Kinetics of alloy formation and densification in Fe-Ni-Mo microfilaments extruded from oxide- or metal-powder inks

Acta Materialia 2020 Volume 193, Pages 51-60

3D ink-extrusion of powders followed by sintering is an emerging alternative to beam-based additive manufacturing, capable of creating 3D metallic objects from 1D-extruded microfilaments. Here, in situ synchrotron X-ray diffraction and tomography are combined to study the phase evolution, alloy formation and sinter-densification of Fe-20Ni-5Mo (at.%) microfilaments. The filaments are <200 µm in diameter and are extruded from inks containing a blend of Fe2O3+NiO+MoO3 µm-sized oxide particles. Blended oxide inks show rapid reduction and homogenization during heating to 1373 K in H2 accompanied by fast densification and interdiffusion. The resulting homogenous Fe-20Ni and Fe-20Ni-5Mo alloys reach near-full density within minutes at 1373 K. When using Ar-5%H2, co-reduction and interdiffusion are slower and the sequence of reduction is changed. During H2 co-reduction, Fe2O3 and NiO show synergistic effects. The onset temperature of reduction is mutually reduced and the conversion rates to Fe3O4 and Ni are increased. Fe-20Ni-5Mo microfilaments printed with coarser, elemental powder (~3 µm) show slower sintering and compositional homogenization as compared to inks of blended oxide, as the coarser metal particles provide lower surface/volume ratio and higher diffusion distances. Using micron-size oxide powders (rather than coarser metallic particles) accelerates kinetics of reduction, sintering and interdiffusion which reduces costs and energy in 3D ink-printing, improves filament surface quality, but doubles the extent of shrinkage