Abstract
Electrically assisted manufacturing is based on the electro-plastic effect induced by electricity on the material flow during deformation and represents an alternative method for forming materials. Several studies have pointed out the real effectiveness of this technique, but no relations among microstructure, electrical resistivity, crystal structure and deformation-mode have been revealed. In the present work, the stacking fault energy (SFE) was taken into account and three FCC materials possessing different SFEs were strained in electrically assisted uniaxial tension under continuous current application. The results showed an advantageous electric contribution only in the highest SFE material, whereas no enhancements in formability were revealed in the investigated low- and intermediate-SFE metals.
Introduction
In the past years, many efforts have been spent on the study of the effect of electricity within metals during deformation, in order to understand in which extent the application of continuous and pulsed electrical current can induce enhancements on their formability. Starting from the pioneering works of Conrad and Troitskii,1–3 the researchers involved in the study of the so-called electroplasticity have performed numerous experiments on a wide range of metals and alloys,4–24 leading to the development of theories explaining the phenomenon known as electro-plastic effect (EPE). These works have revealed that the passage of electricity can cause a modification of the materials flow properties by improving their workability, therefore making this method suitable to be adopted in forming operations. In this perspective, the EPE-based technique known as electrically assisted manufacturing (EAM) may represent an alternative method for materials manufacturing, allowing for the obtainment of increased formability and aiding to overcome the limitations imposed by conventional techniques such as hot- or cold-working processes. 20 EAM is applicable to all conductive metals and several studies have pointed out its real effectiveness, especially in forging, 5 where reductions in the required stresses and improvements in the strain at rupture have been observed.
Recently developed theories consider the concept of dislocations/electrons interaction as the driving force for EPE and three concurrent effects have been found to induce EPE. Localised resistive heating, kinetic energy from direct dislocations–electrons interactions and the excess of electrons are considered responsible to allow dislocations for passing lattice obstacles more easily, thus causing an enhancement in materials workability by improving the total elongation and decreasing the flow stress. 20 Among these factors, the localised resistive heating at atomic-level caused from the passage of electricity is considered to enhance dislocations motion, owing to the occurrence of diffusional contribution. On the other hand, dislocation–electrons interactions can take place when flowing electrons impact the dislocation lines, assisting in pushing dislocations and further enhancing plastic deformation and materials ductility (the so-called electron-wind force). Finally, the addition of excess electrons will assist in breaking and reforming of metallic bonds and, as they are able to break and reform easier, the ductility of the metal is improved. 20 Moreover, changes in electronic structure of the lattice near the core region of defects may be an important factor on affecting and controlling EPE, since the rearrangement of conduction electrons occurring during plastic deformation causes local changes in electronic symmetry. Therefore, in addition to elastic and electric–dipole interactions, others interactions can occur if the dangling bonds that may exist in some dislocation cores capture flowing electrons. 25
Nevertheless, the unavoidable Joule-heating effect due to the passage of an electric current may hinder the induced EPE during EAM, since heat and electricity are contemporarily both present. The material property affecting the extent of the Joule effect is the electrical resistivity, which increases during deformation owing to the generation of new dislocations, point defects and interfacial defects within the lattice, thus causing a non-uniform distribution of electron collisions. Therefore, if a proper cooling device is not set up, the application of high current densities may lead to deformation processes that are performed at relatively high temperatures, altering the observed behaviour because to the presence of thermally activated deformation mechanisms. In this regard, the knowledge of a real electrical contribution on materials formability requires a separation of thermal effects from those deriving from current application, since temperature risings can alter the observed behaviour when an external load is applied.
EPE has been observed in materials possessing different crystal structure, electrical resistivity and thermal conductivity, and deformed under different conditions.1–24 In a recent work, 20 a decrease of the electrical threshold as the material resistivity increased has been noted, but no relations between lattice type and EPE were revealed; this reduction in the threshold value may support the theory of localised heating from electrons scattering-off from lattice obstacles, allowing the lattices to expand easier. Further, in some cases, it has been observed a possible effect on the material flow stress that appears to be solely due to the electrical influence, beyond what would be expected from the temperature effects. However, even if EPE has been found to be more significant in BCC materials respect to FCC ones, 16 a direct correlation among crystal structure, electrical resistivity and EPE seems not to be the key point for the occurrence of electroplasticity. On the contrary, by analysing the available literature data on EAM,4–24 a metallurgical parameter that is likely to be more connected to EPE could be the stacking fault energy (SFE) of the tested material – or that of the constituting phases. SFE is a property of the considered metal or alloy and it depends on the electron-to-atom ratio (i.e. the ratio of valence electron concentration to concentration of atoms in an alloy), which has been also found to control several physical properties of materials. 26 SFE is known to govern the deformation mechanisms in FCC materials,27,28 playing a fundamental role in plastic deformation by determining a preferred dissociation of dislocations into partials when its value is low, thus acting as a further obstacle against dislocation motion within lattices. Moreover, SFE influences the atoms mobility during annealing at high temperatures, leading high-SFE materials to preferentially recover rather than recrystallise since dislocations rearrangements are less hindered. 27
The present work is a part of an interdisciplinary project recently started at the Industrial Engineering Department (DII) of the University of Padova and is aimed to investigate the various aspects of the EPE in metals during tensile tests for its application to an industrial scale. In this manuscript, the experimental results on three FCC metals possessing different SFEs are discussed, considering the occurrence of EPE in terms of both reduction in the ultimate tensile strength (UTS) and enhancement in the strain at fracture with respect to room-temperature tests (i.e. improvement in formability). The materials were strained in electrically assisted uniaxial tension by registering the reached temperatures, and the observed mechanical properties under continuous current application were compared to those obtained in purely thermal regimes, in order to determine the presence of an effective contribution of electricity apart the heating due to Joule's losses. This attempt in separating the electrical contribution from the thermal one to EPE is rather new, since few records in literature report comparisons of this kind.
Materials and methods
The tested materials were an AISI 316L austenitic stainless steel (SFE ∼40 mJ/mm2) provided by the Italian division of Outokumpu S.p.A., a C11000 nearly pure copper (SFE ∼90 mJ/mm2) and a 99.5% purity aluminium AA1050-H24 (SFE ∼200 mJ/mm2). The materials, supplied in form of sheets having thicknesses in the order of 1–1.2 mm, were strained under uniaxial tension using a strain rate of 10−2 s−1. The adopted deformation rate was chosen by considering that AISI 316L, like others low-SFE austenitic steels, is sensitive to the TRIP (TRansformation Induced Plasticity) effect, which involves a strain-induced phase transformation during deformation at room temperature, leading to improvements in the mechanical characteristics of the material.29–31 Thus, the steel was subjected to a preliminary characterisation at various strain rates (from 10−1 s−1 to 10−4 s−1), since the occurrence of TRIP may hinder the observation of EPE. In the AISI 316L under study, the TRIP effect has been observed for deformations rates equal and lower than 10−3 s−1 and, therefore, the selected experimental strain rate was 10−2 s−1. Conversely, copper and aluminium do not suffer of such strain-induced transformation, and the selected strain rate did not cause any particular alteration of their mechanical behaviour.
The uniaxial tensile tests were performed on a universal 5 t MTS™ testing machine adapted for electric current testing, as shown in Fig. 1. The electrical power was provided through a DC current generator, composed by six modules that grant a maximum power of 60 kW at low tension (10 V) and high current (6000 A). The same generator was provided with an in-line current intensity control with an accuracy of 1 A. The tensile specimens were connected to the high current circuit by two copper jaws, and four slices of high-density wood with a thickness of 5 mm were interposed between the sample and the clamps, in order to guarantee the electrical insulation. The temperature was measured by a laser pyrometer pointed in the middle of the specimens, which were painted with black opaque paint to avoid reflection and stabilise emissivity. Moreover, during the electric tensile test, cold air at eight bar of pressure was blown through two nozzles, to maintain the sample heating as low as possible. The temperatures recorded in current regime were employed to reproduce tests in thermal conditions in a climatic oven, with the aim to separate heating-related effects to those induced by electricity; in the so-called thermal tests, a climatic chamber applied at the testing machine was used to heat up the tensile specimens, with an accuracy of 1°C (Fig. 1).
Experimental settings for uniaxial tension: a electrically assisted and b thermal
The outcomes from the performed electrically assisted and thermal tests were then compared each other and related to the room-temperature ones (at 25°C, hereinafter called baselines), in order to achieve a discrimination between electrical and thermal effects. However, depending on materials resistivity, the continuous current application to a metal workpiece leads to an unavoidable rising in temperature that was different in the materials under study, thus forcing to test them in different current-density ranges (Table 1 and Fig. 2). Therefore, in order to limit excessive specimens heating, the testing temperatures were imposed to not exceed a homologous temperature TH of 0.5 ( Temperature rise in the tested materials as a function of the employed current density Materials resistivity and experimental current/temperature ranges
– being Tm the absolute melting temperature of the material and T the testing one). Indeed, too high temperatures can cause the lowering of the activation free energy threshold for unpinning dislocations from relatively weak constrains, leading to important diffusional contributions on deformation mechanisms and making more complicated the interpretation of the results. Note that the use of the homologous temperature allows for a proper comparison of the mechanical properties at various temperatures of materials having very different melting points, since it is representative of comparable metallurgical states. In this regard, it is worth to note that for the three considered materials the straining at room temperature is different in terms of homologous temperature, since 25°C corresponds to TH of about 0.18, 0.22 and 0.32 for AISI 316L, C11000 and AA1050-H24, respectively.
The results deriving from electrical and thermal tests were evaluated by considering the absolute reduction in the UTS and the relative variation of the electrically assisted strength with respect to the thermal one (UTS
R
):
Moreover, in order to compare possible improvements in the strain at fracture, the relative variation in the true strain 
was evaluated using the following relation:
is the true strain at fracture obtained from room-temperature tests while 
is the one deriving from the considered thermal/electrical test. In each electrical and thermal test, very narrow repeatability were obtained, with maximum standard deviations in the order of 5 MPa for UTS and about 0.001 for total elongation.
Experimental results
AISI 316L austenitic stainless steel
The material was tested using continuous current densities from 5 to 20 A/mm2 (Table 1), since the adopted cooling device did not allow for the application of higher current densities because of the high electrical resistivity and low thermal conductivity of the steel. Among the tested materials, AISI 316L possesses the lowest SFE value, and the performed electrical and thermal tests provoked a reduction of the mechanical properties with respect to the baseline. Concerning the observed total elongations, a clear difference between the tests performed in current regime and the thermal counterparts can be revealed, showing that electricity affected the steel properties mainly by worsening the elongation at rupture (Fig. 3). On the other hand, the UTS reduction in current regime with respect to the thermal ones was not significant up to 10 A/mm2 and reached a maximum of 40 MPa at the highest current density (20 A/mm2). The observed behaviour is in agreement with a previous study on AISI 304L austenitic stainless steel,
13
where a material softening as a consequence of the current application has been noted, but for which the thermal comparisons have not been performed.
Stress–strain curves of AISI 316L: a 5 A/mm2 and b 10 A/mm2
In AISI 316L, the same overall behaviour was observed in the entire experimental current-density range; thermal and electrical curves always stand below the baseline and the elongations obtained in the current regime were always lower than the thermal counterparts (Fig. 4). The strain at fracture was increasingly worsened as the current density was increased and, even if the material exhibited a softening in terms of mechanical strength, no enhancements in formability with respect to the baseline were observed. However, besides the possible contribution of SFE on EPE, the absence of an electrical effect can also be ascribed to the presence of an activation threshold for electroplasticity, as noted by Conrad.
1
Indeed, in the case under study, the applied currents were not enough to overcome such limit and the employed current densities only caused a reduction in the material flow properties.
Relative variation of true strain in steel AISI 316L as a function of the homologous temperature
C11000 copper
The lower electrical resistivity and higher thermal conductivity of copper with respect to AISI 316L allowed for a more effectiveness of the cooling device and for the application of higher current densities (up to 65 A/mm2, see Table 1), without excessive specimens heating during the electrically assisted tests. For this intermediate-SFE metal, significant thermal variations with respect to room temperature were not observed when relatively low current densities were applied, and the testing temperatures were maintained around 40°C (Table 1). However, despite the low temperatures, a slight worsening of the copper properties with respect to the baseline was observed in this low current-density range, both in electrical and thermal regimes, but no differences between the two types of tests were revealed. On the contrary, when the applied current was increased over 20 A/mm2, the elongations at rupture in the electrically assisted tests were reduced respect to the thermal counterparts (Fig. 5), but no significant variations in UTS were observed. Even in this case, thermal and electrical curves always stand below the baseline, denoting a similar material softening if compared to the room-temperature behaviour.
Stress–strain curves of C11000: a 33 A/mm2 and b 50 A/mm2
In copper, the obtained results were almost similar to those of the stainless steel in terms of worsening in elongation and no improvements in formability were revealed in the entire experimental current-density range (Fig. 6). However, conversely to what happened in the steel, no differences in UTS were registered between the two types of test. For temperatures up to 200°C (TH = 0.35), a similar decreasing trend in both electrical and thermal regimes was revealed, denoting an effect of electricity similar to that observed in the AISI 316L. Even in this case, the lack of an effective EPE may be addressed to the presence of a current threshold,
1
which was not reached in the performed experimental tests.
Relative variation of true strain in C11000 as a function of the homologous temperature
AA1050-H24 aluminium alloy
The physical properties of the high-SFE aluminium alloy permitted to strain the material adopting current densities from 5 to 65 A/mm2, as for the copper, and the cooling device allowed for a suitable lowering of the testing temperatures. In this material, conversely to what happened in copper and steel, the application of electricity caused a progressive enhancement in the total elongation as the current was increased, reaching a peak in formability at 25 A/mm2 (Fig. 7). Moreover, in this low current-density range, electricity also increased the UTS of aluminium respect to the baseline, whereas the thermal tests revealed a slight material softening. However, a trend inversion in the achievable strain at fracture was observed for current densities higher than 25 A/mm2, and aluminium exhibited a progressive reduction in total elongation. In particular, over 45 A/mm2 (TH = 0.37), the material behaviour was similar to that of the other tested FCC materials, showing a decrease in formability with respect to the thermal counterparts. In this regard, a dramatic electrical-to-thermal difference in elongation was registered at 65 A/mm2 (Fig. 7), although for this material the elongation is supposed to increase when the temperature is raised up to 200°C,
32
as confirmed by the thermal straining.
Engineering curves of AA1050-H24: a 25 A/mm2 and b 65 A/mm2
In the experimental current-density range, aluminium exhibited a clearly different behaviour respect to the other FCC materials (Fig. 8), highlighting a real effectiveness of electricity when relatively low current densities were applied, even if its electro-plastic threshold reported in literature is slightly higher and of the same order of magnitude to that of steel and copper.
1
However, this favourable effect of electricity was only present for current densities lower and equal to 25 A/mm2, whereas the solely thermal straining was found to be more effective in improving the material workability when homologous temperatures over 0.4 were involved.
Relative variation of true strain in AA1050-H24 as a function of the homologous temperature
Discussion
Among the tested materials, EPE in uniaxial tensile deformation only occurred in the high-SFE aluminium, denoting a possible contribution of this metallurgical parameter on electroplasticity in FCC metals. The results showed that, as the current densities (and the testing temperatures) were increased, the low- and intermediate-SFE materials exhibited a decreasing trend in the relative variation of total elongation within the entire experimental current-density range (Fig. 9), and the overall mechanical properties in electrically assisted tension were always pejorative respect to both the baselines and the thermal counterparts. On the contrary, aluminium showed very favourable enhancements in elongation when relatively low current densities were applied (Fig. 9) and, up to 10 A/mm2, the employed cooling device permitted to maintain the thermal regime close to room temperature (Fig. 2), thus avoiding excessive specimens heating. In aluminium, the greatest improvement in strain was registered at 25 A/mm2 whereas, for higher current densities, the elongations were reduced respect to the thermal counterparts. Further, over 45 A/mm2, aluminium started to behave similarly to the other FCC materials, exhibiting a worsening in total elongation respect to the corresponding thermal test (Fig. 7).
Relative variation of true strain in the tested FCC materials as a function of the current density
Regarding the mechanical strength, a material softening with respect to the thermal tests was observed in the stainless steel under current application, which exhibited a progressive UTS decrease in the entire experimental current-density range, reaching a reduction of ∼10% at the maximum current density (Fig. 8). Conversely, no noticeable variations between electrical and thermal UTSs were registered in copper, being the obtained values very close to each other and within the experimental error (Fig. 10). On the other hand, the UTS of aluminium showed a trend similar to that of its elongation, exhibiting improvements up to 25 A/mm2 and a progressive reduction when such current density was overcome, also underlining a further worsening with respect to the baseline over 45 A/mm2 (Fig. 10).
Relative variation of the electrically assisted UTS with respect to the thermal tests
In any case, the performed experimental tests revealed the presence of an extra-effect added to that of temperature, especially in elongation, which in turn opposed (medium and low SFE) and eased (high SFE) the formability of the materials. In AISI 316L (low SFE), the negative effect of electricity was strongly pronounced as the current density was increased, whereas in copper (medium SFE) the decrease in formability was quite smoothed and tended to reach a plateau when high current densities were involved. On the contrary, for AA1050-H24 (high SFE), electricity led to a considerable improvement in the material formability that, however, was only present when relatively low current densities were applied. In EPE, the variables involved are several, belonging to physical/metallurgical properties and testing condition. Nevertheless, fixing the deformation mode (tension) and the crystal structure (FCC), a comparison among the tested materials can be performed by taking into account their SFE, also considering that copper and aluminium possess comparable electrical resistivity (Table 1) but exhibited very different behaviours in the low current-density range (Figs. 9 and 10). Moreover, it must be noticed that aluminium possesses a considerably lower melting temperature respect to that of copper and stainless steel, which may have some implications if heating is involved, since straining this material at room temperature equals to deform it closely to the end-boundary of its cold-working regime.
The experimental results revealed a clear splitting of the materials behaviour as a function of their SFE, especially if elongation is taken into account (Fig. 9), which can therefore be considered a representative parameter in contributing to EPE. The passage of continuous current within the tested materials altered their mechanism of deformation, owing to both the presence of scattering electrons and the increase in temperature caused by localised resistive heating. In electrically assisted tension of steel and copper an additional term to the thermally activated deformation mechanism seemed to be present in controlling dislocations dynamics, probably allowing for the overcoming of some long-range barriers and acting as a further thermal contribution. In these materials, the anticipation of the breakage conditions in current regime can be ascribed to the premature reaching of the critical dislocation density at the grain boundaries, but the force required to reach these critical conditions remained almost unaltered, without considerably compromising the mechanical strength. This fact is supported by the scanning-electron metallography performed in a recently published work, 33 in which the same AISI 316L tested upon current application exhibited a minor number of slip bands within the grains near the fracture zone respect to the thermal counterparts, owing to a partial suppression of bands formation caused by the induced localised resistive heating.
On the other hand, in aluminium, electricity locally rise the temperature toward considerably high values, thus favouring important diffusional contributions, such as those governing recovery; indeed, in this material, recovery at high temperatures is facilitated, since dislocations motion is less hindered owing to its high SFE. 27 Thus, the observed improvements can be explained considering that the localised resistive heating induced by current may have affected its mechanical behaviour by easing recovery and allowing for the annihilation of existing dislocations. Hence, new dislocations were generated in the partially recovered microstructure permitting the material to withstand greater deformations before the final rupture, enhancing the total elongation. Therefore, the high SFE value of aluminium may be considered responsible for the observed behaviour, favouring recovery when localised resistive heating occur, and this correlation between EPE and SFE can also justify the observed behaviours of copper and AISI 316L, possessing both of them considerably lower SFEs. However, the low melting point of aluminium implies that such recovery conditions are readily reached for lower current densities respect to the intermediate-SFE copper (which possesses a similar electrical resistivity), leading aluminium in a favourable position for the occurrence of EPE (at least in tension). Moreover, the hypothesised role of SFE on EPE may also justify the more proneness of BCC metals to improve their formability in electrically assisted deformation, since SFE is generally greater in BCC respect to FCC materials. Thus, this fact can explain the recently observed enhancements in elongation respect to the thermal tests of an AISI 430 stainless steel electrically strained in uniaxial tension. 34
The role of SFE in EPE suggested from the results obtained in the present work must be considered valid if uniaxial tension is performed, since literature data from tensile and compressive tests on same materials revealed different behaviour when continuous current is applied,4,5 highlighting a strong contribution of the deformation mode on electroplasticity. In fact, besides the material type, it must be considered that the mechanisms of deformation in tension and compression are dissimilar. In compression, the cross-sections are continually increased and the deformation causes cracks and micro-voids to be either reduced or closed; conversely, tensile deformations favour the expansion of such defects, causing necking to occur and, in EAM, an excessive heating due to localised peaks of current density. Therefore, the effect of current on the maximum achievable deformation can be significantly different. Analysing the available literature works, EPE has been found to be not very effective in tension, whereas extraordinary results have been obtained in compression of Magnesium, Titanium and Aluminium alloys, making EAM very suitable in forging operations. 5 Actually, if taken as pure metals, Al, Mg and Ti are all high-SFE materials arranged in close-packed structures (FCC and HCP), and the addition of alloying elements to these materials lowers their SFE that, however, remains considerably higher than that of copper or austenitic steels, validating the hypothesis of a role of SFE in electroplasticity.
Nevertheless, together with SFE and deformation mode, a further contribution of the microstructure must be taken into account in analysing EPE, in terms of number of phases, crystal structure, grain size and precipitation-hardening conditions. In this regard, extraordinary improvements in compressed titanium has been revealed on grade Ti-6Al-4V, 5 which is biphasic (HCP+BCC), whereas no favourable effect has been observed in compressing pure titanium subjected to the same current densities.11,18 Conversely, straining pure titanium in uniaxial tension has led to considerable enhancements in formability, 18 while the electrically assisted tension tests on grade Ti-6Al-4V only revealed slight improvements. 4 These results on pure and alloyed titanium, both of them possessing a high SFE, clearly denote a direct interaction among EPE and the involved variables (i.e. SFE, deformation mode and microstructure). Another example concerning the influence of the presence of different phases is represented by the EPE observed in a biphasic 60/40 brass (FCC+BCC),4,5 which is considered to be a low-SFE material if this property is measured in the alpha (FCC) phase, but which possesses two phases having different SFEs. In this case, the presence of a BCC high-SFE phase may have contributed to alter the material behaviour by inducing the observed little EPE. Moreover, even the grain size may have some effects on the extent of EPE, 12 since a coarser copper microstructure has been found to favourably contribute to EPE, being the observed total elongation greater to that of the corresponding thermal tests owing to localised heating effects. Finally, the different hardening conditions of the aluminium alloys reported in literature4,5 represent a further variable in altering the materials response in electrically assisted deformation, explaining the observed differences in formability. Therefore, a combination of several variables must be considered in studying electroplasticity and thermal comparisons are mandatory in order to reveal a real effectiveness of the EAM experiments during continuous current application.
As a final note, it is stressed that a further effect of electricity in the elastic regime was noted in each tested material, resulting in a more or less pronounced elastic stiffening in the electric tests respect to the thermal counterparts (Figs. 3, 5 and 7). In AISI 316L, this variation was slightly visible and within the experimental errors, but such stiffening was clearly present in copper at any current density (Fig. 5) and in aluminium for densities overcoming 55 A/mm2 (Fig. 7). Nevertheless, in this work, the Young moduli were not measured using strain-gages, but were estimated by considering the raw data obtained from the tensile machine; therefore, the real presence of such stiffening requires further studies, which were not the aim of the present investigation.
Conclusions
In the present work, an analysis of EPE in three FCC materials having different SFE was performed (AISI 316L stainless steel, C11000 copper and A1050-H24 aluminium alloy). The results obtained from electrically assisted uniaxial tension (current regime) were compared to those performed at room temperature (baselines) and to those conducted at the corresponding temperature reached upon current application (i.e. the thermal counterparts). As was noted in the experiments, the passage of electricity through the materials changed their response to uniaxial plastic deformation respect to both baselines and thermal tests, denoting a further (positive or negative) effect on the mechanical response ascribable to SFE.
Among the tested materials, EPE during tensile deformation was found to occur only in aluminium, which possesses the highest SFE. However, the increase in formability was only registered in the low current-density range, where the material experienced the maximum increase in total elongation respect to the baseline (173% at 25 A/mm2); conversely, higher densities led to a progressive worsening in its strain at fracture. On the other hand, copper (intermediate-SFE) and stainless steel (low-SFE) always exhibited a progressive reduction in mechanical properties as the current density was increased, and their formability was always reduced respect to baselines and thermal tests.
From the obtained results and from the analysis of literature data, both SFE and deformation mode can be accounted as playing an important role in the occurrence of EPE under continuous current application and, among the effects inducing EPE, it seemed that the localised resistive heating could be the most effective one in leading to EPE in high-SFE materials. Indeed, resistive heating within the microstructure can locally shift the thermal regimes toward those temperatures at which recovery is determinant in controlling dislocation dynamics, thus permitting the annihilation of defects and dislocation and easing the formation of defect-free areas in which new dislocation can be generated by plastic deformation. However, the presence of others EPE-controlling factors (electron-wind force and excess of electrons) cannot be neglected and must be deeply analysed and verified. On the contrary, in low- and intermediate-SFE materials seemed that electricity enhanced dislocations mobility by only allowing an anticipate reaching of the forests dislocation respect to what observed in thermal regime, thus acting as a further contribution to thermally activated mechanisms and leading to a premature failure of the material in terms of total elongation.
Nevertheless, it must be noticed that the description of EPE within metals is not a straightforward process, since several material-related variables can affect in different extent the variation in formability of the materials. Among these, the presence of a biphasic microstructure, the grain size, the thermo-mechanical history of the material and their mutual correlation must be considered in studying EPE, and the effect of temperature risings caused by Joule heating under current application must be always verified by testing the materials in pure thermal regime.