Many researchers have studied the process parameters of polymers which welded via FSW, while material parameters are still encountering obstacles, particularly in polymer-based nanocomposites. On the other hand, because of the lower thermal conductivity of polymers in comparison with Al alloys, higher heat input is required to increase joint efficiency where special design tool is attracted much more attention. So, I think both tool design and material properties are the current trends.
Dear , joining of polymer is typical but not impossible you can join the polymer by FSW and new trends is T joint, pipe joint dissimilar and corner joint, etc. , please look into the article,
Article Friction stir welding/processing of polymers and polymer mat...