# Effect of Local Gas Flow in Full Penetration Laser Beam Welding with High Welding Speeds
## 3 Results
### 3.3. _Spatter Reduction by Use of Local Gas Flow_
#### 3.3.2 Conditions of the Gas Flow  
In order to visualize the effect of the local gas flow in more detail, the process was recorded by high-speed schlieren technique. A schematic of the setup is shown in Figure 6a. Based on the difference in density and refractive index of the escaping gas and the surrounding atmosphere, it was possible to  
Figure 5: Spatter formation mechanism representing in time of: (**a**) Reference process; (**b**) Gas-assisted process.  
visualize the flow of the escaping metal vapor as well as of the gas flow of argon. Using a plane-parallel orientation (0\({}^{\circ}\) relative to the specimen surface), the flow conditions were recorded on the top side as well as on the bottom side simultaneously. For a general understanding, the results presented below are carried out for all three process regimes of Rosenthal regime (8 m/min), single wave regime (12 m/min), and elongated keyhole regime (16 m/min, see Section 3.1) using the parameters of Table 3. Based on the same process characteristics at welding speeds of 10 m/min and 12 m/min, Figure 6 illustrates exemplary only schlieren images of 12 m/min.  
At a welding speed of 8 m/min (Rosenthal regime), the metal vapor escapes quite symmetrical from the keyhole for the reference process (Figure 6b). Thereby, the intensity of the metal vapor was much higher on the top side then on the bottom side of the specimen. The flow field was significantly affected by the local gas supply, which is schematically depicted by orange respectively green arrows, representing the flow on top and bottom side (Figure 6c). Following, the flow of the metal vapor and argon was changing orientation against welding direction at the top side. This effect could also be seen on the bottom side of the specimen. Thereby the flow orientation was also changing, but in a less intense manner compared to the top side.  
At a welding speed of 12 m/min (single wave regime, Figure 6d) and 16 m/min (elongated keyhole regime, Figure 6f), the flow orientation of the reference process was changing into an orientation against welding direction. This can be attributed to the increase in keyhole inclination at high welding speeds (see Section 1), whereby the flow conditions of the escaping gas can be affected. In contrast to the changes of the top side, the bottom-sided flow orientation of the reference process was not affected. Applying the gas flow at welding speeds of 12 m/min (Figure 6e) and 16 m/min (Figure 6g), the top-sided flow was also influenced by the increase in keyhole inclination. Thus, the flow changed against welding direction, while the bottom-sided flow pattern did not show changes. It can be stated, that the effect of the local gas supply on the flow field is almost independent from the welding speed.  
Figure 6: (**a**) Schematic of used schlieren setup; (**b**–**g**) Schlieren records of the reference and gas-assisted process (argon) depending on welding speed.  
Following, the gas reflected from the keyhole towards the top side results in a change of the main flow direction. The trailing orientation of the nozzle supports this behavior. In order to clarify the effect of the changed flow conditions on the melt pool dynamics, the velocity of the detached spatter was determined as function of flow rate and welding speed.