Binder Jetting and Heat Treatment of Ferrous Alloys

Abstract

Binder Jetting (BJ) is a manufacturing process that involves iterative and layer-wise deposition of powder material and an adhesive binder to construct geometrical features. It can be used to construct freeform and complex objects out of metal or ceramic powders. Printed BJ products typically must undergo sintering at high temperatures to fuse together the powder particles and create a dense structure. BJ is a relatively new technology that requires more research and development to promote its adoption into the industrial space. It is attractive because of its comparatively low cost yet high customizability and scalability. This is especially the case for metal part production.

In this thesis, process development was undertaken for BJ and heat treatment of ferrous metal powders. Iron (Fe) and silicon-iron (Fe-Si) powders were studied as materials of interest. The goals of this work were to establish process maps for BJ and sintering to achieve respectively high densities, as well as to better understand the significance of the relevant parameters. In the BJ process, studies to tailor the parameters and then to optimize for green density were conducted. The effect of powder morphology was discussed. Statistical significance of parameters and their interactions was noted. Regression analysis formed the basis of the optimization. Expressions for green density and powder packing behavior were derived in terms of the parameters. Green densities of 49.7% were achieved for the irregular Fe powder and 71.3% for the spherical Fe-Si powder.

Beyond green density optimization, the importance of debinding temperature and duration was explored. Sintering was investigated in two different modes: in the solid state for Fe and in the liquid phase for Fe-Si. Sintered densities of 91.3% were achieved for Fe and 94.7% for Fe-Si. For the Fe dataset, it was found that sintering at high temperatures diminished local variability in green density. Observations of sinter necks and the density values indicated that sintering occurred in the intermediate-final stages. For the Fe-Si dataset, rapid densification was achieved within minutes of reaching the liquid phase. The influence of sintering hold duration was found to be small, which is consistent with the theoretical understanding. Sinter necks were observed to be in the form of a Si-rich liquid and appeared well-progressed. The Si segregation phenomenon seemed to be amplified by temperature and more importantly by the presence of surface oxides. The work done on BJ and sintering of Fe and Fe-Si forms the basis for further studies on densification. Master Sinter Curves (MSCs) and Master Sinter Surfaces (MSSs) are planned as future work that will increase the utility of BJ in industrial applications to produce high and predictable densities.