Sintering simulation framework for 316L stainless steel components manufactured by binder jetting

Doktorsavhandling, 2024

Binder Jetting (BJT) is an additive manufacturing (AM) technology allowing mass-production of small to medium-size metal components. BJT is a multi-step AM technology, where the geometrical shape of components is provided during the printing step, and the final properties are achieved through a second consolidation step - sintering. The sintered properties are achieved through densification of the porous (~40-60%) BJT parts, resulting in large volumetric shrinkages during the process. Furthermore, shape distortions are common due to sintering at high temperatures, close to the melting point of the metallic powder.

The first part of the thesis focusses on establishing an experimental characterization procedure for the sintering of BJT parts. The dimensional evolution from dilatometry experiments reveals the anisotropic sintering shrinkages with larger shrinkages along the building direction. Debinding does not induce substantial shrinkages (< 0.5%) or shrinkage anisotropy. Density fluctuations along the building direction related to the printing layer thickness of 42 µm were revealed, which decrease during sintering. An increase in shrinkage rate above ~1310°C was observed, related to the formation of
δ-ferrite phase detected in samples sintered at 1370 °C.

The second part of the thesis focusses on the development and implementation of a phenomenological model of sintering based on experimental input from the first part. The model is based on the continuum theory of sintering and describes the particularities of the 316L stainless steel BJT components during sintering. The bulk viscosity dependency on porosity is studied by using different expressions with and without fitting constants. Also, a new material shear viscosity expression is proposed, which explicitly accounts for the effect of δ-ferrite formation on the sintering behavior.

The last part comprises the implementation of the model expressions in a commercial FEA software and the sintering simulation of BJT geometries that showcases predictions with less than ~1 mm deviation on the shape distortions due to gravity. Demonstrator components were designed, including overhang structures that lead to large shape deformations. Different sintering models’ results are compared, where the ROH (Rios-Olevsky-Hryha) sintering model shows the best performance revealing small deviations of ~0.56 mm related to the isotropic assumption of the model.

This works paves the ground for its expansion to component manufactured using different BJT printers and stainless steel powders. Moreover, it has a high application potential to other sinter-based manufacturing technologies.