Simulation and experimental verification of flexible roll forming of steel sheets

Roll forming is a sheet metal forming process that has been used for decades. Usually roll-formed sections have a constant cross section. Flexible roll forming is a brand new forming process that produces parts with variable cross sections, in which the rollers translate back and forth in a direction that is perpendicular to the sheet feeding direction. Theoretical analysis gives an explanation of the plane strain state, compressive stresses, tensile stresses, and shear stresses in flexible roll forming. In order to analyze the mechanics and the deformation characteristics of flexible roll forming, the finite element method (FEM) model of a 17-step flexible roll forming process is established. The yield criterion used in the FEM simulation is Hill 48, and the parameters of which are solved with the yield stresses under different loading conditions and are firstly verified with a plane strain tensile test. The complicated roller paths are realized with data extracted from the computer-aided design (CAD) files with VC++ programs developed by the authors. We developed the first flexible roll forming prototype machine in China, with which the roll forming experiment of a side door beam is performed. Final shapes of the experimental and numerical results are compared. It is shown that the numerical results based on Hill 48 yield criterion that is solved with yield stresses agree well with the experimental results, which indicates that the simulation model can well reflect the real forming process. Detailed analysis of the distribution and history of plastic strain, longitudinal strain, shear strain, and thickness of both the constant cross section and the variable cross section is performed, which is of great help to understand this forming process.     

Numerical and experimental investigation of inward tube electromagnetic forming

In this work, experimental and numerical simulation of high-speed inward forming of tubes on a die in electromagnetic forming (EMF) system using a compression coil is presented. A 2D axi-symmetric electromagnetic model is used to calculate magnetic field and magnetic forces. Modified loose-coupled simulations of electromagnetic and structural aspects of EMF process are reported and emphasized in this paper. During the simulation, in each time step, the transient magnetic forces are obtained from the electromagnetic model and used as input load to the mechanical model. Based on the calculated deformation, in each step, the tube geometry in the electromagnetic model is updated to calculate the electromagnetic forces in proceeding step. Tube material, AA 6061-T6, is assumed to obey the Johnson-Cook (J-C) rate-dependent model. Displacement and thickness variations of workpieces along the tube length are presented and discussed experimentally and numerically. The results demonstrate that various workpiece zones could be thickened or thinned based on various process parameters. In addition, it is seen that the increase of discharge voltage decreases the thickness at die radius and reverses the thickening trend at tip of the bead.     

An experimental study of the effect of sheet thickness and grain size on limit-strains in biaxial stretching

Effects of grain size on the limiting strains attained in biaxial stretching have been investigated using 70% Cu-30% Zn brass sheets of 1.0 and 0.5 mm thickness with regular grain structures. The limit-strains were closely related to the ratio of sheet thickness, t_o, to average grain diameter, d_o, over a range of t_o/d_o between 2 and 30. In the earlier stages of stretching, growth in the amplitude of surface roughening was independent of t_o and close to being proportional to d_oε₃ where ε₃ is thickness strain in equibiaxial stretching. The diffuse instability strain was rather insensitive to the effects of the short-range strain inhomogeneities developed during such stable extension. Surface roughening became sensitive to t_o at an applied strain ε_3c which decreased with decreasing t_o/d_o. In stretching beyond ε_3c macroscopic strain localisation became established in the form of a network of surface grooves. It is concluded that the influence of grain anisotropy on limit-strains in biaxial stretching cannot be predicted in the terms of available continuum models of defect growth. An attempt is made to identify the dominant material and geometric factors which control the onset of localised necking when grain anisotropy is the principal source of inhomogeneous straining.     

Numerical and experimental approach to reduce bouncing effect in electromagnetic forming process using cushion plate

Electromagnetic forming (EMF) is a high strain rate forming process that uses Lorentz force. In this study, electromagnetic forming with a rectangular block shape in the center of the forming die was examined to determine the possibility and applicability of EMF. However, the high speed of the process in the absence of a medium between the coil and the workpiece results in bouncing of the workpiece, which may result in poor forming. So, in this study, the use of a cushion plate is proposed as a means of reducing the degree of bounce in an EMF process. A 3D electromagnetic numerical model using a spiral forming coil was considered. An RLC circuit, coupled with the spiral coil, was numerically simulated to determine the deformation behavior and design parameters, such as the input current and the magnetic forces. A cushion plate was used between the forming coil and the sheet to be deformed to reduce the extent of bounce. In the numerical simulation, the sheet was found to be well fitted to the objective die with the cushion plate. The simulation results showed that the extent of bounce was drastically reduced because of the velocity direction of the workpiece and the cushion plate. The experiment was performed using 24 kJ to deform Al 1100 with a thickness of 1.27 mm, based on the simulation results. The deformed sheet was well formed, and closely fitted the objective die with a minimum of wrinkling, relative to the results obtained without a cushion plate. As a result, an EMF process with a middle-block die was successfully established both numerically and experimentally to reduce the bouncing.     

Numerical simulation and experimental investigation of multistage incremental sheet forming

Multistage forming is usually adopted to form those parts which have steep angles or even vertical walls during incremental sheet forming (ISF) process. In order to study the multistage incremental forming further, based on a finite element method model which was experimentally verified, different forming strategies were adopted to form a frustum of cone with a wall angle of 30° to research the influence of the number of forming stages (n) and the incremental wall angle between the two adjacent stages (?α) on the formability of ISF. The simulation results including the thickness distribution, the equivalent plastic strain, and the magnitude of springback were analyzed in detail. It was found that with the growth of n, the minimum thickness increases largely, and more uniform thickness distribution is achieved, but the quantity of springback becomes larger in contrast with a single-pass process because of the accumulation of springback during each forming stage. Furthermore, an expression to figure out the appropriate value of n was given. In addition, the maximum thickness reduction decreases initially and then increases as the value of ?α grows. Meanwhile, it indicates that there is no relation between ?α and the quantity of springback.     

Numerical and experimental investigation of preform design in non-axisymmetric warm forming

This paper describes a numerical and experimental investigation of preform design in non-axisymmetric warm forming in order to achieve a large reduction in the volume of flash. A titanium TA2 watch-case-like component was used as an example. Sixteen different shapes of hollow preforms were specially designed for finite-element simulation. Their diameters of the center-holes were designed based on the values of the diameters of end-face configurations of punches and ejectors. The corresponding thicknesses of the preforms were determined by the assumption of constant volume. Under the same processing conditions, the diameter of center-hole of the preform should be the inner diameter of ejector such that its volume of flash was able to be greatly reduced to 6% of the volume of the formed component whereas the volume of flash was approximately 25% in the conventional implementation. Experiments were subsequently performed to verify the simulation results. This study rationally demonstrates the success of the preform design for warm forming of non-axisymmetric components, and provides great improvement in the utilization of material in the bulk forming process. Thus, the achievement is a tremendous saving in materials, more than a fourfold, particularly for these rare and expensive alloys.     

A study on thick plate forming using flexible forming process and its application to a simply curved plate

In this study, design and fabrication of a flexible forming machine are carried out for the purpose of manufacturing a prototype of curved plate block for hull structure used in shipbuilding industry. Flexible forming dies which consist of numbers of punches in an array form for upper and lower sides are designed in view of thick plate forming. A punch has formation of male and female screws to adjust its length with regard to a given surface, and all punches are supported by each other in punch housing. Software for process configuration and punch control are developed to operate the novel flexible forming machine. The software are composed of the punch height calculation part which uses an offset surface scheme. Prior to manufacturing of a prototype, numerical simulations for a saddle-typed thick plate forming process including metal forming and spring-back analyses are carried out to predict the forming performance. Experiments are also carried out to validate and confirm the feasibility of flexible forming technology in view of practical application of thick plate forming process. Curvature radii observed in the simulation and experiment are investigated and compared. Consequently, development and practical application of flexible forming technology to the thick plate forming process are described from design of the forming machine to manufacturing of the prototype. It is confirmed that the flexible forming technology suggested in this study has enough feasibility in new application of thick plate forming in shipbuilding structures which has been formed through expensive and laborious conventional line heating.     

Numerical simulations on reducing the unloading springback with multi-step multi-point forming technology

Multi-point forming (MPF) is an innovative flexible manufacturing technology for three-dimensional sheet metal forming. It replaces the conventional solid dies with a set of height-adjustable discrete punches, called the “punch group”. A set of punches can construct various three-dimensional curved surfaces freely and conveniently, through adjusting the relative position of each punch. MPF technology not only saves a significant amount of money and time in the design, manufacture, and adjusting of the dies but it can also be applied to change the deformation path and to improve the forming quality. Unloading springback is an inevitable phenomenon in sheet metal forming using MPF. To control and reduce springback, numerical simulations for the MPF process and the unloading springback are carried out using the explicit-implicit coupled finite element method. Subsequently, influencing factors such as thickness, deformation amount, and material properties of MPF springback are researched to investigate the MPF springback tendency. Next, the multi-step MPF technology is introduced to reduce MPF springback. Based on numerical simulation analysis, it is obviously validated that the unloading springback is decreased when the multi-step MPF method is applied. Finally, it is verified that the equidifferent deformation path and small deformation amount of each forming step can improve the workpiece stress state and minimize the unloading springback effectively by an evaluation result of the deformation path effect on the multi-step MPF.     

An experimental study of the in-plane stretching of sheet metal

Experimental results are presented of the limit strains for the in-plane stretching of steel, aluminium and brass sheets. The development of surface strains is studied in incremental tests and attention is drawn to “crazing” of the surface of sheet specimens and to “banding” in aluminium specimens. Despite the wide difference in the mechanical properties of the three materials the values for the limit strain are remarkably similar. For steel and aluminium the limit strains are greater than the predicted instability strains, but for brass the limit strains are considerably less than the predicted instability strains. The wide scatter in the experimental limit strains perhaps indicates that failure is largely influenced by random effects.     

Application of flexible forming process to hull structure forming

The conventional line heating method has been applied for the manufacture of various curved blocks used for hull structures in the ship-building industry. However, it has low economical efficiency and productivity because most of its processes are based entirely on the skillful experience of experts. In this study, a flexible forming process is proposed and utilized to substitute for the conventional manual process. In the proposed process, numbers of punches which have round shapes on the contact tip are adopted to configure an equivalent forming surface according to the arbitrarily curved objective surface. A simple punch height determination algorithm that considers the discrete surface and its spatial planar equations is applied to determine the discrete forming surface composed of the adjusted punches. Punch height data are transferred to a numerical simulation model using ANSYS parametric design language, and finite element analysis is conducted to check the formability of the process. Further, experimental investigations are carried out to verify the feasibility of the process using a flexible forming apparatus. Consequently, it is confirmed that a double-curved thick plate could be obtained by the flexible forming process.     

Finite element simulation of flexible roll forming with supplemented material data and the experimental verification

Flexible roll forming is a promising manufacturing method for the production of variable cross section products. Considering the large plastic strain in this forming process which is much larger than that of uniform deformation phase of uniaxial tensile test, the widely adopted method of simulating the forming processes with non-supplemented material data from uniaxial tensile test will certainly lead to large error. To reduce this error, the material data is supplemented based on three constitutive models. Then a finite element model of a six passes flexible roll forming process is established based on the supplemented material data and the original material data from the uniaxial tensile test. The flexible roll forming experiment of a B pillar reinforcing plate is carried out to verify the proposed method. Final cross section shapes of the experimental and the simulated results are compared. It is shown that the simulation calculated with supplemented material data based on Swift model agrees well with the experimental results, while the simulation based on original material data could not predict the actual deformation accurately. The results indicate that this material supplement method is reliable and indispensible, and the simulation model can well reflect the real metal forming process. Detailed analysis of the distribution and history of plastic strain at different positions are performed. A new material data supplement method is proposed to tackle the problem which is ignored in other roll forming simulations, and thus the forming process simulation accuracy can be greatly improved.     

Numerical and experimental study on the effect of vibration of the tool in ultrasonic assisted EDM

In this paper the effect of ultrasonic vibration of the tool in the process of electrical discharge machining is investigated. The ultrasonic vibration of the tool has significant effect on the dynamic behaviour of the vapour bubble generated between the tool and the workpiece due to the electrical discharge. The computational simulation of the bubble behaviour is carried out by employing the boundary integral equation method. Results show that when the electrical discharge occurs in the closest position of the tool to the workpiece, the vapour bubble expands to the largest maximum volume of the bubble and the lifetime of the bubble is the longest. This in turn makes the pressure inside the bubble decrease rapidly to the lowest magnitude and causes melted material at the sparked point vaporize and leave the crater on the surface of the workpiece.     

金属薄板激光弯曲成形的工艺参数研究

以不锈钢薄板的激光弯曲成形为研究对象,研究其成形的工艺过程及影响因素;介绍实验设备和激光调焦方法。通过实验研究了激光能量因素、扫描速度因素、激光光斑大小和扫描次数等因素对弯曲成形角度的影响。     

Numerical and experimental study of multi-contact on an elastic half-space

This paper deals with the numerical and experimental study of multiple contacts at the surface of an elastic half-space. A two-scale iterative method is proposed for solving the problem. First a procedure that takes into account interaction gives the contact forces at the tips of the asperities from which the pressure distribution at the contact interface is then calculated using an iterative scheme. Numerical results in the case of two and seven spherical indentors show that the method is as accurate as classical methods and very time efficient even for close proximity contacts. Additionally contact forces and pressures between a rubber block and several spherical indenters were measured. The differences between experiments and theoretical predictions were below 10%. This means that the proposed method can be a reliable tool to model contact problems for which such an accuracy is enough.     

尼龙6粉末选区激光烧结中能量累积的数值模拟与实验研究

选区激光烧结的逐行逐层的增材制造是一个能量的持续作用过程。对不同的线能量密度、不同预热温度的尼龙6粉末烧结过程进行建模和仿真,结合反映烧结样品热氧老化程度的黄蓝指数b~*值的测量,对烧结过程的能量变化进行了分析。结果表明:不同线能量密度的单层烧结,连续5行扫描烧结区能量增加约3.4%～7.82%,扫描能量较小时累积较为明显;预热温度从25℃增加到180℃时,烧结区能量的增幅为9.17%～5.66%;多层烧结中10层以下能量累积较快,增幅为11.66%,10～20层仅增大0.67%;虽然均表现出由于热平衡而使得在较高能量下的累积减缓,但是,实验烧结样品外观的黄蓝指数测量显示,对应b~*值从25℃的2.15增大到180℃的11.65;烧结4～20层时则由6.81上升到31.02,样品黄色程度在加深,表明烧结样品的热氧老化也越严重;故了解烧结过程的能量变化及相关工艺参数优化,将有助于提高烧结样品的综合质量。     

柔索驱动的玻璃幕墙清洗机器人研制及实验研究

玻璃幕墙由于保温防潮性能好、实用美观,在城市建筑物中被广泛采用.但是目前对于玻璃幕墙的清洗,还主要是采用人工吊篮作业,工人劳动强度大、危险性高、效率低.受蜘蛛在空中拉动吐丝实现移动的启发,研制了一种由2根柔索驱动的玻璃幕墙清洗机器人.介绍了机器人的基本结构组成,设计了采用蜗杆减速器实现自锁保护功能、多槽主从动辊轮增大绳索摩擦力的机器人驱动部分,由储水罐、控制阀、喷头、清洗刷组成的清洗部分,以及由旋翼实现的机器人墙壁吸附部分.进行了机器人的工作空间计算、机器人运动时柔索收放控制分析和旋翼工作时机器人的受力分析.最后基于双柔索驱动的玻璃幕墙清洗机器人模型样机,进行了沿给定轨迹移动的实验和清洗实验.实验证明机器人满足了设计功能,达到了满意的清洗效果,结果验证了该模型样机的有效性和实用性.     

Continuous flexible roll forming for three-dimensional surface part and the forming process control

Continuous flexible roll forming (CFRF) is an effective sheet metal-forming process used to manufacture three-dimensional surface parts. To obtain a final part with desired shape, the CFRF process should be systematically and accurately controlled in the longitudinal and transverse directions simultaneously. The major purpose of the present work is to establish a system of methods for controlling the continuous forming process. The target surface of desired part is represented by triangular mesh; the transverse control model for adjusting the shape of flexible roll is proposed based on the cubic B-spline curves; and the formulations to determine the downward displacement of upper roll is presented. Finally, the software for controlling CFRF process is developed, and the validity of the presented methods and the effectiveness of the developed software are demonstrated by CFRF test for a typical surface part.     

An experimental and numerical study of forming limits of AA5182-O

The International Journal of Advanced Manufacturing Technology - Forming limit curve (FLC) has been applied for evaluating the formability of sheet metal in a standardized way. Various experimental...