(734c) Rheology Scaling of Metal Particle Reinforced Polymer Matrix Composite (PMC) for 3D Printing of Dense Metal Parts Via Fused Filament Fabrication

Although extrusion-based metal additive manufacturing i.e. FFF (Fused filament fabrication) is plagued by porosity and dimensional shrinkage after post-processing compared to the mainstream metal 3D printing technologies (i.e. powder bed fusion), it has its own merits i.e. low tooling cost, user-friendliness and multi-material printing capabilities with the potential of disseminating prototyping opportunity among hobbyists, home and office users. The cardinal part of metal FFF 3D printing is the “filament” – an extruded PMC typically made of gas-atomized metal powders, compatible polymers and its melt rheological properties, which dictates the maximum metal content of the printed green composite that is critical to improve the printed part porosity and dimensional shrinkage. There remained a specific knowledge gap on how to scale the rheology of these composite’s melt suspension to achieve the highest possible metal packing without losing its ability to extrude in a hot-melt extruder. In our study, two distinct feedstock mixing strategies were examined, both from the industrial perspective and scientific viewpoint. While 63.4 vol% metal filament was successfully extruded from the solution mixing approach, solid-state physical mixing only reached up to 54 vol% and showed unstable extrusion. By conducting flow sweep rheology test, the rheology scaling of the composite's melt suspension was achieved as a function of metal content and mixing strategies, which fitted pretty well with the existing Krieger-Dougherty analytical model. An insight of the jamming physics was also elucidated inside the extruder die leading to extrusion failure by doing small amplitude oscillatory shear (SAOS) test. During the flow sweep test, at higher shear rate (>100s^-1), a distinct shear thinning behavior was observed for 62 vol% solution-mixed filaments due to improved wall adhesion. In comparison, physically mixed filament failed to sustain more than 10s^-1 shear rate proving the point that they were more prone to wall slippage at higher shear rate. Apart from this, a quantifiable proof of homogeneity of metal dispersion inside the polymer matrix was shown by showing the spatial variance along the as-extruded filament length using thermogravimetric analysis and by analyzing micro-computed X-ray tomography images with machine-learning based algorithm. Sintered and 3D printed part of our highest extrudable and windable solution mixed filaments yielded 76% less shrinkage compared to the physically mixed filament. Above all, this was a correlative study based on reverse engineering where a systematic way to achieve highly dense metal part was illustrated by exploiting the rheological signature using extrusion-based metal additive manufacturing.