Electropulsing Treatment Research Articles

Rapid reversion of martensite in low-alloy steel under electropulsing treatment: Exploring feasibility of reversible transformation

Reversibility is an essential feature of martensitic transformation but is always believed impossible in FeC alloys since martensite (α') is unstable, which decomposes and forms cementite (θ) before reversion begins. Here we used electropulsing treatment (EPT) to inhibit θ precipitation and explored the feasibility of reversible transformation without α' decomposition in low-alloy steels (22MnB5 and 35CrMo steels). The reversion behaviors of the electropulsed specimens were analyzed by morphology, orientation, elemental distribution, and final microstructure. The result shows that in low carbon, low-alloy steel (22MnB5 steel), acicular γ abruptly grows through the whole pack without composition change. The prior γ grains are reproduced with the same grain size, shape, and orientation, indicating that inhibiting the θ precipitation does make steels with low carbon content achieve the reversibility of martensitic transformation. This phenomenon mainly results from reducing the effective driving force for θ precipitation by the current. However, in low-alloy steel with higher carbon content (35CrMo steel), 3D orientation distributions reveal that the prior γ grain is divided into equiaxed grains with various orientations and irregular grains with slightly changed orientations. This result suggests that when carbon content increases above a limit, high dislocation densities lead to recrystallization and destroy the original orientation relationship.

Improved Prediction for Nonlinear Thermal History of Mg Alloy Subjected to Multiple Electropulses

The repeated rapid heating and nonlinear cooling associated with electropulsing treatment (EPT) hinder the accurate modeling of temperature variations based on a constitutive approach using physical models. This objective is not attainable through conventional machine learning (ML) approaches as well, such as artificial neural network (ANN) and long short‐term memory (LSTM). The present study investigates the intricate nonlinear thermal history of Mg alloys induced by EPT to improve a predictive model under multipulse conditions. Furthermore, it enhances the predictivity and generalizability of a given problem using an ML model called i‐LSTM by specifically tailoring a dataset. The i‐LSTM model performance surpasses that of the constitutive approaches and conventional ML models when predicting the complicated thermal history of EPT specimens in a wide variety of processing parameters. The mechanisms underlying this improvement are discussed based on several experimental datasets. This study not only addresses challenges in existing models but also offers a promising avenue for understanding and predicting the thermal dynamics of Mg alloys subjected to the multipulse EPT process.

Electropulsing-induced recrystallization of cold-rolled SUS304 and effects of duration on mechanical properties

A cold rolled SUS304 stainless steel was treated by electropulsing treatment (EPT) with different time and duration, and then, microstructural evolution and texture development was studied by electron backscattering diffraction (EBSD) technique. EPT enhances the recrystallization process of cold rolled SUS304 stainless steel and the completed recrystallization state of sample can be obtained at low temperature below 530 K for a short time of 100 s. The best strength-ductility combination can be obtained when 210 μs of pulse duration applying. Pulse duration has been found to have a significant impact on the microstructure and tensile properties of SUS304. EPT is not a simple heat treatment process and electrical effects should be responsible for the recrystallization of the EPT samples.

Electropulsing anisotropy of cold-rolled Grade 2 titanium sheet: Effect of electric current direction on recrystallization and hardness

This study investigated the effects of electropulsing treatment (EPT) condition on the microstructure and microhardness of cold-rolled Grade 2 Ti sheet. Particular attention was paid to the dependence of the material properties on the electric current direction, referred to as electropulsing anisotropy. Direct-current (DC) EPT was applied along either the rolling direction (RD) or transverse direction (TD) of the rolled sheet. EPT along the TD resulted in a higher heating rate and maximum temperature, whereas that along the RD accelerated the static recrystallization (SRX) process under identical electropulsing parameters. Such a discrepancy was interpreted using the elongated grain structure and basal texture in the cold-rolled Ti sheet. These results were further supported by microhardness measurements, confirming the presence of electropulsing anisotropy during DC EPT in Ti alloys for the first time. In addition, the employed EPT was compared to a traditional furnace heat treatment (FHT) with rigorous temperature and time control, wherein the optimum EPT process spent only 13% of the processing time to complete SRX with a lower thermal energy requirement. This indicates a significant athermal contribution of EPT to electropulsing anisotropy.

Improving the wear resistance of Ti–6Al–4V alloy through electro-pulsing combined with laser shock peening

In this study, the effects of surface treatment namely electro-pulsing followed by laser shock peening (EP-LSP) on wear resistance properties of Ti–6Al–4V alloy are investigated. It was compared with samples treated solely with laser shock peening (LSP) and electro-pulsing treatment (EP) to investigate the effects of different processes on the friction and wear properties of Ti–6Al–4V alloy. In addition, the friction coefficient, wear rate, wear morphology were also investigated. Finally, the wear mechanism was described in detail. The results demonstrated a significant reduction in grain size for the samples after EP-1-LSP treatments. Furthermore, compared with the as-received samples, the maximum hardness of EP-1, LSP and EP-1-LSP treated samples increased by 6.5%, 5.9%, and 19.6%, respectively, and the compressive residual stress on the surface increased by 2.4%, 15.0%, and 37.2%, respectively. Under the same friction conditions, the depth of wear scar of the samples after EP-1, LSP, and EP-1-LSP treatments decreased by 7.2%, 10.6%, and 18.5% than that of the as-received sample. The as-received samples mainly exhibited severe oxidative and adhesive wear, and the LSP and EP-1 treated samples showed slight adhesive and oxidative wear. The EP-1-LSP treated samples predominantly showed signs of abrasive wear.

Effective toughening of dissimilar Ti alloy friction welded joint by combining electropulsing thermal and athermal effects

To address the low toughness issue in titanium alloy friction weld joints, this study employs electropulsing treatment to synergistically control the microstructure of TC11/TC17 linear friction welded (LFWed) joint through thermal and athermal effects, transforming equiaxed grains into a basketweave structure in the weld zone. The toughness of the joint increased from 9.3 J/cm2 to 33.5 J/cm2, while the strength maintained surpasses that of both parent materials.

Enhanced strength–ductility synergy in Ti55531 titanium alloys through gradient microstructural design strategy

The strength-ductility trade-off dilemma still exists for titanium alloys, which is a challenging and pressing issue within the realm of microstructure design and property control. In the present work, a typical axial gradient microstructure was prepared in Ti55531 titanium alloy through a composite strengthening process involving rapid electropulsing treatment (EPT) combined with step-quench treatment (SQT). The results show that a significant improvement of 450 MPa (54 %) and 6.3 % (100 %) for the strength and ductility are obtained respectively for gradient microstructure by EPT + SQT (GMES) compared to that gradient microstructure prepared by EPT (GME) only. This indicates that the enhanced strength-ductility synergy observed in GMES is attributed to the characteristic Twinning induced Plasticity (TWIP) mechanism employed in this novel design of gradient microstructure. Furthermore, a detailed analysis and discussion are provided on the physical deformation mechanism and crack initiation behaviour of the gradient microstructure, providing valuable insights for the design of high-strength, high-ductility metals such as beta titanium alloys, advanced steels, and multi-phase alloys.

Accelerated precipitation and enhanced mechanical properties of Cu-3.5Ti alloy via electropulsing treatment

Aging is an important step in improving the comprehensive properties of Cu-3.5Ti alloy. However, both prolonged holding times and high heat treatment temperatures lead to significant oxidation and substantial energy consumption. The effects of heat treatment (HT) and electropulsing treatment (EPT) on the microstructure and mechanical properties of the solid solution Cu-3.5Ti alloy were comparatively examined in this study. The findings demonstrated that EPT quickly completed the processes of spinodal decomposition and short-range ordered to long-range ordered phase transformation, thereby fully precipitating fine and diffusive β′-Cu4Ti phase, which greatly enhanced the precipitation strengthening effects. In addition to enhancing the strength of the alloy, EPT significantly reduced the peak aging time (to just 1/12 of that of HT) and greatly increased the plasticity (elongation to failure rose by 47 % compared to HT). The main reason for the rapid completion of the spinodal decomposition and the full precipitation process by the EPT was the additional free energy provided by the electropulsing, which significantly accelerated the Ti atomic diffusion and lowered the nucleation barrier of the β′-Cu4Ti phase. EPT is a toughening heat treatment process for metallic materials, which offers the advantages of high efficiency, low energy consumption, and low cost.

Strengthening and Toughening of ZG25SiMn2CrB Steel without Tempering Brittleness via Electropulsing Treatment.

High-strength low-alloy steels are widely used, but their traditional heat-treatment process is complex, energy-intensive, and makes it difficult to fully exploit the material's potential. In this paper, the electropulsing processing technology was applied to the quenching and tempering process of ZG25SiMn2CrB steel. Through microstructural characterization and mechanical property testing, the influence of electropulsing on the solid-state phase transition process of annealing steel was systematically studied. The heating process of the specimen with the annealing state (initial state) is the diffusion-type transition. As the discharge time increased, the microstructure gradually transformed from ferrite/pearlitic to slate martensite. Optimal mechanical properties and fine microstructure were achieved after quenching at 500 ms. The steel subjected to rapid tempering with 160 ms electropulsing exhibited good, comprehensive mechanical properties (tensile strength 1609 MPa, yield strength 1401.27 MPa, elongation 11.63%, and hardness 48.68 HRC). These favorable mechanical properties are attributed to the coupled impact of thermal and non-thermal effects induced by high-density pulse current. Specifically, the thermal effect provides the thermodynamic conditions for phase transformation, while the non-thermal effect reduces the nucleation barrier of austenite, which increases the nucleation rate during instantaneous heating, and the following rapid cooling suppresses the growth of austenite grains. Additionally, the fine microstructure prevents the occurrence of temper brittleness.

Electropulsing of low-density duplex steel for strengthening and ductilization by microstructural refinement

The present investigation finds the effect of electropulsing treatment on the mechanisms of refining and reprecipitation of ordered L20 structure (B2) and its impact on tensile properties of low-density duplex steel of Fe-18Mn-10.5Al-1 C-6Ni composition. The selected composition without manganese is melted at 1600°C by induction heating in a vacuum but Mn is added into the melt at 1560°C, in the argon atmosphere to reduce the loss. The melt is cast into a plate form in a copper mold. The cast steel is homogenized, hot rolled, and annealed (PD1-A sample) to get elongated bands, spheroids, and platelets of B2 phase with an austenitic matrix which results in a yield strength of 1015 MPa, and tensile strength of 1285 MPa with a plastic elongation of 16.3%. Electropulsing partially dissolves the B2 phase at a temperature much lower than the equilibrium solvus by reducing barrier energy because of electron wind. As a result, the sizes of bands and spheroids are reduced, and platelets are disintegrated. Electron wind force deforms coarse precipitates, and fragments as well as spheroidized it. Electropulsing of annealed steel (PD1-AE) mobilizes dislocations, and recovers it in B2, but induces localized recrystallization of austenite. At a later time of the signal, the temperature rapidly falls and the matrix becomes supersaturated but electropulsing accelerates low-temperature precipitation of the B2 phase. Therefore, electropulsing can be adopted as a fast manufacturing process to dissolve, deform, fragment, spheroidize and reprecipitate high temperature phase at much lower temperature by dominant athermal effect of electron wind energy over thermal effect. Electropulsing also induces recovery and recrystallization at a relatively low temperature and reduces defect density. Reduction in the overall size of B2 and its distribution towards better uniformity provided an increased yield strength of 1077 MPa, tensile strength of 1355 MPa, and a plastic elongation of 21.6%. The selected duplex low-density steel follows two-stage work hardening of Ludwigson model with an additional stage of dynamic recovery. Fractography analysis confirms that both samples show mixed modes of ductile and brittle fracture, with an increase in size and area percentage of dimples.

Reconstructing brittle hydrides using pulsed electric current to restore the degraded performance of zirconium alloys

When zirconium alloy undergoes water corrosion reaction during the service process, it absorbs hydrogen to form brittle hydrides, which is one of the main reasons for the deterioration of the mechanical properties of zirconium alloy. There is an urgent need to find an effective method to regulate brittle hydrides to restore the properties of hydrogen-containing zirconium alloys. This study conducted electropulsing treatment on the hydrogen-charged nuclear grade Zr-4 alloy, and the results showed that the electropulsing process caused the hydrides to transform from large-sized brittle stripe-shaped δ-phase to the small-sized ζ-phase. The hydrogen content in the alloy is significantly reduced, which basically restores the mechanical properties of the alloy. However, in the comparative experiment of isothermal heat treatment at 400 °C, brittle hydrides did not dissolve and their mechanical properties did not recover. A mathematical model was established for the characteristics and morphological changes of strip hydrides, and the current density distribution and introduced free energy during the microstructure evolution process were calculated and analyzed. The calculation results indicate that the pulsed current promotes the decomposition of hydrides through the introduction of free energy, reduces the activation energy of hydrogen atom diffusion, accelerates the diffusion and desorption of hydrogen atoms, and realizes the repair of mechanical properties of hydrogen-charged zirconium alloy.

A promising approach to enhance fatigue life of TC11 titanium alloy: Low dislocation density and surface grain refinement induced by electropulsing

The fatigue life of components can be significantly enhanced by the formation of the surface hardness layer through surface strengthening technology. To avoid the geometric distortion of thin-walled components caused by strengthening, the strengthening energy is limited and the ideal strengthening effect cannot be obtained. This work aims to propose a novel approach to address this issue effectively. The surface layer with high-density dislocations was obtained by a low-energy surface strengthening method (shot peening) at first. Then the surface strengthening mechanism changes from dislocation strengthening to grain boundary strengthening after electropulsing treatment (EPT). The evolution of residual stress and microstructure was analyzed using multi-scale characterization techniques. The results demonstrate that EPT followed by surface strengthening makes a remarkable 304 % increase in fatigue life of TC11 titanium alloy. The enhancement of fatigue life can be attributed to the grain refinement accompanied by the formation of nanotwins and sub-grains in the surface-strengthened layer, as well as the reduction in dislocation density within the substrate after EPT. This study demonstrates the significant potential of EPT in further enhancing the fatigue life of surface pre-strengthened thin-walled components.

Crack healing via electropulsing treatment applied to additive-manufactured TiC/316L stainless steel composites

Three coupons of TiC/316L stainless steel composites were fabricated by laser-based directed energy deposition (DED-LB). One coupon is crack-free, and the other two coupons have cracks inside. Electropulsing treatment (EPT) was applied to heal the cracks, with various current densities and 75 pulses on each sample. The cracks with widths of 0–200 μm can be healed by EPT. The microstructure of the healed region consists of refined fractured TiC particles and resolidified fine austenite grains. The EPT is able to potentially be applied to heal the defects or cracks in additive-manufactured metal-matrix composites.

Electropulsing-enhanced atomic diffusion and recrystallization to optimize mechanical properties of Al/Cu laminated metal composites

This work adopted the combined processing of electropulsing bonding (EPB) and electropulsing rolling (EPR) to prepare high-performance Al/Cu laminated metal composites (LMCs). The results show that electropulsing (EP) significantly accelerates the diffusion of Al and Cu atoms, thus facilitating the formation of diffusion layer. Through EP treatment, the recrystallization behavior of the Al and Cu layers is enhanced. The microstructure composed of a diffusion layer with suitable thickness and fine recrystallized grains contributes to the mechanical properties. Due to the introduction of intermetallic compounds (IMCs), the hardness and elastic modulus of Al, Cu, and diffusion layers exist large differences. The hard diffusion layer can exert a rigid deformation constraint on the ductile Al and Cu layers, and promote the interface deformation. Meanwhile, the tensile strength of as-EPRed samples increases and then decreases with the increasing EP frequency, the maximum is ∼215.2 MPa at 20 kHz. Theoretical analysis indicates that the local Joule heating and athermal effect should be the principal factors for microstructure evolution and property optimization. These results are expected to clarify the EP-related mechanism on atomic diffusion and recrystallization, which provides guidance for the fabrication of Al/Cu LMCs.

Mechanism of Electropulsing Treatment Technology for Flow Stress of Metal Material: A Review

Residual stress is caused by non–uniform deformation caused by non–uniform force, heat and composition, which is of great significance in engineering applications. It is assumed that the residual stress is always the upper limit of the elastic limit, so the reduction of the flow stress will reduce the residual elastic stress. It is particularly important to control the flow stress in metal materials. Compared with traditional methods, the use of electropulsing treatment (EPT) technology stands out due to its energy–efficient, highly effective, straightforward and pollution–free characteristics. However, there are different opinions about the mechanism of reducing flow stress through EPT due to the conflation of the effects from pulsed currents. Herein, a clear correlation is identified between induced stress levels and the application of pulsed electrical current. It was found that the decrease in flow stress is positively correlated with the current density and the duration of electrical contact and current action time. We first systematically and comprehensively summarize the influence mechanisms of EPT on dislocations, phase, textures and recrystallization. An analysis of Joule heating, electron wind effect, and thermal–induced stress within metal frameworks under the influence of pulsed currents was conducted. And the distribution of electric, thermal and stress fields under EPT are discussed in detail based on a finite element simulation (FES). Finally, some new insights into the issues and challenges of flow stress drops caused by EPT are proposed, which is critically important for advancing related mechanism research and the revision of theories and models.

Friction-mediated tuning of shear deformation and microstructure in asymmetrically rolled Mg–1Gd alloy strips

Asymmetric rolling (ASR) is distinguished from conventional symmetric rolling (SR) process as the mismatched rolling speeds of two rolls introduce additional shear deformation to the rolled strips. Friction between the rolls and strip plays an important role in the formation of shear during ASR, which further impacts the microstructures of metallic strips. In this study, finite element (FE) simulations are used to probe the effects of friction in ASR of magnesium (Mg) alloy strips with varied roll speed ratios. The results on shear deformation distributions through the thickness of the strips reveal that the increase of friction coefficient or roll speed ratio induces greater shear deformation simultaneously. It is also found that the roll speed ratio can only promote the shear formation up to a threshold value which is dictated by the friction coefficient, further increasing the mismatch of the roll speeds provides negligible improvement on the plastic deformation of the strips. Experimentally, Mg–1Gd alloy strips are processed by SR and ASR, followed by electropulse treatment (EPT) to induce static recrystallization (SRX). The predictions of the numerical models are verified via EBSD and XRD observations on the grain refinement and texture weakening in Mg–1Gd alloy, which are the direct microstructure consequences of plastic deformations during rolling. At the roll speed ratio of 1.67, the alloy shows a weakened basal texture with an average grain size of 6.0[Formula: see text][Formula: see text]m after SRX. Further increasing the roll speed ratio to 5, however, does not enhance the grain refinement or texture weakening significantly.

Ultrafast annihilation of irradiation-induced defects using pulsed electric current for damage performance regeneration

As the most important irradiation-induced defects, dislocation loop and copper-rich nanocluster are the major contributors to the embrittlement of the neutron-irradiated reactor pressure vessel steels. In this study, such nano-defects were introduced into the material by 3 MeV Fe ions up to the dose of 1 dpa at high temperature (290 °C) to simulate neutron irradiation. It was found that pulsed electric current can effectively reduce 95 % of irradiation-induced hardening. Correspondingly, the characterization results showed that almost all the dislocation loops disappeared and the quantity of copper-rich nanoclusters also reduced greatly at relatively low temperature (450 °C), and the process took only 20 min. Meanwhile, it was qualitatively proved by positron annihilation spectroscopy that the number of irradiation-induced vacancy-type defects and solute-enriched clusters was significantly decreased after electropulsing. Furthermore, under the pulsed electric field, the rapid annihilation of the dislocation loops due to their accelerated collision with vacancies can remove the nucleation sites of the copper-rich nanoclusters and make them become dispersed, further promoting the nanoclusters that lack nucleation sites dissolving faster. Therefore, this electropulsing treatment provides a practical ‘‘in-situ” performance repair technology to extend the service life of reactor pressure vessel steels by regulating the interaction between vacancies, interstitial atoms and irradiation-induced defects.

Strength and ductility improvement of the axial gradient microstructure TC21 titanium alloys manufactured by electropulsing plus step-quenching treatment

In order to reduce the weak area caused by traditional connection method and improve the structural efficiency of aircraft, the design concept of an integral component with dual microstructure is proposed. The electropulsing treatment (EPT) was used to produce an axial gradient microstructure (AGM) in TC21 titanium alloy, followed by the step-quenching treatment (SQT) to tailor the precipitation behavior of the α phase. Results demonstrate that the AGM produced by the combination of EPT and SQT achieves a simultaneous enhancement in both strength and plasticity of the alloy. Compared to EPT alone, the SQT results in the dissolution of the bimodal microstructure region with small-sized α phase, and an increased density of secondary α phase precipitation in both the transition and lamellar microstructure regions. During room temperature tensile deformation, coordinated distribution of deformation occurs sequentially in various regions, the 101̅0112̅0type of prismatic slip and deformation nanotwins exist in the equiaxed and lamellar α phase, respectively. Moreover, the deformation gradually declines from the bimodal region to the lamellar region, leading to the fracture of AGM in the bimodal region and an overall augmentation of the strength and plasticity of TC21 alloy. Hence, the results of this paper can provide research basis for the strength and plasticity improvement and deformation damage mechanism of high strength and toughness titanium alloy.

Mechanical properties and microstructural evolution of 5052 aluminumalloys by multi-pass stretching and induced electro-pulsing treatment

A compound-forming method that combines multi-pass stretching and induced electro-pulsing treatment (IEPT) is proposed. The effects of induced current on the microstructure and mechanical properties of material were investigated using transmission electron microscopy and electron backscatter diffraction measurements. The samples stretched by 5% each time showed 145% increase in elongation after 4 stretching and IEPT cycles. The IEPT caused “stress drop” in pre-deformed sheets, while the plasticity increased. Meanwhile, the dislocation density decreased, the deformation structure changed to the substructure, and the grain size increased slightly. The material anisotropy was reduced and a new 《111》//TD texture appeared after IEPT. The combination of electrical effect and Joule heating enhanced dislocation mobility and vacancy movement. The temperature distribution, with a maximum temperature reaching 144.7 °C, was simulated using ANSYS software. The theoretical calculation results were consistent with experimental data.

Improve the microstructure and properties of Al0.5CoCrFeNi compositionally complex alloy fabricated by selective laser melting with electropulsing treatment

In order to improve the microstructure of the Al0.5CoCrFeNi compositionally complex alloy (CCA) prepared by selective laser melting (SLM), the electropulsing treatment (EPT) has been used as a post processing in this study. The strengthening and softening mechanism was analyzed by comparing the microstructure of SLM-Al0.5CoCrFeNi CCAs under different EPT parameters. At a 10 s power on time, the substructure size of CCA decreases continuously with increasing voltage. Under 90V/10s EPT, the yield strength increases from 658 ± 10 MPa in the printed state to 748 ± 17 MPa, and the microhardness increases from 265.1 ± 5.6 HV0.1 to 318.9 ± 5.0 HV0.1. At 90V/20s EPT, the cellular substructure disappears, and a large number of uniformly distribute BCC phases precipitates inside the FCC matrix. At 90V/30s EPT, recrystallization occurs and the BCC phase coarsen. Dislocation network is the main factor that affects the strength of SLM-CCA. EPT can change the dislocation network structure to enhance the strength of SLM-CCA. Simultaneously, the EPT effectively promotes the transition from FCC phase to BCC phase in a short time. Joule heat and electronic wind force are the main reasons for changing structure and properties of SLM-Al0.5CoCrFeNi CCA. This work provides a post-processing method named as EPT to effectively control the microstructure in additive manufacturing CCA products.