Engineered bi-material lattices with thermo-mechanical programmability

Lightweight meta-structures can reveal combinations of properties that do not occur in conventional structures. In this paper, a 2D bi-material unit cell is proposed based on a re-entrant architecture. Two 3D unit cells, socalled star and cubic architectures, are then constructed using the proposed 2D unit cell. Finally, lattice-based unit cells are periodically arranged to develop a meta-structure. The thermo-mechanical functionalities of the developed meta-lattices can be programmed by tailoring either architectural parameters (i.e., strut length, diameter, shape, and orientation) or the material combination of struts. The influence of the architectural parameters and material combinations on the effective elastic stiffness, Poisson's ratio (PR), and coefficient of thermal expansion (CTE) is systematically investigated. A finite element (FE) model is established to perform a goal-driven multi-objective optimization and to attain an uncommon combination of PR and CTE (e.g. negative PR up to ?1.8 or near-zero CTE) with uncompromised stiffness. Despite the complicated design (many struts are diagonal with respect to the printing bed), the low-cost additive manufacturing process (i.e., fused filament fabrication) parameters are optimized to fabricate a selected number of bi-material meta-lattices with a reasonable time and quality. By employing the FE model, the optimized architectural parameters are determined for a series of applied engineering applications such as structurally robust aerospace structures, energy absorbable structural elements, and shape transforming structures. The developed lightweight meta-lattices with tunable thermo-mechanical properties can offer the material selection charts with more economical material options for engineering applications such as antennas and precision instruments.