Разработана продвинутая техника взрывной сварки

1. Journal of Materials Processing Technology 214 (2014) 865–875
Contents lists available at ScienceDirect
Journal of Materials Processing Technology
journal homepage: www.elsevier.com/locate/jmatprotec
Vaporizing foil actuator used for impulse forming and embossing of
titanium and aluminum alloys
A. Vivek∗
, R.C. Brune, S.R. Hansen, G.S. Daehn
Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210, USA
a r t i c l e i n f o
Article history:
Received 6 August 2013
Received in revised form 4 November 2013
Accepted 7 December 2013
Available online 15 December 2013
Keywords:
Embossing
Impulse forming
Urethane pad
Vaporizing foil actuator (VFA)
Elecrohydraulic forming (EHF)
Titanium
Aluminum
a b s t r a c t
Electrically driven rapid vaporization of thin conductors is known to produce short-duration pressure
pulses 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 aluminum
foils of various thicknesses, which were vaporized using a capacitor bank discharge with a maximum
charging voltage of 8.6 kV. Peak current was delivered on the order of 100 kA with rise times of about
12 ␮s. In this work, polyurethane was used as a medium to transfer pressure from the aluminum foil
vaporization zone to the workpiece. Fundamental experiments, where AA 3003-H14 aluminum alloy
was 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 foil
before 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 into
cavities 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 that
combined a quasistatic pre-forming step with a vaporizing foil forming step. Sheets of 0.508 mm thick AA
2024-T3 aluminum alloy were embossed into a die with features of varying depths. Aluminum foils with
straight and curved active sections were used as actuators. The curved-section foils resulted in higher
conformation of the workpiece to the die in the center region, while the straight-section foils produced
better conformity to the die features on the ends.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Passage of an electrical current of large magnitude through a
thin conductor can cause its rapid vaporization. The rapid expan-
sion of the gases creates a high-pressure pulse. Generally the
discharge source is a capacitor bank and the conductor is in the
form of a thin wire or foil. “Exploding” conductors have been
studied before, but the emphasis has mainly been on understand-
ing the phenomenon itself or its application in shock physics and
explosive detonation. Several studies were performed that utilized
rapid vaporization of metal foils to achieve high velocities in flyer
plates. Keller and Penning (1962) achieved 4–5 km/s velocities in
thin dielectric flyers that were impacted with target plates. The
impact results revealed shock properties of the dielectric and the
target plates between 1 and 10 GPa. Stroud (1976) later produced
100 GPa pressures with an improved method and used the resulting
shock 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).
pressures, upward of 500 GPa, were then reached by Chau et al.
(1980) through use of a thin metal layer attached to the dielectric
plate. This improved results in two key ways. Additional driv-
ing force was created due to the momentary repulsion between
the magnetic field of the foil and the conductive metal workpiece
layer, 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 greater
shock impedance, resulting in overall higher impact pressures.
These improvements produced parallel flyer motion for the first
5 mm of travel and resulted in velocities near 20 km/s. This method
was used to characterize shock detonation of explosive materials,
but much greater range than before was possible, as detonation
pressures up to 28 GPa were attained. Other applications, such as
quick action fuses, discharge pulse sharpening, and nanopowder
manufacturing, have also been explored in the past.
Use of rapid metal vaporization in the metal working industry
has been limited. Electrohydraulic forming (EHF) is implemented
by underwater electrical detonation of bridgewires or, more com-
monly, by high voltage dielectric breakdown across spark gaps. As
early as 1961, electrohydraulic forming was researched for use in
0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jmatprotec.2013.12.003

2. 866 A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865–875
metal deformation, as Felts (1961) first demonstrated how pressure
impulses large enough to form sheet metal could be produced with
this method. The underwater current discharge was successfully
used to form metal objects into parts of relatively small struc-
tural mass. Woetzel et al. (2006) demonstrated that experiments
which used aluminum wires as a chemically reactive detonation
source produced comparable deformation to those initiated with
secondary explosives, such as PETN. Daehn (2006) commented that
this forming method has so far been difficult to commercialize, but
that some organizations still successfully use this type of process
for formation of small batches of parts. Golovashchenko (2010a)
demonstrated the use of pulsed electrohydraulic discharges for cal-
ibration of a partially formed metal part onto the forming surface of
a die. In a related work, Golovashchenko (2010b) used underwater
electric discharge for high speed trimming of metallic blanks, with
discharge energies ranging from 5 kJ to 50 kJ.
Since then, Vohnout et al. (2010) conducted experiments to
determine that the water used in the EHF process should be free
of impurities and gases to prevent any unwanted cavitation dur-
ing detonation. Cavitation can cause non-uniformity in pressure
distribution and reduce overall efficiency if it is not avoided. For
that reason, Vivek et al. (2013a) replaced water with polyurethane
as a pressure transfer medium. Polyurethane was chosen because
it maintains a high Poisson’s ratio (thus low compressibility) at
pressures ranging up to 4.2 GPa (Kanel et al. (2004)). That work
was focused around fundamental studies via instrumented tube
expansion experiments. In more recent work, however, flat sheets
of polyurethane were used to transfer the pressure to the work-
piece. Additionally, thin foils were used as actuators instead of
wires so that a planar pressure pulse could be produced for form-
ing flat sheets. Vivek et al. (2013b) demonstrated the use of these
vaporizing foil actuators without the urethane pad to implement
collision welding of dissimilar metals at much smaller scales than
explosive welding.
According to Thiruvarudchelvan (1993), use of a polyurethane
pad for pressure transfer during forming operations introduces
multiple advantages, such as elimination of alignment and mis-
match problems, minimization of springback, and accommodation
for thickness variations. Additionally, the same flexible pad can
be used for forming into different shaped dies. Lubrication is
often unnecessary, and the workpiece surface in contact with
the polyurethane pad is unharmed. Some disadvantages of using
polyurethane, including higher press capacity, possibility of wrin-
kling, and shorter working life, are also considered in comparison
to corresponding tools used in conventional forming processes. A
more recent paper by Thiruvarudchelvan (2002) gives an account
of different configurations in which urethane pads are currently
being used for forming applications.
This article has been divided into two main parts: (1) fundamen-
tal studies focused on understanding the effect of foil thickness on
efficiency of pressure pulse generation and magnitude of forming,
(2) application of the technology to practical use, such as forming of
a depression and embossing. Procedures, results, and discussions
are presented together for each section. At the end, a summary of
results and key lessons from this work have been discussed.
2. Parametric studies
Tube expansion experiments described by Vivek et al. (2013a,b)
showed that, if end effects are ignored, use of the rapid metal vapor-
ization technique can result in uniform axisymmetric deformation
over a length of 76.2 mm. This can be ensured if the frequency of the
discharge source is high enough and the diameter of the wire is uni-
form. Applying the same technique in a flat configuration, however,
is a challenge. In the present work it will be shown how vaporiz-
ing a thin aluminum foil under a constrained elastomer sheet can
create relatively uniform pressures over an area larger than the foil
itself.
2.1. Methods for indirect pressure estimation
During impulse forming operation, because the pressure pulse
lasts for a very short duration, its measurement requires sensitive
gauges with low response times. There are some methods by which
pressure can be estimated indirectly as well. Feature heights on
resultant workpieces from extruding or punching through a per-
forated plate and those from impression die embossing can be
used 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 investigated
by examining the resultant workpieces from pressure estimation
experiments.
2.1.1. Bulging into a perforated plate
Bulging 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 (Rinehart
and Pearson, 1963) and electrohydraulic forming (Knyazyev et al.,
2010). The height of each formed hemispherical dimple is inversely
proportional to its radius of curvature. By modeling each dimple as
a thin-walled pressure vessel, the amount of pressure created at
each location can be estimated, according to Laplace relation for
spherical shells in Eq. (1):
=
P × r
2t
(1)
Assuming constant flow strength, , as larger pressures, P, are
exerted at different locations, the radius, r, will be driven to smaller
values, thus resulting in dimples of different curvatures, and ulti-
mately different heights. Because the material continues to form
into the perforation until the generated pressure balances the flow
strength of the material, it is inversely proportional to the radius
of curvature, hence directly proportional to the height of the dim-
ple. Therefore, the height of a dimple is directly proportional to the
pressure experienced by that area of the sheet metal. This is a sim-
plistic model which works on the assumption that the thickness
of the sheet metal is much smaller than the radius of the dimple,
which is not the case here. SanJose et al. (2012) provide a more
detailed analytical model for pressure estimation based on this
method. Important factors such as cavitation time and transition
from elastic to plastic deformation are also considered by Rinehart
and Pearson (1963).
2.1.2. Punch out
During the extrusion of the workpiece into a perforated plate, it
is also possible that the dimple gets punched out before it reaches
its maximum height. The pressure required to punch out a circle
of radius r from a sheet of thickness t, and shear strength , can be
estimated 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 always
punch out before it can form into a hemisphere. Therefore, if the
dimple is punched out, then the minimum pressure can be esti-
mated with Eq. (2), otherwise it can be estimated by Eq. (1) which
takes into consideration the dimple height.
2.1.3. Coining
Monaghan (1988) provides an upper bound analysis of the pres-
sure required during the coining stage of a closed die axisymmetric

3. A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865–875 867
Fig. 1. Different configurations for estimating driving pressure.
cold-forming application. Their numerical model is based on the
energy dissipated due to several factors: internal deformations
in bulk and near the surface, friction between punch and work-
piece and between die and workpiece, and velocity discontinuity
between the workpiece material in the bulk and near the surface.
The energy per unit volume then gives an estimate of pressure. It
was seen that the ratio of forging pressure to the flow strength of
the material, P/ , remains close to 1 until the die fill out ratio, d/D,
is less than 0.8, after which it ramps up to close to 3. Their model
was verified against experiments with high conductivity copper
and aluminum magnesium silicon alloy. Therefore, under assump-
tions of closed die forging, the pressure can be up to three times
the flow strength of the workpiece material if close conformity to
minute die features is observed (Fig. 1).
In the aforementioned methods, the estimated values of pres-
sure depend on the flow and shear strength of the membrane being
formed. It should be noted that these material parameters can vary
significantly under dynamic conditions. Further work on measur-
ing strain rates more accurately will help in more accurate pressure
estimations based on constitutive properties of materials at high
strain rates.
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 foil
dimensions are shown in Fig. 2(C). A 0.0508 mm thick polyester
sheet was taped around the foil to provide electrical insulation.
The ends of the foil were left uncovered to enable connection to
the copper terminals of the forming set up, which in turn were
connected to those of the capacitor bank. The characteristics of the
capacitor 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, which
was insulated with polyimide tape and G10 insulation plating.
A 12.7 mm thick, 80 A grade polyurethane puck with dimensions
as shown in Fig. 2(C) was placed on top of the foil. A 12.7 mm
thick steel restraint was then placed around the polyurethane
puck to prevent it from expanding laterally during forming. A
76.2 mm × 101.6 mm × 0.508 mm sheet of AA 3003-H14 aluminum
alloy, which exhibits a yield strength of 145 MPa and shear strength
of 96.5 MPa, was chosen as the workpiece material and placed
above the urethane puck. A perforated steel plate with a thickness
Fig. 2. (A) Schematic representation showing different parts of the forming set up, (B) actual set-up, (C) overlayed schematic of the positioning and dimensions of the steel
restraint, urethane puck and aluminum foil actuator.

4. 868 A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865–875
Table 1
Capacitor bank characteristics.
Capacitance Inductance Resistance Maximum charging voltage Maximum charging energy Short circuit current rise time
426 ␮F 100 nH 10 m 8.66 kV 16 kJ 12 ␮s
of 1.143 mm and perforation diameters of 2.311 mm was placed on
top of the workpiece and then backed by a thick steel block. The
entire assembly was held together using four 12.7 mm diameter
threaded rods and nuts. Once fixtured, the assembly was attached
to the capacitor bank via the copper terminals. A high electrical
current was discharged through the conductors, thereby vaporiz-
ing the foil in the active area and creating sufficient pressure to
form the workpiece into the perforations in the plate. Foils were
vaporized at various input energy levels. Current and voltage were
measured with a 100 kA:1 V Rogowski coil and 1000:1 V probe. The
polyurethane pad was replaced for different foil actuator thick-
nesses.
2.3. Results and discussion
Assuming that the workpiece material could fully form into a
perforation in the plate without shearing, a perfect hemisphere
would be created. Using the thin-walled pressure vessel calcula-
tion, a driving pressure of 127 MPa would be needed to achieve
this. To reach that pressure over the whole area of the polyurethane
puck, a force of 726 kN would be required. Although an ideal case
for pressure measurement was considered, quantitative pressure
measurement using the perforated plate technique was rendered
impossible in many cases because the workpiece either pushed
through the perforations and flattened out against the backing
block or simply sheared through the holes before reaching a full
hemispherical shape. From Eq. (2), it could be estimated that a pres-
sure of 84 MPa would be required for punching out those dimples.
Therefore the minimum pressure experienced by the workpiece
would have been 84 MPa, which would require a force of 500 kN
over the area of the polyurethane. Although exact calculation of
pressure was not possible, qualitative interpretation of results was
still possible with the final workpieces, which are shown in Fig. 3
and Table 2. It should be noted that the part number (A, B, C, or D)
do not necessarily represent a sheet formed with the same input
electrical energy.
The 0.0508 mm thick foil actuators resulted in maximum
pressure at 1.6 kJ input energy and did not show significant
improvement as input energies were increased to 4.8 kJ, exhibiting
similar or less amounts of deformation of the workpieces. Thicker
0.0762 mm thick foils gave increasingly higher impulse as the input
energy was increased. 0.127 mm thick foils also showed a sim-
ilar trend. However, neither of the latter two foil types burst at
1.6 kJ and each produced very little pressure. The pressure wave, as
expected, originated along the length of the foil; but the urethane
puck assisted in spreading that pulse over a larger area.
When the foils vaporize, they undergo an intermediate liquid
phase as well. Therefore in order to fully vaporize a certain volume
of a conductor, the electrical energy deposited into the foil before
the burst must exceed both the latent heat of fusion and vapor-
ization of the foil material. This deposited energy, Ed, is called the
action integral and can be calculated by integrating the product of
current, i(t), and voltage, v(t), over time, t, until the conductor burst
time, tb.
Ed =
tb
0
v(t) × t(t)dt (3)
The burst event occurs when voltage increases and current
decreases rapidly as shown in Fig. 4. This is due to a sudden
increase in resistance of the foil during its vaporization. In the
highly resistive state, inductive energy stored in the circuit could
be a significant driving force for the current (Chace and Moore,
1959). This inductance causes the voltage to rise above the initially
charged value.
The latent heat of fusion of pure aluminum is 396 kJ/kg, while
its 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 foil
requires 1 kJ, a 0.0762 mm thick foil requires 1.5 kJ, and a 0.127 mm
thick foil requires 2.5 kJ of energy for complete vaporization.
The difference between the action integral and heat of formation
of the vapor phase is proportional to the magnitude of the vapor
pressure that needs to be overcome before boiling or the burst phe-
nomenon, as discussed by Cho et al. (2004). The vapor pressure is
hydrostatic and is applied not only on the foil but also on its sur-
roundings. Excess energies in all the experiments are also noted
in Table 2. Here, it has been assumed that most of the energy is
deposited into the narrow region of the foil.
Another source of energy, and hence additional pressure, are
the various exothermic reactions that occur between the metallic
vapors 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 active
volume of the foils in consideration generates 1.4 kJ of heat per
0.0254 mm of thickness. Hence the 0.0508 mm, 0.0762 mm and
0.127 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 to
the instant of the burst. In this case, a bubble of aluminum vapor
forms under the urethane pad, and the oxidation reactions begin to
take place in that bubble. These exothermic reactions can then cre-
ate more pressure as the bubble expands, pushing the polyurethane
toward the die. As the delayed pressure pulse travels through the
polyurethane it can get shocked up as discussed by Cooper (1996).
In addition, as the initial input energy to the aluminum foil is
increased, the vaporization reaction rate is also increased, caus-
ing the evolution of the delayed pressure pulse due to oxidation
to occur more quickly. It is thus evident that there are many fac-
tors which contribute to pressure evolution due to vaporization of
aluminum foils.
Pressure is directly proportional to energy and inversely propor-
tional to volume, and a dimensional analysis reveals that energy per
unit volume has the same dimensions as pressure. So we estimate
pressure 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 the
energy released by oxidation, Eloss is energy loss due to sound, vis-
cosity of the polyurethane, etc., and A · x is the volume of the
bubble underneath the polyurethane puck, which increases in size
as the pressure increases. If the final pressure experienced by the
workpiece can be measured, the final volume of the bubble can be
calculated. In many of the experiments presented here, the pres-
sure clearly exceeded the measurement scale, as several shearing
events occurred in some cases. Using a stronger workpiece material
should be considered if the perforated plate technique is pursued
for measurement of driving pressure.

5. A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865–875 869
Fig. 3. Resulting workpieces formed by different foil actuators at various input energy levels.
3. Applications
It is of significant interest to explore the possibility of using these
pressures produced through use of rapidly vaporized thin conduc-
tors for practical applications. Experimental work has shown that
this process can be successfully used to emboss fine features into
thin metal workpieces and form deep drawn features into small
die cavities. Embossing tests have shown that features greater than
half the material thickness can be simply made with the small
device shown in Fig. 2. Compared to the traditional requirement
of a large multi-ton press, the ability to make such features with
this small setup is quite remarkable. Forming trials revealed that
deeply drawn part geometries can be formed using an array of
workpiece materials, including aluminum, titanium, and stainless
steel. Additionally, no punch is needed for the forming process,
as the polyurethane puck conforms to the die geometry when the
metal vapor rapidly expands. Even fine features on the die surface
can be successfully transferred with this method. When coupled
with quasistatic pre-forming, this process has the potential to form
highly strained parts, as the material drawn in during pre-forming
is then uniformly stretched during the high velocity component of
the process. Traditional quasistatic forming limits can be exceeded
in this way.
3.1. Forming into a cavity
Forming a depression into a sheet metal part is of interest
in many industries, including automobiles, electronics, household
appliances, and medical equipment. In this example, the vaporizing
foil technique was used for forming a sheet metal workpiece into a
cell phone case die.
The die that was used in these experiments was supplied by
MIRDC (Taiwan) and is shown in Fig. 5. It was previously used by
Kamal 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 study
Kamal et al. showed how the material can be stretched beyond
the limits of quasistatic forming limit diagrams and also demon-
strated that even the fine features on the die can be translated to
the sheet metal workpiece due to the high impact pressures that are
produced. The UP actuator, however, like many other electromag-
netic forming apparatus, is complicated by longevity issues when
Table 2
Results of the experiments done for determination of pressure magnitude and distribution.
Foil thickness (mm) 0.0508 0.0762 0.127
Part (refer to Fig. 2) A B C D A B C D A B C D
Input energy (kJ) 0.8 1.6 3.2 4.8 1.6 3.2 4.8 6.4 1.6 4.8 5.6 8.0
Excess energy (kJ) 0.12 0.53 0.75 0.95 0.28 1.16 2.24 3.26 0.00 1.65 3.00 3.62
Burst current (kA) 42 50 46 52 41 50 65 80 – 81 87 102
Burst time (␮s) 19.4 16.7 16.5 14.5 31 24 22 19 – 30 28 24

6. 870 A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865–875
Fig. 4. Temporal record of current and voltage from a typical vaporizing conductor experiment. Energy deposited in the foil before burst can be calculated as a time integral
of the product of current and voltage until time, tb.
repeatedly used at high voltages, pressures, and cycle frequencies.
Additionally, it is difficult to form common types of metals that
have high resistivity using this technique, due to the poor magnetic
coupling that occurs as a result.
In this study, forming of 0.5 mm thick Grade 2 com-
mercially pure titanium (yield strength = 345 MPa, tensile
strength = 486 MPa) sheets was attempted. The setup is very
similar to the one used for perforated plate experiments, shown in
Fig. 2. Dogbone shaped foils (Fig. 2) with 50.8 mm long and 12.7 mm
wide active sections were cut from 0.127 mm thick 1000 series
aluminum sheets and used as actuators. While discharge current
and voltage were measured during every experiment, workpiece
velocity was measured in separate dedicated experiments. A port
was drilled through the die to allow velocity measurement through
Photonic Doppler Velocimetry (PDV).
Fig. 5. Cellphone case die provided by MIRDC (Taiwan).
At 8 kJ input energy, the titanium sheet tightly conformed to
the fine features of the die, but it sheared severely around the die
entry radius (Fig. 6) and some of the embossed features. It is known
from the work of Klepaczko and Klosak (1999) that high speed
encourages shearing by reducing the energy required for it. Since no
material was drawn in from the flat die region, the strain in the cell
phone wall increased well beyond the tensile limits of the material,
which was also noticed by Kamal et al. (2007) in their work with the
UP actuator. They conteracted this problem by forming the sheet
in two steps:
(i) A small discharge (3.2 kJ) to flange the workpiece relatively
slowly to allow sufficient draw-in while avoiding tearing at die
entry.
(ii) Two high-speed flanging steps (5.6 kJ each) implemented by
impacting copper drivers onto the back surface of the pre-
formed aluminum workpiece, to form it fully into the cellphone
case die.
A similar method was employed in the current work. The tita-
nium sheet was first quasistatically formed using a hydraulic press
up to a force of 15 kN which caused a draw-in of nearly 7% along the
width of the sheet. The preformed sheet was then formed using the
vaporizing foil actuator with 6.4 kJ of input energy, which resulted
in the workpiece shown in Fig. 7. Clearly, the addition of a quasi-
static pre-forming step allowed the final part to closely conform to
the die geometry, while avoiding the unwanted edge shearing that
is illustrated in Fig. 6. Minor shape imperfection can be noticed in
the center of each of the formed workpieces. This can be attributed
to bounce back due to the impulse nature of the process and the
air trapped in the die, since the space between workpiece and the
die was not evacuated. While the final workpiece is comparable in
shape to the one shown in the work of Kamal et al. (2007), it must be
noted that with the vaporizing foil actuator, a much stronger mate-
rial was formed with half the input electrical energy used with the
UP actuator.
Data from the diagnostic experiments are presented in Fig. 8. The
flat workpiece was accelerated to a peak velocity of 270 m/s, while
the pre-formed workpiece was launched to a maximum velocity of
540 m/s. Repetition of these diagnostic experiments yielded very
reproducible results. The two cases are clearly different since in the

7. 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.
Fig. 7. CP titanium sheet formed by hybrid forming technique at input energy of 6.4 kJ.
first case the pressure pulse travels through the urethane pad and
gets transferred on to the workpiece, whereas in the second case,
the urethane pad impacts the surface of the sheet and launches it
at a velocity of nearly twice its own velocity. This phenomenon will
be investigated in more detail in the future, as it introduces some
very interesting possibilities with configurations for flyer launch.
3.2. Embossing
In order to demonstrate the capabilities of the vaporizing foil
actuator for embossing applications, a die with three distinct
feature regions was milled from a 4120 tool steel block with initial
dimensions of 101.6 mm × 76.2 mm × 20.0 mm. To represent the
academic institution at which the authors conduct their work, the
die design featured an outline of the state of Ohio, the word “Ohio”
in scripted letters, and a buckeye leaf design. The state outline
was created using a Ø0.5 mm ball-nosed end mill on a single-line
engraving 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 overlapping
offset tool path to a depth of 0.38 mm, which resulted in leaving
patterned machining marks on the letter surfaces. The buckeye leaf
area was milled with a Ø0.40 mm flat end mill on an overlapping
Fig. 8. Current, voltage and velocity records during forming of 0.508 mm thick CP-Ti sheets into an embossed cavity. Flat sheets were launched with 8 kJ and pre-formed
sheets were launched with 6.4 kJ input energy into the vaporizing foil actuator.

8. 872 A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865–875
Fig. 9. Features in the die and the two different shapes of foil actuators used for
embossing experiments.
offset tool path to a depth of 0.23 mm, which left a smooth finish
on the internal area. Since AA 3003-H14 alloy is prone to welding
with the die material at high speed impact, AA 2024-T3 was used
as the workpiece material for these experiments.
Perforated plate experiments were again implemented to obtain
pressure distribution information. For these experiments, the
active region of the foil had a length of 63.5 mm, which is equal
to the length over which the features on the die are machined. The
geometries of the foils are shown in Fig. 9, which were cut from
0.127 mm thick aluminum sheets. This thickness was used because
experiments discussed in Section 2 revealed that an increasing
amount of energy could be deposited in that thickness while
simultaneously increasing impulse generation. The input electri-
cal energy was set at 8.0 kJ. Current and voltages were recorded to
detect arcing if it occurred between components of the assembly.
Perforated plate experiments showed that the pressure was
higher in the central region when a curved-section foil was used
as an actuator, whereas it was higher toward the ends of the active
region of the straight section foils. It should, however, be noted from
Fig. 10 that there was a significant pressure even in other regions.
Results of the embossing experiments with these foil shapes is
shown in Fig. 11, which shows two large pictures on either side. The
picture on the left corresponds with the straight section foil results,
and the picture on the right corresponds with the curved section foil
results. The entire embossed area is shown in these pictures. Two
boxes were drawn around areas of interest in each picture, which
exhibit the forming characteristics of each foil shape near the sam-
ple center and near the sample end. The top pictures focus on the
end features by highlighting the southern border of the Ohio state
outline. The bottom set of pictures details the end of the straight
portion of the letter “h” in the script Ohio area to exhibit the form-
ing characteristics near the middle of the foil. Images in the middle
two columns of the figure were recorded using a composite micro-
scope technique, whereby three dimensional data was extrapolated
from sets of several two dimensional images taken at different
focus levels. By associating each focused area with a certain depth,
the detailed three dimensional plots were produced for each area
examined. Approximately 50 two dimensional images were com-
bined to make each three dimensional image, with a 25 ␮m depth
of focus difference between each image. Profile curves were also
extracted from each of these three dimensional data sets along the
vertical lines shown in the highest magnified pictures. The graphs
in the center of the figure plot the profiles of each of these curves.
The phenomenon of heterogeneous pressure distribution along the
length of the active region of the foil is evident from these results.
With the curved design foil, the workpiece experienced a very high
pressure 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 the
ends, as the Ohio state outline was only slightly formed into the
sheet. The straight-section foil demonstrated the opposite effect,
as evident from the figure. The Ohio state outline showed much
better forming while the features on in the script Ohio area were
not as sharp.
Another important variable of these forming experiments is
whether or not there was an alternate electrically conductive path
before or after the foil burst. This can be detected by the presence of
a current going in the negative direction even after vaporization of
the foil. Fig. 12 shows each case: (A) when arcing occurred and the
current found an alternate path after foil burst, and (B) when there
was no arcing and the current dropped to zero after the burst. It was
found that the experiments with no current reversals were more
efficient, as evidenced by the final results of the script O shown for
each trial. Straight-section foils were used in these experiments.
This set of experiments depicts the possibility of foil design
and testing with basic perforated plate experiments and then
transitioning into a more practical operation such as forming or
embossing.
3.3. Pertinent issues with application of VFA
One important consideration is the lifetime of the urethane pads
used in these applications. While Thiruvarudchelvan (1993) esti-
mated that a single urethane pad can be used 50,000 times in a
normal stamping process, the method used here is much more
aggressive in terms of the damage inflicted on the urethane pad.
Fig. 13 shows that the side facing the vaporizing foil experienced
significant surface damage after 10 experiments, at which time the
pads were replaced. Some imprints of the die can also be seen on the
side facing the workpiece. However, a transverse cross-section cut
showed that despite the surface damages, the pads sustained very
little through-thicknessdamage. The effect of urethane pad’s super-
ficial damage on efficiency and the quality of formed parts needs
to be studied to understand its reusability based on its appearance.
Such information would support a thorough cost analysis of this
process.
Another issue that should be considered is the lifetime of the
dies used here. During forming experiments, impact velocities as
high as 500 m/s were measured. While high velocity impact can
help achieve close conformity with minute die features, it needs to
be optimized to avoid damage to the die. Shock-resistant tool steels
such as S7-grade can be used for such an application. Although
spot welding was not observed in the cases reported here, it can
occur under a range of conditions of impact angles and velocities
for a given combination of die and workpiece materials. Coating the
impact surface of die with ceramics can also help reduce damage
with welding or wear.
There will likely be instances where the advantages of EHF
make it the method of choice. Shaped polyurethane pads are
not needed and it is often easy to use several electrohydraulic
impulses 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 real
advantages. First, handling water can be difficult to manage if
any leakage occurs, and it must be periodically replaced due to
property changes that occur from it becoming filled with fine

9. A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865–875 873
Fig. 10. Resultant AA 2024-T3 workpieces from perforated plate experiments showing higher pressure toward the ends of the active area of a straight-section foil actuator,
and around the center with a curved-section foil actuator.
sediment from the ablated or vaporized wires or electrodes. Second,
since polyurethane is a solid, it is immune from the cavita-
tion effects that can cause unwanted pressure nonuniformity in
electrohydraulic forming. These heterogeneities can cause dam-
age to workpieces and dies. Third, because the foil generates
pressure over a surface area (instead of a region that approxi-
mates a point), the workpiece can be relatively close to the foil
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 be
controlled much more easily than EHF by changing foil geome-
try.
Fig. 11. Results of the embossing experiments done with straight- (left) and curved-section (right) aluminum foil actuators.

10. 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.
Fig. 13. Condition of the urethane pads after experiments: (a) side facing straight section foil actuator after 10 shots, (b) side facing curved section foil actuator after 10 shots.
The absence of any cracks through the thickness of the sectioned urethane pad should be noted, (c) side facing workpiece after a die cavity forming experiment.

11. A. Vivek et al. / Journal of Materials Processing Technology 214 (2014) 865–875 875
4. Conclusions
• Urethane pad-assisted vaporizing foil actuator has a potential to
be an agile, robust, inexpensive, and efficient tool for impulse-
based forming. It counters the longevity issue presented by
electromagnetic actuators, and this technique can be applied in
a small laboratory or traditional industrial environment, unlike
explosive forming.
• Forming of AA 3003 H14 sheets into perforated plates, using the
urethane pad assisted vaporizing foil actuator, showed that the
magnitude of driving pressure is proportional to the excess elec-
trical energy deposited into the foil before it bursts. Thinner foils
gave diminishing returns on investment in terms of pressure
when subjected to increasing input energy. Thicker foils pro-
duce higher masses of vapor than thinner foils, so the amount of
total heat generated from the subsequent exothermic oxidation
reaction is higher.
• Urethane pad assisted forming of commercially pure titanium
sheets into an embossed cavity was implemented by a single and
a two-step procedure. An 8.0 kJ discharge caused the launch of a
flat workpiece up to a velocity of 270 m/s, but resulted in tear-
ing along the die edge. The impact of the urethane pad launched
by a 6.4 kJ discharge into the vaporizing foil actuator caused a
quasistatically pre-formed workpiece to be accelerated to a peak
velocity of 540 m/s. Using this method, the workpiece formed
into the die without tearing, and picked up all the minute surface
features.
• AA 2024-T3 sheets were embossed into a die with varied features.
It was seen that the workpiece formed better in the center with a
curved-section foil, and better on the ends with a straight-section
foil. Additionally, any arcing reduced the overall efficiency of the
process.
The development of this technique is not intended to replace
the existing high rate forming methods such as EHF, EXF or
EMF; rather, the purpose is to supplement them where their
application is not suitable. Significant work needs to be done
in terms of modeling, parameter optimization, and cost analysis
before this technique can be regularly applied. The parametric
studies and practical implementation of forming and emboss-
ing done in this work are a few steps toward developing a
better understanding of this process. Control of pressure distribu-
tion through foil shape variation and by introducing intentional
defects will be investigated further. Use of pressure sensors
for direct measurement of pressure is also intended for future
work.
Acknowledgements
This material is based upon work supported by Department of
Energy under Award number DE-PI0000012. The authors would
also like to thank the ALCOA foundation, which supported the work
through the Advancing Sustainability Research Initiative.
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