# Effect of Local Gas Flow in Full Penetration Laser Beam Welding with High Welding Speeds
## 5 Conclusions and Outlook  
In this paper, the effect of local gas flows on spatter formation, melt pool dynamics, and associated loss of mass was investigated for the full penetration laser beam welding of stainless austenitic steel AISI 304 (1.4301, X5 CrNi18 10) at high welding speeds between 8 m/min and 16 m/min. For this purpose, high-speed video recordings, image processing, high-speed Schlieren imaging, as well as computational analysis data were used. For the investigation, a new type of gas nozzle unit was designed, which was used for supplying argon locally by flow rates from 1.6 L/min up to 32 L/min. Based on the characterization of the reference process without gas supply, the effect of an affected pressure balance due to an exerted dynamic pressure was investigated. By a variation of flow rate and welding speed, different process regimes were determined, which are characterized by different seam qualities, respectively, different welding imperfections (defect-free, humping, melt ejection), as well as by a significant variance in loss of mass. By flow rates up to 9.6 L/min, which corresponds to a dynamic pressure of 9.3 kPa, the loss of mass could be reduced by up to 70% by realizing defect-free welds. A melt pool elongation as well as the formation of a reverse melt flow could be overserved by high-speed videography. This can be attributed to the exerted dynamic pressure as well as to the exerted compressive force inside the keyhole. Further, by using the gas flow, the rear-sided melt pool swelling showed less movement, whereby a collapse of the keyhole could be prevented more often. In the case of a keyhole collapse, spatters were detached following the same mechanisms as in the reference process. However, the detached spatters were characterized by a lower kinetic energy, which is why the spatter were mostly re-transported into the melt pool. Based on high-speed schlieren imaging, it was possible to determine a change in flow pattern while applying the gas flow. Following, the flow orientation at the top and bottom-sided field of view changed against welding direction. A rise in particle speed (2.0-2.5 m/s up to 5 m/s) could be proven by image processing while increasing the flow rate up to 9.6 L/min. By analyzing the dimensions of the keyhole, an increase in keyhole length by up to a factor of 1.9 could be found by flow rates of \(>\)8 L/min. Further, a local widening of the weld seam top was determined, which can be attributed to the affected heat balance at the surface area of the melt pool. Despite the spatter reduction, no influence on the frequency of keyhole fluctuation could be found. In contrast, the normalized amplitude of the fluctuation was rising while using the gas flow as well as while increasing the welding speed.  
Further investigations will focus on the importance and the relation of the gas flow affected physical quantities of the additional exerted expansion pressure inside the keyhole, as well as the affected spatter formation mechanism due to the temperature dependence of thermophysical properties, respectively, to the changes in surface tension.  
Conceptualization, L.S. and K.S.; methodology, L.S. and K.S.; formal analysis, L.S. and C.J.; investigation, L.S.; resources, L.S.; data curation, L.S.; writing--original draft preparation, L.S.; visualization, L.S.; supervision and discussion, K.S. and J.P.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.