Electroplastic Effect Research Articles

Unconventional mechanical responses and mechanisms of Ti-6Al-4V sheet subjected to electrically-assisted cyclic loading-unloading: Thermal and athermal effects

For the sake of unveiling the macro-micro behaviors and underlying control mechanisms of electroplastic effect (EPE) of sheet metals during recent thriving electrically-assisted incremental forming (EAIF) processes, electrically-assisted cyclic loading-unloading tension (EACT) experiments were carried out at a current density of 14～20A/mm2 followed by forced and unforced cooling. The electro-thermo-mechanical responses and the evolution rules of dislocation configurations, fracture modes, phase composition and local orientation distribution were characterized by IR imaging, SEM, quantitative EBSD and TEM tests. Moreover, the thermal and athermal components of the electrically-induced stress drop throughout the EACT processes were decoupled and systematically analyzed via temperature control tests and isothermal correction. The results show that thermal effect of electric current possesses a great proportion of 75 % in the overall stress drop and plays a key role in elastic modulus degradation and ductility enhancement by activating pyramidal {101‾1}⟨12‾10⟩ slip systems, weakening (0001) texture and promoting dynamic recrystallization (DRX) and α→β→α′ phase transformation. While the athermal effect only operates after reaching both threshold current density and plastic strain, it increases monotonically with current density, brings about “hot spots” effect, local charge imbalance and weakened atomic bonding, and further causes intensified local strain gradient, substructure formation and DRX nucleation. The ratio of athermal stress drop first decreases and then increases significantly with loading cycle at the later stage of EACT under the combined effect of local Joule heating effect and flow localization. Additionally, the athermal effect only activates short-range solute diffusion and localized diffusional α→β phase transformation, while the combined thermal and athermal effects promote long-range solute diffusion and an increasing rate in β content of 261.8 %. The present work provides theoretical basis for optimizing the EAIF forming process and realizing extreme manufacture of light-weight thin-walled complex components.

Application of electroplastic effect in mechanical processing

Application of electroplastic effect in mechanical processing

Critical Parameters of the Athermal Electroplastic Effect in Metallic Materials

Introduction. Plastic deformation and electric current, acting separately, usually have opposite effects on the deformation behavior and flow stresses in electrically conductive materials. In the case of the combined action of plastic deformation and applied electric current, the result is not pre predictable. The study of the synergistic effect of deformation and electric current can be used for metal forming.Aim of the Study. The study is aimed at demonstrating the existence of impulse current threshold parameters at which the athermal electroplastic effect manifests itself in various materials.Materials and Methods. Tensile tests were performed at various current modes, which exclude the increased contribution of the thermal effect to the reduction of flow stresses – current density and duty cycle. The fractographic features of the fracture surface were studied using raster scanning microscopy. There were found the threshold values of current parameters at which stress jumps associated with the electroplastic effect occur.Results. The influence of the density and duty cycle of the impulse current on the manifestation of the electroplastic effect is shown. Both parameters have threshold values, above which the electroplastic effect becomes observable (at density j jкр ) or athermal (at duty cycle Q Qкр). All types of tension are accompanied by a viscous fracture and void formation, which is most intensively formed, when current is injected.Discussion and Conclusion. In alloys with low electrical resistance, the threshold impulse current density corresponding to the occurrence of the electroplastic effect is higher than in alloys with high electrical resistance. Increasing the duty cycle of the impulse current reduces the temperature of the deformed sample that allows considering the electroplastic effect as athermal.

Electroplasticity mechanism of Ti2AlNb alloys under hot compression with the application of a lower electric current

In this study, a lower electric current (1.0–2.0 A/mm2) was applied during hot compression of Ti2AlNb alloys, and the mechanical response, microstructure evolution, and electroplasticity mechanism were investigated. The results showed that the maximum flow stress drop could reach 18.7 % after applying a lower electric current of 2.0 A/mm2, which confirms the existence of electroplasticity. The stress drop increased with increasing electric current density. However, the flow stresses of the samples in the isothermal compression test and those subjected to a 1.0 A/mm2 electric current compression test were similar, indicating that there is a threshold value for the electroplasticity effect on flow stress. The lower strain hardening rate caused by the electric current confirmed the electroplasticity effect, promoting the softening effect. Additionally, the lower average activation energy Q, affected by the electric current, implied the improvement of the deformability of the alloy due to the deformation mechanism of dynamic recovery (DRV), dynamic globularization, or dynamic recrystallization (DRX). The gradual reduction of dislocation density with increasing electric current density resulted in the stress drop, which can be ascribed to the directional drifting electrons colliding with dislocations, which promoted the movement of dislocations by arranging the dislocations in parallel. The local high temperature induced by the local Joule heating effect at the O lamella had a strong promoting effect on the globularization of the O lamella by producing uniform lattice distortion/local strain at the O/B2 interface. With increasing furnace temperature, the drifting electrons accelerated the transformation of LAGBs to HAGBs which promoted the appearance of dynamic recrystallization grains.

Modeling and characterization on electroplastic effect during dynamic deformation of 5182-O aluminum alloy

The coupling effects of electrical pulse, temperature, strain rate, and strain on the flow behavior and plasticity of 5182-O aluminum alloy were investigated and characterized. The isothermal tensile test and electrically-assisted isothermal tensile test were performed at the same temperature, and three typical models were further embedded in ABAQUS/Explicit for numerical simulation to illustrate the electroplastic effect. The results show that electric pulse reduces the deformation resistance but enhances the elongation greatly. The calibration accuracy of the proposed modified Lim−Huh model for highly nonlinear and coupled dynamic hardening behavior is not much improved compared to the modified Kocks−Mecking model. Moreover, the artificial neural network model is very suitable to describe the macromechenical response of materials under the coupling effect of different variables.

Study of the microstructure evolution of alloy structural steel and inhomogeneity effect of the microscale pulsed currents during current-assisted plane strain compressions by modeling a novel cellular automata method

The current-assisted forming process is considered to be a novel process that utilizes the electroplasticity effect to reduce the deformation resistance of difficult-to-deform metals and to refine the grain size while improving the mechanical properties of the part. However, the lack of understanding of the mechanisms by which pulsed current affects grain refinement still makes it difficult to experimentally develop analytical or empirical models of the relationship between grain size, pulse current parameters, and deformation amount. It will seriously hinder the further development and application of the current-assisted forming process. To reveal the mechanism of grain refinement coupling with electroplasticity, a series of Electron Back Scattering Diffraction observation tests were carried out after current-assisted plane strain compressions. The results show that the grain orientation spread value of the pulse current condition is lower than that of the non-current condition, while the refined grain area percentage is higher than that of the non-current condition. Based on the microstructure observation results, a cellular automata model for grain refinement is proposed. This consists of a dislocation density evolution, sub-grains generation, grain fragmentation, and a grain orientation rotation algorithm. To study the electroplasticity of the grain refinement process, the cellular automata model is also embedded with a finite difference numerical algorithm for solving the microscopic current density distribution of the model. The cellular automata model simulation results show that the error of grain size accuracy of this cellular automata model under non-current and pulse current conditions is 10.48% and 8.55%, respectively. It shows that the model can accurately characterize the grain refinement behavior of the current-assisted plane strain compression. The simulation results reveal the deep mechanism of pulsed current promotion on grain refinement, i.e., an inhomogeneous microstructure leads to uneven distribution of pulse current density. The inhomogeneous current distribution will further increase the grain refinement rate in the coarse-grained regions, thus increasing the proportion of the refined grain regions in the microstructure and leading to a relatively more homogeneous microstructure under pulse current conditions. The cellular automata model accurately reveals the mechanism of electroplasticity on the grain refinement. The model will serve as a crucial theoretical reference in designing current-assisted forming process routes to ensure excellent microstructure and properties in manufactured parts. It will enhance the widespread utilization of current-assisted forming process.

Study on electric pulse-assisted plastic deformation behavior of 5182-O aluminum alloy

Plastic model-based neural networks are emerging phenomenological models with excellent calibration accuracy and interpretability of parameters, and they do not significantly increase the finite element calculation time of neural network models with a simple structure optimized by the algorithm. Because aluminum and magnesium have low ductility at room temperature, complex components made from these alloys need to be forged at high temperatures. In comparison with traditional thermoforming, advanced current-assisted processing provides energy savings, efficiency and green production. Experimental and constitutive modeling are carried out in this study to investigate the coupling effects of electrical pulse, temperature, strain rate, and strain on flow behavior and ductile fracture. It has been shown that electric pulses induce Joule heating effects and electroplastic effects, which reduce deformation resistance and improve formability. Electrical pulses suppress negative strain rate effects caused by dynamic strain aging, which is the main reason for the non-monotonic relationship between temperature and strain rate. The behavior of plasticity and fracture initiation can be described by simulations based on a neural network-based evolving plasticity model that incorporates stress states, current density, temperature, and strain rates.

Development of a method for measuring pulse currents of large magnitude

The paper presents the results of a study on the search for correct methods for measuring a high-value current pulse, which will be used to conduct research on the electroplastic effect. The electroplastic effect is the effect of electric current pulses on the plastic flow of metals. Electroplastic metal forming technology is a relatively new metal forming process that is energy efficient, environmentally friendly and versatile. In particular, it can be used to process metals or alloys that are difficult to process using conventional manufacturing processes. For the experimental study of the electroplastic effect, it became necessary to measure pulse currents of large magnitude, not only in amplitude, but also in the shape of the pulse. The pulsed current causes the formation of an alternating electromagnetic field near the conductors, so it can be measured with a Rogovsky current transformer. The results of the work present a schematic electrical diagram and a photograph with the appearance of an experimental installation for the study of the electroplastic effect. The results of measurements of the current value, the voltage drop on the sample and the dependence of the peak voltage values on the sample on the peak current value are shown. After making calculations and renormalising the data for the voltage drop on the sample according to the peak value of the current obtained on the transformer, the authors obtained the desired current values. The error of this method is estimated by calculating the total capacitance of capacitors, which does not exceed 2%.

Structure and properties of low-alloy steel 10G2FBYu after rolling in embossed rolls under conditions of electroplasticity

The article describes the features of grain structure formation and mechanical properties of low-alloy steel 10G2FBYu after rolling in flat and embossed rolls under the conditions of ordinary and electroplastic deformation. When rolling in embossed rolls, a significant non-uniformity of deformation is achieved over the rolling cross-section, expressed in localized macroshifts directed at an angle of 45° to the rolling plane. It is shown that local shear deformation during rolling in embossed rolls leads to an increase in the ultimate strength of the steel under study with a decrease in plasticity of the rolled material. Rolling 10G2FBYu steel in embossed rolls under conditions of electroplasticity provides maximum strength characteristics with a high hardening coefficient at the stage of macrodeformation. At the same time, the plasticity is maintained at a level sufficient for technological purposes. Structural metallographic and electron microscopic studies showed that increase in strength of steel when rolling in embossed rolls under conditions of electroplastic effect is caused by the refinement of ferrite grains to sizes less than 0.5 µm. Fractographic studies revealed changes in the nature of fracture in steel during rolling in embossed rolls, which is expressed in appearance of areas of brittle fracture in the rolled samples. Rolling under conditions of electroplasticity increases the proportion of ductile fracture and ductility of 10G2FBYu steel.

An electroplastic constitutive model of fcc metals and their alloys under high current density

The mechanical behavior of fcc (face center cubic) metals and their alloys such as copper-based and aluminum-based materials changes obviously under high current density, but there is a lack of suitable constitutive model to describe the electroplastic behavior of these materials, which limits the accuracy of the mechanical response prediction in applications, such as electromagnetic launch. In this paper, a novel nonlinear constitutive model for electroplastic behavior under the high current density of fcc metals and their alloys is proposed. The model proposed is based on the theory of dislocation evolution, energy transfer theory and electron transport theory and takes into account the effects of high current density on dislocation density, strain rate and resistivity. Considering the evolution of forward and reverse dislocations caused by pulsed current, the relationship between the evolution rate of reverse dislocation and strain is established. Due to the sensitivity of the electroplastic effect to strain rate, the strain rate under electric current is obtained by combining the theory of dislocation thermal activation and energy transfer. According to the concept of thermal resistance and dislocation resistance, the relationship between resistance and current density is derived. The quantitative results demonstrate that this model can well capture the flow stress softening under high current density (>3000 A/mm2) and the transient hardening after current removal because of the consideration of a more suitable strain rate and dislocation evolution. The critical current density that has a significant effect on the strain rate and the current density corresponding to the subsection point of the resistivity are quantitatively obtained. Especially, it is shown that the evolution rate of the reverse dislocation decreases nonlinearly with the increase of strain after the current is removed. These results will be helpful to study the mechanical behavior of the rail during electromagnetic launch and further optimize the design of electromagnetic launch device.

Electroplastic Effect in Titanium Alloys Under Tension

Electroplastic Effect in Titanium Alloys Under Tension

A Pulsed Current Application to the Deformation Processing of Materials.

A review of studies on the electroplastic effect on the deformation process in various conductive materials and alloys for the last decade has been carried out. Aspects, such as the mode and regimes of electric current, the practical methods of its introduction into materials with different deformation schemes, features of deformation behavior accompanied by a pulsed current of different materials, structural changes caused by the combined action of deformation and current, the influence of structural features on the electroplastic effect, changes in the physical, mechanical, and technological properties of materials subjected to plastic deformation under current, possible mechanisms and methods of physical and computer modeling of the electroplastic effect, and potential and practical applications of the electroplastic effect are considered. The growing research interest in the manifestation of the electroplastic effect in such new modern materials as shape-memory alloys and ultrafine-grained and nanostructured alloys is shown. Various methods of modeling the mechanisms of electroplasticity, especially at the microlevel, are becoming the most realistic approach for the prediction of the deformation behavior and physical and mechanical properties of various materials. Original examples of the practical application of electropulse methods in the processes of drawing, microstamping, and others are given.

Electro-plastic effect in Ti-6Al-4V: An experimental and numerical study

Electric pulse aided deformation is gaining importance in plastic deformation processes because of its ability to form difficult-to-form materials like Ti-6Al-4V at much lower temperatures than hot/superplastic forming processes. Applying electric pulses with suitable parameters during plastic deformation reduces the flow stress near instantaneously (stress-drop) due to thermal (expansion and softening) and electro-plastic effects. To quantify the electro-plastic effect, one needs to predict thermal effects accurately. In the present work, electrically assisted uniaxial tensile tests on Ti-6Al-4V are carried out both in elastic and plastic regions. Flow stress reduction due to thermal effects are predicted using finite element analysis. Comparison of predicted thermal effects with that of experimentally measured in elastic region revealed that they are in excellent agreement, as it is well known that thermal expansion only plays a role in the elastic region. In the plastic region, a considerable difference between measured (thermal and athermal) and predicted (only thermal effects) stress-drop values is observed, and this difference is due to the electro-plastic effect. The effect of different process parameters on electro-plastic effect is studied, and the same is quantified.

The electroplastic effect in coarse-grained and ultrafine-grained titanium

One of the well-known features of the external action of the electric current in the process of plastic deformation is the electroplastic effect manifesting in a decrease in flow stresses and an increase in plasticity (deformability). Understanding the nature of the electroplastic effect provides targeted regulation and application of the effect to improve the efficiency of metal working processes or to change the structure and properties of materials. The deformation behavior of commercially pure titanium under the impact of an electric current of critical density from 12 to 400 A/mm2 is considered. The electroplastic effect in coarse-grained (d = 50 μm) and ultrafine-grained (d = 500 nm) VT1-0 titanium has been studied under a combination of tensile deformation and applied current of various modes and regimes, including the single-pulse, multipulse and direct current modes. It is shown that a decrease in the grain size contributes not only to an increase in the strength characteristics, but also to a decrease in the electroplastic effect, the mechanism of which is closely related to the density of mobile dislocations. It has been shown that the manifestation of the electroplastic effect in titanium is controlled by the grain size, and a decrease in the grain size leads to its electroplastic degradation and finally to the complete disappearance in the amorphous state due to a decrease in the density of free dislocations.

Electro-plastic effect on the indentation of calcium fluoride

Calcium fluoride (CaF2) possesses excellent optical properties, making it a promising material for many optical components. However, due to its brittleness, the intrinsic plasticity of CaF2 needs to be improved to enhance its machinability. The adoption of the external electric field is a feasible scheme, but the relevant electric effect on the mechanical performances of non-conductors remains little explored. This study aims to investigate the relationship between the applied electric field and the plasticity of CaF2 using both experimental and numerical approaches. Micro-indentation tests are conducted under varying electric field conditions to identify the effect of the electric field on the plasticity of CaF2. Molecular dynamics simulations are employed to investigate the deformation mechanisms under the electric effect. Specifically, the study investigates the correlated influence of the electric field direction and field intensity on plastic behavior. Combining the experimental and simulation results, it is demonstrated that the applied electric field significantly enhances the plasticity of CaF2. The field direction influences the growing direction and distribution of plastic deformations while the field intensity affects the degree. The systematic study deepens the understanding of the electro-plasticity of non-conductive ceramics, further informing the development of electric field-assisted machining techniques.

Neural network-based ductile fracture model for 5182-O aluminum alloy considering electroplastic effect in electrically-assisted processing

Complex components made of 5182-O aluminum alloy are usually formed at high temperatures due to their low ductility at room temperature, and the advanced current assisted processing can achieve energy-saving, high-efficiency, and green production compared with traditional hot forming. In this study, the coupled effects of electrical pulse, temperature, strain rate, and strain on flow behavior and ductile fracture are comprehensively investigated by experiments and constitutive modeling. In order to investigate the influence of electroplastic effects in different stress states, shear, hole and notched specimens are tested by isothermal tensile test and electrically-assisted isothermal tensile test in temperature ranges from 300 to 423 K and strain rates from 0.001 to 0.1/s. The evolution of the microstructure is characterized by electron backscatter diffraction to reveal the mechanism of the experimental phenomenon observed. An artificial neural network model is developed to characterize the dynamic hardening behavior and ductile fracture, and further embedded in ABAQUS/Explicit for numerical simulation. The results show that the experimental results are highly non-linear and coupled for the flow curves. At the same temperature, electric pulses have an independent effect on suppressing the Portevin–Le Chatelier effect and can reduce the deformation resistance, thereby forming products at low energy. The non-monotonicity of the fracture loading paths obtained from the hybrid experimental–numerical analysis is affected by strain rate hardening, thermal softening and electroplasticity. The neural network-based plasticity model is calibrated and validated to describe fracture initiation considering stress state, current density, temperature and strain rate.

Coupled effect of pulsed current and ultrasonic vibration on deformation behavior of Inconel 718 sheet: phenomena and modeling

Multi-energy fields provide higher dimensional precision control and desirable performance for microforming processes. The constitutive model under low strain level was established to describe the coupled effect of pulsed current and ultrasonic vibration based on the Kocks-Mecking (K-M) model. Combined with the crystallographic theory, the model introduced acoustic softening effect and electroplastic effect, and described the interaction between pulsed current and ultrasonic vibration under coupled field from the perspective of energy absorption. In the research, the uniaxial tensile test of Inconel 718 sheet with thickness of 0.3 mm was carried out. The ranges of the current density and ultrasonic energy density were 20.9–50.9 A mm−2 and 15.84–57.64 J cm−3, respectively. The results showed that: the facilitation of ultrasonic vibration on dislocation generation was limited with the increase of strain, and the energy generated by ultrasonic vibration was more reflected in the change of Helmholtz free energy; when the energy generated by the current at athermal condition tended to be saturated, the increase of heat became the key to improve the electroplasticity, and the heating model caused by Joule heating effect was established. The constitutive model of coupling electric pulse and ultrasonic treatment was established, combining the characteristics of ultrasonic vibration and pulsed current; the flow stress was described by using the parameters of ultrasonic energy density and current density; The trigonometric function was used to quantitatively describe the competitive relationship between ultrasonic vibration and pulsed current in external work. Two sets of experiments were used to demonstrate the reliability of the model.

Role of the Pulse Current Duty Cycle during Titanium Tension

The effect of a pulsed current on titanium tensile deformation obtained by postdeformation annealing after cold rolling of the coarse-grained and ultrafine-grained states has been considered. The effect of the duty cycle of the pulse current over a wide range on the shape of the stress–strain curves and mechanical properties has been studied. It is shown that an increase in the duty cycle results in an enhancement in the thermal effect of the current and a decrease in the flow stresses, strength, and plasticity, as well as in intense necking. A decrease in the duty cycle leads to the absence of heating and the occurrence of the electroplastic effect and an increase in the strength and plasticity, which depends on the structural state of coarse-grained titanium and the method of titanium production. The possible physical mechanisms of hardening associated with twinning, strain aging, and low-cycle fatigue have been considered.

Electroplasticity Mechanisms in hcp Materials

Herein, the mechanisms of the electroplastic effect (EPE) in different hexagonal close‐packed (hcp) metals under varying loading conditions and current densities through the analysis of flow curves and microstructural changes are investigated. The investigations show a significant change in the forming behavior of the hcp materials as a result of superimposed electric current impulses. This behavior could be attributed to two effects. On the one hand, additional dislocation types are activated; on the other hand, new characteristic twin bands are formed. This is shown for all three hcp materials under investigation: Ti, Mg, and Zn. Furthermore, the hypothesis of the existence of a critical value of the current density at which a significant change in the plastic behavior occurs is verified by the experiments. The magnitude of this critical value for the analyzed hcp materials corresponds approximately to the theoretical values reported to be in the range of 1.6 to 2.0 kA mm−2. In addition to the current density, the duration of the pulses also has an influence on the EPE. Understanding the correlation between the individual activated deformation mechanisms during electric pulse treatment can be crucial for controlling the electroplastic forming processes in a systematic and targeted manner.

Evaluation of electrically-assisted vacuum diffusion bonding of a TiBw/TA15 composite by microstructural analysis and mechanical testing

Electrically-assisted diffusion bonding (EADB) of a TiBw/TA15 composite with a novel network architecture was studied under a vacuum degree of 3 × 10−3 Pa in this work. Traditional diffusion bonding (TDB) was also investigated to verify the electroplasticity effect. Characterization of the bonding interface revealed that the interface bonding ratio and shear strength increased with bonding time, and the maximum shear strength of the EADB sample could reach 786 MPa when bonded at 4.23 A/mm2 and 2 MPa for 60 min. However, when bonded in the same condition, the EADB sample achieved higher bonding quality than the TDB sample did. Moreover, the potential mechanism through which electropulsing promoted diffusion bonding was studied in detail. The findings provide ideas for EADB of titanium matrix composites.