Heat treatment , thermomechanical treatment, burnishing, there are many methods as well as when choosing one you should see at both material FSWed and working characteristics needed.

If you can solved the hooking crack problem in the FSWed materials, you will achieve high tensile strength

As FSW/FSP process possesses capability of producing nano-composite as a result of material flow in the stir zone, the mechanical properties of stir zone can be improved significantly. In fact, if you create a groove at the edge of the sheets, which would be joined together, and put nano-particles into it, these nano-particles can be dispersed during FSW process.