High Speed Forming 2004

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High Speed Forming of the Light-Weight Wrought Alloys
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Abouridouane, M.; El-Magd, E.
The deformation and fracture behaviour of the Al-alloy AA7075, Mg-alloy AZ80, and Tialloy Ti-6Al-4V were investigated in quasi-static and dynamic uniaxial compression and tension tests at strain rates in the range of 0.001 s^(-1) ≤ ε̇ ≤ 5000 s^(-1) and temperatures between 20°C and 500°C. Shear tests with hat shaped specimens of AZ80 were carried out by quasi-static and dynamic loading in the shear rate range of 0.01s^(-1) ≤ γ̇ ≤ 116000s^(-1) at a temperature of 20°C. For strain rates of ε̇ ≤ 10 s^(-1), the tests were carried out using a computer numerical controlled hydraulic testing machine. High strain rate experiments with ε̇ ≥ 1000 s^(-1) were performed on a Split Hopkinson Pressure Bar. Using the experimentally determined flow curves, the effect of strain rate and temperature on the compressive deformation at fracture was determined, showing that the forces required for forming as well as the limits of the possible deformation are controlled by strain rate und temperature. Under dynamic loading, both AA7075 and AZ80 show an increase of the deformation degree at fracture with increasing strain rate, whereas the Ti-6Al-4V shows a decrease of it. The investigated mechanical material behaviour (strain hardening, strain rate sensitivity, and thermal softening) and metallographic investigations of the deformed specimens in dynamic compression tests allow an explanation for character, formation, and evolution of damage in the deformed material. Constitutive material laws, whose parameters are determined from the experimental data, can be applied to describe the influence of strain rate and temperature on the mechanical material behaviour in compression, tension and shear tests. These material laws are to be implemented into the FE simulation, in order to determine the local state of stress and strain at time of the fracture. Through combination of experiment and simulation, a failure criterion for ductile fracture could be determined for AA7075 under quasi-static and dynamic tensile loading.
Determination of Material Characteristics using Electromagnetic Forming and Weak Coupled Finite Element Simulations
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Brosius, A.; Kleiner, M.
The aim of this approach is to determine material characteristics of aluminium alloys (in the present case: AA5747) at very high strain rates, more precisely the relationship between yield stress, plastic strain and strain rate is figured out. To achieve high strain rates up to 10^4 s^(-1) the electromagnetic forming process (EMF) is applied, where a pulsed magnetic field is used to form materials with a high electrical conductivity during a process time between 10µs - 50µs. The advantage that EMF is a non-contact forming process can be used to determine material characteristics without any influence of friction. Additionally, in contrast to other testing methods the assumption of a mean strain rate over the process time is not needed, because the evaluation is done by finite element simulations. To compute the associated flow curve array, where the strain rate is the third dimension, a method will be proposed combining an online measurement technique and iterative finite element simulations. During EMF of the tube specimen, the radial displacement of at least one significant point at the tube surface is measured online. These data are used as reference values for the iteration scheme. The iteration starts with the material data of a quasistatic tensile test. In order to minimise the deviations between online measurement and simulation result an automated data modification scheme is implemented. The kernel of this scheme consists of an optimisation algorithm and two finite element codes. The first one is used to compute the deformation process of the specimen in a conventional transient way. The second code is implemented to calculate the body force distribution by a harmonic electromagnetic analysis. These two codes are coupled in a weak staggered approach.
Stress-Strain Curves of Sheet Material in High-Rate Forming Processes
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Golovashchenko, S.; Mamutov, V.
Electromagnetic forming technologies are based on high-voltage discharge of capacitors through the conductive coil. Two methods of testing and the results of dynamic coefficient kd for aluminum alloys, copper, brass, steel, and some other materials are presented. The first method is based on expansion of rings machined from tubular blanks, which are designated for further stamping operations. The displacement of the ring was registered by using the light shading method. Parameters of the discharge electric current running through the electromagnetic coil were measured with a Rogowski gauge. The acceleration stage of the ring expansion process was used for more accurate calibration of the inductive gauge defining the parameters of electromagnetic pressure. Registering the kinematics of the ring during the inertial stage of the deformation process provided the information on dynamic behavior of the studied material. The second method employed in this paper for dynamic yield stress measurement was based on transverse pulsed loading of a sheet sample clamped by its ends. Shapes of the samples during their deformation were photographed using a high-speed camera. The specifics of the sheet sample deformation under the pulsed transverse load are the following: the sample has near-trapezoidal shape; the middle part of the sample has almost the same velocity ??0 through the whole process; the angle between two inclined parts of the sample and the horizontal middle area ?? has minor variation during the deformation process. In some cases, hybrid stamping processes including conventional forming on the press and final shape calibrating with pulsed forming technique require additional information about the influence of the static preliminary deformation of the sheet on dynamic yield stress. Experiments with different levels of material prestrain were conducted for this purpose.
A Review of the Techniques Available for Obtaining the Mechanical Properties of Materials at High Rates of Strain
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Field, J. E.; Proud, W. G.; Walley, S. M.
Material Behaviour at High Strain Rates
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Meyer, L. W.
An overview is given of high rate mechanical testing procedures and the material behaviour under tensile, compression, and shear loading as well as under biaxial loading states tension/compression with torsion resp. shear.
Effects of Electromagnetic and Hydraulic Forming Processes on the Microstructure of the Material
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Bach, F. W.; Kleiner, M.; Risch, D.; Walden, L.
Over the past few years, various papers have been published in the field of high speed forming processes. The focus was mainly on the technological aspects of metal forming, however. Therefore, the present contribution puts an emphasis on transmission electron microscopy analyses. The present research work describes the effects of the two forming processes upon the aluminum microstructure and their influence on the material properties. The objective is to characterise the micro processes determining the plastic deformation with both forming velocities the electromagnetic high speed forming process with strain rates of 10,000 s^(-1) and the bulge test, having deformation rates of less than 0.1 s^(-1) as a quasistatic process. In this article sheet metals out of technical pure aluminum 99.5% with a thickness of 1 mm were investigated. To this end, sample specimens were taken from manufactured workpieces along the radius at various distances from the center. Because of the similarity of the forming paths, two places on the specimens manufactured at different forming rates were evaluated and compared to each other: immediately next to the blankholder and from the area of maximum strain. Metallographic tests of the structures, the sheet thickness, and the micro hardness distribution of the initial state and the formed sheet metals were executed in advance.
Recent Enhancements to Determine Flow Stress Data in High Speed Compression Tests
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Kopp, R.; Rehrmann, T.
The accuracy in numerical simulation and physical models is increasing continually. For this reason, the attention to the measurement of flow stress fields at hot forming conditions rises presently. The determination of flow curves at small and middle strain rates up to 100/s has become a standard procedure. In the wide range between strain rates of 100/s up to 500/s no accurate experimental data of compression tests by the use of servo-hydraulic testing systems exists to date. In this context, the IBF investigated how compression tests especially on servo hydraulic testing-machines can be developed in order to expand the range of flow-curve fields clearly above 100/s with strains up to phi = 0.7. The maximum tool speed of the IBF testing machine is 3000 mm/s. This presentation shows the advances realised in this field. During the research activities, the existing servo-hydraulic high-speed testing machine has been optimised and new post processing techniques have been developed. Hence, valid flow stress values can now be determined up to strain rates of 300/s in compression tests on the utilised servo hydraulic testing system. Present investigations have the aim to determine absolutely reliably flow stress data even for strain rates up to 500 /s.
Numerical Simulation of Pulsed Electromagnetic Stamping Processes
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Bessonov, N.; Golovashchenko, S.
In earlier published papers simulation of electromagnetic forming (EMF) was often conducted assuming that pulsed electromagnetic load can be replaced by the pulse of mechanical force calculating its parameters similar to R-L-C electric circuit. However, in many practical cases, parameters of this circuit are variable during the process because of the displacement of the blank and from one operation to another due to the accumulation of heat in the coil. The distribution of electromagnetic forces is also non-uniform and may affect the quality of the part being stamped. In our opinion, the accuracy of the simulation of EMF can be significantly improved if the formulation of the problem includes Maxwell equations of the electromagnetic field propagation, equations of dynamic elastic-plastic deformation, and heat transfer equations all coupled together. In addition, this approach may provide knowledge of electromagnetic coil deformation, which was investigated earlier with significant simplifications. The complexity of the problem is defined by mutual dependence of all three physical processes (electromagnetic field propagation, dynamic elastic-plastic deformation, and heat transfer) and variable boundary conditions. The propagation of the electromagnetic field is defined by quasi-stationary Maxwell equations transformed in Lagrangian form. The dynamics of elastic-plastic deformation is modeled using the solid mechanics equation of motion, the modified theory of elastic plastic flow, and the Von Mises yield criterion. The energy conservation law is employed for the simulation of heat transfer, which is important to define the appropriate stamping rate without overheating the coil. The developed methodology is illustrated by 2D examples of cone formation from sheet using a flat coil and the conical die and 2D plane strain sheet formation by direct propagation of the electric current through the metal bar, serving as a coil, and through the deformed sheet.
Development of Multi Field Software Solutions and their Application for the Optimization of Electromagnetic High Speed Forming processes
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Blum, H.; Stiemer, M.; Suttmeier, F. T.; Svendsen, B.; Unger, J.
The simulation of complex processes in engineering solids involving coupled mechanical and non-mechanical fields represents a challenge to physicists, mathematicians, and engineers. Both, the formulation of such models and their numerical implementation involve a great number of difficulties. Electromagnetic forming is one example of such a process, whose modelling and simulation requires a coupled electromagnetic-thermomechanical model. The purpose of this contribution is to discuss some key issues associated with the modelling and simulation of electromagnetic metal forming (EF) and the corresponding development of a finite-element-based simulation tool for EF. In particular, the modelling is based on a thermodynamically-consistent electromagnetic-thermoelastoviscoplastic material and field model in which the energy and momentum balance are coupled to the quasi-static form of Maxwell s equations via the electromotive intensity and Lorentz force, respectively. On the algorithmic side, questions like the choice of meshes, the element formulation, the numerical treatment of nonlinearities, possible model simplifications, different discretisation strategies, realisation of the non-linear coupling etc. are discussed for the presented software solution. Such issues are investigated with the help of benchmark simulations that have been developed for this purpose. Finally, as an example of an application of the developed software tool, a computer aided manufacturing (CAM) problem is considered. Here, the size of the tool coil and the peak value of the current in the tool coil circuit are optimised in order to achieve the prescribed work-piece form within the given tolerance.
Validation of Different Approaches to Coupled Electrodynamic-Structural Mechanical Simulation of Electromagnetic Forming
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Blum, H.; Joswig, A.; Klocke, M.; Kulig, S.; Stiemer, M.; Suttmeier, F. T.
Electromagnetic forming (EF) is a high speed forming process in which strain rates of over 103 s^(-1) are achieved. The workpiece is deformed by the Lorentz force resulting from the interaction of a fast varying electro magnetic field with the eddy currents induced by the field in the workpiece. Within a research group (FOR 443) funded by the German Research Foundation (DFG) an object oriented simulation tool for this multi physical process has been developed (SOFAR), that can handle the fully coupled simulation in a single software environment. In this contribution, details of the algorithmic implementation of the electromagnetic side of the coupled model are discussed and validated. Basis of this validation are benchmark simulations developed for this purpose. In particular, the implementation of transient field computation for coupled problems within SOFAR is compared with an experienced FD-code (FELMEC) developed at the Institute of Electrical Machines, Drives and Power Electronics.
A new Finite Element Technology for the Numerical Simulation of High Speed Forming Processes
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Leppin, C.; Reese, S.
In this contribution we propose a new solid-shell element formulation based on the concept of reduced integration with hourglass stabilization. Due to the absence of shear locking thin structures can be computed with only one element layer over the thickness. This enhances the computational efficiency in two ways. First of all the number of elements is reduced. Secondly, working with an explicit scheme, a larger critical time step is obtained. The damping and the mass matrix are not affected by the element technological treatment. The formulation is validated at first by typical element examples as well as two forming simulations.
Influence of Different Material Models on the Result of Numerical High Speed Cutting Simulations
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Halle, T.; Meyer, L. W.
Extreme conditions for the workpiece and the tool can occur in high speed cutting processes. Temperatures above 1000 °C at very high strains over 3 and strain rates near 105 1/s are not unusual. In the first part of this paper an overview about the well known and new developed testing methods for these extreme conditions is given. For numerical simulations it is necessary to formulate closed material models which include strain, strain rate, and the temperature. In the second part some well known material models are presented and compared. Furthermore, advantages and disadvantages are named. The flow stress behaviour of two types of steel (1.1191, 1.2311) as a function of strain rate and temperature is presented. A Johnson-Cook and a Zerilli-Armstrong model is used for the comparative numerical simulations of an orthogonal cutting process. To indicate the process of chip segmentation, a damage model is often used. The influence of various damage models with different damage parameters and failure modes is shown. The calculated cutting forces and the shape of the chips are compared with results determined at a quickstop cutting device with integrated force measurement. Additionally, the calculated chip formation is compared with the measured shape by means of highspeed photography. The temperatures, forces, and chip shape for both used models are presented and the influence of different material models are evaluated and named.
Electro-magnetic Tooling for Metal Forming and Powder Compaction
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Kladas, A. G.; Koumoutsos, A. K.; Mamalis, A. G.; Manolakos, D. E.
The multipurpose FE Code ANSYS is employed to simulate an electro-magnetic forming process. A complicated compression coil with a ferromagnetic outer screen and a stepped field shaper is considered. Details on FE model building are thoroughly discussed. The calculated parameters are the magnetic flux density around the conductors as well as the Lorentz forces developed in both the field shaper and the workpiece. A simplified analysis of the workpiece deformation characteristics is also provided. An equivalent circuit method is employed in order to validate the results from the electro-magnetic FE model. Results from both analyses are in good agreement, denoting that the FE results are valid from an engineering point of view.
Direction Change of the Force Action upon Conductor under Frequency Variation of the Acting Magnetic Field
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Batygin, Yu.; Khimenko, l.; Lavinsky, V.
The present work is dedicated to the description of the thin-walled conductor attraction effect by the pulse magnetic field. This phenomenon was displayed experimentally. The effect pointed out relates to the direction of the pulse magnetic fields energy practical usage for the different technologies in manufacture. In the scientific literature this direction is known as the magnetic pulse metal forming. A hypothesis about the physical essence of the displayed phenomenon is suggested.
Improved Crimp-Joining of Aluminum Tubes onto Mandrels with Undulating Surfaces
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Daehn, Glenn S.; Eguia, Inaki; Zhang, Peihui
Over its history electromagnetic forming (EMF) has probably seen far and away more application in assembling tubes or rings onto (or into) nominally axisymmetric mating elements. The vast majority of these assemblies does not require any significant structural integrity or strength. However, a small fraction of these are designed and fabricated for mechanically-demanding applications. There are two key factors (which seem to be largely independent) that are key in the design and performance of a crimpedelectromagnetic tube joint. First is the state of residual stress that exists after the crimped joint is created. A natural interference fit seems to be a fairly general feature of EMF crimp joints. This interference gives a backlash-free joint that will not fret. The second key issue is the configuration of the joint. The fabrication of designed interlocking geometries is required to create a joint that maximizes mechanical strength while minimizing the electromagnetic energy and forces required to create it. Both of these issues will be considered here. Here we consider crimping onto textured surfaces such as screw threads and knurls. We show experimentally that approaches of this type can give joints that exceed the strength of the tube both in torsion and axial loading. Analysis methods based on coupling impact-indentation and break-before-strip criteria can be used to compare joints made in this way with those based on the more traditional large scale deformation of the tube. One of the advantages of forming onto 'textured' surfaces is that a number of small pulses (possibly generated by small and inexpensive capacitor banks) can be used to create a joint that has the strength of the parent tube, without any heat affected zones or distortion. Again, the natural interference fit developed by impact eliminates the potential for fretting.
Electromagnetic Compression as Preforming Operation for Tubular Hydroforming Parts
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Beerwald, C.; Homberg, W.; Kleiner, M.; Psyk, V.
With the aim to extent the forming limits of tube hydroforming a concept of using a previous electromagnetic compression operation will be introduced. One important limit for the possibilities of tube hydroforming is set by the initial circumference and the maximum tangential strain of the used material, whereby the initial circumference is typically determined by the smallest local circumference of the workpiece. The application of an appropriate contoured preform makes it possible to use tubes with a larger initial circumference. In the paper the investigation of the suitability of electromagnetic tube compression for the production of such a preform will be presented. The valuation is based on geometric criteria and material properties of the resulting preform which are strongly influenced by the process parameters. The discussed aspects are the roundness of the preform and the strain hardening of the material.
Non-thermal Laser Forming of Sheet Metal
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Schulze Niehoff, H.; Vollertsen, F.
In this paper the results of some preliminary experiments are presented on non-thermal microforming of thin metal sheets with laser induced optical breakdown shock waves. Three sheet metal forming processes are realized with this method. The most deeply investigated process is laser stretch-forming, since the influence of parameters like defocussing, power density, pulse energy, number of pulses, and material could be worked out. The results show that uniform shaped domes with a dome height over 250 µm with diameters of 1.4 mm could be produced. Additionally, first investigations on laser stamping and laser embossing have been carried out, but are not presented in this paper.
On the Significance of the Die Design for Electromagnetic Sheet Metal Forming
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Beerwald, C.; Brosius, A.; Kleiner, M,; Risch, D.
Electromagnetic Forming is a high speed forming process using a pulsed magnetic field to form metals with high electrical conductivity, such as copper or aluminium alloys. During the process, typical pressure peaks up to 200 MPa and velocities in the range of 300 m/s can be achieved. As significant process parameters the pressure maximum as well as the local and temporal varying pressure distribution have been identified. As of a certain drawing depth and distance between workpiece and tool coil, the pressure does not act any longer on the workpiece, but the deformation process is still driven by the inertia forces. It has been found out that the velocity distribution within the sheet metal during the forming stages as well as at the time of impact with a die significantly influences the forming result. Additionally, a special undesired effect is the rebound behaviour of flat workpiece areas being in contact with the die. To investigate the influence capability of the die concerning this effect, the parameters stiffness and damping properties have been varied by means of simulation using a mechanical substitute model.
Formability and Damage in Electromagnetically Formed AA5754 and AA6111
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Golovashchenko, S.; Imbert, J. M.; Winkler, S. L.; Worswick, M. J.
This paper presents the results of experiments carried out to determine the formability of AA5754 and AA6111 using electromagnetic forming (EMF), and the effect of the tool/sheet interaction on damage evolution and failure. The experiments consisted of forming 1mm sheets into conical dies of 40° and 45° side angle, using a spiral coil. The experiments showed that both alloys could successfully be formed into the 40?? die, with strains above the conventional forming limit diagram (FLD) of both alloys. Forming into the higher 45° cone resulted in failure for both materials. Metallographic analysis indicated that damage is suppressed during the forming process. Micrographs of the necked and fractured areas of the part show evidence that the materials do not fail in pure ductile fracture, but rather in what could be a combination of plastic collapse, ductile fracture and shear band fracture. The failure modes are different for each material; with the AA5754 parts failing by necking and fracture, with significant thinning at the fracture tip. The AA6111 exhibited a saw tooth pattern fractures, a crosshatch pattern of shear bands in the lower half of the part, and tears in the area close to the tip. Both areas showed evidence of shear fracture. This experimental study indicates that there is increased formability for AA5754 and AA6111 when these alloys are formed using EMF. A major factor in this increase in formability is the reduction in damage caused by the tool/sheet interaction.
Improved Formability by Control of Strain Distribution in Sheet Stamping Using Electromagnetic Impulses
(Institut für Umformtechnik - Technische Universität Dortmund, 2004) Daehn, G. S.; Shang, J.; Vohnout, V. J.
Stamping failures consist of, broadly speaking, either tearing (excessive local strain energy) or wrinkling (insufficient or inappropriate local strain energy). Good parts are produced when the strain energy or plastic work is effectively distributed during the forming process such that tears and wrinkles are eliminated. The process window framed by tearing and wrinkling limits can be rather small for some materials, notably aluminum alloys. At present, there are no established methods of directly controlling the forming energy distribution within the tool during a stamping operation. All current commercial methods attempt plastic strain control at the sheet boundary by various binder geometries and pressure profiles. While improvements by active control of draw beads and binder pressure have led to improved stamping performance, these methods still broadly rely on tool geometry to set the energy distribution. We have recently developed and demonstrated a method for more directly controlling the distribution of forming energy in a stamping operation based on an extension of electromagnetic (EM) impulse forming. We now have techniques for embedding and operating EM pulse actuator coils in stamping tools. These coils can be operated in a single high power pulse or as a series of lower energy pulses occurring several times during the forming stroke. A single high power pulse can provide the advantage of increased material forming limits of high velocity forming. However, applying a series of lower power pulses can increase forming limits without exposing the tooling and coil to large shock loads. Multiple pulses reduce the maximum strain levels by engaging more of the part material in the forming process which mimics (eliminates) the use of lubricants. Conventional production stamping rates are technically obtainable with proper integration of the EM impulse circuit with the forming press and tooling. This paper focuses on the basic design approach of our multiple pulse technique and integrated process forming results. Comparisons to other augmented stamping processes as well as conventional stamping are presented in terms of both simple metrics, such as draw depth and strain distributions.

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