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Journal of Materials Processing Technology 214 (2014) 865– 875

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

jou rn al hom epage: www.elsev ier .com/ locate / jmatprotec

aporizing foil actuator used for impulse forming and embossing ofitanium and aluminum alloys

. Vivek ∗, R.C. Brune, S.R. Hansen, G.S. Daehnepartment of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210, USA

r t i c l e i n f o

rticle history:eceived 6 August 2013eceived in revised form 4 November 2013ccepted 7 December 2013vailable online 15 December 2013

eywords:mbossingmpulse formingrethane padaporizing foil actuator (VFA)lecrohydraulic forming (EHF)itaniumluminum

a b s t r a c t

Electrically driven rapid vaporization of thin conductors is known to produce short-duration pressurepulses of high magnitude. This impulse can be used for applications such as high strain rate forming, shear-ing, collision welding, and springback calibration. Mechanical impulse was developed from aluminumfoils of various thicknesses, which were vaporized using a capacitor bank discharge with a maximumcharging voltage of 8.6 kV. Peak current was delivered on the order of 100 kA with rise times of about12 �s. In this work, polyurethane was used as a medium to transfer pressure from the aluminum foilvaporization zone to the workpiece. Fundamental experiments, where AA 3003-H14 aluminum alloywas formed into perforated plates, show that for a given foil thickness, a limit existed over which sup-plying higher electrical energy from a given capacitor bank did not necessarily result in higher pressure.The magnitude of generated pressure was proportional to the excess Joule heat deposited into the foilbefore it burst. Although the polyurethane layer helped spread the pressure pulse over a larger area,the resulting pressure distribution remained heterogeneous. Practical applications, such as forming intocavities and embossing into shallow dies, were possible with this method. Sheets of 0.508 mm thick com-

mercially pure titanium were nearly fully formed into a cellphone case die using a hybrid process thatcombined a quasistatic pre-forming step with a vaporizing foil forming step. Sheets of 0.508 mm thick AA2024-T3 aluminum alloy were embossed into a die with features of varying depths. Aluminum foils withstraight and curved active sections were used as actuators. The curved-section foils resulted in higherconformation of the workpiece to the die in the center region, while the straight-section foils producedbetter conformity to the die features on the ends.

. Introduction

Passage of an electrical current of large magnitude through ahin conductor can cause its rapid vaporization. The rapid expan-ion of the gases creates a high-pressure pulse. Generally theischarge source is a capacitor bank and the conductor is in theorm of a thin wire or foil. “Exploding” conductors have beentudied before, but the emphasis has mainly been on understand-ng the phenomenon itself or its application in shock physics andxplosive detonation. Several studies were performed that utilizedapid vaporization of metal foils to achieve high velocities in flyerlates. Keller and Penning (1962) achieved 4–5 km/s velocities inhin dielectric flyers that were impacted with target plates. Thempact results revealed shock properties of the dielectric and the

arget plates between 1 and 10 GPa. Stroud (1976) later produced00 GPa pressures with an improved method and used the resultinghock pressure as a detonation source for explosives. Even greater

∗ Corresponding author. Tel.: +1 608 332 4892.E-mail addresses: vivek.4@osu.edu, vivekanupam@gmail.com (A. Vivek).

924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jmatprotec.2013.12.003

© 2013 Elsevier B.V. All rights reserved.

pressures, upward of 500 GPa, were then reached by Chau et al.(1980) through use of a thin metal layer attached to the dielectricplate. This improved results in two key ways. Additional driv-ing force was created due to the momentary repulsion betweenthe magnetic field of the foil and the conductive metal workpiecelayer, which increased with increasing applied electrical current.The material properties of the layer itself were also important,because although it resulted in increased mass, it offered greatershock impedance, resulting in overall higher impact pressures.These improvements produced parallel flyer motion for the first5 mm of travel and resulted in velocities near 20 km/s. This methodwas used to characterize shock detonation of explosive materials,but much greater range than before was possible, as detonationpressures up to 28 GPa were attained. Other applications, such asquick action fuses, discharge pulse sharpening, and nanopowdermanufacturing, have also been explored in the past.

Use of rapid metal vaporization in the metal working industry

has been limited. Electrohydraulic forming (EHF) is implementedby underwater electrical detonation of bridgewires or, more com-monly, by high voltage dielectric breakdown across spark gaps. Asearly as 1961, electrohydraulic forming was researched for use in

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66 A. Vivek et al. / Journal of Materials Pr

etal deformation, as Felts (1961) first demonstrated how pressurempulses large enough to form sheet metal could be produced withhis method. The underwater current discharge was successfullysed to form metal objects into parts of relatively small struc-ural mass. Woetzel et al. (2006) demonstrated that experimentshich used aluminum wires as a chemically reactive detonation

ource produced comparable deformation to those initiated withecondary explosives, such as PETN. Daehn (2006) commented thathis forming method has so far been difficult to commercialize, buthat some organizations still successfully use this type of processor formation of small batches of parts. Golovashchenko (2010a)emonstrated the use of pulsed electrohydraulic discharges for cal-

bration of a partially formed metal part onto the forming surface of die. In a related work, Golovashchenko (2010b) used underwaterlectric discharge for high speed trimming of metallic blanks, withischarge energies ranging from 5 kJ to 50 kJ.

Since then, Vohnout et al. (2010) conducted experiments toetermine that the water used in the EHF process should be freef impurities and gases to prevent any unwanted cavitation dur-ng detonation. Cavitation can cause non-uniformity in pressureistribution and reduce overall efficiency if it is not avoided. Forhat reason, Vivek et al. (2013a) replaced water with polyurethanes a pressure transfer medium. Polyurethane was chosen becauset maintains a high Poisson’s ratio (thus low compressibility) atressures ranging up to 4.2 GPa (Kanel et al. (2004)). That workas focused around fundamental studies via instrumented tube

xpansion experiments. In more recent work, however, flat sheetsf polyurethane were used to transfer the pressure to the work-iece. Additionally, thin foils were used as actuators instead ofires so that a planar pressure pulse could be produced for form-

ng flat sheets. Vivek et al. (2013b) demonstrated the use of theseaporizing foil actuators without the urethane pad to implementollision welding of dissimilar metals at much smaller scales thanxplosive welding.

According to Thiruvarudchelvan (1993), use of a polyurethanead for pressure transfer during forming operations introducesultiple advantages, such as elimination of alignment and mis-atch problems, minimization of springback, and accommodation

or thickness variations. Additionally, the same flexible pad cane used for forming into different shaped dies. Lubrication isften unnecessary, and the workpiece surface in contact withhe polyurethane pad is unharmed. Some disadvantages of usingolyurethane, including higher press capacity, possibility of wrin-ling, and shorter working life, are also considered in comparisono corresponding tools used in conventional forming processes. A

ore recent paper by Thiruvarudchelvan (2002) gives an accountf different configurations in which urethane pads are currentlyeing used for forming applications.

This article has been divided into two main parts: (1) fundamen-al studies focused on understanding the effect of foil thickness onfficiency of pressure pulse generation and magnitude of forming,2) application of the technology to practical use, such as forming of

depression and embossing. Procedures, results, and discussionsre presented together for each section. At the end, a summary ofesults and key lessons from this work have been discussed.

. Parametric studies

Tube expansion experiments described by Vivek et al. (2013a,b)howed that, if end effects are ignored, use of the rapid metal vapor-zation technique can result in uniform axisymmetric deformationver a length of 76.2 mm. This can be ensured if the frequency of the

ischarge source is high enough and the diameter of the wire is uni-orm. Applying the same technique in a flat configuration, however,s a challenge. In the present work it will be shown how vaporiz-ng a thin aluminum foil under a constrained elastomer sheet can

ng Technology 214 (2014) 865– 875

create relatively uniform pressures over an area larger than the foilitself.

2.1. Methods for indirect pressure estimation

During impulse forming operation, because the pressure pulselasts for a very short duration, its measurement requires sensitivegauges with low response times. There are some methods by whichpressure can be estimated indirectly as well. Feature heights onresultant workpieces from extruding or punching through a per-forated plate and those from impression die embossing can beused to estimate the range of pressure during those operations.While sensors are good for finding the temporal history of the pres-sure pulse, the spatial distribution can be more easily investigatedby examining the resultant workpieces from pressure estimationexperiments.

2.1.1. Bulging into a perforated plateBulging of a sheet metal workpiece into a perforated steel plate

is often used as a technique for indirect measurement of the mag-nitude and distribution of pressure in explosive forming (Rinehartand Pearson, 1963) and electrohydraulic forming (Knyazyev et al.,2010). The height of each formed hemispherical dimple is inverselyproportional to its radius of curvature. By modeling each dimple asa thin-walled pressure vessel, the amount of pressure created ateach location can be estimated, according to Laplace relation forspherical shells in Eq. (1):

� = P × r

2t(1)

Assuming constant flow strength, �, as larger pressures, P, areexerted at different locations, the radius, r, will be driven to smallervalues, thus resulting in dimples of different curvatures, and ulti-mately different heights. Because the material continues to forminto the perforation until the generated pressure balances the flowstrength of the material, it is inversely proportional to the radiusof curvature, hence directly proportional to the height of the dim-ple. Therefore, the height of a dimple is directly proportional to thepressure experienced by that area of the sheet metal. This is a sim-plistic model which works on the assumption that the thicknessof the sheet metal is much smaller than the radius of the dimple,which is not the case here. SanJose et al. (2012) provide a moredetailed analytical model for pressure estimation based on thismethod. Important factors such as cavitation time and transitionfrom elastic to plastic deformation are also considered by Rinehartand Pearson (1963).

2.1.2. Punch outDuring the extrusion of the workpiece into a perforated plate, it

is also possible that the dimple gets punched out before it reachesits maximum height. The pressure required to punch out a circleof radius r from a sheet of thickness t, and shear strength �, can beestimated from Eq. (2).

P = � × 2t

r(2)

Since shear strength, �, is generally less than the flow strength, �,of a material, it can be expected that a dimple will almost alwayspunch out before it can form into a hemisphere. Therefore, if thedimple is punched out, then the minimum pressure can be esti-mated with Eq. (2), otherwise it can be estimated by Eq. (1) whichtakes into consideration the dimple height.

2.1.3. CoiningMonaghan (1988) provides an upper bound analysis of the pres-

sure required during the coining stage of a closed die axisymmetric

A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865– 875 867

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Fig. 1. Different configuratio

old-forming application. Their numerical model is based on thenergy dissipated due to several factors: internal deformationsn bulk and near the surface, friction between punch and work-iece and between die and workpiece, and velocity discontinuityetween the workpiece material in the bulk and near the surface.he energy per unit volume then gives an estimate of pressure. Itas seen that the ratio of forging pressure to the flow strength of

he material, P/�, remains close to 1 until the die fill out ratio, d/D,s less than 0.8, after which it ramps up to close to 3. Their model

as verified against experiments with high conductivity coppernd aluminum magnesium silicon alloy. Therefore, under assump-ions of closed die forging, the pressure can be up to three timeshe flow strength of the workpiece material if close conformity to

inute die features is observed (Fig. 1).In the aforementioned methods, the estimated values of pres-

ure depend on the flow and shear strength of the membrane beingormed. It should be noted that these material parameters can varyignificantly under dynamic conditions. Further work on measur-

ng strain rates more accurately will help in more accurate pressurestimations based on constitutive properties of materials at hightrain rates.

ig. 2. (A) Schematic representation showing different parts of the forming set up, (B) acestraint, urethane puck and aluminum foil actuator.

estimating driving pressure.

2.2. Experimental procedure

1000 series aluminum foils of thicknesses 0.0508 mm,0.0762 mm and 0.127 mm were cut in the shape of a dog-bone, which featured a central active vaporization section. The foildimensions are shown in Fig. 2(C). A 0.0508 mm thick polyestersheet was taped around the foil to provide electrical insulation.The ends of the foil were left uncovered to enable connection tothe copper terminals of the forming set up, which in turn wereconnected to those of the capacitor bank. The characteristics of thecapacitor bank used for all the experiments are listed in Table 1.

The foil was placed on top of a 38.1 mm thick steel block, whichwas insulated with polyimide tape and G10 insulation plating.A 12.7 mm thick, 80 A grade polyurethane puck with dimensionsas shown in Fig. 2(C) was placed on top of the foil. A 12.7 mmthick steel restraint was then placed around the polyurethanepuck to prevent it from expanding laterally during forming. A76.2 mm × 101.6 mm × 0.508 mm sheet of AA 3003-H14 aluminum

alloy, which exhibits a yield strength of 145 MPa and shear strengthof 96.5 MPa, was chosen as the workpiece material and placedabove the urethane puck. A perforated steel plate with a thickness

tual set-up, (C) overlayed schematic of the positioning and dimensions of the steel

868 A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865– 875

Table 1Capacitor bank characteristics.

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Capacitance Inductance Resistance Maximum chargin

426 �F 100 nH 10 m� 8.66 kV

f 1.143 mm and perforation diameters of 2.311 mm was placed onop of the workpiece and then backed by a thick steel block. Thentire assembly was held together using four 12.7 mm diameterhreaded rods and nuts. Once fixtured, the assembly was attachedo the capacitor bank via the copper terminals. A high electricalurrent was discharged through the conductors, thereby vaporiz-ng the foil in the active area and creating sufficient pressure toorm the workpiece into the perforations in the plate. Foils wereaporized at various input energy levels. Current and voltage wereeasured with a 100 kA:1 V Rogowski coil and 1000:1 V probe. The

olyurethane pad was replaced for different foil actuator thick-esses.

.3. Results and discussion

Assuming that the workpiece material could fully form into aerforation in the plate without shearing, a perfect hemisphereould be created. Using the thin-walled pressure vessel calcula-

ion, a driving pressure of 127 MPa would be needed to achievehis. To reach that pressure over the whole area of the polyurethaneuck, a force of 726 kN would be required. Although an ideal caseor pressure measurement was considered, quantitative pressure

easurement using the perforated plate technique was renderedmpossible in many cases because the workpiece either pushedhrough the perforations and flattened out against the backinglock or simply sheared through the holes before reaching a fullemispherical shape. From Eq. (2), it could be estimated that a pres-ure of 84 MPa would be required for punching out those dimples.herefore the minimum pressure experienced by the workpieceould have been 84 MPa, which would require a force of 500 kN

ver the area of the polyurethane. Although exact calculation ofressure was not possible, qualitative interpretation of results wastill possible with the final workpieces, which are shown in Fig. 3nd Table 2. It should be noted that the part number (A, B, C, or D)o not necessarily represent a sheet formed with the same inputlectrical energy.

The 0.0508 mm thick foil actuators resulted in maximumressure at 1.6 kJ input energy and did not show significant

mprovement as input energies were increased to 4.8 kJ, exhibitingimilar or less amounts of deformation of the workpieces. Thicker.0762 mm thick foils gave increasingly higher impulse as the inputnergy was increased. 0.127 mm thick foils also showed a sim-lar trend. However, neither of the latter two foil types burst at.6 kJ and each produced very little pressure. The pressure wave, asxpected, originated along the length of the foil; but the urethaneuck assisted in spreading that pulse over a larger area.

When the foils vaporize, they undergo an intermediate liquidhase as well. Therefore in order to fully vaporize a certain volumef a conductor, the electrical energy deposited into the foil beforehe burst must exceed both the latent heat of fusion and vapor-zation of the foil material. This deposited energy, Ed, is called thection integral and can be calculated by integrating the product ofurrent, i(t), and voltage, v(t), over time, t, until the conductor burstime, tb.∫ tb

d =0

v(t) × t(t)dt (3)

he burst event occurs when voltage increases and currentecreases rapidly as shown in Fig. 4. This is due to a sudden

tage Maximum charging energy Short circuit current rise time

16 kJ 12 �s

increase in resistance of the foil during its vaporization. In thehighly resistive state, inductive energy stored in the circuit couldbe a significant driving force for the current (Chace and Moore,1959). This inductance causes the voltage to rise above the initiallycharged value.

The latent heat of fusion of pure aluminum is 396 kJ/kg, whileits latent heat of vaporization is 10,888 kJ/kg (Dean and Lange,1999). Therefore, to fully vaporize the active volume of the foil(the narrow region), which weighs 44 mg/0.0254 mm of thickness,approximately 500 J of energy is required. So, a 0.0508 mm thick foilrequires 1 kJ, a 0.0762 mm thick foil requires 1.5 kJ, and a 0.127 mmthick foil requires 2.5 kJ of energy for complete vaporization.

The difference between the action integral and heat of formationof the vapor phase is proportional to the magnitude of the vaporpressure that needs to be overcome before boiling or the burst phe-nomenon, as discussed by Cho et al. (2004). The vapor pressure ishydrostatic and is applied not only on the foil but also on its sur-roundings. Excess energies in all the experiments are also notedin Table 2. Here, it has been assumed that most of the energy isdeposited into the narrow region of the foil.

Another source of energy, and hence additional pressure, arethe various exothermic reactions that occur between the metallicvapors and the atmosphere, forming oxides, nitrides and carbides.One of the most exothermic reactions is the reaction between alu-minum vapor and oxygen to form alumina. Oxidizing the activevolume of the foils in consideration generates 1.4 kJ of heat per0.0254 mm of thickness. Hence the 0.0508 mm, 0.0762 mm and0.127 mm thick foils produce 2.8 kJ, 4.2 kJ and 7 kJ of heat, respec-tively, upon full oxidation. However, as shown by Lee and Ford(1988), these oxidation reactions are quite delayed in relation tothe instant of the burst. In this case, a bubble of aluminum vaporforms under the urethane pad, and the oxidation reactions begin totake place in that bubble. These exothermic reactions can then cre-ate more pressure as the bubble expands, pushing the polyurethanetoward the die. As the delayed pressure pulse travels through thepolyurethane it can get shocked up as discussed by Cooper (1996).In addition, as the initial input energy to the aluminum foil isincreased, the vaporization reaction rate is also increased, caus-ing the evolution of the delayed pressure pulse due to oxidationto occur more quickly. It is thus evident that there are many fac-tors which contribute to pressure evolution due to vaporization ofaluminum foils.

Pressure is directly proportional to energy and inversely propor-tional to volume, and a dimensional analysis reveals that energy perunit volume has the same dimensions as pressure. So we estimatepressure after burst, P(t) as:

P(t) = Exs + Eox + Eloss

A · �x(4)

where Exs is the excess energy deposited in the foil, Eox is theenergy released by oxidation, Eloss is energy loss due to sound, vis-cosity of the polyurethane, etc., and A · �x is the volume of thebubble underneath the polyurethane puck, which increases in sizeas the pressure increases. If the final pressure experienced by theworkpiece can be measured, the final volume of the bubble can becalculated. In many of the experiments presented here, the pres-

sure clearly exceeded the measurement scale, as several shearingevents occurred in some cases. Using a stronger workpiece materialshould be considered if the perforated plate technique is pursuedfor measurement of driving pressure.

A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865– 875 869

rent f

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Fig. 3. Resulting workpieces formed by diffe

. Applications

It is of significant interest to explore the possibility of using theseressures produced through use of rapidly vaporized thin conduc-ors for practical applications. Experimental work has shown thathis process can be successfully used to emboss fine features intohin metal workpieces and form deep drawn features into smallie cavities. Embossing tests have shown that features greater thanalf the material thickness can be simply made with the smallevice shown in Fig. 2. Compared to the traditional requirementf a large multi-ton press, the ability to make such features withhis small setup is quite remarkable. Forming trials revealed thateeply drawn part geometries can be formed using an array oforkpiece materials, including aluminum, titanium, and stainless

teel. Additionally, no punch is needed for the forming process,s the polyurethane puck conforms to the die geometry when theetal vapor rapidly expands. Even fine features on the die surface

an be successfully transferred with this method. When coupledith quasistatic pre-forming, this process has the potential to formighly strained parts, as the material drawn in during pre-forming

s then uniformly stretched during the high velocity component of

able 2esults of the experiments done for determination of pressure magnitude and distributio

Foil thickness (mm) 0.0508 0.0762

Part (refer to Fig. 2) A B C D A

Input energy (kJ) 0.8 1.6 3.2 4.8 1.6

Excess energy (kJ) 0.12 0.53 0.75 0.95 0.28

Burst current (kA) 42 50 46 52 41

Burst time (�s) 19.4 16.7 16.5 14.5 31

oil actuators at various input energy levels.

the process. Traditional quasistatic forming limits can be exceededin this way.

3.1. Forming into a cavity

Forming a depression into a sheet metal part is of interestin many industries, including automobiles, electronics, householdappliances, and medical equipment. In this example, the vaporizingfoil technique was used for forming a sheet metal workpiece into acell phone case die.

The die that was used in these experiments was supplied byMIRDC (Taiwan) and is shown in Fig. 5. It was previously used byKamal et al. (2007) for forming 0.80 mm thick AA 2219-O sheets(yield strength = 76 MPa, tensile strength = 172 MPa) via an electro-magnetic uniform pressure actuator, or UP actuator. In that studyKamal et al. showed how the material can be stretched beyondthe limits of quasistatic forming limit diagrams and also demon-

strated that even the fine features on the die can be translated tothe sheet metal workpiece due to the high impact pressures that areproduced. The UP actuator, however, like many other electromag-netic forming apparatus, is complicated by longevity issues when

n.

0.127

B C D A B C D

3.2 4.8 6.4 1.6 4.8 5.6 8.01.16 2.24 3.26 0.00 1.65 3.00 3.62

50 65 80 – 81 87 10224 22 19 – 30 28 24

870 A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865– 875

F expero

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ig. 4. Temporal record of current and voltage from a typical vaporizing conductor

f the product of current and voltage until time, tb .

epeatedly used at high voltages, pressures, and cycle frequencies.dditionally, it is difficult to form common types of metals thatave high resistivity using this technique, due to the poor magneticoupling that occurs as a result.

In this study, forming of 0.5 mm thick Grade 2 com-ercially pure titanium (yield strength = 345 MPa, tensile

trength = 486 MPa) sheets was attempted. The setup is veryimilar to the one used for perforated plate experiments, shown inig. 2. Dogbone shaped foils (Fig. 2) with 50.8 mm long and 12.7 mmide active sections were cut from 0.127 mm thick 1000 series

luminum sheets and used as actuators. While discharge currentnd voltage were measured during every experiment, workpieceelocity was measured in separate dedicated experiments. A port

as drilled through the die to allow velocity measurement through

hotonic Doppler Velocimetry (PDV).

Fig. 5. Cellphone case die provided by MIRDC (Taiwan).

iment. Energy deposited in the foil before burst can be calculated as a time integral

At 8 kJ input energy, the titanium sheet tightly conformed tothe fine features of the die, but it sheared severely around the dieentry radius (Fig. 6) and some of the embossed features. It is knownfrom the work of Klepaczko and Klosak (1999) that high speedencourages shearing by reducing the energy required for it. Since nomaterial was drawn in from the flat die region, the strain in the cellphone wall increased well beyond the tensile limits of the material,which was also noticed by Kamal et al. (2007) in their work with theUP actuator. They conteracted this problem by forming the sheetin two steps:

(i) A small discharge (3.2 kJ) to flange the workpiece relativelyslowly to allow sufficient draw-in while avoiding tearing at dieentry.

(ii) Two high-speed flanging steps (5.6 kJ each) implemented byimpacting copper drivers onto the back surface of the pre-formed aluminum workpiece, to form it fully into the cellphonecase die.

A similar method was employed in the current work. The tita-nium sheet was first quasistatically formed using a hydraulic pressup to a force of 15 kN which caused a draw-in of nearly 7% along thewidth of the sheet. The preformed sheet was then formed using thevaporizing foil actuator with 6.4 kJ of input energy, which resultedin the workpiece shown in Fig. 7. Clearly, the addition of a quasi-static pre-forming step allowed the final part to closely conform tothe die geometry, while avoiding the unwanted edge shearing thatis illustrated in Fig. 6. Minor shape imperfection can be noticed inthe center of each of the formed workpieces. This can be attributedto bounce back due to the impulse nature of the process and theair trapped in the die, since the space between workpiece and thedie was not evacuated. While the final workpiece is comparable inshape to the one shown in the work of Kamal et al. (2007), it must benoted that with the vaporizing foil actuator, a much stronger mate-rial was formed with half the input electrical energy used with theUP actuator.

Data from the diagnostic experiments are presented in Fig. 8. The

flat workpiece was accelerated to a peak velocity of 270 m/s, whilethe pre-formed workpiece was launched to a maximum velocity of540 m/s. Repetition of these diagnostic experiments yielded veryreproducible results. The two cases are clearly different since in the

A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865– 875 871

Fig. 6. CP titanium sheet formed by one-step urethane pad assisted vaporizing foil forming with input energy of 8 kJ.

id for

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Fig. 7. CP titanium sheet formed by hybr

rst case the pressure pulse travels through the urethane pad andets transferred on to the workpiece, whereas in the second case,he urethane pad impacts the surface of the sheet and launches itt a velocity of nearly twice its own velocity. This phenomenon wille investigated in more detail in the future, as it introduces someery interesting possibilities with configurations for flyer launch.

.2. Embossing

In order to demonstrate the capabilities of the vaporizing foilctuator for embossing applications, a die with three distinct

ig. 8. Current, voltage and velocity records during forming of 0.508 mm thick CP-Ti sheheets were launched with 6.4 kJ input energy into the vaporizing foil actuator.

ming technique at input energy of 6.4 kJ.

feature regions was milled from a 4120 tool steel block with initialdimensions of 101.6 mm × 76.2 mm × 20.0 mm. To represent theacademic institution at which the authors conduct their work, thedie design featured an outline of the state of Ohio, the word “Ohio”in scripted letters, and a buckeye leaf design. The state outlinewas created using a Ø0.5 mm ball-nosed end mill on a single-lineengraving path at a depth of 0.15 mm. The script Ohio area was

milled with a Ø0.79 mm ball-nosed end mill on an overlappingoffset tool path to a depth of 0.38 mm, which resulted in leavingpatterned machining marks on the letter surfaces. The buckeye leafarea was milled with a Ø0.40 mm flat end mill on an overlapping

ets into an embossed cavity. Flat sheets were launched with 8 kJ and pre-formed

872 A. Vivek et al. / Journal of Materials Processi

Fe

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ig. 9. Features in the die and the two different shapes of foil actuators used formbossing experiments.

ffset tool path to a depth of 0.23 mm, which left a smooth finishn the internal area. Since AA 3003-H14 alloy is prone to weldingith the die material at high speed impact, AA 2024-T3 was used

s the workpiece material for these experiments.Perforated plate experiments were again implemented to obtain

ressure distribution information. For these experiments, thective region of the foil had a length of 63.5 mm, which is equalo the length over which the features on the die are machined. Theeometries of the foils are shown in Fig. 9, which were cut from.127 mm thick aluminum sheets. This thickness was used becausexperiments discussed in Section 2 revealed that an increasingmount of energy could be deposited in that thickness whileimultaneously increasing impulse generation. The input electri-al energy was set at 8.0 kJ. Current and voltages were recorded toetect arcing if it occurred between components of the assembly.

Perforated plate experiments showed that the pressure wasigher in the central region when a curved-section foil was useds an actuator, whereas it was higher toward the ends of the activeegion of the straight section foils. It should, however, be noted fromig. 10 that there was a significant pressure even in other regions.

Results of the embossing experiments with these foil shapes ishown in Fig. 11, which shows two large pictures on either side. Theicture on the left corresponds with the straight section foil results,nd the picture on the right corresponds with the curved section foilesults. The entire embossed area is shown in these pictures. Twooxes were drawn around areas of interest in each picture, whichxhibit the forming characteristics of each foil shape near the sam-le center and near the sample end. The top pictures focus on thend features by highlighting the southern border of the Ohio stateutline. The bottom set of pictures details the end of the straightortion of the letter “h” in the script Ohio area to exhibit the form-

ng characteristics near the middle of the foil. Images in the middlewo columns of the figure were recorded using a composite micro-cope technique, whereby three dimensional data was extrapolatedrom sets of several two dimensional images taken at differentocus levels. By associating each focused area with a certain depth,

he detailed three dimensional plots were produced for each areaxamined. Approximately 50 two dimensional images were com-ined to make each three dimensional image, with a 25 �m depthf focus difference between each image. Profile curves were also

ng Technology 214 (2014) 865– 875

extracted from each of these three dimensional data sets along thevertical lines shown in the highest magnified pictures. The graphsin the center of the figure plot the profiles of each of these curves.The phenomenon of heterogeneous pressure distribution along thelength of the active region of the foil is evident from these results.With the curved design foil, the workpiece experienced a very highpressure in the script Ohio area and picked up all the machin-ing marks from the die, demonstrating very sharply formed edges.However, the magnitude of forming was highly reduced toward theends, as the Ohio state outline was only slightly formed into thesheet. The straight-section foil demonstrated the opposite effect,as evident from the figure. The Ohio state outline showed muchbetter forming while the features on in the script Ohio area werenot as sharp.

Another important variable of these forming experiments iswhether or not there was an alternate electrically conductive pathbefore or after the foil burst. This can be detected by the presence ofa current going in the negative direction even after vaporization ofthe foil. Fig. 12 shows each case: (A) when arcing occurred and thecurrent found an alternate path after foil burst, and (B) when therewas no arcing and the current dropped to zero after the burst. It wasfound that the experiments with no current reversals were moreefficient, as evidenced by the final results of the script O shown foreach trial. Straight-section foils were used in these experiments.

This set of experiments depicts the possibility of foil designand testing with basic perforated plate experiments and thentransitioning into a more practical operation such as forming orembossing.

3.3. Pertinent issues with application of VFA

One important consideration is the lifetime of the urethane padsused in these applications. While Thiruvarudchelvan (1993) esti-mated that a single urethane pad can be used 50,000 times in anormal stamping process, the method used here is much moreaggressive in terms of the damage inflicted on the urethane pad.Fig. 13 shows that the side facing the vaporizing foil experiencedsignificant surface damage after 10 experiments, at which time thepads were replaced. Some imprints of the die can also be seen on theside facing the workpiece. However, a transverse cross-section cutshowed that despite the surface damages, the pads sustained verylittle through-thickness damage. The effect of urethane pad’s super-ficial damage on efficiency and the quality of formed parts needsto be studied to understand its reusability based on its appearance.Such information would support a thorough cost analysis of thisprocess.

Another issue that should be considered is the lifetime of thedies used here. During forming experiments, impact velocities ashigh as 500 m/s were measured. While high velocity impact canhelp achieve close conformity with minute die features, it needs tobe optimized to avoid damage to the die. Shock-resistant tool steelssuch as S7-grade can be used for such an application. Althoughspot welding was not observed in the cases reported here, it canoccur under a range of conditions of impact angles and velocitiesfor a given combination of die and workpiece materials. Coating theimpact surface of die with ceramics can also help reduce damagewith welding or wear.

There will likely be instances where the advantages of EHFmake it the method of choice. Shaped polyurethane pads arenot needed and it is often easy to use several electrohydraulicimpulses to assure full conformation between a workpiece and die.However, there are also several instances where a polyurethane

pressure transfer medium could be preferable and this has realadvantages. First, handling water can be difficult to manage ifany leakage occurs, and it must be periodically replaced due toproperty changes that occur from it becoming filled with fine

A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865– 875 873

F ing ha

ssteapm

ig. 10. Resultant AA 2024-T3 workpieces from perforated plate experiments shownd around the center with a curved-section foil actuator.

ediment from the ablated or vaporized wires or electrodes. Second,ince polyurethane is a solid, it is immune from the cavita-ion effects that can cause unwanted pressure nonuniformity in

lectrohydraulic forming. These heterogeneities can cause dam-ge to workpieces and dies. Third, because the foil generatesressure over a surface area (instead of a region that approxi-ates a point), the workpiece can be relatively close to the foil

Fig. 11. Results of the embossing experiments done with straight

igher pressure toward the ends of the active area of a straight-section foil actuator,

as opposed to the large standoffs that are typical in electrohy-draulic forming. Because much smaller volumes are pressurized,higher pressures can be generated at smaller plasma energies.

This reduces the size of the capacitor bank needed. Fourth, as ini-tially shown in this work, with VFA pressure distribution can becontrolled much more easily than EHF by changing foil geome-try.

- (left) and curved-section (right) aluminum foil actuators.

874 A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865– 875

Fig. 12. Urethane pad assisted embossing done with straight-section vaporizing foil actuators: (A) arcing, detected by current reversal, reducing the efficiency, (b) no arcing;the workpiece conforms better to the die features.

FT

ig. 13. Condition of the urethane pads after experiments: (a) side facing straight section

he absence of any cracks through the thickness of the sectioned urethane pad should be

foil actuator after 10 shots, (b) side facing curved section foil actuator after 10 shots.

noted, (c) side facing workpiece after a die cavity forming experiment.

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A. Vivek et al. / Journal of Materials Pr

. Conclusions

Urethane pad-assisted vaporizing foil actuator has a potential tobe an agile, robust, inexpensive, and efficient tool for impulse-based forming. It counters the longevity issue presented byelectromagnetic actuators, and this technique can be applied ina small laboratory or traditional industrial environment, unlikeexplosive forming.Forming of AA 3003 H14 sheets into perforated plates, using theurethane pad assisted vaporizing foil actuator, showed that themagnitude of driving pressure is proportional to the excess elec-trical energy deposited into the foil before it bursts. Thinner foilsgave diminishing returns on investment in terms of pressurewhen subjected to increasing input energy. Thicker foils pro-duce higher masses of vapor than thinner foils, so the amount oftotal heat generated from the subsequent exothermic oxidationreaction is higher.Urethane pad assisted forming of commercially pure titaniumsheets into an embossed cavity was implemented by a single anda two-step procedure. An 8.0 kJ discharge caused the launch of aflat workpiece up to a velocity of 270 m/s, but resulted in tear-ing along the die edge. The impact of the urethane pad launchedby a 6.4 kJ discharge into the vaporizing foil actuator caused aquasistatically pre-formed workpiece to be accelerated to a peakvelocity of 540 m/s. Using this method, the workpiece formedinto the die without tearing, and picked up all the minute surfacefeatures.AA 2024-T3 sheets were embossed into a die with varied features.It was seen that the workpiece formed better in the center with acurved-section foil, and better on the ends with a straight-sectionfoil. Additionally, any arcing reduced the overall efficiency of theprocess.

The development of this technique is not intended to replacehe existing high rate forming methods such as EHF, EXF orMF; rather, the purpose is to supplement them where theirpplication is not suitable. Significant work needs to be donen terms of modeling, parameter optimization, and cost analysisefore this technique can be regularly applied. The parametrictudies and practical implementation of forming and emboss-ng done in this work are a few steps toward developing aetter understanding of this process. Control of pressure distribu-ion through foil shape variation and by introducing intentionalefects will be investigated further. Use of pressure sensorsor direct measurement of pressure is also intended for futureork.

cknowledgements

This material is based upon work supported by Department ofnergy under Award number DE-PI0000012. The authors would

ng Technology 214 (2014) 865– 875 875

also like to thank the ALCOA foundation, which supported the workthrough the Advancing Sustainability Research Initiative.

References

Chace, W., Moore, H., 1959. Exploding Wires, vols. 4. Plenum Press, New York.Chau, H.H., Dittbener, G., Hofer, W.W., Honodel, C.A., Steinberg, D.J., Stroud, J.R.,

Weingart, R.C., Lee, R.S., 1980. Electric gun: a versatile tool for high-pressureshock wave research. Rev. Sci. Instrum. 51 (12).

Cho, C.Y., Murai, K., Suzuki, T., Suematsu, H., Jiang, W.H., Yatsui, K., 2004. Enhance-ment of energy deposition in pulsed wire discharge for synthesis of nanosizedpowders. IEEE Trans. Plasma Sci. 32 (5), 2062–2067.

Cooper, P., 1996. Qualitative description of a shockwave. In: Explosives Engineering.Wiley-VCH, pp. 167–174.

Daehn, G.S., 2006. High velocity metal forming. In: ASM Handbook, vol. 14B, Metal-working: Sheet Forming. ASM International, Materials Park, Ohio, pp. 405–418.

Dean, J.A., Lange, N.A., 1999. Thermodynamic properties. In: Lange’s Handbook ofChemistry, 15th ed. Mc-Graw-Hill Inc., New York, 6.107.

Felts, R., 1961–62. Application of the electrohydraulic effect to metal forming. In:Creative Manufacturing Seminars. Advanced High Energy Rate Forming, PaperSP62-08, 10 pp.

Golovashchenko, S.F., 2010. Electrohydraulic trimming, flanging and hemming ofblanks, US Patent: US 7,810,366, Issue Date: October 12, 2010.

Golovashchenko, S.F., 2010. Pulsed electro-hydraulic calibration of stamped panels,US Patent: US 7,827,838 B2, Issue Date: November 9, 2010.

Kamal, M., Shang, J., Cheng, V., Hatkevich, S., Daehn, G.S., 2007. Agile manufacturingof a micro-embossed case by a two-step electromagnetic forming process. J.Mater. Process. Technol. 190 (1), 41–50.

Kanel, G.I., Razorenov, S.V., Fortov, V.E., 2004. Shock Wave Phenomenon and TheProperties of Condensed Matter. Springer, New York, pp. 67–68.

Keller, D.V., Penning, R.J., 1962. Exploding foils – the production of plane shock wavesand the acceleration of thin plates. Exploding Wires 2, 263–277.

Klepaczko, J.R., Klosak, M., 1999. Numerical study of the critical impact velocity inshear. Eur. J. Mech. A: Solids 18, 93.

Knyazyev, M., Zhovnovatuk, K., Ya, S., 2010. Measurements of pressure fields withmulti-point membrane gauges at electrohydraulic forming. In: Proceedings of4th International Conference on High Speed Forming, pp. 75–82.

Lee, W., Ford, R., 1988. Pressure measurements correlated with electrical explosionof metals in water. J. Appl. Phys. 64 (8), 3851–3854.

Monaghan, J.M., 1988. An upper-bound analysis of an axisymmetrical coining pro-cess. J. Mech. Work. Technol. 16 (2), 175–192.

Rinehart, J.S., Pearson, J., 1963. Explosive Working of Metals. Mac Million, New York,pp. 85–90.

SanJose, J., Perez, I., Knyazyev, M., Zhovnovatuk, K., Ya., S., 2012. Pressure fieldsrepeatability at electrohydraulic pulse loading in discharge chamber with singleelectrode pair. In: Proceedings of 5th International Conference on High SpeedForming, pp. 33–42.

Stroud, J.R., 1976. A New Kind of Detonator, The Slapper. Lawrence LivermoreNational Labs.

Thiruvarudchelvan, S., 1993. Elastomers in metal forming: areview. J. Mater. Process. Technol. 39 (October (1–2)), 55–82,http://dx.doi.org/10.1016/0924-0136(93)90008-T.

Thiruvarudchelvan, S., 2002. The potential role of flexible tools in metalforming. J. Mater. Process. Technol. 122 (March (2–3)), 293–300,http://dx.doi.org/10.1016/S0924-0136(02)00077-8.

Vivek, A., Taber, G.A., Johnson, J.R., Woodward, S.T., Daehn, G.S., 2013a. Electricallydriven plasma via vaporization of metallic conductors: a tool for impulse metalworking. J. Mater. Process. Technol. 213 (8), 1311–1326.

Vivek, A., Taber, Hansen, S.H., Liu, B.C., Daehn, G.S., 2013b. Vaporizing foil actuator:a tool for collision welding. J. Mater. Process. Technol. 213 (12), 2304–3231.

Vohnout, V.J., Fenton, G., Daehn, G.S., 2010. Pressure heterogeneity in small displace-

ment electrohydraulic forming processes. In: Proceedings of 4th InternationalConference on High Speed Forming, pp. 65–74.

Woetzel, M., Löffler, M.J., Spahn, E., Ritter, H., 2006. Preliminary examination of highvelocity metal-shaping with electrical wire explosion. In: 1st Euro-Asian PulsedPower Conference, September 18–22, Chengdu, China.