# Investigations of the Absorption Front in High-Speed Laser Processing Up to 600 m/min
## 1 Introduction  
The development of remote technology enables very high processing speeds in laser material processing, which leads to an enormously enlarged field of applications. Depending on the processing speed, applications ranging from welding to cutting are possible. For example, gas-free sheet cutting [1] or highly precise mass corrections of metallic compounds for balancing processes can be realized via the use of cw-mode laser radiation in combination with high processing speeds. Compared to pulse-mode ablation methods [2], the cycle time can be shortened due to higher removal rates. As such, the removal mechanism is primarily based on melt pool ejections (spatters) when the process is combined with high feed rates. This approach is already applied for cutting thin sheet metal without an additional cutting gas supply [1; 3]. The effect of spatter formation occurs in deep penetration laser welding if a local volume of melt is exposed to a certain amount of momentum perpendicular to the melt surface to extract the melt pool [4]. For this purpose, an equation is known from the literature, which specifies the condition for spatter detachment (see Equation (1) [5]):  
\[\rho V_{dr}\left|\vec{V_{fh}}\right|^{2}>\rho V_{dr}\left|\vec{V_{dr}}\right|^ {2}+2\sigma O_{dr}. \tag{1}\]  
Following this relation, the extraction of a single spatter requires the kinetic energy of the fluid element to be greater  
material and process characteristics, whereby some factors can be influenced significantly. The velocity of the melt flow depends strongly on welding speed but is significantly higher than the welding speed itself, as shown in [6]. This effect can be explained by the pressure that is exerted on the melt by the vaporized material of the keyhole front wall [4, 7]. Therefore, it is a common approach to scale up the processing speed if a certain loss of mass [8] or a resulting cut kerr is to be achieved [9] by means of spatter formation. However, the application of processing speeds of several meters per second leads to changes in the characteristic keyhole model. Characterizations of the keyhole in deep penetration laser welding have already been made in many studies. In [10], for example, high-speed X-ray imaging was used up to welding speeds of 7 m/min to determine the inclination of the keyhole front, among other values. Another method that is used to perform process observations in deep penetration laser welding with welding speeds up to 10 m/min is shown in [11] for coated steel. Here, a glass plate was fixed tightly to the metal sheets, while the laser spot was placed with two-thirds of its diameter on the sample and one-third on the glass plate. This setup allowed for a lateral view inside the resulting keyhole when the laser focus was moved along the edge of the sample. Several studies have shown that a huge enlargement of the keyhole in the feed direction occurs already at welding speeds of 20 m/min due to the increased inclination of the keyhole front wall. As such, the changed reflection conditions of the laser beam at the keyhole front wall lead to significant reductions in the penetration depth [12]. Further increases in welding speed up to 50 m/min led to the formation of an \"absorbing front,\" where most of the interaction with the incident laser must take place [13]. In [14], these assumed shapes of the keyhole could be confirmed using trials with welding speeds up to 70 m/min by means of high-speed X-ray radiography. Correspondingly, in different studies on remote laser cutting, a significant change in the welding regime occurs when it comes to processing speeds of about 200 m/min [1]. Up to this velocity, deep penetration welds with single spatter detachments of the melt are produced. Further increased welding speeds lead to significantly reduced penetration depths and less remaining melt that comes along with the formation of cut kerris [1, 3].