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

激光焊接作为一种灵活、无接触的连接技术，越来越受到人们的重视。大熔点合金的凝固开裂倾向给其加工带来了挑战。凝固裂纹的形成是由于枝晶组织的临界应力和应变状态以及枝晶间熔体。尽管与工业高度相关，但只有从冶金或结构方面解决问题的单一方面的办法。“激光束焊接过程中的凝固开裂——高性能工艺的高性能计算”研究单位旨在对凝固开裂机理及其与工艺参数的关系进行定量的工艺理解。本子项目范围内的计划任务之一是在其他研究小组或研究所进行必要的实验和测量，以验证数值模型。此外，该子项目的一个目标是使用外部加载热裂试验，结合自行开发的光学测量技术，以确定凝固开裂所需的局部临界机械和热条件。该子项目的主要目的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根据热和力学边界条件的凝固开裂。在这里，主要关注的是熔池中流体动力学过程对所得温度分布的影响，最重要的是，固-液相边界的几何排列对糊状区局部应力和应变分布的影响。由于裂纹起裂发生在脆性温度范围（BTR）内，因此在有限元模型中对该区域的正确表征是保证其准确性的重要要求。该模型将建立在两相域(L-delta)/(L-gamma)或多相域（L-delta-gamma）基于接近现实的材料模型在中观尺度。为此，粘弹塑性材料定律将被应用到宏观力学模型中。在宏观模型中引入了材料的温度和相依赖规律以及等效热源后，考虑到材料的塑性曲线，在BTR中也要引入凝固裂纹模型。这些进一步用于预测裂纹萌生和裂纹扩展。

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.