Study on the material flow behavior, microstructure evolution and strengthening mechanism of arc-assisted friction stir weld of Ti6321 alloy

Abstract

In this study, 6 mm thick Ti6321 alloy plates were butt welded using conventional friction stir welding (FSW) and arc-assisted friction stir welding (AAFSW). The defect formation and suppression, microstructure evolution, and plastic deformation and failure mechanisms of the joints were systematically studied. The results showed that a defect-free joint could be achieved with a significant reduction of 50 % in both axial and forward forces for AAFSW. The formation of the bottom defect was attributed to the non-convergence of vertical flow components and the circumferential flow along the pin. The applied arc enhances the material flow along the thickness direction, thereby promoting the transformation of the microstructure at the bottom of the stir zone (SZ) from a bimodal structure to a fully lamellar structure. Texture analysis reveals a shift from B and Y₂ to C₁ components with increased heat input and strain. Plastic deformation in the SZ is primarily dominated by slip, accompanied by twinning deformation in both joints. And the mechanism involving dislocation decomposition and stacking fault formation leading to dynamic phase transformations has been further confirmed. The AAFSW joint exhibits a more uniform hardness distribution and higher joint efficiency (96 %) with an average ultimate tensile strength (UTS) of 771 MPa and an elongation of 7.8 %. The joint failure mechanism is influenced by microhardness and uncoordinated plastic deformation arising from dislocation slip and twinning effects along the thickness. These insights confirm that AAFSW is a new choice for achieving efficient and high-quality connections in medium-thick Ti alloys.