Electrochemical additive manufacturing of micro/nano functional metals

Conventional additive manufacturing (AM) technology driven by thermal energy have enabled reliable fabrication of macro/micro metal structures. Thermally printed metal components often undergo tedious post-treatments to remove the binders and improve the structural density. On the other hand, drastic changes in processing temperatures may lead to thermal expansion and redistribution of residual stresses in the metal parts, which triggers deformation and defects on the metal surfaces. While electrochemical additive manufacturing (ECAM) technology has rapidly developed in recent years, allowing the direct fabrication of dense and non-porous metal structures based on anode motion-guided metal reduction at the micro/nano scales. Nevertheless, since the anode movement and power supply variations affect the metal nucleation behaviour, leading to significant differences in their end-product mechanical performance, it is necessary to systematically elaborate on 3D ECAM technology in different operating modes. This review offers a comprehensive overview of 3D ECAM technology including micro/nano and vertical/lateral fabrication processes. The deposition principles, operation modes, nucleation mechanisms, process optimization, and potential applications of micro-3D ECAM technologies, including localized electrodeposition (LECD), meniscus-confined electrodeposition (MCED), and jet electrodeposition (JECD), are mainly discussed. The technical characteristics and printable geometries of nano-3D ECAM technology represented by electrohydrodynamic (EHD) printing and tip-based nanofabrication (TBN) are described in detail. Ultimately, we provide an in-depth comparison between 3D ECAM and thermal AM technologies in terms of treatments, printable materials and their maximum surface hardness to fully evaluate and prospect the feasibility of 3D ECAM as an alternative to thermal 3D printing for the advanced manufacturing of functional metals.