Abstract Scope |
To minimize the shrinkage effect after debinding and sintering during metal additive manufacturing by Fused Filament Fabrication (FFF), metal content closer to the random close packing (~0.64) were homogeneously dispersed in polymer matrix in a hot melt extruder. This was done by re-engineering the feedstock mixing, which influences the melt rheology and controls the filament extrusion domain in terms of shear rate susceptibility. In this correlative study, a framework of rheological scaling was presented for two different feedstock mixing method and it was shown that, even with the high zero shear viscosity metal composites prepared by solution mixing process, showed controlled shear thinning behavior to be in the targeted extrusion zone and hence we were able to extrude 63.4 vol.% metal filaments without the use of any additives. Our experimental outcome of zero shear viscosity scaling fits well with the theoretical model of Krieger and Dougherty. Thermogravimetric analysis and micro-computed X-ray tomography were employed to quantify the homogeneity in both global and micro-scale. SAOS (Small amplitude oscillatory shear) test was carried to elucidate the composite filament's transition point from viscoelastic to solid state which creates the onset of jamming. Finally, it was shown that, due to the increased green part density, 3D printed and sintered part exhibited 76% linear shrinkage reduction while compared to the traditional method. This mixing strategy and analysis gave us insight on how to scale and modulate the feedstock melt rheology to create dense 3D printed metal parts with negligible shrinkage using Fused Filament Fabrication (FFF) technique. |