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.

Modern solid-state lasers enable a continuous increase in welding speed due to higher possible laser beam powers. However, welding speeds above 8 m/min for high-alloy steels lead to altered melt flow dynamics and increased spatter formation, resulting in spatter adhesion and particularly in undercuts along the weld seam. In contrast to the cost- or time-intensive methods described in the state of the art to reduce these effects, modifying the intensity distributions appears to be a simple and effective approach that is independent of the welding direction. In particular, using a superimposed intensity allows a detailed investigation by adjusting the intensity with varying spot sizes or laser powers. Using a superimposed intensity to widen up the melt pool is especially effective in reducing the velocity of the melt flow around the keyhole and, consequently, spatter formation. However, its effect on other melt pool flows, especially the upward directed melt flow along the keyhole rear wall, remains largely unknown. To investigate these effects, high-speed synchrotron X-ray imaging in combination with tungsten carbide tracer particles was used to visualize the melt flow dynamics. Generally, two melt vortices are observed in the area behind the keyhole, one in the upper and one in the lower part of the melt pool. The backward melt flow at the melt pool bottom is unaffected by the additional energy input due to the limited penetration depth of the second laser. Furthermore, the upward directed flow along the keyhole rear wall remains unchanged. When increasing the second laser power, the keyhole widens and fluctuates at the rear wall. As a result, the upward melt flow at the rear wall is not discernible. In general, the superimposed intensity alters the melt flow behind the keyhole. However, these changes appear to be minor significant for spatter formation compared to the flow around the keyhole.