Effects of oscillating molten pool flow on the weld formation and mechanical performance of titanium alloy joints

•Four oscillating laser patterns were employed to join Ti6Al4V plates.•The oscillating laser caused the molten pool to fluctuate in the vertical direction.•The phase difference of oscillating laser spot and molten pool was quantified.•The mechanism of the molten pool flow affecting the weld formation was discussed. An oscillating laser was applied to weld 4 mm-thick Ti6Al4V titanium alloy, and four beam oscillating patterns, namely linear, circular, 8-shaped, and reverse 8-shaped, were studied. In circular and 8-shaped oscillations, the tensile strength was improved by 4 ∼ 8 % compared to the joint without oscillation. The melt flow behaviors under beam oscillating were discussed to explain the differences in mechanical performance. Under linear oscillation, the liquid metal in the molten pool spilled out of the pool due to excessive kinetic energy, resulting in the most severe undercut defect, which was 447 % of the none oscillation. The liquid metals under circular and 8-shaped oscillation were limited to the molten pool range by the vortex of directional motion, and the mass loss was small, so the weld seams were well-formed. The ability of R 8-shaped oscillation to transport molten metal to the front and rear of the laser irradiation range was strong, resulting in the optimal impact and grain refinement effect, which reduced the average grain size by 23.8 % compared to the conventional laser welding.