Enhanced microstructure and toughness in narrow-gap oscillating laser-TIG hybrid welded Ti-6Al-4V plates via high-frequency pulsed current

• NG–OL–TIG with 10 kHz pulsed arc refines HAZ width and β grain size within the HAZ. • Current pulsing over 10 kHz promotes columnar-to-equiaxed grain transition of FZ. • Pulsed current mitigates Al segregation and promotes α′ refinement in FZ. • 10 kHz current pulsing makes low-temperature toughness of FL higher than BM. Welding of thick Ti-6Al-4V plates often produces coarse columnar grains and a wide heat-affected zone (HAZ), degrading joint strength and reliability. Recently, a tailored narrow-gap oscillating laser-TIG hybrid welding (NG-OL-TIG) technique has shown promise for high-performance thick Ti-6Al-4V welds. In this study, 100 Hz and 10 kHz pulsed currents were incorporated into NG-OL-TIG welding for 25 mm-thick Ti-6Al-4V plates to optimize microstructure and mechanical properties. Compared with direct-current (DC) welding, pulsed current improved microstructural uniformity and mechanical performance. At a high-frequency pulsed (HFP) current of 10 kHz, the HAZ width decreased by 26 % and the primary β-grain width by nearly 50 %. In the fusion zone (FZ), the HFP current promoted a columnar-to-equiaxed transition (CET) by intensifying thermal fluctuations, characterized by a temperature gradient of ∼370 K/mm, a cooling rate of ∼300 K/s, and a solidification rate of ∼2.6 mm/s. α’-martensite were refined by about 60 %, maintaining tensile ductility with only 3.7 % hardness increase. The −40 °C impact toughness at the fusion line (FL) reached 30 ± 2 J, exceeding that of the base metal (BM). This study proves HFP’s role in achieving high-quality thick Ti-6Al-4V welds during the NG-OL-TIG process.