Sintering anisotropy of binder jetted 316L stainless steel: part II - microstructure evolution during sintering

Green density of binder jetted parts are typically equal or lower than the powder tap density. Also, anisotropic green porosity distribution is expected because of the characteristics of the binder jetting (BJ) printing process. In this study, the microstructure evolution in terms of phases and porosity characteristics was studied. A transition from irregular-shape interconnected porosity in pre-sintered samples to closed quasi-spherical porosity for samples sintered at 1370 degrees C was observed. EBSD phase map showed similar to 2.73% of delta-ferrite in sample sintered at 1370 degrees C. The anisotropic porosity distribution was revealed by a higher area fraction of aligned large pores (>35 mu m), within the cross-section perpendicular to the building direction. Chemical analysis showed an increase of C, O and N on the green sample, while a strong decrease was found after sintering when compared with the powder chemistry. delta-ferrite onset, from phase equilibrium calculations, varies from similar to 1250 degrees C (sintered sample chemistry) to similar to 1350 degrees C (powder chemistry).