PNNL

A spinning tool plunges into two pieces of metal, rotating at a high rate of speed. As the tool begins to move, it softens and mixes the metal, creating a powerful weld—one that can securely join similar and dissimilar materials (particularly metals and alloys) without rivets, fasteners, or adhesive. This advanced manufacturing technique—called friction stir welding—requires only a fraction of the energy required by conventional techniques, but isn’t used on many assembly lines today.

Why? Because friction stir welding exerts tremendous force (up to 5,000 pounds) and something needs to capture that force. Currently, the process requires a rigid, perfectly shaped anvil underneath the material being welded. For many assembly lines, that requirement is tough to meet.

“When the friction stir tool heats the material, it exerts a massive amount of force that we need to constrain to ensure a precise and secure weld,” said Mitch Blocher, a mechanical engineer at the U.S. Department of Energy’s (DOE’s) Pacific Northwest National Laboratory (PNNL). “For basically the entire history of friction stir, the way you do that is by putting a rigid anvil underneath the material.”

Now, a breakthrough at PNNL could free friction stir from those constraints—and open the door for increased use of the advanced manufacturing technique on commercial assembly lines.

Here’s the rub

Currently, friction stir welding is not widely applicable on most assembly lines.

“There is some friction stir welding that’s done in vehicle manufacturing,” said Piyush Upadhyay, senior materials scientist at PNNL. “But typically, it’s limited to two flat sheets welded on top of a rigid anvil.” 

Case in point: ten years ago, PNNL worked with several companies—including General Motors—to apply friction stir welding in the production of car doors. The process involved welding flat sheets before stamping them into the 3-D shape of a car door.

That approach doesn’t work for larger, more complex car parts that can’t simply be stamped into shape: for instance, roof rails and the metal frames that surround the doors. 

“If you want to friction stir weld anything that isn’t flat, you’re going to need an anvil in the shape of that part,” Upadhyay explained. “If you’re welding a roof rail, you’ll need a roof rail-shaped anvil. For a real-world assembly line, that’s too cumbersome.”

Many components manufactured for vehicles still rely on spot welding and adhesives for joining. The PNNL team knew that if they could design a new, more maneuverable fixturing system for friction stir welding, manufacturers could produce those same components with lighter materials, stronger welds, and lower energy costs. 

Self-fixturing friction stir

Enter the new method: self-fixturing friction stir welding. 

“We started by saying: ‘alright, let’s get rid of the anvil,’” Blocher said. “Of course, it wasn’t that simple.”

Friction stir tools have been attached to robotic arms in the past, but they always required a separate anvil. Self-fixturing friction stir, however, uses an attachment for a robotic arm that includes both the friction stir tool and a miniature backing plate. If the old approach was an arm holding a pencil, the new approach is an arm holding both a pencil and a clipboard. 

The new attachment essentially pinches the target material between the friction stir tool and the backing plate, exerting the necessary force and eliminating the need for a separate, custom-shaped anvil.

The goal: freely moving, maneuverable friction stir welding that is deployable on the robotic arms used on typical commercial assembly lines.

However, there’s still the issue of the thousands of pounds of force exerted by the friction stir tool. Because self-fixturing friction stir uses a built-in backing plate, rather than an anvil, the system must not only exert, but also withstand, that force. 

There’s just one problem: most assembly lines don’t employ welding robots that are strong enough to handle that.

“Most of the welding in vehicle manufacturing requires very minimal force, since the material is melted in the process. Friction stir doesn’t melt the material, so pushing into and across the material requires a significant amount of force,” Upadhyay said. 

The PNNL team is in the process of adding another capability to its self-fixturing friction stir tooling: a hydraulic system that powers the attachment and creates a closed loop for the force it generates. Currently, the hydraulic system can capture the force from the tool pressing and/or tilting. The researchers are now developing new mechanisms to capture additional degrees of movement and developing a system that allows the attachment to pull material into the tool.

“Once this is perfected, there will be no fixturing, no anvil, and no force transmitted into the assembly line,” Blocher said. “The only job of the robot will be to hold the friction stir attachment in place and to maintain the correct position.”

After that, the researchers will package self-fixturing friction stir into a more ergonomic, “industry-hardened” form so that the technology can be applied on real-world assembly lines.

This work was supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy’s Vehicle Technologies Office.