# Effect of Local Gas Flow in Full Penetration Laser Beam Welding with High Welding Speeds
## 3 Results  
### Reference Process  
In the first step, the welding process was characterized without gas supply (reference process). For this purpose, the resulting loss of mass was measured and analyzed by means of high-speed videography (Figure 2).  
Figure 1: **(a)** Schematic experimental setup; **(b)** Helium flow pattern visualized by schlieren imaging at plane-parallel camera position (0°); **(c)** Specs of used gas nozzle unit.  
Figure 2: Loss of mass and melt pool dynamics of the reference process as function of welding speed.  
At a welding speed of 8 m/min, the lowest loss of mass of 0.56 mg/mm \(\pm\) 0.01 mg/mm could be determined. Based on the high-speed recordings, a stable and slightly fluctuating keyhole was observed. The aperture of the keyhole formed circularly and was surrounded by a bead of melt. The rear side of the keyhole was characterized by the formation of a small melt pool swelling. However, this melt accumulation does not contribute to spatter formation. Instead, the spatter separation takes place inside the keyhole. Subsequently, the spatters were already detached from the melt pool within the keyhole wall. This resulted in the formation of small spatters, which were detached from the keyhole in undirected trajectories. In literature, this process regime is well known as \"Rosenthal regime\" [4]. High-speed recordings of the bottom side showed similar melt pool dynamics as well as a similar spatter formation mechanism as seen on the top side before. The weld seam on the top side was characterized by small adhering spatters, while the bottom side showed no adhering spatter. By increasing the welding speed to 10 m/min and 12 m/min, the melt pool dynamics changed. Following, the keyhole was fluctuating stronger, while a melt pool swelling was formed at the upper side of the keyhole rear wall. Due to movements of this melt pool swelling, the keyhole was temporarily constricted. Based on the change in outgassing conditions, the pressure inside the keyhole was increasing. By exceeding a critical threshold (Equation (2)), the keyhole was collapsing, and spatter formation occurred. The separated spatter was specified by larger dimensions and directed against the welding direction. Based on the state of art, these process characteristics can be related to the \"single-wave regime\" [4]. The changed process characteristics had also an effect on the resulting loss of mass. While the loss of mass at a welding speed of 10 m/min was only slight affected by an increase of 9% (0.60 mg/mm \(\pm\) 0.02 mg/mm), the loss of mass was abruptly rising to 1.26 mg/mm \(\pm\) 0.02 mg/mm at a welding speed of 12 m/min. Due to this significant increase, the weld seam top sides of 12 m/min is underfilled. However, at both welding speeds, it was possible to determine an enhanced number of adhering spatters at the metal surface. On the bottom side, it was not possible to determine any effect of the increased welding speed on the melt pool dynamics and the seam quality. By increasing the welding speed to 14 m/min, the loss of mass reaching a maximum of 1.31 mg/mm \(\pm\) 0.02 mg/mm. For increasing the welding speed up to 16 m/min, the loss of mass was decreasing to 1.24 \(\pm\) 0.01 mg/mm. At these welding speeds, the melt pool dynamics can be related to the \"elongated keyhole regime\" [4]. Following, the keyhole was characterized by a significant elongation against the welding direction. Similar to the single-wave regime, a fluctuating melt pool swelling at the keyhole rear wall was formed. By the collapse of such melt pool swelling, a spatter detachment in filamentary form could be observed. Based on the resulting loss of mass, the formation of underfill was determined on the weld seam top side. However, the increase in welding speed also led to a reduction in adhering spatters, whereby the spatters showed a reduced size compared to 10 and 12 m/min.