Droplet transfer behavior and its regulation mechanism of high-pressure laser welding with filler wire of Ti6Al4V titanium Alloy

Under high-pressure conditions, the weld forming quality of Ti6Al4V (TC4) titanium alloy in laser welding with filler wire (LWF) deteriorates significantly. As ambient pressure increases from 5 kPa to 700 kPa, weld forming continuity drops by around 70%. High-speed imaging was employed to observe droplet transfer and plasma characteristics, and a computational fluid dynamics (CFD) numerical model applicable to a wide range from reduced pressure to high pressure was established for LWF. The predicted weld surface morphologies at 5 kPa, atmospheric pressure and 700 kPa agree well with experimental results. The thermomechanical mechanism governing droplet transfer under different pressure conditions was elucidated. With rising pressure, the high-temperature zone shifts from the molten pool leading edge to the droplet; metal vapor recoil pressure and molten pool length both decrease, and the transfer mode changes from small droplet combined liquid-bridge transfer to large droplet transfer. Relative to atmospheric pressure, 700 kPa ambient pressure extends the droplet transfer period by 8.9-fold and raises its critical transfer size by 2.3-fold, which stems from the enhanced droplet detachment resistance at high pressure. A method reducing wire-to-plate spacing is proposed to lower gas pressure resistance. It makes surface tension dominate droplet transfer, realizing a full liquid-bridge transfer mode at 700 kPa. Accordingly, weld forming continuity rises from 32% to 100%, and the overall welding process stability is greatly enhanced.