretch forming mode are bigger than that in the discrete-gripper stretch forming mode; springback quantity and average shape error of formed parts are gradually increasing as the length of transition zone increase; springback gradually become smaller as the magnitude of pre-strecth increasing, and the corresponding average shape error is slowly reduced. Once the magnitude of pre-stretch exceeds1%, it has no influence on springback and the shape error, so 1% of pre-stretch is a priority; as sheet metal gets thicker, springback is reduced and the shape error is also gradually reduced; the greater the elastic modulus of the material, the smaller the springback of the formed parts gets after unloading, and the less the average shape error is. A method for correcting springback by multi-point stretching die surface compensation was presented, and this method was practiced by numerical simulation and experiment, which validates the feasibility and practicability of the method.
6. Numerical simulations on the contact forces between sheet metal, grippers and punch
Numerical simulations were performed for cylindrical, spherical and saddle parts of different materials and different thicknesses by using finite element method. The distribution of contact force between sheet metal and grippers was studied. The results show: the greater the elastic modulus and the yield strength of the material, the greater the deformation resistance of the sheet metal has, and the bigger the contact force is needed between sheet metal and grippers in stretching process. The thickness of sheet metal has a relative bigger influence on contact force: the thicker the sheet metal is, the more difficult it is for the material to deform, and the bigger the contact force is needed between sheet metal and grippers. The situation of contact force between sheet metal and punch was analyzed by simulating the stretching process of forming spherical parts with different materials, different thicknesses and different deformation. The results are: the contact force applied to the punch tends to increase as the material elastic modulus and yield strength increase; given the same dimensions of the same material, the thicker the sheet metal, the greater the contact force; the bigger the target curvature of the parts, the smaller the contact force.
本文研究主要内容与结论如下:
1.板材离散夹钳柔性拉形工艺和成形装置的研究
研究了板材离散夹钳柔性拉形方法与传统板材拉形方法的区别和联系,指明板材离散夹钳柔性拉形的技术特点。采用解析法对板材柔性拉形过程进行了力学分析,推导出板材柔性拉形过程中应力、应变、厚度减薄和拉力的计算公式。详细阐述了板材离散夹钳柔性拉形实验装置的设备组成、工作原理和结构特点等,并指出该设备区别于传统拉形机的技术特点,不仅可以降低设备造价,还可以提高材料利用率。
2.板材柔性拉形有限元模型的建立
基于金属板材塑性成形的相关理论,建立了板材柔性拉形的有限元模型,采用动态显式算法模拟板材柔性拉形过程,静力隐式算法进行拉形件回弹过程分析。详细阐述了板材柔性拉形有限元模型建立过程中单元选择、网格化分、约束条件、接触与摩擦处理等若干问题,并对不同材料的本构关系进行了分析,板材采用双线性等向强化弹塑性模型,聚氨酯弹性垫采用超弹性材料模型,模具和夹钳定义为刚体模型,为板材柔性拉形数值模拟研究奠定了基础。
3.夹钳离散数量与夹钳形状的数值模拟研究
以球形件为研究对象,采用有限元法对板材柔性拉形工艺过程进行了数值模拟,探讨了夹钳离散数量对工件应力与应变分布、板厚变化、工件夹持面形状以及板料流动趋势的影响,结果表明:整体平夹钳拉形的工件应力与应变分布不均匀,板厚减薄严重,而离散夹钳拉形的工件应力与应变分布相对均匀,并且夹钳的离散数量越多,工件应力与应变分布愈加均匀化,工件夹持面的弯曲弧度越与模具曲率相接近,工件的成形质量也越好。分析了夹钳形状对工件应力与应变分布、板厚变化以及工件夹持面形状的影响,结果显示:圆形夹钳拉形时可以明显改善工件过渡区的应力、应变与板厚的分布状态,能够防止工件产生拉裂现象;圆形夹钳拉形时工件夹持面的弯曲弧度与模具的曲率更加接近,工件成形质量比较好,并对相关的数值模拟结果进行了实验验证。
4.不同过渡区长度的数值模拟研究
利用有限元软件对不同过渡区长度的球形件与鞍形件拉形进行了数值模拟,研究了整体平夹钳与离散夹钳两种拉形方式对工件贴模效果、成形缺陷及材料利用率的影响,结果显示:整体平夹钳需要较长的过渡区才能成形,导致球形件在过渡区产生多条皱纹,鞍形件会在成形区产生单一皱纹;而离散夹钳拉形只需较短的过渡区即可成形,工件表面光滑,没有皱纹,可以减小拉形工艺余量,提高材料利用率。并且离散夹钳拉形件的压痕深度明显小于整体平夹钳拉形件的压痕,只需较薄的弹性垫就能够抑制工件的表面缺陷,减小了工件的成形误差。研究了变形量、板材厚度与过渡区长度对球形件和鞍形件厚度变化的影响,结果显示:其他条件相同时,曲率半径越大,工件减薄率越小;板材厚度越大,工件减薄率越小;过渡区长度越大,工件减薄率越小。采用整体平夹钳拉形机和离散夹钳拉形机对球形件进行了拉形实验,实验结果与模拟结果基本吻合。
5.回弹的数值模拟及模具型面修正回弹方法的研究
采用显式-隐式算法对球形件在整体平夹钳与离散夹钳两种拉形方式下的回弹现象进行了研究,分析了夹持方式、过渡区长度、拉伸量、板材厚度、材料性能等因素对工件回弹量与形状误差的影响。结果表明:整体平夹钳拉形的工件比离散夹钳拉形的工件回弹大,成形误差也大;工件的回弹量和平均形状误差随着过渡区长度的增加而逐渐增大;回弹量随着预拉伸量的等量增加而逐渐减小,相应的平均形状误差也逐渐减小,当预拉伸量超过1%以后,回弹量和形状误差几乎不受影响,优先选用预拉伸量为1%;随着板材厚度的增加,回弹量下降的趋势逐渐变缓,形状误差逐渐减小;材料的弹性模量越大,工件卸载后回弹量就越小,平均形状误差也越小。提出了修正回弹误差的多点模具型面补偿计算方法,并通过数值模拟和实验对模具型面补偿方法进行了应用,验证了多点模具型面修正补偿方法的可行性和实用性。
6.板材与夹钳以及板材与冲头之间接触力的数值模拟研究
通过有限元方法对不同材质、不同板厚的柱形件、球形件以及鞍形件拉形过程进行了模拟,探讨了板材与夹钳之间接触力的分布规律,结果表明:材料的弹性模量和屈服强度越大,板材的变形抗力就越大,拉形时板材与夹钳之间的接触力就越大。板材的厚度对接触力的影响比较大,板材越厚,变形越不容易,拉形时板材与夹钳之间的接触力就越大。还探讨了拉形过程中冲头与板材的接触受力状态,通过模拟不同材料、不同板厚和不同变形量的球形件成形过程,研究了板材与冲头之间接触力的变化情况,结果显示:冲头接触力随着板材弹性模量和屈服强度的增大而增大;对于相同尺寸、相同材料的板材,板材越厚,冲头接触力就越大;目标曲率半径越大,冲头接触力则越小。
Stretch forming technology is mainly used in manufacturing of aircrafts skin and due to its low material utilization and relatively high cost,application of stretch forming technology is limited in other industry field. However, stretch forming technology has the advantages of small springback and high precision of formed parts,which make it still applicable in many other fields that have small demands and variant products.To solve the problem of low material utilization and to broaden the application range for stretch forming technology,a new technology is needed. To meet this need, a flexible stretch forming technology is developed for fabricating sheet metals. Flexible stretch forming technology is realized by interaction of the discrete-gripper with multi-point die. This technology combines self-coordination of the discrete-gripper with reconfigurability of the multi-point die, which can meet the demand in high flexibility, high productivity and low cost, respond swiftly to the increasingly demands of individualized products, quickly improve the ability level of fabricating sheet metals in plastic processing field.
The discrete-gripper flexible stretch forming technology is a new technology for forming sheet metals. With the hydraulic drive, the multiple grippers can work in a self-coordination mode, instead of the mode that gripper moving as a whole in traditional stretch forming process. In clamping process, the multiple grippers are arranged in a line and hold tightly at both sides of the sheet metal; during the stretching forming, the multiple grippers, based on the variation of the die configuration, can automatically adjust displacements and rotations of each gripper jaws correspondingly to make the clamped edges curvatures in accordance with the die-face. So this process can make the sheet metal easily attach the die with less stretching margin and have high material utilization. In this study, numerical simulations on sheet metal flexible stretch forming process were performed by LS-DYNA software, the influences of gripper structure and transition zone length to the quality of formed parts were investigated, springback and contact force were discussed and relative verification experiments were carried out. The main contents and conclusions of this study are as follows:
1. Research on sheet metal discrete-gripper flexible stretch forming process and its forming device
The difference and connection between discrete-gripper flexible stretch forming process and current sheet metal stretch forming process was studied, and the technical features of discrete-gripper flexible stretch forming process were pointed out. Mechanics analysis of flexible stretch forming process was done by using analytical method with derived equations for computing the stress, strain, tensile force and thickness reduction in stretching process. The composition, working principle and structural characteristics of discrete-gripper flexible stretch forming device were elaborated and the technical advantages compare with traditional stretch forming equipment were given, for example, lower equipment building cost and higher material utilization.
2. Building finite element model of flexible stretch forming process
Based on relative theories of sheet metal plastic forming, the finite element model of flexible stretch forming process was established, and dynamic explicit algorithm was chosen for numerically simulating its stretching process and static implicit algorithm for analyzing the springback of formed parts. Problems in establishing the finite element model of flexible stretch forming process such as element selection, meshing, constraint conditions,contact and friction were elaborately discussed. Constitutive relationships of different materials were analyzed, in which sheet metal is taken as bilinear isotropic hardening elastoplastic model, polyurethane elastic cushion is taken as hyper-elastic material model and the die along with grippers is taken as rigid model, these results are foundation of numerical simulations in flexible stretch forming process.
3. Numerical simulations on the quantity and configuration of discrete grippers
Taken spherical parts as investigating objects, numerical simulations on flexible stretching process by finite element method were done. And the influences of discrete gripper’s quantity to the stress, strain distributions, thickness reduction, shape of clamped parts sides, and material flow in sheet metal were studied. The results show: with the flat integral-gripper, the stress and strain distributions of formed parts are uneven, and accompany with serious thickness reduction; while, with the discrete-gripper, the stress and strain distributions are relatively even, moreover, the more the grippers, the more even the stress and strain distributions are, the closer the curvatures of the parts clamped sides approach the die and the better quality the formed parts has. The influences to the configuration of grippers on the parts stress and strain distributions, thickness reduction and the shape of clamped sides of the parts were analyzed, and the results show that the circular grippers can not only improve the distributions of stress, strain and thickness within the transition zones, but also prevent the parts crack. With the circular grippers, the curvatures of clamped sides of parts are closer to the die and the forming quality of the parts is better. Relative experiments were done to verify the simulation results.
4. Numerical simulations on different transition zone length
By means of finite element method, numerical simulations were done for flexible stretching process of spherical parts and saddle parts with different transition lengths, and the influences of both flat integral-gripper and discrete-gripper on sheet metal attaching die, forming defects and material utilization were studied. The results show: the flat integral-gripper need a long transition length for forming, which can cause multiple wrinkles in the transition zones of spherical parts and single wrinkle in the forming areas of saddle parts; the discrete -gripper need a relatively shorter transition length and the formed parts have smooth surface with no wrinkle, so the process margin is reduced and the material utilization is improved. Moreover, the dimpling of the discrete-gripper on the formed parts are less deep than that of the flat integral-gripper, and the surface defects of formed parts can be prevented by using a thinner elastic cushion accompany with decreased forming errors. The impacts of deformation quantity, thickness of sheet metal, and the influence of transition zone length on the thicknesses of the spherical and saddle parts were investigated. Obviously, when other conditions are same, the results are: the greater the curvature, the smaller the maximum thinning rate of the parts thickness is; the thicker the sheet metal, the smaller the maximum thinning rate; the longer the transition zone, the smaller the maximum thinning rate. Verification experiments were done by using both the flat integral-gripper and discrete-gripper forming machines to form spherical parts, and the experimental results are basically agree with the simulated results.
5. Research on springback simulation and the method of correcting springback by die surface compensation
The springback of spherical parts in both flat integral-gripper and discrete-gripper stretch forming processes were investigated by using explicit-implicit algorithm, and the impacts to springback quantity and shape errors caused by clamping mode, length of transition zone, stretching increment and materials were studied. The results show: springback and forming errors in the flat integral-gripper stretch forming mode are bigger than that in the discrete-gripper stretch forming mode; springback quantity and average shape error of formed parts are gradually increasing as the length of transition zone increase; springback gradually become smaller as the magnitude of pre-strecth increasing, and the corresponding average shape error is slowly reduced. Once the magnitude of pre-stretch exceeds1%, it has no influence on springback and the shape error, so 1% of pre-stretch is a priority; as sheet metal gets thicker, springback is reduced and the shape error is also gradually reduced; the greater the elastic modulus of the material, the smaller the springback of the formed parts gets after unloading, and the less the average shape error is. A method for correcting springback by multi-point stretching die surface compensation was presented, and this method was practiced by numerical simulation and experiment, which validates the feasibility and practicability of the method.
6. Numerical simulations on the contact forces between sheet metal, grippers and punch
Numerical simulations were performed for cylindrical, spherical and saddle parts of different materials and different thicknesses by using finite element method. The distribution of contact force between sheet metal and grippers was studied. The results show: the greater the elastic modulus and the yield strength of the material, the greater the deformation resistance of the sheet metal has, and the bigger the contact force is needed between sheet metal and grippers in stretching process. The thickness of sheet metal has a relative bigger influence on contact force: the thicker the sheet metal is, the more difficult it is for the material to deform, and the bigger the contact force is needed between sheet metal and grippers. The situation of contact force between sheet metal and punch was analyzed by simulating the stretching process of forming spherical parts with different materials, different thicknesses and different deformation. The results are: the contact force applied to the punch tends to increase as the material elastic modulus and yield strength increase; given the same dimensions of the same material, the thicker the sheet metal, the greater the contact force; the bigger the target curvature of the parts, the smaller the contact force.
引文
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