Deformation behavior of 5052 aluminum alloy sheets during electromagnetic hydraulic forming
Graphical abstract
Introduction
In the hydroforming (HF) process, liquid is used as a forming force to make the material adhere to the mold. HF is widely used in the plastic processing industry as it can accurately control the shape of complex parts. Zhou et al. [1] performed both numerical and experimental studies on single- and dual-layer sheets using the HF process to investigate the thickness and wrinkling behavior of the sheets. The authors concluded that the dual-layer sheet HF process improves the component by reducing wrinkles and is helpful to obtain a uniform thickness distribution. Dong et al. [2] combined artificial aging and heat treatment of aluminum alloys into the HF process, which resulted in excellent formability. Liu et al. [3] investigated the feasibility of utilizing fiber metal laminates in the automobile industry using the HF technique. Chen et al. [4] proposed an analytical method to avoid failures, e.g., wrinkling, by implementing the reverse bulging effect and demonstrated that exceptionally thin-walled sheets can be formed without wrinkles. Han et al. [5] proposed a trimmed layered tube hydroforming process that allows the fabrication of hollow tubular parts of dissimilar thicknesses without welding. Ra et al. [6] manufactured wire-reinforced aluminum tubes by hydroforming process. Based on finite element analysis, appropriate loading paths were derived to manufacture without defects tube.
However, HF is a low-rate deformation method, and it is challenging to increase the forming limit of materials. Ye et al. [7] reported that, when the HF process was employed to manufacture two-layer four-wave annular bellows, the pipe fittings were prone to rupture due to wall thinning. Wang et al. [8] utilized the HF process to manufacture T-shaped tubes and demonstrated that the actual pressure inside the tubes adopts the form of linear loading. The experimental results revealed that such a loading mode can easily lead to tube fracture. Therefore, a step-loading method was proposed, where the thickness distribution of the T-shaped tube is more uniform, and no fold or fracture of the parts occur. Omar et al. [9] manufactured mild steel tubes with an outside diameter of 57.15Ā mm and a wall thickness of 1.6Ā mm using the HF process. When the outer diameter of the pipe fitting is determined, the cracking of the tube can be prevented by changing the height. Feng et al. [10] proposed hydroforming overlapping method which uses overlapping tubular rather than closed cross-sectional tubes to prevent wrinkling defects. It is feasible to reduce the forming pressure and the wall thinning using an overlapping tube.
Electromagnetic forming (EMF) is a type of high-strain-rate forming method that can significantly improve the material forming limit. Golovashchenko et al. [11] conducted a large number of experimental investigations using different types of molds. The forming limits of 6111-T4 and 5754 aluminum alloys using the EMF method were improved compared with those obtained via quasi-static forming. Imbert et al. [12] studied the electromagnetic free-forming process of AA5754 sheets and observed that the maximum strain at high rates was 35ā45%, whereas the maximum strain under quasi-static conditions ranged from 20% to 30%. Jin et al. [13] elaborated on the deformation behavior of AA2195-T6 sheets during EMF. The total effective strain was 46.3% under electromagnetic bulging, reflecting an increase of 189% relative to that (16%) under rigid-punch bulging.
There is currently a growing demand for high-precision miniaturized parts, components, structures and systems in various industries [14]. However, the limited capability of controlling the shape has become the main obstacle to the widespread use of the EMF method. Noh et al. [15] studied EMF of a closed die and demonstrated that the fitting between the deformed metallic sheet and the die was poor due to the bounce and high strain rate of the sheet metal. Wu et al. [16] proposed the inner-field uniform pressure actuator (UPA) and applied it to fabricate titanium bipolar plates. It provides an open reinforced environment for increasing the coil strength without affecting the forming efficiency. However, the maximum channel depth of titanium bipolar plate is 0.294Ā mm, which corresponds to 73.46% of the die depth (i.e. 0.4Ā mm). In order to improve the forming accuracy, electromagnetically assisted sheet metal stamping (EMAS) through a two-step method was proposed. During the EMAS process, quasi-static stamping was carried out first, and the local area of the material was then corrected using an electromagnetic force. Iriondo et al. [17] utilized the EMAS method for L-shaped parts and evaluated the influence of the discharge energy on the spring-back effect of 5754 aluminum alloy sheets. They managed to reduce the spring-back angle of the sheets and improve their forming accuracy. Cui et al. [18] utilized the finite element model to analyze the spring-back reduction mechanism of U-shaped parts using the EMAS method. Their results revealed that the residual stress in the bending zone decreased after the EMF process. In the two-step forming method, Kamal et al. [19] utilized the electromagnetic force to obtain a phone face. Su et al. [20] studied the electromagnetic flanging process of an inclined hole and found that the flanging parts obtained via a single EMF process had poor forming accuracy. Furthermore, they proposed a two-step flanging process to improve the forming accuracy of these parts. However, the main deformation in the EMAS process occurs under quasi-static conditions, while the two-step EMF method cannot easily be used to manufacture complex parts. Li et al. [21] proposed gradient electromagnetic forming method which utilizes the gradient electromagnetic force to improve tube forming accuracy. However, different parts require different coils. Cao et al. [22] proposed the Lorentz-force-driven sheet metal stamping. Due to the presence of steel punch, the forming accuracy of parts was improved. However, the stamping process requires matching convex and concave die structure for forming complex parts. If the part shape changes, the structure of convex and concave die must be changed. Compared with HF process, poor flexibility was in stamping. Moreover, steel punch has greater mass than water. To obtain same high strain rate of sheet, a higher discharge energy was required to drive steel punch.
Electrohydraulic forming (EHF) is another high-strain-rate forming process, in which the electrical energy stored in a pulsed power generator suddenly discharges between two electrodes immersed in a liquid. The rapid heating and expansion of the plasma channels between electrodes produce strong pressure waves in the water, causing a plastic deformation of the metallic sheets. Woo et al. [23] carried out free forming experiments on 6061 aluminum alloy. The obtained FLD showed that the material could have improved formability at EHF condition compared with the quasi-static condition. Woo et al. [24] carried out forming experiments and numerical simulations of complex closed dies using the EHF process and demonstrated that parts obtained via the EHF process have a high forming accuracy. However, there are still some limitations restricting the development of EHF. Golovashchenko et al. [25] pointed out that the electrode is gradually corroded and the distance between electrodes increases due to corrosion, leading to the repeated adjustment or replacement of the electrode. Bonnen et al. [26] found that particles are released in the fluid due to the electrode corrosion; these particles hit the forming part during the subsequent explosion and cause damage to the deformed part. Consequently, the fluid also needs to be replaced frequently. Therefore, the HF, EMF, and EHF processes all have some limitations.
Herein, the electromagnetic hydraulic forming (EMHF) process was proposed to overcome the aforementioned limitations. The deformation behavior of a 5052 aluminum alloy sheet during the HF and EMHF processes was systematically investigated. The deformation mechanism during the EMHF process was revealed via macroscopic and microscopic experiments as well as numerical simulations. The forming mechanism of the EMHF process was also revealed.
Section snippets
Experimental design of forming limit
Fig. 1(a) shows a schematic illustration of the forming process with a free forming die. The discharge coil has a helical structure, and the number of coil turns is 13. The distance between the turns is 1.5Ā mm, and the cross-section size of the wire is 3Ā ĆĀ 10Ā mm2. The inner diameter of the die is 80Ā mm, and the radius of the fillet is 5Ā mm. Water is used in the liquid chamber. The height of the liquid is 30Ā mm. The material of the driven plate is red copper. Seal rings are installed in the
Finite element model
The forming sheet was made of 5052 aluminum alloy and the initial sheet thickness was 0.5Ā mm. The density, modulus of elasticity, and Poisson's ratio of 5052 aluminum alloy are 2700Ā kg/m3, 70,000Ā MPa, and 0.33, respectively. The drive sheet was made of 1060-O aluminum alloy and the initial sheet thickness was 1.0Ā mm. The density, modulus of elasticity, and Poisson's ratio of 1060-O aluminum alloy are 2700Ā kg/m3, 69,000Ā MPa, and 0.33, respectively. The chemical composition of the materials used
Analysis and discussion of forming limit
Compared with HF, EMHF has the ability to significantly improve the forming limit of 5052 aluminum alloy sheet. In this section, the deformation mechanism is analyzed using the mechanical theory. For aluminum alloys, the uniaxial tensile deformation process can be divided into a stable deformation stage and a localized necking stage. For the weakest region (Section A), with the strain at the moment of necking () set as the limit, the deformation that occurs before the strain reaches is
Conclusions
A novel EMHF method was developed and investigated in this work. The deformation behavior of 5052 aluminum alloy in EMHF process was analyzed from experiment, simulation and mechanical theory. The main conclusions are as follows:
- (1)
Compared with the free forming results obtained via quasi-static HF, for case-1, the sheet forming height was increased by 30.2%, and the major and minor strains were increased by 62.9% and 64.3%, respectively. For case-2, the sheet forming height was increased by
Author statement
ZiQin Yan: Methodology, Investigation, Simulation, Experiments, Writing original draft. Ang Xiao: Collected data, Investigation, Microstructure analysis. Peng Zhao: Collected data, Experiments. Xiaohui Cui: Methodology, Investigation, Writing - review &editing. Hailiang Yu: Investigation. Yuhong Lin: Investigation.
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.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant Number: 51775563 and 51405173), Innovation Driven Program of Central South University (Grant number: 2019CX006), the Project of State Key Laboratory of High Performance Complex Manufacturing, Central South University (ZZYJKT2020-02).
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