DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

4.3.5. Sintering and Metallography
After sintering in reducing atmosphere at 900 °C, Ni parts fabricated using the
φsolids=0.472 ink have sintered density >99.0% of theoretical, as measured by Archimedes
method. Example sintered nickel rods (from a cylindrical mold) and lattices (fabricated
by Robocasting) are shown in Figure 4.6. Metallography reveals good microstructure
with small grain sizes and little porosity. Several factors contribute to the high sintered
density and good microstructure, including an appropriate sintering profile, a high green
density, and homogeneity in particle packing; all these factors act concertedly to yield a
high sintered density.
Archimedes method indicates 99.1±0.1% of theoretical density for solid
specimens, and a calculated 100±0.1% density for latticed specimens. Short diffusion
distance for pore exclusion in the radial direction of filaments likely contributes to the
higher density of the lattice specimens. Metallography of the solid specimens reveals
small grain sizes with little porosity, as shown in Figure 4.7a. This microstructure shown
has grain sizes no larger than 20 µm with an ASTM grain size number of n = 12.6
(Planimetric Procedure, ASTM E112 Section 9) and nominal grain diameter of 4.6 µm.
For comparison, the best result reported in literature for powder metallurgy formation of
Ni Components has a microstructure with grain size range from 3-30 µm, an average
grain size of 5 µm, but at slightly lower density of 97%. 136
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