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abstract
Electromagnetic forming is an impulse or high-speed forming technology using pulsed magnetic field to
apply Lorentz’ forces to workpieces preferably made of a highly electrically conductive material without
mechanical contact and without a working medium. Thus hollow profiles can be compressed or expanded
and flat or three-dimensionally preformed sheet metal can be shaped and joined as well as cutting operations
can be performed. Due to extremely high velocities and strain rates in comparison to conventional
quasistatic processes, forming limits can be extended for several materials. In this article, the state of the
art of electromagnetic forming is reviewed considering:
• basic research work regarding the process principle, significant parameters on the acting loads, the
resulting workpiece deformation, and their interactions, and the energy transfer during the process;
• application-oriented research work and applications in the field of forming, joining, cutting, and process
combinations including electromagnetic forming incorporated into conventional forming technologies.
Moreover, research on the material behavior at the process specific high strain rates and on the equipment
applied for electromagnetic forming is regarded. On the basis of this survey it is described why
electromagnetic forming has not been widely initiated in industrial manufacturing processes up to now.
Fields and topics where further research is required are identified and prospects for future industrial
implementation of the process are given.
Introduction
Electromagnetic forming is an impulse or high-speed forming
technology, which uses pulsed magnetic fields to apply forces to
tubular or sheet metal workpieces, made of a material of high
electrical conductivity. The force application is contact free and
no working medium is required. The principle is based on physical
effects described by Maxwell (1873). Maxwell explained that
a temporarily varying magnetic field induces electrical currents
in nearby conductors and additionally exerts forces (the so-called
Lorentz forces) to these conductors. Northrup (1907) reported
accordingly that “in passing a relatively large alternating current
through an non-electrolytic, liquid conductor contained on
a trough, that the liquid contracted in cross-section and flowed
up hill lengthwise of the trough, climbing up upon the electrodes”
was observed. With increasing current a contraction of
the cross-section and a depression in the liquid was found. The
first one who generated magnetic field strengths which were suf-
ficient to deform solid conductors was Kapitza (1924). Thus, he provided the foundation for the electromagnetic forming process.
However, the earliest work on technologically exploiting this principle
for a target-oriented forming of metals began in the 1950s
with the patent of Harvey and Brower (1958). A more detailed
description including examples of applications is given in Brower
(1969).
Depending on the arrangement and the geometry of the coil
and workpiece, different applications of electromagnetic forming
are achieved: compression and expansion (also called bulging) of
tubular components or hollow profiles as well as forming of initially
flat or three-dimensional preformed sheet metals (see Fig. 1).
According to these three different variants of the process, different
types of coils for the electromagnetic forming process can be distinguished.
During tube compression the coil encloses the workpiece,
while in the setup for the expansion it is the other way around.
According to Belyy et al. (1977) tubes with a diameter in the range
of 3 mm up to 2 m and with thicknesses of up to 5 mm can be processed.
For electromagnetic sheet metal forming flat coils are used.
Here, the area of the formed workpiece can be in the range of 10−4
up to 0.02 m2 and the sheet thickness can be up to 5 mm (Belyy
et al., 1977). However, the charging energy depends on the area
to be formed, so that a machine with higher maximum charging
energy is required if large tubes or sheets shall be processed.
Apart from these three major process variants, which are frequently
discussed in the literature, some special variants are
mentioned in Furth and Waniek (1962). These are electromagnetic
forming with direct electrode contact. While in most cases
a required current in the workpiece is realized via induction, Furth
andWaniek (1962) suggest passing the current directly to the metal
through electrodes. They claim this method to be more efficient
than the conventional procedure and they recommend using electrodes
with flexible extensions in order to prevent sparking or
erosion. A second idea presented in Furth and Waniek (1962) deals
with electromagnetic forming by pulling. While in typical applications
the workpiece is always pushed away from the tool coil, here
a special setup including two different coils is suggested in order
to establish pulling forces, which allows forming bulges on hollow
objects or large sheets, where a force application on the inner or
reverse side is not possible.
Another special process variant is suggested in Brower (1966)
for the first time. In this variant the electromagnetic forces act on
the workpiece via an elastic medium. For this purpose the setup
for electromagnetic sheet metal forming illustrated in Fig. 1 is supplemented
by a pressure concentrator and an elastomeric punch,
which is positioned between the tool coil and workpiece. In contrast
to the more conventional electromagnetic forming variants, this
process is not limited to workpieces made of an electrically conductive
material. In Livshitz et al. (2004) a comparison between direct
electromagnetic forming and electromagnetic forming through an
elastic medium is given. It is pointed out that using the elastic
medium the current oscillation frequency should be lower then
in case of direct electromagnetic forming (a frequency of 5 kHz is
advised, here). Furthermore, information about the suitability of
elastomers of different modulus of elasticity are given. It is said
that an elastomer of higher modulus of elasticity allows using an
open die while in case of an elastomer of lower modulus of elasticity
has to be applied in a closed system in order to achieve good
efficiency.
Bühler and von Finckenstein (1971) claimed the joining of
tubular workpieces to be the most widespread and economically
promising field of application. Bauer (1980) even stated that only
the process variant of the electromagnetic compression has advantages
compared to conventional forming processes at all. However,
according to Beerwald (2005) a kind of renaissance of the electromagnetic
forming can be observed over the last years, which
is related to the increasing trend of implementing lightweight construction concepts especially in the automotive industry. As
recently stated by Schäfer and Pasquale (2010) as well as by Zittel
(2010), at the moment joining operations are still the most relevant
ones, but according to Löschmann et al. (2006), the significance
of the electromagnetic sheet metal forming can be expected to
increase within industry until 2012.
The electromagnetic forming process has several advantages
in comparison to conventional, quasistatic forming processes. The
major ones are summarized in the following:
• Due to the contact-free force application, it is possible to form
covered semi-finished parts without destroying the layer as
stated by Bertholdi and Daube (1966). No mechanical contact
between the tool coil and workpiece exists, so that no impureness
or imprint occurs on the workpiece surface.
• According to Erdösi and Meinel (1984) the process is environmentally
friendly, because no lubricants are used. Additionally,
this results in a simplification of the workpiece processing,
because there is no need to clean the workpiece.
• A high repeatability can be achieved by adjusting the forming
machine once. According to Daube et al. (1966) the adjustment
of the applied forces via the charging energy and the voltage,
respectively is very accurate. Belyy et al. (1977) quantify that the
forming energy can be dosed precisely up to 1%. According to
Bertholdi and Daube (1966) reworking operations are usually not
necessary.
• Joining of dissimilar materials including material combinations
of metals and glass, polymers, composites or different metals is
possible. This is shown in Al-Hassani et al. (1967) on the example
of a metallic cap joined to a glass bottle and in Rafailoff and
Schmidt (1975)for the example of a joint between a metallic tube
and a porcelain component.
• In contrast to the conventional sheet metal forming the electromagnetic
sheet metal forming process uses only one form
defining tool. Hence, the tool costs can be decreased significantly
(Plum, 1988).
• Springback is significantly reduced in comparison to conventional
quasistatic forming operations. This simplifies the die
design significantly.
• According to Saha (2005) high production rates can be achieved.
In the case of manual feeding the production rate is limited by the
time required for loading and unloading of the part. As mentioned
in Brower (1969) production rates of 350–400 parts per hour can be achieved if closing a coil cover directly initiates the chargeand-fire
cycle. In a more recent publication a charging time of
approximately 8 s is reported for modern pulse generators. Belyy
et al. (1977) state that the process can be easily automated and
mechanized and mention an output capacity of 3600 operations
per hour or even more.
• Due to the fact that the magnetic forces penetrate low-conductive
materials like glass, ceramics and polymers, applications within a
vacuum, an inert gas atmosphere or under clean room conditions
are possible as predicated by Belyy et al. (1977) as well as by
Dengler and Glomski (1991). So the forming of sensitive materials
can be realized.
• The process can be operated by remote control and the pulsed
power generator need not physically be in the same room as the
tool coil. According to Zittel (1976) this can be exploited, e.g.
in order to close nuclear fuel waste containers in a radioactive
environment.
• Due to the high workpiece velocities (about 250 m/s) and the
high strain rates in the range of 104 s−1 the mechanical properties
of the workpiece material can be improved compared to
the quasistatic ones. Details about investigations on the material
behavior are presented in Section 4.
• Belyy et al. (1977) also pronounce that the process offers a
high technological flexibility, because the same coil can be used
to form workpieces of different configurations. Moreover, they
claim that it is possible to perform EMF in hard-to-reach areas,
because the coil can be connected to the capacitor by a flexible
bus bar.
Nevertheless, there are some disadvantages of the electromagnetic
forming process:
• The process is most suitable for materials with a high electrical
conductivity and low flow stress. Wilson (1964) as well as
later on Bertholdi and Daube (1966) specified that the maximum
specific resistance should not be lower than 15 cm. This corresponds
to a specific electrical conductivity of about 6.7 MS/m.
In a recent publication, Schäfer and Pasquale (2009) refer to the
conductivity of mild steel, as a limiting value which conforms to
the earlier statements. However, according to Belyy et al. (1977)
lower conductive materials can be formed successfully if EMF
machines with high discharge frequency (60–100 kHz) or a socalled
driver foil is used. They claim that well-annealed copper
is the best material for a driver. However, Dengler and Glomski
(1991) recommend the application of aluminum foil for this purpose.
• Only a small part of the charging energy is used for the plastic
deformation resulting in a comparable bad efficiency (Weimar,
1963). Bertholdi and Daube (1966) found that the ratio of deformation
energy and capacitor charging energy is not higher than
20%. In Bauer (1969) an efficiency of only 2% is reported.
• Significant requirements regarding safety aspects are necessary,
because high currents and high voltages resulting in strong magnetic
fields can occur (Plum, 1988).
• As mentioned already in Boulger and Wagner (1960) the main
limitation for the process is the mechanical and the thermal loading
of the tool coil. Up to now efforts have been made to build coils
which can withstand this load long-term. A promising concept of
a durable flat coil is presented in Golovashchenko et al. (2006a)
and some results of lifetime tests on a realized coil are shown in
Golovashchenko et al. (2006b).
• Belyy et al. (1977) state that it is difficult to realize a deep drawing
state by electromagnetic forming. They explain that in order to
reach this strain state it is necessary to form the workpiece by
various coils which must fit to the shape of the workpiece.
In the following a review about electromagnetic forming is presented.
After a description of the process principle and process
variants mentioned in the literature (see Section 2), information
about basic research considering the process analysis is given in
Section 3. Thereby especially:
• the determination of the transient process parameters, i.e. the
magnetic pressure and the workpiece deformation,
• the interactions in-between these process parameters,
• the energy transfer during the process, and
• the influence of the electrical conductivity.
are considered. Differentiations according to the process variants
are made wherever this was appropriate.
Subsequently, the material behavior at the process specific
high strain rates is regarded in Section 4. Information about the
equipment necessary for electromagnetic forming is summarized
in Section 5. A comprehensive overview regarding applicationoriented
research work as well as some industrial application
examples is presented in Section 6. Thereby, special focus is set
on applications in the field of:
• forming,
• joining,
• cutting, and
• process combinations as well as process chains including electromagnetic
forming operations.
The review is completed by a brief summary and some recommendations
for future work in Section 7. A list of the symbols used
in this article and the according meanings is composed in Table 1.
2. Principle of the electromagnetic forming process
The typical setup of an electromagnetic forming configuration
corresponds to a resonant circuit. The high magnetic fields, which
are necessary to form metals with a high electrical conductivity, are
achieved via a pulse generator. According to Ertelt (1982), the tool
coil-workpiece-unit characterizes a transformer. Equivalent circuit
diagrams of different degrees of simplification have been used in
literature to represent this setup, but with regard to the direct
transferability of the typical components the design suggested by
Bauer (1967)is themost descriptive one. Here, the formingmachine
is represented by a serial circuit consisting of a capacitor C, an
inductance Li as well as a resistor Ri. The tool coil is represented
by its resistance Rcoil and its inductance Lcoil, both connected in
series to the pulse power generator (see Fig. 2).
Within this high speed forming process, pulsed magnetic fields
are used to form metals with a high electrical conductivity. Thereby,
the stored charging energy of a capacitor battery Ec(t), which
according to Eq. (1) can be calculated from the capacity C and the
charging voltage U(t), is suddenly discharged by closing the high
current switch:
EC(t) = 1
2CU(t)
2, (1)
The resulting current I(t), which is a highly damped sinusoidal
oscillation, is determined by the electrical properties of the resonant
circuit. For the calculation the different inductances and
resistances can be summarized and represented by an equivalent
inductance Lrc and an equivalent resistance Rrc. If the capacitance
C, the inductance Lrc and the resistance Rrc of the resonant circuit
are known the differential equation for the description of damped