Effect mechanisms of double-pulsed current on microstructure and mechanical properties of thick-plate narrow-gap MIG welded Ti–6Al–4V

: Narrow-gap welding of thick Ti-6Al-4V titanium alloys remains challenging due to insufficient sidewall fusion, continuous columnar grain growth, and pronounced microstructural heterogeneity, all of which degrade joint performance. This study proposes a double-pulsed metal inert gas (DPMIG) welding strategy to enhance sidewall fusion by directing more heat toward the groove sidewalls, without the need for external magnetic control. Comparative experiments were conducted under similar average heat input conditions between DPMIG and conventional single-pulsed MIG (SPMIG) welding to elucidate the effects of DPMIG on weld formation, microstructural evolution, and mechanical properties. Results show that DPMIG induces more pronounced periodic arc morphological variations and stronger molten pool stirring, promoting molten metal flow toward the groove sidewalls and improving sidewall fusion. The weld width increased by ∼8%, while the heat-affected zone (HAZ) width decreased by ∼43%. DPMIG also significantly refined prior-β grains, reducing their average size by 30.3% and 58.9% in the first and second weld layers, respectively, and facilitated the formation of finer α′/α laths. Additionally, DPMIG weakened α-variant selection and reduced texture intensity, resulting in a more randomized crystallographic orientation. Consequently, DPMIG joints exhibited more uniform hardness distribution and superior tensile properties, with the ultimate tensile strength and elongation of the second-layer specimens reaching 948.8 MPa and 4.30%, respectively. This study provides an effective narrow-gap waveform modulation strategy for thick Ti-6Al-4V weldments, offering a practical approach to improving microstructural homogeneity and mechanical reliability.