Stability enhancement mechanisms of keyhole and molten pool in high-power single-mode laser welding of titanium alloy

Single-mode fiber lasers offer superior beam quality and ultra-high energy density, giving them considerable potential to improve welding stability compared to conventional multi-mode lasers. This study aims to elucidate the mechanisms by which single-mode laser welding enhances process stability. Using a combined experimental and numerical approach, the differences in keyhole and molten pool stability between single-mode and multi-mode laser welding of titanium alloy, and the mechanisms behind these differences, are investigated. Welding experiments confirm that single-mode laser welding substantially enhances process stability. Compared to conventional multi-mode welding, the average fluctuation amplitude of the keyhole opening is reduced by 50%, and the fluctuation range of the molten layer thickness on the keyhole front wall decreases by 54%. The superior stability arises from a distinct thermo-mechanical distribution mechanism. Unlike the single-point sliding support mechanism typical of multi-mode welding, the ultra-high energy density of the single-mode laser produces a multi-point overlapping thermo-mechanical support along the keyhole depth. This energy distribution yields two synergistic effects. First, multiple reflections of the ultra-high-energy beam within the deep, narrow keyhole create high-energy multi-point coverage from top to bottom, which reinforces keyhole support and optimizes melt flow. Second, the intensified evaporation driven by the high energy density accelerates keyhole recovery after collapse. The synergy of these mechanisms fundamentally enhances the overall stability of the welding process.