Fundamental mechanisms of spatter formation during full penetration laser welding revealed by high-speed synchrotron X-ray imaging

Abstract Laser beam welding of AISI 304 stainless steel leads to spatter formation as a result of the interaction between the dynamics of keyhole, melt pool, and metal vapor. Spatter formation on the top side for partial penetration welding has already investigated in the state of the art. However, as full penetration welding is used for most industrial applications, spatter formation at the sheet bottom side is particularly critical for these applications. Spatters adhering to the inside of tubes and containers cannot be removed after the welding process and may consequently result in the rejection of the entire component. A description of the fundamental mechanisms resulting in spatter formation on the bottom side is therefore essential for developing effective strategies to reduce it. In this study, high-speed synchrotron X-ray imaging is used to investigate the welding process, providing high spatial and temporal resolution of the detachment of spatters. In addition, this investigation method enables direct visualization of the keyhole, particularly fluctuations of its aperture on the bottom side. Using this method, two distinct process regimes are identified as a function of welding speed during full penetration welding with an open keyhole on the bottom side. At welding speeds below 12 m min -1 , a melt swelling forms behind the keyhole aperture on the bottom side. Spatters detach from this melt swelling and are directed opposite to the welding direction. As welding speed increases, this rear melt swelling disappears and transfers to melt accumulations on the sides of the keyhole aperture with spatter formation in lateral direction. Also, in both regimes spatters continuously detach from the keyhole front on the bottom side. Due to the keyhole aperture, the melt can only partially flow around the keyhole on the bottom side or be ejected out of the melt pool directly at the keyhole front. This promotes spatter detachment directly from the front, like the melt ejection from the front in laser remote fusion cutting. These two different process regimes appear to be primarily the result of an increase in melt flow velocity around the keyhole at the bottom as the welding speed increases. The higher melt flow velocity increases the dynamic pressure within the melt pool, forcing the melt out laterally.