Abstract
Additive manufacturing (AM) of metals can enable rapid development of functional parts of complex geometry, with potential applications in the aerospace, automotive, and biomedical fields [1–3]. Typical metal additive manufacturing techniques are based on expensive laser melting or sintering processes which are often highly anisotropic, limiting the development and use of these methods. Furthermore, few additive manufacturing techniques focus on high temperature materials, ceramics, and fabrication of machine elements. The recent introduction of Hydrogel-Infusion Additive Manufacturing (HIAM) may reduce some of these barriers, enabling potential applications in high performance metal and ceramic devices and components. The HIAM process involves 3D printing a polyethylene oxide (PEO) photo-resin using vat polymerization, immersing the polymer in a metal salt solution which allows ionized metal cations to bond to the polymer backbone, followed by calcination to combust the polymer leaving a metal oxide that takes the same functional structure as the original polymer. Finally, the metal oxide is reduced using a forming gas (95% N2/5% H2) to give a metal product that maintains a scaled-down version of the complex as-printed architecture. This technique enables architected features with microscale resolution by use of a single photoresin simply by varying post-processing conditions. As a first step in the fabrication of machine elements and devices, this paper outlines an attempt to fabricate springs made from silver metal via HIAM. Silver nitrate is infused into an additively manufactured polymer spring structure. Based on the relative differences in the standard free energy of the oxides, Silver Oxide (AgO) is readily reduced to metallic silver under a single thermal processing step: calcination/reduction at 500°C without the need for forming gas. A variety of analytical techniques confirm HIAM processing obeys chemical kinetics of single-step calcination and reduction in accordance with literature and results in fabricated components of low microstrain (8.40E−7 ± 2.78E−9) crystalline silver with average crystallite size of 514.95 ± 5.32 Å and lattice parameter of 4.09 ± 5.23E−5 Å. Thermal analyses such as Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) elucidate the mass loss and reactions that occur during the furnace processing. Springs were subjected to quasi-static and cyclic loading using a Dynamic Mechanical Analyzer (DMA). A range of ∼2–20 N/mm stiffness was measured in unloading for different coil diameters and produced springs show consistency of part stiffness following compaction under cyclic loading.
