Achieving uniformly refined grain structure in fusion-based metal additive manufacturing: Experimental demonstration and analytical model development

Abstract

Adding nucleants is a common method for achieving columnar to equiaxed transition (CET) in laser metal additive manufacturing (AM). However, the resulting microstructure often exhibits heterogeneity due to variations in solidification conditions across different melt pool locations, which introduces uncertainties in mechanical properties. Here, we achieved uniformly refined equiaxed grain structure at every location of the melt pool during laser powder bed fusion (LPBF) of Al6061 by adding TiC naoparticles. To elucidate the underlying mechanisms of columnar to equiaxed transition at different melt pool locations, we employed experimentally validated thermo-fluid dynamics simulations to capture the dynamic evolution of solidification conditions. Analysis using Hunt’s CET model revealed that TiC-induced heterogeneous nucleation can facilitate columnar to equiaxed transition only at the melt pool center. Further grain refinement at the melt pool boundary, characterized by a low solidification rate and a high temperature gradient, was achieved through particle-induced grain growth restriction. Given previous analytical CET models do not explicitly account for particle-induced growth restriction effects, we developed an analytical model that integrates the particle induced growth restriction mechanism for predicting grain structure. The developed model accurately predicts the grain morphology evolution at different melt pool locations observed in our Al6061+TiC samples. Our research provides quantitative insights and material design guidelines for achieving uniformly refined grain structures in fusion-based metal AM processes.