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INTRODUCTION
High Velocity Forming:-
High velocity forming can be defined simply as moving a work piece at high rates
(over 200 m/sec) and transforming the associated kinetic energy into plastic
deformation as the work piece contacts the die surface. High velocity forming
only uses a one sided die to produce parts, so the issues associated with
tolerances, machining and alignment as discussed earlier with traditional
stamping [1] are significantly reduced. High velocity forming methods can provide
improved formability, more uniform strain distribution in a single operation, and
lightweight tooling equipment [1]. With more uniform strain distribution, strain
hardened alloys can be more readily shaped with reduced intermediate
annealing operations. As stated previously, there are several methods that can
be considered for high velocity forming; among these are explosive forming,
electromagnetic forming, and electro hydraulic forming [2]. Figure (1) shows the
methods available through high velocity forming [2]. High velocity forming uses
several different methods for exerting force on the sheet stock including;
explosive, electromagnetic and electro-hydraulic. Each only requires a one sided
die, which significantly reduces several of the problems associated with the
traditional stamping method.
Electromagnetic forming EMF:-
The EMF technique was first used in this country in the 1950’s and 60’s, due to
its advantages in enabling the fabrication of many complex geometry parts and
enhancing the formability of low ductility materials. Numerous applications of
EMF have been implemented in industrial production; among the more
spectacular applications are engine nacelles made in a single piece,
electromagnetic riveting guns and hammers (developed by NASA in the mid
1980s) used in the assembly of aircraft skins, and dent pullers1. Recent
advances in electronics and energy storage make EMF technology ripe for mass
Pressure actuator production and plans are well under way for the large scale
manufacturing of fuel cell plates and tubular frames for the automotive industry.
One of the most promising recent applications is the manufacturing of fuel cell
plates (Figures 2 and 3), where conventional stamping methods have failed and
only the EMF technique can deliver the final shape without wrinkling or tearing
deeper channels[3].
Electromagnetic Forming procedure:-
Electromagnetic forming has only been around since the 1960s, but it is the most
common method of high-energy rate forming (HERF). In this process, electrical
energy is converted to mechanical energy with the use of a magnetic field. When
an electrical current is rapidly introduced through a conductor (wire), a magnetic
field is created around the wire. The sudden introduction of a magnetic field
creates eddy currents that flow in opposite direction in any conductor nearby.
The eddy currents develop their own magnetic field and cause a repelling force.
The repelling force is then used as a means of forming sheet metal into different
shapes. Figure (6) a schematic showing an electrical circuit, two magnetic fields,
and a compression coil. When the capacitor is charged by the power supply, the
second switch is closed causing the capacitor to discharge and send a sudden
surge of current through the conductor. This is the process used to create the
first magnetic field. The magnetic field creates eddy currents in the nearby
conductor which creates an opposing magnetic field as well. The part to be
formed is then placed between the two magnetic fields where it will be forced to
take shape due to the repulsive force from the two opposing magnetic fields (see
Figure (7). The two types of coils used in this process are called compression
and expansion coils. These coils are capable of withstanding up to 60,000 psi
and 15,000 psi respectively (see Figure 6). The advantage of electromagnetic
forming is that the magnitude of the fields can be controlled with extreme
accuracy. The process has a high repetition rate with exact consistency. Forming
dies are relatively inexpensive and most applications only require a single die
because the magnetic force replaces the punch portion of a die
The tooling quality is extremely important for this process. Only one side of the
tooling is used to fabricate parts, which causes tooling marks to show up on one
side of the part. When a die is made up of metal, induced current can create
electrical arcing between the die halves. Using dies made from nonconductive
and impact-resistant plastics can eliminate electrical arcing
EMF Advantage:-
As the experience in introducing this method has indicated, the electromagnetic
metal forming has the following advantages compared to other metal forming
techniques:-
1. A significant amount of energy (usually between 5 and 200 kJ,) is stored in a
large capacitor, or bank of capacitors, by charging to a high voltage (usually
between 3,000 and 30,000 volts).[7]
2. Easy to use, the process is easy to implement and require no special operator
skill.
3. Improved strain distribution and repeatability from work piece to work piece.
4. The great technological flexibility of the process. The same inductor can be
used to form the work pieces of different configurations.
5. Simplicity of the technological equipment. Only one die or plunger is used.
Review of New Work In EMF
1- Formability and Damage in Electromagnetically Formed AA5754 and
AA6111.
((J.M. Imbert1, S.L. Winkler1, M.J. Worswick, S.Golovashchenko))/2004
Abstract:
This paper presents the results of experiments carried out to determine the
formability of AA5754 and AA6111 using electromagnetic forming (EMF), and the
effect of the tool/sheet interaction on damage evolution and failure. The
experiments consisted of forming 1mm sheets into conical dies of 40° and 45°
side angle, using a spiral coil. The experiments showed that both alloys could
successfully be formed into the 40° die, with strains above the conventional
forming limit diagram (FLD) of both alloys. Forming into the higher 45° cone
resulted in failure for both materials. Metallographic analysis indicated that
damage is suppressed during the forming process. The failure modes are
different for each material; with the AA5754 parts failing by necking and fracture,
with significant thinning at the fracture tip. The AA6111 exhibited a saw tooth
pattern fractures, a crosshatch pattern of shear bands in the lower half of the part,
and tears in the area close to the tip. Both areas showed evidence of shear
fracture. This experimental study indicates that there is increased formability for
AA5754 and AA6111 when these alloys are formed using EMF.
Experimental Procedure:-
The experiments consisted of forming 1mm sheet into conical dies of 40° and 45°
side angle, using a spiral coil. A Magnepress system with a maximum storage
capacity of 22.5 kJ at 15 kV, capacitance of 200 μF and inductance of 230 nH
was used. The conical cavity dies were made from tool steel hardened to 50 Rc.
A vacuum port was provided to evacuate the air before each part was formed.
Figure (17) shows a schematic of the experimental apparatus. The material was
cut into 165x165 mm (6.5”x 6.5”) squares. The AA5754 was provided with a solid
film lubricant which was removed. No lubrication was used in the experiments.
Circle grids were used to measure the engineering strains. Grids with a nominal
diameter of 2.5 mm were applied using electrochemical etching. The strains were
measured in the rolling direction using a digital grid measurement system.
Result and conclusions:-
Safe parts were produced from both alloys with the 40º cone, with charge
voltages of 8.0 kV for the AA5754 and 9.0 kV for the AA6111. All the parts
formed with the 45º cone failed at charge voltages of 9.0 and 10.0 kV for AA5754
and AA6111, respectively. Figure (18) show AA5754 parts formed with the 400
.
Buckling was observed in all of the formed parts. In the AA5754 samples the
buckling is localized in the area of the vacuum hole (Figure19), whereas for
AA6111 it was more evenly distributed Figure (20).