Size and build strategy effects for the L-PBF process applied to Inconel 625 vertical struts: a combined numerical and experimental approach

Abstract

A combined numerical and experimental analysis of melt-pool dimensions and resulting solidification conditions was carried out on small laser powder bed fusion (L-PBF) struts (0.2 mm to 2 mm diameters), considered as single constitutive parts of the structure lattice. In the beginning, the high-speed imaging monitoring of melt pools was performed on a dedicated instrumented L-PBF set-up for various scan strategies. In the subsequent stage, a numerical thermal model was employed on COMSOL Multiphysics® to determine the alteration of the melt pool by the struts' diameter and scanning strategy for constant (power, scan speed) conditions. A good agreement was obtained between experimental and numerical melt-pool areas. This allowed validation of calculated local cooling rates and thermal gradients near the solidification front. A clear difference was shown between outside-in or inside-out strategies, and contour-hatching in terms of local solidification conditions. Higher cooling rates were obtained for outside-in conditions, especially near the external part of struts whereas inside-out conditions promoted more uniform cooling rates and thermal gradients. Moreover, a reduction of strut diameter induced the formation of a single melt-pool on the full strut’s surface, which promoted lower and more uniform cooling rates and a highly textured built material. A fairly good agreement was found between simulated thermal data and local microstructure development at the scale of solidification cells. Finally, the current work provides a deeper understanding on size and L-PBF strategy versus microstructure formation, and allows adapting build conditions on strut diameters.