Interaction between Local Shielding Gas Supply and Laser Spot Size on Spatter Formation in Laser Beam Welding of AISI 304

Background. Spatter formation at melt pool swellings at the keyhole rear wall is a major issue for laser deep penetration welding at speeds beyond 8 m/min. A gas nozzle directed towards the keyhole, that supplies shielding gas locally, is advantageous in reducing spatter formation due to its simple utilization. However, the relationship between local gas flow, laser spot size, and the resulting effects on spatter formation at high welding speeds up to 16 m/min are not yet fully understood. Methods. The high-alloy steel AISI 304 (1.4301/X5CrNi18-10) was welded with laser spot sizes of 300 μm and 600 μm while using a specially designed gas nozzle directed to the keyhole. Constant welding depth was ensured by Optical Coherence Tomography (OCT). Spatter formation was evaluated by precision weighing of samples. Subsequent processing of high-speed images was used to evaluate spatter quantity, size, and velocity. The keyhole oscillation was determined by Fast Fourier Transform (FFT) analysis. Tracking the formation of melt pool swellings at the keyhole rear wall provided information on the upward melt flow velocity. Results. The local gas flow enabled a significant reduction in the number of spatters and loss of mass for both laser spot sizes and indicated an effect on surface tension by shielding the processing zone from the ambient atmosphere. The laser spot size affected the upward melt flow velocity and spatter velocity.