# Effect of Local Gas Flow in Full Penetration Laser Beam Welding with High Welding Speeds
## 3 Results
### Process Regimes Depending on Flow Rate  
Starting from the reference process, the effect of the local gas flow at different welding speeds, and therefore spatter regimes, is investigated. An argon gas flow was applied to the keyhole with flow rates from 3.2 L/min to 32 L/min in steps of 3.2 L/min. In order to generalize the investigations, the flow rates are attributed to the change in dynamic pressure, which have been adjusted between 1 kPa up to 107 kPa due to the change in flow rate. The associated calculation follows Equation (4). The loss of mass is related to the flow rate as well as to the dynamic pressure in Figure 3. By increasing the flow rate, the resulting loss of mass drops to a minimum of about \(\leq\)0.25 mg/mm as the flow rate respectively the dynamic pressure increases. In the range from 9.6 to 22.4 L/min, this level remains almost constant. Above these flow rates, the mass loss increases significantly. At a maximum flow rate of 32 L/min, the loss of mass exceeds the initial values of the reference process by a factor of up to five. For different flow rates, in addition to the loss of mass, further characteristic features can be identified which representing different defect regimes:  
* Reduced spatter formation (3.2 L/min/1 kPa to \(\leq\) 6.4 L/min/4 kPa): The first regime is characterized by a significant reduced loss of mass. By using the local gas flow, a considerable reduction in spatter formation and adhering spatters could be observed on the top side of the specimen. Nevertheless, at a welding speed of 12 m/min and 16 m/min, some weld defects in form of small humps occurred on the specimen top side at a flow rate of 6.4 L/min. Below this flow rate, no weld seam defects could be determined. In contrary, the bottom side of the specimen was not affected by the gas flow regarding the weld seam quality or the spatter formation mechanism. This can be explained by the differing spatter formation mechanism at the bottom aperture of the keyhole. Based on the spatter detachment within the keyhole, the spatter formation can only be affected by changing the windage of the formed steps on the keyhole wall [5]. For this purpose, the forces for a melt displacement must exceed a critical threshold, depending on the acting dynamic melt flow pressure and the rheological properties of the melt. Up to flow rates of 6.4 L/min, the resulting dynamic pressure of \(\leq\)4 kPa seems to be not sufficient to fulfill this physical condition. * Regime A (\(\geq\)9.6 L/min/9 kPa to \(\leq\)16 L/min/26 kPa): This process regime is characterized by the formation of humps on the top side of the specimen. Compared to the reduced spatter formation process regime, the enhanced flow rate contributed to a significant elongation of the keyhole and melt pool against welding direction. Based on the trailing flow orientation, the gas flow interacts primarily with the keyhole rear side. Following, an impulse of the gas flow can be transferred to the keyhole rear wall, whereby melt is displaced and accelerated due to the compressive force. This explains the elongation of the keyhole as well as the increase in kinetic energy of the melt flow can contribute to the melt pool extension. At the end of the melt pool, it was possible to observe the formation of drop-shaped melt pool swellings, which solidified on the surface of the weld seam (humps). In literature, humping formation is typically observed at higher welding speeds \(\geq\)18 m/min [3; 4]. In case of the local gas flow, the formation of humps may be attributed to the melt flow acceleration. At the bottom side of the specimen, it was not possible to determine any effect of the gas flow. * Regime B (\(\geq\)19.2 L/min/38 kPa to \(\leq\)22.4 L/min/51 kPa): The further increase in flow rate resulted in the formation of humps at the bottom side of the specimen. Due to the enhanced flow rate, it was possible to observe an elongation of the keyhole and the melt pool as well.