The objective of this research is to reveal relationship between melt flows and spatter reduction by angle of incidence and defocusing distance in partial-penetration welding of a SUS304 stainless steel plate with a 6-kW power laser beam. In welding speeds from 50 mm/s to 250 mm/s, underfilled weld beads with spatters were obtained more than 150 mm/s. Accoriding to three-dimensional X-ray transmission in-situ observation of melt flows at 150 mm/s in welding speed with tungsten carbide (WC) tracers, the melt flews achieved approximately 2.3 m/s in speed and made convex molten-pool surface behind a keyhole inlet grow higher, resulting in spattering over 0.1 mm in the diameter. A 2-mm inner defocusing distance or a 20-degrees angle of advance decreased the number of spatter over 0.1 mm in the diameter by half or one third in comparison with that at focal point and zero angles. The X-ray transmission images demonstrate that the appropriate defocusing distance and angle of incidence made the speed of the melt flow decrease and the melt flow behind a keyhole inlet circulate, which led to not only suppressing the convex surface but also improving the frequency that the convex surface went back to the molten pool.
