Synchrotron radiation imaging-based understanding of spatter reduction condition in laser beam keyhole welding of austenitic stainless steels through superimposed intensities

High welding speeds above 8 m/min during laser beam welding of high-alloy steel lead to spatter formation, resulting in material losses and spatter adhesion that significantly degrade seam quality. Superimposing the main laser intensity with a second laser intensity leads to a reduction of such effects, as melt flow conditions around the keyhole are influenced. In this configuration, the area hit from the superimposed laser must be notably larger than the main laser spot, in order to enlarge the melt pool on the top side. However, the basic interactions of keyhole, melt pool, and metal vapor using a superimposed intensity are largely unknown.
In order to identify and understand the related effects with combined laser intensities a welding speed of 12 m/min was chosen, while through high-speed synchrotron radiation imaging visualization of keyhole behavior and melt flow characteristics by tracking tungsten carbide particles was carried out. The superimposed intensity widens the melt pool on the sheet top side, thereby increasing the cross-sectional area of the melt flow, while the keyhole geometry and its fluctuations remain comparable to those observed without superimposed intensity. This reduces the melt́s kinetic energy of the flow around the keyhole and finally spatter formation. Raising power of the superimposed laser intensity, the keyhole width expands over its entire depth, and a bulge on the keyhole rear wall due to vaporizing material is formed. This bulge causes fluctuations of the melt pool and leads to lateral spatter formation due to an upward directed melt flow at the keyhole walls. Therefore, in order to reduce spatter formation, the power of the superimposed intensity has to be chosen consequently in order to enhance melt pool dimensions and avoid bulging.