Elucidation of melt flows and spatter formation mechanisms during high power laser welding of pure titanium

This study was undertaken to clarify the relationship among the laser-induced plume ejected from the keyhole, the melt flows in the molten pool, and the formation mechanisms of spatters ejected from the molten pool during 10 kW laser welding of a pure titanium plate. High-speed video camera observation results showed that laser-induced plumes occurred at intervals of about 0.5 ms, and that the maximum plume ejection velocity reached 250 m/s. Three-dimensional X-ray transmission in-situ observation of the weld molten pool with tungsten carbide tracers revealed that the melt flowed mainly along the bottom of the molten pool from the keyhole tip to the rear part and then from the rear to the front near the surface of the molten pool at high speeds, while the melt in front of a keyhole flowed upward along the keyhole wall at a velocity of less than 0.6 m/s and then was accelerated to 2.1 m/s at the height of about 2 mm above the keyhole inlet. One-way upward melt flows were continuously piled up at the tip of the elongated melt, resulting in spattering as droplets from the molten pool due to the strong ejection of laser-induced plumes. Spatters were formed from part of a molten metal elongated around the keyhole inlet, and approximately 20 ms was required to form spatters. About 80% of spatters were generated from melts of the front or sides of the keyhole at the speeds of less than 50 mm/s. When the welding speed increased from 50–100 mm/s to 300 mm/s, the ratio and the size of spatters occurring from the rear part of a keyhole increased from 20% to 80% and became smaller than 1 mm.