Experimental investigation into the electropulsing assisted pulsating gas forming of CP-Ti tubes

Journal Pre-proof Experimental Investigation into the Electropulsing Assisted Pulsating Gas Forming of CP-Ti Tubes Zejun Tang, Jian Chen, Kexin Dang, Gang Liu, Kemei Tao

PII:

S0924-0136(19)30465-0

DOI:

https://doi.org/10.1016/j.jmatprotec.2019.116492

Reference:

PROTEC 116492

To appear in:

Journal of Materials Processing Tech.

Received Date:

16 March 2019

Revised Date:

26 October 2019

Accepted Date:

4 November 2019

Please cite this article as: Tang Z, Chen J, Dang K, Liu G, Tao K, Experimental Investigation into the Electropulsing Assisted Pulsating Gas Forming of CP-Ti Tubes, Journal of Materials Processing Tech. (2019), doi: https://doi.org/10.1016/j.jmatprotec.2019.116492

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Experimental Investigation into the Electropulsing Assisted Pulsating Gas Forming of CP-Ti Tubes

Zejun Tanga, Jian Chen a, Kexin Dang c, Gang Liu b,c*, Kemei Tao a

a

College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao

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Street, Nanjing 210016, PR China b

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology

c

School of Materials Science and Engineering, Harbin Institute of Technology

*

Corresponding author at: State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology,

Harbin, 150001, RP China. Tel:+86 13936491818.

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E-mall address: [email protected] (Gang Liu).

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Graphical Abstract

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Abstract

A novel electropulsing assisted pulsating gas forming (EAPGF) process of commercial pure titanium (CP-Ti) tubes by using pressurized argon filled into a sealed tube was developed to optimize the process scheme and reduce the cost of die heating. The formability of CP-Ti tubes was tested on the self-designed innovative gas forming platform by applying three different frequencies of electropulsing. Based on the uniaxial tensile tests assisted by the electropulsing, the principle of EAPGF was modeled and interpreted by the generalized Johnson-Cook model. At first, the temperature variation of the experi1

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mental tubes assisted by electropulsing of three frequencies was measured and analyzed. High-frequency electropulsing will make the temperature rise steadily. Low-frequency electropulsing will cause a serration-shaped growth of the temperature. Moreover, the influences of the pulse duration and frequency on the forming pressure, the relative load, the expansion ratio, the shape deviation and the thickness distribution in the middle section were investigated. The relative load fluctuates periodically with an oscillation of temperature as well as internal pressure induced by electropulsing. In terms of the three frequencies, the higher bulging limit and more uniform deformation were observed in the condition of low frequency. From the result of microstructure analysis through EBSD, the low-angle grain boundaries (LAGBs) exhibit a strenuous increase attributed to the deformation assisted by electropulsing, with the higher increment at a lower frequency. Obvious dynamic recrystallization was observed at the electropulsing frequency of 1/20 Hz from the KAM distribution in the specimens. Finally, a modified EAPGF was suggested by adjusting pulsating hydroforming parameters by regulating the parameters of electropulsing. The relative load was adjusted to pulsate around 0.0577, and a typical adjustable CP-Ti tube with 60% expansion ratio was successfully fabricated.

Keywords

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Gas forming; CP-Ti tubes; Formability; Electropulsing; Pulsating; Relative load 1. Introduction

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With the development of the aerospace, nuclear field, sports equipment, chemical industry, automobile field, electronic industry, and modern medical equipment, as well as the increasing awareness of environmental protection and energy conservation, titanium and titanium alloy parts are attracting more and more attention. Titanium alloys have wide application prospects due to its outstanding properties such as low density, high special strength, excellent elevated temperature performance, good corrosion resistance and superior welding property (Leyens and Peters (2003)). Titanium alloy parts have to be deformed at elevated temperature because of its low plasticity, high strain hardening exponent, and serious springback at room temperature (Zhan et al. (2015)). However, the high temperature will lead to the serious wear and tear of the die, reduced tool life, high cost and low production efficiency (He et al. (2013)). Internal High Pressure Forming (IHPF) is a cost-effective way to volume-produce hollow, lightweight, and structurally rigid components. Under the joint action of the internal pressure and the die cavity, the standard sheet or pipe is formed into a single thin-walled structure of the complex structure. In comparison with traditional welding or casting process, IHPF simplifies the production sequence and utilizes material efficiency maximally. For structural parts subjected to bending and torsional loads, hollow 2

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cross-section components are urgently needed to reduce weight and make full use of strength and stiffness of materials. Warm and hot hydroforming is commonly used in industrial aluminum alloy and magnesium alloy workpieces, which not only improves the formability but also realizes the optimization of parts. Keigler et al. (2005) investigated the enhanced formability of aluminum alloy tubes via warm and hot hydroforming. Kim et al. (2007) numerically analyzed the hydroformability of an AA6061 extruded tube at 200℃ and 300℃. Yuan et al. (2006) had developed thermal hydroforming test of 5A02 aluminum tube. Manabe et al. (2010) verified the finite simulation model of AZ31 magnesium alloy tube in warm hydroforming. Nevertheless, it is sometimes challenging to fabricate titanium alloy tubes through conventional hydroforming as a result of the limitations of the heating temperature of pressure media. In contrast to pressure media such as hydraulic oil and water, hyperbaric gas not only eliminates the limitation of forming temperature attributed to its favorable temperature resistance, but also can be filled into the sealed hollow body without preheating as a result of low specific heat capacities of gases (Neugebauer et al. (2006)). Hot metal gas forming (HMGF) is an advanced outgrowth of hot blow forming (HBF) and superplastic forming (SPF) techniques during the span of the tubular products processing history, especially in the fields of aerospace and automobile industry. Among the researches of HMGF process, Kim et al. (2010) studied the fracture and forming behavior of an Aluminum alloy tube with the aid of finite element simulation (FEM), into which the failure criterion based on Zener-Hollomon parameter was incorporated. Wu (2007) observed transient creep behavior during the HMGF of AZ31 magnesium alloy and analyzed its application in gas forming techniques. Dykstra (2002) investigated the expansion of a steel tube and an aluminum alloy tube. He found that the expansion ratio of a steel tube reached 150% at 1100℃, and the expansion ratio of an aluminum alloy tube reached 50% at 480℃, respectively. Vadillo et al. (2007) showed that a high strength steel pipe was successfully fabricated with a high expansion ratio up to 55% under the pressure of only 1.4MPa. Liu et al. (2014) presented the thickness distribution at various pressure levels, which proved the production readiness for forming hollow titanium structures with complicated geometrical features efficiently. Moreover, the research results provided by Wang et al. (2016a) indicated that the microstructure was obviously refined after the HMGF because of the shorter forming time and the lower forming temperature, which is entirely different from traditional SPF. Although pressurized gases overcome the confinement of the temperature and forming process, fabricating titanium alloy tubes faces new technical difficulties. Since the amount of energy required for die heating is too large, the heating time is prolonged, resulting in low production efficiency.

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Therefore, it is particularly urgent and necessary to propose a minimal cost and novel HMGF to tackle the problems mentioned above. The electrically assisted forming is an alternative forming technology in which the mechanical behaviors were promoted by introducing the intermittent application of high-density electric current to conductive metal components during plastic deformation as stated by Roth and Cao (2014). The decreasing flow stress, increased ductility (Troitskii (1969)), reduced residual stress(Kim et al. (2014)), omitted annealing process and reductive springback (Xie et al. (2015a)), which was ordinarily known as electroplasticity effects, has been experimentally analyzed in plastic deformation under a pulsed electric current. The modeling and simulation of the electrical response of materials have attracted widespread attention. For example, a numerical model to quantify the current-induced effect on mechanical properties and an analytically thermo-electrical-mechanical finite element study under pulsed electric current was conducted to decouple the thermal effect and the electroplasticity by Hariharan et al. (2015). An analytic flow stress model consisting of strain and strain rate hardening, thermal softening as well as solution-dislocation interaction calibrated by uniaxial micro-tension successfully and effectively predicted the thermal and mechanical behaviors at a different current density(Wang et al. (2016b)). Therefore, in view of material characteristics such as poor plasticity and high strength of high strength lightweight materials, electropulsing could abate the work hardening and eliminate the anisotropy partly proposed by the author of this manuscript in an earlier publication (Tang et al. (2018)). Comprehensive analysis showed that the improvement was attributed to the healing and arrest of microcracks and microvoids, local refinement of microstructure, the reduction of strengthening phases under the action of electropulsing (Tang et al. (2019)). Inspired by the aforementioned researches of electrically assisted techniques, efforts have been taken to verify the practicability and feasibility of the process of an electrically assisted process for warm and hot gas forming. He et al. (2012) had developed a novel experimental set-up for free bulging test heated directly at the location where the deformation is demanded. The hardness and microstructure evolution of AA6061-O alloy extruded tubes were analyzed before and after the experiments. Maeno et al. (2009) introduced an innovative hot bulging process utilizing auxiliary electric current, which gave rise to a significant benefit of the short heating time to the required temperature for processing, which circumventing both the long heating period and the expensive furnace. Three parameters were analyzed in his research to evaluate the expansion ratio of the bulging region considering the impacts of initial air pressure, current density, and velocity of axial feeding (Maeno et al. (2014b)). Afterward, an ultrahigh strength steel V-shape hollow component was successfully fabricated in conjugation with the die-quench of shaped punch and rapid resistance heating (Maeno et al. (2014a)). Similarly, Michieletto 4

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et al. (2014) designed an innovative experimental set-up to elevate the formability with the aid of current assisted heating. At the same time, in order to have a further improvement of forming ability of the tube, a new approach in forming complex hollow products is the pulsating hydroforming. Hama et al. (2004) had conducted the pulsating hydroforming process numerical studies of a three-dimensional automotive component by FEM. Simulation results indicated that, to some extent, pulsating pressure seems to be a rough agreement on the mechanism as that of reducing the friction coefficient between the tube and die. In the pulsating hydroforming process, Mori et al. (2007) concluded that the occurrence of defects was prevented and a uniform expansion was obtained. The improvement of formability was interpreted from the repeating appearance of little wrinkling and the variation of stress components. The pulsing hydraulic loading path which was actualized by the fluctuating internal pressure has been demonstrated to be useful to obtain a promoted expansion ratio (Yang et al. (2014)). To characterize the effects of pulsating pressure on tubes’ shape accuracy, experiments using Taguchi design were employed by Ashrafi and Khalili (2016) that the linear regressions between wrinkling, expansion height and thickness were extracted. By reference to the idea of the conventional pulsating hydroforming, a novel electropulsing assisted pulsating gas forming (EAPGF) method was therefore proposed to bulge the commercially pure titanium (CP-Ti) tubes in the present article. The unique mechanism and plastic deformation behaviors in EAPGF process subjected to hyperbaric gas and three different frequencies of electropulsing were studied. Influences of electropulsing pulse duration and frequency on the forming pressure, the relative load (see Eq. (2)), the expansion ratio, the shape deviation and the thickness distribution in the middle section were experimentally and analytically investigated. The microstructure evolution under different frequency of electropulsing assisted gas bulging was characterized and analyzed by electron backscattered diffraction (EBSD).

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2. Principles of electropulsing assisted pulsating gas forming

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In order to investigate the principle of electropulsing assisted pulsating gas forming, quasi-static uniaxial tensile tests assisted by the 80A/mm2 electropulsing of three different frequencies, were carried out at the nominal strain rate of 0.002 by using the experimental setups illustrated in Fig.1 (a). To carry out tensile tests with pulsed electric current, an electronic universal testing machine (type: UTM5000) with a capacity of 10 tons was used. Electropulsing was generated by VIS-200A power and applied to the specimen through the red copper electrodes. As shown in Fig.1 (b), the first pulse of electropulsing was applied to the specimen after yielding. The initial amplitude of electric current density was 80 A/mm2 according to the cross-sectional area of the initial specimen. The elec5

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tric current density during the electropulsing assisted tensile process increased continuously due to the gauge consistently decline in the effective cross-sectional area. From the tensile test results in Fig.1 (c), true stress profiles of 50Hz and 1Hz are similar to some extent. It is noted that the true stress shows the characteristics of pulsation at the low frequency of 1/20 Hz. In the case of 1/20 Hz, the flow stress decreases during the duration of a pulsed electric pulse and increases between the pulses. Overall, the maximum stress in different periods shows a downward trend as a result of the decrease in the effective cross-sectional area of the specimen. Meanwhile, the ultimate elongation increases while the electropulsing frequency decreases. Therefore, it is inferred that the ultimate tensile elongation is enhanced by the low-frequency electropulsing. A similar influence contributing to the enhancement of the ultimate expansion ratio in EAGF is also expected for the condition of low frequency. Because the flow stress fluctuates along with the electropulsing (as shown in Fig.1 (c)), the relative load (the ratio between the internal pressure p and the yield stress) in electropulsing assisted gas forming (EAGF) fluctuates with electropulsing even though the internal gas pressure is constant. In this way, the effect of internal pressure on the deformation of the material shows fluctuating characteristics under the action of the low-frequency electropulsing. Consequently, the fluctuation of relative load gives the process a pulsating loading characteristic, which was demonstrated the prevention of defects and the promotion of uniform expansion (Mori et al. (2007)). So we will provide a more in-depth analysis of the pulsation in EAGF. For a better understanding of EAPGF, the model of the relative load in EAGF was derived as follows. Magargee et al. (2013) analyzed the tension tests of CP-Ti (Grade 2) sheets in conjunction with air-cooling to eliminate the effect of Joule heating, and observed that the flow stress of air-cooled experiment specimen was consistent with that of specimen at room temperature, which led to a conclusion that Joule heating effect is dominant in the electrically-assisted forming while the pure electroplastic effect is negligible. The mechanical behavior of CP-Ti under a pulsed current can be effectively predicted by utilizing an empirical thermal-mechanical model. Jonson-Cook model (Johnson and Cook (1983)) was adopted in the current study to quantify the influence of pulse current on mechanical properties, as Eq. (1) shows. 

  b n exp   

Tm  T   Tm  TR 

(1)

Where b is the strength constant, n is the strain hardening constant,  is the equivalent plastic strain, TR is the room temperature, Tm is the melting temperature of materials,  and

 are the thermal softening parameters.

The melting temperature of CP-Ti is 1665 °C, and the room temperature is 25 °C (at which the yield stress and strain hardening coefficient are determined). Meanwhile, the 6

instantaneous temperature is obtained through real-time measurements in the experiment. The strength constant and strength hardening constant are adopted from the generalized Jonson-Cook model which predicted the mechanical behavior of CP-Ti at the elevated temperature effectively by Sheikh-Ahmad and Bailey (1995). The calculated yield stress fits well with the experimental results. The coefficients in the generalized Johnson-Cook model are summarized in Table.1. The deformation resistance of materials decreases with the temperature rising generally. Therefore, the effects of the same internal pressures at different temperatures are different. The concept of the relative load is introduced in this paper. The relative load  r during the EAPGF can be obtained as p

s



p  T T  b n exp   m   Tm  TR 

(2)

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r 

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The optimization of the electropulsing parameters and the pressure in EAPGF can achieve a fuzzy load control algorithm of conventional pulsating hydroforming, which can improve the formability of tubes. As shown in Fig.2, the ratio of relative stress in increasing to decreasing time A (see Eq. (3)) can be used to adjust the pulsating characteristics in EAPGF, which is similar to that in the conventional pulsating hydroforming process (Ashrafi and Khalili (2016)).

td ts

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A=

(3)

lP

Where t d is pulse duration, t s is the time between pulse.

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Moreover, the electrical pulse period in EAPGF T is consistent with the period in conventional pulsating hydroforming. The oscillation amplitude B can be enlarged by increasing the current density or increasing the pulse duration interpreted from the thermoelectric model (Xie et al. (2015b)). The number of relative load peaks is equal to the number of electric pulses. The pulsating hydroforming characteristics can be adjusted by regulating the pulse parameters through the programmable current controller of an electric generator. The above analysis for the characteristics of EAPGF provided a deep insight into the effects of pulse duration and frequency on plastic behavior during the EAPGF process. In the following sections, additional experimental investigations on the EAPGF process of the CP-Ti tube will be deployed. 3. Material and Experimental Setup 3.1. Material

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The research material in present experiments is CP-Ti (Grade 2) with a nominal thickness of 1mm, an external diameter of 25mm and length of 118mm. The chemical compositions are tabulated in Table 2. 3.2.experimental setup

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Fig.3 shows the schematic diagram of the self-developed EAGF platform for electrically assisted hot gas bulging of CP-Ti (Grade 2) tubes. The air was compressed by DM-7A air compressor and filtered before filling into the argon pressurization system, which utilized the driving force provided by pressurized air to supercharge the argon gas into a high-pressure gas cylinder. Application of cold compressed argon with a maximum output pressure of 70MPa to bulge the heated tubes was developed. The precision pressure regulator located between the high-pressure gas cylinder and the tube controls and maintains the argon pressure in a range of 0 to 70MPa, which is adequate for gas forming tests of resistance-heated titanium alloys tubes. Meanwhile, the pressure sensor was used to synchronize the experiment with the measurement of the internal pressure. In order to display the internal structure, only the lower die is included. In the experiment, one end of the tube was linked with the argon pressurization system, and the other end was connected with a globe valve to seal the tube. In order to detect the temperature in the die, two holes were distributed on both sides of the dies, on both of which stainless steel pipes were welded. On the one hand, the thermocouple could be inserted into the die. On the other hand, the low-pressure argon flowed through the cavity when the tube diameter was increasing. In this condition, the stainless steel pipes played the role of exhausting excess gas and preventing the outer surface from oxidation. The electropulsing applied in tubes created through an external 20kW rated VIS-200A power generator with a programmable current controller, and VIB-10KB transformer is mounted on the specimen by four semicircular red copper electrodes, which clamp the tube at the vicinities of both ends through the clamping force provided by a hydraulic press and thru-hole bolting. The electrodes embedded in the die elaborately have proper contact with the outer wall of the tube to ensure the uniform distribution of electropulsing. Mica insulators were granted as pointed out in Fig.3 to isolate the electric current from the machine frame during EAPGF. For electrical insulation and thermal insulation, ZS-1091 high-temperature ceramic insulating varnish is coated in the concave die cavity. The schematic diagram of the die set of bulging is illustrated in Fig.4. The length of the bulging area in this study was 48mm, which was 1.6 times the external diameter of the as-received tube, and the radii of die entrance were 2mm and 4mm, respectively. The die was fabricated with Cr12MoV quenchable die steel hardening to an average hardness of approximately 50HRC and yield strength of about 750MPa. The concave die was sandwiched in the middle of the frame so that different tube lengths and hollow sections could be realized by means of the concave die’s straightforward interchange. Each end of the as-received tube was sealed 8

by a built-in set drawbar, the outstretched part of which arranged a lug boss distributed between a screw head and a screw rod. The sealing of the apparatus was realized with the joint action of compression rings, polyurethane ring, and nuts. After the experiment, the polyurethane was damaged because of rapid heating while the built-in set drawbar was not heated due to the non-directly contact. 3.3.Experimental scheme and procedure

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In order to experimentally investigate the unique mechanism and plastic deformation behaviors in EAPGF process, DC electropulsing of rectangular waves with three different frequencies were induced in this study, as shown in Fig.5. The initial amplitude of electric current density was 80A/mm2 according to the cross-sectional area of the as-received tubes. The experimental procedures are as follows.

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To start with, the tube was positioned in the die cavity, and the experimental setups were installed on the platform. Next, the polyurethane ring closing the inner wall was held in place elaborately by the compression ring and nuts. Then, the argon pressurization system and globe valve were linked to both ends. Meanwhile, compressed argon was filled into the tube until the pressure reached the set value. The low-pressure argon was filled into the die consistently. Afterward, the pre-programmed electropulsing was introduced into the tube by the semicircular red copper electrodes. The tube’s temperature was detected by K-type thermocouples. Conditions of EAPGF of CP-Ti tubes are shown in Table.3. Each electrical parameter was performed twice, and the average bulging ratios were indicated. Because the power generator provided a constant rectangular wave, the current density of electropulsing attached to the tube was based on the cross-sectional area of the un-deformed tube. In this process, the control scheme is simplified because the pressure control is eliminated. At the same time, the production cost is decreased without the preceding die heating. In this paper, the bulging ratio of expansion tube is obtained from the following equation:

=

lm  l0 l0

(4)

where lm is the outer circumference of the expansive tube and l0 is the primitive circumference of the tube. To evaluate the influence of electropulsing on the geometry uniformity in the middle section,  is induced to characterize the shape deviation ratio and it is expressed as follows: 9

=

d1  d 2 2(d1  d 2 )   d1  d2  / 2 d1  d2

(5)

where d1 and d 2 are the maximum diameter and minimum diameter in the middle section respectively. Under normal conditions, the shape deviation ratio in middle section increased along with the bulging height. To further evaluate the uniformity of circumferential thickness distribution, the standard deviation of wall thickness measurements S is introduced as follows: S

1 N ( xi  x ) 2  N 1 1

(6)

where N is the number of the measurement point, xi is the measured thickness, x is

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the average value of wall thickness measurement. 4. Experiment results and discussion 4.1. The temperature history of tubes

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It is noted that the temperatures at the point P1, P2, P3, and P4 (as shown in Fig.6 (a)) on the external surface were recorded through K-thermocouples, the measurement accuracy of which was ±2.5°C under 1300 °C. Considering that the temperature is approximately axisymmetric about the middle section, all the temperature measuring points were distributed on the tube of one-half. The points P1, P2 and P3, were fixed in the middle section. The K-type thermocouples were coupled to the XSR-50 multichannel paperless recorder with a scan frequency of 10Hz, as shown in Fig.6 (b). Temperature profiles of three electropulsing frequencies along the axial direction are shown in Fig.7. For the case of the high frequency of 50Hz, the temperature presented the exponential rise in the preceding period and the reduced growth rate, with the balance at about 55s. The temperature for the electropulsing of 10ms duration was apparently higher than that of 5ms duration. Longer duration led to a higher temperature rise. In the case of the intermediate frequency of 1Hz, the trend of temperature curves was similar to high frequency. The maximum temperature at P1 increased exponentially in the first 30s and got balanced gradually in a minute. In terms of the low frequency of 1/20Hz, the temperature exhibited an exponential growth during the period of electropulsing and then continuous decay accompanied by a momentary change of rising rate because of the heat dissipation. On the whole, the temperature profile presented a serration-shaped growth. To summarize, temperature profiles show no fluctuation characteristics at high frequency and an intermediate frequency, and only obvious oscillations occur at low frequency. Other than heating by the furnace with minimal temperature gradients in traditional HMGF, applying a pulsed electric current to CP-Ti tubes can elevate the temperature during the plastic deformation process and influence the necking behavior (Lee et al. 10

(2017)). However, applying the electropulsing in the course of the bulging process accompanies specific individual mechanical considerations, such as the inhomogeneous of temperature distribution and gradients, which will not happen in typical HMGF. It will be experimentally evaluated in the following sections that the three different electropulsing frequencies directly influence the mechanical property, formed shape and microstructure evolution of inflated tubes. 4.2. Results of electropulsing assisted gas forming

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Bulging experiments of the TA2 tubes in the case of the high frequency were firstly carried out for the comparison, although there is no pulsating characteristic in the case of the high frequency. Experimental burst tubes for different duration of the high-frequency electropulsing are shown in Fig.8. Observing macroscopic features of experimental tubes, the length of fracture rip elongated with the increase of pulse duration. At the pulse duration of 5ms, fracture rip is small and slight tears, whereas the fissure is large and severe at the pulse duration of 10ms and 15ms. It is of particular remarkable that the burst always starts from the top of the middle section. Because of the thermal convection, the maximal temperature distributed in the upper tubular part. As shown in Fig.7 (a), the temperature of P1 is slightly higher than that of P4 with an amplitude of 20-100℃ as a result of the rising of heated gas. Therefore, the thickness in the upper part decreases faster than other places until the fracture occurs. The expansion ratio and the shape deviation ratio in the middle section of expansion tubes under three different duration of electropulsing are shown in Fig.8 (b). As the pulse duration increases, the expansion ratio increases and reaches the maximum value of 21.1% at 10ms. However, all ultimate expansion ratios are limited under the high-frequency electropulsing with large shape deviation ratio, indicating a non-uniform deformation in the middle section. Fig.8 (c) presents the circumferential thickness distribution in the middle section with the high-frequency electropulsing. The place of 0° represents the point at the vertex, and the horizontal ordinate represents the clockwise angle from the starting point as displayed below the diagram. The place in the vicinity of vertex endures serious thinning due to the higher temperature, which resulting in a less thickness compared to the opposite area. Finally, all the tubes burst at the vertex, which is coincident with the circumferential thickness distribution. The measured thickness fluctuates slightly at 90° to 270° along the circumferential direction at the bottom. With the increase of pulse duration, it can be observed obviously that the bottom thickness decreases. However, the highest expansion ratio of high frequency is still far less than conventional HGMF (He et al. (2012)). From the bulging results above, it can be seen that the expansion ratios of three tubes are relatively low with a non-uniform deformation. It is vital to note that the pulsation of relative load only occurred under conditions at a low frequency. Therefore, the EAGF experiment under different frequencies of electropulsing will be explored to provide an alternative perspective for the pulsating theory. The bulged tubes under three dif11

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ferent frequencies of electropulsing are given in Fig.9 (a), where the comparison shows the potential of improving expansion ratio and facilitating the deformation behavior of the lower frequency. More concretely, the ultimate expansion ratio and shape deviation ratio of expansion tubes under three different frequencies of electropulsing are shown in Fig.9 (b). It is evident that the ultimate expansion ratio increases as the frequency decreases, despite the fact that the temperature of high frequency was higher than that of low frequency (see Fig.7). Meanwhile, the shape deviation ratio decreases slightly as the frequency decreases, indicating that the roundness in the middle section is better in 1/20Hz. The ultimate expansion ratio of low frequency with the highest temperature of 617℃ in the whole process reached 38.9%, which is close to the ultimate expansion ratio of the CP-Ti tubes in conventional gas forming of 40% at 770℃(He et al. (2012)). In the case of the modified EAPGF in subsection 4.4, a typical adjustable CP-Ti tube with 60% expansion ratio was successfully fabricated at 767℃, demonstrating the ultimate expansion ratio of more than 60%. The experimental results indicate that the ultimate expansion ratio is improved as compared to the conventional hot metal gas forming at the approximate temperature, where the comparison shows the potential of improving expansion ratio and facilitating the deformation behavior of the low-frequency electropulsing. The circumferential wall thickness distributions under three different frequencies of electropulsing are shown in Fig.9 (c). The wall thickness in the lower frequency case exhibited a more uniform distribution with the maximum expansion ratio due to the oscillation of the relative load. Yang and Chen (2009) studied the conventional pulsating hydroforming process by FEM and concluded that the cause of the uniformity and homogeneity for circumferential wall thickness was interpreted from the periodic appearance and disappearance of tiny wrinkles in the bulging area. The temperature difference along the circumferential and axial directions of the tubes using three different frequencies of electropulsing are given in Fig.10, where the uniformity of temperature distribution is reflected. The experimental results demonstrated that the temperature difference increases with time. During the EAGF process, the heating and the periodic homogenization of temperature appearing in the deformed tubes of the low frequency, which avoid the happening of local necking. The cause of more uniform expansion in EAPGF is shown in Fig.11 (a). During the duration of the electropulsing, the temperature is increased, and the upper part is deformed firstly because of the highest temperature, which results in strain hardening and the conductivity degradation in this region. In the cooling time of nearly 20s between pulses, the temperature of the tube decreases and tends to be uniform. Moreover, the area near the original deformation region begins to deform during the next duration of the electropulsing. By repeating the behavior of c-e, a uniform section profile is obtained before the occurrence of plastic instability, and thus the formability in improved in terms of the low-frequency electropulsing. Therefore, the uniform wall thickness distribution in EAPGF could be attributed to two factors: one is the periodic temperature homogeneity 12

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along the circumferential direction in terms of low frequency, the other one is the periodic hardening and softening of materials caused by the cyclical rise and fall of the overall temperature of tubes. In order to verify the deformation mode, the experiment with the large duration of 300ms of low-frequency electropulsing under comparable processing condition was conducted. As shown in Fig.11 (b), the maximum deformation is concentrated in the vicinity of vertex, while the deformation at the bottom is limited. Due to uneven temperature before deformation and the large duration, huge deformation started at the top of the middle section, where the temperature inhomogeneity is aggravated by the increase of local resistivity after huge deformation. The localized deformation is too fast to extend to the lower part. Therefore, serious local thinning created in case of excessive electropulsing duration imparted sufficient factors to directly burst. The experiment result provides further evidence that the successive localized deformation mode probably is the general situation in EAPGF. The historical curves of high internal pressure for three different frequencies of electropulsing are shown in Fig.12 (a). For conciseness, only the experimental group with the maximum expansion ratio of different duration in each frequency is included. In the beginning, compressed argon was filled into the tube until the pressure reached 20MPa. For the case of the frequency of 50Hz and 1Hz, once the electropulsing applied to the bulging area, the high internal pressure rises accompanied with heating profile, then remained stable and fluctuates slightly, and decrease due to the expansion of the tube, then the tube burst under the joint effect of excessive pressure and elevated temperature. In terms of the low frequency of 1/20Hz, the pressure profile presented a serration-shaped. The temperature increased when the high-density pulse was conducted to the tube and dropped between the pulses. With the decrease of frequency, the internal pressure fluctuation becomes obvious.

ph  ( p0  0.1)

Th  273 1   0.1 Tm  273 1  Vr

(7)

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where ph is the increased internal pressure after heating, Th and Tm are the temperatures of the pressurized air before and after heating, Vr is the reduction of internal

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volume. It is shown in Fig.12 (b) that the relative load  r (see Eq.(2)) presents the overall trend of growth, due to an increase of internal pressure and a decrease in yield stress of materials since the beginning of EAPGF process. Three lines of relative load were nearly coinciding in the early stage and deviated from each other as the bulging deformation continuing. For the case of high-frequency and intermediate-frequency electropulsing, the temperature rose fast with little deformation at the early stage, so the internal pressure was also in the rising state due to the more obvious influence of temperature. In the middle and late stages of deformation, the temperature change tended to be gentle due to the effect of heating and heat transfer. On the other hand, the influence of the volume change 13

of the tube was more considerable as a result of accelerated deformation speed, so the pressure exhibited constant decay until the bursting. In terms of the low-frequency electropulsing of 1/20Hz, the internal pressure behaved more dynamically than that of high frequency because of the more substantial proportion of the cooling phase and the temperature fluctuation. Moreover, the relative load fluctuated periodically around the amplitude of 0.2 accompanied by an oscillation of temperature as well as internal pressure. To further investigate the deformation mechanism of EAPGF, the microstructure of different frequencies was contrasted in details through EBSD in the next subsection. 4.3.Microstructures of electropulsing assisted gas forming

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In order to further investigate the deformation mechanism of EAGF with and without pulsating, the microstructures of different frequencies were contrasted in details through EBSD. The standard grinding and polishing techniques were used to prepare the metallographic as-received basis material; then the specimens were elect-polished at -30°C with a solution consisting of 10% perchloric acid, 30% butanol and 60% carbinol (vol.%). The crystal phases and microstructure in the orthogonal planes were observed using electron backscattered diffraction (EBSD) on a scanning electron microscope (Quanta 200FEG-SEG) equipped with an EBSD detector. Fig.13 shows the microstructures of the basal material before and after EAGF under three frequencies of electropulsing. For conciseness, only the microstructures under the conditions of high frequency (50Hz) and low frequency (1/20Hz) are included. The schematic diagram of the sampling location of EBSD was shown in Fig.13 (d). The microstructure of as-received tube depicted in Fig.13 (a) is composed with a large number of equiaxial fine α grains (the ratio of length to diameter is less than 2), and the initial grain size of basal material varies in a wide range with the mean size of 2.68μm in the scanned area. Fig.13 (b) and Fig.13 (c) present the microstructures in the middle section for the current frequency of 50Hz and 1/20Hz, respectively. It can be easily seen that the grain size of the surface in the middle section of EAGF is obviously increased, which is the result of the rotation and deformation of the grain under the action of biaxial tensile conditions. In the low-frequency group, there is clear dynamic recrystallization around the grain boundary of the coarse grain, around which newly formed fine  grains are interconnected and circled. The grain size distribution of the basal material in the middle section assisted by three different frequencies of electropulsing is shown in Fig.14. The low-angle grain boundaries (LAGBs) can be classified as the low scalar degree of intragrain misorientation (2° < θ < 15°) between the adjacent pixels (Brewer et al. (2006)). The average grain size in the middle section is slightly decreased with the frequency decreases because of the grain refinement. As shown in Fig.15, the LAGBs exhibit a strenuous increase regardless of the pulse frequency, indicating higher stored energy inside the grains, which was demonstrated the promotion of dynamic recrystallization (Humphreys and 14

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Hatherly (2004)). It is well worthy of being concerned that the fraction of LAGBs in the initial tube was just 22.1% because the initial tube is annealed. The initial structure gradually transformed into a deformation substructure due to the accumulation of dislocation during the process of EAPGF, and the fraction of LAGBs in the material increased prominently with the increasing expansion ratio. An exhaustive analysis of the misorientation evolution of three different frequencies suggested a continuous increase of LAGBs with the frequency decrease, which is consistent with the dynamic recrystallization phenomenon in the low frequency. However, as shown in Fig.16, the grain coarsened obviously under the electropulsing of 50Hz. There are some red lines distributed in the grain interior, indicating that a few of LAGBs existed in the materials. The content of the LAGBs increased under the electropulsing of 1/20Hz compared with the high-frequency electropulsing as shown in Fig.16 (b). With the increase of deformation amount, the dynamic evolution of grain boundaries begins to take place. At first, the density of LAGBs increases remarkably and spreads over all the original grains indeed. During the EAPGF process, the dislocations are accumulated near the grain boundary. Accompanied by the rotation and deformation of the grain, the density of newly formed HAGBs increases. Under these conditions, the transformation from cell wall to LAGBs starts, and therefore convert to a subgrain structure (Mcqueen (2004)). Lots of newly formed fine  grains nucleated around the grain boundaries of coarse grains. These fine  grains are connected around initial grains with

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a circular distribution, thus forming necklace structure, which is the typical feature of recrystallization (Fan et al. (2011)). The existence of a large amount of LAGBs in the deformed structure as well as the high temperature caused by the electropulsing can easily lead to the nucleation of recrystallization. Most of the fine “newly formed” grains nucleated near the grain boundaries and are interconnected and circled and free of LAGBs interior, which is the strong evidence of dynamic recrystallization (Momeni et al. (2014)). In order to verify the occurrence of dynamic recrystallization under the low-frequency electropulsing, the kernel average misorientation (KAM) is used to estimate the generation of the geometrically necessary dislocations (GNDs), which accumulated at the grain boundaries to accommodate the bulging deformation inhomogeneity. (Ashby (1970)). There are usually two methods to estimate GND density in the scanned area: one is based on KAM method from the EBSD data (Calcagnotto et al. (2010)); the other is the tensor component method (Pantleon (2008)). EBSD misorientation data was usually used to calculate the GND density, which dominated the whole dislocations density in the bulging deformation of polycrystalline material (Liu et al. (2015)). The GND density calculated by KAM method can be expressed as:

GND 

 b x 15

(8)

where GND is the GND density,  is the constant which depends on the boundary

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types formed by dislocation, b stands for the magnitude of the Burgers vector, x is the scanning step. The KAM distribution of the basal material in the middle section under the high-frequency and low-frequency electropulsing are shown in Fig.17, where the values of KAM from 0 to 5 are shown in the figure from deep blue to red. It is clear that the KAM value increased as the frequency decreased. The grain size was obviously increased with relatively low KAM value in the high frequency, demonstrating the as-received grain was coarsened at the early stage of the bulging process under the heating effect of electropulsing. Additionally, many fine grains emerged at the grain boundaries of the initial grains with high KAM value and the corresponding GND density as shown in Fig.17. (b). Furthermore, It can be seen that the newly formed grains emerged at the grain boundaries with the lower KAM value interior, which is the typical phenomenon of dynamic recrystallization (Field et al. (2007)). Therefore, the main microstructure evolution of TA2 tubes under the low-frequency electropulsing was dynamic recrystallization. The KAM value distributions and the average KAM value distribution in the middle section under the high-frequency and low-frequency electropulsing are shown in Fig.17 (d-e). The peak frequency point moves to higher KAM value, and the curve is broadened gradually as the frequency decrease. So the average KAM value of material under the low-frequency electropulsing is higher than that of the high-frequency electropulsing, which is consistent with the higher expansion ratio for the condition of low frequency. According to the discussion of the microstructure above, the microscopic mechanism of dynamic recrystallization in the low frequency is proposed as follows to elaborate on the phenomenon and origin. The schematic microstructure evolution during dynamic recrystallization in EAPGF is illustrated in Fig.18. At first, there are few dislocations scattered in the as-received tube due to the plastic deformation in the production process. The density of dislocations is increased prominently with the increasing expansion ratio. In the meantime, the improvement of the mobility of dislocations due to the effect of electrical activation dislocation introduced by Conrad and Okazaki (1978) can facilitate for the conversion of intertwined dislocation with high density to the low angle grain boundary. Frommert et al. (2009) observed a cell structure transformed to a subgrain structure during the steady-state deformation at elevated temperature. In comparison to cell walls that comprise immobile intertwined dislocation, subgrain boundaries which are essentially LAGBs have certain kinetic and thermodynamic properties such as mobility and surface tension. It is worth noting that the transformation from the cell wall to LAGBs starts when the deformation approaches a steady state. The intertwined dislocation as the heat source because of the higher electrical resistivity will promote the nucleation of dynamic recrystallization under the electropulsing. The surface tension of subboundaries induces an imbalance and remotion of grain boundaries, which produce a favorable condition for the bulging and nucleation. Because dynamic recrystallization grains undergo the same development, the newly  grains are interconnected and therefore formed the successive necklace structure as introduced by 16

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Graetz et al. (2014). Since the short duration of the electropulsing compared with the period time and the superior heat dissipation condition, the newly  grains have no chance to grow coarser so that the microstructure refinement was observed in the low frequency. To summarize, there are three essential factors to interpret the obvious dynamic recrystallization phenomenon in the middle section of the EAGF process: 1) Steady-state deformation. During the EAPGF process, the dislocations are accumulated near the grain boundary. The dislocations as the heat source because of the higher electrical resistivity will promote the nucleation of dynamic recrystallization under electropulsing. 2) High fractions of LAGBs after substantial bulging deformation. The LAGBs derived from intertwined forest dislocations spread over the original grains under the electropulsing. The surface tension of subboundaries which are essentially LAGBs induces an imbalance and remotion of grain boundaries. 3) Low-frequency electropulsing. Due to the short duration and the long cooling time of the electropulsing, the newly  grains have no chance to grow coarser. In terms of high frequency, the grains grow coarser, and no local refinement was observed in the EBSD map because of the durative resistance heating. From the above discussion, there is obvious dynamic recrystallization and microstructure refinement in the case of the low-frequency EAPGF, which is significantly improved compared with the traditional hot metal gas forming (HMGF) for lower temperature (<700 °C) and shorter forming time (105s). Consequently, the uniformity and homogeneity of EAPGF were improved attributed to the joint action of the electroplasticity and dynamic recrystallization under the low-frequency frequency. 4.4.Modified electropulsing assisted pulsating gas forming

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From the study above, pulsation characteristics of EAPGF process were analyzed under the same internal pressure (20MPa). The expansion ratio and deformation uniformity of tubes assisted by low-frequency electropulsing were improved attributed to the pulsation characteristics of EAPGF. In order to further study the practical application of the EAPGF, the process parameters were optimized in this section to form a typical adjustable tube with 60% expansion ratio based on the principle of new technology. Considering the effect of strain rate at elevated temperature in the EAPGF process, the loading path and the starting time of pulsation were modified to improve the formability of CP-Ti tubes as shown in Fig.19. From the previous subsection, we know that the expansion ratio was higher at low frequency with the pulsating relative load, so the frequency was kept at 1/20Hz. Meanwhile, the duration of pulsed current was promoted to 300ms to achieve the proper forming temperature range of CP-Ti, and the loading of internal pressure was started from the third electric pulse. Furthermore, as a result of the strain rate sensitivity of titanium alloys at elevated temperature, the pressure was increased from 0MPa, and the loading rate was controlled around 0.05MPa/s. In consideration of the parameters of pulsating hydroforming in modified pulsating EAPGF, the period of pulsating hydroforming T (see in Fig.2) is consistent with the electropulsing period (20s). Meanwhile, the oscillation amplitude B was enlarged by increasing the pulse duration to 300ms. The relative load was increased for the elevated 17

temperature at the beginning of the bulging test. Fig.20 shows the bulged tube of modified pulsating EAPGF. Fig.21 shows the temperature profile, internal pressure, and the corresponding relative load history during EAPGF process for J=80A/mm2, td =300ms, t p =20s. The tem-

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perature profile presented a serration-shaped growth induced by the electropulsing. When the temperature reached 770 °C after the third pulse, the temperature gradually became stable periodic oscillation. The loading path was nearly a straight line, and the pressure applied to tubes was smoothly increased to 4MPa in 80s from the third pulse. Then the tube was sealed, and the internal pressure fluctuated periodically along with an oscillation of temperature. In the early stage of EAPGF process, the deformation rate is slow due to the high deformation resistance and lower temperature. As the initial temperature and the relative load increased, the deformation rate was promoted since 120s. Compared with the study above, the relative load presented the reduced growth rate in the modified EAPGF. The loading path plays an important role in HMGF because it not only determines the wrinkling form directly but also affects the wall thickness distribution of tubes. Serious local thinning created in case of the high-speed loading produced by excessive pressure or/and slow heating rate imparted sufficient factors to directly burst or to cause a pre-existing microcrack in the tube propagated to the critical point at which burst occurs. The initial pressure was constant and the relative loading speed was comparatively fast in subsection 4.2 to analyze the pulsation characteristics of EAPGF process. In terms of the modified EAPGF, the declined relative loading speed due to the loading speed of 0.05MPa/s and the acceleration of temperature rise caused by the promoted duration of electropulsing prevents the occurrence of cracks and improve wall thickness uniformity. To calculate and fit the relative load in the present analysis, the Fourier fit in the fitting area was induced as shown in Fig.22. The general model Fourier function can be expressed as: f ( x)  0.5766  0.01911  sin(0.3096 x)  0.00898cos(0.3096 x)  0.00299cos(0.6192 x)

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0.0098sin(0.6192 x)  0.0061cos(0.9288 x)  0.0024sin(0.9288 x)

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The relative load exhibited a stable periodic fluctuation around the amplitude of 0.577 accompanied with an oscillation of temperature as well as internal pressure induced by the electropulsing.

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5. Conclusions

In order to simplify the control scheme, reduce die wear as well as omit the preceding die heating of IHPF of high strength lightweight material tubes, such as commercial pure titanium alloy tubes, during the bulging process, the experimental apparatus and platform were designed and developed to carry out the investigation of electrically assisted gas bulging of CP-Ti (Grade 2) tubes. The electropulsing-induced pulsation effect of CP-Ti tubes was experimentally and analytically investigated by electropulsing assisted gas forming combined with the uniaxial tensile experiment and subsequent microstructural observation through EBSD. Three different frequencies of 18

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electropulsing were applied to the tubes filled with pressurized argon to investigate the unique mechanism and plastic deformation behaviors in EAPGF process. The variation of the temperature distribution, the expansion ratio, the shape deviation ratio, the circumferential thickness distribution, the relative load and the microstructure evolution with electropulsing frequency were studied experimentally. Furthermore, a modified pulsating EAPGF was proposed to provide a comprehensive understanding of the amelioration of the formability by adjusting pulsating hydroforming parameters by regulating the parameters of electropulsing. The experimental results are as follows: (1) From the experimental results of electropulsing assisted uniaxial tension, the flow stress decreased drastically when electropulsing was applied during the tensile deformation. The ultimate tensile elongation is enhanced with the decrease of the frequency. The model of the relative load in EAGF was derived and interpreted with the generalized Johnson-Cook model. Besides, a fuzzy load control algorithm of pulsating hydroforming, which can improve the formability of tubes, can be achieved by optimizing of the electropulsing parameters and the pressure in EAPGF. (2) When evaluating the bulging limit of tubes assisted by electropulsing of three different frequencies, it is evident that the expansion ratio increased as the frequency decreased. For the case of low frequency of 1/20Hz, the relative load behaved much dynamically than that of high frequency. In this study, the bulging limit of seamless CP-Ti tubes during 1/20Hz EAPGF was 38.9%. The experimental results reveal that the higher bulging limit and more uniform deformation were realized for the condition of low frequency. (3) The experimental results demonstrated that the temperature difference increases with time. In terms of the low-frequency electropulsing of 1/20Hz, the internal pressure behaved more dynamically than that of high frequency because of the more substantial proportion of the cooling phase and the temperature fluctuation. Moreover, the internal pressure and the relative load fluctuated periodically around the amplitude of 20MPa and 0.2, respectively. Uniform wall thickness distribution in EAPGF could be attributed to the periodic temperature homogeneity along the circumferential direction, the repeated hardening and softening of materials, and the oscillation of the relative load. (4) The microstructure of expansive tube shows that the grain grew coarser after the bulging process as a result of the rotation and deformation of the grain under the biaxial tensile conditions. The LAGBs exhibit a strenuous increase compared with the as-received tube, indicating higher stored energy inside the grain, which was demonstrated the promotion of dynamic recrystallization. Lots of the newly formed fine  grains nucleated around the grain boundaries of coarse grains.

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Declaration of interests

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(5) From the result of microstructure analysis around the necklace structure, the occurrence of dynamic recrystallization was confirmed. The formability of CP-Ti tubes was promoted, and microstructure was refined attributed to the dynamic recrystallization assisted by low-frequency electropulsing. To summarize, the obvious dynamic recrystallization phenomenon in the middle section of the EAGF process may be attributed to the steady-state deformation, high fractions of LAGBs after substantial bulging deformation and the superior heat dissipation under the low-frequency electropulsing. (6) Concerning the perspective of the enhancement of the formability of EAPGF, the loading path and the pattern of applying electropulsing were modified. The relative load was adjusted to pulsate around the amplitude of 0.0577, and a typical adjustable CP-Ti tube with 60% expansion ratio was successfully fabricated. The present research provides significant insights into electropulsing assisted pulsating gas forming. From the experimental results, the formability and uniformity are significantly improved in the low-frequency case. A beneficial aspect of applying electropulsing in bulging is that it simplifies the control scheme. Meanwhile, the pulsation effect of electropulsing of low frequency has a positive influence on improving the bulging limit, reducing the shape deviation, enhancing the forming uniformity, improving the microstructure and mechanical properties in the middle section of expansion tubes. Consequently, EAPGF is a promising process.

The authors declare that they have no known competing financial interests or personal relationships

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that could have appeared to influence the work reported in this paper.

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The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Acknowledgments

The work reported in this paper was supported by the State Key Lab of Advanced Welding and

Joining, Harbin Institute of Technology (No. AWJ-19M05) and Program of National Natural Science 642 Foundation of China (No.51105203).

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Figure captions Fig.1 (a) Illustrations of the experimental setups, (b) schematic diagram of electropulsing during tensile tests, and (c) stress-strain and temperature curves under electropulsing. Fig.2 The relationship between the pulsating parameters and electropulsing parameters. Fig.3. Schematic of experimental apparatus for electrically assisted hot gas bulging. Fig.4. Die set and sealing structure of electrically assisted hot gas bulging. Fig.5. The schematic diagram of three different frequencies of electropulsing: (a) High frequency, (b) medium frequency, (c) low frequency. Fig.6. (a) The schematic diagram of temperature measuring points and (b) the experiment apparatus of temperature measurement.

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Fig.7. Temperature profiles of three electropulsing frequencies of tubes: (a) 50Hz, (b) 1Hz, (c) 1/20Hz. Fig.8. (a) Bulged tubes for different durations of electropulsing, (b) the expansion ratio and shape deviation of bulged tubes, (c) the circumferential wall thickness distribution.

Fig.9. (a) Bulged tubes for different frequencies of electropulsing, (b) the expansion ratio and shape deviation of bulged tubes, (c) the circumferential wall thickness distribution.

Fig.10. The temperature difference along the circumferential and axial directions: (a) The temperature difference be-

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tween P1 and P3 (circumferential directions), (b) the temperature difference between P1 and P4 (axial directions). Fig.11. The schematic diagram of the successive localized deformation mode in EAPGF.

Fig.12. Historic curves of three different frequencies of electropulsing: (a) Internal high pressure, (b) relative load.

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Fig.13. Microstructure of the basal material in the middle section of different frequencies of electropulsing: (a) As-received, (b) 50Hz, (c) 1/20Hz.

Fig.14. Grain size distribution in the middle section of tubes under different frequencies of electropulsing: (a) Overall

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grain size distribution, (b) average grain size distribution.

Fig.15. Misorientation distribution in the middle section of tubes under different frequencies of electropulsing: (a) Overall misorientation distribution, (b) average misorientation distribution. Fig.16. The microstructure in the middle sections under different frequencies of electropulsing: (a) 50Hz, (b) 1/20Hz.

na

Fig.17. KAM distributions in the middle section under electropulsing of different frequencies: (a) 50Hz, (b) 1/20Hz, (c) demonstration of DRX, (d) KAM value distribution, (e) average KAM value distribution. Fig.18. Schematic microstructure evolution during dynamic recrystallization in EAPGF: (a) Emerge of dislocation, (b)

ur

arrangement of LAGBs, (c) grain boundary remotion, (d) bulging and concurrent subgrain generation, (e) formation of the first necklace structure, (f) development of the continuous necklace structure. Fig.19. The schematic diagram of the low frequency of electropulsing for modified EAPGF.

Jo

Fig.20. Bulged tubes of modified EAPGF. Fig.21. The temperature profile, internal pressure, and relative load histories during EAPGF process for J=80A/mm2, td =300ms, t p =20s. Fig.22. Fourier fitting of the relative load for J=80A/mm2,

td =300ms, t p =20s.

24

ro of -p

re

Fig.1 (a) Illustrations of the experimental setups, (b) schematic diagram of electropulsing during tensile tests, (c)

ts

ur

na

Period

Duration

Relative load

lP

T

td

B

Current density

stress-strain and temperature curves under electropulsing.

Time(sec)

Jo

Fig.2 The relationship between the pulsating parameters and electropulsing parameters.

25

Argon Argon pressurization system

High pressure argon

Tube

Bulging area Globe valve

le

Electropulsing

Pressurized Gas filtration air system

up oco m er

Th

ro of

Mica insulator

na

lP

re

-p

Fig.3. Schematic of experimental apparatus for electrically assisted hot gas bulging.

Jo

ur

Fig.4. Die set and sealing structure of electrically assisted hot gas bulging.

26

ro of

Fig.5. The schematic diagram of three different frequencies of electropulsing: (a) High frequency, (b) medium fre-

lP

re

-p

quency, (c) low frequency.

Fig.6. (a) The schematic diagram of temperature measuring points and (b) The experiment apparatus of temperature

na

measurement.

1000

5ms,P2

800

700

5ms,P3

600

ur

Temperature/℃

5ms,P4

10ms,P1

600

10ms,P2 10ms,P3

400

10ms,P4

Jo

Temperature/℃

800

5ms,P1

200 0

0

20ms,P1

20ms,P2

20ms,P3

20ms,P4

30ms,P1

30ms,P2

30ms,P3

30ms,P4

500 400 300 200 100

10

20

30 Time/s

40

50

0

60

(a)

0

10

20

30 40 Time/s

(b)

27

50

60

700

100ms,P1

100ms,P2

100ms,P3

100ms,P4

200ms,P1

200ms,P2

200ms,P3

200ms,P4

Temperature/℃

600 500 400 300 200 100 0

0

20

40 60 Time/s

80

100

(c)

re

-p

ro of

Fig.7. Temperature profiles of three electropulsing frequencies of tubes: (a) 50Hz, (b) 1Hz, (c) 1/20Hz.

Fig.8. (a) Bulged tubes for different durations of electropulsing, (b) the expansion ratio and shape deviation of bulged

ur

na

lP

tubes, (c) the circumferential wall thickness distribution.

Fig.9. (a) Bulged tubes for different frequencies of electropulsing, (b) the expansion ratio and shape deviation of

Jo

bulged tubes, (c) the circumferential wall thickness distribution.

28

Temperature difference between P1 and P3/℃

50Hz,10ms 1Hz,30ms 1/20Hz,200ms

160 140

50Hz,10ms 1Hz,30ms 1/20Hz,200ms

80

120

60

100 80

40

60 40

Periodic homogenization

20

20 0

Temperature difference between P1 and P4/℃

100

180

0

10

20

30 40 Time/s

50

60

70

80

0

0

10

(a)

20

30 40 50 Time/s

60

70

80

(b)

Fig.10. The temperature difference along the circumferential and axial directions: (a) The temperature difference be-

lP

re

-p

ro of

tween P1 and P3 (circumferential directions), (b) the temperature difference between P1 and P4 (axial directions).

Jo

ur

na

Fig.11. The schematic diagram of the successive localized deformation mode in EAPGF.

Fig.12. Historic curves of three different frequencies of electropulsing: (a) Internal high pressure, (b) relative load.

With the increment of temperature, the gauged pressure in the sealed tube can be expressed by the following equation:

29

ro of -p

re

Fig.13. Microstructure of the basal material in the middle section of different frequencies of electropulsing: (a) As-received, (b) 50Hz, (c) 1/20Hz.

na

Average grain size/μm

lP

(b) 45 35 30

27.38

25 20 15.22

15 10 5

ur

39.62

40

0

2.68 BM

50Hz

1Hz

1/20Hz

Jo

Fig.14. Grain size distribution in the middle section of tubes under different frequencies of electropulsing: (a) Overall grain size distribution, (b) average grain size distribution.

30

Average misorientation/°

(b) 50

46.76

40 30 20 13.81

11.95

10 0

BM

50Hz

1Hz

9.59

1/20Hz

Fig.15. Misorientation distribution in the middle section of tubes under different frequencies of electropulsing: (a)

lP

re

-p

ro of

Overall misorientation distribution, (b) average misorientation distribution.

Jo

ur

na

Fig.16. The microstructure in the middle sections under different frequencies of electropulsing: (a) 50Hz, (b) 1/20Hz.

31

(a)

(b)

(c) Fine grains

Microstructure

The increase in LAGBs Necklace structure Free of LAGBs interior Low KAM value interior Dynamic recrystallization (d) 35

(e) 1.2 BM 50 1/20

20 15 10

0.5685

0.6 0.4

0.3948

0.2

5 0

1 2 3 4 Kernel averaged misorientation/°

0.0

5

BM

50Hz

-p

0

0.8

ro of

25

1.0141

1.0

Average KAM

Frequency/%

30

1/20Hz

Fig.17. KAM distributions in the middle section under electropulsing of different frequencies: (a) 50Hz, (b) 1/20Hz, (c)

Jo

ur

na

lP

re

demonstration of DRX, (d) KAM value distribution, (e) average KAM value distribution.

32

Fig.18. Schematic microstructure evolution during dynamic recrystallization in EAPGF: (a) Emerge of dislocation, (b) arrangement of LAGBs, (c) grain boundary remotion, (d) bulging and concurrent subgrain generation, (e) formation of

ro of

the first necklace structure, (f) development of the continuous necklace structure.

re

-p

Fig.19. The schematic diagram of the low frequency of electropulsing for modified EAPGF.

Fig.20. Bulged tubes of modified EAPGF. P2

P3

8

600 500

6

400

Unloading

200

2

2 80A/mm ……

Relative load

0

Fitting area

0.05

Loading

Unloading

0.00

2 80A/mm ……

ur

100 0

4

Loading

300

0.10

10

na

Temperature/℃

700

Pressure

Internal high pressure/Mpa Relative load

800

12

P1

lP

900

0 20 40 60 80 100 120 140 160 180 200 220 Time/s

0 20 40 60 80 100 120 140 160 180 200 220 Time/s

Jo

Fig.21. The temperature profile, internal pressure, and relative load histories during EAPGF process for J=80A/mm2,

td =300ms, t p =20s.

33

td =300ms, t p =20s.

Jo

ur

na

lP

re

-p

ro of

Fig.22. Fourier fitting of the relative load for J=80A/mm2,

34

Table captions Table1 Material coefficients for constitutive models in electrically assisted tension tests. Table2 Chemical compositions of TA2 titanium alloy (mass fraction, %).

Jo

ur

na

lP

re

-p

ro of

Table3 Conditions for EAPGF process of CP-Ti tubes.

35

Table1 Material coefficients for constitutive models in electrically assisted tension tests.

Model

Coefficient

Value

Johnson-Cook

b

796.1

n

0.124



0.01



4.56

Table2 Chemical compositions of TA2 titanium alloy (mass fraction, %).

C

N

H

0.080

0.01

0.009

0.008

O 0.10

Table3 Conditions for EAPGF process of CP-Ti tubes.

Current density

electropulsing

pressure p0 (MPa)

J (A/mm2)

50Hz

20

80

1Hz

20

80

1/20Hz

20

Pulse period

-p

Initial internal

re

Frequencies of

Jo

ur

na

lP

80

36

Ti

ro of

Fe

Bal.

Duration

tp (s)

td (ms)

0.02

5/10

1

20/30

20

100/200