For veteran aircraft technicians, the very idea of friction stir welding can be disturbing. “How can you weld two pieces of metal together without using fusion or filler material to bind them together?” they might ask. “Wouldn’t it be wiser to do things the old way, or perhaps just go back to rivets? And how the heck are we supposed to fix a friction stir welded skin, anyway? By not using fusion or filler materials again?”
To the concerned, AMT says relax: Once you understand the mechanics behind friction stir welding, the process makes sense. Meanwhile, repairing damaged aircraft skins that have been friction stir welded is a no-brainer, since rivets and metal patches are used to make such repairs.
Friction stir welding explained
Eclipse Aviation (www.eclipseaviation.com) is currently using friction stir welding to assemble its Eclipse 500 VLJ (Very Light Jet). “Compared to conventional welding and riveting, robotic friction stir welding is faster yet just as good,” says Brent Christner, Eclipse’s manager of Materials and Processes Engineering. “In fact, friction stir welding is five times faster than automated riveting, and faster still than manual riveting or welding.” The secret lies in the process: Friction stir welding robots are connected directly to the CAD/CAM files for the aircraft being assembled. In this way, manufacturers can ensure precise manufacturing directly from plans; speeding up production while minimizing error.
But how can two pieces of metal be welded together without using a third fusion or filler material? Again, the answer lies in the process. In friction stir welding, the two pieces of aluminum are mated together, then heated to the point of plasticity (800 F), but not liquefied (1,200 F). At the same time, pressure is applied to the mated metal pieces by an automated rotating tool. Traveling across the seam, this tool’s pressure effectively causes the two pieces’ metal edges to intermingle on a molecular level, thus binding them together.
To explain this metaphorically, the combination of heat and pressure causes the metals’ edges to form lots of little pits and hooks that can interlock; just like a piece of Velcro. This is why a friction stir weld is so strong, and why it does not require the use of fusion or filler material. “To enhance the seam, we include corrosion-resistant material in the weld,” Christner says. “The result is a seam that is stronger than rivets, and less prone to the kind of metal fatigue cracking common in older riveted skins.”
Implications for aircraft maintenance
In an industry where composite materials have substantially changed the task of aircraft maintenance and repair, the advent of friction stir welding is yet another change for aircraft technicians to keep up with. Thankfully, however, friction stir welded skins do not represent the same degree of difficulty as composites. In fact, in a strange way, friction stir welding can be said to be ‘neutral’ when it comes to its impact on aircraft repair techniques.
The reason is that friction stir welds are not something that can be duplicated in the repair shop. It’s not just the requirement for computer-controlled robotic welders that makes the difference; it’s also the fact that this process is really a form of manufacturing rather than assembly. Think of it this way: A friction stir welded skin is akin to plastic injection molding, in that both create something new that is not designed to be taken apart later. In contrast, rivets can be removed after the fact and replaced, as can screws and bolts.
What this means is that repairing a damaged piece of aircraft skin that has been friction stir welded cannot be done by using the same approach. Instead, more conventional techniques such as patching and riveting are called for; just as if the skin had originally been assembled in this way.