Electrohydraulic Forming Process Research Articles

Analysis and Optimization of the Electrohydraulic Forming Process of Sinusoidal Corrugation Tubes

Analysis and Optimization of the Electrohydraulic Forming Process of Sinusoidal Corrugation Tubes

Experimental investigation of the pulse duration on the efficiency and electrode wear of electrohydraulic forming process

Electrohydraulic Forming (EHF) is a high velocity sheet metal forming technique using high-energy electrical discharges in a water chamber to shape complex parts. Despite its potential benefits, the EHF process is not well understood, and further research is needed to comprehend the underlying physical mechanisms and optimize the process parameters. One of the critical factors is the control of energy discharge within the forming chamber in terms of arc duration. The proposed study aimed to analyze its impact on the characteristics of electrohydraulic forming focusing on the efficiency of the operation and electrode wear. As a result, the height of the formed parts was compared and used to evaluate the efficiency of the operation. The electrode wear was estimated by a phenomenological model. By comprehending the relationships between these factors and operational efficiency, a tradeoff was established to enhance the EHF process, taking into account electrode wear, thereby expanding its industrial applications.

Comparative Analysis of Electrohydraulic and Electromagnetic Sheet Metal Forming against the Background of the Application as an Incremental Processing Technology

High-speed forming processes such as electromagnetic forming (EMF) and electrohydraulic forming (EHF) have a high potential for producing lightweight components with complex geometries, but the forming zone is usually limited to a small size for equipment-related reasons. Incremental strategies overcome this limit by using a sequence of local deformations to form larger component areas gradually. Hence, the technological potential of high-speed forming can be exploited for large-area components too. The target-oriented process design of such incremental forming operations requires a deep understanding of the underlying electromagnetic and electrohydraulic forming processes. This article therefore analyzes and compares the influence of fundamental process parameters on the acting loads, the resulting course of deformation, and the forming result for both technologies via experimental and numerical investigations. Specifically, it is shown that for the EHF process considered, the electrode distance and the discharge energy have a significant influence on the resulting forming depth. In the EHF process, the largest forming depth is achieved directly below the electrodes, while the pressure distribution in the EMF depends on the fieldshaper used. The energy requirement for the EHF process is comparatively low, while significantly higher forming speeds are achieved with the EMF process.

Influence of scaleboard on the dispersion deformation behavior of DP600 steel sheets during electrohydraulic forming

The action mechanism of scaleboard on the dispersion deformation behavior of DP600 steel sheets during electrohydraulic forming (EHF) was explored by means of experiments and numerical simulation. The results showed that a maximum major strain of 60% was achieved during the pure EHF process, which was improved by 70% compared with that during the quasi-static condition, but the scaleboard played a negative role and decreased the hyperplasticity effect of EHF by 20%. Under the action of the energy absorption of the scaleboard and its interaction with the tested specimen, the utilization rate of critical discharge energy for crack bifurcation and dispersion deformation was decreased by 55.3% and 26.8%, respectively. Under the instantaneous impact of scaleboard, an additional tensile stress was generated on the central element of the tested specimen, which destroyed the effect of inertial coordination deformation on the elements near failure and decreased the proportion of inertial dispersion deformation by 48.6%. Due to the mechanical impact action of scaleboard in the biaxial tensile stress state and the accompanying bidirectional friction effect, the failed elements changed from the uniaxial tensile stress state to the biaxial tensile stress state gradually, while the central element underwent a fluctuating transition of uniaxial-biaxial-uniaxial tensile stress state. Besides, the curling effect of scaleboard induced the edge elements along the width direction of the tested specimens to change from the uniaxial tensile stress state to the biaxial tensile stress state, which aggravated the uneven deformation along the width direction obviously. In summary, the pure EHF with profiled blank holder is more suitable for industrial application and production than that with scaleboard.

Plasticity enhancement mechanism of DP600 steel sheets during uniaxial quasi-static/dynamic forming

The main goals of this work are to explore the macro- and micro- mechanisms of plasticity enhancement of DP600 steel sheets during the hybrid uniaxial tension consisting of quasi-static forming and then dynamic electrohydraulic forming (EHF). The experimental results showed that the room-temperature plasticity of DP600 sheets during the hybrid forming process was improved significantly: the limit strain was improved by 35 %, and the total percentage elongation was improved by 25 % in contrast to that during the quasi-static forming process. Then, the mechanical behavior, fractography and microstructure evolution of the specimens were analyzed. The quasi-static pre-deformation led to the change of the material constitutive of DP600 steel at high strain rate, improved the formability in the subsequent dynamic step and resulted in an excellent final plasticity. The inertial effect in the dynamic EHF process and the additional stress increment Δσθ caused by quasi-static pre-deformation reduced the velocity gradient of the mass particles along the tensile direction after starting plastic instability, thereby coordinating the subsequent deformation and enhancing the final plasticity of the hybrid uniaxial tension specimens. Under high strain rate, the nucleation from the existing dislocations caused higher dislocation density in the ferrite phase, which offset the easy stacking, winding, and uneven distribution of the dislocations caused by quasi-static pre-deformation. Besides, typical [0 1−1]MT deformation twins presenting in the martensite phase, enhanced the plastic compatibility of the ferrite and martensite phases and contributed to the plasticity improvement.

Combined electrohydraulic and flexible pin die forming: a novel high strain rate forming die setup

Electrohydraulic forming (EHF) is one of the high-velocity metal forming processes that can significantly increase the formability of metals compared with quasi-static forming processes. On the other way, multi-point forming (MPF) is one of the flexible forming methods that provides different sheet metal geometries by varying the height of the pins. The purpose of this study is to take the advantages of both the EHF and MPF processes by presenting a design for flexible dies to be used in the electrohydraulic forming process. As the first step, electrohydraulic free-forming was performed to investigate the reproducibility and to obtain some of the parameters needed for simulations. Then, the ABAQUS finite element software and Coupled Eulerian-Lagrangian method were used to simulate this process. Afterward, the experimental tests were undergone to determine the defects of the forming process using a flexible pin die. Various geometries were produced by conducting experiments and inserting the proper elastic layer to eliminate the dimpling. The reproducibility and validity of the flexible pin die forming simulation was investigated by analyzing the dome height and final profile of the specimens. After ensuring the accuracy of the simulation, a thicker elastic layer was used to remove the dimpling defect completely. The results of the experiments and simulations illustrated that the use of flexible pin die is possible in the process of EHF and the proper final forms can be obtained by applying the appropriate thickness of the polyurethane layer.

Formability and deformation behavior of DP600 steel sheets during a hybrid quasi-static/dynamic forming process

The purpose of this work was to investigate the formability and deformation behavior of DP600 steel sheets during a hybrid forming process composed of quasi-static forming and dynamic high strain rate electrohydraulic forming (EHF) sequence. The results showed that the ultimate limit major strains under the uniaxial tensile strain path were 40–47%, and those under plane strain and biaxial tensile strain paths were 28–31% and 43–57%, respectively. Compared with quasi-static data, the hybrid data under these three strain paths were improved by 30%, 70%, and 36%, respectively, demonstrating a hyperplasticity characteristic. The quasi-static prestrain had little effect on the ultimate formability, and the hyperplasticity effect of the quasi-statically prestrained specimens mainly depended on the inertial effect and the change in constitutive behavior in the high strain rate EHF loading step. Furthermore, the optimized Johnson-Cook constitutive model of the quasi-statically prestrained sheet during high strain rate condition was obtained by means of dynamic Hopkinson tensile tests, and the numerical simulation results based on this material constitutive were in good agreement with that obtained by experiments, showing an error of less than 5%. Then, the numerical simulation reproduced the propagation and evolution process of the pressure wave from ellipsoid to sphere, and quantified the dynamic deformation behavior of the quasi-statically prestrained specimen during an EHF process. The maximum velocity and strain rate of the prestrained specimen during EHF step were 270 m/s and 3700/s, respectively, and the ultimate height of the deformed specimen was 43 mm.

Numerical Estimation of Material Properties in the Electrohydraulic Forming Process Based on a Kriging Surrogate Model

High-speed forming processes, such as electrohydraulic forming, have recently attracted attention with the development of forming technology. However, because the high-speed operation (above 100 m/s) raises safety concerns, most experiments are conducted in a closed die, which hides the forming process. Therefore, the experimental process can only be observed in a numerical simulation with accurate material properties. The conventional quasistatic material properties are improper for high-speed forming simulations with high strain rates (>102 s−1). In this study, the material properties of Al 6061-T6, which reflect the deformation behavior in the high-strain-rate region, were investigated in a numerical approach based on a reduced order model and a surrogate model in which the numerical results resemble the experimental results. The strain rate effect on the material was determined by the Cowper–Symonds constitutive equation, and two strain rate parameters were predicted. The surrogate model takes two material parameters as inputs and outputs a weighting coefficient calculated by the reduced order model. The surrogate model is based on the Kriging method to reduce the simulation cost. Next, the optimal material parameters that minimize the error between the surrogate model and the experiments are estimated by nonlinear least-squares optimization using a genetic algorithm and the constructed surrogate model. The predicted optimal parameters were verified by comparing the results of the experiment, numerical simulation, and surrogate model.

Hyperplasticity mechanism in DP600 sheets during electrohydraulic free forming

Abstract This work discussed the multi-scale mechanisms of hyperplasticity during the high strain rate electrohydraulic forming (EHF) process by exploring the formability of DP600 sheets in a state of uniaxial tensile stress. The experimental results showed that the limit strains and limit dome heights of the deformed specimens obtained by EHF were improved by 15 %–27 % and 22.54 %, respectively, as compared with quasi-static specimens, showing a hyperplasticity characteristic. The inertial effects that occurred during the EHF process were responsible for the macro-scale enhancement in terms of formability, which could generate an additional principal stress along the direction of stretching that slowed the velocity gradient of the necked elements to restrain uneven deformation, resulting in a 60 % broadening of the action zone of maximum Y-displacement. The proportion of inertial effects that contributed to the plastic deformation of the deformed specimens was 87.1 %, indicating that the vast majority of the deformation in the EHF process occurred as a result of inertial effects after the electrical energy was completely discharged. A larger dislocation density and a more uniform dislocation distribution were observed in the EHF specimens, which were regarded as the micro-scale causes of the hyperplasticity in the EHF process. Multiplication and entanglement of dislocations caused by the significant shear stress, together with the extensive nucleation of new dislocations caused by the high strain rates, demonstrated the micro-scale mechanism of hyperplasticity during EHF.

Acquisition of Dynamic Material Properties in the Electrohydraulic Forming Process Using Artificial Neural Network.

Electrohydraulic forming is a high-velocity forming process that deforms sheet metals with velocities above 100 m/s and strain rates more than 100 s−1. This experiment was conducted in a closed space because of safety concerns related to the high-velocity conditions; therefore, we were not able to examine the deformation process of the sheet metal. To observe the electrohydraulic forming process in detail, we performed virtual numerical simulations using accurate material properties. Therefore, in this paper, we obtained the material property of a sheet metal from a numerical estimation by using a surrogate model based on the reduced order model and the artificial neural network. The Cowper–Symonds constitutive equation was selected for the Al 6061-T6 sheet metal, and two strain rate parameters were adopted as the unknown parameters. From the two sampling techniques, the training and test samples were extracted from the specific ranges of two unknown parameters, and a numerical simulation was performed for these samples by using the LS-DYNA program. The z-axis displacements of the deformed sheet metal were obtained from the results of the numerical simulation, and two basis vectors were extracted by using principal component analysis. In addition, to predict the weighting coefficients of the two basis vectors at the defined range of parameters, we used the artificial neural network technique as a surrogate model. By comparing the surrogate model and the experimental results and calculating the root mean square error value, we estimated the optimal parameter for Al 6061-T6. Finally, the reliability of the obtained material parameters was proved by comparing the experimental results, the surrogate model, and LS-DYNA.

Acquisition and Evaluation of Theoretical Forming Limit Diagram of Al 6061-T6 in Electrohydraulic Forming Process

The current study examines the forming limit diagram (FLD) of Al 6061-T6 during the electrohydraulic forming process based on the Marciniak–Kuczynski theory (M-K theory). To describe the work-hardening properties of the material, Hollomon’s equation—that includes strain and strain rate hardening parameters—was used. A quasi-static tensile test was performed to obtain the strain-hardening factor and the split-Hopkinson pressure bar (SHPB) test was carried out to acquire the strain rate hardening parameter. To evaluate the reliability of the stress–strain curves obtained from the SHPB test, a numerical model was performed using the LS–DYNA program. Hosford’s yield function was also employed to predict the theoretical FLD. The obtained FLD showed that the material could have improved formability at a high strain rate index condition compared with the quasi-static condition, which means that the high-speed forming process can enhance the formability of sheet metals. Finally, the FLD was compared with the experimental results from electrohydraulic forming (EHF) free-bulging test, which showed that the theoretical FLD was in good agreement with the actual forming limit in the EHF process.

Microscopic investigation of failure mechanisms in AA5182-O sheets subjected to electro-hydraulic forming

In this study, damage mechanisms in AA5182-O aluminium sheets are investigated using quasi-static (QS) Marciniak tests and high strain-rate electro-hydraulic forming (EHF) process. The results confirm that void nucleation, growth and coalescence are the main damage mechanisms of AA5182-O at both high and low strain rates. The EDS analysis suggests that cracking of Al3(Fe-Mn) intermetallic particles is the main source of void nucleation, whereas Mg2Si particles do not majorly influence void formation. Void growth analysis suggests that specimens deformed under QS conditions contained more voids in areas away from the sub-fracture surface but specimens deformed at high strain rate exhibit more significant rate of void growth close to sub-fracture areas. Void formation is suppressed by increasing the applied energy in EHF. And more significantly, the growth of voids is suppressed due to the high-velocity impact of the sheet against the die which plays an important role in increasing formability of AA5182-O aluminium sheet in EHF process. When the void percentage increase remains less than about 0.5% AA5182-O can be formed safely. However, when the void percentage increases beyond 0.6–0.8% fracture becomes inevitable.

Development of Electrohydraulic Forming Process for Aluminum Sheet with Sharp Edge

Electrohydraulic forming (EHF), high-velocity forming technology, can improve the formability of a workpiece. Accordingly, this process can help engineers create products with sharper edges, allowing a product’s radius of curvature to be less than 2 mm radius of curvature. As a forming process with a high-strain rate, the EHF process produces a shockwave and pressure during the discharge of an electrical spark between electrodes, leading to high-velocity impact between the workpiece and die. Therefore, the objective of this research is to develop an EHF process for forming a lightweight materials case with sharp edges. In order to do so, we employed A5052-H32, which has been widely used in the electric appliance industry. After drawing an A5052-H32 Forming Limit Diagram (FLD) via a standard limiting dome height (LDH) test, improvements to the formability via the EHF process were evaluated by comparing the strain between the LDH test and the EHF process. From results of the combined formability, it is confirmed that the formability was improved nearly twofold, and a sharp edge with less than 2 mm radius of curvature was created using the EHF process.

Enhancement of Formability of AA5052 Alloy Sheets by Electrohydraulic Forming Process

Formability of lightweight materials like Al and Mg alloys is a major concern for their application in automobiles. Forming limit diagram (FLD) and strain distribution are extremely useful in the assessment of overall formability of sheet metals. At very high strain rates, the deformation behavior of Al alloys and the safe forming window could be different from quasi-static conventional forming. In this paper, formability of Al 5052 alloy sheets of 0.5 mm thickness has been assessed in electrohydraulic forming (EHF) in terms of FLD and strain distribution and compared with formability in conventional forming by punch-stretching experiments. EHF is a high strain rate forming process which utilizes energy released from a capacitor bank to generate shockwaves in a fluid medium. Experiments have been conducted at different energy levels to identify the highest safe strains in different modes of deformation. From the experimental results, it has been observed that the limit strains increased by nearly 45-50% in all the three regions of the FLD (tension-tension, plane strain and tension compression). Unlike in the case of conventional forming, no clear necking due to strain localization has been observed prior to failure due to very high strain rates of the order of 103/s. The strain distribution has been found to be more uniform in the case of EHF with a single strain peak at the pole. Absence of friction in EHF also leads to higher degree of biaxiality leading to higher limit strains in biaxial tension. In the case of EHF, the effective strain and hardness are maximum at the pole and their variation correlated well with the findings from the strain distribution analysis. In all modes of deformation, the features of fractured surface in EHF appeared different from a normal ductile failure.

Experiments on electrohydraulic forming and electromagnetic forming of aluminum tube

Electromagnetic forming (EMF) and electrohydraulic forming (EHF) are typical high speed forming technologies characterized with improved formability of sheet metal, single-sided die, and high precision deformation of parts. In this work, the bulging experiments of aluminum alloy tubes by EMF and EHF were performed to investigate the attaching-die capability, wall thickness, working hardness, and residual plasticity of deformed tubes. The results showed that the bulged tube by EMF is of low attaching-die capability, remarkable rebound, serious thinning of wall thickness, and prone to rupture in the outer corner. However, the parts by EHF process are of high attaching-die capability, almost no rebound, relatively uniform thickness, and remarkable fillet filling in the inner corner. The load mechanism contributes to the different deformation results of both processes. With the almost equivalent deformation, the tensile deformation ability of EHF part is better than that of EMF one, and the discharge energy efficiency of the former is higher than that of the latter.

Numerical study on electrohydraulic forming process to reduce the bouncing effect in electromagnetic forming

Electromagnetic forming (EMF) is a high-speed forming process that forms a blank by using Lorentz force. Because it is a high-speed process, the effects of high strain rate can enhance the formability of the blank. However, when the blank is deformed into a complex shape, bouncing occurs at the surface of the blank, thereby generating wrinkles in the metal. Therefore, electrohydraulic forming (EHF) is proposed in this paper to overcome the bouncing effect. EHF is a high-speed forming process that employs high voltage discharge in water. The shockwave resulting from the electric discharge propagates to the blank through the water and deforms the blank into the shape of the die. A numerical simulation of the EHF process was performed using the LS-DYNA program to examine the forming procedure in detail. In the simulation, an arbitrary Lagrange–Eulerian multimaterial formulation was used to describe the interaction between the fluid components. Furthermore, a coupling mechanism was used for modeling the fluid–structure interaction. The numerical results of EHF were compared to those of EMF. This comparison showed that the EHF process can overcome the bouncing effect and deform a blank into a complex shape.

Modeling of electrohydraulic forming of sheet metal parts

Electrohydraulic forming (EHF) is based upon the electro-hydraulic effect: a complex phenomenon related to the high voltage discharge inside the water filled chamber. The resulting shockwave in the liquid is propagated toward the blank, and the mass and momentum of the water in the shock wave accelerates the sheet metal blank toward the die. Methodology of numerical simulation of EHF processes was developed based upon LS-DYNA commercial code using Arbitrary Lagrange–Eulerian (ALE) Multi-Material formulation. The model incorporates energy deposition inside the plasma channel, expansion of the channel driven by high pressure inside of it, propagation of the pressure pulse through the water filled chamber in contact with the rigid walls of the chamber and with the sheet metal blank being deformed. Comparison of the numerical and experimental results was performed on maximum pressure measured on the wall of the cylindrical chamber employing the membrane method.The model was used to simulate multistage EHF of a complex geometry automotive part. Analysis of the results showed the complex nature of multistage EHF process: a clearly recognizable wave picture during the initial stage of the channel expansion which transitions to almost incompressible water flow during later stages.

Formability of dual phase steels in electrohydraulic forming

Electrohydraulic forming (EHF) is based upon the electro-hydraulic effect: a complex phenomenon related to the discharge of high voltage electrical current through a liquid. In EHF, electrical energy is stored in a bank of capacitors and is converted into the kinetic energy needed to form sheet metal by rapidly discharging that energy across a pair of electrodes submerged in a fluid. The objective of this paper is to report the results of formability testing of dual phase steels in EHF conditions and to provide an explanation of the observed formability improvement based on analysis of the experimental results and through the use of a modeling technique developed as part of this work for simulating the EHF process. Comparison of the maximum strains resulting from EHF into a conical die and a v-shape die to the maximum strains resulting from quasistatic LDH testing (limiting dome height) indicated that substantially higher strains can be accomplished in the EHF process. In order to obtain the maximum benefits of increased formability with EHF technology, it is necessary to get to the high strain rate regime and the accompanying high hydrostatic stresses in the sheet, which occurs mostly in the contact area between the blank and the die. If the blank decelerates and gets to quasistatic forming conditions before filling the die cavity completely, very limited improvement in formability can be anticipated. A numerical model of the EHF process has been developed, incorporating four distinct models that are integrated into one: (1) an electrical model of the discharge channel, (2) a model of the plasma channel, (3) a model for the liquid as a pressure-transmitting medium, and (4) a deformable sheet metal blank in contact with a rigid die. The relative improvement in plane strain formability in EHF conditions was between 63% and 190%, depending on the grade of dual phase steel. Numerical modeling showed that the peak strain rates occurring in EHF are approximately 20,000 units per second.

Effect of quasi-static prestrain on the formability of dual phase steels in electrohydraulic forming

Dual phase steels derive their name from their microstructure, which consists of islands of martensite surrounded by a ferrite matrix. These steels are increasingly being used in automobile structures in order to reduce weight, improve fuel economy, and maintain crash safety performance. The higher strength grades of dual phase steels, such as DP780 and DP980, often present significant formability challenges in sheet stamping operations, and therefore any technologies which could alleviate these issues would be of significant value to the automotive industry. Electrohydraulic forming (EHF) is based upon the electro-hydraulic effect: a complex phenomenon related to the discharge of high voltage electrical current through a liquid. In EHF, electrical energy is stored in a bank of capacitors and is converted into the kinetic energy needed to form sheet metal by rapidly discharging that energy across a pair of electrodes submerged in a fluid. During such a discharge, a high pressure, high temperature plasma channel is created between the tips of the electrodes. The resulting shockwave in the liquid, initiated by the expansion of the plasma channel, is propagated toward the blank at the acoustic velocity of the fluid, and the mass and momentum of the water in the shock wave accelerates the sheet metal blank toward the die. The objective of this paper is to report the results of formability testing of dual phase steels under three basic conditions: (1) conventional limiting dome height (LDH) testing; (2) starting with a flat blank and using one pulse of EHF to fill the desired die geometry; and (3) starting with a quasi-static preforming step to partially fill the die cavity and then using one pulse of EHF to fill the remaining area of the die cavity. A hybrid process which combines sheet hydroforming (HF) and EHF as described herein has the potential to reduce the cycle time of the EHF process by replacing the initial EHF forming increments with one quasi-static preforming step. Additionally, a numerical model was developed and employed in order to better understand the sheet deformation process within EHF. The numerical model consists of four distinct models that are integrated into one: (1) an electrical model of the discharge channel, (2) a model of the plasma, (3) a model of the liquid as a pressure-transmitting medium, and (4) a deformable sheet metal blank in contact with a rigid die. Significant improvements in formability were confirmed experimentally for DP780 and DP980 by forming into conical and v-shape dies using EHF from a flat sheet and by using EHF combined with a quasi-static preforming step. Numerical modeling showed that the peak strain rates occurring in both single-pulse EHF the hybrid HF-EHF process are approximately 17,000 units per second.

Electro-hydraulic forming of sheet metals: Free-forming vs. conical-die forming

This work builds upon our recent advances in quantifying high-rate deformation behavior of sheet metals, during electro-hydraulic forming (EHF), using high-speed imaging and digital image correlation techniques. Aluminum alloy AA5182-O and DP600 steel sheets (1mm thick, ∼152mm diameter) were EHF deformed by high-energy (up to ∼34kJ) pressure-pulse in an open die (free-forming) and inside a conical die. The deformation history (velocity, strain, strain-rate, and strain-path) at the apex of the formed domes was quantified and analyzed. The data shows that the use of a die in the EHF process resulted in an amplification, relative to free-forming conditions, of the out-of-plane normal velocity and in-plane strain-rate at the dome apex. This amplification is attributed to the focusing action of the die on account of its conical geometry. Further, while the strain-path at the dome apex was generally linear and proportional, the use of a die resulted in greater strain at the apex relative to the strain during free-forming. The sheet deformation profile in the EHF process was found to be different from that previously observed in electromagnetic forming (EMF) and, thus, the two processes are expected to result in different strain-paths and formability. It is anticipated that quantitative information of the sheet deformation history, made possible by the experimental technique developed in this work, will improve our understanding of the roles of strain-rate and sheet–die interactions in enhancing the sheet metal formability during high-rate forming.