Stretch Bending of Z-section Stainless Steel Profile

The stretch bending properties of a new Z-section stainless steel profile were investigated by simulation.The causes of the forming defects,such as section distortions and poor contour precision,were analyzed,and the corresponding controlling methods were proposed.The results show that the main forming defects for the stretch bending of the Z-section profile were the flange sagging,the sidewall obliquing inward,the bottom surface upwarping,and the bad contour accuracy;the cross-section distortions were mainly induced by the shrinkage of the sidewall,which could be eliminated by increasing the sidewall height of the profile reasonably;the poor contour precision was mainly due to springback,which could be controlled by modifying the die surface based on the springback amount;for the investigated bending beam,the proper sidewall height compensation was 2mm,and the suitable die surface modification amount was 1.2times of the springback amount,when the elongation was 10% of the initial profile length.Stretch bending tests were conducted on a new type of die with adjustable bending surfaces,and high quality components were achieved,which verified the effectiveness of the defect controlling measures.     

Influence of constant and variable restraining force on springback of TRIP700 and stainless steel grades

Newly developed high strength steels (HSS) like dual phase (DP) and transformation induced plasticity (TRIP) grades as well as austenitic grades like AISI 304 and AISI 301 show superior strength compared to the microalloyed grades with the same formability (e.g. HSLA340). However, due to the multiphase microstructure and the austenite martensite transformation during forming, a higher springback will appear. In this paper the influence of different process parameters of six high strength steels on springback after stamping is investigated. The material properties were determined with uniaxial tensile tests. A modified Duncan Shabel test was used to draw U‐profiles with a controlled restraining force. To investigate different process parameters, constant restraining forces of 1 and 5kN and die radii of 3, 6, 9, and 12 mm were applied. In a second step, a shape set process was used. A constant restraining force of 1kN was used until a draw depth of 80 % was reached. Then the restraining was increased to 5.5 kN. The springback and the sidewall force were measured and analysed. An increased restraining force and a reduced die radius increases the sidewall force and reduces the springback. This resulted in a significant decrease in springback. The tests with variable restraining forces, also known as shape set process have shown that it combines a good formability with a reduced springback.     

ARefined Model of Three-roller Elastoplastic Asym-metrical Pre-bending of Plate

The geometry of plate after edge pre-bending mode is compared with that after roll-bending mode and the relationship among edge pre-bending angle, pre-bending edge length, and cylindrical desired radius is presented for a three-roller plate bender with bottom rollers adjustable horizontally. The analytical moment-curvature model and springhaek model for pure bending are established, assuming that the stress-strain relationship of material is linear, and the material is in plain strain and yields according to Mises yield criterion. The mathematical model for three- roller edge pre-bending of plate is developed considering the effect of pre-bending edge length, bottom roller radius, friction between plate and roller, etc. The plate tensile test and plate bending test are done and the numerical results agree well with the test data. The results are shown graphically and analyzed in the following aspects： （1） the error between numerical results and test data of top roiler force; （2） the influence of bottom roller radius, relative curva- ture, and bending arc length on springback angle; （3） the relationship between springhack ratio and edge pre-ben- ding angle.     

Numerical Solution of Thin‐walled Tube Bending Springback with Exponential Hardening Law

The process of manufacturing thin‐walled tubes which show exponential hardening is investigated. The analysis is based on the feedback analysis of bending springback tests. The springback angle is calculated using a formula which is derived from numerical methods. The experiments and finite element calculations prove that the formula agrees well with the test results. However, for tubes with strong hardening characteristics, certain discrepancies exist. The springback angle increases linearly with the ratio of plastic and elastic modulus, and decreases nonlinearly with increasing hardening index. The larger the ratio of plastic and elastic modulus, the greater the amount of reduction as the hardening index increases. The amount of increment in the springback angle incurred by the increase of the normalized bending radius is greater for smaller hardening index values. For thin‐walled tubes, after unloading, the elastic component takes a higher percentage in the total deformation as the relative wall thickness increases, causing the springback angle to increase slightly. However, when the growth rate of the cross section inertia moment is greater than that of the proportion of elastic deformation, the springback angle tends to decrease slightly as the normalized wall thickness increases. The formula will be applied to promote the technical development in springback prediction, control and compensation.     

CT20钛合金薄壁管材数控冷弯成形行为研究

薄壁管材的小弯曲半径数控弯曲成形十分困难,外侧壁厚减薄是弯管成形中的加工缺陷之一,对于钛合金薄壁管尤为严重。采用模拟与实验相结合的方法,对规格为58 mm×1.5 mm的CT20钛合金管材数控弯曲成形过程中弯曲段的壁厚减薄进行了研究,得到相对弯曲半径对壁厚减薄的影响规律。结果表明,CT20钛合金管材冷弯成形时的极限相对弯曲半径(R/D)为2。     

非线性组合硬化条件下镁合金板料变形回弹预测

变形回弹作为金属板料成形的主要缺陷之一,如何提高变应变路径条件下的回弹预测精度一直是研究者们面临的难题.本文针对镁合金变形特点,提出了同时考虑同向硬化、动态硬化和屈服圆畸变的本构模型.以0.8 mm厚AZ31B镁合金板料为研究对象,施加不同预拉伸后进行弯曲变形试验,观察了不同预变形对回弹规律的影响.同时结合有限元分析ABAQUS-Explicit (Vumat)和ABAQUS-Implicit (Umat)对板料的变形及回弹过程进行模拟仿真,对比试验与模拟结果,验证动态硬化对于镁合金板料变形回弹的重要影响.      

Shear Fracture of Advanced High Strength Steels

Failure experiments were carried out through a stretch-bending test system for advanced high strength steels, i.e. dual-phase （DP） steels and martensitic steels （MS）. The die radius in this system was designed from 1 to 15 mm to investigate the failure mode under different geometries. Two failure modes were observed during the ex- periments. As a result, critical relative radii （the ratio of inner bending radius R to sheet thickness t） for DP590 and DP780 steels were obtained. The stretch-bending tests of DP980 display some trends unlike DP590 and DP780 steels, and curve of DP980 in different thicknesses does not coincide well. High blank holder force exhibits more possibility of shear fracture tendency than low blank holder force. The unique character of high strength martensitic steel （1500MS） is that no shear fracture is found especially over small bending radius （R =2 mm） under the same experi- mental conditions. Microstructure analysis indicates that there are obviously elongated grains on shear fracture sur- face. It shows smaller diameter and shallower depth of the dimples than the necking failure.     

Experimental Analysis and Prediction of Springback in V-bending Process of High-Tensile Strength Steels

Experimental setup has been designed, to study the effects of thickness, width, bend angle and machine tool parameters on the springback, in two high-tensile strength steel grades, namely JSC440 and JSC590, during the V-bending process. Relationship between the springback and the parameters are analyzed using plots. Optimal combination of parameters for the minimum springback is evaluated. Analysis of variance has been carried out to analyze the magnitude of influence of these parameters on the springback. Using the experimental results, analytical models for the prediction of springback for the combinations of blank thickness, width, bend angle and machine tool parameters have been developed. Results reveal that in V-bending of JSC440, thickness and width are the dominant factors influencing the springback, whereas in JSC590 steel, insignificant change in springback is observed with the change in width of blank and using the hydraulic press with holding. However, thickness of steel sheet and bend angle influence significantly the springback in JSC590 steels.

     

Transverse Bending Characteristics in U-channel Forming of Tailor Rolled Blank

Research on the formability of tailor rolled blank (TRB)is of good practical significance and application value because of the enormous potential of TRB in the aspect of automobile lightweight.However,the forming of TRB is problematic because of the varying properties;especially,springback is a main challenge.The transverse bending (bending axis is perpendicular to the rolling direction)of TRB U-channel was studied through simulation and experiment.The forming characteristics of TRB U-channel during transverse bending were analyzed.The mecha-nisms of forming defects,including bending springback and thickness transition zone (TTZ)movement,were re-vealed.On this basis,effects of blank geometric parameters on springback and TTZ movement were discussed.The results indicate that springback and TTZ movement happen during transverse bending of TRB U-channel.Nonuni-form stress distribution is the most fundamental reason for the occurrence of springback of TRB during transverse bending.Annealing can eliminate nonuniform stress distribution,and thus diminish springback of TRB,especially springback on the thinner side.Therefore,springback of the whole TRB becomes more uniform.However,annealing can increase the TTZ movement.Blank thickness and TTZ position are the main factors affecting the formability of TRB U-channel during transverse bending.     

Robustness of the sheet metal springback cup test

The robustness of a proposed test for elastic springback characterization of sheet metal has been examined using a matrix of defined experimental errors. A series of flat bottom deep drawn cups made from AISI 1010 steel sheet were examined. It was found that misalignment of the blank over the forming tool and error in the vertical location where the springback ring was cut from the cup sidewall had the largest effect on the resulting springback opening. Other experimental errors involving cup height and ring width were found to be less important. The effect of in-plane anisotropy of mechanical properties on springback was negligible. The results are examined in terms of measured through thickness residual stresses and elastic bending of beams with circumferential thickness gradients.     

轧制差厚板纵向弯曲回弹规律分析

为了推动轧制差厚板在汽车梁结构件上的应用,以U型件为对象,研究了轧制差厚板的纵向弯曲回弹特性。首先完成了差厚板U型零件纵向弯曲成形数值模拟,分析了差厚板的回弹趋势,讨论了差厚板的应力分布,揭示了差厚板弯曲回弹规律,探讨了差厚板等效应力的影响因素,并通过试验对回弹仿真结果进行验证。结果表明,不均匀的应力分布是纵向弯曲的差厚板U型件沿弯曲轴方向上回弹不一致的根本原因,退火处理能够减小差厚板卸载前后的应力差,从而实现抑制差厚板回弹的作用。差厚板的板料尺寸、厚度、过渡区长度均会对差厚板的等效应力造成较大影响,从而改变差厚板的回弹大小及分布。另外,差厚板零件不同厚度部位的回弹相互牵制,使得各部分的回弹量趋向一致,从而导致差厚板的回弹量均介于薄、厚等厚板之间。     

Experimental Study on Springback of Sheet Metal Single Point Incremental Forming

Sheet metal single point incremental forming (SPIF) is a new technology for flexible process.The spring- back phenomenon in single point incremental forming has been discussed.Effects of forming angle and shape of the part are analysed using simple experimental method.Tool diameter, sheet thickness, step size, material parameters and the interaction of them are also analysed by using orthogonal test.The results show that the primary factor af- fecting springback is forming angle.In addition, springback is decreased when the specimen has a larger forming angle.The order of the four factors that influence springback is tool diameter, sheet thickness, step size and materi- al parameters.The forming precision will increase if springabck is decreased by optimizing the forming parameters.     

Forming properties and springback evaluation of copper beryllium sheets

Copper beryllium (CuBe) alloys possess excellent strength and conductivity. They have become the most important materials used for producing high reliability connectors and interconnections for electrical and electronic applications. As demand for high connection density in electrical and electronic products grows, springback behaviors become increasingly critical in fabricating these miniaturized contact components from sheet base materials. In the present article, a study of the springback behavior of CuBe sheets under different heat treatments is presented, with the goal of providing reliable information needed for fabricating more intricate connection parts. Both experimental and analytical techniques were adopted. The tensile tester was first used to study the springback related tensile properties. The governing tensile parameters on springback were identified, and their variations for sheets with different heat treatments were studied. It was found that a bilinear constitutive relationship can best characterize the stress strain behavior of the CuBe alloy. A closed form solution based on this bilinear relationship was formulated to predict the springback for the CuBe sheets at bending conditions. A V-shaped bend tester having an interchangeable punch to accommodate multiple radii was designed and built to evaluate the springback properties of CuBe sheets. A good correlation was found between the analytical predictions and experimental data. A parametric study, as an example, was also performed to provide the springback information needed for designing complicated connectors. formerly Director of R&D with NGK Metals Corp., Reading, PA 19612     

FEM supported optimization of bending processes for vibration damping steel sheets

An FE-model has been developed that allows large relative sheet displacement and sheet splitting as well, in order to obtain more information for the bending process of vibration damping steel sheets. In addition to the evaluation of the material data for the visco-elastic resin core the main process parameters for the die bending are optimized. Best results are realized by using higher forming temperatures in combination with a large die width and different flange length. The variation of the punch force and the punch radius can be used to regulate the spring back and the possible splitting of the sheets.     

大口径直缝焊管自由弯曲回弹试验与仿真分析

主要介绍方大特钢轧钢厂生产线的设备概况,以及在进行直角扁钢品种开发过程中碰到的问题和解决办法。     

轧制差厚板横向弯曲成形仿真与试验

为了抑制轧制差厚板在横向弯曲成形过程中的缺陷问题,采用仿真和试验方法研究轧制差厚板U型件的横向弯曲过程。分析差厚板U型件在横向弯曲过程中的成形特点,探讨弯曲回弹以及过渡区移动等缺陷的发生机理。结果表明,横向弯曲的差厚板U型件在成形过程中除了会产生回弹缺陷外,还会发生厚度过渡区移动;差厚板薄、厚板侧的厚度以及性能差异是差厚板产生缺陷的根本原因;采用退火处理能够减小差厚板的回弹量,但是会导致过渡区移动量增大;板料尺寸越大,回弹量与过渡区移动量也随之增大。     

Spline‐Interpolation and Calculation of Machine Parameters for the Three‐Roll‐Pushbending of Spline‐Contours

Today, bending tasks become more and more complex. Not even constant bending radii are required in the industrial practice. There is a growing demand for bending spline‐contours, too. Such geometries are often produced with Freeform‐Bending procedures like Three‐Roll‐Pushbending. This paper presents a method to interpolate a given spline bending‐contour (by CAD data), in order to calculate its radii distribution, which is needed to determine the machine parameters in certain points for the Three‐Roll‐Pushbending. For the determination of the machine parameters one has to consider the different influences on the bending process. The material springback and the deflection of the bending machine per radius need to be compensated to reach a near net shape bending result. Nevertheless deviations cannot be avoided. To improve the results, a possibility to adjust the pre‐calculated machine parameters is shown. For the investigations tube profiles with constant wall thicknesses were considered. The corresponding plasticity calculations refer to tube cross‐sections. The results were validated by bending a representative spline contour on the bending machine of the Chair of Forming Technology at the University of Siegen.     

钢管连轧过程中机架孔型参数优化

为了优化某限动芯棒连轧管机的孔型参数,利用正交试验参数优化方法,结合钢管热连轧有限元模拟分析,研究轧辊孔型参数(脱离角、脱离比、过渡圆角半径、辊缝值)对轧制所得钢管的尺寸精度和轧制过程中力能参数的影响,分析各参数影响的显著性,确定各因素的优化组合.结果表明:辊缝值对外径椭圆度、壁厚不均度和轧制力矩的影响最为显著,脱离角的影响居于次位,而脱离比对外径椭圆度、轧制力、轧制力矩的影响最小.根据影响规律获得较优参数组合:脱离角35°、脱离比2.25、过渡圆角半径5mm、辊缝值35 mm.对比仿真结果,优化后的钢管尺寸精度较优化前有明显提高.     

Study of Kinetics of Mill Scale Reduction: For PM Applications

In the deep drawing process, material of blank is transformed into the desired complicated shape by a punch. In this paper, a technique for increasing the drawability of AA1200 aluminium alloy cylindrical cups has been developed. Effects of die and punch geometry including die and punch fillet radius, on limiting drawing ratio (LDR), drawing load with respect to punch stroke and strain of the cup wall have been investigated numerically for optimal process design. A commercial finite element simulation package, ANSYS 14.0, is used in order to determine the optimum limiting drawing ratio. An experimental setup is built accordingly with a half cone angle of 18°. In the experimental and finite element analysis, AA 1200 alloy sheets are used. The effects of the original blank thickness (t = 2 mm) on the various LDR and punch load are numerically investigated. The present process successfully produces cylindrical cups with drawing ratio of 2.64.     

Air Bending Process for Load Optimised Profiles

In this paper the air bending of Tailor Rolled Blanks (TRB) to load optimised profiles is investigated. A new flexible modular tool system has been developed in order to locally adapt the die height to the spring back which is more pronounced in thinner areas in comparison to thicker areas of the TRB. With the new tool system the dies can be modulated according to the thickness distribution of the Tailor Rolled Blank, whereas the amount of die lifting depends on the thickness difference and the corresponding variance of mechanical properties. In bending tests with several specimens having a constant thickness a linear relation between the increasing part angle and a stepwise increased die lifting was observed. With this linear dependency the amount of die lifting can be calculated and thus applied on bending of TRB. Special consideration is required for the die adjustment in the area of the linear thickness transition, therefore three different die arrangements have been investigated. The results of different bending tests are compared and evaluated in order to apply the new technique to form a new car body floor structure in profile intensive construction. By using load optimised profiles in combination with a steel 22MnB5 which has been heat‐treated at defined sections the survival space of a car passenger in a side pole impact has been enormously increased by more than 40 %.