Minimizing serrated flow in Al-Mg alloys by electroplasticity

In recent years, with the rapid development of society and economy, the problems of environmental pollution and energy shortage are attracting more and more attention from the government. Especially for the automobile industry, lightweight materials have been used to reduce vehicle weight, which has become the main direction of the development of the automotive industry [1]. Among various lightweight materials, aluminum alloys have become the best choice for automotive lightweight materials because of their excellent properties such as low density, high strength to weight ratio, good corrosion resistance [2]. However, under certain loading strain rate and experimental temperature, aluminum alloys would appear repeated instability during plastic deformation, known as the Portevin-Le Chatelier (PLC) effect (also called the serrated yielding effect) [3], [4], [5]. Unlike the aging strengthening aluminum alloys (for example, 2xxx and 7xxx series alloys), 5xxx series Al alloys cannot reduce the PLC effect by solution aging precipitation. Therefore, the PLC effect of 5xxx series Al alloys is commonly removed by changing the deformation conditions (experimental temperature or loading strain rate). But these methods need higher temperature, which will increase production cost, cause energy waste and is not conducive to industrial production.

Pulsed electric current, as an alternative with high efficiency and low energy consumption, had been applied to the deformation of metal materials [6], [7], [8]. The electroplastic effect appears during the deformation, which represents that the electric current reduces the flow stress and improves the plasticity of the metal. The movement of atoms under the action of electric current was observed in Pb-Sn and Hg-Na molten alloys by Geradin [9], and then the electroplastic effect of metals was discovered when electrons were irradiated to Zn single crystal in uniaxial tensile tests at low temperatures by Troitskii and Likntman [10]. The results showed that when the electrons were irradiated along the crystal slip direction, the ultimate elongation and the plasticity of the sample increased. If the electrons are irradiated perpendicular to the crystal slip direction, the opposite effect occurs. For decades, many scholars have devoted themselves to the research of the mechanism of electroplastic effect, and mainly formed the following viewpoints. By simulating the thermal effects due to Joule heating of 5052 Al alloys in pulsed tensile test, the researchers found that the thermal component had a major influence on the instantaneous stress drop and long-range permanent softening observed in experiment [11]. Conard and others believed that when the current flowed in the metal, a large number of free electrons were blown through the metal, which was called the electron wind. It can accelerate dislocation movement, improve plasticity and toughness of materials, and reduce material flow stress [12]. Molotskii and Fleurov proposed a mechanism to explain the electroplastic effect, which they considered that the increase of metal plasticity was caused by the facilitation of dislocation depinning from the current-induced magnetic field [13]. Although the mechanism for the electroplasticity is not yet clear (that is, thermal effect, electron wind force and dislocation depinning effect), the structural evolution of dislocations under electric current is predictable.

As mentioned above, serrated flow is mainly caused by the interaction between solute atoms and mobile dislocations [14,15]. The solute atoms diffuse towards dislocations during their temporary arrests at local obstacles, and consequently increase the yield stress by additionally pinning dislocations [15]. If the dislocation density is greatly reduced under the action of an electric current, which means that the interaction probability of solute atoms and dislocations is reduced, is the serrated flow minimized? The present work is aimed to study how the electropulsing assisted suppression of serrated flow in 5182 Al alloys, and the evolution of dislocation density will also be explored. The above questions will be addressed in this study.

Here, 5182 Al alloy containing 4.24 Mg, 0.16 Cu, 0.21 Mn, 0.21 Fe, 0.087 Si, 0.054 Cr and 0.016 Ti was used for investigation. The specimens for the uniaxial tensile test were cut along the rolling direction by wire-electrode cutting with the dimensions of 40 mm (long) × 6 mm (wide) × 2.5 mm (thick). The pulsed and non-pulsed tensile tests were carried out on slow strain rate test system and the surface temperature of samples was measured with thermocouple and infrared thermal imaging camera to estimate the Joule heat generated by the pulse. The electric current in tensile test was produced by pulsed constant current generator. All experiments were carried out at the same strain rate of 5 × 10^-5 s^-1, and the pulsed tests are accompanied by varying current intensities, but have the same frequency of 180 Hz and duration of 180 μs. All tensile tests were repeated more than three times to ensure the accuracy of the results. In order to explore the change of dislocation density, transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to analyze the microstructure of investigated samples. All tested specimens were taken from cross section of perpendicular to normal direction.

Fig. 1 shows the stress-strain curves, critical strain and waiting time of pulsed and non-pulsed tensile tests at the same strain rate of 5 × 10^-5 s^-1. For a better view, the curves are separated by an interval of 30 MPa along the ordinate axis in Fig. 1(a). It can be observed that the PLC effect is present only within the current intensity range from 0 A to 150 A. When the current intensity reaches 200 A, the PLC effect is suppressed. This phenomenon can be clearly seen from the magnifying diagram of Fig. 1(b). The variation of the critical strain with current intensity is shown in Fig. 1(c). The critical strain increases with increasing current intensity. Meanwhile, the waiting time for the serrated flow also increases with increasing current intensity, as indicated in Fig. 1(d). However, as can be seen from Fig. 1, the serrated flow in 50 A is similar to the case of non-pulsed sample.

Fig. 2 shows the observed distributions of stress drops in different conditions. The serration amplitude increases with increasing current intensity and is almost independent of strain at all cases. The stress drop number in Fig. 2(a) and (b) is similar, except for a slight increase of serration amplitude in Fig. 2(b). The stress drop number in Fig. 2(c) and (d) is extremely reduced in relative to the untreated state of Fig. 2(a). From the critical strain, waiting time, the statics of stress drops, one conclusion can be reached that the serrated flow can be suppressed with the increasing current intensity.

As previously mentioned, Joule heat generated by electric current may also be the cause of plastic deformation. Therefore, the surface temperature of the specimen of pulsed tensile was measured to understand whether electric effect or thermal effect plays a more important role. The temperature obtained from thermocouple shows that the maximum Joule heat induced by the current at 200 A is only 40 °C. Besides, this result is also confirmed by infrared thermal imaging camera. Fig. 3(a) shows the temperature distribution of the sample during pulsed tensile under the current intensity of 200 A, on the right side of the diagram is a ruler of different colors corresponding to different temperatures. It is clearly seen that the maximum temperature is about 35 °C, which appears in the current contact site at both ends of the sample. The majority area of temperature is evenly distributed at about 30 °C, simply 20 °C higher than the ambient temperature. Considering the thickness of the sample is only 2.5 mm, the temperature gradient in the thickness direction of the sample can be ignored. Studies have shown that when the deformation temperature is higher than 100 °C or below -100 °C, the PLC effect completely disappeared [16].

In order to further confirm the effect of temperature induced by the current on the plastic instabilities, the stress-strain curves at the same strain rate of 5 × 10^-5 s^-1 were observed with the same temperature in Fig. 3(b). For the pulsed and non-pulsed samples at 20 °C, the serrated flow can be greatly suppressed by pulse in comparison to the untreated one. Definitely, as the temperature increases, the number of serrations also decreases as reported [17], but after the application of the pulse, the suppression effect is much better. It suggests that the temperature and the pulse simultaneously suppress the serrated flow, but the effect of the pulse is more pronounced.

The generally accepted explanation for the PLC effect is dynamic strain aging (DSA) theory [18], [19], [20], that the repeated pinning and unpinning between solute atoms and mobile dislocations. When the mobile dislocation is blocked by the obstacles in the crystal during the movement, the solute atoms diffuse to the mobile dislocation and then form the solute air mass to pinning the mobile dislocation. Under the action of stress, the mobile dislocation can overcome the pinning by thermal activation. The repeated action of the pinning and unpinning between mobile dislocation and solute atom appears as serration yielding phenomenon on a macroscopic scale. It is known that electroplasticity can improve the plasticity of metals by promoting the slip of dislocations and enhancing the ability of atomic diffusion. Therefore, DSA theory under pulsed electric current is analyzed by studying TEM bright field images and full width at half maximum (FWHM) based on XRD for the pulsed and non-pulsed tensile. Fig. 4 shows the dislocations in the investigated samples detected by TEM. Dislocations pile-up at grain boundary were observed in both non-pulsed and pulsed tensile (Fig. 4(a) and (b-d)), the distinctly can be seen that the dislocations pile-up under pulsed is much less than that of untreated sample.

In previous studies, the relationship between FWHM and dislocation has been analyzed [21]. They believed that the X-ray diffraction peaks broaden (or the FWHM broaden) when there are a large number of lattice defects such as dislocations in the crystal under the same instrument conditions and finite crystallite size. In other words, the value of FWHM is positively related to the dislocation density. Fig. 5(a) and (b) shows the function image of FWHM and 2θ at the investigates samples and the dislocation density. It can be seen from the Fig. 5(a) that the value of FWHM decreases as the current intensity increases. By calculations, the dislocation density also decreases with increasing current intensity as indicated in Fig. 5(b). These results are consistent with the TEM observations. From this we can conclude that the pulse current reduces the dislocation density of the samples during deformation.

All of the above indicate that the electroplastic effect plays an important role in suppressing the PLC effect of Al alloys by changing the dislocation density during tensile tests. As we all know, the drift electrons in the metal will produce an electron wind under the action of the current, which will act on the dislocations. The electron wind forces the dislocation slip in the crystal and enhances the movement of dislocations. In addition, in order to quantify and calculate the electron wind generated by drift electrons under the action of the electric field, equation accepted by scholars have been proposed [22].

F_ew=αbp_Fn_e(v_e-v)=B_ew(v_e-v)=αbp_F(j/e-n_ev) (1)

where, F_ew is the electron wind force, e is the electron charge, n_e is the electron concentration, j is the current density, b is the Burgers vector, p_F is the Fermi momentum, v_e is the electron drift velocity, v is the dislocation velocity, B_ew is the electron push coefficient, and α is numerical factor ~1/3. It is known by Eq. (1) that when the drift velocity is greater than the dislocation velocity there is a “push”, and when the dislocations are moving faster than the drift velocity there is a drag. In addition, the electron wind acting on a unit length of dislocation is proportional to the current density. The results show that as the current intensity increases, the electron wind applied to the dislocations increases, and then the dislocation slip in the grains increases. Based on the experimental results, the PLC effect of Al alloy is gradually weakened as the current increases from 0 A to 200 A, which can be inferred that as the current intensity increases, the thrust of the electron wind on the dislocation increases, and the dislocation density decreases, hindering the interaction between the solute atoms and the dislocations, and finally achieves the purpose of suppressing the serrated flow.

In summary, the extension of plastic region can be modified by the application of pulsed electric current. Combined with the DSA theory and electroplastic effect, the suppression of serrated flow is attributed to the reduction in dislocation density which affecting the plastic instabilities. Therefore, pulsed electric current, as an alternative with high efficiency and low energy consumption, has a great significance for the plastic deformation of metals and the improvement of the surface quality of aluminum alloys for automobiles.

Acknowledgments

The work was financially supported by the National Natural Science Foundation of China (Nos. 51601011, 51874023), the Fundamental Research Funds for the Central Universities, and Recruitment Program of Global Experts.

The authors have declared that no competing interests exist.

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