A Vision of a Future for Metal Forming
Overview
Over the past several years it has become clear that metals can be stretched to much higher strains at high velocity versus conventional quasi-static stretching. We refer to this extended ductility in high velocity conditions as hyperplasticity. We have worked in both understanding formability in a fundamental way and applying this to practical sheet metal forming. One of the most attractive ways of developing high velocity metal forming is with electromagnetic forming. Electromagnetic forming actuators can be fabricated into a wide variety of configurations and used in conjunction with stamping operations. We believe the end result of this vision will be that many sheet metal components will be able to be fabricated with a smaller number of operations (i.e., less expensive) and that it will enable the fabrication of many components from difficult materials such as aluminum. This ultimately leads to lighter weight and more cost effective land and aerospace vehicles.
How does electromagnetic forming work?
Electromagnetic forming is based on something called the Lorentz force. Basically whenever an electrical current is rapidly imposed within an electrical conductor, it will develop a magnetic field. This change in magnetic field will induce eddy currents in any nearby conductor that generally run in a direction opposite to the primary current (like in a transformer). These eddy currents develop their own magnetic field and cause a mutual repulsion between the workpiece and actuator. This technique is quite general and is suitable for any workpiece made from a good conductor provided the current pulse is of a sufficiently high frequency. These basics are well described in Moon’s book.
The spatial distribution of pressure can be controlled by the configuration of the actuator and the overall force magnitude is largely controlled by the discharge energy.
Has electromagnetic metal forming been used before?
There is a long history in the use of both high velocity forming in general (often developed by explosives) and electromagnetic forming in specific. Hundreds of capacitor banks have been manufactured and sold by Maxwell Magneform. These have been used since the 1960’s to fabricate millions of parts without operator injury. The overwhelming majority of operations that have been carried out in the past involve simple axisymmetric compression or expansion. These operations have usually been used as part of an assembly operation. Improvements in formability and suppression of wrinkling have received scant attention in the older literature. Several reviews of research performed in the heyday of high velocity forming have been written [see ASTME, Austin & Davies].
What are the fundamental benefits of high velocity forming?
Formability is improved. We have observed over 100% plane strain elongation in aluminum subjected to a single room temperature high velocity forming event. A full understanding of high velocity formability is still not available, but it appears one primary reason is a resistance to sheet metal necking that is developed by inertia. These issues are discussed at the web site and 1-D FEM partially explains this [see Hu and Daehn].
Impact has benefits. When sheet metal strikes a tool at high velocity large compressive impact stresses are developed. These coin the sheet into the die surface. This can reduce springback, improve surface finish and enhance formability.
Wrinkling is suppressed. When a sheet is launched with a particular velocity profile, each part of the sheet would like to travel along its launch path. Wrinkling usually necessitates a change in direction. Thus at high velocity wrinkling is inhibited my material momentum. As an example, one can reduce the diameter of slender rings with an electromagnetic impulse by a 2:1 ratio or more.
Can electromagnetic forming be carried out with traditional stamping?
Yes. In principle this is straightforward. Electromagnetic actuators can either be pulsed while the tool punch is advancing. Or a single impulse can be delivered at the bottom of the press stroke. Several actuators might be independently controlled in one press stand.
Concept for a press with several electromagnetic actuators
What issues can Electromagnetically Assisted Stamping Address?
Improved Formability — Forming limits can be significantly increased at high velocity.
Improved strain distribution — In addition to improved formability, the freedom afforded by electromagnetically assisted stamping allows modification of strain distributions. For example small pulses can produce strain in an area that might otherwise be locked out by friction.
Reduction in wrinkling — This has several advantages, for example it greatly widens the window of operation for shrink flanging.
Active control of springback — The use of small pulses at the bottom of a press stroke can be used to ‘tune’ springback behavior to accommodate for differences in material.
Distortions at local features are minimized — The high contact pressures and high velocities associated with electromagnetic forming can minimize the distortions that are typical with restrike operations.
Local coining — When a driven sheet hits a die, impact pressures are high enough that coining, embossing and hitting corners to minimize springback are possible.
Has this approach been demonstrated to be effective on automotive parts?
To a limited extent. We have collaborated with the USAMP materials forming team and examined how an electromagnetic forming operation combined with a softened initial shape could enable the fabrication of a one-piece aluminum door inner. A pre-form with softened corners was first formed by Milford Fabricating, then this was electromagnetically re-formed into a GM Cavalier door inner die. The electromagnetic operation took the corner back to its design radius. This part could not be formed by traditional means with aluminum.
A demonstration of the ability to add character lines without distortion and/or wrinkling was also carried out with full-size aluminum hood.
The Electromagnetic re-forming process carried out on the hinge face of an aluminum door inner. The photographs show a softened initial shape that is electromagnetically reformed to very near the original product shape.
Are numerical simulations of electromagnetic forming possible?
There are some special problems related to solving for motion, plasticity and electromagnetic coupling all at once. The Lawrence Livermore National Lab code, CALE has been shown effective for 2-D problems of this type [Fenton]. Doug Everhart’s Multi-Physics Analysis Code (MAC) has also been shown to show considerable promise for 3-D problems. The correspondence between an experiment and a similar MAC simulation of a simple clamped aluminum sheet with an actuator utilizing a primary central current is shown below
Is this technology advanced enough for use today?
Yes and no. Electromagnetic forming is already used extensively for assembly operations. For operations where relatively small amounts of energy are to be used (such as for incremental forming to alter strain distributions, and active springback control), robust systems could be developed fairly easily. In order to do operations like the door inner shown above in a routine, robust manner without extensive trial and error in design further development is needed. Some of the areas that need development are:
Coil design and durability — There is relatively little experience with the kinds of relatively flat coils that might be used in dies. Best construction practices and durability as a function of use parameters need to be established.
Modeling — Further development and validation of simulation tools such as those mentioned above needs to go further. Such codes will be useful in understanding the electrical and mechanical stresses in coils and will allow the design of coils that give pressure distributions that are required to form a given component.
Material Behavior — Both forming limits and flow behavior of materials under the kinds of conditions seen in electromagnetic forming are poorly understood. These are required in the quantitative design of forming processes.
Process design — We need to develop a framework regarding how to decide how to form a part using electromagnetically augmented stamping. What should the stamped shape look like before electromagnetic forming, for example?
Experience — We need to crawl before we run. We will gain new insight into how to use this approach with each study, trial and production system development.
What facilities and activities exist at Ohio State?
In addition to the usual equipment for the examination of materials properties and structure, the Department of Materials Science and Engineering at Ohio State has two Maxwell Magneform Capacitor banks (16 kJ and 48 kJ) as well as a variety of dies, oscilloscopes and high speed video equipment.
Areas of research emphasis include: high velocity and strain rate formability, electromagnetic forming process development, validation of numerical simulations, and education and dissemination of knowledge about the practice and theory of electromagnetic metal forming.
Why should this vision of electromagnetic forming be pursued?
Sheet aluminum offers many powerful advantages to automakers, but it is also notoriously difficult to form. The development of electromagnetically assisted stamping should make aluminum easier and less expensive to form while affording significantly greater flexibility to automotive designers.
Acknowledgements
This work is largely sponsored by the National Science Foundation, Division of Design Manufacture and Industrial Innovation and the Center for Advanced Materials and Manufacturing of Automotive Components (CAMMAC) at Ohio State. The J-Car Door demonstration was financed by the USAMP Materials Forming Team, and Larry DuBois of GM was active in all phases of that work. Also, The students and post-docs in the high velocity foming group are the ones who have actually performed the research. Particularly important with respect to the work described here are, V. J. Vohnout, A. Tamhane, S. Datta, H. Panshikar, G. Fenton, M. Padmanabhan and V.S. Balanethiram.
References
F.C. Moon, Magneto-Solid Mechanics, John Wiley and Sons Inc., 1984.
High Velocity Forming of Metals, Revised Edition, E.L. Bruno, ed., ASTME, 1968
R. Davies and E. R. Austin, Developments in High Speed Metal Forming, Industrial Press, New York, 1970.
X. Hu and G.S. Daehn, Acta Mater., 44, pp 1021-33, 1996.
G. K Fenton and G. S. Daehn, J. Mater. Proc. Tech., 75, 6-16 (1998).
Copyright, Glenn S. Daehn, Department of Materials Science and Engineering, The Ohio State University, 1999. Prepared 10/1999
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