Architectured microstructures, for example with grain size gradients, can significantly improve a metal’s performance.  Yet, the complexity of such microstructures is limited, because the local control over the microstructure during synthesis is very basic. For example, we can produce grain size gradients or layered structures with nanometer resolution in thin-films by changing the deposition condition during growth. However, the microstructural control over these gradients is limited to a single dimension: the growth direction.
Additive manufacturing (AM) could expand our abilities to fabricate architectured, metallic microstructures to all three dimensions. There are two main prerequisites to enable “microstructure-printing”: first, the resolution of the printing process should be comparable to typical microstructural features: ~10 µm. Secondly, precise control is needed over the synthesized microstructure at voxel-level.
Multiple metal micro-AM techniques are available , which all satisfy the resolution-criterion. Yet, many of them lack the ability to actively control the microstructure of the deposited material. Exceptions are electrochemical and electron-beam based techniques, and, to some extent, approaches that employ in-situ sintering, but the electrochemical methods undoubtedly promise the most elaborate control. This is owed to the fact that electrodeposited microstructures can be a strong function of the deposition potential. Because the in-situ manipulation of this potential is straightforward, the manipulation of the synthesized microstructure is possible at voxel-level.
Here, we demonstrate this basic approach using meniscus-confined electroplating of an electrochemical model-system: electrodeposited CuZn-alloys. In these alloys, the Zn-content is determined by the deposition potential in the range of 0 – 50 at.%. Controlling the local Zn-percentage enables manipulation of the local grain size, as well as the local solid-solution level. Additionally, dealloying of the CuZn-alloy yields nanoporous structures whose porosity replicates the initial Zn distribution. We show printed microscale objects with both grain-size and pore-size gradients and a characterization of their mechanical behavior.
Future possibilities for manipulating microstructures are powerful: nano-twin density and orientation, grain-size, alloy composition or incorporation of reinforcement particles can be changed at will by adjusting the electrode potential. The combination of this strategy with the high resolution of microscale AM could enable the ultimate control over architectured microstructures.
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 L. Hirt, A. Reiser, R. Spolenak, T. Zambelli, Adv. Mater. 2017, 201604211, 1604211.