Friction stir welding fuses engineering research and Wisconsin industry

U.S. naval ships usually conjure images of aircraft carriers or other large vessels far out to sea. The USS Freedom (LCS 1), however, is able to enter water as shallow as 14 feet. This gives sailors an unprecedented level of access to regions where the U.S. military is present, such as the Persian Gulf.

In addition to its extreme shallow-water abilities, the Freedom, which was built by the Wisconsin shipyard Marinette Marine, is unique in another way: It’s the first naval vessel to substantially include friction stir-welded components. The components, which directly contribute to the Freedom’s stealth and speed capabilities, came from Brookfield, Wis.-based Friction Stir Link Inc. (FSL), which has close ties to the University of Wisconsin–Madison College of Engineering.

Those ties include a research partnership with a team of mechanical engineering faculty and students that is yielding insight into the friction stir-welding process. Those insights have broad applications for transportation and manufacturing companies, both in Wisconsin and beyond.

The connection between UW–Madison and FSL started in the 1970s, when FSL vice president of technology John Hinrichs (who earned a master’s degree in metallurgical engineering from UW–Madison in 1964) worked at A.O. Smith Corp., which had partnered with College of Engineering faculty on robotics-related research. Hinrichs collaborated closely with then-student Neil Duffie, who went on to become a UW–Madison professor of mechanical engineering. In the mid 1990s, one of Duffie’s students, Chris Smith, worked on advanced robotics with Hinrichs and eventually joined A.O. Smith after graduating with a master’s in mechanical engineering in 1995.

Smith is now the FSL vice president of engineering and operations. Shortly after starting at A.O. Smith, he and Hinrichs developed techniques for robots to perform friction stir welding. In 2001, the pair founded FSL, which is the only company in the world that uses standard industrial robots for friction stir welding.

The connection to UW–Madison resurfaced in 2003 when Duffie introduced Smith and Hinrichs to mechanical engineering associate professor Frank Pfefferkorn, who now leads a team including Duffie, mechanical engineering professor Nicola Ferrier and assistant professor Michael Zinn in studying the fundamental science behind friction stir welding and the robotics used in the process. The team collaborates with FSL to better understand the production realities of developing and implementing friction stir-welding technologies.

“It really is a team effort,” Hinrichs says. “It’s exciting to be involved in this.”

Friction stir welding is an alternative to traditional fusion-welding techniques, where metals are heated to their melting point and fuse together as they cool and re-harden. Instead, friction stir welding is a solid-state welding technique, where thousands of pounds of pressure are applied via a robotic tool to “stir” metals close to, but below, their melting point. The pressure and heat create atomic-level contact between the metals, and they form new bonds, i.e. weld, without melting.

“This isn’t new — people have done this since the first ironsmiths folded metal over and over again and hammered it together to make swords,” says Pfefferkorn. “The idea is to apply temperature and pressure to bond the metal together.”

As traditional fusion-welded parts cool, the metals around the weld area contract at different rates, which can cause significant structural distortions. Those distortions have to be straightened, and in addition to the time and cost associated with straightening, fusion-welded parts often have weld beads protruding from the surface that must be ground down.

The lower temperatures in friction stir welding mean smaller temperature differences and thermal stresses occurring as the metals cool. Usually, the result is limited distortions in the surrounding metal, eliminating the need for costly straightening. Additionally, friction stir welds are smooth, meaning no weld beads form.

The lack of distortion makes friction stir welding a valuable cost- and time-saving process for large-structure construction, such as ships, railroad cars and semi-trailer truck beds. Currently, friction stir welding is commercially viable for low melting point metals, such as aluminum and magnesium, and research is ongoing to develop processes to friction stir weld steel and other ferrous alloys.

The UW–Madison team has demonstrated promising new methods to monitor and control friction stir-weld temperature. Mechanical engineering Ph.D. student Axel Fehrenbacher, has developed a wireless data acquisition system to measure the temperature exactly where the stir-weld tool touches the joining materials. Those measurements can be monitored to develop predictive models to control the process temperature.

The UW–Madison team is also working on weld quality. Fehrenbacher and Ph.D. students Edward Cole and Ted Shultz have developed experiments and process models for a weld-process control interface for people and computers. The team is studying how to allow FSL to weld together parts that may not be exactly the same size (or fit-up), which could prevent scrapping many expensive, large components that don’t perfectly match.

Additionally, the team is studying methods to allow robots to perform friction stir welding in situ, meaning on site. Right now, large panels have to be friction stir-welded together by custom-engineered machines at FSL. The panels then are shipped for assembly at, for example, Marinette Marine.

“By working so closely with Friction Stir Link, we’ve learned what the biggest challenges are while using friction stir welding in actual production,” says Pfefferkorn.

Friction stir-welded components are an integral part of the USS Freedom design. Madison native Bruce Halverson is a quality assurance manager at Marinette Marine. “The biggest reason we came to Friction Stir Link was the limited distortion, which was critical for radar signature and weight,” he says.

The smoothness of friction stir welds means the aluminum deckhouse of the almost 400-foot-long Freedom is very flat, which, combined with an angular design, makes it difficult for radar systems to spot. (Radar waves can pick up flat surfaces only within a very narrow range of degrees close to 90 degrees.) This stealth capability is crucial for the Freedom as it navigates a variety of water environments.

Additionally, weight is critical for littoral (shallow-water) combat ships, and the especially thin structure of the Freedom means traditional fusion welding would distort it terribly, says Halverson. “Friction stir welding was a huge advancement in technology that was critical for us to be able to build the littoral combat ship structure.”

The Freedom completed its maiden deployment in April, which, among other missions, involved capturing drug-runner boats carrying several tons of cocaine across the Gulf of Mexico. The Freedom can also be used to quickly deliver humanitarian aid to areas in crisis around the world.

The U.S. Navy is planning to commission more than 50 more littoral combat ships, and Marinette Marine is in the bidding process to be the lead shipyard for the order. If the bid is successful, it could yield a significant number of jobs in Wisconsin over the next couple of decades.

“It’s why I do what I do,” says Pfefferkorn of working on research with real-world applications. “Next to seeing students graduate and be successful, the best thing is to see technology you work on applied in industry, and it’s even better if it’s in a Wisconsin company.

“Innovation is what’s going to create jobs and foster industry, and if we can get companies in Wisconsin to implement these technologies, they’ll have an advantage and be able to do something others can’t,” he says.