Influence of laser beam intensity maxima and their relative positions on the keyhole during multi-beam laser deep penetration welding

The intensity distribution of the laser beam during deep penetration laser welding strongly affects the process behavior. While the influences of several specific distributions have been studied, the almost infinite number of possible beam profiles makes it impractical to examine each experimentally. Therefore, a more profound understanding of how intensity maxima at defined keyhole locations interact with the keyhole is required to enable an a priori design of task-specific intensity distributions in the future. This study investigates the hypothesis that the characteristics of individual laser beam processes can coexist independently within a combined process, provided that the additional intensity maximum interacts only locally at the keyhole bottom. An optical system was used that coaxially combines a multi-mode laser beam and a single-mode laser beam, allowing the intensity of the single-mode laser beam to be positioned within the larger keyhole generated by the superimposed multi-mode laser beam in mild steel. The effects on keyhole morphology, weld formation, and process stability were analyzed using optical coherence tomography data, high-speed imaging recordings, and metallographic cross-sectional micrographs. The results confirm the hypothesis by demonstrating that an additional intensity maximum at the keyhole bottom leads to the formation of a secondary keyhole tip while leaving the primary keyhole shape largely unaffected – creating a stable “keyhole-in-keyhole” process. In contrast, placing the additional intensity on the primary keyhole’s front wall alters the inclination angle, shifts the primary keyhole bottom, and increases the occurrence of process irregularities such as spatter formation. These findings advance the understanding-based portfolio for the targeted manipulation of keyholes and represent a step toward an a priori design of laser beam intensity distributions.