Metallic Microlattice Structures

Given the wide ranging subject matter covered in this book, this chapter discusses the content and structure of the book in detail. The book takes an integrated approach to metallic microlattice structural design, as materials and manufacturing processes need to be considered in parallel with the structural realisation and application. The chapter introduces conventional structural theory, selective additive manufacturing processes, material and lattice block characterisation, theoretical methods, specialised additive manufacturing processes, structural applications (with quantified performance) and overall conclusions. The book focuses on the structural functions of sandwich construction and on energy absorption.

The aim of Chap. 2 is to give a number of fundamental ideas on lattice structures, independent of the use of additive manufacturing technology. These ideas have been developed for a number of years for conventional structures. Aspects covered will be (a) lattice structures as a cellular material, (b) general nomenclature for lattice structures, (c) lattice structures as core materials in sandwich panels, and (d) impact energy absorption in conventional metallic structures. In this way, relevant rigorous engineering science ideas will be identified, and the potential for applying these ideas to additively manufactured microlattice structures will be highlighted.

The additive manufacturing processes discussed here have been selected for their significance for the selected structural applications, e.g. core materials and energy absorbing materials. Selective laser melting and electron beam melting are mature (industrial) processes, whereas binder jetting (and associated techniques) is currently under intense development. As far as selective laser melting and electron beam melting are concerned, the controlling parameter is the beam scanning strategy, which defines the dimensions and quality of the microlattice. Also, the parent material will influence the realisation process, the final quality of the microlattice and structural performance. In this discussion, three main materials will be discussed: namely, stainless steel 316L, titanium alloy Ti 64, and aluminium alloy AlSi10/12Mg. Stainless steel 316L is widely discussed in the literature, and Ti 64 and AlSi10/12Mg are lower density but more highly reactive materials.

The aim of this chapter is to describe the measurement of parent material properties, mostly using micro strut tensile tests, and to discuss the failure modes of microlattice blocks of selected topologies, mostly under compressive loading. A number of microlattice parent materials (SS316L, Ti 64, AlSi10/12Mg), topologies (BCC, BCCZ, OT), lattice block loadings (compression, tension, shear, combined impact) will be discussed. Modes of failure of microstruts will be discussed and specific stiffness and strength properties compared. Both static and impact loading will be discussed. Parent material behaviour includes large plastic strains and material rupture.

The focus for applications in this book are core materials in sandwich beams and panels, and energy absorbing devices. Hence, microlattice structures will not only be subject to elastic deformation, but also to plastic deformation, buckling and rupture. Also, these responses may take place under impact loading. Thus, theoretical models developed will have to address these non linear and transient effects. The simplest lattice finite element model is modelling the strut as a set of beams. Such an approach is appropriate for complete modelling of large scale microlattice structures. For more detailed modelling, the selected number of cells can be modelled using three dimensional solid elements. Such models discriminate three dimensional plasticity, material rupture and the interaction between cells. For modelling large scale microlattice structures, homogenisation is also an appropriate approach. This latter approach has been followed for foams for a number of years, but modelling localised plasticity, buckling and rupture is problematic. Analytic modelling of microlattice behaviour is useful for parametric investigation and to define and investigate specific structural cases. The synthesis of optimal microlattice structures is problematic given non linearities in response issues. Formal optimisation approaches are not currently possible, but the distinct approach of generative design is relevant. These approaches are discussed in terms of the specific structural applications of interest in this book.

This chapter discusses two alternative methods for manufacturing microlattice structures. In the first method, ultra violet light is shone into a liquid photopolymer, and the liquid solidifies in the volume that the light beam has irradiated. This means that complex lattice structures can be created out of liquid polymer. This lattice can either be used to create solid metallic microlattice structures, using investment casting techniques, or the lattice can be electroless plated with nickel phosphorus alloy. In the latter case, the polymer core is then removed, to produce ultra lightweight hollow microlattices. This chapter discusses mainly the manufacture, materials and progressive collapse of the ultra lightweight, hollow, microlattices. Such structures are an important class of the emerging field of mechanical metamaterials, and the latter are briefly introduced. The second method discussed is woven metal. In this, the metal wire (of the order of 1 mm in diameter) is shaped in three dimensions, and touching nodes are soldered or brazed. Relative densities of 6–43% can be obtained. Highly complex lattice patterns can be obtained. Both methods can be used to create shell lattice (Shellular) structures. Photopolymer wave guides are discussed first.

This chapter focuses on a small number of specific structural applications relevant to the additive manufacturing processes discussed. The focus here is mainly on two applications, namely core material in sandwich construction and energy absorbing devices. As far as core materials are concerned, two specific cases will then be discussed, namely cores in beam bending and in panels. In the case of energy absorbing devices, the use of lattices will be discussed first and then the use of more complex, shell like, elements will be discussed. The ultimate aim of this discussion is to quantify improvements in structural performance by the effective use of additive manufacture technology.