The Electromagnetic Approach to Forming
One of the problems associated with producing aluminum body panels as compared with making the same old typical steel sheet is that there is a tendency for the aluminum to tear or wrinkle during forming operations. Consequently, those panels that are produced out of aluminum are often comparatively soft in shape, avoiding the sharp edges that can be stamped into steel as a matter of course.
Electromagnetism may be the means by which this limitation is overcome. At least the work being performed at Ohio State University by Glenn S. Daehn, a professor of materials science and engineering, and his colleagues have been working on the past few years on methods to use electromagnetism for forming aluminum. Although the process itself is fairly remarkable, its name is, well, rather pedestrian: “electromagnetically assisted stamping” (EMAS).
Although you might immediately think, “Wait a minute, aluminum isn’t magnetic,” Daehn answers that the real issue here is that of a material being electrically conductive. Which aluminum is. So, too, are materials including high-strength steel, which is a material that can also be difficult to form. In the setup for forming, there is an actuator (e.g., a coil of wire) through which a large pulsed current is passed. “The changing magnetic field created by the transient current induces eddy currents in any conductor nearby,” Daehn explains, adding, “These currents have their own magnetic fields. There is a mutual magnetic repulsion between the two sets of currents. This can cause a very rapid motion of the metal.” And if the motion is down into a mold, such as a sharp create or character line across a body panel, then the result is an aluminum panel that otherwise probably couldn’t be produced. In tests at Ohio State, aluminum sheet that could ordinarily be stretched no more than 30% of its length was stretched 100% without tearing. Which means that complex shapes can be produced in aluminum through EMAS.
And it can be done with comparatively simpler tooling than would be necessary with the conventional stamping approach, which could necessitate the use of multiple die sets in order to attain the required form (a form which isn’t as complex as that which can be attained through EMAS).
EMAS is actually something of a hybrid process, inasmuch as the conventional stamping press and tooling are still part of it. Inserts are integrated into the tooling. According to Daehn, design work remains to be done on the configuration of the setup as well as on the strategy used to deploy the electromagnetic force (e.g., there could be a big push of electromagnetism when the ram is at the bottom of the stroke, or there could be a series of small impulses, which is a process that’s called “bump forming,” which Daehn and OSU post-doc researcher Vincent J. Vohnout developed with Ishikawajima-Harima Heavy Industries of Japan). Still, he suggests that the actuators can be easily protected and so it shouldn’t be a troublesome issue. He is particularly bullish on the bump forming approach, about which he notes, “I think lots can be done with bump forming with very little technology development.”
While it might seem that something formed more slowly might resist tearing, Daehn acknowledges that while it isn’t particularly intuitive, there are solid reasons why faster is better. For one thing, when the pulse occurs and launches the metal in a uniform way, the only way for a tear to occur is if there is a local change in velocity. “Inertia resists this—at high speed, things want to keep moving in the direction in which they were launched.
“Second, when a sheet of metal strikes a die surface at high speed, large compressive stresses are developed at impact. The forming process resembles forging more than sheet forming at impact.” Once again, tearing is typically circumvented. “Third,” Daehn concludes, “when we use electromagnetic forming with stamping we can alter the strain distribution in a part very significantly. We can also exploit this to move strain away from regions where a part is likely to tear.”
One of the other beneficial aspects that they’ve discovered with relation to EMAS is that they can do it without utilizing lubricant, so there is an environmental gain.
Although this is something that is going on in a lab at Ohio State, Daehn says that if there was a manufacturing company that was interested in aggressively pursing EMAS, he thinks that it could become commercially viable in about 12 months.
Done In One With Innovative Fineblanking Process
The closer you can get to a finished part in one process, the better off you are from a variety of points of view, ranging from inventory and logistics to capital equipment and manpower requirements. Consequently, for those who make parts with sheet metal, the Forming, Fineblanking and Stamping (FFS) technology developed by Feintool System Parts ought to be of particular interest.
Consider: FFS is high-volume, precision-part production that is, in effect, from coil-to-component in a single press. The Feintool Systems Parts facility in Nashville, TN, for example, has a unique 500-ton hydraulic press that is fitted with multiple tooling modules fed by a CNC part transfer system with the result that all operations, including in-tool deburring, are conducted in a single pass. This setup combines both stamping modules and fine-blanking modules to get the job done—and this single-pass job includes operations that can eliminate the need for secondary operations including bending, milling, grinding, broaching, and drilling.
The types of parts that it can handle include racks, clutch plates, and seat recliner hardware, just to name a few. It can handle ferrous and nonferrous materials up to 5.0-mm thick. The dimensional accuracy that can be attained is +/-0.025 mm, and flatness is held to 0.025 mm/25 mm for parts sized from 150- to 200-mm square. (Fineblanking, if you’re not familiar with it, is a cold extrusion process, not a stamping process per se: the material isn’t ripped.)
An example of the operations-consolidation capability of FFS is an HSLA U-shaped housing for a seat belt retractor. It had been fineblanked, belt sanded, washed and dried, offset bended, and U-bended. Apparently, this multiple-process, multiple-handling process took so long that sometimes there was actually part corrosion before the part was completed.
With the FFS approach and a redesign of some aspects of the part, however, these individual steps are replaced by a clever arrangement of modular tools that complete 15 parts per minute. The tooling includes a deburring operation in which burrs are actually pushed back into the component so that a clean edge is left on the part. While there was originally a need for a semi-piece operation followed by the spin riveting of a reinforcing washer on the side of the housing, the revised design calls for a coining operation that produces a locking profile that performs like a washer without the additional part.
One operation that had been tricky was creating the U-bends because there was a tendency for the width of the bends to be inconsistent, which meant rework. Thanks, in part, to CNC control of the transfer in the tooling, the U-bend outside width is held to 60 mm +/-0.1 mm.