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Welding

Joined at the bit

By Nick Wright

At BYU, friction bit joining fuses steel and aluminum as car companies look on

November 2013 - When it seems every method of joining metal has been invented, another one surfaces that could change the way products are built. Spurred by government automobile fuel efficiency regulations, the School of Technology at Brigham Young University in Provo, Utah, recently developed friction bit joining, which forms a tight bond of aluminum and ultra-high-strength steel. 

Taking pages from the playbooks of welding, riveting and friction stir welding, the bit joining process is similar to friction stir welding, which uses friction-made heat and pressure to whip two aluminum parts together seamlessly, for example. Apple has used friction stir welding to form the case of its newer iMacs. However, friction bit joining can FFJ-1112-webex-welding-image1bond [separate metals] [different types of metal], making it ideal for car companies where lightweighting at the material level is their focus.

Michael Miles, a manufacturing engineering technology professor, and retired professor Ken Kohkonen collaborated with MegaStir Technologies, based in Orem, Utah, a company developing friction stir welding technology.

“Initially we were using friction bit joining to weld ultra-high-strength steel together,” says Miles. “But then thought a better application might be dissimilar metals, like aluminum to steel. And it works so well that we dropped the steel joining and focused on dissimilar metals.”

Unlike the butt joint of friction stir welding, bit joining works with lap joints. Using a standard alloy steel consumable bit that has a cutting tip, rotating at several thousand rpm, the Torx driver pushes the bit into the surface of the aluminum. As the cutting tip removes material, the bit descends through the aluminum and contacts the steel. At that point, the cutting tip gets blunted, which starts to form a friction bond. The whole process takes about 2 to 3 seconds.

The friction bond between the bit and the steel’s backside is the goal, but a flange from the driver side compresses the aluminum against the steel. That is, it’s primarily a metallurgical bond between the tip of the bit and the sheet sheet, but there’s also a mechanical bond as the flange holds the aluminum to the steel’s backside, somewhat like a rivet.

Car challenge

While it can be done, joining dissimilar metals reliably is a challenge engineers are dealing with, especially the auto industry, says Miles. “A lot of this is still in research and development,” he says. Friction stir welding, too, is coming of age. But tool life is a limiting factor because the process is rough on tools. “The interest hasn’t gone away—there’s still high interest in friction stir spot welding for ultra-high-strength steel.”

Other methods, like self-pierce riveting, work for lower strength steels. As long the ductility is good and the material strength isn’t too high, up to 600 to 700 MPa, self-pierce riveting works. Above that, it becomes difficult to use friction bit joining.

“It’s nice because it doesn't require a ductility where you can put light metal on top, then cut through it, and friction bond it to the backside, which is typically a dual-phase steel or hot-stamped boron steel. So you can use friction stir spot welding for ulta-high-strength steel, and friction bit joining for other metals to ultra-high-strength steel.”

FFJ-1112-webex-welding-image2

The processes aren’t quite yet implemented in production settings, but opportunities are ripening. Federal CAFE regulations mandate an equivalent 54.5 mpg fuel economy by 2025 for cars and light duty trucks, with benchmarks for automakers to meet along the way. By 2016—less than three years outthe fleet average should be 34.5 mpg. That timeframe could be a realistic target for friction bit joining to make an impact, says Miles. 

The National Science Foundation is backing the friction bit joining research in conjunction with Oak Ridge National Laboratory in Tennessee as the the “principal investigator,” he says. As automakers shave weight from cars with lighter materials and the friction process matures, welding companies could be chomping at the bit for a technology boom. FFJ

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