Electromagnetic Forming and Joining for Automotive Applications

In this paper some options of how electromagnetic forming (EMF) can assist to expand the capabilities of conventional forming and joining technologies are discussed. Three different areas where EMF has the potential for a significant expansion of capabilities of traditional technologies are reviewed: 1) restrike operation to fill sharp corners of automotive panels; 2) low energy method of springback calibration; 3) joining of closed frames with an openable coil. Each of these applications was demonstrated in laboratory conditions and the description of the tooling is provided in the paper. Suggested design of a flat concentrator collecting induced electric currents from a flat coil was demonstrated for a corner filling operation and a springback calibration. An efficient technique of fabricating the flat coil from a flat plate by using water jetting technology enables a cost effective coil design, which can be reinforced by a system of non-conductive bars. The insulation of the coil is produced from the flat sheet of insulation material. Suggested design allows the coil to be repaired if a shortcut or fracture of insulation strips happens. A technology of low-energy calibration of stamped parts provides an option of working with a wider variety of materials including aluminum alloys, mild steels, and advanced high-strength steels. This technology is demonstrated for calibration of U-channels

Numerical Simulation of Pulsed Electromagnetic Stamping Processes

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

Stress-Strain Curves of Sheet Material in High-Rate Forming Processes

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

Design and Testing of Coils for Pulsed Electromagnetic Forming

Coil design influences the distribution of electromagnetic forces applied to both the blank and the coil. The required energy of the process is usually defined by deformation of the blank. However, the discharge also results in a significant amount of heat being generated and accumulating in the coil. Therefore, EMF process design involves working with three different problems: 1) propagation of an electromagnetic field through the coil-blank system and generation of pulsed electromagnetic pressure in specified areas, 2) high-rate deformation of the blank, and 3) heat accumulation and transfer through the coil with the cooling system. In the current work, propagation of an electromagnetic field in the coil, blank, die and surrounding air was defined using a consistent set of quasi stationary Maxwell equations applying a corresponding set of parameters for each media. Furthermore, a deformation of the blank driven by electromagnetic forces distributed through the volume of the blank was modeled using a solid mechanics equation of motion and the elastic plastic flow theory. During the discharge of capacitors the process was considered to be adiabatic due to the short duration of the pulse, so a heat transfer during the discharge time was neglected. The distribution of electric current density integrated during the discharge process defines the increase of temperature at every element of the coil. The distribution of temperature was calculated as a function of time using the energy conservation law

Analysis of Blank-Die Contact Interaction in Pulsed Forming Processes

During recent decade, significant efforts were dedicated to increasing the amount of Aluminum Alloys in automotive parts in order to reduce the net weight of cars. Processes of pulsed forming are known to expand the capabilities of traditional stamping operations. Propagation of pulsed electromagnetic field can be defined by quasi-stationary Maxwell equations, solved numerically using a non-orthogonal Lagrangian mesh. Suggested formulation included modelling of contact interaction of the blank with deformable die. Mild contact model based on introduction of acting-in-vicinity forces repelling the surfaces to be in contact was employed. It was tested by analyzing the elastic impact of bars and then was applied to the corner filling operation. This operation was analysed as a single pulse and as a multi pulse forming process. It indicated that some compromise between the blank formability enhancement and level of contact stresses on the die surface can be found. In addition, some examples of tubular parts pulsed press fitting using tube expansion with pulsed pressure were analyzed. Specific attention was paid to the analysis of factors playing important role in residual contact pressure between the exterior and interior tubes in pulsed press fitting operation

Contributing Factors to the Increased Formability Observed in Electromagnetically Formed Aluminum Alloy Sheet

This paper summarizes the results of an experimental and numerical program carried out to study the formability of aluminum alloy sheet formed using electromagnetic forming (EMF). Free-formed and conical samples of AA5754 aluminum alloy sheet were studied. The experiments showed significant increases in formability for the conical samples, but no significant increase for the free-formed parts. It was found that relatively little damage growth occurred and that the failure modes of the materials changed from those observed in quasi-static forming to those observed in high hydrostatic stress environments. Numerical simulations were performed using the explicit finite element code LS-DYNA with an analytical EM force distribution. The numerical models revealed that a complex stress state is generated when the sheet interacts with the tool, which is characterized by high hydrostatic stresses that create a stress state favourable to damage suppression increasing ductility. Shear stresses and strains are also produced at impact with the die which help the material achieve additional deformation. The predicted peak strain rates for the free formed parts were on the order of 1000 s^(-1) and for the conical parts the rates are on the order of 10,000 s^(-1). Although aluminum is typically considered to be strain-rate insensitive, the strain rates predicted could be playing a role in the increased formability. The predicted strain paths for the conical samples were highly non-linear. The results from this study indicate that there is an increase in formability for AA5754 when the alloy is formed into a die using EMF. This increase in formability is due to a combination of high hydrostatic stresses, shear stresses, high strain rates, and non-linear strain paths

Application of Electrohydraulic Forming for low volume and prototype parts

Electrohydraulic forming process enables forming of panels from Dual Phase steels in case the strain level required to fill the shape exceeds formability limit. Filling of the die cavity was conducted in nine discharges to allow for smoother materials flow from the flanges. Additional formability benefit was obtained by preforming operation which was based on bulging the areas of low strain adjacent to heavily stretched areas of the blank. Filling of all the radii was achieved during final higher energy discharges

Electrohydraulic Forming of Light Weight Automotive Panels

This paper describes the results of development of the electrohydraulic forming (EHF) process as a near-net shape automotive panel manufacturing technology. EHF is an electro-dynamic process based upon high-voltage discharge of capacitors between two electrodes positioned in a fluid-filled chamber. This process is extremely fast, uses lowercost single-sided tooling, and potentially derives significantly increased formability from many sheet metal materials due to the elevated strain rate. Major results obtained during this study include: developing numerical model of the EHF; demonstrating increased formability for high-strength materials and other technical benefits of using EHF; developing the electrode design suitable for high volume production conditions; understanding the limitations on loads on the die in pulsed forming conditions; developing an automated fully computer controlled and robust EHF cell; demonstration of electrohydraulic springback calibration and electrohydraulic trimming of stamped panels; full scale demonstration of a hybrid conventional and EHF forming process for automotive dash panel

Formability and Damage in Electromagnetically Formed AA5754 and AA6111

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

Design and Testing of Coils for Pulsed Electromagnetic Forming

Abstract Coil design influences the distribution of electromagnetic forces applied to both the blank and the coil. The required energy of the process is usually defined by deformation of the blank. However, the discharge also results in a significant amount of heat being generated and accumulating in the coil. Therefore, EMF process design involves working with three different problems: 1) propagation of an electromagnetic field through the coil-blank system and generation of pulsed electromagnetic pressure in specified areas, 2) high-rate deformation of the blank, and 3) heat accumulation and transfer through the coil with the cooling system. In the current work, propagation of an electromagnetic field in the coil, blank, die and surrounding air was defined using a consistent set of quasi stationary Maxwell equations applying a corresponding set of parameters for each media. Furthermore, a deformation of the blank driven by electromagnetic forces distributed through the volume of the blank was modeled using a solid mechanics equation of motion and the elastic plastic flow theory. During the discharge of capacitors the process was considered to be adiabatic due to the short duration of the pulse, so a heat transfer during the discharge time was neglected. The distribution of electric current density integrated during the discharge process defines the increase of temperature at every element of the coil. The distribution of temperature was calculated as a function of time using the energy conservation law