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.     

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.     

Investigation of the Geometry of Rectangular Cross Section Aluminum Parts in Low Pressure Tube Hydroforming Process

Tube hydroforming process is used for producing thin integrated tubular parts with different cross sections. In this paper, production of aluminum tube parts with a rectangular cross section using low-pressure hydroforming process has been investigated numerically and experimentally. In order to achieve the lowest geometric defect of the part with minimum corner radius, the low-pressure hydroforming has been simulated and the effect of different loading curves on the deformation has been studied. The obtained numerical results indicate a good agreement with the experimental results. According to the results, the ratio of internal pressure variation to the movement of the upper die has a large effect on the cross-sectional profiles of the final part. The lowest corner radius also is formed when a high internal pressure applies on the internal surface of the tube before moving down the upper die.     

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.     

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

The main scope of the presented work is to demonstrate the potential of load optimized tubes with a varying thickness distribution in circumferential direction produced by roll forming. As initial material a so called Tailor Rolled Strip (TRS) sheet metal coil produced by Strip Profile Rolling (SPR) method was used instead of plain sheet. The TRS sheet metal is manufactured in a continuously working process by rolling one or more groves in transverse direction into the sheet metal coil. In this paper, the secondary forming of the TRS sheet metal to TRS tubes is investigated by means of FE‐simulations and roll forming experiments. To simulate the manufacturing process of the TRS tube by FEM, an integrated consideration of the process is necessary because of the large local strain hardening in the groves of the initial SPR sheet metal. In experimental roll forming operations welded tubes could be manufactured successfully. The geometrical and material properties of these tubes are analyzed. The reprocessing of TRS tubes by hydroforming is investigated by means of tube bursting tests. It has been found that an additional annealing process is necessary to achieve deformations in the grooved area during the hydroforming process.     

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.     

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.     

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.     

Precise Material Properties as a Prerequisite for FE‐Analysis of the Hydroforming of Tailored Welded Blanks

Tailored welded blanks are already used in the series production of deep drawn parts, and are now considered also for other forming technologies, like for example hydroforming. As new materials come into the market, also their combinations in the production of new tailored welded blanks increase, improving on the one hand the possibilities of matching material and functional requirements, but making the material characterisation more difficult on the other hand. For this reason new methods for a fast, precise and flexible characterisation of the mechanical and tribological properties of tailored welded blanks are needed. In particular the FE‐analysis, a powerful instrument for the definition and optimisation of forming processes, needs precise and reproducible material data in order to deliver accurate results. Therefore, when working with tailored welded blanks, it is crucial that the weld line is modelled using mechanical and tribological properties which refer to its structure and surface. This work summarises the methods used for the characterisation of the mechanical properties of the weld line at the chair of manufacturing technology in Erlangen and presents the results obtained for four different steel grades (FeP04, ZStE340, DP450 and TRIP800) and the tailored welded blanks obtained by their combinations. The accuracy of the results obtained in tensile and in friction tests were verified in simple experiments by comparing results from the tests with results from the finite element analysis. These were seen to be in very good accordance, confirming the accuracy of methods used in the material characterisation.     

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.     

高强钢先进成型技术和本构模型研究现状与发展趋势

高强钢是汽车车身轻量化的首选材料,但其成型问题一直是限制其推广应用的重要因素。对高强钢的先进成型技术,如液压成型、激光拼焊、热冲压成型等的原理、特点及最新研究进展进行了论述。同时,对先进高强钢的本构模型进行了阐述,以期对改进高强钢成型性能提供依据。     

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。      

Increase of Process Stability of Sheet Metal Hydroforming due to Feed Back Control

Sheet metal hydroforming of single sheets is a deep drawing process. In contrast to deep drawing with rigid tools, an active fluid medium performs the forming operation. The process is chiefly influenced by the control of the clamping force against the increasing fluid pressure. Although the optimum force‐pressure progression is close to the sealing line, it is initially unknown and dependent on different system parameters. To perform the process at the ideal curve, the system parameters need to be detected online and led back to the feed back control unit. An optical system is used for the detection of the sealing state. The detection system is a CCD‐camera system that allows for the continuous recording of occurring leakages. Acquired data permit the application of different strategies to control the forming process along the sealing line. This paper provides an overview of this developed technology, advantages and problems with regard to the process stability and performance, the parts quality, and recent results comparing control strategies.     

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.     

Effects of Hydro‐Impulse Forming of Blanks

This work presents the outcomes of investigations on dynamic effects and parameters of Hydro‐Impulse Forming (HIF) and their influence on the shaping process. Parameters for the blanks exposed to hydro‐impulse forming are defined by FE‐simulations, which use AUTODYN 2D‐3D software. These simulations enable a clear visualization of the processes that occur in the material and the working media. Results are presented for the simulated deep drawing of a semi‐sphere for the aluminium alloy A2024 and the steel QStE340. A larger influence of dynamic forces on the forming process was detected by numerical simulations on the “transmission medium ‐ die ‐ blank” scheme. These outcomes were confirmed by experimental investigations and a classification of the typical malformations during HIF (unusual in conventional forming processes). As shown by the performed experiments and numerical simulations, HIF offers great advantages in comparison with common forming processes, e.g. with regard to the creation of deeper reliefs and low residual springing.     

Effect of Isothermal Bainite Treatment on Microstructure and Mechanical Properties of Low‐Carbon TRIP Seamless Steel Tube

In this work, a low‐carbon transformation‐induced‐plasticity (TRIP) seamless steel tube (Fe–0.15C–1.34Si–1.45Mn–0.029Nb–0.024Ti, in wt%), having potential in application of hydroforming process, has been successfully manufactured by using piercing, cold‐drawing, and two‐stage heat‐treatment process. The optimal heat‐treatment conditions, inter‐critical annealing (IA), and isothermal bainite treatment (IBT) were firstly obtained to maximize the volume fraction and stability of the retained austenite (RA). The effects of temperature and holding time IBT on the microstructures of the TRIP steel tube were studied via optical microscopy (OM), scanning microscopy (SEM), transmission electron microscopy (TEM), and X‐ray diffractometer (XRD). The mechanical properties in the axial direction and hydroformability were also evaluated by conventional tensile test and flaring test, respectively. Two‐stage heat‐treatment was finally performed to achieve the required mechanical properties for the hydroformed tube. The results shows that the RA volume fraction increased at first and then decreased with the increase of IBT holding time and IBT temperature for a particular set of IA temperature and IA holding time. It was also demonstrated that high tensile strength of 618 MPa, total elongation of 35.5%, n‐value of 0.23, and better hydroformability could be successfully produced in this TRIP steel tube at IA temperature of 800°C, holding for 10 min, and IBT of 410°C for 4 min holding time.     

Innovative Technology for Preparation of Seamless Nitinol Tubes Using SHS Without Forming

This paper presents innovative technology for the production of seamless Ni-Ti tubes using self-propagating high-temperature synthesis (SHS). The proposed production technology is a unique method which removes the need of forming operations, reduces machining processes, and at the same time it eliminates the negatives of production Ni-Ti alloys by conventional melting methods. The proposed process consists in SHS reaction in evacuated silica tube with the use of extremely high heating rate (over 300 K min^?1).     

3D‐FE Modelling and Simulation of Multi‐way Loading Process for Multi‐ported Valves

Multi‐ported valves are widely used in the marine, sanitary, petrochemical and power industry. Multi‐way loading forming technology provides an efficient approach for integral forming of high strength multi‐ported valves, such as tee pipe coupling, high‐pressure cross valves, large‐scale complex valves, and so on. Since the multi‐way loading process is a very complicated plastic forming process due to the complexity of loading path, finite element numerical simulation is adopted to investigate the multi‐way loading process in order to predict and control the multi‐ported valve forming process. A reasonable model of the process is developed under DFEORM‐3D environment based on the coupled thermo‐mechanical finite element method. Then the reliability of the model is validated with respect to geometry development and forming defects. Numerical simulations of multi‐way loading forming for a tee valve and a cross valve have been carried out via using the developed model. Further, the forming processes of tee valve and cross valve have been compared. Moreover, the modelling method is also suitable for multi‐way loading processes of other complex components.