## 1 Introduction
In addition, the geometry of the keyhole was affected for partial [33] and full penetration welding [34] and for the welding of lap joints [35]. The influence of the gas flow on the pressure balance in the keyhole was estimated in [33] combining investigations in [36; 37]. Also, in [38] the gas flow was mentioned as a mechanism to stabilize the keyhole and to reduce spatter formation. Humps appeared at the weld seam due to the increased momentum transfer from gas flow to melt pool at flow rates \(\geq\)6.4 L/min. Using even higher flow rates of up to 32 L/min resulted in an extended formation of the weld seam. Therefore, these flow rates should be avoided due to the reduced weld seam quality [33]. However, the reduction of spatter formation cannot be attributed solely to the dynamic pressure of the gas flow, also the influence of the surface tension must be considered [39]. Accordingly, the surface tension must be high enough to prevent spatter detachment [22]. The surface tension of the melt pool depends particularly on the oxygen level of the melt [40], its Sulphur content, and temperature [41]. Furthermore, elements with limited solubility in liquid metals and considerably weaker intermolecular bonding forces are most likely highly surface active [40]. Due to the high surface activity of oxygen, the surface tension is decreased also at low oxygen levels [42]. Consequently, a weld process under the exclusion of oxygen showed an altered melt flow in previous investigations [43]. A reduction of weld seam imperfections was also achieved by increasing the laser spot size. Larger laser spot sizes result in an increase in the melt pool [44] and keyhole size [11], whereas an elongated melt pool suppresses the spatter formation [44] because of a calmer melt pool behavior [45]. An increase in laser spot size at the same power and feed rate results in a steeper ejection angle of the spatters. This is a result of an altered metal vapor flow due to a greater keyhole front inclination [11]. In addition to the reduction of spatters, the suppression of humping by means of larger laser spot sizes was already demonstrated in [45]. Furthermore, the keyhole closing Laplace pressure decreases as the keyhole radius increases [36]. The keyhole stability is often related to spatter formation. However, keyhole oscillation does not seem to be the cause of spatter detachment [46]. The earliest calculations of the keyhole oscillation by means of appropriate models showed a radial keyhole frequency of 500 Hz [47]. Further models to describe the process dynamics determined oscillation frequencies of the keyhole depth between 2000 Hz and 5000 Hz [48]. Measurements of the laser-induced plume by photodiodes, evaluated by means of Fast Fourier Transform (FFT) for a simplified calculation of the amplitude and frequency [49], revealed significant amplitudes for 1000 Hz which resulted from keyhole oscillations [50]. [PERSON] calculated the oscillation over the keyhole depth, whereby the oscillation frequency was most pronounced in the keyhole center. While varying the laser power and the beam profile, oscillations above 100 kHz were achieved by welding an aluminum alloy. It appeared that the frequency behaves contrary to the oscillation amplitude. If the frequency decreases, the amplitude increases [46]. [PERSON] et al. demonstrated that material ejections could be comparatively low even at high oscillation amplitudes [22]. For the use of a local gas flow, a similar keyhole oscillation frequency but an increased keyhole oscillation amplitude were determined in comparison to the reference process without gas flow [38]. In addition to high-speed imaging for process visualization, the measurement of the keyhole depth by means of Optical Coherence Tomography (OCT) is a promising method to explore the keyhole dynamics [51]. Weld penetration depths of up to 5 mm were measured reliably for steel [52] and aluminum [53] using data filtering.