Reverse effect of tensile force on sidewall curl for materials with tensile/compressive strength difference

Sidewall curl occurring by the removal of tool surfaces after forming is one adverse phenomenon that should be effectively reduced in sheet metal forming operations. Among several process parameters controlling sidewall curl, a constraint tensile force is widely used along with attainable formability by introducing blank holder and drawbead. The classic but common knowledge is that sidewall curl is suppressed for conventional sheet metals as the constraint tensile force increases. Interestingly, however, for magnesium alloy sheets that have unusual asymmetry in tension and compression it has been recently reported that springback increases as the tensile force increases within a specific range of tension. The major deformation in the sidewall usually consists of bending and unbending under tensile force. Therefore, this unique stress-strain response of sheet materials with strength-differential, including magnesium alloys, should be considered for an accurate estimation of sidewall curl. In the present study, a semi-analytical bending/unbending theory incorporating characteristic constitutive behavior of magnesium alloys was developed to evaluate the moment-curvature relationship for various levels of constraint tensile forces. The present analysis proved that the reverse effect of constraint tensile force on sidewall curl was caused by the lower resistance to plastic yielding in compression with proper combination of applied tensile force.     

复杂截面铝合金型材拉弯成形有限元模拟

本文对矩形管截面铝合金型材拉弯成形工艺进行了研究 ,建立了位移控制拉弯工艺的有限元模型 ,采用ABAQUS软件对几种不同截面的铝合金型材拉弯成形过程进行了数值模拟 ,分析了不同补拉量对型材的截面畸变和回弹的影响以及不同截面形状抵抗拉弯成形时截面畸变的能力。其结果和方法对铝合金型材的拉弯成形工艺具有参考价值。     

Prediction of spring-back and side-wall curl in 2-D draw bending

Accurate prediction of spring-back is essential for the design of tools used in automotive sheet-stamping operations. The 2-D draw bending operation presents a complex form of spring-back occurring in sheet-metal forming since the sheet undergoes stretching, bending and unbending deformations. These three sets of deformation can create complex stress-strain states in the sheet which result in the formation of side-wall curls after the sheet is allowed to unload. Accurate prediction of the side-wall curl requires using finite-element shell models which can account for curvature and stress variation through the thickness caused by bending and unbending of sheet. Since such models are generally computationally intense, an alternative and efficient method of predicting side-wall curls is desirable. This paper describes a novel and robust method for predicting spring-back and side-wall curls in 2-D draw bending operations, using moment-curvature relationships derived for sheets undergoing plane-strain stretching, bending and unbending deformations. This model makes use of the membrane finite-element solution to calculate spring-back. The accuracy of the model is verified by comparison with finite element (ABAQUS) and experimental results.     

汽车门框上条制件拉弯成形模具设计简

汽车门框、窗框多由各种复杂截面型材弯曲成形后焊接而成，型材截面复杂，并且形状多为空间三维曲面，该类复杂截面型材的高精度弯曲成形是制造汽车门框和窗框的关键技术。通过分析某汽车门框上条制件的成形工艺特点，综合比较各种弯曲方式后选取了张臂式拉弯成形工艺，并设计了拉弯成形模具、切断机构、冲孔及辅助成形装置，利用该模具可以成形出符合制件质量要求的车门框上条工件。     

Numerical analysis of draw bending of load-adapted sheet metal profiles

In comparison to traditional roll forming processes, draw bending process represents a flexible and inexpensive production of open sheet metal profiles. Draw bending technologies allow the fabrication of load-adapted parts for applications in lightweight design. It enables the fabrication for predominantly bearing-like parts with adjustable cross sections along the longitudinal axis. This paper introduces the draw bending process with its functional principle and specifies the most important characteristics. It includes the key results of some research projects dealing with draw bending, which prove the applicability of this technique to produce load-adapted profiles. The fabrication of u-shaped profiles with various dimensions and adjusted cross sections as well as the fabrication of hat-shaped profiles with various widths of flange and adjusted longitudinal warping was investigated. The profiling is, in contrast to deep drawing processes, not bound to the form of the tools. The control of the draw bending process defines the contour of the profile during its production. Thereby an increased flexibility is achieved which is particularly useful for small series. Moreover numerical results are presented. The production of load adapted profiles and the buckling of the bottom area, as a result of a cross-sectional variation along the longitudinal axis are in the focus of the numerical and experimental investigations.     

Development of a new model for plane strain bending and springback analysis

A new mathematical model is presented for plane strain bending and springback analysis in sheet metal forming. This model combines effects associated with bending and stretching, considers stress and strain distributions and different thickness variations in the thickness direction, and takes force equilibrium into account. An elastic-plastic material model and Hill’s nonquadratic yield function are incorporated in the model. The model is used to obtain force, bending moment, and springback curvature. A typical two-dimensional draw bending part is divided into five regions along the strip, and the forces and moments acting on each region and the deformation history of each region are examined. Three different methods are applied to the two-dimensional draw bending problems: the first using the new model, the second using the new model but also including a kinematic directional hardening material model to consider the bending and unbending deformation in the wall, and the third using membrane theory plus bending strain. Results from these methods, including those from the recent benchmark program, are compared. University of Michigan, Dept. of Mechanical Engineering and Applied Mechanics, Ann Arbor, Mi 48109, USA.     

基于灰色系统理论的汽车横梁充液成形用坯料优化

类似汽车横梁这些复杂管类结构件的充液成形,是一个不均匀的变形过程,坯料的相对形状及尺寸,是影响金属流动的关键因素,从而对充液成形零件的最小壁厚、最大壁厚等评价对象有重要影响。以坯料的弯曲角度、弯曲半径、轴线高度差为因子,建立正交试验设计方案,进行有限元模拟,获得不同坯料尺寸下的最小壁厚、最大壁厚参考序列。利用灰色系统理论,计算各个参考序列与比较序列之间的关联系数,将多目标转化为以关联度为目标的单目标。进一步计算各坯料结构尺寸的平均关联度,获得优化的坯料尺寸组合参数,以此进行有限元模拟和生产实验。验证结果表明,该优化方法能够获得满足质量要求的充液成形的汽车横梁。     

Ductile fracture behavior of 5052 aluminum alloy sheet under cyclic plastic deformation at room temperature

Ductile fracture behavior of a 5052 aluminum alloy sheet undergoing cyclic plastic deformation is investigated in order to clarify the effect of cyclic plastic deformation on formability enhancement in incremental stretch sheet forming at room temperature. In the incremental forming, formability markedly increases owing to strain distribution and accumulation effects. The former effect is activated when the deformation region expands along tool paths. Thus, localization of deformation, which leads to necking or fracturing, can be prevented. On the other hand, local strain is accumulated without fracturing when a blank sheet is repeatedly subjected to out-of-plane deformation at the same position. In this paper, the effect of the strain accumulation due to cyclic deformation generated by bending and unbending is primarily focused on to discuss the effect on deformability. To apply cyclic plastic straining to the specimen, a cyclic stretch-bending test was adopted. A cyclic tensile test was also conducted for larger bending curvature. The experimental results show that cyclic bending–unbending affects the ductility of sheet metals. The fractography obtained by scanning electric microscopy also indicates that fewer and smaller voids are observed particularly on bending the inner side than on the outer side.     

薄壁管纯弯曲塑性成形分析及回弹计算

回弹是薄壁管数控弯曲常见的质量缺陷。文章基于薄壁管弯曲变形微体受力分析,建立了薄壁管圆周方向上力的平衡微分方程和切向应力在薄壁管横截面上的分布函数。由此可确定中性层的位置,求出外加弯矩,并用虚功原理解得回弹角度。论文针对不同几何参数的薄壁管进行了数控弯曲实验,分析了弯曲角度和相对弯曲半径对回弹角度的影响规律,并验证了理论计算结果与实验结果基本相符。利用该文介绍的计算方法可在实际弯曲之前计算出回弹补偿量,以提高管件弯曲成形精度。     

轿车转向节臂零件内高压成形工艺研究

研究了某型号轿车转向节臂零件的内高压成形工艺,并成功试制出样件。样件横截面主要为矩形空心截面,且沿轴线截面形状尺寸变化很大,因此需要采用管材通过预成形和终成形两个工序来完成。在预成形中,通过内部液压和轴向力的合理匹配,使最大膨胀量达到70％的预成形件;然后在终成形模具中进行压弯、合模和内高压成形,获得壁厚、外形尺寸均满足设计要求的最终零件形状。由于管材截面连续,无焊接边,因此整体刚度、强度均比冲压焊接件有所提高,并有助于提高疲劳强度。     

薄壁矩形管弯曲过程截面畸变的三维有限元分析

薄壁矩形管的弯曲成形是一个受材料性能和工艺参数等诸多因素交互作用的复杂成形过程,在弯曲过程中极易产生截面畸变等缺陷,导致成形质量难以达到要求。为此,文章提出了薄壁矩形管弯曲成形过程截面畸变的描述方法,并基于ABAQUS/Explicit软件平台,从截面形状变化、截面畸变量大小和弯曲过程中的能量变化等方面,研究了芯棒与管坯间隙和管坯与防皱块之间的摩擦对截面畸变的影响规律。该研究对薄壁矩形管弯曲过程工艺参数的选取,提供了理论依据。     

管材向外翻卷变形机理的研究

圆管在模具上受轴向压缩，翻转并非唯一的变形模式，还有如失稳、撕裂、扩口、卷曲等多种变形模式。本文应用能量观点分析了管材向外翻卷变形过程，提出了周向变形能增量控制管端翻卷变形的规律，并用实验加以验证，这为有效地控制翻管变形提供了理论依据。     

材料性能波动下高强TA18钛管绕弯过程截面畸变行为分析

为了研究材料性能参数对管材数控绕弯过程截面畸变的影响规律及敏感性大小,基于ABAQUS非线性有限元平台,建立了高强TA18钛管数控绕弯成形过程有限元模型,并通过文献中的实验结果验证了模型的可靠性,然后采用该模型模拟研究了材料性能波动下高强TA18钛管数控绕弯成形过程的截面畸变行为。结果表明:截面畸变率随着弹性模量、硬化指数的增加或强度系数、厚向异性指数的减小而减小,其减小率分别为11.76%,23.67%,12.07%和23.51%;不同材料性能参数波动下,高强TA18钛管绕弯过程截面畸变率的最大值均小于4.00%,并且在弯曲平面附近,截面畸变率出现负值;截面畸变率对材料性能参数的敏感性大小依次为强度系数、弹性模量、硬化指数和厚向异性指数。     

四辊卷板机弯卷过程中侧辊位移计算

四辊卷板机弯卷过程中,筒形零件的曲率半径主要决定于侧辊位移.实际生产过程中,侧辊位移的确定依据工人工作经验反复试凑,具有一定盲目性.本文以弹塑性力学基本理论为基础推导了四辊卷板机在弯卷过程中板材回弹半径公式.通过力学-几何关系分析提出了一种用于指导生产实践的侧辊位移计算方法.在此基础上,使用VC++语言编程实现整个计算...     

Local flange buckling and its relation to elastic springback in forming of aluminium extrusions

This paper presents a numerical and experimental investigation of interactions between local buckling and elastic springback in forming of thin-walled aluminium extrusions. The cross sectional distortions that are present in typical aluminium sections prior to elastic unloading cannot be disregarded. This is the case even when these beams are bent with relative large radii. The numerical analyses presented herein have shown that both element type and formulation have to be chosen carefully in order to correctly predict elastic recovery. Full integration tri-linear brick elements in combination with the assumed strain formulation have proven to be preferable to linear thick-shell, conventional full integration brick elements as well as reduced integration elements.     

飞机Z形型材滚弯回弹实验与研究

通过飞机Z形型材滚弯回弹的研究,分析了影响飞机Z形型材滚弯回弹的因素。实验证明:板料弯曲的平剖面假设理论、中性层理论对型材零件同样适应。研究结果表明:随着弯曲半径的增大,回弹半径也增大;回弹半径随型材厚度的增大而减小;屈服应力愈高、应变刚模量D值愈大,滚弯回弹愈大;自由弯曲回弹量大,校正弯曲回弹量小;形状复杂、相互牵扯多回弹量小;冷作硬化后回弹量大。     

热镀锌法提高B82 MnQL材料的抗拉强度研究

目的增强B82Mn QL材料的抗拉强度。方法以B82Mn QL盘条作为热镀锌的试验材料,利用拉伸试验机、反复弯曲试验机对其力学性能进行测试,借助金相显微镜、扫描电镜分析几种镀锌工艺下的横截面和轴向截面以及拉拔断口的组织形貌,同时测试显微硬度。结果 B82Mn QL盘条拉拔后钢丝的抗拉强度由原来的1250 MPa提高至1670 MPa。断面收缩率由32.5%下降至7%。采用新镀锌工艺和助镀配方,B82Mn QL钢丝镀锌后抗拉强度下降幅度为8%,低于传统的10%~15%。稳定化后B82Mn QL的扭转次数为20次左右,反复弯曲次数为6次左右,强度由1650 MPa提高至1710 MPa,硬度值比镀锌后有较大提高,由镀锌后的430HV升高至450HV,而且分布均匀。结论通过优化镀锌工艺,能大幅提高B82Mn QL盘条的抗拉强度。     

双孔模型材挤压过程的有限元分析

本文采用大变形弹塑性有限元理论 ,对双孔模型材挤压成形过程进行了有限元分析。模拟了双孔模非对称角铝型材挤压变形过程 ,获得了挤压变形时网格畸变、流速、应力和应变分布 ,展示了型材挤压件变形不均匀细节。并与单孔模型材挤压过程进行了比较 ,发现双孔模[1] 挤压时 ,变形体内存在两个互为相反方向的涡流场 ,它们相互抵消 ,从而可以消除型材挤压过程中产生的扭拧、波浪、弯曲等缺陷。为提高挤压件质量 ,优化设计多孔型材挤压模以及制定合理的工艺 ,提供了理论依据     

Prediction of frequency response function (FRF) of asymmetric tools from the analytical coupling of spindle and beam models of holder and tool

The prediction of chatter stability diagrams in milling requires accurate frequency response functions (FRF) at the tool – workpiece contact zone. Traditionally, the most accurate FRFs are best obtained through the experimental modal testing of each tool, which is costly. This paper presents analytical modeling and coupling procedures for spindle–holder–tool assemblies with asymmetric tools. Tools and holders are analytically modeled with continuous Timoshenko beams, while considering variation of the cross section geometry of the fluted sections and helix angle. While each solid part segments with varying geometry are assembled with rigid receptance coupling, the holder–spindle and tool–holder are coupled using contact stiffness and damping. The asymmetric cross sections of the helical end mills cause the variation of FRF as a function of the spindle's angular position. It is experimentally proven that the proposed method can predict the FRFs as the asymmetric tools rotate.