This was interesting to see. Particularly the forming.
I feel like when I’ve seen friction welding process instead of slow the rotating part down, the stationary part is either loosened or allowed to spin together once the metals have heated sufficiently to weld. In this case I was thinking one could loosen the tail stock chuck. Or maybe the amount of pressure is higher and it’s kept on as the spinning force is removed so the spinning part slows quickly…I’ve wanted to learn more about the friction forming processes but have never had a chance.