# Effect of Local Gas Flow in Full Penetration Laser Beam Welding with High Welding Speeds
## 4 Model of Effect Mechanism  
The results show a wide range of influencing variables and interaction relationships, which are presented in the following and transferred into a model concept. Based on the reference process for full penetration welding at high welding speeds \(\geq\)8 m/min (Figure 11a), the use of the gas flow results in a manipulation of the pressure balance (Equation (3)). Following this, depending on flow rate, a dynamic pressure is exerted which results in a general pressure increase inside the keyhole (Figure 11b). Further, based on the trailing flow orientation, the kinetic energy of the gas causes the formation of a compressive force on the keyhole rear wall. Due to the acting forces, the keyhole rear-sided melt pool swelling shows a much less frequently movement. Instead of a keyhole collapse, a reverse melt flow from the keyhole rear wall into the downstream melt pool is formed. Based on the  
Figure 10: Cross sections of weld seam geometries depending on flow rate and welding speed. conditions of a continuous melt flow propagation, the melt pool is extensively elongated. Thereby, this effect can be intensified by the gas flow induced acceleration of melt. Nevertheless, in the case of melt pool movement, spatter are detached in the same way as in the reference process. In contrast, the detached spatter have a significantly lower kinetic energy. This can be attributed to a decreased impulse transfer from the metal vapor to the rear-sided melt, due to a reduced windage of the formed melt pool swelling at the keyhole rear side. Based on the orientation of the gas flow and the escaping metal vapor on the top side of the specimen, the outgassing conditions of the metal vapor flow are affected. Depending on flow rate, the interaction of the contra-orientated flows results in the formation of a heat accumulation due to an impeded degassing. Following, the weld seam top is locally widened. According to the law of continuity (Q = v \(\cdot\) \(\rho\) \(\cdot\) A = const. ), this change in flow rate can cause a reduction in flow velocities of the keyhole surrounding melt. Consequently, the melt flow induced dynamic pressure (Equation (4)) is falling. Due to a spatter detachment from the melt pool within the keyhole, the bottom-sided spatter formation mechanism is not affected by a change in pressure balance, as no significant change in keyhole dimensions could be observed at lower flow rates \(\leq\)9.6 L/min. [4]  
Besides the mentioned model description, there are further physical quantities which can be affected by the local gas supply. * Pressure/force balance: Based on the interaction of the gas flow and the metal vapor, the gas can be heated up. Thus, the volume of the gas increases, which results in the formation of an expansion pressure. This contributes to another increase in opening force. An additional heating of the gas due to laser adsorption can be classified as insignificant based on the optical properties of argon [32]. Additionally, the force balance of the keyhole can be affected by the kinetic energy of the gas. Due to the trailing flow orientation, the gas flow exerts a compressive force on the keyhole rear wall. * Melt pool temperature: Besides the discussed effect of heat dissipation due to forced convection and changing outgassing conditions, the melt pool temperature is affected by a change in thermophysical properties. Based on the difference in specific heat capacity, density and thermal conductivity of the gas and the surrounding atmosphere, the heat transfer coefficient is changing. Figure 11: Schematic illustration of the effect mechanism: (**a**) Reference process; (**b**) Local gas flow. This can result in a manipulation of the Marangoni flow due to changed temperature gradient at the melt pool surface [33]. * Surface tension: Depending on the type of gas used, the surface tension of the melt changes due to adsorption and desorption its chemical components [34; 35].