Impact of binder on part densification: Enhancing binder jetting part properties through the fabrication of shelled geometries

In binder jetting (BJT) additive manufacturing (AM), jetted liquid binder binds powder particles and provides structural integrity to the printed green parts. Following printing, the binder is pyrolyzed before densification to final part. While the effects of binder saturation on the green part quality have been explored, the study of the impact of binder on part densification and subsequent part properties is limited. In this study, the impact of binder on densification is studied through a new approach of binder jetting termed as “shell printing” to vary the amount of binder content in a green part. In this approach, binder is only deposited around the part surface, which effectively traps packed powders inside the bound shell geometry. Post-process sintering consolidates both bound (printed shell) and unbound powders and densifies the part. Manipulation of the shell thickness enables exploration of the effects of binder content on process-structure-property relationships. Using pure copper as an exemplar material, and analyzing parts with varying shell thicknesses, it was found that shell printing significantly affects green part density (3.7% increase), final part density (∼5% increase), grain size (∼290% increase) and tensile strength (8.84% increase) when compared to traditional strategies of homogeneous binder placement. While the traditional binding approach improves green part strength and reduces part slumping during sintering, it also hinders densification, constrains grain growth, and induces porosity at the grain boundaries, as compared to the shell printing approach.