Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 149598

149598 - Carbon Reduction of 3d-Ink-Extruded Oxide Powders for Synthesis of Equiatomic Cocufeni Microlattices 

Additive manufacturing (AM) has recently been used to create high-entropy alloy objects with complex 3D shapes, leveraging the benefits of good material reuse, greater efficiency for production, and reduced defect density. Ink-printing is one of the AM techniques employed to manufacture metallic alloys, where a green body is first 3D-extruded, in the air at ambient temperature, from an ink containing metallic, oxide, or hydride powders and a polymeric binder. The green body is subsequently debound and densified by sintering under simple process control. If oxide particles are used, a reduction step with hydrogen or solid carbon is needed, creating metallic particles which, if (sub-)micron-size, can improve sintering kinetics and surface quality, as compared to AM methods using coarser powders. As compared to fine metallic powders, micron-size oxide powders contribute to cost reduction, eliminate pyrophoric risks, and simplify handling and storage procedures.

In this study, we demonstrate an approach to fabricate equiatomic CoCuFeNi microlattices via carbothermic reduction of a 3D-extrusion-printed ink containing a blend of Co₃O₄, CuO, Fe₂O₃, NiO, and graphite powders. In-situ X-ray diffraction (XRD) is used to analyze the sequential oxide reduction and metallic phase evolution during carbon reduction. The phases and microstructures after sintering are characterized by ex-situ XRD and scanning electron microscopy, starting with loosely-packed, as-printed oxide powders and ending with a fully-reduced, dense metallic CoCuFeNi alloy. The compressive mechanical properties of sintered microlattices with two different strut diameters are measured at ambient temperature.

Equiatomic CoCuFeNi high-entropy alloy microlattices are created by 3D-extrusion printing with an ink containing a blend of binary oxides (Co₃O₄+CuO+Fe₂O₃+NiO) and graphite (C) powders. After printing, the green parts are subjected to a series of heat-treatments under Ar leading to: (i) carbon reduction of the oxides to form metallic particles, (ii) interdiffusion of metal particles to form an alloy, and (iii) sintering to remove porosity. The phase evolution in individual extruded filaments (similar to struts in the microlattices) is observed by in-situ XRD, showing that intermediate suboxide phases (Cu₂O, CoO, Fe₃O₄, CuFeO₂, and FeO) form as the original oxides are reduced by carbon, before the final metallic alloy is formed. At 830 °C, extruded filaments comprise a face-centered cubic (FCC) CoCuNi(+Fe) alloy with unreduced FeO inclusions. After reduction and sintering at 1100 °C, homogenous, densified, equiatomic CoCuFeNi microlattices are achieved, containing small amounts of a Cu-rich phase. At room temperature, the compressive strength of these CoCuFeNi microlattices increases as strut diameter decreases from ~260 to ~130 µm, as expected from an observed drop in strut porosity resulting from a more complete sintering. This is consistent with easier escape, from thinner struts undergoing reduction and sintering, of CO+CO₂ gas created during carbothermic oxide reduction. This new printing method can be applied to a variety of alloy systems, including Cupronickel and Constantan (Cu-Ni) and Invar (Fe-36Ni) alloys, steels, and Ni- or Co-based (super-)alloys.

Presenting Author: Ya-Chu Hsu Northwestern University

Presenting Author Biography: At Northwestern, I am working on 3D printing of high-entropy alloys and reduction of steel alloys.

Authors:

Ya-Chu Hsu Northwestern University
Dingchang Zhang Northwestern University
David Dunand Northwestern University