Experimental and numerical investigation of the formation mechanisms of the bulging effect and its influence on the development of centerline cracking defects in high power laser beam welding of low-alloyed steels of high thickness

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In this follow-up project, a multiphysical coupled numerical model describing the formation of the so-called bulging-region in high-power laser beam welding of low-alloy steels with a high sheet thickness will be extended to improve the accuracy of the ray tracing algorithm for the input of the laser energy. For this purpose, two methods are considered to describe more accurately the absorbed energy and its distribution in the vapor capillary. The first method is based on a physical adaptive mesh refinement and the second on a virtual mesh refinement. Both methods increase the accuracy of the calculation of the reflection points of the sub rays, while the adaptive mesh refinement method as well improves the accuracy of the melt pool flow and the temperature distribution. Thus, a contribution to the yet unexplained formation mechanisms of the bulging of the weld at higher process speeds will be provided under consideration of a transient local calculation of the vapor capillary. Furthermore, with the aid of the numerical model, an investigation of the influence of the bulging-region on the formation of the centerline cracking defect will be carried out. By studying the influence of different experimental parameters such as process speed, laser power and focal position, the critical factors for the occurrence of the bulging effect will be examined. The obtained results will be used for the quantification of the hot cracking susceptibility. Here, the sensitivity of the examined parameters will be evaluated on the development of the bulging region and the tendency to centerline cracking defects, whereby in particular also the difference between partial penetration and full penetration welding will be worked out. The aim of the project is to identify the formation mechanisms of the bulging-region by means of a combination of numerical and experimental studies and to clarify the relationship between location and intensity of the bulge of the weld and the formation of centerline cracking defects. Finally, a numerical model being able to be adapted in terms of process optimization to avoid the formation of the studied bulging-region and thus as well the centerline cracking will be available.