Microstructure evolution and mechanical property enhancement of friction stir additive manufactured Ti6Al4V alloy

Friction stir additive manufacturing (FSAM) is a promising and cost-effective technology that has found extensive applications in the fabrication of lightweight alloy components, particularly aluminum and magnesium. However, the exploration of FSAM for titanium alloys is still at an early stage. In this study, the microstructure evolution and mechanical properties of the dual-phase Ti6Al4V alloy fabricated by FSAM were investigated. The microstructural characteristics under various process parameters were determined using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The experimental findings revealed that the stir zone (SZ) exhibited a refined and equiaxial grain structure with a gradient microstructure when the tool rotation speed was set at 150 rpm and the travel speed at 50 mm/min, which was attributed to the combined effects of mechanical fragmentation and dynamic recrystallization (DRX) behavior. Moreover, upon employing a tool rotation speed of 300 rpm and a travel speed of 100 mm/min, acicular products were observed in conjunction with lamellar (α+β) structures located at both the top and center regions within the SZ. The microhardness and tensile properties were improved after FSAM treatment (150 rpm-50 mm/min). Furthermore, the dominant mechanisms of microstructure evolution as well as mechanical property enhancement in friction stir additive manufactured (FSAMed) Ti6Al4V were revealed, thereby providing valuable insights for titanium alloy additive manufacturing processes.