## 3 Results  
### Reference Process  
The loss of mass was utilized as a describing parameter for the weld quality of the reference process without applying local shielding gas. Figure 7a depicts the loss of mass over the welding speed for the laser spot size of 300 um. The lowest loss of mass of 0.04 \(\pm\) 0.02 mg/mm was measured at a welding speed of 8 m/min. The high-speed images show melt pool swellings at the rear wall of the keyhole opening. The melt pool swellings occurred periodically and subsequently merged with the melt pool. No spatters detached  
Figure 6: Evaluated weld seam parts for spatter and keyhole investigation with static process behavior for the welding speed of 12 m/min. from the melt pool swellings at this welding speed which is typical for the \"Rosenthal\" regime. Contrary to the state of the art, no spatters were emitted from the keyhole rim [12]. This is attributed to the lower energy input in the current investigation compared to the study in [12]. However, the beginning formation of melt pool swellings already indicates a transition to the \"single-wave\" welding regime as described in [12]. The characteristic and distinct melt pool swellings of the \"single-wave\" regime steadily formed behind the keyhole at a welding speed of 12 m/min and spatters were constantly detached. The spatter detachment reduced the sample mass by 1.10 \(\pm\) 0.05 mg/mm. Melt columns formed, which dissipated into spatters due to the Plateau-Rayleigh instability  
Figure 7: Loss of mass, keyhole and melt pool behavior of the reference welding process for various welding speeds: (**a**) laser spot size of 300 \(\upmu\)m; (**b**) laser spot size of 600 \(\upmu\)m.  
condition [22]. Consequently, the increase of the welding speed from 8 m/min to 12 m/min changed the mechanism of spatter detachment resulting in an enhanced spatter quantity. A further increase in welding speed to 16 m/min led to a minor reduction in loss of mass to 1.06 \(\pm\) 0.02 mg/mm. The process behavior in this case was assigned to the \"elongated keyhole\" regime due to the observed elongation of the keyhole with increasing welding speed [12]. Spatters detached from melt pool swellings by necking the melt columns as already described in [22] similar to the process at 12 m/min. The welding speed of 8 m/min also showed the lowest loss of mass with 0.07 \(\pm\) 0.03 mg/mm using the laser spot size of 600 \(\upmu\)m. However, the melt pool swellings were less pronounced. Therefore, the process behavior was assigned to the \"Rosenthal\" regime. There was also a larger melt pool in front of the keyhole compared to the smaller laser spot size. This was attributed to a changed beam caustic due to the defocusing to increase the laser spot size. The loss of mass rose to 0.85 \(\pm\) 0.05 mg/mm as the welding speed was increased to 12 m/min for the larger laser spot size. However, compared to the smaller laser spot size the loss of mass was slightly lower at this welding speed. A further increase of welding speed to 16 m/min led to a significant decrease in loss of mass to 0.55 \(\pm\) 0.05 mg/mm. This effect was attributed to the keyhole elongation with increasing welding speed typically known as the \"elongated keyhole\" regime [12]. A keyhole elongation with increasing welding speed from 12 m/min to 16 m/min of 37% (0.50 mm to 0.69 mm; median values for 1000 measured keyhole lengths) was determined for the laser spot size of 300 \(\upmu\)m while the laser spot size of 600 \(\upmu\)m revealed an elongation of 46% from 0.59 mm to 0.86 mm. An extended keyhole length possibly decreased the interaction between escaping metal vapor and melt pool swellings. As a result, despite the increased flow velocity around the keyhole due to the higher welding speed [8], fewer spatters detached from the melt pool swellings, and the loss of mass decreased. A detailed discussion of the changing keyhole size is described in detail in Section 3.4.