Tailored Profiles Made of Tailor Rolled Strips by Roll Forming – Part 1 of 2

To fully exploit the potential of sheet and profiles, various technologies to produce tailored blanks and profiles have been developed [1,2]. In earlier work [3] it has been shown that Strip Profile Rolling (SPR) can be used to produce metal strips with a predefined thickness distribution across the width of the strip. Building upon this knowledge, in two subsequent papers the extended goal of the project is presented. Therein is demonstrated that Strip Profile Rolling can be applied very effectively using a combination of roll forming (part 1) as well as further processing by roll forming (see part 2) to allow for the production of profiles with varying wall thickness in their cross section. To achieve the goal of part 1, a numerical model describing SPR was developed and used to study the influencing process parameters on spread and bulge formation. As a result of this parametric study, an optimized roll design and rolling sequence was developed to produce a demonstrator strip on a 12 stand roll forming mill manufactured by the company Dreistern [4]. Starting with a conventional strip out of DC01 steel (width 170 mm, thickness 2.5 mm), 29 rolling passes were necessary to achieve the desired geometry (width 186 mm, thickness 2.5 mm with a longitudinal groove being 64 mm wide where the thickness is reduced to 1.5 mm). In the second part of the process chain the coils produced by Strip Profile Rolling were successfully roll formed into a circular tube of 60 mm.     

Peter Groche

Tube hydroforming (THF) is a relatively new but established technology among metal tube forming processes. It is the technology of forming closed sections, hollow parts with different cross‐sections by applying an internal hydraulic pressure and sometimes additional axial compressive loads to force a tubular blank to conform to the shape of a given die cavity. Material properties have a significant influence on the process stability. Often roll‐formed, non‐heat treated tubular materials made of steel with longitudinally oriented welding lines are used in tube hydroforming. Different production processes involve a change of the material properties from the initial flat sheet to the hydroformable tube. Testing methods such as tensile tests and conventional forming limit diagrams do not accurately reflect the state of stress and strain conditions seen in the tubular blank during the hydroforming process. Thus, inaccuracies in FEA predictions and design failures occur. Test methods were developed to characterize the relevant geometrical and mechanical properties of tubular semi‐finished products.     

Development of a Robot‐Based Sheet Metal Forming Process

This paper describes a new sheet metal forming process for the production of sheet components for prototypes and small lot sizes. The generation of the shape is based on kinematics and is implemented by means of a new forming machine consisting of two industrial robots. Compared to conventional sheet metal forming machines, this newly developed forming process offers a high geometrical form flexibility, and comparatively small deformation forces enable high deformation degrees. The principle of the procedure is based on flexible shaping by means of a freely programmable path‐synchronous movement of the two robots. The final shape is produced by the incremental infeed of the forming tool in depth direction and its movement along the contour in lateral direction at each level of the depth direction. The supporting tool with its simple geometry is used to support the sheet metal and follows the forming tool at the rear side of the sheet metal. The sheet metal components manufactured in first attempts are of simple geometry like frustum and frustum of pyramids as well as spherical cups. Among other things the forming results are improved by an adjustment of the movement strategy, a variation of individual process parameters and geometric modifications of the tools. In addition to a measurement of the form deviations of the sheet with a Coordinate Measurement Machine, screened and deformed sheets are used for deformation analyses. Furthermore, the incremental forming process is analysed with assistance of the finite element method. In total the results show that a robot‐based sheet metal forming with kinematic shape generation is possible and leads to acceptable forming results. In order to be able to use the potential of this process, a goal‐oriented process design is as necessary as specific process knowledge. In order to achieve process stability and safety, the essential process parameters and the process boundaries have to be determined.     

Forming evaluation of complex tube hydroformed components

Abstract

Currently, the main feedstock of tubular blank material for tube hydroforming is derived from slit steel coil, which has been roll formed and electric resistance welded (ERW). From ERW tubular blank, a number of intermediate processing stages such as prebending and preforming may be necessary to achieve the final formed component. The end result is a component that may have experienced a complex strain path history. The traditional forming limit (strain) curve (FLC) is inherently affected by non-linear strain paths; hence, a suitable alternative forming limit criterion has been studied and is proposed for complex tube hydroforming processes. The criterion proposed is the forming limit stress curve (FLSC), which is theoretically independent of strain path and prestrain history. The methods used to generate the FLC and FLSC for tubular blanks are described, and prebending is used as an example to demonstrate how complex tubular component forming operations, and subsequent formability, may be evaluated through use of the FLSC.     

Roll Forming Strategies for Welded Tubes

Tubes are used as semi‐finished products as well as final components in almost all areas of the engineering industry. Roll forming of tubes with longitudinally oriented welding lines is one of the most efficient and economic tube production processes. However, numerous roll forming strategies already exist. Each strategy involves a characteristic change of the material properties from the initial slit strip to the final tube. A classification of the different roll forming strategies, which is given in this paper, aims to provide a systematic overview. A finite element analysis of the roll forming process is presented to identify specific forming loads and property changes.     

Process Modeling for Precision Forming of Discrete Parts – State of the Technology and Critical Issues

This paper presents an overview on the application of FE simulation as a virtual manufacturing tool in designing manufacturing processes for precision parts. The processes discussed include forging, sheet metal forming and hydroforming. Determination of reliable input parameters to simulate a process is a key element in successful application of process simulation for process design in all the mentioned areas. These issues are discussed in detail. Practical examples of application of FE simulation are presented for improvement of the existing metal forming process and/or designing new metal forming process for manufacturing discrete precision parts in forging, sheet metal forming and hydroforming.     

Investigation on the Hydroforming Characteristics of Double‐layered Tubes

Double‐layered tubes consist of an inner tube and an outer tube. Both are similar in material, contact closely and deform simultaneously when subjected to external force. Hydroforming assembly technology has several advantages in the manufacturing of double‐layered tubes. In this study, the hydroforming characteristics of double‐layered tube are investigated. Free bulging tests are performed to produce formability diagrams of double‐layered tubes at various forming pressures and feeding amounts. In addition, the hexagonal‐shape hydroforming test is performed to estimate the dimensional accuracy of double‐layered tubes through the corner filling ratio and the gap between the inner and outer tubes. Besides experimental analyses, an analytical model that can predict internal pressure for the hydroforming of double‐layered tubes is proposed and experimentally validated in this study.     

Design of hydroforming process for an automobile subframe by FEM and experiment

Tube hydroforming technology has shown the attention of the automotive industry due to its advantages over conventional stamping and welding methods.In this study,the tube hydroforming process including tube bending,preforming and hydroforming process for an automobile subframe is analyzed and designed by the simulation software AutoForm of a finite element method (FEM) program.A parametric study is carried out to obtain the effect of the forming parameters such as initial tube size and loading path on the forming results.The simulation results are also compared with experiment results.The research indicates that the multiple forming operation of the tube hydroforming process can be simulated accurately by using the implicit code AutoForm,and the formability of tube hydroforming can be improved by designing suitable forming parameters.     

Effects of Anisotropy in Drawing and Extrusion Processes of Bulk Metal Forming

The effects of anisotropy of axisymmetric materials (round bars, tubes) on metal forming processes are discussed. These effects are strongest for thin‐walled hollow materials in metal forming processes when the wall thickness is not predetermined by the die (tube drawing without mandrel, free extrusion of hollow components). Similarly to the normal anisotropy of sheet metal, a high radial anisotropy increases the resistance against a variation of wall thickness in tube drawing. There are also effects in forming solid materials such as forward extrusion of bars whereby the buckling of cross sections is influenced through the variation of radial anisotropy with the distance from the axis. The favourable anisotropy properties depend on the actual priorities. If, for example, for a metal forming process the material anisotropy results in high compressive stresses this may be favourable for increasing the ductility of the material whereas the increase of the load acting on the tool reduces tool life.     

方矩形管连续辊弯成型金属的流动规律

 为了研究方矩形管辊弯成型时金属的流动规律,结合某厂实际生产工艺,建立了方矩形管辊弯成型的三维弹塑性非线性有限元模型,基于该模型得到的仿真结果与现场轧制结果基本一致,通过对仿真结果的分析分别得到了方矩形管辊弯成型时长边、角部、短边和纵向上的内、中、外层的金属流动规律。研究结果表明：在厚度方向稳定段处,从外层到内层的金属流动规律是不一致的,长边处的不一致性要比短边处明显;在纵向上前、中、后段处,外层上的金属流动规律存在较大的差异,前段与后段的外、内层上的金属流动规律是相反的。分析结果为预测最终成型尺寸精度与辊弯成形工艺参数的制定提供了理论依据。     

方矩形管端部成型机制

 利用非线性有限元方法,结合实际生产中方矩形管辊压成形过程,建立了有限元仿真模型。基于该模型对7机架实际辊压成型过程进行了模拟运算,得到了成形过程中稳定段处节点位移矢量、应力和应变的分布情况。通过对仿真计算结果的分析,得到管头端面上在角部和边部的过渡区存在着“位移中性面”,在端面上角部和边部的中心处伴有明显的金属“外翻”和“内翻”的情况,仿真结果与现场轧制结果相符。     

Experimental Investigation on Hydromechanical Deep Drawing of Aluminum Alloy with Heated Media

Temperature controlled sheet hydroforming is known as the innovative processing of warm/hot sheet hydroforming. Cylindrical cup hydromechanical deep drawing (HMD) at elevated temperature is the typical process for basic research. Warm HMD process was carried out on a warm sheet hydroforming experiment platform to investigate the influences of key processing parameters on formability. The process window of successful forming versus liquid pressure was obtained, which was manifested as a shape of pyramid. The region of successful forming in warm/hot sheet hydroming is a father set of that in cold sheet hydroforming. The microstructure evolution of cups formed by using warm HMD under the effect of temperature was investigated. The grain growth was observed compared with cold HMD. The hardness of hydroformed cup was tested and no apparent reduction of hardness was detected.     

Controlled Material Flow in Hydroforming by Multipoint Drawing Technology

Sheet hydroforming, which is based on an active working medium, results in advantages over conventional forming techniques that make this technology interesting for the production of components with a large surface area. In order to expand the range of applications for this method, the current limits must be extended and the obstacles eliminated. One important aspect here is finding a solution to the conflict between a reliable tool sealing and a controlled material flow, particularly in the filling and preforming phases of the hydroforming process. One way of achieving progress in this area is to employ multipoint technology. In order to exploit multipoint cushion technology ‐ the potential of which has been proved in conventional deep‐drawing operations ‐ to extend the limits of sheet hydroforming, this technology has to be developed further, similarly to the multipoint cushion systems used in deep‐drawing, and adapted to the process‐specific conditions of sheet hydroforming.     

DP590/DP780高强钢管液压成形的性能

为对生产进行指导,研究了DP590/DP780高强钢焊管在液压成形过程中的变形行为;使用场发射扫描电镜观察管材周向的横截面以确定基体的组织,通过VMHT30M显微硬度计确定管材的焊缝及热影响区的大小,以便研究液压成形破裂行为;采用液压成形试验机对两种管件进行液压成形研究。实验结果表明:管材在胀形过程中的破裂压力比理论计算公式得到的破裂压力大,破裂位置全部位于靠近焊缝及热影响区的母材区域;随着管径的增大和长径比的增大,管材的极限膨胀率呈现下降趋势;在自由胀形过程中,管材的焊缝区域基本上不发生减薄,最小壁厚位于管材的热影响区和基体的过渡区域,并且壁厚的减薄率在胀形最高点所在截面最大,越靠近管材夹持区,壁厚的减薄率越小。最终得到以下结论:管材液压成形实验是准确获得管材力学性能参数的途径;提高焊接质量有助于控制失效破裂位置;合理选择管材的长径比有利于管材性能的充分发挥;通过合理控制各处的减薄有利于降低液压成形件的破裂风险。      

Induction Heat Treatment of Sheet‐Bulk Metal‐Formed Parts Assisted by Water–Air Spray Cooling

In order to produce components with massive secondary functional elements from sheet metal bulk forming operations, termed sheet‐bulk metal forming, can be applied. Owing to high, three‐dimensional stress and strain states present during sheet‐bulk metal forming, ductile damage occurs in the form of micro‐voids. Depending on the material flow properties, tensile residual stresses can also be present in the components' formed functional elements. During service, the components are subjected to cyclic loading via these functional elements, and tensile residual stresses exert an unfavorable influence on crack initiation and crack growth, and therefore on the fatigue life. Following the forming process, temperature and microstructurally related compressive residual stresses can be induced by local heat treating of the surface. These residual stresses can counteract potential crack initiation on the surface or in the subsurface regions. In the present study, the adjustability of the residual stress state is investigated using a workpiece manufactured by orbital cold‐forming, which possesses an accumulation of material in its edge region. Based on residual stress measurements in the workpiece's edge region using x‐ray diffractometry, it is possible to verify the compressive residual stresses adjusted by varying the cooling conditions.     

Investigation on a Deep Drawing and In‐Process Electromagnetic Calibration

Extending forming limits is one of the most important aims of research work in production engineering. One possibility to improve material formability is the application of high strain rates, which can be realized e.g. by means of electromagnetic forming (EMF). A further extension of the forming limits can be achieved by a beneficial combination of EMF and quasistatic forming operations, which allow exploiting the complementary advantages of the different technologies involved. This approach will be described on the basis of a deep drawing and inprocess electromagnetic sheet metal forming calibration in this paper. Thereby, the design as well as the subsequent analysis of the components as well as the combined process plays a distinctive role. Furthermore, the stages of development regarding the integrated tool coil will be presented and the resulting examples discussed. Finally, the setup of the integrated process as well as the feasibility will be shown on an exemplary semi‐industrial workpiece.     

Failure in internally pressurized bent tubes

The analysis and modeling of tube-hydroformed components is more complicated than that employed for sheet-metal panels, due to the lengthier process sequence and variable strain path—from flat-rolled sheet to tube; from straight tube to bent tube; and from bent tube to hydroformed component. These additional process steps make it difficult to determine whether post mortem analyses of tube failure during hydroforming can, and should, be conducted with the same tools and databases as used for simple stampings. To provide a partial answer, the properties of commercially fabricated welded straight tubes were evaluated using a free-expansion internal pressure test and compared with those of free-expansion internal pressure tests on bent tubes. The results demonstrated that the behavior of the bent tube was consistent with the mechanical properties of the as-received tube, provided due notice was accorded to the complex strain history of the bent tube. However, due to the strain-path changes occurring at the failure location, conventional approaches for monitoring strain history would yield (apparently) anomalous results.     

Numerical Simulation and Experimental Study on Interlock Deformation for Roll Formed U-Section Steel Piling

The interlock of a roll formed U-section sheet steel piling under loading was analyzed by means of numerical simulation, and meanwhile the tensile failure experiment was conducted. The results indicated that under the same load, the interlock corners of roll formed steel piling are not only the regions with the lowest safety factor, but also the regions with the highest stress; there are two slippages in the tensile instability process of interlock. Each slippage can be regarded as a failure, and different types of failure mode should be used to evaluate the performance of steel pilings according to different applications. Due to the work hardening effect during the roll forming process, the hardness of the interlock material increases by 16% compared with that of the original sheet steel. It was also found that the instability strength obtained in tensile failure test is only 15.6% of the tensile strength of the original sheet steel.