Thermokinetics-driven evolution of grain morphologies during laser direct energy deposition

Abstract

The additive manufacturing of non-rare-earth permanent magnetic alloys such as Alnico-8H using laser directed energy deposition has consistently exhibited equiaxed grain morphologies, despite columnar structures being more desirable for enhanced coercivity. This necessitates a process induced thermokinetics–microstructure modeling framework to elucidate the governing mechanisms underlying equiaxed grain formation. The present study addresses this gap by integrating an experimentally validated high-fidelity thermo-fluidic model with CALPHAD-informed analytical Kurz-Giovanola-Trivedi model and Hunt's columnar-to-equiaxed transition criterion. Emphasis is placed on the physical interpretation of constitutional undercooling and nucleation volume density, which significantly influence morphology transitions but are challenging to quantify experimentally. Through this integrated approach, the influence of Marangoni-driven melt convection and rapid thermokinetics on spatial variations in thermal gradients and cooling rates is examined. A comparative analysis with conventionally studied alloys, SS316L and IN718, reveals that the grain morphology in Alnico is governed by markedly different threshold conditions due to its complex multi-component solidification behavior. The study highlights the dominant role of nucleation density in predicting microstructural evolution and provides a thermokinetic based rationale for the persistent equiaxed morphologies observed in Alnico. These findings offer valuable insight into tailoring grain morphology through process control in additive manufacturing of compositionally complex alloys.