Electrically-assisted Forming Research Articles

Study on deformation behavior of Ti60 alloy based on multi-physics coupling

Titanium alloys are extensively employed in aerospace due to their superior performance, but their development is constrained by limited formability at room temperature, an electrically assisted forming (EAF) process emerges as a promising solution to reduce flow stress, improve formability, and accelerate material recrystallization. This study focuses on elucidating the interplay between pulse current, temperature, and flow stress in Ti60 alloy under combined electrical, thermal, and stress fields. To this end, electrically assisted tensile (EAT) tests were performed on Ti60 alloy using six different current densities. Currently, there is no unified theory for electro-plasticity in metal materials, and the absence of a suitable model hinders finite element simulation in EAF. The investigation encompasses four key areas: the impact of multi-physics coupling on temperature variations, the stress-strain curve characteristics, the microstructural analysis of tensile fractures, and the modification of the Johnson-Cook (MJC) model. Notably, temperature distribution proves more uniform across the width than the length. Tensile elongation initially rises and then falls with increasing current density, peaking at a critical current density (J=30 A/mm2). Higher current densities lead to a transition in fracture features, with deep, numerous dimples and tiny voids supplanting tearing ridges and facets. Despite this, brittle fracture remains the dominant mode, accompanied by granular oxide formation at J= 35, 40 A/mm2. The MJC model aligns closely with experimental outcomes, showing R and EAARE values of 92.7% and 7.15%, respectively. This model accurately predicts Ti60 alloy's deformation under multi-physics conditions, marking a significant advancement in integrating electro-plasticity into constitutive modeling.

Modified constitutive models for Inconel 718 considering current density and temperature in electrically assisted forming process

Electrically Assisted Forming (EAF) technology has obvious advantages in material forming. To develop an effective constitutive model considering electrical effects, room temperature and electrically assisted quasi-static uniaxial tensile tests were conducted using ultrathin nickel-based superalloy plates with a thickness of 0.25 mm. The research focused on the two most widely recognized effects: the Joule thermal and the electric athermal effects. The mechanism of current action can be divided into two scenarios: one considering the Joule thermal effect only, and the other considering both effects simultaneously. Two basic constitutive models, namely the Modified-Hollomon model and the Johnson-Cook (J-C) model, were selected to be optimized through the classification of two different situations, and four optimized constitutive models were proposed. It was found that the J-C model with simultaneous consideration of the Joule thermal effect and electric athermal effect had the best prediction effect by comparing the results of these four models. Finally, the accuracy of the optimization model was verified by finite element simulation of the electrically assisted stretching optimization model. The results show that the constitutive model can effectively predict the temperature effect caused by the Joule heat effect and the athermal effect of current on the material.

Electrical-thermal-mechanical coupled modeling and simulation on deformation behaviors of Ti6554 alloy in electrically-assisted micro-tension

The electrically assisted forming (EAF) technology can reduce the load & stress and improve the formability of difficult-to-deform alloys due to its coupled thermal and athermal effects. To decouple the two effects and to elucidate each mechanism for deeply understanding and optimizing the EAF processing, we developed an electrothermalmechanical (ETM) macro-meso multiscale model by combing a modified Johnson-Cook (JC) constitutive model with the dislocation density based crystal plasticity finite element (DD-CPFE). The modified JC was used to describe the macroscopic electro-assisted deformation, while the meso-scale DD-CPFE was used to deal with the thermal and athermal effects on the slip rate. Statistically stored dislocations (SSDs) were introduced into the Orowan rate equation to explain the thermal effect part, and the athermal part was reflected through the effect of free energy change on the dislocation motion in the rate equation, due to the collision between free electrons and metallic ions. The simulated results show that the equivalent stress, strain, and dislocation densities are distributed heterogeneously in a meso-scale and sensitive to the current density. Local thermal effect due to geometrically necessary dislocations (GND) exerts no effect on the temperature gradient because of rapid heat conduction between grains. The thermal effect is dominated by the thermal expansion obviously reducing the flow stress in the single-phase and two-phase fields (0 to 70 A/mm2), while the athermal effect is effective in the two-phase field (corresponding to the current density range of 0 to 20 A/mm2) in reducing the flow stress, which is reasonably explained by the change of bonding strength in defect zones, such as grain boundaries, of the HCP structure via the first principle calculations.

The effect of pulsed electric current on the structural and mechanical behavior of 6016 aluminium alloy in different states of hardening

This study presents the effect of high current pulses on the structural and mechanical behavior of the 6016 aluminium alloy in three different states of hardening: naturally aged, super saturated, and annealed. The 6016 aluminium alloy was used for the first time in terms of electrically-assisted forming. The influence of the application of different current parameters on the material behavior was conducted. The study of electrically-assisted tensile tests showed that the application of current pulses results in a distinct response of the material, depending on the hardening state. Although in a hardened state, the mechanical properties and plasticity are deteriorated, in the solution treated state, they are improved. For the changes of the material properties is responsible the interaction of the flowing current with the precipitates and the aging process. The new parameters were proposed to describe the distinctions in the material properties between the different states of hardening of the aluminium alloy during the electrically-assisted tension. The material examination was conducted using light and scanning electron microscopy, using also electron backscattered diffraction methods. The application of, for example, the grain orientation spread parameter demonstrated the presence of recrystallized grains, in electrically-assisted specimens.

The anomalous negative electric current sensitivity of a precipitation hardened Al alloy during electrically-assisted forming

It has been known for decades that the formability of metal sheets can be improved effectively by electrically-assisted (EA) forming. However, it remains controversial as to the detailed mechanisms and the manifestation of electrical effects can vary in different materials. In this paper, the EA flow behavior and mechanism of a precipitation hardened AA7075-T6 were studied. The term “electric current density sensitivity” (ECDS) was introduced to describe the athermal effect. The results show that Joule heating is the main influence factor, while the athermal effect varies with the electric–thermal parameters. Here, we report an anomalous negative electric current sensitivity in AA7075-T6, i.e. the flow stress increases with current density under the same temperature and strain rate when the temperature is ≤ 150 °C and strain rate is ≥ 0.005/s. The above phenomena are attributed to the local Joule heating and promoted atom diffusion under the action of the current, leading to the dissolution of GPZ, the formation of fine precipitate and Cr segregation-induced vacancies. Further, an extended dislocation density based constitutive model was proposed to cover the ECDS and softening effect, which could predict the flow behavior of 7075-T6 under EA forming, including the negative electric current sensitivity. The above work provides a viable method to simultaneously control both the shape and performance of the metal part. Meanwhile, the corresponding mechanism shed new light into the understanding of electroplasticity.

An experimental and modelling study of cyclic tension-compression behavior of AA7075-T6 under electrically-assisted condition

Electrically-assisted (EA) forming is an effective method to improve forming quality of sheet metal. In order to reveal the effects of current on material flow behavior under complex strain paths and develop corresponding constitutive model, the EA tension-compression cyclic loading tests with particularly designed setup were carried out. Then, the effects of current on the elastic modulus degradation and cyclic deformation were investigated. Further, corresponding modulus degradation model and constitutive model were developed. The results showed that the flow stress under EA uniaxial tension was lower than that under warm uniaxial tension. Moreover, the current-induced growth of precipitate and the decrease of dislocation occur under EA uniaxial tension. Regarding the EA cyclic deformation behavior, the elastic modulus decreases with the increase of plastic strain, temperature and current density, which is described by a proposed modified modulus degradation model. The Bauschinger effect and permanent softening effect are weakened with the increase of temperature and current density. The asymmetric tension-compression behavior becomes more obvious with the increase of temperature and current density. Based on above results, a modified Y-U model was established and verified with EA draw-bending process. The above research work can provide some research basis for the application of electrically-assisted forming.

One Step Forward to Electrically Assisted Forming Mechanisms and Computer Simulation: A Review

The electrically assisted forming (EAF) is a hybrid manufacturing process that utilizes electric current energy to improve the productivity and efficiency of the forming process. Many studies have been accomplished to investigate the effect of process parameters on its forming abilities, including pulse current effect, grain size, and current duration. There is currently a lack of agreement on the underlying mechanisms of the electroplastic effect to the formability of metallic materials through EAF, specifically on the Joule heating and electric wind's contributions. One promising solution to this dilemma is the application of computer simulation techniques for a better understanding of the physics of EAF. Therefore, this paper provides a brief review of the previous research studying the modeling of EAF and offers new solutions aiming to further the understanding of the underlying physics of this process through computer simulation approaches.

Electroplasticity in electrically-assisted forming: Process phenomena, performances and modelling

Electrically-assisted forming has been proven to be an efficient process in terms of reducing forming flow stress, improving formability and supressing the springback for different hard-to-deform alloys. However, there is a controversial debate about whether the flow stress reduction with the introduction of electric current into a metallic specimen undergoing plastic deformation is attributed to Joule heat, additional athermal effect, namely the electroplasticity, or their combination thereof. There is a lack of thorough understanding of different mechanisms inducing the flow stress reduction and formability improvement, which would hinder the development and application of innovative electrically-assisted forming processes. Most prior works have examined only one or two materials with similar electroplastic behaviours, making it difficult to resolve this debate. In this study, three typical materials with diverse electroplastic behaviours, i.e. copper, SS304 and Ti6Al5V, were investigated and compared via electrically-assisted tensile tests and thermally-assisted tensile tests. The flow stress of the copper specimens in the electrically-assisted tensile test was lower than that in the thermally-assisted tensile test, which verifies the existence of electroplasticity. However, the flow stresses of the SS304 specimens in the electrically-assisted tensile test and thermally-assisted tensile test were similar at the same temperature, indicating that the athermal effect on the flow stress is not evident. For the Ti6Al4V specimens, the athermal stress reduction was not significant at lower current densities. However, the electrically-assisted and thermally-assisted tensile tests results began to differ when the current density exceeded 37.6 A/mm2. According to the further discussion on the microstructure evolution of different materials, the introduced current improves the dislocation motions and recovery and consequently reduces the flow stress in simple face-centered cubic structure materials such as copper. However, these effects can be suppressed by stronger effects such as the solution hardening in stainless steels. The current also affects the phase transition, which in turn influences the hardening behaviours of Ti6Al4V. By clarifying the cause of conflicting electroplastic effects, a uniform analytical method calculating the short- and long-range stresses that considers the correlation of electrical and thermal effects on dislocation motion, solution hardening and phase transformation was developed for electrically-assisted forming processes. The model was also experimentally verified.

The Effect of Direct Electric Current on the Plastic Behavior of AA7075 Aluminum Alloy in Different States of Hardening.

Electrically-Assisted Forming (EAF) techniques are interesting and promising for the automotive industry. Electrically-assisted tensile tests were carried out on specimens of AA7075 aluminum alloy in different states of hardening, namely T6 (the as-received state) and the supersaturated solid solution state. All the tests were carried out in quasi-static conditions under the application of direct electric current (DC) in the range of 90 to 540 A. The experimental results showed that with a DC density of 10 A/mm2 the uniform strain and strain at fracture increased when the AA7075 was in the supersaturated solid solution state. A correlation between the mechanical results and microstructural features analysed through transmission electron microscopy was assessed. An explanation of the investigated phenomena based on the electron wind theory, heterogeneous microscale Joule heating and the Portevin–Le Chatelier (PLC) effect was finally proposed.

Prediction of Electropulse-Induced Nonlinear Temperature Variation of Mg Alloy Based on Machine Learning

Electropulse-induced heating has attracted attention due to its high energy efficiency. However, the process gives rise to a nonlinear temperature variation, which is difficult to predict using a traditional physics model. As an alternative, this study employed machine-learning technology to predict such temperature variation for the first time. Mg alloy was exposed to a single electropulse with a variety of pulse magnitudes and durations for this purpose. Nine machine-learning models were established from algorithms from artificial neural network (ANN), deep neural network (DNN), and extreme gradient boosting (XGBoost). The ANN models showed an insufficient predicting capability with respect to the region of peak temperature, where temperature varied most significantly. The DNN models were built by increasing model complexity, enhancing architectures, and tuning hyperparameters. They exhibited a remarkable improvement in predicting capability at the heating-cooling boundary as well as overall estimation. As a result, the DNN-2 model in this group showed the best prediction of nonlinear temperature variation among the machinelearning models built in this study. The XGBoost model exhibited poor predicting performance when default hyperparameters were applied. However, hyperparameter tuning of learning rates and maximum depths resulted in a decent predicting capability with this algorithm. Furthermore, XGBoost models exhibited an extreme reduction in learning time compared with the ANN and DNN models. This advantage is expected to be useful for predicting more complicated cases including various materials, multi-step electropulses, and electrically-assisted forming.

Investigation of Electrically-Assisted Rolling Process of Corrugated Surface Microstructure with T2 Copper Foil.

Electrically-assisted (EA) forming is a low-cost and high-efficiency method to enhance the formability of materials. In the study, EAF tensile tests are carried out to study the properties of T2 copper foil in an annealed state, and the effect of the electric current on the forming quality of corrugated foils is further studied in the EA rolling forming process. The result shows that the current reduces the flow stress and the fracture strain, which is different from the result of rolled samples. The joule heating effect on mechanical properties is significant in EA tension, and the softening effect of the surface layer can be observed at tensile strength, due to the grain size effect. Moreover, the current can weaken the grain size effect. In the rolling forming process, the influence of different electrical parameters on the forming height is remarkable, especially for the rolled T2 copper. The appropriate electrical parameters can improve the forming height, while keeping a small thickness thinning. Nevertheless, the high current density will lead to local rupture. This study proves that the current can improve the forming quality of the corrugated foils and is a promising surface texture forming process.

Mechanical behavior and shear banding of electropulsing-assisted micro-scale shear-compression in Ti-6Al-4V alloy

Various electrically-assisted forming (EAF) technologies have been developed as a promising and new metal forming process, but few studies pay attention to the effect of electropulsing on shear deformation behavior and shear banding evolution. Here the electropulsing-assisted micro-scale shear-compression (EAMSC) experiments were conducted in Ti-6Al-4V alloy with lamellar and equiaxed microstructures under different current densities. Numerical simulation was used to investigate the field distributions during EAMSC. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to comparatively investigate the shear bands evolution and microcracks nucleation of two samples. The results show the electropulsing heating temperature was homogeneously distributed in the shear plane and formed a distinct gradient in the matrix. In lamellar microstructure, shear bands show a distribution of fork-shaped and the localised diffusive rotation occurs at grain boundary α (αGB) layer. The local fluctuations in stress/strain and temperature can contribute to the initiation of shear bands, and the microcracks nucleation will happen in the grain boundaries (GBs). While the formation of elongated dislocation cells, formation of elongated subgrains and break-up of elongated subgrains are the evolutionary process of shear bands for equiaxed microstructure, and the microvoids are nucleated at the breakpoint of subgrains with excessive elongation. In addition, the flow curves of EAMSC display obviously strain softening for lamellar microstructure, while hardening for equiaxed microstructure. It should be mainly attributed to that the equiaxed microstructure EAMSC caused the high dislocation pile-up and deformation twins (DTs). In contrast, the predominance of shear deformation during shear-compression deformation can cause the softening in lamellar microstructure.

Investigation on Two Electrically-Assisted Forming Processes of AZ31B Magnesium Alloy Sheets

Two new types of electrically-assisted forming (EAF) processes of AZ31B magnesium alloy sheets, i.e., electrically-assisted blanking (EAB) and electrically-assisted sheet upset-extruding (EASUE), are proposed in this paper. During EAB, pulsed current (PC) decreases blanking load and improves blanked surface quality, and higher frequency has a more effective improvement; when the frequency reaches 600 Hz, the whole punched hole fracture is improved to smooth zone. Moreover, EAB does not influence the microstructure and does not induce obvious annealing, and this process is advantageous to the materials that cannot change microstructure. During EASUE, PC can reduce extrusion load and enhance extrusion height, and higher frequency also has a more effective improvement. Both processes indicate that introducing PC can effectively improve the part quality and have an application prospection.

Investigation of Electroplastic Effect on Four Grades of Duplex Stainless Steels.

Since the late 1950s, an effect of electrical current in addition to joule heating on the deformation of metals called the Electroplastic Effect (EPE) has been known. It is used nowadays in the so-called Electrically Assisted Forming (EAF) processes, but the understanding of the phenomenon is not very clear yet. It has been found that EPE increases the formability of high stacking fault energy (SFE) materials, while low SFE materials reach fracture prematurely. Since Duplex Stainless Steels (DSSs) possess a microstructure consisting of two phases with very different SFE (low SFE austenite and high SFE ferrite) and they are widely used in industry, we investigated EPE on those alloys. Tensile tests at 5 A/mm2, 10 A/mm2 and 15 A/mm2 current densities along with thermal counterparts were conducted on UNS S32101, UNS S32205, UNS S32304 and UNS S32750. The DSS grades were characterized by means of optical microscopy, X-ray diffraction and their mechanical properties (ultimate tensile strength, total elongation, uniform elongation and yield stress). An increase in uniform elongation for the electrical tests compared to the thermal counterparts as well as an increase in total elongation was found. No differences were observed on the yield stress and on the ultimate tensile strength. Un uneven distribution of the current because of the different resistivity and work hardening of the two phases has been hypothesized as the explanation for the positive effect of EPE.

Electroplastic behaviour in an aluminium alloy and dislocation density based modelling

Electroplasticity refers to the application of controlled electric pulses during plastic deformation of materials. The electroplasticity phenomenon in metallic materials has led to the development of electrically assisted forming (EAF) process with improved formability. The lack of a suitable constitutive model to describe this electroplastic behaviour is a serious limitation in modelling and optimizing the EAF process. In the present work, a dislocation – density based constitutive model is developed for electroplastic deformation and is capable of predicting the effect of continuous and pulsed electric current during plastic deformation. Single- pulse electroplastic deformation experiments conducted on Al 5052 reveal similar mechanical behaviour as that predicted by the proposed model. The proposed model is also validated against published results for multiple electric pulses using Al 5052. The predicted results correlate well with the experimental data. Based on the predicted results, it is demonstrated that the long range softening observed in certain experiments results from the frequent application of electric pulses and is not due to any other internal softening mechanism.

Influence of Continuous Direct Current on the Microtube Hydroforming Process

Research of the microtube hydroforming (MTHF) process is being investigated for potential medical and fuel cell applications. This is largely due to the fact that at the macroscale the tube hydroforming (THF) process, like most metal forming processes, has realized many advantages, especially when comparing products made using traditional machining processes. Unfortunately, relatively large forces compared to part size and high pressures are required to form the parts so the potential exists to create failed or defective parts. One method to reduce the forces and pressures during MTHF is to incorporate electrically assisted manufacturing (EAM) and electrically assisted forming (EAF) into the MTHF. The intent of both EAM and EAF is to use electrical current to lower the required deformation energy and increase the metal's formability. To reduce the required deformation energy, the applied electricity produces localized heating in the material in order to lower the material's yield stress. In many cases, the previous work has shown that EAF and EAM have resulted in metals being formed further than conventional forming methods alone without sacrificing the strength or ductility. Tests were performed using “as received” and annealed stainless steel 304 tubing. Results shown in this paper indicate that the ultimate tensile strength and bust pressures decrease with increased current while using EAM during MTHF. It was also shown that at high currents the microtubes experienced higher temperatures but were still well below the recrystallization temperature.

Temperature-Controlled Forming of 7075-T6 Aluminum Using Linearly Decaying Direct Electric Current

7075-T6 aluminum suffers from limited elongation during tensile forming; electrically assisted forming (EAF), which uses direct current to improve formability, is a viable candidate process to improve this effect. In past electrical tension testing by various authors, two types of waveforms have been examined: continuous current and square waveforms. For tension, it was shown that the applying current using square waveforms was able to extend formability beyond what continuous current could do, due to reducing the overheating in the necking region. The goal of this paper is to model the temperature and flow stress effects of saw tooth waves by modifying an existing square wave temperature prediction model and combining it with a theoretical flow stress model. Nondecaying and linearly globally decaying saw tooth waveforms are used in an attempt to control the temperature of the necking zone to allow for increased strain at fracture. Comparisons between saw tooth waveforms and square waveforms are exhibited, and it is found that the saw tooth waveforms are inferior to square waves for increasing strain at fracture for 7075-T6.

The Effect of Pulse Electric Current on the Mechanical Properties and Fracture Behaviors of Aluminum Alloy AA5754

The influences of pulse electric currents at energy density levels of 0.105 J/mm3 and 0.150 J/mm3 on AA5754's flow stress and elongation are investigated. Different combinations of current density and pulse duration are carried out for each energy density. The non-Joule heating effects in electrically assisted forming (EAF) are revealed since the temperatures generated by the electric currents of the same energy density are identical. It is observed that a pulse current helps reduce AA5754's flow stress and increase its elongation. At the same level of energy density, as the current density increases, the instant drop of stress increases as well as the elongation, although the maximum flow stress remains almost unchanged. A theoretical model is proposed that can predict the stress drop during electrically assisted forming. The fracture surfaces of AA5754 subject to pulse currents are observed and analyzed. The dimples of fracture continue to decrease until they completely disappear as the density of pulse current increases. The suppression of voids nucleation and growth by a pulse current leads to the increase of total elongation.

Analysis of the Electric and Thermal Effects on Mechanical Behavior of SS304 Subjected to Electrically Assisted Forming Process

Significant improvements in deformation resistance and ductility of metals are observed in the electrically assisted forming (EAF) process. Both electroplastic effect (EPE) induced by electric current and thermal effect associated with Joule heating have been proposed to explain the phenomenon. However, there are still arguments in the contribution of the EPE in EAF process. In this paper, both electrically assisted tension tests (EAT) and thermally assisted tension tests (TAT) were conducted on SS304 specimens at the same temperature. The existence of EPE is investigated, and the contribution of EPE is also distinguished with thermal effect numerically by considering the initial yield stress, dislocation hardening, and martensite phase transformation. It is shown when the temperature is around 34 °C, the electric current of 50 A/mm2 in EAT induces additional stress reduction of 16% in the short-range internal stress (effective stress) involved in the initial yield stress and volume reduction of 45.2% in martensite formation compared with results in TAT. However, the effect is not obvious for the cases of 100 A/mm2 and 150 A/mm2 when the temperature is above 100 °C. By comparing the storage coefficient and recovery coefficient of dislocation in EAT and TAT, it indicates that electric current has no additional activation effect on dislocation movement of SS304.

Electroplastic forming in a Fe-based metallic glass ribbon

Electric current has been applied to lower the temperature and loading required for metalworking of extremely strong or brittle alloys. However, limited studies have been conducted on the electrically-assisted forming of metallic glasses. Our previous study revealed the feasibility of electroplastically deforming a Fe78B13Si9 metallic glass ribbon. Yet the drawback is that pre-stressing was required prior to electropulsing and large elastic recovery limited the dimensional accuracy of the forming process. In the present work we demonstrate a setup in which metallic glass ribbons were subjected to punching, with electropulsing applied simultaneously in various combinations of frequency and punching speed. Process temperature was measured to be significantly below the glass transition temperature and the process only takes 20–30 s. Lower load was required to punch the ribbon with electrical pulses applied. All the ribbons were fully deformed without spring back after the punch was retracted. X-ray diffraction revealed that all ribbon were essentially amorphous, with only some rare cases suffering from embrittlement. It is concluded that electric current provided a directional force acting as an aid to overcome the activation barrier of shear transformation zones; however, as a side effect joule heating also accelerated beta relaxation causing the embrittlement. This study revealed a novel way of shaping metallic glasses with the potential of industrial scale application.