4D-printed NiTi auxetic metamaterial with enhanced functionality via deposition of nano-Ni layer

Abstract

The aerospace and energy conversion industries utilize superelastic metallic metamaterials fabricated by additive manufacturing (AM). Currently, printing samples with “residual particle-free” surfaces requires post-cleaning techniques such as chemical and electrochemical etching. However, the high concentration of stress and cracking near the nodes and sharp edges can cause brittle fracture, strength reduction, and energy absorption. Moreover, topological optimization can significantly increase the weight of a structure. To overcome these challenges, in this study, we utilized the benefits of multicomponent structures and nanomaterials possessing high capillary effects and low melting points. Specifically, a thin layer of nano-Ni was deposited on the surfaces of NiTi auxetic structures printed using micro-Ni–Ti powders. The results showed that high-performance functional structures with reliable surfaces, high strength, high energy absorption, and constant Poisson’s ratios for a given deformation could be achieved. This was attributed to a low stress-induced martensite formed during laser processing and a low residual stress, computed experimentally and by simulations. Although the loss factor or dissipated energy during superelastic cycling was enhanced, the superelastic reproducibility was lowered in the samples fabricated using the developed method. This was attributed to the higher strain hardening and dislocation generation, despite the developed approach resulting in a very high superelastic recovery in the initial cycles. The findings of this study provide a framework for designing high-performance functional structures using AM and nanomaterials for energy-absorbing devices with high resistance to vibration.