Linkage of Macro- and Microscale Modeling Tools for Additive Manufacturing of Steels

Additive manufacturing (AM) offers several benefits including the capability to produce unique microstructures, geometrical freedom allowing for material and energy savings, and easy production lines with fewer post-processing steps. However, AM processes are complex and phenomena occurring at different length and time scales need to be understood and controlled to avoid challenges with, for example, defects, residual stresses, distortions, and alloy restrictions. To overcome some of these challenges and to have more control over the final product, computational tools for different length scales need to be combined. In this work, an 18Ni300 maraging steel part is studied to understand the link between the process parameters and the as-built microstructure. The temperature evolution during laser powder bed fusion is simulated using the MSC simulation software Simufact Additive. This result is then linked to microscale models within the Thermo-Calc software package to predict the elemental micro-segregation, martensite start (M-s) temperature, and martensite fraction. The different values of the key process parameters such as laser speed, laser power, heating efficiency, and baseplate temperature are considered, leading to different thermal histories. The thermal histories affect the elemental segregation across the solidification structure, which in turn results in different M-s temperatures at different locations of the built part. It is found that higher laser energy generally causes higher temperatures and higher cooling rates, which results in a larger degree of elemental segregation and lower M-s temperatures in segregated regions. Furthermore, the segregated regions are predicted to have M-s temperatures below 200 degrees C, which would result in retained austenite when using a baseplate temperature of 200 degrees C. On the other hand, by using a baseplate temperature of 100 degrees C, all regions would reach temperatures below the M-s temperature, and an almost fully martensitic structure would be possible. In summary, it is demonstrated how the linkage of macro- and microscale modeling tools for AM can be used to optimize the process and produce the desired microstructure, thereby achieving the desired mechanical properties.