Fundamental mechanisms and modeling of microstructure evolution during beam and powder bed-based additive manufacturing

• Applicant: Dr.-Ing. Matthias Markl
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 327889862

Beam-based additive manufacturing (AM) of metals in a powder bed not only offers the opportunity to build complex, custom-made components of high-performance materials, but also to adjust the local material properties by proficient processing. The variation of solidification conditions enables the modification of microstructure length scales. Additionally, latest research results indicate, that also the texture of the components is adjustable during manufacturing. Therefore, entirely new perspectives are opened regarding optimization of light weight components, because not only the topology, but also the texture of the material is adjustable to the local loads on the component. In order to comprehend and control the texture evolution, the hydrodynamic non-equilibrium solidification process (grain growth, selection and nucleation) needs to be fundamentally understood. Experimental investigations show that especially the mechanisms of nucleation under the extreme conditions of AM are insufficiently resolved and are not reproduced by classical models.The aim of this proposal is to identify, to fundamentally understand and to physically model the microstructure evolution, especially the nucleation under the special solidification. This model should be implemented in existing software, which is developed at our chair. Modeling and verification are experimentally substantiated basing on additively manufactured samples of IN718. At the end of the project the model should predict the solidification structure, grain structure and texture evolution during beam and powder bed-based AM.The project draws on our software for simulating the consolidation process during beam and powder bed-based AM. The software contains a lattice Boltzmann method to describe the hydro- and thermodynamics during melting and solidification. This method s coupled to a cellular automaton modeling the grain structure evolution during solidification neglecting currently grain nucleation. Our new theoretical ansatz contains besides the temperature gradient and the solidification front velocity for the first time additional information about the texture of the previous layers (orientation, spacing of cells/dendrites, segregation) and the local composition of the melt at the interface to the currently processed layer. It should be investigated, how orientation changes at the solidification front in combination with the present segregations in the rapidly melted material (memory of melt) induce grain nucleation by local undercooling. These findings are mathematically utilized for a grain nucleation model.

• DFG Programme: Research Grants
• Co-Investigator: Professorin Dr.-Ing. Carolin Körner