Friction process for joining aluminum panels

Coldwater Machine is developing a solid-state, friction-stir process to mimic automotive resistance spot-welding for aluminum components.

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July 15, 2015
By Robert Schoenberger

At first glance, it looks like the typical spot-welding robot that would send sparks flying at any assembly line in the world. The first clue that something’s different comes when the robotic pincers meet. Instead of the typical process of applying pressure, passing an electrical current, and generating those telltale sparks, a high-pitched whine fills the air, like a pneumatic socket wrench pulling lug nuts off of a NASCAR racer.

“Our SpotMeld machine is designed to have the same basic footprint and work envelope as a resistance spot-weld robot. It’s a context that the automotive OEMs are very familiar with,” says Tim McCaughey, president of Coldwater Machine in Coldwater, Ohio. By using a spinning tool, the system needs less energy and may require fewer joints than competing systems for aluminum. Coldwater is working with German welding company Harms & Wend and Germany’s Helmholz Institute, both of which were instrumental in the initial development of this technology.

With the worldwide automotive fleet replacing steel panels with aluminum, manufacturers are investing development dollars on finding more efficient ways to shape and weld the lightweight metal. Ford uses several joining technologies including rivets and adhesives to hold critical components of its F-150 pickup together. General Motors uses similar techniques and others including specialized screw fasteners, aluminum spot welding, and aluminum laser techniques on its upcoming Cadillac CT6 and existing vehicles such as the Chevrolet Corvette.

Those techniques all work, but they fall short of a critical goal, says John Seaman, friction stir welding engineer at EWI, a non-profit manufacturing consulting and research group in Columbus, Ohio. Most joining techniques require new processes and equipment, while most automakers want to stick with time-tested metal-joining methods.

EWI has been working with Coldwater to test SpotMeld, a brand name Coldwater Machine engineers created, because while it acts like a welding process, the machine doesn’t melt metal – solving the cracking problem that aluminum develops when the metal melts in the presence of hydrogen.

Seaman says the new system holds promise both because it solves critical problems that stem from aluminum, and because the machine will fit many existing plant operations. “It’s a technology that’s really just waiting for an industry to adopt it and see the benefits.”


 

Friction-stir welding

The equipment undergoing testing at Coldwater Machine and EWI is a refill friction-stir spot-welder (RFSSW). Similar to a traditional resistance spot-welder, a robotic arm holds two or more metal panels together to create the joint. Instead of resistance welding electrodes locally melting sheets together, a rapidly spinning tool pushes into the metal panels. The friction from the tool spinning against metal generates heat, and the panels soften but stay cooler than the melting point. A pin in the center of the rotating tool then retracts into the machine, pulling softened metal from the metal sheets into the tool. The pin then pushes the displaced material back down, refilling the hole it created.

Linear friction-stir welding

While Coldwater Machine is working on friction-stir technology to replace some automotive spot welding, other researchers are experimenting with spin welding for large linear seams, such as car door panels.

Researchers at General Motors, Alcoa, TWB Co., and the Department of Energy’s Pacific Northwest National Laboratory (PNNL) recently published a technical paper (http://goo.gl/7lqCa9) in The Journal of The Minerals, Metals, & Materials Society detailing new processes to speed up the process of linear friction-stir welding, making it a suitable technology for creating custom stamping blanks for automakers. Linear friction-stir welding uses a rapidly spinning steel tool that pushes into the joint of two metal panels. As with refill friction-stir processes, the spinning tool softens and mixes metal from both panels, creating a weld-like seam.

The problem has been speed. In their paper, researchers note that linear friction-stir welding for aluminum blanks has been limited to 0.5m/min with existing technology, while laser welding typically is faster than 6.0m/min. Though still slower than laser welding, researchers were able to friction- stir-weld different thicknesses of aluminum sheet together at 3m/min.

PNNL researcher Yuri Hovanski says, “The faster the weld, the better the quality and strength of the join, thus the significant increase in speed.”

TWB, a company that makes blanks for most major automakers, has built a 250,000-unit-per-year friction-stir machine at its Monroe, Michigan, plant, built around PNNL’s process.

As with Coldwater Machine’s SpotMeld system, the next step for PNNL researchers is developing processes for linear friction-stir welding of dissimilar metals. www.pnnl.gov

Nate Wenning, engineering manager at Coldwater Machine, says, “You displace material out of a very defined area, you blend that material together, and you insert it back into the hole. It looks like a rivet, but you’re not using a fastener. You’re pulling material off of the panels and inserting it back into the joint.”

James Cruz, engineering manager at EWI says several other toolmakers are developing friction-stir processes for joining aluminum automotive panels. One process inserts a spinning tool into the sheets then turns at an angle to increase the surface area of the joint. Another inserts a spinning probe through the metal sheets, creating a joint at the hole.

According to Cruz, an advantage of the Coldwater system is that the tool doesn’t fully penetrate one of the metal layers, leaving a surface that’s nearly smooth enough to use for vehicle exteriors.

McCaughey says compared to laser welding and other aluminum-joining techniques, SpotMeld is more forgiving, adding, “It’s a much easier process to fixture, and it’s much more tolerant of imperfections. I could join panels covered with lubricant, and I might only have to add one tenth of a second in processing time.”
 

Long-term costs

McCaughey says it’s too early to talk pricing on SpotMeld systems, but they say several developments should keep the machines competitive with other aluminum-joining systems. The company has spent years developing the process to make it reliable and repeatable, and most of the machine’s components are standard products, sourced from major providers, so the equipment has few expensive, custom pieces.

Cruz says it’s important to consider applications. Coldwater Machine’s RFSSW isn’t going to be used for steel components on most cars because resistance spot-welding is fairly inexpensive and very reliable. The first applications will be with aluminum, and most of the metal-joining techniques used for aluminum are expensive.

“The frequency with which you have to change your welding electrodes with resistance spot welding is much higher than you have with steel. When you’re resistance welding, you have a fairly high tip-dress or tip-change frequency, and the welding consumables you’re using are copper, and copper isn’t a cheap material,” Cruz explains.

The head and retracting pin on SpotMeld machines are made of tool steel, so they’re more durable than copper. Part of the testing being done at Coldwater Machine and EWI is determining how many joints a tool can make before operators will need to swap heads. Cruz says the fact that it’s steel means, “there’s a better wear characteristic. Also, with copper, you have an impact of thousands of amps of electricity passing through. That creates a lot more heat, which is another mechanism for wear of the electrodes. You don’t have that in steel tools used in RFSSW.”

McCaughey says Coldwater Machine is working with at least one major automaker to put a SpotMeld machine into a complete, automated joining system in the near future, but he declined to offer a name or a timeline for getting the first unit running.

“We still have to prove that it all works together with the automation and plant equipment. It needs to fit into the total manufacturing process. That’s what we’re developing now,” McCaughey says.


 

Future of SpotMeld

While Coldwater Machine is promoting SpotMeld as a way to join aluminum sheets, the system could be used to join dissimilar metals – steel to aluminum, aluminum to copper, composites to steel, or some other combination.

John Seaman, friction stir welding engineer at EWI, says, when combining dissimilar metals, keeping the temperature low reduces the likelihood of the creation of an intermetallic at the joint, reducing the risk of corrosion where dissimilar metals meet.

Coldwater Machine President Tim McCaughey says the ability to easily join aluminum with steel could simplify the design and build of the multi-metal cars that automakers are developing. Instead of using an almost all-steel frame and almost all-aluminum body, companies could mix and match materials to meet specific goals.

“There are a lot of potential uses for this technology. The main barrier is education. Many in the industry associate friction-stir technology with big parts” such as flat-bed freight trailers, McCaughey says, adding that automakers have used spin-stir processes on big parts for decades. “We need to get out there and explain what we can do with spot-melding techniques.”

 
 

Coldwater Machine
www.coldwatermachine.com

EWI
www.ewi.org


About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or rschoenberger@gie.net.