Influence of high feed rates during laser beam welding on the capillary geometry and the resulting weld seam quality

At high feed rates, the laser beam welding process is very efficient and stable, resulting in a high weld seam quality. However, at a certain feed rate, the weld seam begins to suffer from undercuts, and, when further increasing the feed rate in addition humping occurs. It is known that the increase of the feed rate leads to an elongation of the capillary opposite to the feed direction. This has a significant influence on the melt flow and therefore on the formation of undercuts and humps. In the present work, the geometry of the capillary was studied as a function of the feed rate by means of X-Ray imaging during welding. It was found that if a critical feed rate was exceeded, the capillary geometry switched from a U-shape to a wedge-shape. The wedge-shaped capillary was found to be directly related to the occurrence of undercuts. It was also found that the critical feed rate for the formation of a wedge-shaped capillary increases with the reduction of the laser beam diameter. Furthermore, a possibility to shift the critical feed rate to higher feed rates by the means of beam shaping is shown.