Tailoring microstructure and deformation behavior of Ti-6Al-4V through boron-assisted fast-frequency double-pulse wire arc additive manufacturing

The mechanical performance of wire arc additively manufactured (WAAM) Ti-6Al-4V alloys is limited by coarse prior β grains, strong texture, and microstructural heterogeneity. Here, boron was introduced into a fast-frequency double-pulse WAAM (FFDP-WAAM) process to regulate the microstructure and mechanical properties of Ti-6Al-4V deposits. The effects of boron addition levels and low-frequency pulse parameters on microstructure evolution and mechanical properties were systematically investigated. Boron addition combined with FFDP-WAAM reduced the average prior β grain size to 130.8 μm at 0.1 wt% B. A 10 Hz low-frequency pulse promoted constitutional and thermal undercooling as well as melt-pool stirring, facilitating grain refinement and homogeneous TiB whisker distribution. Boron also refined the α phase, decreased its aspect ratio to 2.3, and weakened α-variant texture by reducing the maximum pole figure intensity to 15.6. The optimized deposit exhibited a microhardness of 377.1 Hv, an ultimate tensile strength of 983.6 MPa, and an elongation of 8.1%. Grain refinement and uniform TiB dispersion increased interfacial density and hindered dislocation motion. Neutron diffraction and TEM analyses revealed progressive dislocation accumulation during deformation, dominated by -type dislocations on the basal slip system and -type dislocations on the pyramidal slip system. These findings establish the microstructure–property relationship of boron-modified FFDP-WAAM Ti-6Al-4V alloys and provide an effective strategy for achieving high strength and ductility.