Understanding the mechanism of columnar-to-equiaxed transition and grain refinement in additively manufactured steel during laser powder bed fusion

Controlling the columnar-to-equiaxed transition (CET) is one of the key mechanisms to design microstructures in additively manufactured metals. Nanoparticles are effective sites for heterogeneous nucleation during additive manufacturing (AM) of ferritic stainless steels promoting fine grain structures and influencing CET. However, the combined mechanisms of grain refinement, CET, and Zener pinning originating from these nanoparticles are not yet well understood. In this study, the process-structure relationship was quantitatively analysed for a Fe-18Cr steel containing TiN particles. A combined finite-element method (FEM) and multi-phase field method (PFM) approach was used to reveal the mechanisms that govern the microstructure evolution during laser powder bed fusion (LPBF). TiN particles were considered implicitly through the seed-density model in the phase-field model, including their influence on heterogeneous nucleation as well as the Zener pinning effect. It was found that undercooling, required for particle activation as nucleus for solidification, was the dominant parameter for CET in the particle-inoculated Fe-18Cr steel. The undercooling was controlled by process parameters and particle properties. The process parameters during LPBF controlled the temperature gradient and solidification velocity across the melt-pool and influenced the respective undercooling. Concurrently, the particle properties that are controlled by the free growth criterion determined the undercooling required for activation of a particle as a nucleus during solidification. The reliability of the model was validated by experimental data from literature through the effect of process parameters and TiN particles on the CET and microstructure characteristics. The influence of the solidification behaviour as well as particle size, volume fraction, and distribution on the microstructure evolution during LPBF will be discussed based on activated nuclei density plots. This is the first PFM model of solidification during LPBF that considers the heterogeneous nucleation in inoculated alloys including the growth constraint without any rough approximations for the grain evolution during solidification.