Macro-mechanical modelling of the material behavior during laser beam welding under mechanical load

激光焊接作为一种柔性、无接触的连接技术，日益受到重视。由于合金的凝固开裂倾向，大熔点合金的加工是一个挑战。凝固裂纹是由于枝晶组织与枝晶间熔体的临界应力和应变状态所致。尽管具有很高的工业相关性，但只有从冶金或结构上解决问题的单一方面的办法。研究单位“激光焊接过程中的凝固裂纹--高性能工艺的高性能计算”旨在发展对凝固开裂机理及其与工艺参数的关系的定量工艺理解。本次级项目范围内计划的任务之一是进行必要的实验和测量，以便在其他研究小组或研究所验证数值模型。此外，这个子项目的一个目标是使用外部加载的热裂试验和自行开发的光学测量技术来确定凝固裂纹所需的局部临界机械和热条件。这个子项目的主要目标是开发一个宏观力学有限元模型，用于预测激光焊接过程中的凝固裂纹，该模型取决于激光焊接过程中的热和力学边界条件。这里主要讨论熔池中流体动力学过程对温度分布的影响，尤其是固-液相边界的几何布置对糊状区局部应力和应变分布的影响。由于裂纹萌生发生在脆性温度范围内，在有限元模型的背景下对该区域进行正确的描述是其准确性的重要要求。该模型将在两相域(L-三角洲)/(L-伽马)或多相域(L-三角洲-伽马)中建立，其基础是在细观尺度上进行近真实的材质建模。为此，在宏观力学模型中引入了粘弹塑性材料定律。在宏观模型中实施了与温度和相有关的材料定律和等效热源之后，考虑到材料的延性曲线，在边界条件下也建立了凝固裂纹模型。这些结果进一步用于预测裂纹的萌生和扩展。

As a flexible and contact-free joining technology, laser beam welding has increasingly gained importance. Processing of alloys with large melting range poses a challenge due to their solidification cracking tendency. Solidification cracks form due to critical stress and strain states of the dendritic microstructure with interdendritic melt. Despite the high industrial relevance, there are only approaches addressing single aspects of the problem, metallurgically or structurally oriented. The research unit "Solidification Cracking during Laser Beam Welding – High Performance Computing for High Performance Processes" aims at developing quantitative process understanding of the mechanisms of solidification cracking and their relation to process parameters. One of the planned tasks within the scope of the present subproject is to perform the necessary experiments and measurements for the validation of the numerical models at other research groups or institutes. Furthermore, one goal for this subproject is to use an externally loaded hot cracking test in combination with a self-developed optical measurement technique to determine the local critical mechanical and thermal conditions required for solidification cracking. The main aim of this subproject is the development of a macro-mechanical FEM model for the prediction of the solidification cracking, depending on the thermal and mechanical boundary conditions during laser beam welding. Here, the main focus is on the influence of the fluid dynamical processes in the melt pool on the resulting temperature distribution and, above all, the geometric arrangement of the solid-liquid phase boundary on the local stress and strain distribution in the mushy zone. Since crack initiation takes place within the brittle temperature range (BTR), a correct characterization of this region within the context of an FEM model is an important requirement for its accuracy. The model is to be built up in the two-phase domain (L-delta)/(L-gamma) or multi-phase domain (L-delta-gamma) based on a near-realistic material modelling at the meso scale. For this purpose, visco-elasto-plastic material laws are to be implemented into the macro-mechanical model. After the implementation of the temperature and phase-dependent material laws and the equivalent heat source in the macro model, a solidification crack model is also to be implemented in the BTR, considering the determined ductility curve of the material. These are further used to predict crack initiation and crack growth.