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

在这个后续项目中，一个多物理耦合的数值模型，描述了形成的所谓的鼓包区域的高功率激光束焊接低合金钢与高板厚将被扩展，以提高精度的射线跟踪算法的输入的激光能量。为此目的，两种方法被认为是更准确地描述吸收的能量及其在蒸汽毛细管中的分布。第一种方法基于物理自适应网格细化，第二种基于虚拟网格细化。这两种方法都提高了子射线反射点的计算精度，而自适应网格细化方法也提高了熔池流动和温度分布的精度。因此，在考虑蒸汽毛细管的瞬态局部计算的情况下，将提供在较高工艺速度下对焊缝的隆起的尚未解释的形成机制的贡献。此外，借助于数值模型，研究了鼓包区对中心裂纹缺陷形成的影响。通过研究不同的实验参数，如加工速度，激光功率和焦点位置的影响，发生胀形效应的关键因素将被检查。所得结果将用于热裂纹敏感性的量化。在此，将根据胀形区域的发展和中心线裂纹缺陷的趋势来评估所检查参数的灵敏度，特别是还将计算出部分熔透和全熔透焊接之间的差异。该项目的目的是通过数值和实验研究相结合的方式来确定鼓包区域的形成机制，并阐明焊缝鼓包的位置和强度与中心线裂纹缺陷形成之间的关系。最后，一个数值模型能够适应的过程优化，以避免形成的研究膨胀区，从而以及中心线开裂将是可用的。

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.