High Density Electric Current Pulse Research Articles

Solidification microstructure characteristics of Cu–Pb alloy by ECP treatment

Abstract The effects of high-density electric current pulse (ECP) treatment on the solidification of Cu–37.4 wt% Pb monotectic alloy melt were investigated. Compared to the method of molten glass purification combined with cyclic superheating, ECP treatment created finer microstructures of Cu–Pb alloys, with more homogeneous distribution of the Pb phase in the matrix. This phenomenon can be explained by the cluster theory for liquid metals and the non-equilibrium diffusion theory. First, ECP treatment could cause the fission of larger atomic clusters to increase the undercooling that enabled a large number of smaller clusters to grow and reach the critical nucleation radius. Second, ECP treatment could reduce the diffusion energy barrier to enhance the non-equilibrium diffusion of solute atoms and suppress the segregation of Pb.

Effect of High Density Current Pulses on Microstructure and Mechanical Properties of Dual-Phase Wrought Superalloy

In this study, high density electric current pulse (ECP) treatment was introduced instead of the conventional solution treatment, and the γ′ phase was completely dissolved under the ECP treatment within only several milliseconds at 1148 °C. Due to the extremely short treatment time and high cooling rate, the growth of γ-phase matrix grain and γ′ phase precipitate was effectively retarded. By comparing with the conventional heat process, the grain size of ECP treated sample was controlled to about 15 μm, the size of the re-precipitated γ′ phase reduced from 65 to 35 nm, and the number density of γ′ precipitate increased from 1.46 × 108 to 3.03 × 108/mm2. The Vickers hardness, ultimate tensile strength and yield strength of the ECP treated sample were significantly improved. According to the theoretical derivation of kinetics, the ECP treatment introduces an extra electrical free energy which promoted the dissolution of γ′ phase. The ECP treatment may provide a new method for solution treatment of the Ni-based superalloy.

Effects of high energy on steel structure in deformation

The author of the article investigates how microwave radiation affects the processes of active deformation and mechanical stress relaxation in stressed stainless steel specimen under the electric current and when vector E (of microwave radiation) moves in different directions along the axe of the specimen. When vector E of microwave radiation was oriented in a longitudinalis way and electric current was passed, the softening of metal increased from 22 to 30 %. Multi-criteria analysis of steel samples with electric current and samples with no electric current was made. Analysis findings showed that external energy deposition influenced greatly on deformation of steel crystals. The action of high-density electric current pulses and microwave radiation on a sample loaded above the yield strength increases the ductility of stainless steel, and its strength characteristics, the microstructure is modified. Phase composition of steel was also investigated. The studies showed that the content of martensitic and austenitic phases in steel changed significantly. Moreover, the results showed that there was an additional mechanism of electroplastic deformation in the crossed fields of microwave radiation and magnetic field of current.

Effect of High Density Electric Current Pulse on Solidification of Cu-37.4 wt.%Pb Monotectic Alloy Melt

The effect of high-density electric current pulse (ECP) on the solidification of Cu-37.4 wt.%Pb monotectic alloy melt was investigated. The microstructure formation mechanisms of ECP were clarified according to liquid metal cluster theory. The results demonstrated that with ECP treatment, the microstructure of Cu-Pb monotectic alloy became finer, the distribution of Pb phase in the matrix was more even and the solute trapping was significantly apparent. Based on the metal liquid cluster theory under ECP, the solid solubility increase result might be due to the salvation clusters increase under the action of pulse current, leading to the binding force increase among solute atoms and solvent atoms. Simultaneously, the aforementioned results were verified through the Differential Scanning Calorimetry (DSC) curve analysis. The results of hardness test, anti-friction test and wear- resistance test show that the ECP can enhance the hardness, improve the properties of anti-friction and wear-resistance of the alloy.

Microstructure and Crystallography of β Phase Formed through Electric Current Pulse (ECP) Treatment in Cold-Rolled Cu-40%Zn Alloy

Most of the studies on phase transformation in metallic materials have focused on transformations during cooling processes due to the easiness of the conservation of the product phase. However, for phase transformation happening during heating processes, the experimental investigations have been indirect if the product high temperature phase could not be preserved to the convenient observation temperature, for example the room temperature. The high density Electric Current Pulse (ECP) treatment allows the phase transformation during heating process and the preservation of the high temperature phase to the room temperature, offering possibilities for direct experimental examinations. Thus, in the present work, a cold-rolled Cu–40%Zn alloy was ECP treated and the microstructure of the product phase and the transformation orientation relationship were investigated. Results show that during the ECP treatment, the high temperature beta phase with BCC structure formed in the parent alpha phase with FCC structure. Especially, two kinds of orientation relationships could be detected between the parent alpha phase and the product beta precipitates. The one is the Kurdjumov-Sachs orientation relationship (K-S OR), and the other is the Nishiyama-Wasserman (N-W). In addition, the amount of beta precipitates obeying the K-S OR is more than that of precipitates obeying the N-W OR. The results of this work provide new fundamental information on phase transformation of metallic materials.

Crystal defect associated selection of phase transformation orientation relationships (ORs)

Phase transformation in solids always follows specific orientation relationships (ORs). The OR usually ensures a minimum lattice deformation for the structure change. However, in many cases different ORs are respected by the same transformation. The selection role and the link between the OR and the existing crystal defects needs further investigation. Thus, in this work, the α to β heating phase transformation induced by high density Electric Current Pulse (ECP) treatments in an annealed Cu–40%Zn alloy was investigated. Results show that the β phase obeys the K-S OR when formed along the α grain boundaries or in their vicinities, or the N-W OR when formed in the α grain interiors. In the former sites, the {111}α/<11¯0>α dislocation arrays were frequently observed, whereas in the latter, the {111}α/<112¯>α stacking faults were often found. Transformation strain analyses revealed that under the K-S OR the maximum lattice deformation required is a shear on the {111}α plane in the <11¯0>α direction, whereas under the N-W OR the maximum deformation is a shear on the {111}α plane in the <112¯>α direction. Thus the existing {111}α/<11¯0>α dislocation arrays along the α grain boundaries and in their vicinities provide pre-strain required by the transformation via the K-S path, whereas the {111}α/<112¯>α stacking faults boarded by {111}α/<112¯>α partial dislocations offer pre-strain facilitating the transformation via the N-W path. The present results provide new information on the role of crystal defects on phase transformation strain path and the selection of transformation ORs.

Effects of High-Density Electric Current Pulse on the Undercooling of Fe-B Eutectic Alloy Melt

The solidification microstructure of Fe-B eutectic alloy under high undercooling and high-density electric current pulse (ECP) was investigated with the technique of molten glass slag purification combined with cyclical superheating and the ECP treatment. The effects of high-density ECP on the undercooling of Fe-B eutectic alloy melt were analyzed by the DSC method. The analysis results showed that the solidification microstructure of Fe-B eutectic alloy under ECP was similar to that obtained by the high undercooling technique. The undercooling obtained under two experimental conditions was basically the same, proving that the high undercooling of the metallic melt could be realized by the ECP.

Effect of high-density electric current pulses on precipitation and mechanical properties of a Cu–Zn alloy

ABSTRACTBy applying high-density electric current pulses (ECP) on the Cu–Zn alloy, the high-temperature precipitated phase is explored and the related mechanical properties are investigated. Results show that the nucleation rate of the high-temperature β precipitation can be greatly accelerated due to the ECP treatment. Especially, an obvious orientation relationship can be detected between the matrix α phase and the product β precipitation by increasing the electric current density. In addition, for the samples with a dense distribution of refined β precipitation, a slight decrease in the tensile strength and a great increase in the elongation-to-failure can be observed.This paper is part of a themed issue on Materials in External Fields.

The surprising influence of continuous alternating electric current on recrystallization behaviour of a cold-rolled Aluminium alloy

Pronounced acceleration of recrystallization in cold worked metals is generally observed during annealing with additional high density electric current pulses. In this work, continuous alternating current of low density was applied to heat cold-rolled samples in a Gleeble machine. The recrystallization behaviour of these samples is compared with those heated in a conventional furnace using the same temperature-time path. It is shown for the first time that strongly accelerated recrystallization can also be achieved with low density alternating electric current. This effect is sensitive to prior cold-deformation and current direction, which provides new degrees of freedom for thermo-mechanical process design and microstructure control.

Phase Transformations in Metals Stimulated by a Pulsed High-Energy Electromagnetic Field

Processes occurring in metals with microdefects when metallic specimens are treated by short high-density electric current pulses are considered. The variations in the electric and temperature fields in the material and their influence on the phase transformations and the stress–strain state in the vicinity of microdefects in the form of plane cracks with linear sizes of the order of 10 μm are studied. A mathematical model of the effect of an electromagnetic field on a predamaged thermoelastoplastic material with an ordered system of defects is proposed. The model takes into account melting and evaporation of the material and the dependence of all of its physical and mechanical properties on temperature. The solution of the resulting system of equations is sought by the finite element method on moving grids with the use of the combined Euler–Lagrange method. The dependence of the processes on the boundary conditions of the model is considered. We estimate the error that occurs when solving the problem for one representative cell rather than for the whole sample with an ordered system of defects. The influence of the distance between the cracks on the deformation and healing of microdefects is investigated. Numerical modeling has shown that a high-density current with large field gradients arises in the vicinity of microdefects, which leads to intensive local heating accompanied by thermal expansion and melting of the metal on the tips of the microcracks. This results in high compressive stresses near microcracks, intensive plastic flow of the material and, as a consequence, clamping of microcrack sides, decrease in microcrack length, and ejection of the molten material into the crack. As a result, the microcrack is completely healed. The numerical results obtained by the proposed model agree with experiments. Computations showed that if the distance between microcracks is equal to or greater than ten of their lengths, then the time for complete defect healing weakly depends on the distance between defects and the micro-defect interaction can be neglected. The interaction between microcracks in the metal significantly affects their healing process if the distance between them is reduced to about 5÷6 of microcracks lengths. With further decrease in the distance between the defects up to one microcrack length, the healing process does not change qualitatively, but slows down significantly: the ejection of molten material into the crack still happens, but the crack size reduction, especially in the transverse direction, is substantially smaller.

Microstructural Evolution of AZ31 under the Application of High Density Electric Current Pulses

The microstructural evolution in annealed Mg-3Al-1Zn (AZ31) magnesium alloy during high density electric current pulses (ECP) treatment is investigated by using a same current density with different processing numbers. It is found when the processing number of the ECP treatment is not greater than four times, the grains are refined and more homogenized, and the texture intensity obtained from the (0002) pole figure appears an obvious enhancement from 11.22 to 22.88. However, increasing the repeated ECP processing number will cause the coarsening of grain size and the decreasing of the texture intensity. The mechanism of the microstructural evolution during ECP treatment is discussed from the point of view of grain boundary motion.

Microstructure evolution in metals induced by high density electric current pulses

In this paper, microstructure evolution in metals induced by high density electric current pulses (ECPs) treatment, such as grain refinement, formation of oriented microstructure and redistribution of inclusions, is carefully reviewed. The mechanism for these microstructure evolutions is discussed. It is suggested that there are usually several factors working during ECP: Joule heating (including temperature rise, the heating rate and the cooling rate), stress (including thermal compressive stress, skin effect, electron wind force) and free energy change caused by the current. The Joule heat and the stress usually affect the dynamics, enhance the microstructure evolutions rate, while the free energy change affects the thermodynamics and lowers the barrier of the phase transformation or recrystallisation. The experimental results indicate that high current density ECP provides an effective approach to refine the microstructure of polycrystalline materials and to improve the mechanical properties of the materials.

Non-uniform carbon segregation induced by electric current pulse under residual stresses

As-quenched carbon steel samples were treated with high density electric current pulses (ECP). After that, the carbon distribution and microstructures were examined, and the residual stresses were measured. The carbon segregation was apparent in the near-surface region, and in the internal region no carbon segregation was displayed after ECP treatment. The lath-shaped martensite remained identical before and after ECP treatment. The residual stress gradient was relatively large in the near-surface region, and small in the internal region. The theoretical analysis proved the drag force on carbon atoms due to drift electrons had a minimal effect on carbon diffusion; the Joule heat due to ECP and the quenched residual stresses were determined to be the primary contributing factors. The skin effect of ECP existed. Facilitated by the large stress gradient in the near-surface region, the metastable carbon atoms in the martensite were activated by high density ECP and diffused toward into grain boundaries and dislocations. In the internal region, the stress gradient was small, and the Joule heat due to ECP was insufficient for the diffusion of carbon atoms. The behavior of non-uniform carbon segregation is attributed to the non-uniform residual stresses and Joule heat of the ECP.

Simulating the effect of a high density electric current pulse on the stress field during plastic deformation

A certain pulse of electric current combined with plastic deformation is a powerful tool for improving the formability of hard-to-deform metal alloys. In recent years, much research has indicated that the current not only improves macro-mechanical properties but also influences microstructural-level phenomena such as recrystallization, local phase transformation, grain refinement, and even amorphous nanocrystallization. Despite the huge experimental dataset, virtually no focus has been placed on phenomenon’s computing. In this fashion, present paper concerned with the continuum-level numerical analysis of the pulse impact on the stress field during plastic deformation. Computation conducted herein has shown that a high density electric current pulsing weakens the stress field during plastic deformation. Ultimate results of the study should be useful in developing the novel metal processing technology.

β‘ Phase Precipitation in a Cold Rolled Cu-Zn Alloy under Electric Current Pulses

β' phase precipitation in a cold rolled Cu-Zn alloy under high density electric current pulses was studied in the present work. The results showed that the precipitation of β' phase was controlled by the angle between the current direction and rolling direction. When the angle was 45º, the application of electric current could refine α phase without β' phase precipitation, while at 0º or 90º, β' phase precipitated from α phase boundaries and distributed along the rolled direction. It was proposed that the precipitation of β' phase during the application of high density electric current was determined by the electron wind force and anisotropic electrical resistivity of the grain boundaries.

Influence of electric current on the mechanical characteristics of rail steel

We present the results of investigations into the influence of high-density electric current pulses on the mechanical characteristics of rail steels in the initial state and after a long in-service operating time. The effect of in-service loading conditions and modes of exposure to electric current pulses on the steel mechanical properties and their anisotropy is shown.

The electroplastic effect in the cold-drawing of copper wires for the automotive industry

The paper presents a comparison between conventional cold wire drawing and wire drawing combined with the application of high-density electric current pulses. The investigations were carried out having in mind the manufacture of copper wire for cables used in the automotive industry. A specially built experimental stand is described. The experiments showed a decrease in the pull force for wire drawing combined with the application of current pulses. The material properties of the drawn wires were tested. It was found that the plastic properties of the wire and its surface quality improved in the case of wire drawing combined with current pulses. The proposed technique of wire drawing is highly promising and potentially can replace the conventional on-line continuous annealing of the material.

On the nature of the low-field electromagneto-plastic effect

In the present study, reduction in the linear density of twinning dislocations resulting from the simultaneous application of high-density electric current pulses (40 A/mm2 for 400 μs) and constant magnetic field (with induction in 0.2 T) to bismuth crystals has been found. Current pulses and constant magnetic field were applied to the specimen after the deformation was introduced by a concentrated load and pileups of twinning dislocations were created. It has been clearly shown that the decrease in the linear density of twinning dislocations results in the elongation of the deformation twins. This effect occurs despite the increase of the number of twinning dislocations with an increase in the concentrated load. The experimental facts are in good accord with the modern concept of the electroplastic effect.

Thermally activated plastic flow of metals and ceramics with an electric field or current

The effects of high density electric current pulses (10 3–10 6 A cm −2) on the flow stress of metals at low homologous temperatures and of a modest external electric field on the flow stress of fine-grained oxides at high temperatures is presented. The results in both cases are evaluated in terms of thermally-activated plastic deformation processes. In the case of the metals, the influence of an electron wind on each of the parameters in the equation for the thermally-activated motion of dislocations was determined, the largest effect being on the pre-exponential. The derived electron wind push coefficient was one or more orders of magnitude larger than the value normally accepted for the electron drag coefficient. In the case of the oxides, the substantial effect of an applied electric field on the flow stress was evaluated in terms of its influence on the electrochemical potential of vacancies in the space-charge cloud adjacent to the grain boundaries. Both the derived space-charge cloud width and the electric potential/stress parameter Δ∅/Δ σ are in reasonable accord with theoretical predictions.

Electroplasticity in metals and ceramics

The influence of an electric field or corresponding current on the plastic deformation of metals and ceramics is reviewed. Regarding metals, the following are considered: (a) the effects of high density electric current pulse on the flow stress at low to intermediate homologous temperatures; and (b) the effects of an external electric field on superplasticity at high temperatures. The major effect of the current pulses was to reduce the thermal component of the flow stress. This resulted from the combined action of an electron wind force, a decrease in the activation enthalpy for plastic deformation and an increase in the pre-exponential, the last making the largest contribution. Besides giving a reduction in the flow stress during superplastic deformation, an external electric field reduced cavitation and grain growth. The influence of the external field appears to be on the migration of vacancies or solute atom-vacancy complexes along grain boundaries to the charged surface. In the case of ceramics, the effects of an internal electric field on the plastic deformation of polycrystalline NaCl at 0.28–0.75 T M and on the superplasticity of fine-grained oxides (MgO, Al 2O 3 and ZrO 2) at T>0.5 T M are considered. Regarding NaCl, at T≤0.5 T M an electric field E≥10 kV cm −1 is needed to enhance dislocation mobility in single crystals. However, a field of only 1 kV cm −1 significantly reduced the flow stress in polycrystals, which is concluded to result from an enhancement of cross slip. At T>0.5 T M, there occurred a decrease in the flow stress of polycrystalline NaCl along with a reduction in the rate-controlling diffusion activation energy. Regarding the fine-grained oxides at T>0.5 T M, an internal electric field E≤0.3 kV cm −1 gave an appreciable, reversible, reduction in the flow stress by an enhancement of the rate-controlling diffusion process. Limited work suggests that a field may also retard grain growth and cavitation in ceramics.