Metamaterial Design and Additive Manufacturing

By Bo Song, Aiguo Zhao, Lei Zhang and 3 more

Metamaterial Design and Additive Manufacturing covers optimization design, manufacturing, microstructure, mechanical properties, acoustic properties, mass-transport properties and application examples of PMs fabricated by selective laser melting additive manufacturing technology. The book introduces the definition and concept of pentamode metamaterials and then describes their characterization, including manufacturing fidelity, mechanical response, acoustic properties and so on. Final sections analyze research situations, problems and applications of additive manufacturing pentamode metamaterials.

• Covers design and optimization methods of pentamode metamaterials
• Describes manufacturing fidelity, microstructure and physical properties of pentamode metamaterials fabricated by AM
• Includes recent applications for pentamode metamaterials, along with research situations and potential problems

• Language: English
• Publisher: Academic Press
• Release date: Apr 24, 2023
• ISBN: 9780443189012

Bo Song

Professor Bo is based at the School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, China. Specialties and research interests: Additive manufacturing, Powder metallurgy, Design of materials and structure, Advanced functional parts and coatings

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Chapter 1: Introduction

Bo Song; Yusheng Shi    School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, China

Abstract

The concept of metamaterials originates from the proposal of left-handed materials with negative refractive index, which was followed by varieties of metamaterials with unusual properties that cannot be found in natural materials, such as zero/negative Poisson's ratio, and electromagnetic/acoustic/thermal cloaking effect. According to their application fields, metamaterials are roughly classified into four categories: mechanical metamaterials, acoustic metamaterials, thermal metamaterials, and biological and microlattice metamaterials. By designing structures and arranging the distribution of materials with different physical parameters, the function of metamaterials can be realized in theory. Additive manufacturing (AM) technology provides a more direct and efficient way to achieve a sample of metamaterial and experiment verification, due to the significant advantages in fabricating complex structures. This chapter introduces typical metamaterials in different application situations and describes their design methods. Particular focus is placed on the fabrication of metamaterials and the application status of AM technology in them.

Keywords

Metamaterials; Negative refractive index; Negative Poisson's ratio; Additive manufacturing; Selective laser melting

1.1: Origin and classification of metamaterials

Metamaterials refer to artificial structures or composite materials with extraordinary physical properties that cannot be found in natural materials, such as electromagnetic/acoustic cloaks [1–5], zero/negative Poisson's ratio [6–10], negative refractive index [11–15], etc. To make use of these promising phenomena, some carefully designed singlematerial or multimaterial structures were put forward, consisting of identical or gradually changing cell arrays. Similar to atoms or molecules in natural materials, these cells are basic units that determine the properties of metamaterials (MMs); they are therefore called artificial atoms. According to their functionalities, the MMs that are currently developed can be roughly classified into four categories: mechanical metamaterials, acoustic metamaterials, thermal metamaterials, and biological and microlattice metamaterials, as shown in Fig. 1.1.

Fig. 1.1

Fig. 1.1 The classification of MMs based on their functionalities.

The research of metamaterials derived from the proposal of left-handed materials by Soviet physicist Veselago in 1968 [11]. These materials led to many very interesting physical phenomena, such as negative refraction, electromagnetic stealth or absorption, etc. Another significant research achievement of electromagnetic metamaterials was the proposal of the photonic crystal [16,17], in which electromagnetic waves of a certain frequency range cannot propagate because there is a photonic bandgap in their band structure, just like the electronic bandgap in semiconductors. This made photonic crystals promising in applications involving lasers and high-quality microwave antennas. However, before electromagnetic metamaterial structures with negative permittivity and permeability were designed and manufactured [12,18], research in this field remained theoretical. Subsequently, the word metamaterials [19] was used to refer to materials that achieved unusual performance beyond the limits of natural materials, and the term was widely accepted. These exciting works attracted more and more researchers to devote themselves to the field of electromagnetic metamaterials. Varieties of electromagnetic metamaterials with unusual properties were proposed, such as electromagnetic cloaks [1–3], electromagnetic wave absorbers [20–22], terahertz electromagnetic metamaterials [23–25], etc.

Inspired by photonic crystals, Liu et al. [26] proposed locally resonant photonic crystals, which opened the door to the study of acoustic metamaterials. With further research, acoustic metamaterials with both negative equivalent modulus and equivalent density—the two most important parameters for acoustic materials—were realized by dispersing soft rubber in water [13]. These double negative acoustic metamaterials could achieve a negative refractive index. It was widely accepted that all acoustic metamaterials relied on locally resonant units to realize their extraordinary properties [27], until Norris [28] proposed that pentamode metamaterials with effective density and bulk modulus could be regulated in a large range without a resonant phenomenon. These pentamode metamaterials had fluid properties and could decouple effective bulk modulus and density, meaning that the two most important parameters could be designed separately without affecting each other. Afterward, varieties of acoustic metamaterials, such as acoustic cloaking metamaterials [5,29–31], acoustic absorption metamaterials [32–35], and acoustic focusing metamaterials [36–38], were proposed.

With further research into metamaterials, significant progress has been made in the study of mechanical, thermal, biological, and microlattice metamaterials. The term auxetics refers to materials with a negative Poisson's ratio [6,39] and was first introduced by Evans in 1991 [40], playing an important role in mechanical metamaterials (MMMs) [41]. This phenomenon is mainly attributed to the materials’ unique microstructure and composites designs [40,42], and can improve mechanical properties such as enhanced shear moduli, indentation resistance, fracture toughness, and impact energy absorption [41]. Research on thermal metamaterials was mainly targeted at controlling the heat flux by carefully arranging the distribution of different materials and designing microstructures. Furthermore, thermal metamaterials could be combined with other kinds of metamaterials to achieve multifunction. A biological metamaterial is a structural material that can provide a unique combination of mechanical, mass transport, and biological properties, and can meet the particular requirements of hard substrates, such as bone implants and dental implants. Microlattice metamaterial is a composite structure with multiple physical properties arranged in an atomic lattice configuration, which can realize components with multiple performance requirements, such as sound absorption and load-bearing structure, biological scaffold structure, etc.

1.2: Additive manufacturing metamaterials

The advantageous properties of metamaterials are mainly realized by designing the structure and combining various materials. Traditional processing methods to fabricate them, such as casting, welding, and molding, were inconvenient, and some sophisticated lattice structures simply cannot be manufactured. Thanks to the advent of additive manufacturing (AM) technology, the fabrication and experiment verification of metamaterials can now be realized more conveniently and efficiently. By discretizing a 3D model of an object into several thin layers and accumulating them layer by layer, AM technology is theoretically capable of fabricating any complicated structures. In addition, AM has been greatly developed over the past 40 years; it was patented in 1979, and more than 20 AM technologies have now been recognized [43]. Moreover, new technologies are still emerging [44–50]. For example, Daniel Oran et al. [47] proposed a nanoscale AM technology that involved volumetric deposition and controlled shrinkage, which can use more than one functional material at the same time. Moreover, materials with different properties, including metals, semiconductors, and biomolecules, can be utilized.

On the whole, according to the forming material state, AM technologies can be divided into wire-based, liquid-based, powder-based, and mixed liquid-powder-based types. The applicable materials contain metals, polymers, and ceramics [43], and the manufacturing dimension ranges from nanoscale [45,47,49,51–54] to meter-scale, which is very effective in meeting the ultrahigh requirements of most metamaterials. Moreover, AM technologies can work by automatic control software and equipment, which offers significant labor savings. Table 1.1 lists examples of applications of AM technologies in metamaterials. However, it should be noted that different AM technologies have different characteristics: the forming materials, size, resolution, and surface quality may all vary significantly. When manufacturing metamaterials, it is necessary to select the appropriate technology according to the structure and characteristics of the required material. Fig. 1.2 illustrates the manufacturing characteristics (mainly relating to manufacturing size and resolution) and applied materials of some typical AM technologies, which reveal the limitations of AM technologies in fabricating metamaterials with various dimensions. Although the figure is mapped with reference to acoustic and electromagnetic waves, it has general applicability in balancing the manufacturing resolution and size of metamaterials. It should be noted that although AM technology is developing rapidly, there are still some limits in terms of of manufacturing some types of metamaterials, such as ultrafine nanoscale complex structures, multimaterial systems, ultralarge structures, and so on. These problems will be discussed in detail in Chapter 1.3.

Table 1.1

Notes: DLW, direct laser writing; FDM, fused deposition modeling; SLA, stereolithography apparatus; SLM, selective laser melting.

Fig. 1.2

Fig. 1.2 Manufacturing characteristics and applied materials of some typical AM technologies. The triangle, circle, and square represent the maximum part size, minimum part size, and manufacturing resolution, respectively. The position of the symbol represents the dimension corresponding to the wavelength. The distance between two symbols represents the achievable amount of unit cells per AM technology.

1.3: Selective laser melting

With the development of additive manufacturing (AM) technology, and particularly the continuous progress of metal AM technology, AM technology provides an innovative idea and new research approach for the design and manufacture of high-performance complex components. Powder-bed fusion additive manufacturing (PBF-AM) is one important type of AM technology. PBF-AM is a typical technology that is commonly used to manufacture small parts with high surface accuracy requirements, and shows good reproducibility in manufacture. A high-energy beam scans the powder bed at a designed rate and trajectory. This beam directly interacts with the powder to produce tiny molten pools, which enables complex components to be formed by continuous melting and solidification layer by layer, with good forming accuracy and surface quality. Electron beam and laser beam are the two main energy beams used in the PBF-AM process, and are respectively applied in electron beam melting (EBM) and selective laser melting (SLM)/selective laser sintering (SLS). The difference between SLM and SLS is that the SLM powder goes through a solid and liquid transformation process, whereas the SLS powder hardly goes through a liquid state. In metal AM technology, SLM is the best means of forming metal complex components; the technology is to realize high-performance parts with complex shapes by continuously diffusing powder and regionally selective laser melting powder, point by point, line by line and surface by surface. It has high manufacturing accuracy and great potential in forming topologically controllable metal lattice structure. It can meet the specific functional requirements of load-bearing or energy absorption applications [66–69]. SLM technology is an effective method for the design, manufacture, and application of high-performance powder materials. As shown in Fig. 1.3, after using the laser to scan a layer of raw material powder, the forming platform moves down a displacement of a layer thickness in the vertical direction. Then the powder cylinder is moved upward the same displacement in the vertical direction. The powder is spread by the scrapper to work chamber to prepare for next manufacturing layer. The process is repeated, layer by layer, until the printed part is fully formed. The printing process takes place in a protective atmosphere, such as in a working chamber filled with inert gas argon (Ar) or nitrogen (N2), to prevent oxidation during the manufacturing process.

Fig. 1.3

Fig. 1.3 Schematic diagram of the forming principle for selective laser melting additive manufacturing.

SLM forming metal parts has the characteristics of fine grain size, excellent mechanical properties, high density, and high forming accuracy. At present, research units focusing on SLM technology are mainly concentrated in the USA, the UK, Belgium, Germany, Australia, South Korea, and other developed countries. Famous SLM technology companies include SLM Solutions and EOS in Germany, Renishaw in the UK, and GE in the USA. These companies have launched a number of SLM devices and special powder materials. Harvard University and the University of Texas at Austin are among the universities that are studying this area most. Many domestic enterprises and units are also engaged in relevant research, such as Xi’an Blite Laser Forming Technology Co., Ltd., Wuhan Huake 3D Technology Co., Ltd., Shenzhen Sunshine Laser & Electronics Technology Co., Ltd., Guangzhou Lejia Additive Technology Co., Ltd., Northwestern Polytechnical University, Xi’an Jiaotong University, Huazhong University of Science and Technology, South China University of Technology, Beijing University of Aeronautics and Astronautics, Nanjing University of Aeronautics and Astronautics, and other universities.

SLM forming metal materials include titanium alloy, aluminum alloy, stainless steel, nickel alloy, copper alloy, and other materials. These commonly used materials have an established process and posttreatment scheme, and new materials for the SLM process are also being explored, including iron matrix composites, titanium matrix composites, and other new materials. The majority of SLM worktop sizes are 250 mm × 250 mm and below, indicating that the SLM process is mainly targeted at small, fine metal components. With the development of technology and the improvement of product requirements, the metal AM process is gradually being adapted toward large table and multilaser, SLM forming metal powder raw materials, moving away from the traditional single metal material in favor of a multimaterial approach. In 2017, EOS launched the EOS M400–4 SLM device. Its forming table size is 400 mm × 400 mm × 400 mm. Four independent lasers can form four parts at the same time, greatly improving production efficiency. In the same year, Concept Laser launched the Xline 2000R dual laser SLM system. The maximum forming part size of the equipment theoretically reached 800 mm × 400 mm × 500 mm, and its printable volume increased by 27% compared with the previous generation 1000R, making it the largest metal 3D printer worldwide at that time. The XDM750 equipment independently developed by the domestic enterprise Suzhou Sidimo 3D Technology Co., Ltd. adopts a mobile laser galvanometer system. Its maximum forming part size is 750 mm × 750 mm × 500 mm, which is globally top in terms of forming size and equipment size. Due to the unique technical advantages of SLM, the technology has been partially applied and developed rapidly in aviation, aerospace, medical equipment, automobile manufacturing, and other fields. SLM shows the following three trends:

(1)from forming homogenous mechanical structural parts to multiscale complex key components;

(2)from a single uniform structure to a multimaterial gradient mixed structure; and

(3)production philosophy is shifting from bottom-up to top-down design to meet structural integration, functionalization, and modularity requirements.

The surface quality and performance control of SLM forming components are significant challenges. The principles of the SLM process involve a laser and powder interaction being used, the metallurgy process occurs in a very short time, and the material undergoes repeated rapid heating and cooling, from the powder state to the solid state, in typical nonequilibrium thermodynamics, Therefore, nonuniform microstructure, nonuniform thermoplasticity, and phase change plasticity are inevitable, which also leads to spheroidization, microcracks, pores, and other defects in the SLM forming process. From a macro point of view, SLM formed components often have large residual stress, resulting in deformation and cracking, which is a difficult problem for surface quality control of SLM components. In addition, many unmelted or semimelted particles adhere to the surface of SLM forming components, and the surface is not smooth. In cases of SLM forming large size and low complexity components, uneven surfaces can be improved by posttreatment processes such as chemical corrosion, shot blasting/sandblasting, slow wire cutting, etc. Due to small and delicate internal complex components, it is difficult for traditional surface treatment technology to control the internal surface structure's quality, such as chemical etching to internal and external structure synchronization of uniform corrosion. It is also challenging for shot peening/abrasive particles of sand blasting process to enter the internal structure, and the cutting tool cannot enter the internal constraints, such as structure. As a result, surface quality control remains an important research problem.

AM technology is an effective means to realize the forming of multimaterial cross-scale high-performance components, which changes the design from being process-oriented to performance-oriented, and greatly improves the design space. AM can provide manufacturing technology for various kinds of metamaterial components with complex shapes. SLM technology has the ability to form high-precision metal components. Laser AM technology for metamaterial components is one of the development directions of advanced manufacturing in the future.

1.4: Book outline

This chapter discusses the current state of the art in metamaterials and available additive manufacturing (AM) technology, and provides the necessary background for this dissertation. The opportunities and limitations of metal AM technologies, specifically selective laser melting (SLM), are discussed, in particular their applications in manufacturing complex metamaterials. An overview is also provided of metamaterial design and additive manufacturing for engineering applications.

Chapter 2 presents a study on topology optimization and bamboo-inspired mechanical metamaterials and additive manufacturing. Mechanical metamaterial is a structural material with singular mechanical properties, which can realize impressive mechanical responses such as negative Poisson's ratio, high specific strength, low shear modulus, and negative stiffness. Firstly, the level set-based topology optimization method is adopted to design novel mechanical metamaterials. A study on the mechanical properties and energy absorption capability of topology-optimized mechanical metamaterials is conducted through quasistatic compressive testing. Finally, inspired by atom packing and bamboo morphologies, a combinatorial biomimetic strategy is presented to realize mechanical metamaterials with simultaneous lightweight, high-strength, and isotropic properties. The underlying mechanisms of the hollow strut effect on the mechanical properties and stress distributions are also discussed.

Chapter 3 performs a study on various acoustic metamaterial designs and additive manufacturing. Acoustic metamaterial is a structural material with an abnormal reflection and refractive index that do not exist in conventional materials. This chapter focuses mainly on two types of acoustic metamaterials: broadband sound absorbing metamaterials, and underwater acoustic stealth metamaterials.

Chapter 4 considers a thermal metamaterial that imitates pomelo peel. By imitating the microstructure of pomelo peel, a multifunctional thermal metamaterial with energy absorption and heat insulation is realized. Through simulation and experimental verification, it is confirmed that the designed metamaterial has excellent heat insulation and energy absorption capacity.

Chapter 5 presents a study on metamaterial-based bone scaffold design and additive manufacturing. A double cone metamaterial design strategy is proposed to reduce the stress shielding of diamond-like porous metallic biomaterials while maintaining the same porosity, which is conducive to matching the mechanical properties of the bone scaffold with the host bone and avoiding stress shielding. Bone scaffolds are constructed with pentamode metamaterials to balance porosity, mechanical, and mass-transport properties. Compared with traditional biomaterials, pentamode metamaterial-based bone scaffolds have the characteristics of graded pore distribution and appropriate strength, which significantly improve cell seeding efficiency, permeability, and impact resistance, promote in vivo osteogenesis, and have broad application prospects in cell proliferation and bone regeneration.

Chapter 6 discusses microlattice metamaterial design and additive manufacturing. A design strategy of anisotropic metamaterials imitating crystal structure is proposed. By constructing microlattice metamaterials with different crystal planes and crystal orientations, independent control of elastic response and mass-transport performance is realized. The results show that the coupling relationship between mechanical and mass transport properties is weakened, and it is dependent on the crystal plane and orientation direction of the microlattice metamaterial. Inspired by the Hall-Petch relationship in crystal materials, microlattice metamaterials are constructed with decoupling mechanical-transport properties to meet the needs of artificial bone scaffolds. The innovative design method of the microlattice-like structure provides infinite possibilities for the development of multiphysical field coupled metamaterials widely used in engineering.

Chapter 7 presents research on plate lattice metamaterials. Plate lattice metamaterials with half-open-cell topology are proposed and fabricated via the laser powder bed fusion technique. To investigate their mechanical performance and deformation behaviors, numerical simulations and experimental tests are performed on finite element models and as-built specimens, respectively. The porous architecture, lightweight body, and superior and easily tunable mechanical performance of half-open-cell plate lattice metamaterials provide them with application potential in the fields of load-bearing, energy absorption, and biomedical engineering. In bone scaffolds, the mechanical performance provides the load-bearing capability, and the mass-transport performance, presented as permeability, dominates the nutrients/oxygen transportation efficiency. Body-centered-cubic and face-centered-cubic plate lattice scaffolds with mechanical and mass-transport performance that are very similar to human bones are proposed.

Finally, Chapter 8 sets out the applications and prospects of additive manufacturing metamaterials.

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