Ink-based additive manufacturing of high-entropy alloys from oxide and hydride powders

Northwestern University, Evanston IL

Investigators

Abstract

Non-technical Abstract Additive manufacturing (3D printing) of metals is revolutionizing US manufacturing for civilian and defense applications, as it allows the creation of complex-shaped objects in a single operation, typically by melting metal powders with a laser. Inspired by polymer 3D printing, this work focuses on a novel 3D printing method - 3D ink-extrusion - to create strong and lightweight metallic objects. A liquid ink containing metal-hydrogen (hydride) precursor powders and a binder is extruded into solid filaments, layer by layer, which is then heated to remove the solvent, binder and hydrogen, leaving behind a skeleton of metal powders. This skeleton is heated at a high temperature to densify the metal powders to form a strong and lightweight metallic structure. This promising approach has many key advantages as compared to the more established manufacturing methods, including lower costs, improved energy efficiency due to room-temperature printing in air, and formation of more robust metal components. The overall goal of this project is to identify and predict the critical parameters of this promising advanced manufacturing method. Furthermore, a web-based free-access video game, PRIMA (Printing Robots with Inks of Metals and Alloys), is being developed for youngsters (grades 6-12) that conveys the design and scientific principles of 3D-printing techniques in order to excite, educate, and motivate them to pursue STEM and manufacturing education and careers. Technical Abstract This study explores the fundamental physical phenomena associated with a novel additive manufacturing approach, metallic ink-extrusion printing. In this method, inks containing mixed (Co,Cr,Fe,Ni) oxide particles or mixed (Hf,Nb,Ta,Ti,Zr) hydride particles are extrusion-printed into filaments (stand alone or assembled into micro-lattices), which are then reduced/decomposed to metals, inter-diffused to form high-entropy alloys (HEA), and sintered. Systematic studies of the co-reduction (for oxides) or co-decomposition (for hydrides), metal inter-diffusion and porosity evolution are performed as a function of particle composition, size and packing fraction, reducing species (H2 or C) for mechanistic insights. Also, the evolution of pores (remaining from partial sintering or added via foaming/space-holders) during the extrusion, homogenization and sintering of fibers, struts and micro-trusses are examined by both ex situ metallographic techniques and in situ synchrotron x-ray diffraction and tomography. These experimental data allow for the development of diffusion-based models and finite-element models in order to predict and optimize this novel manufacturing approach for specific mechanical properties based on the synthesized micro-lattices, strut porosity, phases, and geometry. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.