A process to spatially control the fraction of SS420 and bronze phases in binder jet infiltrated parts

Abstract

Additive manufacturing (AM) is a useful tool to fabricate components with unique spatial control over material composition, phases, and geometry. However, spatially controlling the material phases in AM metals typically requires changing powder during the printing process or using multiple nozzles, and most methods only offer control in one dimension. This requires a significant printer hardware upgrade. This paper presents a method that exploits the binder jet additive manufacturing process to spatially control the fraction of stainless steel 420 (SS420) and infiltrated bronze. In our method, we modify the CAD file of the binder jet part to include completely enclosed voids within the printed part, which result in enclosed regions of loose steel powder without binder. This process does not induce extra porosity, nor require hardware modifications. Post-sintering, we use optical microscopy to show that zones of enclosed SS420 without binder contain up to a 25 % increase in SS420 phase compared to the zones printed using the conventional method (i.e., with binder). To understand the mechanisms behind the spatial distribution of phase fraction, we employ the method to print parts with higher SS420 concentration channels of different thicknesses in horizontal and vertical directions. Results show that the relative phase fraction of SS420 and bronze depends on channel size, channel orientation, and sintering direction. Further, the SS420 percentage can be uniform or spatially graded inside the channels depending on the sintering direction relative to the printing directions. We hypothesize that the underlying mechanisms of phase fraction variation in this method are powder spreading, packing during the print process as well as the effect of gravity on loose powder during sintering step. To establish a possible application of the method, the spatial variation of Rockwell B hardness was measured in selected samples. The hardness results show that our method obtains spatial variation of mechanical properties, and qualitatively corroborate the phase fraction variation obtained from optical microscopy. Although here we show phase fraction control over two dimensions, the method could be applied to obtain a 3D phase fraction control. The method only requires changes in the CAD model of the printed part, without any hardware or software modifications of the typical binder jet AM process.