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

• Applicant: Professor Dr.-Ing. Michael Rethmeier
• Subject Area: Joining and Separation Technology
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 434946896

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.

• DFG Programme: Research Units
• Subproject of: FOR 5134: Solidification Cracks during Laser Beam Welding: High Performance Computing for High Performance Processing
• International Connection: Russia
• Cooperation Partner: Privatdozent Sergey Volvenko, until 3/2022