Prehistoric man, accustomed to making fire from striking two rocks together, would have understood the general idea of friction welding. But today’s concept uses friction to make something less than fire—it creates plasticity in metals, but keeps them several hundred degrees under their melting temperatures, then forges them together.
As a result, unlike other welding processes, friction welding retains the workpieces’ base metal properties. The process can join metals unweldable by other means and is controllable and repeatable. Friction welding, a solid-state process, includes several variations—some well-established and others developed more recently. All require machine tools, usually automated, instead of manual processes, and capital investment is higher than conventional welding.
One of the three types of friction welding is friction stir welding (the others are rotary friction welding and linear stir welding). Friction stir welding (FSW) was invented in 1991 by Wayne Thomas at The Welding Institute (Cambridge, UK) and produces high-quality welds. In FSW, a rotating, nonconsumable tool spins to create friction with stationary parts. The patents covering the basic process and methods of enhancing material flow recently expired, making it less costly to use FSW.
In addition to temperature control, tool material choice for FSW is critical. For example, 7000 series aluminum can be FSWed with pin tools made from H13 tool steel or MP159, a cobalt-based tool steel, because the temperature needed to make the material flow is not excessively high, according to Russell Steel, director of business development, MegaStir Technologies LLC (Provo, UT), a unit of oil and gas services firm Schlumberger. However, steels, stainless and nickel-based alloys begin to flow at higher temperatures, creating wear issues with tool steels, and may require polycrystalline cubic boron nitride (PCBN) tools, which even at 1200°C are stable and with a 3600 Vickers hardness.
Orvilon, a fabrication unit of Holtec International (Jupiter, FL), used FSW to join Metamic-HT, a nanoparticle reinforced metal matrix composite that serves both as the neutron absorber and as the structural material of the fuel baskets in Holtec’s canisters and transportation casks, which are used to store spent nuclear fuel. The process replaced a metal inert gas (MIG) welding process. Using a PCBN tool from MegaStir, Orvilon welded the honeycomb structure, which is 14′ (4.3-m) tall and 16′ (4.9 m) in diameter.
Another promising application is pipeline welding; FSW requires just 10% of the manpower used for conventional welding, according to Steel. However, few industrial codes, such as the API 1104 specification for girth welds on pipelines, cover the process, so suppliers and oil and gas companies are working on FSW specs. While the cost/benefit ratio for pipeline welding is low for FSW in flat, easily accessible areas, it increases considerably for hard-to-access regions such as parts of Brazil or the Arctic, he added. And, since it requires much less equipment than conventional welding, FSW has less impact on the pipeline right-of-way—an important consideration in environmentally sensitive areas.
With multiple new applications, interest in friction welding is growing. And, as applications change, its advantages become clearer. As Steel of MegaStir said, “The easy oil is gone. Drillers are having to go deeper and hotter, and with the materials they are using, conventional welding can’t get the job done. Friction welding can.”
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