Combined experimental and modeling study of melt pool dynamics and characteristics governing columnar-to-equiaxed microstructure transition during single-track laser remelting of Inconel 625

Accurate prediction of melt pool behavior is essential for understanding microstructure evolution in Laser Powder Bed Fusion (LPBF). In this study, the term melt pool characteristics refers not only to melt pool geometry, but also to temporal and thermal metrics, specifically melt pool lifetime, cooling time, and cooling rate. These quantities, together with melt pool shape and flow dynamics, govern local solidification conditions and subsequent grain evolution. A high-fidelity thermo-fluid model was developed to simulate the single-track laser remelting of an IN625, incorporating key physical phenomena including heat transfer, fluid flow, and phase transformation. The model was further linked to a MATLAB-based computational framework for post-processing, enabling the quantitative evaluation of melt pool lifetime, cooling time, and cooling rate at different spatial locations within the melt pool. The model was validated against experimental measurements of melt pool geometry obtained from single-track laser remelting of Inconel 625. Predicted melt pool dimensions, including width, depth, and overall morphology, showed good agreement with experimental observations. The results reveal a distinct melt pool flow pattern, where fluid circulation is concentrated in the upper and lower regions, while a narrow central zone exhibits near-zero velocity. The position and extent of this low-velocity region vary with scanning speed and corresponding melt pool shape. Due to limited fluid motion, this region experiences accelerated solidification, which suppresses columnar grain growth and promotes the formation of equiaxed grains near the top surface of the melt pool. Furthermore, significant spatial variations in melt pool characteristics (lifetime, cooling time, and cooling rate) were observed, strongly governed by melt pool geometry and flow behavior. These findings provide deeper insight into the coupling between melt pool dynamics and solidification conditions, contributing to a more precise understanding of microstructure evolution in LPBF.