Electromagnetically activated high-speed hydroforming process: A novel process to overcome the limitations of the electromagnetic forming process

G Model CIRPJ 524 No. of Pages 10

CIRP Journal of Manufacturing Science and Technology xxx (2019) xxx–xxx

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Electromagnetically activated high-speed hydroforming process: A novel process to overcome the limitations of the electromagnetic forming process Rasoul Jelokhani Niaraki, Ali Fazli* , Mahdi Soltanpour Advanced Forming Technologies and Materials Lab, Mechanical Engineering Department, Faculty of Engineering and Technology, Imam Khomeini International University, Qazvin, Iran

A R T I C L E I N F O

A B S T R A C T

Article history: Available online xxx

In this paper, a novel electromagnetically activated high-speed hydroforming process (EAHF) is introduced. The EAHF process is compared with the electromagnetic forming process (EMF) in the forming of a low-conductive steel sheet, forming of a high-conductive thin aluminum sheet and also, in the multistage forming of an aluminum sheet. Furthermore, the effect of the process parameters on the EAHF process is investigated by the Taguchi design of experiments. Based on the results it can be concluded that the presented process is an efficient high-speed forming process that can be used for metals and non-metals with low electrical conductivity. This process has more uniform pressure and hence more uniform geometry compared to the EMF process. Furthermore, by a proper design of the process, EAHF can be used for the simultaneous forming and trimming operation. © 2019 CIRP.

Keywords: Electromagnetically activated High-speed hydroforming Electromagnetic forming Low-conductive sheet forming Multistage forming Simultaneous forming and trimming

Introduction In order to increase safety and reduce the energy consumption in moving devices, the industries are seeking for the materials with a higher strength to weight ratio. The materials such as aluminum alloys and advanced high strength steels are suitable candidates to increase the strength to weight ratio of the components. However, such metals have low formability, and conventional forming processes have limitations for use in the forming of these high strength metals. The studies in recent decades show that highspeed forming methods can cause an increase in the sheet metal formability [1]. Therefore, such application and development of the high speed forming processes are being increased. It is reported [2] that Daniel Adamson in 1878 in the United Kingdom was the first one who applied the explosive to the forming processes. In this process, the shock wave initiated by the explosion of a charge hits the sheet metal and forms it. This process is versatile and requires low capital investment [2]. However, this process has some drawbacks. The explosive forming process cannot be automated. The process requires explosive storage and handling which can be unsafe and costly. In addition, the energy

* Corresponding author. E-mail address: [email protected] (A. Fazli).

produced from the explosion cannot be easily adjusted, and therefore, the forming may not have high precision. The other high-speed forming process is the electromagnetic forming (EMF) process. In this process, high-density electric current passes through a conductive coil by discharging a capacitor bank and produces a transient magnetic field that induces eddy currents in a nearby metal sheet. The mutually repulsive electromagnetic pressure resulting between the stationary coil and the metal sheet can deform or accelerate the latter [3]. The process has many advantages. This process applies the forming force without contact to the workpiece which enables the forming of semi-finished parts without destroying the surface layer. No lubricant is used, and the process is environmentally friendly. Furthermore, by adjusting the forming machine, high repeatability can be achieved [4]. However, the process suffers from some limitations. The electromagnetic forming requires the metal sheet to have high conductivity and cannot be applied to the nonconductive material. In order to form a non-metal sheet or a low conductive metal using EMF, a driver sheet with high conductivity should be used. However, the driver sheet cannot be reused easily. Moreover, upon the first step of the forming process, the formed part is repelled from the coil, and the increased distance between the formed part and the coil prevents an electromagnetic field to be produced within the formed part in further steps of the process. Therefore, EMF cannot be performed in multiple steps. In addition, the electric current induced in the sheet metal heats it, and hence,

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the electromagnetic forming process cannot be used for the forming of thin heat-sensitive sheets like an aluminum blister [5]. The electrohydraulic forming (EHF) is another high-speed forming process where the electrical energy stored in a pulse power generator is suddenly discharged between two electrodes submerged in a fluid. The rapid heating and expansion of the plasma channel between the electrodes create an intense pressure wave in the water, which then accelerates the blank toward the tool [6]. The advantages of the EHF is its precise control over the amount and rate of the released energy in comparison to the explosive forming and its capability for the multi-pass forming process in comparison to the EMF [6]. However, in the electrohydraulic forming process, the electrodes are gradually corroded, and the distance between electrodes increases due to corrosion. Therefore, the electrodes require repeated adjustment. The electrodes are consumed during the process, and after multiple uses, the electrodes should be replaced with new electrodes. Furthermore, the particles released in the fluid due to electrode corrosion can rapidly hit the formed part in the next explosions and cause scratching and damaging to the formed part [7]. In addition, the volume of the fluid is an important parameter in the forming process, and any change in the liquid volume may require a new die to be designed and made. Due to the limitations and drawbacks of the mentioned highspeed processes, in recent years development of the presented methods or introducing some novel processes are being considered to overcome their limitations. Ratjen and Takamatsu introduced another high-speed forming process named hydropunching [8]. In this process a compresed air system accelerates a plunger to a high velocity of about 30 m/s. The high velocity plunger hits the fluid and pushes it and the sheet metal toward the die cavity. Wielage et al. [9] proposed the laser shock forming for the micro deep drawing of thin foils. Despite the conventional laser forming processes, this process takes place through a nonthermal mechanism. In this process, the workpiece is supplied with a laser pulse of high power density. The temporary pressure on the surface initiates an elasto-plastic shock wave onto the workpiece, and in consequence, the shock wave pressure forms the worksheet. Wielage and Vollertsen [10] made the first attempt to classify the laser shock forming process. Based on their study, the strain rate is about 2
103 s1 which is in the order of other high speed forming processes like EMF. Beerwald et al. [5] presented a new method for generating a pressure pulse in the fluid by using an electromagnetically accelerated driver plate. They successfully applied this method to very thin metallic and hybrid materials used in pharmaceutical and food packaging industry. Meng et al. [11] combined the EMF and warm forming techniques to improve the formability of the magnesium alloy sheets. With the same discharging energy, increasing the temperature up to 150  C decreases the forming height while increasing the temperature above the 150  C increases the forming height. Liu et al. [12] applied a laser-driven flyer for the micro-embossing operation. In this process, a short and intense laser pulse is irradiated onto a flyer surface and instantaneously vaporizes a part of the flyer surface into a high-temperature and high-pressure plasma. This plasma absorbs the incident laser energy and acts as the working fluid to accelerate the remaining flyer. When the driver hits the workpiece, forms it to the micro-groove of the die. Li et al. [13] developed the space–time–controlled multi-stage pulsed magnetic field. This process consists of multiple coils where each coil can be addressed individually by its associated power supply with precise timing control. Using this strategy high magnetic flux density in the order of 40–80 T can be achieved. Cui et al. [14] presented the electromagnetic incremental forming process which can be used to produce large-scale parts with a small

working coil and small discharge energy. Lai et al. [15] developed a dual-coil system to overcome the limitations of the electromagnetic forming systems for deep drawing of sheet metal with large drawing ratio. This process has an additional coil that produces a radial inward force in the flange region to enhance the material flow of the flange. Guo et al. [16] presented the electromagnetic incremental forming (EMIF) process with coil shifting for forming panels. Experimental results indicate that the EMIF is feasible to form panels. However, the loading path, capacitance, discharge voltage, and position distance are the effective parameters of the process. Xiong et al. [17] proposed an axially movable electromagnetic forming system, in which the designed coil can be moved down along the axial direction to decrease the distance between the coil and deformed sheet in the multi-stage forming of large components. Since the size of the forming coil is limited by the cavity of the die, a relatively small but high-field coil was designed to generate an enough strong electromagnetic force. Due to the use of a small diameter coil, the coil radial distance with the formed part increases in the later forming stages and hence, the accuracy of the side wall of the formed part decreases. Therefore, in their other work, they [18] proposed a solenoid filed shaper to increase the applied electromagnetic load on the side wall of the formed part and improve the forming accuracy of the workpiece. Lai et al. [19] introduced a pulsed electromagnetic blankholder system which could be applied in the electromagnetic forming process. They used one coil system for generating the forming force and another coil system which consist of an upper coil and a lower coil, for supplying a pulsed blankholder force (BHF). After discharging the capacitor in the blankholder coils, a high pulsed attractive electromagnetic force is generated between the upper and lower coils which is transmitted to the flange region of the metal sheet as the blank-holding pressure through the blankholder. The discharge of the forming coil would be started when the BHF nearly reaches its peak value. Cui et al. [20] combined the electromagnetic incremental forming with the stretch forming process to produce large-scale components. The coil takes the place of the rigid tool of the conventional two-point incremental forming process and is moved along a 3D trajectory and is discharged at different positions to deform the sheet locally. Iriondo et al. [21] applied the electromagnetic forming process for the shape calibration and correction of the springback. They presented this process to correct the sidewall curl of previously deep drawn high strength steel specimens. Xiong et al. [22] made an in-depth study to determine the causes of geometrical deviation during the EMF process and possible improvement methods. The effective parameters on the generation and evolution of the morphology of work-piece were determined to be the electromagnetic force distribution, the update of the force, the impact of the sheet with the die and the duration of the coil current pulse. Long et al. [23] proposed a novel technology named electromagnetic superposed forming for forming large scale and small-curvature sheets. The electromagnetic force is applied on a sheet that is placed above a punch matrix. Positioning the coil and punch matrix along the sheet surface incrementally, a uniformly distributed deformation matrix is generated. Luo et al. [24] proposed a novel multi-layer coil that can generate a big skin depth on the workpiece and increase the energy efficiency of the EMF process. This type of coil is suitable for the electromagnetic forming of a large and thick-walled component. Choi et al. [25] combined the deep drawing and electromagnetic sharp edge forming to improve the formability of advanced high strength steel sheets. The deep drawing process is performed as the preform stage to produce a box with a large corner radius, and then the electromagnetic sharp edge forming is employed to sharpen the corner radius. Hajializadeh and Mashhadi [26] investigated

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the electrohydraulic forming of a tube by numerical simulation. The results indicated that the discharge energy has the major effect in the electrohydraulic forming process. Zia et al. [27] introduced the warm electrohydraulic forming process. In their novel method, the blank is heated, and then the warm blank is formed using the electrohydraulic forming process. In order to prevent blank cooling due to the contact with water, an air gap is prepared between the blank and the water in the chamber. With the same discharging energy, in the blank temperatures of 110  C, the forming height was higher than the conventional electrohydraulic forming process. The EMF process has limitations in the forming low conductive sheet metals and multistage forming process. Also the pressure distribution in the EMF is non-uniform. The EHF process also have some limitations like, electrode corrosion and necessity of the electrode adjustment after several shots [7]. Also corroded particles of the electrodes in EHF impact the blank, degrading the final part surface quality [28]. Therefore the introduction of a new process without these drawbacks is of interest. In the present paper, an electromagnetically-activated high-speed hydroforming process (EAHF) is introduced which can overcome to most of the mentioned limitations. The presented method is compared with the electromagnetic forming in some aspects. Also, the process parameters of the presented method are investigated. Materials and experiments Experimental setup The schematic of the electromagnetically-activated high-speed hydroforming process is shown in Fig. 1(a). As shown in this figure, the energy stored in a pulse power generator discharges into the flat coil and creates an electromagnetic field in the flat coil; this time-varying electromagnetic field creates an opposite electromagnetic field in the driver which is a high conductive plate and is placed on top of the coil. The repelling force between the two electromagnetic fields moves the driver upward suddenly. The high-speed movement of the driver is transferred via the piston to the fluid stored in the container and initiates a shock wave within the fluid. The produced shock wave is transferred to the sheet and forms it to the shape of the die. It should be noted that the die cavity is vacuumed by connecting its vacuum port to a vacuum pump. The prepared experimental setup is shown in Fig. 1(b) which is placed on top of the flat coil connected to the pulse power generator.

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Table 1 The dimensions of the electromagnetic coil used in the experiments. Coil material Coil inside dia. Coil outside dia. Number of turns Coil cross section dimensions

Copper alloy 10 mm 120 mm 15 4 mm
15 mm

The pulse power generator used in this study had a maximum energy capacity of 8 kJ with arred via the piston to the fluid stored in the container and initiates a shock wave within the fluid. The produced shock wave is transferred to the sheet and forms it to the shape of the die. It should be noted that the die cavity is vacuumed by connecting its vacuum port to a vacuum pump. The prepared experimental setup is shown in Fig. 1(b) which is placed on top of the flat coil connected to the pulse power generator. The pulse power generator used in this study had a maximum energy capacity of 8 kJ with a total capacitance of 250 mF. The frequency of the discharged current in the coil is 1.95 kHz. The dimension of the used electromagnetic coil is shown in Table 1. Experimental plan for the comparison of EAHF and EMF The EMF process has limitations in the forming of low conductive sheet metals and multistage forming process. Also the pressure distribution in the EMF is non-uniform. The EHF process also hve some limitations like, electrode corrosion and necessity of the electrodes adjustment after several shots [7]. Also corroded particles of the electrodes in EHF impact the blank, degrading the final part surface quality [28]. However, EAHF process has overcome to most of these limitations. In order to show the capability of the presented method, it is compared to the EMF process rather than EMF. The reason is that the proposed method uses the same power source of the EMF process which is a flat coil. Therefore to eliminate the effect of the power source parameters on the comparison, the EAHF process is compared with EMF. While in comparison with EHF process, the EHF itself has lots of process parameters, like, the electrical discharge energy, the electrodes gap, stand-off distance, the electrodes diameter, the chamber shape and dimensions [29]. These parameters can affect on the efficiency of the EHF. Then comparison of the EAHF process with the EHF process is more complex and makes the judgment of the results more difficult.

Fig. 1. (a) Schematic of the electromagnetically activated hydroforming process; (b) the setup prepared for the experiments.

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approach is used for the design of experiments (DoE). Process parameters considered in the parametric study of the EAHF process are:

In this section the EAHF process is compared with the electromagnetic forming process. In both processes, the same electromagnetic coil and the same discharge energy is used. Therefore the applied energy of both processes are the same, and the achieved deformation could be a measure of the efficiency of the present method. The comparison is performed in three aspects:

    

 Forming of low-conductive steel sheet: For the first comparison, the EAHF and EMF are compared in the forming of a low conductive 0.5 mm thickness steel sheet. Due to the low conductivity of the steel sheet, in the EMF process, a 0.6 mm thickness aluminum sheet is used as the driver, while the driver is not necessary for the EAHF process.  Forming of high conductive aluminum sheet: in this comparison, the EAHF and EMF forming of a thin 0.3 mm thickness aluminum sheet are compared to each other.  Multistage forming of the aluminum sheet: in this set of experiments, two-stage forming of a 0.3 mm thickness aluminum sheet is performed using EAHF and EMF and compared with each other.

The The The The The

discharge energy viscosity of the fluid fluid volume in the pressure chamber trapped air volume in the pressure chamber driver material

The levels considered for each parameter is shown in Table 3. It can be seen that two levels are considered for the driver materials, and four levels are considered for the other investigated parameters. The material used in these tests is the 0.6 mm thickness aluminum sheets, shown in Table 2. Taguchi’s design of experiments suggests 16 experimental conditions based on the considered process parameters and the levels of each parameter. The experimental conditions for each test are listed in Table 4. The experiments are performed, and the results are statistically analyzed using the Minitab 17 software.

The material properties of the steel AISI 1008 and aluminum AA1100 sheets used in these experiments are shown in Table 2. The driver plate used in this tests is aluminum, the pressure chamber is filled with 150cc water.

Results and discussion Forming of low conductive steel sheet For the first comparison, the EAHF and EMF are compared in the forming of a low conductive 0.5 mm thickness steel sheet. Due to the low conductivity of the steel sheet, in the EMF process, a 0.6 mm thickness aluminum sheet is used as the driver, while no

Design of experiments for the parameter study of the EAHF In order to study the effect of the process parameters and their importance on this new high-speed forming process, the Taguchi Table 2 The material properties of sheet metals used in the experiments. Material

Thickness (mm)

Yield stress (MPa)

Ultimate strength (MPa)

Elongation

AA1100 AA1100 AISI 1008

0.3 0.6 0.5

43 45 258

112 88 375

0.28 0.24 0.23

Table 3 The parameters and their level used in the parameter study using Taguchi approach. Parameter

Level 1

Level 2

Level 3

Level 4

Discharge energy (kJ); Kinematic viscosity of the fluid at 40  C, mm2/s (fluid type) Fluid volume in the pressure chamber (cc) Trapped air in the pressure chamber (cc) Driver material

2.5 0.8 (water) 50 0 Aluminum

4 41 (SAE 10W) 83 17 Copper

5.5 148 (SAE 40) 117 33 –

7 460 (Grease) 150 50 –

Table 4 The design of experiments based on the Taguchi approach. Experiment no.

Discharge energy

Fluid type

Fluid volume (cc)

Trapped air volume (cc)

Driver material

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

2.5 2.5 2.5 2.5 4 4 4 4 5.5 5.5 5.5 5.5 7 7 7 7

Water SAE 10W SAE 40 Grease Water SAE 10W SAE 40 Grease Water SAE 10W SAE 40 Grease Water SAE 10W SAE 40 Grease

50 83 117 150 83 50 150 117 117 150 50 83 150 117 83 50

0 17 33 50 33 50 0 17 50 33 17 0 17 0 50 33

Aluminum Aluminum Copper Copper Copper Copper Aluminum Aluminum Aluminum Aluminum Copper Copper Copper Copper Aluminum Aluminum

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driver is necessary for the EAHF process. The test is performed in two different discharge energies of 2.65 kJ and 6.65 kJ, and the results are shown in Figs. 2 and 3. As shown in these figures, the parts formed using the EAHF process has much smoother and uniform surface than the parts formed using EMF. This is due to the uniform pressure applied to the blank in the EAHF process, while the pressure distribution in the conventional EMF process is non-uniform [30]. The non-uniform pressure distribution in the EMF, causes a non-uniform velocity distribution of the sheet metal which results into a non-uniform surface of the formed sheet. Risch et al. [31] showed that in the EMF process due to the not-uniform velocity distributions, the puckers even may form in the closed die forming process, while the uniform pressure and velocity distribution in the EAHF process eliminates the puckers from the formed sheet. Also the profiles of the formed parts are determined using a milling machine and a dial indicator and compared to each other in the Fig.4(a) and (b). The formed part is fastened to the table of the milling machine and the dial indicator is fastened to the tool head of the milling machine. The dial indicator is moved on a diametric direction of the formed part such that the dial indicator show the same value in locations with 5 mm distance to each other. The positions of the dial indicator in these locations are recorded and drawn in CATIA software. The obtained profile is offset equal to the radius of the spherical head

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of the dial indicator and the profile of the formed part is determined. It can be seen that the parts formed by EAHF were 13% and 38% higher than the parts formed using EMF process in forming energies of 2.65 kJ and 6.65 kJ respectively. So it may be concluded the current introduced process not only eliminates the need to driver sheet in forming of non-conductive sheets, but also its process efficiency was considerably higher that EMF with a driver. This is due to the fact that in the EMF process using a driver, part of the energy is consumed to form the driver sheet. In the EAHF process, although part of the energy is initially consumed to accelerate the driver and piston, it is recovered during deceleration of the piston at the end of the process when its speed reaches zero again. While in the EMF process, the part of the energy is used to form the driver sheet which is not recoverable. Therefore, the efficiency of the EAHF is higher than the EMF. It should be noted that this conclusion is based on the driver thickness of 0.6 mm. However, in the EMF process, using a driver sheet with the optimum thickness, the higher forming height can be achieved. The optimum driver thickness in the EMF process, depends on the workpiece thickness, workpiece conductivity, the yield stress and the conductivity of the driver material [32]. The optimum driver thickness for an aluminum driver is about the skin depth. While in the above experiments, the driver thickness was about one-third of the skin depth.

Fig. 2. The shape comparison of the forming of a 0.5 mm steel sheets by 2.65 kJ forming energy. (a) Formed by the EAHF (b) formed by EMF using a 0.6 mm aluminum driver sheet.

Fig. 3. The shape comparison of the forming of a 0.5 mm steel sheets by 6.65 kJ forming energy. (a) Formed by EAHF. (b) Formed by EMF using a 0.6 mm aluminum driver sheet.

Fig. 4. Comparison of the profile of the 0.5 mm thickness steel sheet formed using EMF with a driver and EAHF. (a) Forming energy of 6.65 kJ, (b) forming energy of 2.65 kJ.

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Therefore, it is possible to obtain higher forming height using thicker driver sheet. However, since the driver sheet deforms itself in the forming process, increasing its thickness results in the higher waste material which increases the production costs. Forming of thin high conductive aluminum sheet In another comparison, the EAHF and EMF forming of a thin 0.3 mm thickness aluminum sheet are compared to each other as shown in Fig. 5. Both parts formed by discharging the same energy of 4 kJ into the coil. Again the sheets formed using the EAHF process has a smoother surface than the sheets formed using EMF due to the uniform pressure distribution in the EAHF process compared with the EMF process. Also, the profiles of the formed parts are compared to each other in Fig. 6. It can be seen that the height of the part formed by EAHF was 41% higher than the part formed using EMF process. So it can be concluded that in the forming of thin conductive aluminum sheets, the efficiency of the EAHF processes is considerably higher than the EMF process. This is due to the skin depth effect. In the EMF process, the depth of the penetration of the current into the workpiece is called skin depth, (d) which mainly depends on the specific electrical conductivity of the workpiece (k), and the frequency (f) of the closed electrical circuit [4]: 1

d ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pf km0 mr

ð1Þ

where m0 represents the magnetic permeability in a vacuum and mr is the relative permeability of the material which the eddy currents is induced in it. The EMF test conditions are shown in Table 5.It can be seen that the skin depth for the test conditions was 1.87 mm which is about six times the sheet thickness. In EMF when the skin depth is greater than the thickness of the

sheet, part of the magnetic fields leaks through the workpiece, thus the efficiency of the forming process reduces [33]. Therefore in order to increase the efficiency of the EMF process in the forming of thin conductive sheet metals, as can be seen in Eq. (1), in order to decrease the skin depth, it is necessary to use a very high discharge frequency equipment [5] which are more expensive. However, in the EAHF process due to the large thickness of the driver (Fig. 1), there is no leakage of the magnetic fields even using equipment with medium and low discharge frequencies, which increases the efficiency of the process in forming of thin metal sheets. Therefore, the higher thickness of the driver makes it possible to use even the equipment with lower frequencies in forming low thickness sheet metals. The other reason in the higher efficiency of the EAHF compared with the EMF could be the difference in the diameter of the die (80 mm) and the forming coil (120 mm) (Fig. 1). Due to this difference, in the EMF process, the Lorenz force applied to the flange region of the formed part is not used to form the part. However, in the EAHF process the diameter of the driver is equal to the forming coil and all the Lorenz force is applied to the driver and transferred to the forming area of the blank through the piston and the fluid. Hence, although the same forming coil and the same discharge energy is used in both the EMF and EAHF, the forming height of the part in the EAHF is higher than the EMF process. It could be concluded that the EAHF tooling acts as a concentrator and transmits the discharged energy to the forming area which increase its efficiency compared with EMF. On the other hand, in the EMF process the eddy current induces in the sheet metal itself. In the forming of polymer-aluminum foils, the induced current causes a strong heating of the aluminum layer and melting of the polymer layer. Therefore the conventional EMF process cannot be used in the forming of the hybrid thin polymer-

Fig. 5. The shape comparison of the forming of a 0.3 mm aluminum sheets by 4 kJ forming energy. (a) Formed by the electromagnetically activated hydroforming (b). Formed by electromagnetic forming.

Fig. 6. Comparison of the profile of the 0.3 mm thickness aluminum sheet formed using EAHF and EMF.

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m0 (H/m) 4p
10

7

mr 1.000022

k(Siemens/m) 6

36,9
10 (for aluminum)

dðmmÞ 1.87

aluminum foils [5]. However, in the EAHF process since the induced current acts on the driver sheet not the forming sheet, this problem can also be solved. Multistage forming of the aluminum sheet In the third comparison, the two-stage forming of a 0.3 mm thickness aluminum sheet is performed using EAHF and EMF and compared with each other. For both tests, the first and second stages are performed using the forming energy of 2.65 kJ and 6.65 kJ respectively. The test results are shown in Fig. 7 and the profile of the formed cups are shown in Fig. 8. It can be seen that in the first forming stage, EAHF formed the sheet slightly higher than EMF, while in the second stage, the increase of the forming height in the EAHF is such that the sheet is completely filled the die cavity,

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while the increase of the forming height in the EMF is negligible. The main drawback of the EMF is that when the sheet metal forms in the first forming step, it became far from the coil and it is not possible to continue its forming in next steps with the same coil. The reason can be described using the electromagnetic equations. The force acting on a sheet metal workpiece in axial direction can be calculated by Eq. (2) [4]: F z ðr;  zÞ ¼ mHðr;  zÞ

@Hðr;  zÞ @z

ð2Þ

where F z is the applied load in different locations in z direction,  m is the permeability and H is the magnetic field strength. The magnetic field strength and its derivative in a point have an inverse relation to its distance from the coil. Therefore when the sheet is located in farther locations than the coil (which is inevitable in forming a formed sheet using a flat coil), the applied load is negligible and the forming is not possible. Therefore in the multistage forming using the EMF a special coil with the inside shape of the part is needed to decrease the distance of the coil and the sheet metal. However, in the EAHF, due to the fact that the high conductive driver is not formed, it is easily positioned on the flat coil again, and the distance is removed; then, the next forming steps of the metal sheet are applied to it successfully. Therefore the

Fig. 7. Comparison of two stage forming of a 0.3 mm aluminum sheet using EMF and EAHF. (a) EAHF first stage, (b) EMF first stage, (c) EAHF second stage, (d) EMF second stage.

Fig. 8. Comparison of the profile of the 0.3 mm thickness aluminum sheet formed using EAHF and EMF in two stages. (a) EAHF, (b) EMF.

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R. Jelokhani Niaraki et al. / NULL xxx (2019) xxx–xxx Table 6 The formed height of the components in various experiments. Experiment number

Forming height

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

9.98 9.09 9.38 8.97 9.98 12.45 15.58 14.71 18.96 19.50 15.66 22.85 27.13 28.58 25.83 17.86

introduced EAHF process can be easily applied in the multi-step forming of a metal sheet. Investigation of the effect of process parameters In order to study the effect of the process parameters, the experiments are performed based on the Taguchi design of experiments reported in Table 4 and the forming height of the

components are determined which is reported in Table 6 for each test. Experiments failed in their perimeter in the test number 11 and 16 which are shown in Fig. 9. Investigating the conditions for these two test reveals that the chamber volume (fluid volume + trapped air volume) is low. Then due to high discharged energy in these conditions (5.5 kJ and 7 kJ respectively), the piston reached the blanks and trimmed it in its perimeter. Therefore, the piston could not move more in the chamber toward the sheet metal, and the forming is limited. Furthermore, it could be explained that part of the energy is used for trimming the formed part which could result in a decrease in the height of the formed parts. This could increase the error on the determination of the effective process parameters. By the way, these experiments itself be a suggestion for a hybrid design of the forming and trimming operations. By the appropriate design of the chamber volume and discharged energy, the forming and trimming can be performed in the last forming stage in a single setup. It should be noted that in experiment 16, an unwanted deformation can be seen in the conical wall. The trapped air in this experiment was higher (50cc) which is compressed during the forming operation. When the electromagnetic force is dissipated, the compressed air is going to expand and push the fluid and punch down. In this phase, it may enforce the fluid to go to the outer surface of the formed component through the trimming line, which results in the observed unwanted deformation. The influence of the process parameters on the formed height of the components are shown in Fig. 10. According to this figure, the

Fig. 9. Components failed in their perimeter in the Taguchi design of experiments. (a) Experiment 11, (b) experiment 16.

Fig. 10. Influence of process parameters on the forming height of the components, (a) discharge energy (kJ), (b) fluid type, (c) fluid volume, (d) trapped air volume, (f) driver material.

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is larger than the aluminum driver. However, the density of the copper is 3.33 times of the aluminum. Hence, the increased electromagnetic force is unaffected by the increased accelerating mass. Conclusions A novel electromagnetically activated high-speed hydroforming process (EAHF) is introduced and compared with the electromagnetic forming process (EMF). Also, Taguchi design of experiments is performed to investigate the effective parameters of the process. Based on the test results, it can be concluded that:

Fig. 11. Significant factors in the EAHF process.

most effective process parameters on the forming height of the component is the discharge energy, and the effect of the other process parameters are negligible. Also, Fig. 11 show that the effects of the other parameters are less than the determined error of the experiments. This is an interesting result which indicates that the process affecting parameters are less than what it thought. As is expected, increasing the discharge energy increases the transferred energy to the sheet metal; Then, the more the discharge energy, the more the forming height. The trapped air volume is the second effective parameter on the forming height of the component. When there is trapped air in the chamber, the shock wave should be transferred throw the air medium. The trapped air acts as a damper and being compressed, reduces the peak load and increases its duration. The reduction of the peak load reduces the impact load and decreases the forming height of the sheet metal. Therefore, increasing the trapped air in the chamber decreases the efficiency of the process and decreases the forming height. However, in the experiments, increasing the trapped air up to 33cc decreases the forming height while increasing the trapped air to 50cc, increased the forming height in comparison to 33cc. As mentioned above, two components having trapped air has ruptured in their perimeter which resulted in the lower forming height of these components and error in the determination of the effect of the trapped air. Since the volume of the fluid has a considerable effect on the other high speed forming process like electro-hydraulic forming [34], the volume of the fluid is the other investigation parameter to see if it has any effect on this novel process or not. The results indicate that the fluid volume does not have any effect on the forming height, except in the minimum tested fluid volume of 50cc. When the forming volume is 50cc, the drawing height is less than the other conditions due to the trimming effect (experiments 11 and 16) discussed above. Therefore, it could generally be said that the fluid volume does not have any effect on the process and hence, in the determination of the fluid volume, just the required result (“forming” or “forming + trimming”) should be considered. Increasing the viscosity of the fluid increases its resistance against accelerating. However, the shock propagation speed in the viscose material is higher. These two different effects in the EAHF process frustrate each other, and hence, the viscosity of the fluid has no considerable effect on the forming height of the component. For the investigation of the effect of driver material on the forming height, it should be noted that the conductivity of the copper is 63% larger than the aluminum driver. Therefore, the applied load on the copper driver

 The sheets formed using the EAHF process has a smoother surface than the sheet formed using the EMF.  The EAHF eliminates the need for the consumable driver sheet in the forming of low-conductive and non-conductive sheets.  The efficiency of the EAHF was considerably higher than the EMF in the forming of non-conductive sheets and thin high conductive sheet.  The EAHF can be successfully applied for the multistage forming of sheets while the EMF has some limitations and difficulties (like needing a special coil) in multistage forming process.  The most effective parameter on the forming height of the component in the EAHF process is the discharge energy, while the fluid viscosity and volume, the trapped air volume and the driver material have no considerable effect on the forming height.  By appropriate design of the EAHF process, the process can be used for simultaneous forming and trimming operations. In this method, the chamber volume is equal to the volume of the final part. Then after deforming the sheet metal, the piston acts as the punch and trims the formed part. Conflict of interest The authors declare that they have no conflict of interest. References [1] Balanethiram, V.S., Daehn, G.S., 1994, Hyperplasticity: Increased Forming Limits at High Workpiece Velocity. Scripta Metallurgica et Materialia, 30/4: 515–520. [2] Mynors, D.J., Zhang, B., 2002, Applications and Capabilities of Explosive Forming. Journal of Materials Processing Technology, 125–126/March: 1–25. [3] Kim, D., Il Park, H., Lee, J., Kim, J.H., Lee, M.G., Lee, Y., 2015, Experimental Study on Forming Behavior of High-strength Steel Sheets under Electromagnetic Pressure. Proceedings of the Institution of Mechanical Engineers Part B: Journal of Engineering Manufacture, 229/4: 670–681. [4] Psyk, V., Risch, D., Kinsey, B.L., Tekkaya, A.E., Kleiner, M., 2011, Electromagnetic Forming — A Review. Journal of Materials Processing Technology, 211:787–829. [5] Beerwald, C., Beerwald, M., Dirksen, U., Henselek, A., 2010, Impulse Hydroforming Method for Very Thin Sheets from Metallic or Hybrid Materials. 4th International Conference on High Speed Forming, 150–158. [6] Gillard, A.J., Golovashchenko, S.F., Mamutov, A.V., 2013, Effect of Quasi-static Prestrain on the Formability of Dual Phase Steels in Electrohydraulic Forming. Journal of Manufacturing Processes, 15/2: 201–218. [7] Bonnen, J.J.F., Golovashchenko, S.F., Dawson, S.A., Mamutov, A.V., 2013, Electrode Erosion Observed in Electrohydraulic Discharges Used in Pulsed Sheet Metal Forming. Journal of Materials Engineering and Performance, 22/12: 3946–3958. [8] Ratjen, R., Takamatsu, M., 1971, Hochenergie-Umformverfahren Hydropunch. Werkstatt und Betrieb, 104/1: 41–45. [9] Wielage, H., Niehoff, H.S., Vollertsen, F., Niehoff, S., 2008, Forming Behaviour in Laser Shock Drawing. 3rd International Conference on High Speed Forming, 213–222. [10] Wielage, H., Vollertsen, F., 2011, Classification of Laser Shock Forming Within the Field of High Speed Forming Processes. Journal of Materials Processing Technology, 211/5: 953–957. [11] Meng, Z., Huang, S., Hu, J., Huang, W., Xia, Z., 2011, Effects of Process Parameters on Warm and Electromagnetic Hybrid Forming of Magnesium Alloy Sheets. Journal of Materials Processing Technology, 211/5: 863–867. [12] Liu, H., Shen, Z., Wang, X., Li, P., Hu, Y., Gu, C., 2012, Feasibility Investigations on a Novel Micro-embossing Using Laser-driven Flyer. Optics & Laser Technology, 44/6: 1987–1991.

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[13] Li, L., Han, X., Qiu, L., Zhou, Z., Xiong, Q., 2012, Space-Time-Controlled MultiStage Pulsed Magnetic Field Forming and Manufacturing. 5th International Conference on High Speed Forming, 53–58. [14] Cui, X.H., et al, 2014, Electromagnetic Incremental Forming (EMIF): A Novel Aluminum Alloy Sheet and Tube Forming Technology. Journal of Materials Processing Technology, 214/2: 409–427. [15] Lai, Z., et al, 2015, Radial Lorentz Force Augmented Deep Drawing for Large Drawing Ratio Using a Novel Dual-coil Electromagnetic Forming System. Journal of Materials Processing Technology, 222:13–20. [16] Guo, K., Lei, X., Zhan, M., Tan, J., 2017, Electromagnetic Incremental Forming of Integral Panel under Different Discharge Conditions. Journal of Manufacturing Processes, 1–10. [17] Xiong, Q., et al, 2016, Axially Movable Electromagnetic Forming System for Large-Scale Metallic Sheet. IEEE Transactions on Applied Superconductivity, 26/4: 1–4. [18] Xiong, Q., Huang, H., Deng, C., Li, L., Qiu, L., Tang, H., 2018, A Method to Improve Forming Accuracy in Electromagnetic Forming of Sheet Metal, 57. p. 367–375. [19] Lai, Z., et al, 2016, Design, Implementation, and Testing of a Pulsed Electromagnetic Blank Holder System. IEEE Transactions on Applied Superconductivity, 26/4. [20] Cui, X., Mo, J., Li, J., Xiao, X., Zhou, B., Fang, J., 2016, Large-scale Sheet Deformation Process by Electromagnetic Incremental Forming Combined with Stretch Forming. Journal of Materials Processing Technology, 237:139–154. [21] Iriondo, E., Alcaraz, J.L., Daehn, G.S., Gutiérrez, M.A., Jimbert, P., 2013, Shape Calibration of High Strength Metal Sheets by Electromagnetic Forming. Journal of Manufacturing Processes, 15/2: 183–193. [22] Xiong, W., Wang, W., Wan, M., Li, X., 2015, Geometric Issues in V-bending Electromagnetic Forming Process of 2024-T3 Aluminum Alloy. Journal of Manufacturing Processes, 19:171–182. [23] Long, A., et al, 2018, Forming Methodology and Mechanism of a Novel Sheet Metal Forming Technology — Electromagnetic Superposed Forming (EMSF). International Journal of Solids and Structures, 151:165–180.

[24] Luo, W., Huang, L., Li, J., Liu, X., Wang, Z., 2014, A Novel Multi-layer Coil for a Large and Thick-walled Component by Electromagnetic Forming. Journal of Materials Processing Technology, 214/11: 2811–2819. [25] Choi, M.K., Huh, H., Park, N., 2017, Process Design of Combined Deep Drawing and Electromagnetic Sharp Edge Forming of DP980 Steel Sheet. Journal of Materials Processing Technology, 244:331–343. [26] Hajializadeh, F., Mashhadi, M.M., 2015, Investigation and Numerical Analysis of Impulsive Hydroforming of Aluminum 6061-T6 Tube. Journal of Manufacturing Processes, 20:257–273. [27] Zia, M., Fazli, A., Soltanpour, M., 2017, Warm Electrohydraulic Forming: A Novel High Speed Forming Process. Procedia Engineering, . 207. [28] Golovashchenko, S.F., 2008, Electrohydraulic Forming of Near-net Shape Automotive Panels Executive Summary.Ford Motor Company, Dearborn, Michigan . [29] Zohoor, M., Mousavi, S.M., 2018, Evaluation and Optimization of Effective Parameters in Electrohydraulic Forming Process. Journal of the Brazilian Society of Mechanical Sciences and Engineering, 40/11. [30] Kamal, M., Daehn, G.S., 2007, A Uniform Pressure Electromagnetic Actuator for Forming Flat Sheets. Journal of Manufacturing Science and Engineering, 129/2: 369. [31] Risch, D., Beerwald, C., Brosius, A., Kleiner, M., 2004, On the Significance of the Die Design for Electromagnetic Sheet Metal Forming. 1st International Conference on High Speed Forming, 191–200. [32] Gies, S., Weddeling, C., Tekkaya, A.E., 2014, Experimental Investigations on the Optimum Driver Configuration for Electromagnetic Sheet Metal. 6th International Conference on High Speed Forming, 315–324. [33] Zhangaand, H., 1995, Effects of Various Working Conditions on Tube Bulging by Electromagnetic Forming. Journal of Materials Processing Technology, 48:113–121. [34] Mamutov, A.V., Golovashchenko, S.F., Mamutov, V.S., Bonnen, J.J.F., 2015, Modeling of Electrohydraulic Forming of Sheet Metal Parts. Journal of Materials Processing Technology, 219:84–100.

Please cite this article in press as: R. Jelokhani Niaraki, et al., Electromagnetically activated high-speed hydroforming process: A novel process to overcome the limitations of the electromagnetic forming process, NULL (2019), https://doi.org/10.1016/j.cirpj.2019.09.002