Fundamental understanding of spatter behavior at laser welding of steel

Abstract

Based on results of thin disk and fiber laser experiments it has been shown in earlier publications that strong focusability at high laser power can be used for an expansion of the application range. A strong spatter formation in laser welding of steel has been observed at high velocities (enabled by strong focusability), however. In order to understand the underlying processes, measurements of the front keyhole inclination supported by simplified calculations have been performed. It was observed that the achievable welding depth depending on welding speed, laser power and focus diameter affects the inclination of the front keyhole wall. At increased welding speed the inclination of the capillary becomes larger and the evaporation process perpendicular to the capillary front wall is very important. Therefore the expanding metal vapor impinges at the rear keyhole wall and causes spatters. The ejection direction of the spatters can be directly correlated with the inclination of the front keyhole wall and the welding depth as well. Because the ejection direction of the spatters is not equal to the inclination of the keyhole we conclude that an additional driving force in upward direction must be existent. Against this background it is possible to find methods for influencing the melt flow in a positive manner. It can be shown that the spatter formation can be reduced by an increase of the inclination angle of the keyhole in forward direction (caused by an inclination of the laser beam) or by a variation of the focal position.Based on results of thin disk and fiber laser experiments it has been shown in earlier publications that strong focusability at high laser power can be used for an expansion of the application range. A strong spatter formation in laser welding of steel has been observed at high velocities (enabled by strong focusability), however. In order to understand the underlying processes, measurements of the front keyhole inclination supported by simplified calculations have been performed. It was observed that the achievable welding depth depending on welding speed, laser power and focus diameter affects the inclination of the front keyhole wall. At increased welding speed the inclination of the capillary becomes larger and the evaporation process perpendicular to the capillary front wall is very important. Therefore the expanding metal vapor impinges at the rear keyhole wall and causes spatters. The ejection direction of the spatters can be directly correlated with the inclination of the front keyhole wall and th...