# Effect of Local Gas Flow in Full Penetration Laser Beam Welding with High Welding Speeds
## 1 Introduction  
New developments in the field of solid-state lasers has led to the availability of higher power classes at relatively low investment costs. These power classes offer the potential to significantly increase the processing speed in various applications result in a considerable improvement of economic efficiency. However, the potential cannot yet be utilized for laser beam welding of high-alloyed steels. When welding with high welding speeds \(\geq\)8 m/min, process instabilities occur, which negatively affect welding quality [1]. In this context, spatter formation must be particularly emphasized as the loss of mass results in adhering spatters as well as in affected mechanical properties. The process of spatter formation can be attributed to two basic mechanisms [1; 2; 3; 4]. The first mechanism is significant for welding speeds up to approx. 8 m/min and assumes that the spatter detachment takes place within the keyhole. While the keyhole is characterized by the formation of steps on the front and the rear side of the keyhole wall, the escaping metal vapor interacts with the molten material [5]. Based on the windage of the steps, an impulse is transferred from the escaping metal vapor to the molten material. As a result, spatters can be detached in undirected spatter trajectories. When increasing the welding speed beyond 8 m/min, the key mechanism in spatter formation changes. According to the increase in keyhole inclination at high welding speeds, the energy absorption at the keyhole front is rising [3]. As a result, the metal vapor flow induced is directed towards the keyhole rear wall. Based on the interaction of the metal vapor and the rear wall, the melt pool fluctuates and starts to move along welding direction. If the transferred impulse exceeds a critical threshold, spatter formation occurs [4; 6; 7]. This relation can be given by the condition of spatter formation (Equation (1)). For the extraction of one single spatter, the kinetic energy of the fluid element in the melt (\(E_{kin,fl}\)) must be greater than the sum of the kinetic energy of the droplet (\(E_{kin,dr}\)) and the surface energy of the melt (\(E_{surf,fl}\)) [8; 9]. \[E_{kin,fl}>E_{kin,dr}+E_{surf,fl} \tag{1}\]  
Another representation of this relation can be given based on droplet volume \(V_{dr}\), velocity of the fluid element \(v_{fl}\), velocity of the droplet \(v_{dr}\), surface tension \(\sigma\), and surface area of the droplet \(O_{dr}\) (Equation (2)) [8]. \[\frac{\rho V_{dr}\abs{v_{fl}}^{\rightarrow}}{2}>\frac{\rho V_{dr}\abs{v_{dr}} ^{\rightarrow}}{2}+\sigma O_{dr} \tag{2}\]  
However, the formation of spatters depends significantly on the stability of the keyhole, which can be derived by the pressure balance inside the keyhole (Equation (3)) [10; 11]. Thereby, the equilibrium includes the physical quantities of the recoil pressure \(p_{abl}\), differential vapor pressure \(p_{dp_{-}v}\), atmospheric pressure \(p_{0}\), surface tension pressure \(p_{\sigma}\), hydrostatic pressure \(p_{h}\) and hydrodynamic pressure \(p_{dyn}\). Following this, the equilibrium gives the relation of the opening pressures to the closing pressures of the keyhole. \[p_{abl}+\Delta p_{dp_{-}0}=p_{\sigma}+p_{h}+p_{dyn_{melt}};\;\Delta p_{dp_{- }v}=p_{\sigma}-p_{0} \tag{3}\]  
In order to form a stable and less fluctuating keyhole, the opening pressures must be at least equal or greater than the closing pressures of the keyhole [12]. In this context, numerous parameters have been investigated for manipulating the pressure balance. Despite solutions using laser beam shaping [13; 14; 15; 16], power modulation [17; 18], or vacuum systems [19; 20], the use of local gas flows represents a technically feasible solution. By using the gas flow, the pressure balance is affected by an additionally exerted dynamic pressure depending on the density \(\rho_{gas}\) and the flow velocity \(v_{gas}\) of the supplied gas (Equation (4)) [21]. As a result, the opening pressures are amplified by the relative dynamic pressure \(p_{dyn_{gas}}\).