In-situ monitoring of sintering and analytical modeling of densification and shrinkage in binder jetted 316 L stainless steel

Abstract

In binder jetting, shrinkage and deformation occur during the sintering step, both of which are affected by the green density of the binder jetted materials. The study innovatively introduces a cost-effective, practical, and in-process monitoring system for visualizing shrinkage and deformation on larger samples than conventionally observed using small-scale specimens in dillatometry equipment. The powder characteristics and binder jet printing process itself influence the initial green density. The comprehensive analysis of powder flowability and packing density, densification behavior, and shrinkage reveals that the consolidated parts using virgin powder (with a green density of 55 %) can achieve a relative density above 99.9 % with an anisotropic shrinkage in the Z > X > Y direction. In contrast, the used or recycled powder exhibits a lower green density of ∼48 %, higher shrinkage rate in all three dimensions, and a decreased degree of anisotropy. Using in-process imaging and experimental data on the grain size attained through optical microscopy and electron backscatered diffraction imaging, the material's shear and bulk viscosities were determined. The formation of delta-ferrite and its impact on densification were discussed in the context of solid-state and supersolidus liquid phase sintering. The model relied on the continuum sintering theory formulated by Skorohod and Olevsky. The strain evolution from the in-situ imaging of sintering process is correlated with porosity based on the used feedstock and applied sintering temperatures. The outcomes of this study offer valuable perspectives on anisotropic sintering mechanisms, bridging the knowledge gap regarding the relationships between structures produced through binder jetting and subsequent sintering of materials.