Particularly the automation and industrialization of thermal material processing with lasers of radiation of 1 µm makes high demands on the resulting welding quality. Indeed, current laser systems with strong focusability exhibit a high innovation potential in many application ranges, for example the possibility of adjusting the welding depth to even small material thicknesses. However, at such high focusability there are also disadvantages observed especially in laser welding of steel expressed in a strong spattering generated at the rear keyhole wall. This undesired instability mainly occurs by using lasers with radiation of 1 µm in contrast to CO2 lasers which is why it is essential to understand its fundamentals to push the industrial use of laser systems of 1 µm.
In order to improve the spatter behavior, a sound knowledge of the interaction between the front keyhole wall inclination and the melt pool is required. In this study emphasis is laid on how to manipulate the inclination of the front keyhole wall by changing the focal position. In doing so, the keyhole formation is observed by a novel process monitoring system which allows determining the spatial inclination of the capillary. According to this, a displacement of the focal position below the surface yields a reduced number of spatters due to the steepening of the front keyhole wall. By using additional high-speed cameras it is possible to distinguish the main driving forces for spatter generation and to accent the importance of the inclination of the front keyhole wall.