# 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_  
#### 3.3.1 Influence of Flow Rate  
In order to achieve a spatter-reduced welding without additional weld seam defects caused by humping or melt ejection, the effect of the gas flow was examined more precisely for the spatter reduced process regime and the neighboring transition zone to regime A. Therefore, the increment size of the flow rate was reduced to 1.6 L/min to provide more precise information about the effect of the gas flow and the limits of the spatter reduced process regime by flow rates up to 9.6 L/min (Figure 4). The investigation started with the lowest flow rate of 1.6 L/min. Thereby, it was possible to determine a significant reduction in the loss of mass by up to 67% in comparison to the reference process. With a further increase of the flow rate, the loss of mass fell to a minimum of \(\approx\)0.2 mg/mm at 9.6 L/min, independent of the welding speed. However, an increase in flow rate from 6.4 L/min onwards contributed to the formation of some weld defects. These parameters are highlighted by a red outline in Figure 4. In this context, the top side of the weld seam showed the formation of small and irregular occurring humps (cf. regime A). In order to avoid weld defects, the following investigations were carried out using the sound parameters at the lowest loss of mass (Table 3). Figure 3: Process window of weld seam qualities depending on welding speed and flow rate. The following discussion takes place exemplary for a welding speed of 12 m/min and a flow rate of 4.8 L/min. The described effects are generally valid and can also be applied to welding speeds of 8 m/min, 10 m/min and 16 m/min. By the use of the gas flow, the top side of the specimen showed a significantly reduced spatter formation. In contrast to the much frequently spatter detachment of the reference process (Figure 4a), the formation of a reverse melt flow from the keyhole rear wall into the downstream melt pool could be determined (Figure 4e). Further, an extensive elongation of the melt pool was observed. In comparison to the high amount of adhering spatter on the specimen top side of the reference process (Figure 4b), a noticeable reduction in adhering spatter was found due to the decreasing loss of mass (Figure 4f). Following, the specimen top side was characterized by a homogenous weld seam quality, whereby the formation of underfill could be avoided. Moreover, the use of the gas flow resulted in a local widening of the weld seam top side. This can have an effect on the fluid dynamics of the melt pool. According to the law of continuity (Q = v. p \(\cdot\) A = const. ), the widening of the seam results in a change of the flow field. Subsequently, the flow velocities of the keyhole surrounding melt can be reduced, which can result in decreasing spatter formation. On the bottom side of the specimen, the gas flow had no influence on the spatter formation mechanism neither on the melt pool dynamics. Following, the reference process (Figure 4c) and the gas-assisted process (Figure 4g) are characterized by the same spatter formation mechanism. As a result, small spatters are detached within the keyhole based on the windage of formed steps on the keyhole wall (see Section 1). Consequently, the surface quality of the specimen bottom side was affected by small adhering spatters (Figure 4d,h). In order to explain the effect of the gas flow on the top-sided melt pool dynamics and spatter formation mechanism in more detail, the process characteristics are illustrated by representing points in time for a welding speed of 12 m/min (Figure 5). Figure 4: Loss of mass depending on flow rate and process characteristics for: (**a–d**) Reference process; (**e–h**) Gas-assisted process.