Electromagnetic forming—A review

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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 sufficient 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 and Waniek (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 charge-and-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 μΩ 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 so-called 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 application-oriented 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.

Section snippets

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

Process analysis

For a target-oriented dimensioning of electromagnetic forming processes, knowledge about the relevant process parameters as well as their influences and dependencies is essential and has therefore been a topic of intensive research since the 1960s up to now. Within this work especially the determination of the acting loads and the resulting deformation of the workpiece is an important aspect. The acting loads depend on many different parameters which can be classified as machine parameters,

Material behavior at high strain rates

During electromagnetic forming, high strain rates and elevated temperatures are achieved. Therefore, a physical, coupled understanding of these effects is required to characterize the material behavior for numerical simulations and analytical modeling.

Over the years it has been shown several times that the forming behavior of some materials can significantly differ from the quasistatic forming behavior if high forming velocities are applied. Hu and Daehn (1996) point out that the earliest

Equipment for the electromagnetic sheet metal forming process

As known from conventional forming processes, also in the case of electromagnetic forming the applied equipment significantly influences the forming process and the achievable forming result. Essential components of this equipment are the forming machine (pulsed power generator) and the tool coil including a fieldshaper if applicable, as well as potential form defining tools, an assembly fixture and component transportation devices (Bertholdi and Daube, 1966). However, von Finckenstein (1967)

Applications and application-oriented research work

By Neubauer et al. (1988) the EMF technology can be applied in various different manufacturing processes. Especially in the field of sheet metal processing, applications frequently consider electromagnetic forming in the classical meaning, but it is also possible to perform joining or cutting operations or even a combination of the mentioned applications. A detailed review focusing on the potential of primary shaping by means of powder compaction is given in Mamalis et al. (2004), so that this

Conclusion and future research directions for electromagnetic forming

As this literature review shows electromagnetic forming aroused lively interest in the first years after being invented in the late 1950s. Several publications originate from that timeframe. Important fundamental research work considering the process analysis and the analytical calculation of significant parameters, which is still relevant today, was already published at that time. However, numerous papers are limited to describing the process principle and listing potential process advantages

References (279)

  • S.T.S. Al-Hassani

    Magnetic pressure distributions in sheet metal forming

  • S.T.S. Al-Hassani et al.

    On the parameters of the magnetic forming process

    Journal Mechanical Engineering Science

    (1974)
  • S.T.S. Al-Hassani et al.

    Analysis of electromagnetic forming process

  • Arendes, D., 1999. Direkte Fertigung gerundeter Aluminiumprofile beim Strangpressen. Dr.-Ing.-Dissertation, Universität...
  • Auerswald, W., 1968. Spule zum magnetischen verformen von langen bzw. sperrigen Werkstücken. Patent...
  • G. Babat et al.

    Concentrator of Eddy currents for zonal heating of steel parts

    Journal of Applied Physics

    (1940)
  • Fr.-W. Bach et al.

    Verhalten von Aluminiumwerkstoffen bei der elektromagnetischen Blechumformung

  • M. Badelt et al.

    Process analysis of electromagnetic sheet metal forming by online-measurement and finite element simulation

  • M.A. Bahmani et al.

    3D simulation of magnetic field distribution in electromagnetic forming systems with field-shaper

    Journal of Materials Processing Technology

    (2008)
  • K. Baines et al.

    Electromagnetic metal forming

    Proceedings of the Institution of Mechanical Engineers

    (1965)
  • L.M. Barker et al.

    Laser interferometer for measuring high velocities of any reflecting surface

    Journal of Applied Physics

    (1972)
  • P. Barreiro et al.

    Strength of tubular Joints made by electromagnetic compression at quasistatic and cyclic loading

  • D. Bauer

    Messung der Umformkraft und der Formänderung bei der Hochgeschwindigkeitsumformung rohrförmiger Werkstücke durch magnetische Kräfte

    Bänder Bleche Rohre

    (1965)
  • Bauer, D., 1967. Ein neuartiges Messverfahren zur Bestimmung der Kräfte, Arbeiten, Formänderungen,...
  • D. Bauer

    Zur Physik der Metallumformung mit schnellveränderlichen Magnetfeldern

    ETZ-A

    (1969)
  • D. Bauer
    (1973)
  • D. Bauer

    Elektromagnetisches Umformen: Entwicklungsstand und, Tendenz

    Maschinenmarkt

    (1980)
  • Becker, D., 2009. Strangpressen 3-D-gekrümmter Leichtmetallprofile. Dr.-Ing. Dissertation, Technische Universität...
  • Beerwald, C., 2005. Grundlagen der Prozessauslegung und—gestaltung bei der elektromagnetischen Umformung. Dr.-Ing....
  • C. Beerwald et al.

    Impulse hydroforming method for very thin sheets form metallic or hybrid materials

  • C. Beerwald et al.

    New aspects of electromagnetic forming

  • C. Beerwald et al.

    Einfluss des magnetischen Druckes bei der elektromagnetischen Blechumformung

    2. Kolloquium Elektromagnetische Umformung, Dortmund

    (2003)
  • C. Beerwald et al.

    Aspekte der Prozessführung beim Kalibrieren mittels elektromagnetischer Umformung

    (2001)
  • Beerwald, H., 2003. Mehrwindungsspule zur Erzeugung starker Magnetfeldimpulse. German Patent DE 100 20 708C...
  • Beerwald, M., Henselek, A., 2003. Spiralförmige Spule zur magnetischen Umformung von Blechen. German Patent De 102 07...
  • Beerwald, M., Henselek, A., 2005. Teilbare Einwindungsspule zur Erzeugung starker Magnetfeldimpulse. German Patent De...
  • Bely, I.V., Gorkin, L.D., Dmitriev, V.S., Khvorost, V.J., Khimenko, L.T., Mezhuev, A.T., 1975. Inductor for...
  • Belyy, I.V., Fertik, S.M., Khimenko, L.T., 1977. Spravochnik Po Magnitno-impul’ Snoy Obrabotke Metallov...
  • W. Bertholdi et al.

    Die elektrohydraulische und die elektromagnetische Umformung von Metallen

    Urania—Gesellschaft zur Verbreitung wissenschaftlicher Kenntnisse

    (1966)
  • N. Bessonov et al.

    Numerical simulation of pulsed electromagnetic stamping process

  • D. Birdsall et al.

    Magnetic Forming!

    American Machinist/Metalworking Manufacturing

    (1961)
  • F.W. Boulger et al.

    High Velocity Metal Working Processes Based on Sudden Release of Electrical Energy

    (1960)
  • Bradley, J.R., Schroth, J.G., Daehn, G.S., 2005. Electromagnetic Formation of Fuel Cell Plates. Patent US 2005/02173334...
  • Breitling, J., 1998l. The challenges and Benefits of High-Speed-Blanking. Dr.-Ing. Dissertation, Universität Stuttgart....
  • Brosius, A., 2005. Verfahren zur Ermittlung dehnratenabhängiger Fließkurven mittels elektromagnetischer Rohrumformung...
  • A. Brosius et al.

    Extended finite element modeling of electromagnetic forming

  • Brower, D.F., Hayward, G.B., 1966. Electromagnetic Devices. US patent...
  • Brower, D.F., 1966. Forming Device and Method. US Patent...
  • D.F. Brower
    (1969)
  • W.F. Brown et al.

    Pulsed magnetic welding of breeder reactor fuel pin and closures

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