Investigation on keyhole stability and surface quality in laser welding of TC4 titanium alloy with adjustable ring-mode (ARM) laser

The paper aims to contrastively study the keyhole stability, spatter and surface quality in laser welding of TC4 titanium alloy with different adjustable ring-modes. The high-speed camera and "sandwich" observational structure were used to monitor the dynamic changes of the molten pool and keyhole during the welding process. Spatter number, inlet area of keyhole and the geometric parameters of the longitudinal section of the keyhole were quantified by using image measurement and statistical software to analyze the stability of the keyhole and the weld seam surface quality in different modes. The experimental results show that the hollow and spatters of the weld formed in the pure center laser mode are caused by the fluctuations of keyhole, and the central bulge forms at the pure ring laser mode for the low central energy density, while at the dual-mode shapes the ideal welding surface quality. The keyhole entrance formed in the dual-mode welding is like the crescent shape, with the largest width and the most stable morphology in the three different welding modes with the same power of 2000 W. Effective coordination of the core laser and the outer ring laser in double mode, and the poor thermal conductivity of titanium alloy caused the temperature gradient and surface tension gradient to decline of the molten pool, which effectively suppressed the spatter of the welding process and ensured the surface quality of the weld seam.