Experimental Investigation on Pulsed Current Influence on

U-bending of AZ31B Magnesium Alloy

H Li

, L Wang

, D W Ao

, X R Chu

1, *

and J Gao

School of Mechanical, Electrical&Information Engineering, Shandong University,

Weihai, Weihai 264209, Shandong, China

Weifang GeorTek Electronics Co., Ltd. Weifang 261205, China

Corresponding author and e-mail: X R Chu, xrchu@sdu.edu.cn

Abstract. The electropulse effect (EPE) is combined with the bending forming of magnesium

alloy sheet. Through electropulse assisted (EPA) magnesium alloy U-bending test, the

influence of current parameters on the U-bending properties of magnesium alloy sheet was

studied. Through the analysis of the microstructure of magnesium alloy sheet, the mechanism

of EPE on the bending forming performance of magnesium alloy sheet was explored. It was

found that the pulse current promotes the occurrence of dynamic recrystallization of the

magnesium alloy, improves the forming ability of the magnesium alloy sheet, and reduces the

springback angle of the magnesium alloy sheet bending.

1. Introduction

Magnesium and magnesium alloys are known as “green material” in the 21st Century for their

excellent mechanical properties. Magnesium and magnesium alloys are widely used in aerospace,

automotive, electronics, communications, and other fields due to their good performance [1].

However, due to the hexagonal crystal structure of magnesium alloys, magnesium alloys have poor

plastic forming ability at room temperature, which limits the application of magnesium alloys in the

industrial field and has become a major factor restricting the development of magnesium alloys in the

industry [2].

Guoyi Tang et al. [3-5] studied the electropulse drawing and rolling of magnesium alloys found

that pulsed current can promote the movement and climbing of movable dislocations, open the

dislocation tangles, change the dislocation density, reduce the flow stress of the material, and find

that the pulse current can promote dynamic recrystallization of magnesium alloys at lower

temperatures. Perkin [6] studied the forming properties of aluminum alloys and other metals through

electropulsing upsetting experiments, and found that the electropulsing significantly improved the

plastic forming ability of aluminum alloys. The deformation before fracture was significantly

improved, and the deformation resistance decreased significantly. Bao et al. [7] studied the

electropulsing assisted(EPA)magnesium alloy incremental forming experiments found that the

parameters of the electropulse have a significant influence on the progressive forming limit of the

sheet. With the increase of the pulse current, the forming limit is significantly improved, and the

forming ability is significantly enhanced. It proved the existence of pure EPE. Conrad [8] proposed

that the pulsed current can increase the internal energy of the material and thus promote the thermal

604

Li, H., Wang, L., Ao, D., Chu, X. and Gao, J.

Experimental Investigation on Pulsed Current Inﬂuence on U-bending of AZ31B Magnesium Alloy.

In Proceedings of the International Workshop on Materials, Chemistry and Engineering (IWMCE 2018), pages 604-609

ISBN: 978-989-758-346-9

activation process of the dislocation movement. Molotskii [9] explains the mechanism of pure

electroplastic effect through induced magnetic field theory.

In this paper, the EPE is combined with the bending forming of magnesium alloy sheet, based on

a self-designed experimental mold. Through EPA magnesium alloy bending forming test, the

influence of current parameters on the U-bending properties of magnesium alloy sheet was studied.

Through the analysis of the microstructure of magnesium alloy sheet, the mechanism of EPE on the

bending forming performance of magnesium alloy sheet was explored.

2. Test equipment and materials

The test specimen is AZ31B magnesium alloy, dimensions 90mm x 20mm x 1.2mm, the test device

was shown in Figure 1. In order to prevent the test piece and the concave-convex die from being

incompletely contacted, when the punch reaches 1mm, the device which is shown in the Figure 1

starts to energize until the end of the test. To study the influence of pulse current on the U-bending

forming, the adopted punch fillet radius is set to 4mm, die clearance is set to 1.1t, t is the thickness of

the sheet, the impulse current generator and electropulsing waveform Figure 2, the forming rate is

15mm/min. In order to ensure the accuracy of the test results, each set of tests was performed three

times. The angles of the two right-angled sides of the formed test piece were measured by a universal

protractor. The average value of the three test results was taken as the forming angle value. After the

test was completed, the microscopic metallographic test was performed by an Axio Lab A1 optical

microscope.

Figure 1. U-bending die setup schematic diagram and setup device.

Figure 2. Impulse current generator and pulse current schematic diagram.

Experimental Investigation on Pulsed Current Inﬂuence on U-bending of AZ31B Magnesium Alloy

605

3. Experimentation results and discussion

3.1. Experimentation results

The forming angle of the magnesium alloy test without current is taken as a reference. The forming

angles of U-bending tests under different pulse current were shown in Table 1. The formed

specimens were presented in Figure 3.

Table 1. The results of EPA U-bending.

Sample

Voltage(V)

Frequency(Hz)

Forming angle(°)

114.5

200

112.3

200

110

350

108.5

350

107

450

106.2

350

103.3

Figure 3. The specimens of EPA U-bending.

With reference to Figure 3. and Table 1., it can be found that with the increase of the pulse current

parameters, the forming angle of the U-shaped bending part of the magnesium alloy gradually

decreases. That is, with the increase of the pulse current parameter, the springback angle of the U-

shaped bending of the magnesium alloy gradually decreases. The U-shaped bending angle curve of

the EPA magnesium alloy under different pulsed electric parameters was shown in Figure 4.

Figure 4. Forming angle curves under different electric parameters.

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Figure 4 shows that with the increase of electrical parameters, the forming angle gradually

decreases, and in the selected range of electrical parameters, the U-shaped bending angle of the

magnesium alloy basically shows a linear decrease trend.

3.2. Investigations on microstructure

For U-bending process, before the calibration stage, for the main deformation region of the test piece,

the interior region of fillet radius is under compression stress state and the exterior region of fillet

radius is under tension stress state. The microstructure of the main deformation zone (fillet radius) of

the U-bending specimen was obtained as shown in Figure 5.

Experimental Investigation on Pulsed Current Inﬂuence on U-bending of AZ31B Magnesium Alloy

607

Figure 5. The microstructure of specimens under EPA U-bending (EX

represents the extrados of the specimen, IN represents the intrados of

the specimen. (a) (b) (c) (d) (f) are No current, 50V-200Hz, 70V-

200Hz, 70V-350Hz, 70V-450Hz, 90V-350Hz).

Figure 5 (a) and (b) show the microstructures of the external and internal fillet radius zone at

room temperature. Figure 5 (a) presents that the large grains are stretched and twinning occurs in

some coarser grains. Slip is the main deformation mechanism for the external zone. In Figure 5 (b),

many twins that marked with ellipses can be observed, even in some small grains. Twin is the main

deformation for the internal zone. The difference between the internal and external deformation

mechanisms determines the microstructures. Figure 5 (c) and (d) show when the pulse current

parameter is small, the microstructure of curved specimens has not changed much compared to the

U-bending at room temperature. Figure 5 (c) and (d) have similar microstructures. A large number of

twin grains occur in the interior. It is meant that twinning is the main deformation for the internal

zone, and slip is the main deformation mechanism for the external zone. With the increase of the

electrical parameters until 70V-350Hz, the microstructure does not change significantly, meaning

that EPE is not significant. The fine grains are generated at the grain boundaries of some coarse

grains at the internal fillet radius zone above 70V-350Hz. It is indicated that the dynamic

recrystallization of Magnesium alloy begins to occur under pulsed current. Conrad et al. [10] found

that pulse current has "thermal effect" and "althermal effect". In this paper, their common actions are

considered. The general deformation mechanism of a material having the same HCP structure as that

of magnesium are twinning and dynamic recrystallization. The process of dynamic recrystallization

is opposite to static recrystallization, the nucleation and growth of new grains occur during

deformation rather than later as part of a separate heat treatment [11]. The fine crystallites produced

by dynamic recrystallization play an important role in coordinating the plastic deformation,

contributing to improve the plastic forming ability and reduce the springback angle of the U-shaped

specimen. As the electrical parameter increases to 70V-450Hz, the dynamic recrystallization begins

to occur at the outside fillet radius zone of the U-bending specimen. A large number of fine equiaxed

grains are observed in the interior region. The dynamic recrystallization occurs both at the inside and

outside zones of the U-shaped specimen. The plastic forming ability of the magnesium alloy can be

further improved, and the springback was reduced.

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4. Conclusions

The pulse current can promote the occurrence of dynamic recrystallization of the magnesium alloy,

improves the forming ability of the magnesium alloy sheet, and reduces the springback of the

magnesium alloy sheet bending.

Acknowledgement

The authors would like to acknowledge the financial support from the Natural Science Foundation of

Shandong Province (No. ZR2016EEM25).

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