Development of friction stir spot brazing (FSSB)

As a modified process of friction stir spot welding (FSSW), friction stir spot brazing (FSSB) was developed by (i) using a pinless tool to eliminate keyhole, and (ii) preplacing a braze to extend bond area for one pass, to eliminate hook, and to eliminate the lack of mixing. An unweldable assembly of Cu/Cu by resistance spot welding was FSSBed with a Zn braze foil. At the interface, even in air, excellent wettability could be readily achieved by mechanically disrupting oxide film and dissolution of Cu into molten Zn to form a 2-4 mu m thick Cu-rich interfacial layer containing 40 at.% Zn with the aid of torqueing action of the rotating tool, and no hook was observed. However, solidification crack was present within thick boundary seam with higher Zn content (60 at.%), and shrinkage cavities were present within thin central seam with lower Zn content (40 at.%). By adding an upsetting procedure after wetting reaction, the crack was eliminated and shrinkage cavities were reduced. Thus, the torqueing and upsetting actions of the rotating pinless tool can readily improve joint microstructure and property in early and later stages, respectively, as compared with the case of static and constant pressure. (C) 2016 Elsevier Ltd. All rights reserved.