Ultrasonic-magnetic coaxial hybrid GTAW of Ti-6Al-4 V: Shielding gas flow evolution and joint strength-ductility synergy

This work introduced a novel non-contact ultrasonic-magnetic coaxial hybrid GTAW (U-M-GTAW) process to resolve the inherent trade-off between strength and ductility in the Ti-6Al-4 V joints. A full factorial design was used to systematically investigate the effects of ultrasonic power (900–1200 W) and magnetic field current (3–5 A) on the Ti-6Al-4 V joints. Both multi-physics simulations and experiments reveal that high-power ultrasonic (≥1050 W) excitation induces intense acoustic streaming, triggering laminar flow instability in the shielding gas and forming large-scale asymmetric vortices that ultimately lead to severe environmental oxygen ingress. Oxygen ingress causes severe oxidation of the joint, leading to a sharp decline in mechanical properties. Conversely, the introduction of a 5 A axial magnetic field enables the Lorentz force to substantially bolster the fluid stiffness of the plasma arc, successfully reconstituting a perturbation-resistant shielding gas flow. Under the optimal parameters of 1200 W and 5 A, the synergistic effect of the re-established shielding gas flow and the ultrasonic-magnetic fields achieves a 50.0% increase in the aspect ratio while thoroughly dismantling variant growth heredity, promoting highly uniform nucleation of the α phase. This orientation-randomized microstructural topology effectively dissipates localized stress concentrations, elevating the ultimate tensile strength of the joint to 1137.4 ± 7.5 MPa and achieving an elongation of 10.2 ± 0.3%. By bridging macroscopic hydrodynamics, microstructure, and mechanical properties, this work advances the fundamental understanding of multi-physics hybrid processes in welding and additive manufacturing.