Metal stamping and electromagnetic forming: New process improves material formability, reduces wrinkling

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Research done at The Ohio State University during the past several years has shown that when material is stretched at high speed, it is less apt to tear than if it is stretched slowly. This is because the action of the material tearing causes a change of direction. If this is done very quickly (as happens at high speeds), inertia and acceleration resist this change in velocity. Glenn S. Daehn, Ph.D. and professor in the Department of Materials Science and Engineering at Ohio State, refers to this extended ductility in high-velocity conditions as hyperplasticity.

Working with manufacturers and industrial partners, Daehn's group has developed a number of strategies that can be used to form aluminum components in a cost-effective manner, illustrating that electromagnetic forming (EMF) offers elegant approaches to forming complex components and can improve forming limits in metals.

How EMF Works

High-velocity sheet metal forming can improve material formability (the amount of stretch available without tearing), and wrinkling can be greatly suppressed. "EMF is a very convenient way of flexibly producing high-velocity deformation," says Daehn.

In EMF, when an electrical current is imposed rapidly within an electrical conductor, it develops a magnetic field. "This magnetic field induces eddy currents in any nearby conductor, and the eddy currents develop their own magnetic field and cause a mutual repulsion between the workpiece and the actuator," says Daehn.

Daehn describes the technique as suitable for any workpiece made from a good conductor, provided the current pulse is of a sufficiently high frequency.

This technique's creation is the result of "a lot of serendipity," says Daehn. "About 12 years ago, a company approached The Ohio State University about using electrohydraulic forming for assembly of ventilation components. The equipment was left behind after the project. With my background in hydroplastics, I had an interest in what could be done with it."

He continues, "Since the mid-1970s, there was very little dedicated work in this area. While there still is some explosive forming practiced today, and EMF is used as an assembly technique, the big vision largely was lost by the technical community."

Daehn and his colleagues envision augmenting traditional metal forming processes with high-rate or high-velocity processes and think it will change fundamentally the way sheet metal is formed.

All sheet metal analysis assumes the material is moving very slowly with no inertia. This technique provides an advantageous way to use inertia by adding new degrees of freedom and making parts that are otherwise impossible to make.

With increasing demand for fuel-efficient cars, automakers continue to explore greater fuel efficiency. Weight reduction is by far the most effective method, yet only two major methods exist to reduce auto body weight: optimize the design or use a lightweight material.

Aluminum offers a unique combination of properties, including light mass, with some alloys stronger than structural steel, according to research from Professor Taylan Altan's Engineering Research Center for Net Shape Manufacturing, The Ohio State University.

Honda Research & Development in Tochigi, Japan, stamps aluminum parts for the Acura NSX. "We were impressed by the ease with which it can be shaped using a wide variety of forming methods," says Kazuhiko Tsunoda, chief engineer for aluminum body design.

Aluminum die-cast materials allow complex shapes to be formed out of a single piece, helping to connect extruded parts more rigidly using fewer parts. A rigid body thus can be created using aluminum die-cast materials for connecting elements between structural components. By taking advantage of aluminum's characteristics in this way, increased rigidity and passenger protection can be achieved without sacrificing lightness.

"An aluminum body still is expensive, though its price has decreased greatly since the time we introduced the NSX to the market," says Tsunoda. "We will consider the application of aluminum to future models if the cost of doing so is commensurate with their concept."

Metal Stamping Using EMF

Benefits of metal stamping with EMF include:

1. Reduced number of operations.
2. Improved formability.
3. Improved strain distribution.
4. Less wrinkling.
5. Controlled springback.
6. Less reliance on lubricants.
7. No hazardous chemicals for cleanup.
8. No electromagnetic emissions.
9. Lower energy cost.

"While this is not a new process, some of Dr. Daehn's work is really state-of-the-art," says Michael Plum, CEO of Maxwell Magneform in San Diego. "In conventional dies, you can form compound radii in a steel sheet. If you make the same sheet out of aluminum, you won't have elongation and stretch.

"What this technique brings to the process is the ability for material to achieve some of these contours. At high rates of speed, the material becomes more ductile, creating more elongation."

Maxwell Magneform's equipment is used to make electric intake fuel pumps and fuel rails at Delphi Automotive. "Most recently, we've seen a lot of activity in electric motors and refrigeration. It's also popular for medical devices, because the equipment is noncontact, very clean, and requires no lubrication," says Plum.

Using EMF on Aluminum

Using EMF on aluminum offers some benefits:

1. Materials can be moved in ways that can't be moved otherwise as a result of ductility.
2. Noncontact application of magnetic pressure moves materials without touching them.
3. Operators do not need special training to operate the machinery.

"Maintenance people do need training, though, and we almost insist that a company send a person here to be trained before we let the machine go out to the factory floor," says Plum.

"If two methods are comparable, the primary reason a company would choose this process is repeatability. Because it is electrical and noncontact, nothing wears. Whether you make one piece or a million pieces, they all will look the same," says Plum.

IAP Research, Inc., in Dayton, Ohio, is one of the pioneers in the development of magnetic forming processes for industrial use. IAP has established a Magnetic Pressing Technology Center (MPTC) at its headquarters to help manufacturers to develop component applications using magnetic pressing technologies, such as dynamic magnetic powder compaction (DMPC), EMF, and magnetic pulse welding (MPW) processes.

"Most companies come to the center because they have a problem they can't resolve," says Duane Newman, program manager EMF at IAP. "Some problems are amenable to magnetic solutions, but almost all require some assessment."

"What we try to walk away with in these early efforts is determining whether the project is feasible and economical, and we try to determine requirements for a production system based on studies," says John Barber, IAP president.

"We agree with Dr. Daehn that metal forming was underutilized," says Newman. "From our perspective, we now can do things that were unthinkable in the 1960s. We can move a lot of metal."

Looking Ahead

Perhaps the only problem is that when people see metal stamping and EMF, they think it can solve every problem, but it can't. "We are limited in terms of the kind of materials we can form — only those that have electrical conductivity," says Plum.

With additional development, high-velocity EMF can address significant problems that now exist in forming aluminum and other materials with limited formability. "With a combination of insight and engineering, someday this will become a standard technique in metal forming," says Daehn. "I'd like to see it in my lifetime."