Friction stir welding of ultrafine grained aluminum alloys: a review

Severe plastic deformation (SPD) has been one of promising routes to fabricate ultrafine-grained (UFG) materials, especially aluminum alloys. However, the SPD products often suffer from their small size. This issue implies the necessity of welding of UFG aluminum alloys for making them usable in complex, large forms in industries. Among various welding processes, those based on solid state welding seem to be more consistent with UFG materials. This is associated with the instability of UFG materials upon intense heating cycle that is common in fusion state welding processes. Friction stir welding (FSW) as a well-known process in the category of solid state welding is widely used for welding of aluminum alloys. This review paper provides an overview of the state-of-the-art of FSW of UFG aluminum alloys. To do so, specific attention is given to microstructural and textural evolutions, effect of secondary particles, and cooling medium. Applying cryogenic cooling medium as well as secondary nanoparticles could inhibit excessive grain growth in the stir zone, which were beneficial to improve the strength of the stir zone without remarkable decrease in the ductility. These processing routes did not affect the main recrystallization mechanism of the stir zone.