that's only a test piece. What they're looking at is joining 2 large plates, in this case 25mm thick, single-pass, no prep, and high quality weld. Examples would be say 8 x 4 sheets etc. We work on extrusion presses all the time and the process is suitable for long, relatively low area items, such as tubes etc. The Friction-Stir process was invented by The Welding Institute, of Cambridge, UK some years ago. Used by Aerospace manufacturers for butt welding of Aluminium (sorry, US guys) wing panels etc previously rivetted due to weld ripple problems with TIG/MIG processes.

It is since a lot less heat means the metal properties dont change, they use this a lot in rockets. SmarterEveryDay has a video about "how rockets are made" where the advantages are explained if you're interrested.

It actually is stronger, stronger than the base metal itself, as odd as it sounds. The metal isn't actually melted per-se like a conventional arc welder would, the metal goes through what is called plasticization where the immense forces involved smears the metal together under immense pressure to join them. I was a QC inspector in an industrial machine shop some years back that made friction stir welders for Blue Origin, and I spoke quite a bit with the engineer of the project who told me that friction stir welders have changed the game for rocket manufacturing, the weld is actually _stronger_ than the surrounding billet material, he showed me some videos and pictures of him explosively testing the weld strength between two 10' x 3' x 1.5" thick aluminum plates, which were welded on one of the machines we made, they loaded up some obscene amount of C4 along one side of it then blew it up. All they found was a shitload of small aluminum fragments from the plates and a 10' long twisted snake of friction stir weld bead.

@@D-One than i need see it.