Dynamic mechanical properties of ultra-fine grained copper microlattices fabricated using micro-additive manufacturing
Three-dimensional (3D) metallic printing of macroscale engineering structures is a well-established field that promises precise spatial control and microstructure. The state-of-the-art metallic printer in the market that uses laser sintering has a minimum feature size of 20µm in thickness and 300µm in lateral direction. Thus, the smallest structures that can be built are typically in the millimeter scale. On the other hand, micron scale metal architectures, even with simple extruded two-dimensional shape such as micropillars and micro-cantilevers (with a length scale of ~2-30µm), can be fabricated only using the serial and time-consuming focused ion beam (FIB) based milling process. Thus, there is a critical need to identify a new method of 3D printing metals with microscale resolution to create complex mesoscale geometries for applications in watch-making, MEMS etc.
In this presentation, for the first time, a new manufacturing technique for producing complex 3D metal microlattices using a localized electrodeposition process will be described. Successfully manufactured full-metal 3D copper microlattices (~50µm overall size), with ~2µm feature size, of three different geometries such as octet, kelvin foam and honeycomb will be shown. The microstructure of the copper lattices identified as ultra-fine grain using electron backscatter diffraction (EBSD)/FIB combination will be presented. Subsequently, the first dynamic compressive properties of the copper microlattices, obtained using a state-of-the art piezo-based in situ micromechanical tester, will be presented as a function strain rate from 0.001/s to 200/s. A strong rate-dependency in the yield stress of copper microlattices will be shown as a function of the three different microlattice geometries. It will also be revealed that the copper microlattices deform in an almost ideal plastic manner with a constant plateau-like crushing stress without initial stress peaks and are able to sustain significant strains upto 25% before densification. Further, given that live monitoring of lattice deformation during dynamic compression tests was not possible due to the low frame rates of the SEM (~35fps) finite element analysis (FEA) was used instead to understand the structural evolution of the copper microlattices at high strain rates. Thus, finally the results from FEA, including the determination of the required constitutive laws using high strain rate copper micropillar compressions will be presented.