Elsevier

Journal of Manufacturing Processes

Volume 57, September 2020, Pages 209-221
Journal of Manufacturing Processes

Improvement on formability and forming accuracy in electromagnetic forming of deep-cavity sheet metal part using a dual-coil system

https://doi.org/10.1016/j.jmapro.2020.06.023Get rights and content

Highlights

  • A dual-coil electromagnetic forming (EMF) system assisted by radial Lorentz force was proposed to achieve the forming of deep-cavity sheet metal parts.

  • The deep-cavity workpiece can be formed by the dual-coil EMF with improved formability and more uniform thickness distribution.

  • Due to the flexible controllability of axial and radial Lorentz forces, high forming accuracy with die-fitting gap of 0.35 mm can be attained by the dual-coil EMF.

Abstract

A dual-coil electromagnetic forming (EMF) method assisted by radial Lorentz force was proposed herein to achieve the die forming of deep-cavity sheet metal parts. The objective of this study was to certify the extent to which the formability and forming accuracy can be improved by the proposed method. For comparative study, a single-coil EMF system without radial Lorentz force was also utilized. To develop the deformation process, the experiments and simulations of deep-cavity aluminum workpieces under a wide range of voltage combinations were conducted by the single-coil and dual-coil EMF, respectively. The experimental results showed that the dual-coil EMF resulted in the formation of the deep-cavity workpiece with high forming accuracy, maximum die-fitting gap of 0.35 mm, and more uniform thickness distribution. In contrast, the workpiece tended to crack without contacting the die by the single-coil EMF. Based on the simulation results, dynamic deformation behaviors of the deep-cavity workpieces were demonstrated and the underlying mechanisms of the improved formability and forming accuracy were explained. Moreover, the process window of die-fitting gap control indicated that the fittability could be controlled by adjusting the voltage combinations of the dual-coil EMF, and the corresponding optimum voltage of Coil-2 (V2) that contributed to high forming accuracy decreased with the increase of the voltage of Coil-1 (V1). Therefore, the proposed dual-coil EMF exhibits more flexibility and efficiency for producing the deep-cavity sheet metal part with a higher forming accuracy than the single-coil EMF, extending the potential application of the conventional EMF due to the utilization of multiple coils.

Introduction

Deep-cavity sheet metal parts are widely utilized in various industries, including automobile, aviation, aerospace, and so on. However, for every metallic material, there is a limited forming depth that can be realized without necking or cracking. This limitation is especially obvious for the metal material with poor formability, such as the aluminum alloy that was suggested to be used as an alternative for the mild steel in automobile industries, due to its high strength/weight ratio and good overall performance, as pointed out by Xun et al. [1]. To overcome this limitation, various forming processes including hydroforming, warm forming, high-velocity forming, and so on have been proposed to improve the formability of the metal material. Among these advanced processes, the high-velocity forming has been validated to contribute toward significant improvements of formability and ductility of various materials, due to its unique features of high strain rate (∼1000 s−1) and remarkable inertia stability effect [2]. Daehn [3] pointed out that the high forming velocity can be available by means of explosive forming, electrohydraulic forming, and electromagnetic forming (EMF) processes, among which the EMF stimulates a wider interest for researchers in sheet and tube metal forming community.

Various applications of EMF have been intensively studied to meet the requirements for producing sheet metal parts with high forming quality, deep cavity, large scale, and so on. By combining the conventional stamping process with the EMF, Vohnout [4] achieved local sharper features; and Shang and Daehn [5] realized more uniform strain distribution. By utilizing the Lorentz force to drive a conductive punch, Cao et al. [6] obtained a cylindrical cup with a drawing ratio of 2 and a forming height of 66.4 mm. By implementing only the EMF process, Yu et al. [7] realized the circular hole flanging of QCr0.8 copper alloy sheet, and Lai et al. [8] achieved the forming of aluminum alloy (AA5083 and AA2219) sheet with diameter of 1378 mm into partial ellipsoid shape using ∼800 kJ energy. However, researches on the forming of deep-cavity sheet metal parts by using only the EMF process have rarely been reported, which is mainly limited by the following two issues.

Excessive thinning of the sheet metal is an unfavorable defect. It is a significant issue to avoid excessive thinning in the forming of deep-cavity sheet metal part. During conventional EMF processes, the workpiece region under the projection of the coil is first accelerated to a very high velocity by the Lorentz force, then continually driven by the inertial force, and finally decelerated due to large plastic deformation, which can easily result in necking or cracking. Imbert et al. [9] indicated that the cracking phenomenon first occurred at the bottom region of the conical free-formed part, and the cracking region shifted to the die corner region with the increase in the discharge energy. Golovashchenko [10] demonstrated that the central region of the workpiece was driven and separated by the inertial forces and the fracture area of the central region was enlarged with the increase of the energy level. To improve thinning and inhibit necking or cracking, the Lorentz force distribution that predominates the plastic deformation, should be adjusted appropriately. Oliveira et al. [11] fabricated a flat double spiral coil to avoid “dead spots” that occurred at the center of the winding spiral coil with low magnetic pressure, thereby resulting in relatively uniform pressure distribution, and promoted more uniform major strain distribution and excellent forming height in the forming of AA5754 aluminum alloys. Kamal and Daehn [12] designed a novel electromagnetic actuator that efficiently provided uniform magnetic pressure to drive the workpiece with more uniform velocity distribution, thus better deformation uniformity was obtained. Besides, various electromagnetic actuators for altering the Lorentz force distribution have been proposed for tube forming. Suzuki et al. [13] utilized a field shaper to change the force distribution and control the final deformation profile during tube bulging process, and found that the strain distribution was determined by the geometry of the field shaper and the discharge energy of the coil. Yu et al. [14] conducted tube compression process and found that the ratio of length of tube to that of coil affected the Lorentz force distribution, and there existed a critical value of length ratio corresponding to the relatively uniform radial deformation. Qiu et al. [15] fabricated a concave coil for tube bulging to provide a concave distribution of the radial electromagnetic force, which resulted in better deformation uniformity. However, the effects of coil geometry on improving thinning are limited when it comes to the deep-cavity sheet metal forming, because the stretching deformation is too large and the material flow is difficult, which still leads to severe thinning under the large deflection.

Apart from the severe thinning issue, to ensure a good forming accuracy is another issue that requires a lot more systematic explorations. During conventional free-form EMF processes, the plastic deformation of the workpiece is mainly dominated by the contact-free Lorentz force and inertial force that are difficult to control. It usually leads to a poor forming accuracy, especially for the forming workpiece with deep-cavity and complex-shape. In general, a steel die is adopted to shape the workpiece into desired profile and achieve a good forming accuracy. Imbert et al. [16] implemented conical die experiments and found the “inertial ironing” effect (high through-thickness compressive stress due to impact with the die at high velocity) increased the formability and inhibited necking. Golovashchenko [10] conducted cavity fill experiments using a V-shape die or a conical die, and found that the local strains could be significantly improved due to the interaction with the die. Nonetheless, the rebound phenomenon can significantly reduce the forming accuracy when forming with a die. Risch et al. [17] conducted the EMF experiments using a shallow ellipsoid die, and found that lower discharge energy contributed to a better use of the inertial force before impact with the die, and the kinetic energy of the workpiece should be dissipated after impact with the die, thus to reduce the rebound effect. Yu et al. [18] proposed a two-step EMF method to expand an aluminum alloy tube into square section, and found that a higher voltage than 12.25 kV could easily result in necking or cracking by the single-step EMF forming. However, for the two-step EMF forming, the rebound effect was reduced due to a lower impact pressure. Moreover, the deformation uniformity was improved because of reduced friction, thus effectively enhancing the overall formability and improving the forming accuracy. Feng et al. [19] investigated the effect of discharge voltage on the rebound process using a hemispherical die, and found that the central region of the deformed workpiece exhibited a concave profile at a discharge voltage of 12 kV, which resulted in significant reduction in the fittability. Therefore, the discharge energy influencing the Lorentz force distribution should be adjusted to reduce the rebound effect, thereby promoting the forming accuracy.

This study focuses on the forming of deep-cavity workpieces by a dual-coil EMF that was first proposed by Lai et al. [20]. The dual-coil EMF introduces an additional radial Lorentz force at the flange region to enhance the material flow, thus inhibiting tearing and dramatically increasing the forming depth from 8.44 to 20.28 mm. Lai et al. [21] further investigated the deformation behavior of the proposed method, and found that the maximum thickness reduction decreased from 25.6 % to 6.7 %, and the improved material flow could alter the deformation profile from conical to cylindrical, thus improving the fittability. Further, Chen et al. [22] explored the potential of deformation control by the dual-coil EMF in a wider range of discharge voltage combinations, and realized the deformation profile with convex, flat, or concave bottom. Owing to these advantages of the dual-coil EMF, i.e., the enhancement of material flow and the flexibility in adjusting discharge voltage combinations, the major issues of excessive thinning and forming accuracy in the deep-cavity sheet metal forming may be improved.

To this end, a comparative analysis of the single-coil and dual-coil EMF was carried out by experiments and simulations in this paper. The effects of discharge voltage combinations of the dual-coil EMF on the deformation profile, die-fitting gap, and thickness distribution were investigated. The dynamic deformation behavior of the deep-cavity forming was numerically analyzed to reveal the underlying mechanism of the improved thinning and forming accuracy. Furthermore, a process window in a wider scope of voltage combinations of the two coils was established to provide a better understanding of the die-fitting gap control.

Section snippets

Principle

The schematic diagram of the proposed dual-coil EMF system is shown in Fig. 1(a) and the corresponding experimental setup is shown in Fig. 1(b). The proposed dual-coil EMF system consists of two coils (Coil-1 and Coil-2), an aluminum workpiece, a die, a vacuum-pumping system and a 0.5-ton counterweight. Coil-1 is constrained by the 0.5-ton counterweight while Coil-2 can keep self-balance. The epoxy resin plate attached to the bottom of Coil-1 is working as blankholder and the blankholder force

Numerical simulation

A two-dimensional (2D) axisymmetric model coupled with electromagnetic and mechanical fields was established to investigate the dynamic deformation process by using a commercial software LS-DYNA (version R910), as shown in Fig. 6. In LS-DYNA, the electromagnetic problems were solved using a Finite Element Method (FEM) for the conductors (the coils and the workpiece) and a Boundary Element Method (BEM) for the surrounding air/insulators. Thus, the surrounding air mesh was not necessary, which

Deformation results of the single-coil EMF

For the single-coil EMF, there was only Coil-1 without Coil-2, and its discharge energy was controlled by adjusting V1. Fig. 8(a) shows the deformation profile under various V1. As V1 increased from 4 kV to 6 kV, the forming height significantly increased, but the workpiece profile was always conical that was very common during electromagnetic free-bulging process [27]. Moreover, the fittability between the workpiece and the die cavity was very poor. There was no trend to fit the die cavity as V

Conclusion

A dual-coil electromagnetic forming (EMF) system was proposed in this study to improve the formability and forming accuracy of a deep-cavity workpiece. For comparative analysis, the deformation results of the single-coil and the dual-coil EMF were experimentally and numerically investigated. The main conclusions can be drawn as follows:

  • (1)

    For the single-coil EMF, the final deformation profile of the workpiece was always conical, regardless of the discharge voltage of Coil-1 (V1). As V1 increased

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the China Postdoctoral Science Foundation (2018M632856) and the Young Elite Scientists Sponsorship Program by CAST(YESS, 2018QNRC001).

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