Tool wear is a critical challenge in titanium alloy friction stir welding (FSW). Vortex flow-based FSW (VFSW) is a potential solution for this issue because the VFSW process uses a similar material to the base metal as the tool. In this study, TC4 titanium alloy was welded by VFSW for the first time. Parameter optimization of the vortex flow formation is first done, and then the weld formation and the joints’ microstructure and mechanical performance are investigated at different traversing speeds. The results prove that the plunging velocity is the dominant factor in the vortex depth. A lower plunging speed is beneficial for the formation of a deeper vortex. Full penetration is achieved at traversing speeds of 50–120 mm/min at 300 r/min. At welding speed above 130 mm/min, insufficient penetration defects occur. In the heat affected zone, grain coarsening and β-phase volume fraction increase arise, whereas, in the weld nugget, an α+β lamellar structure emerges. Under the optimized parameters, the joint tensile strength is nearly 98% of the base material, but the elongation decreases significantly. The oxides drawn into the weld by the vortex at the junction between the vortex and the base material are the main reason for the small elongation. This study proves that the VFSW process is feasible for titanium alloys.

Friction stir additive manufacturing (FSAM) technology is the ideal technique for aluminum alloy additive manufacturing from the perspective of defect control, microstructure regulation, and performance optimization. However, there is limited systematic fundamental research on the aluminum alloy FSAM. This study implemented a consumable-tool-based 5A06 FSAM process. By incorporating carbon nanotubes during the FSAM process, our research investigated its impact on grain refinement and the performance of the additive structure. The results show that the well-formed additive structure is composed of multiple layers of stirred metal. The microstructure of the additive structure of AA5A06 consists of refined recrystallized grains and deformed grains within each layer, while the interface between layers is composed of a finer-grain band, with an average grain size of 6 µm, whose tensile strength ranges from 225 MPa to 260 MPa, with an elongation of 26% to 32%. After the addition of carbon nanotubes, although the grain size was refined to 2 µm, there was no improvement in tensile strength, and the elongation was reduced. The tensile strength now ranges from 225 MPa to 270 MPa, with elongation between 12% and 16%.

We have recently published a Special Issue of “Correlation between Microstructure and Macromechanical Properties in Additive Manufacturing and Welding”