Friction stir welding (FSW) is a novel alternative, fusion, welding technique that offers many advantages over current methods including reduced cost, waste, and possible improved performance. This research used direct micro-sample testing, to link local microstructure and localized texture to mechanical properties of FSW titanium with nickel tracking foil. The nickel foil is used to reduce wear on the tool pin, decrease forging force, and to map the complex flow within the stir zone. This was achieved by making direct measurements of the unprocessed base metal and stir zone mechanical properties using micro-scale tensile tests. The local mechanical properties throughout the stir zone, exhibited yield strengths and ultimate tensile strengths within one standard deviation to those of the base metal. This can be attributed to two reasons: High deformation caused by the movement of metal around the tool pin, and high temperatures caused by friction heating of the tool and metal. The strain to failure inside the stir zone showed a fourteen percent decrease than that of the base metal. Scanning electron microscope images were used to visually inspect the fracture surface of the deformed specimens. These observations showed that the material exhibited ductile fracture behavior at the weld center and reduced ductility at the weld interface. These results show that the use of a nickel foil in FSW titanium plates resulted in no reduction in strength while the material became less ductile away from the weld center. These results are important to improving process parameters in FSW as well as providing basic design data. 

This work was funded in part by an Undergraduate Research Award from the UMBC Office of Undergraduate Education and by the Office of Naval Research.