Microstructure simulation of solidification in the weld

作为一种灵活的非接触连接工艺，激光焊接正变得越来越重要。然而，由于合金的凝固开裂倾向，具有大熔化间隔的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，但到目前为止，只有一些办法从冶金或结构上解决了这一问题的各个方面。“激光焊接中的凝固裂纹：高性能工艺的高性能计算”研究小组的目标是对凝固裂纹的形成机制及其与工艺参数的关系进行定量的工艺理解。在该课题组中，该子项目的目标是确定微观组织凝固裂纹形成的准则和影响因素，以便能够对不同工艺条件下金属焊接中的凝固和偏析行为以及凝固和偏析裂纹的形成概率做出可靠的预测。研究工作在于对奥氏体不锈钢焊缝的凝固过程进行建模和模拟，解决了微观组织过程，并考虑了化学-热-机械过程的强耦合作用。在耦合相场力学模拟中，计算了初生枝晶的不同凝固阶段、枝晶阻塞、晶界形成和完全凝固，并预测了局部应力峰值可能导致的微裂纹的形成。化学力学模拟提供了凝固和偏析过程中微裂纹的浓度分布、相分布和应力分布，同时还考虑了局部温度分布和微观组织的溶解。在系统的研究中，与工艺有关的变量，如温度梯度、焊接速度和热输入，都是变化的，分析了热影响区和熔化范围内的枝晶、晶界和相界对裂纹形成的影响，并最终传递到细观尺度上，包括形态信息。微观尺度上的模拟结果有助于在细观尺度上对冶金或机械诱导的热裂纹的形成进行分类，并且对于研究组内的进一步研究是不可或缺的。

As a flexible and contactless joining process, laser beam welding is becoming increasingly important. However, the processing of alloys with a large melting interval is 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 have so far only been approaches that address sub-aspects of this problem - metallurgically or structurally. The research group "Solidification Cracks in Laser Beam Welding: High-Performance Computing for High-Performance Processes" aims to develop a quantitative process understanding of the mechanisms involved in the formation of solidification cracks and the connection with process parameters.Within the research group, the objective of this subproject is to determine the criteria and influencing factors of microstructural solidification crack formation, in order to be able to make reliable predictions about the solidification and segregation behaviour and the probability of solidification and segregation crack formation in metallic welds, under different process conditions. The research work lies in the modelling and simulation of the solidification process in a weld on austenitic stainless steel, with the solution of microstructural processes and the consideration of strongly coupled interactions of chemo-thermomechanical processes. In coupled phase field mechanical simulations, the different solidification stages of the primary dendrites, the dendrite blocking, the grain boundary formation and the complete solidification are calculated and the formation of possible microcracks, resulting from local stress peaks, is predicted. The chemomechanical simulations provide the concentration distributions, phase distributions and stress distributions and, in the future, will also provide the microcrack distributions during the solidification and segregation processes, while dissolving the microstructural structure and considering the local temperature distribution. In systematic studies, both process-related variables, such as the temperature gradient, the welding speed and the heat input, are varied and the influence on the susceptibility to crack formation at the dendritic, grain and phase boundaries in the heat-affected zone and in the melting range is analysed and finally passed on to the mesoscale, including the morphological information. The findings of the simulations on the microscale contribute to the classification of the metallurgically or mechanically induced hot crack formation on the mesoscale and are indispensable for further investigations within the research group.