Experimental study and simulation of sintering of 316L components produced by binder jetting

Licentiate thesis, 2023

Binder Jetting (BJT) is a multi-step Additive Manufacturing (AM) technique that is used for producing components with highly complex geometries and competitive final properties with high productivity when compared to other AM technologies. The first step provides the basic part geometric shape (BJT printing), and the next step (debinding and sintering) consolidates the part to reach final geometry and intended basic material properties. Due to the low density of green BJT components after printing (~50-60%), significant shrinkage (~20%) occurs during the sintering process along different directions. Also, sintering may lead to distortion of the external shape of the components.
During BJT printing, the powder is being deposited layer-by-layer and binder is selectively placed to create a 3D geometry. Therefore, the metal particle’s arrangement of the green BJT components is influenced by the layer-by-layer buildup nature of the printing process. This impacts the behavior of the components during the debinding and sintering process. The first part of this study aims to develop the understanding of densification development during the sintering of 316L stainless-steel BJT samples. The intensity of the dimensional evolution anisotropy was characterized by multi-axial dilatometry experiments. Measured shrinkages were up to 15% higher along the building direction, while minor variation was found between the other two orthogonal directions. Only small shrinkages (<0.5%) were observed during debinding without significant anisotropy. A rapid increase of the shrinkage rate was observed at high temperature (~1310°C), related to the formation of δ-ferrite phase. This boost of densification is critical to achieve high densities (96-99%) of 316L BJT sintered components. The second part consists of the microstructural evolution analysis. The EBSD phase maps showed the formation of δ-ferrite at temperatures >1300°C. The porosity characterization within different cross-sections demonstrated that some anisotropic distribution of porosity may be developed during sintering.
The last part of this study introduces the application of the continuum theory of sintering for modelling the sintering behavior of 316L BJT components. The identification of model parameters was done from dilatometry data. Then, a new material viscosity expression was proposed to account for the effect of δ-ferrite transformation. The model was proved to accomplish good predictions of the density evolution during sintering of BJT samples.