Fundamentals of melt flow dynamics and spatter formation during laser welding in different regimes of high-alloy steels

Laser welding of high-alloy steels leads to characteristic spatter formation at welding speeds above 8 m min-1 within the Single Wave and Elongated Keyhole regimes, which reduces seam quality due to material loss and adhering spatter. Spatter formation is strongly influenced by the melt flow dynamics, particularly by the local flow velocity and direction. Although current literature provides model-based descriptions of the upward melt flow along the keyhole rear wall and the flow around the keyhole, experimental measurements of these flow velocities are of particular interest. In this study, melt flow directions and velocities are quantified using high-speed synchrotron X-ray imaging combined with tungsten carbide particle tracking. As the welding speed increases and the process shifts from the Rosenthal to the Single Wave regime, the upward melt flow along the keyhole rear wall accelerates, resulting in the detachment of comparatively large spatter from a melt swelling behind the keyhole. At even higher welding speeds within the Elongated Keyhole regime, an enhanced flow around the keyhole promotes the detachment of multiple spatter from a single melt swelling.