A combined experimental and numerical assessment of the role of microsegregation and phase formation on hot cracking susceptibility in laser powder bed fusion processed CM247LC

For many high-performance alloys originally developed for the casting route, hot cracking is a serious problem in the Laser Powder Bed Fusion (PBF-LB/M) process and limits the use of e.g. high-γ′ nickel-based alloys such as CM247LC in additive manufacturing. In this work, we investigate the relationship between PBF-LB/M processing parameters and the solidification path, i.e. phase formation and microsegregation, and its potential impact on hot cracking for the high-γ′ alloy CM247LC. We combined experimental microstructural analysis using scanning and transmission electron microscopy, atom probe tomography and diffraction techniques with multiphase-field simulations on μm scale. Process simulations at mesoscale of the melt pool provide the link between the process conditions and the thermal boundary conditions for the microstructural simulations. The study confirms the appearance of carbides, borides and γ′-precipitates in the as-solidified microstructure. The quantity and particle size of these phases as observed in the experimental samples, are in qualitative agreement with the simulation results. Therefore, the simulations can be used to elucidate and quantify the differences in the solidification path for different thermal process conditions. Although the comparison of samples processed with high energy density (cooling rate 65,000 K/s) with those processed with low energy density (cooling rate: 570,000 K/s) show large differences in the crack density observed in the experiments, the microstructural differences and the phase formation at the dendritic scale do not show any remarkable qualitative or quantitative differences. The correlation between processing conditions, microstructure evolution and crack formation is critically discussed and differences to the current understanding presented in existing literature are identified.