Numerical simulation on the local stress and local deformation in multi-point stretch forming process

Multi-point stretch forming (MPSF) is a new flexible forming technique to form aircraft outer skin parts. The multi-point stretching die (MPSD) replaces the traditional fixed shape stretching die, and the sheet metal is formed over a MPSD composed by the punch element. The MPSD is a discontinuous surface of discrete stretching die, and the stress concentration and local strain occur on formed parts. These lead to generate dimples on the surface of formed part. In this paper, a series of numerical simulations on MPSF processes for stretching parabolic cylindrical, spherical, and saddle-shaped parts were carried out. The local stress and local strain in thickness distribution of MPSF part were analyzed by dispersed the blank into solid elements. The forming results of MPSF were compared with those that use traditional stretch forming, and the influences of thickness of elastic cushion and the size of punch element on the stress concentration and local strain were surveyed. The simulation results show the distribution of local stress and local deformation in different layers, and the elastic cushion and the small size of punch element can reduce the stress concentration and local deformation. The results may understand the stress distribution on the sheet and prevent the defect of dimple.     

Numerical investigation of multi-point forming process for sheet metal: wrinkling, dimpling and springback

Multi-point forming (MPF) is a new flexible technique for manufacturing three-dimensional sheet metal parts. In this procedure, a pair of opposed matrices of punch elements substitute for the conventional fixed shape die sets, and the sheet metal can be formed rapidly between the matrices. Extensive numerical simulations of the processes for forming spherical and saddle-shaped parts were carried out by dynamic explicit finite element analysis. The contacting process between sheet metal and punch elements in MPF was investigated, and the variations of forming force with respect to the tool travel were analyzed. The wrinkling processes were simulated, and the MPF limit curves without wrinkles for spherical and saddle-shaped parts were obtained. Dimple is a particular defect in MPF, through the comparison of the thickness strains calculated by solid FE and shell FE, the finite elements appropriate for the numerical analysis of dimpling were detected, and the limit forming force without dimples was determined. Springback processes of MPF were simulated based on explicit-implicit algorithm. The springbacks and their distributions under different conditions were investigated.     

板材多点成形不同工艺方法的数值模拟研究

介绍了板材多点成形不同工艺方法的优缺点,基于有限元方法就多点成形不同工艺对压痕、超皱、回弹以及制品成形精度的影响进行了数值模拟研究,并且对不同厚度板条多点成形模拟结果进行了实验验证。结果表明,与多点模具成形、半多点模具成形和半多点压机成形方式相比,多点压机成形能够有效地抑制压痕和皱纹的产生,并得到变形均匀的制品;反复成开法可有效减小回弹,提高成形精度;边部优先成形法能够有效地抑制制品的直边效应。     

Study of multi-point forming for polycarbonate sheet

The multi-point forming (MPF) process of polycarbonate (PC) sheet is introduced briefly, geometrical relationship between objective surface and punch element is determined, and a simple calculation scheme of punch height is developed. Numerical simulations of spherical and saddle-shaped parts are carried out by dynamic explicit finite element analysis; the effects of forming temperature, forming pressure, punch matrix, and punch radius on the forming quality are investigated, and the suitable forming parameters are determined. Then, the MPF experiments of PC sheet for spherical and saddle-shaped parts based on the forming parameters are done, and the comparisons of shape error between experimental parts and object surfaces are carried out. Consequently, the PC products have good shape accuracy, which confirms that MPF used for forming PC sheet is feasible and the forming parameters obtained by numerical simulation are sensible.     

Estimation of springback in double-curvature forming of plates: Experimental evaluation and process modeling

This paper presents the results obtained from a series of experiments on double-curvature forming of 300 mm square and 15 mm thick plates of type 316L(N) stainless steel to evaluate the inherent springback and also to validate finite element method (FEM) based process model developed for forming of multiple-curvature sectors of large size vessels. The experimental results show that twisting of the plate occurs during pressing, which is unavoidable in an actual forming setup on the shop floor. Twisting increases with increase in slope of the die cavity. Springback in the plate changes in an ascending order towards the centerline of the plate from the edges. The final radius of curvature (ROC) on the pressed plate after springback does not remain constant along a particular axis although the die and the punch had constant ROC along that axis because of varying constraint to opening up of the plate from centerlines to the edges. Springback also increases with reduction in the stiffness of the die and punch. The simulated plate profiles obtained from the FEM process model for multiple-curvature plate forming compared well with the experiments, the maximum error being within 6%. The process model used a sequential dynamic explicit formulation for the plate pressing phase and a static implicit formulation for the unloading (springback) phase in the Lagrangian framework. Reduced integration shell elements were used for the plate and the die and the punch were considered rigid. Dynamic explicit FEM for pressing and static implicit FEM for the unloading phase are adequate and economic for modeling of plate forming process by using FEM. The necessary material and frictional property data needed for the FEM process model were generated in-house. This model can be applied to design of dies and punches for forming the petals of large pressure vessels. The FEM process model predicts the final shape of the product and the residual cold work level for a given die, punch and plate configuration and this information can be used to correct the die and punch shapes for springback to manufacture the petals to the desired accuracy.     

Numerical simulation of multi-point stretch forming and controlling on accuracy of formed workpiece

A finite element model was built for the sheet metal multi-point stretch forming (MPSF), and the model was applied to the MPSF of a typical part. The numerical simulation of the MPSF for a saddle and cylindrical surface workpiece was performed. The effects of non-uniform deformation of elastic cushion and spring back after unloading on the precision of formed workpiece were investigated. The simulation results show that using elastic cushion can effectively suppress generation of dimples, but have an effect on precision of formed workpiece. In addition, the amount of spring back after unloading is small, and the effect of the non-uniform deformation of the elastic cushion on the precision of the MPSF workpiece is larger than that of the spring back. A method based on the iterative corrections according to the numerical simulation results was proposed to compensate the non-uniform deformation of elastic cushion and the spring back to enhance the accuracy of MPSF.     

Prediction and control of pillow defect in single point incremental forming using numerical simulations

Pillows formed at the center of sheets in Single point incremental forming (SPIF) are fabrication defects which adversely affect the geometrical accuracy and formability of manufactured parts. This study is focused on using FEA as a tool to predict and control pillowing in SPIF by varying tool size and shape. 3D Finite element analysis (FEA) and experiments are carried out using annealed Aluminum 1050. From FEA, it is found out that the stress/strain state in the immediate vicinity of the forming tool in the transverse direction plays a determinant role on sheet pillowing. Furthermore, pillow height increases as compression in the sheet-plane increases. The nature of in-plane stresses in the transverse direction varies from compressive to tensile as the tool-end geometry is changed from spherical to flat. Additionally, the magnitude of corresponding in-plane stresses decreases as the tool radius increases. According to measurements from the FEA model, flat end tools and large radii both retard pillow formation. However, the influence of changing tool end shape from hemispherical to flat is observed to be more important than the effect of varying tool radius, because the deformation zone remains in tension in the transverse direction while forming with flat end tools. These findings are verified by conducting a set of experiments. A fair agreement between the FEM and empirical results show that FEM can be employed as a tool to predict and control the pillow defect in SPIF.

     

An elasto-plastic finite element analysis of the sheet metal stretch flanging process

The incremental updated Lagrangian elasto-plastic finite element method (FEM) was employed in this study to analyse the stretch flanging of circular plates with a pre-determined smaller hole at the centre of the sheet metal. An extended r _min technique was employed such that each incremental step size is determined not only by the yielding of an element Gaussian point, but also by the change in the boundary condition along the tool-sheet interface. The experimental results, using a low-carbon (BA-CQ₂) sheet plate with a thickness of 1.0 mm, have been obtained and compared with the corresponding theoretical results. It was found that the flange thickness does not always decrease monotonically from the die shoulder to the flange edge. Reducing the punch diameter and increasing the flange height significantly reduced the flange thickness. Web width does not influence the thickness distribution of the flange. The tendency of flange thickness to thin decreases as punch diameter increases. The reduction of thickness at the die shoulder depends on the die shoulder radius. Simulation results of punch load of stretch flanging, the deformed geometry, and the distribution of thickness are compared with experimental data and found to satisfactorily agree.     

冲裁模具间隙及凹模圆角对凸模应力应变影响的有限元分析

冲裁模具的间隙和凹模圆角,对凸模的应力、应变有重要的影响。运用有限元分析了冲裁模具在不同间隙与圆角下凸模的应力、应变特性。分析结果表明,应力、应变随着间隙和凹模圆角的增加而减小。模拟分析的结果为模具间隙和圆角半径的选择提供了依据。     

Influence of process parameters in mechanical clinching with extensible dies

The influence of clinching tool design in joining metal sheets by the clinching process with extensible dies is investigated. The material flow during the clinching process was examined experimentally and numerically. The geometrical and mechanical characteristics of joints produced under different processing conditions, that is, forming loads, were used to calibrate and validate a 3D finite element model of the clinching process. Then, the model was utilized to evaluate the influence of clinching tool design parameters, namely the punch diameter, the punch corner radius, the fixed die depth, the fixed die diameter, and the die corner radius. The effects of design parameters on the cross section of a clinched joint, the required forming load and the joint strength were analysed and the appropriate processing window was determined. According to the achieved results, the main benefits and drawbacks of each configuration are discussed.     

Investigation of sheet metal forming by bending—Part IV: Bending and wrinkling of circular sheets pressed by a hemispherical punch

This paper reports on an investigation of the bending forming processes in circular sheets pressed by a hemispherical punch into a conical die. Both theoretical and experimental considerations are provided. It is pointed out that the deformation mechanism of such a circular plate is different from that of the workpiece in the conical die cup test studied previously in part III of this series [Int. J. Mech. Sci.31, 327–333 (1989)]. The theoretical analysis shows that a central gap appears between the plate and the hemispherical punch at small deflections and disappears when the punch force increases beyond a certain magnitude. It is then shown that the previous approximate analytical treatment of the distribution of punch force is no longer appropriate to the wrinkling of the plate. It is shown that, as in the case of the conical die cup test, the circumferential wave number for the wrinkling mode corresponding to the critical wrinkling load of the present workpiece is four. In addition, the paper reveals that a doubling of the wrinkling wave may occur in the first stage of the forming process.     

复杂形状的中厚板件拉深成形动力显示有限元分析

将厚向异性屈服函数、单元级双面接触算法的能够反映强烈弯曲效应的M ind lin曲壳单元模型引入动力显式中心差分格式的有限元算法,并组入自主开发的板成形CAE商品化软件KMAS系统中,建立对复杂形状的中厚板件拉深成形有限元模拟分析算法;在此基础上,以轿车摇臂厚板金件为例分析了材料厚向异性参数r值、凹模圆角以及压边力大小对此类成形件成形性的影响。通过数值模拟与实际拉深成形实验的对比,验证了本文方法的有效性;同时,结合实验研究了此类单元对工具弯曲圆角大小的适应性。     

非规则曲面件的多夹钳式柔性拉伸成形研究

应用多夹钳式拉形机具有高柔性化的特点,根据目标件确定毛坯形状,夹钳依据毛坯形状对其进行夹持,尝试柔性拉形中的非矩形毛坯夹持。针对某一非规则曲面件,分别建立矩形毛坯和梯形毛坯的拉形有限元模型,并进行拉形数值模拟,对比分析两种形状毛坯对成形结果的影响。模拟结果表明,梯形毛坯拉形件比矩形毛坯拉形件的应力、应变数值减小且分布均匀;减薄率更小。梯形毛坯更能够节省材料,提高材料利用率。在多夹钳式拉形机上对矩形毛坯和梯形毛坯进行拉形试验,验证模拟结果的正确性。通过数值模拟和拉形试验充分证明了多夹钳式拉形机能够对非矩形毛坯件夹持的可行性,拉形效果更好,可以为其他非规则曲面件拉形提供参考和借鉴,使拉形工艺在工业应用的领域得到进一步拓展,为新型拉形机的设计与开发奠定了基础。     

Quality control on crimping of large diameter welding pipe

Crimping is used in production of large diameter submerged-arc welding pipes. Many researches are focused on crimping in certain manufacturing mode of welding pipe. The application scopes of research achievements become limited due to lack of uniformity in theoretical analysis. In order to propose a crimping prediction method in order to control forming quality, the theory model of crimping based on elastic-plastic mechanics is established. The main technical parameters are determined by theoretical analysis, including length of crimping, base radius of punch, terminal angle of punch, base radius of die, terminal angle of die and horizontal distance between punch and die. In addition, a method used to evaluate the forming quality is presented, which investigates the bending angle after springback, forming force, straight edge length and equivalent radius of curvature. In order to investigate the effects of technical parameters on forming quality, a two-dimensional finite element model is established by finite element software ABAQUS. The finite element model is verified in that its shapes error is less than 5% by comparable experiments, which shows that their geometric precision meets demand. The crimping characteristics is obtained, such as the distribution of stress and strain and the changes of forming force, and the relation curves of technical parameters on forming quality are given by simulation analysis. The sensitivity analysis indicates that the effects of length of crimping, technical parameters of punch on forming quality are significant. In particular, the data from simulation analysis are regressed by response surface method (RSM) to establish prediction model. The feasible technical parameters are obtained from the prediction model. This method presented provides a new thought used to design technical parameters of crimping forming and makes a basis for improving crimping forming quality.     

Hydrodynamic lubrication in hemispherical punch stretch forming—Modified theory and experimental validation

An inlet zone analysis is incorporated in a previously developed hydrodynamic lubrication theory of hemispherical punch stretch forming. Refined calculations of the lubricant film thickness, the hydrodynamic friction and the strains in the sheet metal are carried out. Several sheet materials and liquid lubricants are tested with the use of a punch stretch apparatus, and the strains in the sheet metal are measured. The comparison between the calculated and measured strain distributions confirms the validity of the refined lubrication model, which identifies a dimensionless parameter that characterizes the state of lubrication at the punch-sheet interface.     

汽车覆盖件模具凸型面拉延方法

针对汽车覆盖件这种具有复杂曲面特征的零件,在传统的全型面拉延成形方法基础上提出一种新的凸型面拉延成形方法。该方法根据拉延成形过程中凸、凹模工作型面上只有凸型面接触板料的特点,通过成形仿真获得全型面拉延的成形极限图(Forming limit diagram, FLD)和凸、凹模与板料的接触区域分布。然后将板料相对凸、凹模工作型面上的起皱区域及双面接触区域所对应的单元剔除,将单面接触区域中面积较小且形态狭长的单面接触区域合并转化为双面接触区域,同时将未转化的单面接触区域外周节点拟合成曲线后导入到CAD中对凸、凹模工作型面进行分割获得所需的凸型面。算例表明该方法在保证拉延成形质量的前提下可以有效地减小凸、凹模工作型面的面积,降低拉延模具的生产成本。     

薄板预扣合质量的研究

汽车覆盖件在扣合的过程中,出现的各种质量缺陷是需要解决的主要问题.在扣合过程中出现的质量缺陷主要包括零件边缘轮廓的缩进、卷曲、翘曲和回弹.通过有限元分析方法来模拟平面一曲线轮廓边缘薄板扣合工艺过程,研究薄板在预扣合程中的质量缺陷以及相应的机理.研究结果表明:采用越大的弯曲半径产生的回弹量远小于由此产生的对折边部分弯曲性...     

Three-dimensional effects of the bend–stretch forming of aluminum tubes

Bend–stretch forming is commonly used to shape extruded tubular aluminum parts for automotive applications. The tubes are pre-stretched, pressurized and bent over rigid dies. Tension prevents buckling of the compressed side and significantly reduces springback during unloading. An unwanted byproduct of the process is distortion of the cross section. Small amounts of pressure applied during forming can reduce this distortion. A systematic study of how to select the appropriate amounts of tension and pressure for accurate forming with minimal distortion has been conducted. The problem was first studied experimentally using a custom forming facility. An efficient 2-D model of the process was previously developed which was shown capable of capturing the main deformation features of interest. Its efficiency made this model a useful design tool for optimally selecting the forming parameters. In this paper, a 3-D finite element model of the forming process is used to simulate the complete forming process. By using a specially calibrated non-quadratic yield function, the model accurately reproduces all aspects of the process. The model is used to study 3-D features of the problem such as variations of distortion and springback along the length, the effect of friction, the lifting of a section of the tube around the mid-span off of the die, the effect of post-tension, and forming over a variable radius die.     

Effects of punch load for elliptical deep drawing product of automotive parts

Deep drawing process, one of sheet metal forming methods, is very useful in industrial field because of its efficiency. The production of optimal products using this process is dependent on the process variables such as blank shapes, profile radii of punch and die, and formability of materials. Of the variables, the blank shape is very important since it controls the formability factor. This paper reports the investigations on three kinds of blank shapes and the scribed circle test on three deformation modes. The punch load distribution for elliptical forming processes were measured under different conditions of profile radii of punch and die and discussed here. These experiments clarified the influence of the profile radii of the punch and die and the blank shape on the punch-load distribution for the elliptical deep drawing product of automobile parts. The aim of this study is to investigate the effect that the process variables have on drawability in a non-axisymmetric elliptical deep-drawing process and to obtain useful data from the industrial field.