Lattice metamaterials are a class of engineered materials with repeating 3D structures (lattices) at the macroscale, microscale or nanoscale, designed to achieve mechanical, thermal, acoustic, or electromagnetic properties not found in naturally occurring materials. Their behaviour is governed more by structure (geometry) than composition. The lattice structures enable us to produce tailorable multifunctional properties, including mechanical, thermal, acoustic, vibration, and electromagnetic properties.  There are several key mechanical properties, which are the focus of this study of lattice metamaterials - high strength-to-weight ratio, programmable stiffness and compliance, negative Poisson's ratio, high energy absorption, etc. Some lattices even combine mechanical load-bearing ability with functionalities like heat management, sensing, or actuation. The ability to blend a range of properties makes these materials ideal for various applications, such as aerospace, transport, automotive, marine, biomedical and sports.

In this work, the main aim is to design and fabricate different metallic lattice materials at the macroscale using the Laser Powder Bed Fusion (LPBF process), with the objective of obtaining different types of mechanical properties, considering both strut and surface-based lattices. 

First, the design of conformal lattice structures to encompass complex, three-dimensional geometry was addressed through a comprehensive review and subsequent development of a systematic design process that establishes a step-by-step procedure incorporating lattice geometry and topology generation, as well as compatibility with lattice types and structural boundary integration [1] . 

Next, the focus was on strut-based cellular metamaterial architectures [2], acquiring low-density and high-strength metallic metamaterials. These titanium (Ti-6Al-4V) lattices were designed imitating Wolff’s law of bone remodelling that led to lattice configurations with an exceptional strength-to-density ratio, compared to conventional cellular metamaterials. This was followed by hollow-strut titanium lattice materials, whereby it was found that hollow-strut Ti-6Al-4V lattice materials exhibit consistently higher strength and stiffness (by as much as 60%) compared to solid-strut counterparts of the same relative density [3].

In summary, it was shown that a conformal lattice design founded on either strut-based (cellular) or surface-based (TPMS) lattices, along with the complex geometry mapping capability, can generate highly tailored mechanical properties for a variety of engineering applications, thus revolutionising next-generation optimised designs with exceptional operational performance and structural integrity.

Negative Poisson’s Ratio (or NPR) is the primary mechanical property of auxetic metamaterials that results in an improved mechanical performance compared to regular material arrangements [1]. The distinctive characteristics that are usually improved are stiffness and impact resistance due to selective and controlled densification [2]. The paper explores the effects of changing the parameters of a cylindrical auxetic structure under static and dynamic conditions on the mechanical characteristics. The auxetic cylinder shell was constructed using 2D auxetic unit cells.

Finite Element Analysis (FEA) through the ABAQUS simulation software was used to model quasi-static compression and impact loading for the auxetic structures. This analysis included a mesh-convergence study to ensure the suitability of mesh resolution for the accuracy of the model results. The change in the aspect ratio (ratio of the height to the diameter) of the cylinder structure and the radial thickness of 2D auxetic unit cells were performed, while the size of the unit cells was kept constant. The change in these parameters was found to change the mechanical properties of the overall structure. The cubical configuration with the aspect ratio close to unity provides NPR values close to 1 as well, while the change in the stiffness depends on the scale of the structure. Such processes can be used to tailor the Poisson’s Ratio and stiffness of lightweight cylinder auxetic structures for different applications.