Additive Manufacturing: Science and Technology 9781501518782, 9781501518775

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Emrah Celik Additive Manufacturing

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Emrah Celik

Additive Manufacturing Science and Technology

Author Prof. Emrah Celik Department of Mechanical and Aerospace Engineering University of Miami 1251 Memorial Drive Coral Gables 33146 USA [email protected]

ISBN 978-1-5015-1877-5 e-ISBN (PDF) 978-1-5015-1878-2 e-ISBN (EPUB) 978-1-5015-1098-4 Library of Congress Control Number: 2020939278 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2020 Walter de Gruyter GmbH, Berlin/Boston Cover image: Devrimb/iStock/Getty Images Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

To mom, Adile

Preface Additive manufacturing (also known as 3D printing) is a disruptive technology to manufacture different types of engineering materials layer by layer, in compliance with a previously designed and tessellated CAD model. Additive manufacturing offers cheaper and mobile manufacturing due to the elimination of large stocks of raw materials, semimanufactured parts, labor costs, and allowance to mobilize the printer to the desired location. Besides, due to its difference from conventional methods, it yields zero production wastes or scraps, thanks to its additive nature, rather than subtracting material. Today, 3D printing is pacing toward being a facile, homemade manufacturing method, owing to the rapid spreading of 3D printers all around the world. Despite its unique capabilities and advantages over traditional manufacturing, additive manufacturing techniques still possess significant deficiencies such as fabrication speed, limited number of materials, high cost, weak interface and fatigue performance, and the low reproducibility of material properties. These issues that need to be resolved completely or minimized are extensively described throughout the book and possible remedies are pointed out. In addition to its benefits and limitations, future trends of additive manufacturing according to the author’s perspective and how it will further transform our life will be discussed in this book. The goal of this book is to provide students, researchers, as well as practicing engineers who would like to gain deeper knowledge on current additive manufacturing technologies. This book extensively investigates the theoretical concepts as well as practical considerations of various additive manufacturing methodologies. In this regard, it uniquely combines the underlying science and the state-of-the-art technology aiming to help the readers understand the fundamental concepts and inform them about the latest developments in each manufacturing categories. The structure of this book was organized in such a way that this could be used as an educational tool for an individual as well as a textbook for additive manufacturing curriculum. The first chapter of this book provides an introduction to the concept of additive manufacturing and briefly describes the seven major categories of this technology. This chapter also describes the benefits of additive manufacturing as well as the limitations forbidding it from replacing the conventional manufacturing technologies. Brief history of the additive manufacturing since its inception is also described here to show the readers the development pace and the trends to predict the future of additive manufacturing. The remainder of the chapters are mainly categorized by the type of materials on which additive manufacturing technologies are applied. Being the most commonly used material type, the book starts with polymers in Chapter 2, where additive manufacturing of thermoplastic, thermosetting, and elastomeric polymers are described separately. Similarly, Chapters 3, 4, and 5 investigate additive manufacturing of other engineering materials: polymer composites, metals, https://doi.org/10.1515/9781501518782-202

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and ceramics, respectively. The performance of each type of material system is compared and contrasted against each other by considering different aspects of material performance including manufacturability, cost, mechanical performance, and thermal resistance. Existing material repository for each section as well as the utilized manufacturing technology are described extensively in these chapters to educate the reader on why and how certain material selection is preferred for various applications. Chapter 6 investigates the use of additive manufacturing on tissue engineering applications. A specialized version of additive manufacturing known as bioprinting is described in detail. Its transformative potential and its applicability on various tissues/organs and current success stories are explored. The effect of bioprinting feedstock materials (bioink) on bioprinting quality is evaluated and the currently used bioink materials are described. In addition to the benefits and the current advancements of bioprinting, limitations and the great challenges that are ahead of bioprinting field are evaluated in detail. The true benefit of additive manufacturing lies behind the manufacturing of complex structures that have enhanced performance and/or reduced weight over the unoptimized legacy parts. In other words, if topology is optimized in a way to improve the components that work better than the original design, additive manufacturing can be used to fabricate these concept designs no matter how complex they are. Chapter 7 describes the topology optimization concepts, their benefits, and integration into additive manufacturing world. Chapter 8 discusses the advanced concepts in additive manufacturing field, which has significant potential to transform manufacturing technologies in general. These concepts involve additive manufacturing of nonconventional materials such as thermoelectric systems. Thermoelectric materials convert heat into electricity; therefore, additive manufacturing of these systems will make a great impact in our daily life, considering the tremendous amount of waste heat. Hybrid manufacturing that uniquely combines multiple materials or multimanufacturing systems is also explored in this chapter. Lastly, additive manufacturing of smart manufacturing is described in Chapter 8 using 4D printing technology. Smart material manufacturing using 4D printing is described as the time-dependent transformation of additively manufactured structures in a predicted way. The book ends with the unique technology to pave the way for the development of future concepts such as adding another dimension/functionality on additively manufactured systems. Additive manufacturing is a technology transforming the design concept on manufacturing process. The impact of this technology will be further enhanced in near future as the existing technologies are advanced, manufacturing costs are reduced, and novel concepts are integrated on it such as smart manufacturing and artificial intelligence on material development and processing steps. This book involves the theoretical concepts, latest technologies, and the future trends of additive manufacturing. Therefore, it is hoped that this book will help the students or

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individuals who want to contribute to the additive manufacturing field whether as a user or developer, both of whom are much needed elements to continue the advancement of this technology in the future.

Contents Preface

VII

1 Introduction 1 1.1 A disruptive technology, additive manufacturing 1.2 Advantages of AM over traditional manufacturing 1.2.1 Greater design ability 3 1.2.2 No tooling 3 1.2.3 On-demand manufacturing 4 1.2.4 Rapid prototyping 4 1.2.5 Customization 4 1.2.6 Minimal material waste 4 1.2.7 Low cost for small number of parts 4 1.3 Classification of AM technologies 5 1.3.1 Vat polymerization 5 1.3.1.1 Stereolithography 7 1.3.1.2 Digital light processing 7 1.3.1.3 Continuous liquid interface production 7 1.3.1.4 Volumetric Vat manufacturing 8 1.3.2 Material jetting 9 1.3.3 Binder jetting 10 1.3.4 Material extrusion 11 1.3.4.1 Fused filament fabrication 11 1.3.4.2 Paste extrusion 12 1.3.5 Powder bed fusion 14 1.3.6 Directed energy deposition 15 1.3.7 Sheet lamination 16 1.4 Timeline/history of AM 19 2 2.1 2.1.1 2.1.2 2.1.3 2.2 2.3 2.4 2.4.1 2.4.2 2.5

Additive manufacturing of polymers 22 Classification of polymers 22 Thermoplastics 23 Thermosets 23 Elastomers 23 Selection of polymers for AM 24 AM of thermoplastic polymers 25 AM of thermosets 27 AM of photosensitive thermosets 27 AM of heat-sensitive thermosets 29 AM of elastomers 29

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3 3.1 3.2 3.2.1 3.2.2 3.3 3.4 3.5 3.6

Contents

Additive manufacturing of polymer composites 31 Additive manufacturing of powder-doped polymer composites 31 Additive manufacturing of short fiber-doped composites 34 Short fiber reinforced thermoplastic composites 35 Short fiber reinforced thermoset composites 35 Prediction of mechanical properties of short fiber reinforced composites 38 Alignment of short fibers within additively manufactured composites 40 Additive manufacturing of continuous fiber reinforced composites 41 Mechanical performance comparison of additively manufactured polymer composites 45

4 Additive manufacturing of metals 47 4.1 Feedstock material fabrication for powder bed fusion 48 4.2 Feedstock materials used in metal AM 51 4.2.1 Titanium and titanium alloys 53 4.2.2 Aluminum alloys 53 4.2.3 Other metals 54 4.3 Design considerations in metal AM 54 4.3.1 Void formation 54 4.3.2 Residual thermal stresses 55 4.3.3 Surface roughness 55 4.3.4 Postprocessing 55 4.3.4.1 Stress relief 56 4.3.4.2 Heat treatment 56 4.3.4.3 Hot isostatic pressing 56 4.3.4.4 Machining and surface treatments 56 4.4 Mechanical properties of additively manufactured metals 57 5 5.1 5.1.1 5.1.2 5.2 5.2.1 5.2.2 5.3 5.3.1

Additive manufacturing of ceramics 61 Powder-based ceramic additive manufacturing Binder jetting of ceramics 63 Powder bed fusion of ceramics 64 Slurry-based ceramic additive manufacturing Vat polymerization of ceramics 66 Direct writing of ceramics 67 Bulk solid-based technologies 69 Sheet lamination 70

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Fused filament fabrication 71 Additive manufacturing of polymer-derived ceramics Mechanical properties of AM ceramics 74

5.3.2 5.4 5.5

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6 6.1 6.2 6.3 6.3.1 6.3.2 6.3.3 6.3.4 6.3.5 6.4 6.5

Bioprinting 78 Bioprinting methods 78 Bioink types used in bioprinting 81 Bioprinting applications 82 Bioprinting of blood vessels 84 Skin bioprinting 85 Cartilage printing 85 Cardiac tissue bioprinting 87 Kidney tissue bioprinting 88 Challenges and limitations of bioprinting functional organs Bioprinting in cancer research 89

7 7.1 7.2 7.3

Topology optimization 92 Topology optimization for additive manufacturing 93 Topology optimization methods 96 Solution of topology optimization problem using ANSYS finite element software 99

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8 Advanced concepts in additive manufacturing 101 8.1 Hybrid additive manufacturing 101 8.1.1 Additive/subtractive hybrid manufacturing 101 8.1.2 Additive/additive hybrid manufacturing 102 8.1.3 Hybrid additive manufacturing/scaffolding technologies 104 8.2 Additive manufacturing of thermoelectric materials 106 8.3 Four-dimensional printing with smart materials 113 8.3.1 Four-dimensional printing materials 114 8.3.1.1 Four-dimensional-printed hydrogels 114 8.3.1.2 Shape-memory polymers 115 8.3.1.3 Elastomer actuators 117 8.3.2 Applications of 4D-printed structures 117 References Index

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1 Introduction 1.1 A disruptive technology, additive manufacturing Additive manufacturing (AM) is a revolutionary technology, which is based upon building three-dimensional (3D) objects by adding successive layers of material. Different types of materials can be fabricated with this technology, including polymers, metals, ceramics, composites, or biological materials. Since AM allows fabrication of parts in complex geometries without the use of any tooling, early use of AM focused on visualization models in the form of rapid prototyping. Due to the significant improvement in material library and the quality of the fabricated parts, AM has recently been used to fabricate end products in aerospace, dentistry, medical implantation, automotive, and even fashion design.

Additive Manufacturing

STL File

CAM Software

CAM Software

Tool path Commands file

CAD Model

Tool path 3D Printer Altered Part Commands File

CNC Machine

Quality Control & Inspection

Altered Part

Subtractive Manufacturing Figure 1.1: Additive versus subtractive manufacturing. Figure was reprinted from [1].

Unlike traditional subtractive manufacturing, AM is the process of joining materials to make objects layer by layer. In the past century, subtractive manufacturing has made a great impact on fabrication and prototyping since it was first introduced. Now, manufacturing industry is on the verge of a new revolution due to the new design and fabrication opportunities offered by AM. Figure 1.1 shows the major differences between the subtractive manufacturing and AM technologies. Subtractive manufacturing, as its name implies, involves removing sections from a material by machining or cutting it away. It can be performed manually or via computer numerical control (CNC) machining. AM is also a computer-controlled process but unlike subtractive manufacturing, it adds successive layers of material to create a 3D object. Traditional subtractive manufacturing, in general, has benefits of cost per part (especially for a high-scale manufacturing), speed, component strength, and

https://doi.org/10.1515/9781501518782-001

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material selection. New AM technologies however quickly close the gaps in these areas and bring up a new design space involving on-demand fabrication, creation, customization, and complexity redefining current manufacturing. As it will be described in the next section, there are many AM technologies developed for different manufacturing purposes. Despite their differences, all AM methods share common processing steps as illustrated in Figure 1.2. The AM process starts with the design of the component using a 3D computer-aided design (CAD) modeling software. The model can also be constructed via reverse engineering using a 3D scanner or photography. Once the computer model is created, it is transferred into a slicing software where 3D model is digitally converted into serial 2D sections format. The AM equipment then reads this data and lays down successive layers of liquid, powder, sheet material, or other forms layer-by-layer to fabricate a 3D object. Minimum layer thickness dictates the manufacturing quality and it depends on the machine and AM process type selected. Finally, additively manufactured object is removed from the AM instrument and, if desired, postprocessing such as cleaning, sanding, coating, painting, compacting, or heat treatment is applied to enhance component performance or esthetic appearance. Postprocessing may involve the use of other machines and tools. Step 2. Slicing into AM format

Step 3. Material deposition Step 1. CAD modeling Additive manufacturing Step 4. Postprocessing

Figure 1.2: Main processing steps of additive manufacturing: CAD modeling, AM format slicing, material deposition, and postprocessing.

1.2 Advantages of AM over traditional manufacturing AM finds unique applications in vast variety of fields including electronics, aerospace, automotive engineering, and even fashion design as shown in Figure 1.3. This is due to the advantages of AM over traditional manufacturing technologies. These benefits are summarized as follows:

1.2 Advantages of AM over traditional manufacturing

3

Electronics Transportation

Biomedicine Design Fashion Architecture Figure 1.3: Various applications of additive manufacturing. Concrete printing (architecture) [2], transportation, [3], and fashion [4] application images were reprinted with permission.

1.2.1 Greater design ability AM systems allow fabrication of moving parts (hinges, chains, etc.) in a single print process, and complex structures are unachievable with the traditional manufacturing. In addition, due to the flexibility of design, traditional constraints of manufacturing are eliminated and number of parts in the design can be reduced. Redesign process can be done digitally on the CAD model and the new part can be fabricated in a short period of time. Since AM is a user-friendly technology and not labor intensive, the designers are not limited to only engineers but variety of makers such as architects, artists, and even students adopt this technology in making things. Therefore, AM technologies have created a new design space and found a great interest in the community we live in.

1.2.2 No tooling In AM, unlike many traditional manufacturing techniques, jigs, fixtures, or molds are not required to secure and shape the parts being made. The components are fabricated directly on a printbed or an already-fabricated part. This is a huge cost-saving avenue of AM considering expensive tooling of traditional manufacturing systems.

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1.2.3 On-demand manufacturing Parts can be fabricated anytime as soon as the CAD model is prepared. These CAD models can also be sent digitally and the manufacturing can be done at home or remote locations near consumers eliminating the transportation of the fabricated parts.

1.2.4 Rapid prototyping Compared to the conventional techniques with more geometric limitations, AM can produce models quickly, in hours instead of weeks. The makers/designers can test the prototypes quickly to save significant amount of time in the design process to achieve the final product.

1.2.5 Customization Every part can be fabricated differently with the AM process without adding the cost of fabrication. The customization can be particularly useful for the biomedical applications, where each part can be fully customized for different patients and their special requirements.

1.2.6 Minimal material waste Since AM technology is based on adding material to build the 3D object rather than material removal, it uses the exact amount of material to produce parts and there is usually no waste of material. Support material and extra powder used during the process can often be recycled and reused for the production of the next part. This would reduce material cost due to material waste and waste removal. This would also minimize the environmental implications stemming from the waste in conventional manufacturing.

1.2.7 Low cost for small number of parts With the advancement of technology, AM systems are becoming more affordable and more portable compared to the traditional manufacturing systems. Low investment costs of the AM machines attract more interest on these systems. The cost of fabrication depends highly on the number of fabricated parts and complexity. Usually, traditional manufacturing is beneficial for fabrication of high number of

1.3 Classification of AM technologies

5

geometrically simple parts, whereas AM becomes highly competitive or even cheaper for manufacturing of low number of parts with high complexity.

1.3 Classification of AM technologies There are numerous AM technologies used for the fabrication of different materials. In 2010, the American Society for Testing and Materials (ASTM) group, formulated a set of standards that classify the range of AM processes into seven major categories as shown in Figure 1.4 [5]. Each of these seven methods significantly varies in their method of layer-by-layer AM. Each of these categories is explained as follows.

Additive manufacturing classification

Vat polymerization

Sheet lamination

Material jetting

Directed energy deposition Binder jetting

Powder-based fusion Extrusion

Figure 1.4: ASTM classification of additive manufacturing technologies.

1.3.1 Vat polymerization Vat polymerization (a.k.a Vat photopolymerization) is a 3D printing technology, which relies on selectively curing a liquid photopolymer contained in a vat (or tank) by a light source. Light is used to cure or solidify the resin where required, while a platform moves the built object downward (or upward) after each new layer is cured. The process continues curing the photopolymer layer by layer until building of a 3D physical object is completed. After completion, the resin in the vat is drained and the object is removed. Usually, photopolymerized samples are postcured under ultraviolet (UV) light to achieve complete curing and maximum strength. The majority of the photocurable resins consist of mixtures of monomers combined with oligomers (a few units of monomers) and photoinitiators. As shown in Figure 1.5, oligomers and monomers are disconnected in uncured liquid photopolymer resins. As UV light is applied on these photopolymers, photoinitiators are activated and cross-link the monomer and oligomer units. This chemical process, known as photopolymerization or photocuring, transforms liquid photopolymers into solid components.

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1 Introduction

UV light

Active photoinitiators

Inactive photoinitiators

Figure 1.5: Schematic of photopolymerization process.

Depending on the type of photoinitiator and the polymerization process, there are two major photopolymerization types: (1) free-radical polymerization and (2) ionic photopolymerization. Free-radical photopolymerization is commonly used in photopolymerization-based AM technologies (Vat polymerization, material jetting). Freeradical photopolymerization process takes place in multiple steps including activation of the photoinitiator upon exposure to radiation within an appropriate wavelength range, formation of free radicals by reacting the photoinitiator and monomer molecules, and the propagation by forming long polymer chains to cross-link. In the final step, cross-linking process or photopolymerization is terminated where polymerization comes to an end, usually by one of three mechanisms, including recombination (two chains combine), disproportionation (canceling of one radical by another without joining), or occlusion (free radicals become trapped by the polymer network) [6]. The most widely used UV-curable resins are based on acrylate, which show high reactivity and short reaction times (fraction of a second), and these resins are available in wide range of different monomer and oligomer types [7]. Although free-radical polymerization is the most commonly used photopolymerization process, ionic curing systems are finding increasing applications in AM. The same photopolymerization steps, photoinitiator activation by UV light, propagation, and termination steps, exist in ionic photopolymerization process. The difference, however, is that in ionic curing process, instead of free radicals, reactive ions are the cross-linking agents for the monomers and oligomers. The termination takes place when the ion is neutralized or stabilized. Ionic photopolymerization has distinct advantages over free radicals including no inhibition with oxygen, minimal sensitivity to water, and the ability to polymerize vinyl ethers, epoxides, and other heterocyclic monomers that do not polymerize by a free-radical mechanism [7]. There are four classifications for Vat polymerization technologies. Although the main concept of the polymerization is the same in all of these techniques, differences in the type of light source and the curing process for the polymer material lead to this classification. The operation principles of the three major Vat polymerization techniques are as illustrated in Figure 1.6.

1.3 Classification of AM technologies

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Mirror (A)

(B)

Light source

Liquid photopolymer Platform

SLA

DLP

CLIP

Vat Figure 1.6: (A) Schematic of part fabrication in Vat polymerization technology and (B) different types of Vat polymerization process (SLA, DLP, and CLIP).

1.3.1.1 Stereolithography Stereolithography (SLA) is the oldest AM technology. As a result, it is widely adopted by AM community today. In this method, a focused UV laser beam is directed on photosensitive resin using motor-controlled mirrors. As the light contacts the liquid resin, chemical reaction takes place curing the resin and creating a solid layer of the desired 3D object. 1.3.1.2 Digital light processing This Vat polymerization technique differs from SLA since the light is projected on liquid polymer in terms of a 2D image rather than rastering a focused beam. A digital projector is used to reflect the image over the resin curing the entire 2D layer. Since the entire layer is solidified at once, digital light processing (DLP) process can achieve faster print times compared to SLA process. A modified version of DLP photopolymerization has been developed recently where DLP projector is replaced by a liquid crystal display (LCD) screen. This screen acts as a mask for the UV light coming from an array of light emitting diodes (LEDs) shining through the LCD screen. Similar to the projector, photopolymerization of 2D layer occurs at once resulting in higher speeds compared to SLA. Using LCD screens instead of projectors have significantly reduced the cost and sizes of the projector-based DLP technology. 1.3.1.3 Continuous liquid interface production Continuous liquid interface production (CLIP) is a relatively recent technology introduced by Carbon3D, in 2015, as a novel concept using an oxygen-permeable bottom plate to help speed up the printing process [8]. In SLA and DLP processes, the solidification process occurs at the very bottom of a vat with a clear bottom window. To eliminate the resin to adhere the window surface, the platform is moved up and down creating significant suction to break the adhesion between the object and the window. Recent developments by Carbon3D company and the creation of the

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CLIP process have resulted in inhibiting the solidification of the resin within a certain zone around the clear bottom of the vat, which eliminates this suction force. In CLIP technique, part of the vat bottom is transparent where UV light beam shines through using an LCD screen. While the object is lifted slowly, resin flows under and maintains contact with the bottom of the object. An oxygen-permeable membrane lies below this resin field and creates a dead zone where photopolymerization is inhibited. This liquid interface prevents the resin from attachment of the cured object to the bottom plate. Unlike standard SLA and DLP, the 3D printing process is continuous and therefore much faster. Continuous printing process makes CLIP as one of the fastest AM method. However, similar to all other Vat polymerization techniques, postprocessing is necessary to clean the printed part and fully solidify it via additional postcuring process. 1.3.1.4 Volumetric Vat manufacturing In 2017, volumetric Vat AM was introduced where photopolymerization is performed in a 3D form rather than traditional layer-by-layer fashion [9]. Volumetric Vat manufacturing is similar to the computed tomography (CT) technique where a series of X-ray scans are acquired at different orientations and these 2D images are then processed with a computer algorithm in order to reconstruct the 3D image of the object. Unlike CT imaging, in volumetric Vat 3D printing, 3D CAD model is input as the initial step and this model is converted into 2D projections at different orientations using tomography algorithms. When all these projections are displayed into a homogeneous volume of absorbing material, the cumulative absorbed dose distribution due to the projections reproduces the shape of the 3D object inside the material [10]. When the light is projected in 3D volume at different intensities (different doses), the photopolymer where a high dose of light is applied will solidify whereas other locations will remain below the solidification threshold. Therefore, in volumetric Vat process, light is projected in 3D increasing the speed of the photopolymerization significantly. This technique is still under development stage and better control of the light projection and the photopolymerization processes are expected in near future. Vat polymerization is advantageous over other AM techniques since it is capable of fabricating parts at a high resolution down to the nanoscale level. It is also capable of fabricating large parts using vats with big volumes. Near-transparent objects can also be fabricated with this technique, which are usually not possible with other AM technologies where interface region between the layers leads to high level of light diffraction. Disadvantages include applicability of this technology to only a limited number of UV-curable resins that are not very robust materials in terms of durability, strength, or stability. In addition, this technique usually needs a postcuring process under UV light to complete the curing process. Some photocurable resins have health implications; hence, special gloves and ventilation are needed

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for the printing and postprocessing steps for these toxic materials until full curing is achieved. Depending on the geometry of the part, support structures may be required which adds to the material waste and the fabrication time.

1.3.2 Material jetting Material jetting is a 3D printing manufacturing technique which is similar to the standard inkjet document printing process. However, instead of dispensing ink onto a paper, photopolymer or wax material is sprayed (or jetted) onto a build tray. As the polymer resin droplets are deposited to the build platform, they are cured and solidified using UV light. The process continues to deposit material layer by layer until the 3D object is created. The material jetting process allows fabrication of different materials within the same object. Figure 1.7 shows the schematic of material jetting process along with example parts fabricated with this technology.

(A)

(B) UV light

Inkjet nozzles

Build Support material material

Platform Figure 1.7: (A) Schematic of material jetting technology and (B) fully colored parts fabricated by a material jetting system. Image was reprinted with permission from Stratasys.

Material jetting processes require support, which is often 3D printed simultaneously from a dissolvable material. The support material is then removed during the postprocessing step. Depending on the type of the support material used and the support removal technique, there are two patented technologies; polyjet printing (PJP) and multijet printing (MJP) used by Stratasys and 3D Systems companies. In polyjet technology, the support material is a combination of propylene, acrylic monomer, polyethylene, and glycerin [11]. To remove support material, pressurized water is sprayed over the part and the remaining support material is removed chemically by dipping the part into chemical solvent. On the other hand, MJP technology uses

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meltable paraffin wax as the support structure. To remove the wax support, the printed sample is heated in an oven over melting temperature of the wax followed by wiping out of the wax material out of the sample. Material jetting 3D printing technology is a great choice for making realistic prototypes, providing an excellent level of details, high accuracy, and smooth surface finish. Material jetting allows a designer to print a design in multiple colors and with a number of materials in a single print. These manufacturing systems offer wide range of materials and their combinations to choose from, including rigid to rubberlike materials, opaque to transparent materials, and materials with acrylonitrile butadiene styrene-simulating performance. The main drawbacks to printing with material jetting technologies are the high cost of the UV-activated photopolymers and the degradation of the mechanical properties of these materials over time.

1.3.3 Binder jetting Binder jetting is an AM process where a binding material is sprayed (or jetted) over powder particles to bond them to form a 3D object one layer at a time. Schematic of the AM process is shown in Figure 1.8. Metals and ceramics in powder form are commonly used materials in binder jetting process. During the binder jetting process, inkjet nozzles (similar to the paper inkjet printers) on the printer head spray droplets of a binding material on the powder printbed and bond the powder particles in these areas together. When the layer is complete, the build platform moves down and another powder layer is spread over the printed surface. The process is repeated until the entire part is complete. Some printed parts such as sand-casting cores and molds are typically ready to use after binder jetting process. However, a postprocessing step is required for most applications. This is because binder jetted

Printed part

Building stage

Leveling roller

Feeding stage

Figure 1.8: Schematic of binder jetting technology.

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11

parts have poor mechanical properties and a high porosity when they come out of the printer. In terms of post-processing, metallic parts are sintered at a high temperature enhancing the adhesion between the metal particles or infiltrated with a lowmelting temperature metal such as bronze [12]. Ceramic parts also usually undergo similar sintering and infiltration postprocessing steps to enhance their mechanical strength and reduce porosity stemming from the removal of the binder material. Acrylic coating application on the printed parts is also a common practice to improve visualization and vibrancy of the colors in multicolor printed parts. Binder jetting is a great choice for the applications requiring appealing aesthetic properties and parts for visualization such as architectural models, toys, and figurines since this technique can produce full color 3D-printed parts at a high resolution similar to the material jetting process. The low cost of the powder material feedstock and the high speed of the process are the major benefits of this technology. Binder jetting is generally not suited for functional applications due to the brittle nature of the parts; however, metal-based binder jetting parts have relatively good mechanical properties if the infiltration and/or sintering postprocessing steps are taken. Since the printing process takes place at room temperature in binder jetting, dimensional distortions and warping due to thermal stresses are not a problem in this process. As a result, the build volume of binder jetting machines are larger compared to the all other AM technologies. This allows manufacturing of multiple parts and large objects such as casting molds. Similar to the powder bed fusion systems, binder jetting requires no support structures since the surrounding powder provides the necessary support.

1.3.4 Material extrusion According to ISO/ASTM definition: “material extrusion, is an additive manufacturing process in which material is selectively dispensed through a nozzle or orifice” [13]. Material extrusion is the most commonly adopted AM technology due to the simplicity of this technique, wide range of material selection, low cost of the printer instrument and the feedstock materials, and the functionality of the printed parts. Although there are various different extrusion processes, all of these can be categorized into two major groups: fused filament fabrication (FFF) and paste extrusion. 1.3.4.1 Fused filament fabrication In this technique, a filament preform of thermoplastic material is inserted in an orifice where it is melted and extruded through a nozzle. Once deposited, it cools down rapidly and solidifies into a single line (a.k.a road) as shown in Figure 1.9A. The nozzle and/or the printing platform can move in x-, y-, and z-axes simultaneously to deposit material into 3D geometry matching the digitally designed CAD

12

1 Introduction

Filament moving mechanism

(A)

Printed part

Heated extruder

Fused filament fabrication (FFF )

(B)

Ink

Deposited material

Dispenser

Direct write (DW )

Figure 1.9: Schematic of extrusion technology: (A) fused filament fabrication (FFF) and (B) direct write (DW).

drawing. Most desktop AM machines currently in use are of this type and this is the AM process when most people refer to 3D printing due to its common adoption. 1.3.4.2 Paste extrusion This method is commonly known as direct write (DW), liquid deposition modeling (LDM), or robocasting. In this extrusion method, viscous paste-like material is extruded through a nozzle (or tip). Unlike the melting-based extrusion (FFF), in DW, material is a viscous fluid, not solid when deposited on the printbed. This method relies on the fluid’s yield stress to form self-supporting structures. Fluid viscosity and yield strength are adjusted using rheological modifiers such as nanoclay [14] and fumed silica [15]. The fluid can be extruded via pressure controlled or displacement-controlled system as shown in Figure 1.9B. In displacement-controlled extrusion, a stepper motor precisely moves the piston plunger downward toward the printbed extruding the material. In the pressure-controlled systems, a pump is utilized to apply pressure directly on the material to push it toward the nozzle. Pressure level or the microstep number of the stepper motor is adjusted to control the speed of the extrusion. In DW, rheological properties of the extruded material determine the printability. Highly viscous pastes are preferred as the printing materials since these materials can resist deformation after printing and they can hold shape. In DW, shear thinning is a commonly observed behavior as shown in Figure 1.10A, where material viscosity is decreased as a function of shear rate. In shear thing behaving materials, during extrusion, material viscosity drops significantly, and material can be extruded into an intended geometry at high flowability. After extrusion, material viscosity is recovered (preferably at high speed), and the extruded material can hold its shape under

13

1.3 Classification of AM technologies

(B)

Shear-thinning fluid

Newtonian fluid

Storage modulus (G’) Modulus (G′, G″)

Viscosity

(A)

Solid-like Loss modulus (G″) Modulus crossover

Liquid-like

Shear yield stress

Shear rate

Shear stress

Figure 1.10: Rheology considerations in DW; (A) comparison of a Newtonian fluid and shear-thinning fluid and (B) typical variation of loss and storage moduli as a function of shear rate during extrusion.

gravitational force without sagging. During the extrusion process, shear rate (_γ) is maximal at the walls of the extruder and can be estimated as follows: γ_ max =

4Q_ πr3

(1:1)

where r is the nozzle radius, and Q_ is the volumetric flow rate, calculated as Q_ = Sr2 , with S as the printing speed. Typical values of the shear rate during DW process are 50–100 s−1. In Newtonian fluids, however, viscosity is constant and does not vary as a function of the shear rate. Printability and the rheology relationship can be better identified by quantifying the storage and loss moduli of the DW extruded materials. The storage modulus (G′) relates to the material’s ability to store elastic energy, and the loss modulus (G′′) is the material’s ability to dissipate stress through heat. Figure 1.10B is a representative plot for a viscous, shear thinning paste material indicating the variation of storage and loss moduli as a function of shear stress. In Figure 1.10B, storage modulus is higher than the loss modulus under low shear conditions and therefore the material shows solids-like elastic behavior. On the other hand, under high shear, loss modulus can be higher which indicates a liquid-like behavior of the printed material. At the crossover (or gelation) point, storage, and loss moduli are equal and at this point material yield stress can be measured. High yield stress is preferred for extruded materials since these materials are able to resist deformation and keep their shapes without sagging. However, higher extrusion pressures/forces are required to push these materials with high yield stress through nozzle orifices. In DW method, entire printed structure is deposited at room temperature which significantly reduces the dependence of the mechanical properties on thermal printing history and spatial heating path [16]. Wide range of materials can be additively fabricated via DW. Thermosetting polymers and thermoset composites, ceramics and

14

1 Introduction

ceramic composites, and conductive metal inks (copper, silver, etc.) are printable with this method. After printing, postprocessing is usually required to enhance mechanical strength of the components. Curing is performed at 100–200 °C for thermoset materials. Similarly, heat treatment is necessary for ceramic-based pastes but at elevated 1,000–1,500 °C to facilitate the sintering of the ceramic particles. Postprocessing is usually omitted for printed conductive inks since the applications are limited to planar geometries [17] and high strength is not necessary. DW technique widens the applicability of AM to liquid-based materials, composites, and ceramics. Compared to filament based FFF technique, DW is relatively new and still under development. Currently, it is limited to smaller volumes and height-limited structures. Fabrication of taller 3D structures requires feedstock with extremely high viscosity and yield strength or using support structures.

1.3.5 Powder bed fusion Powder bed fusion is an AM method where a heat source is used to melt and fuse the powder particles together to form a 3D object. As shown in Figure 1.11A, a thin layer of powder is spread over the build platform and heat is directed on the selected region to fuse powder in these areas together. Afterwards, the platform is moved down and another layer of powder is spread across the previous layer using a roller, and the heat-induced powder fusion process is applied again on this layer. The process continues until the entire model is built. After the fabrication is complete, unfused powder is removed and the printed part is detached from the build plate.

Heat source

Mirror

(B) Leveling roller

(A)

(C)

Printed part

Building stage

Feeding stage

Figure 1.11: (A) Schematic of powder bed fusion technology, (B) laser sintering in progress, and (C) octopus figurine fabricated via powder bed fusion (SLS).

1.3 Classification of AM technologies

15

The heat source facilitating the fusion process can be a laser, an electron beam, or a heat lamp. Ceramics, polymers, and metals can be fabricated in 3D geometry with this technology. Powder bed fusion is remarkable at producing overhangs and downward facing surfaces where support structures are not needed as the unbound/unfused powder act as integrated support structure. Cost of powder feedstock and the powderbased fusion machines have reduced significantly over the recent years. In addition, this technology allows recycling of the unfused powder provided that the powder contamination and degradation are closely monitored to achieve the desired part quality. Selective laser sintering (SLS) and selective laser melting (SLM) are the most commonly used powder-based fusion technologies. In SLS, powders are heated near the melting temperatures leading to partial melting and sintering together as shown in Figure 1.11B. Figure 1.11C represents an octopus figurine fabricated with the SLS process. In SLM, however, material is heated above its melting temperature and complete melt of the powder is achieved creating a homogenous, nonporous structure. There are multiple limitations of powder bed fusion technologies. First, high temperatures and heat introduced into the part may cause warpage and residual thermal stresses. In addition, powder bed fusion is one of the slowest AM techniques since it commonly includes powder preheating (to speed up the process/enhance powder fuse), vacuum generation, and material cooling off period. Postprocessing also is common adding to the manufacturing time and cost. Since the parts are made by fusing material powder together, surface quality depends on the grain size of the powder and would be very similar to manufacturing processes like sand or die casting. [18]. The parts are manufactured over a build plate; hence, support removal postprocessing is necessary. Since material melting is necessary (partially or fully), this technique uses significant amount of energy to create parts compared to other AM techniques.

1.3.6 Directed energy deposition Directed energy deposition (DED) is a process where metal wire or powder is melted onto a build plate or an existing part using an energy source as shown in Figure 1.12A schematically. A typical DED system consists of a nozzle mounted on a multiaxis arm inside a closed frame, which deposits melted material onto the workpiece surface, where it solidifies. These robotic arms allow to build objects very quickly from multiple directions as long as the built location is within the reach of the arm. Therefore, the process is similar in principle to the material extrusion AM technique, but unlike FFF, filament is melted right at the deposition surface. In addition, nozzles of DED systems can move in multiple directions, up to five different motion axes are utilized compared to only three for most FFF machines. Material deposition can be accomplished in DED process at a high speed and therefore, this process is claimed to be the fastest AM technology [19]. Fully dense parts in complex geometries can be achieved using robotic arms and no support is required for overhanging features. In addition, DED process

16

(A)

1 Introduction

High-energy laser

(B)

Material nozzle Manufactured part

Platform Figure 1.12: (A) Schematic of DED additive manufacturing process and (B) metallic parts fabricated via DED technology. Published with a permission from AddUp/BeaAM Inc.

can effectively be used to add metal materials to existing metal parts, which makes this technique preferable for welding and repair applications. Different metals can be 3D printed through DED AM technique including aluminum, copper, titanium, tantalum, copper nickel alloys, and steel alloys. The main drawback with this process is the poor surface finish resulting from the melt pools. As the metal melt pools cool down, they leave a very rough surface finish and for this reason, most DED parts require postprocessing steps, usually in the form of secondary machining to improve surface finish. Since the DED process involves local heating of the object at elevated temperatures, thermal stresses are commonly observed. To alleviate these stresses and thermally induced implications, hot isostatic pressure and heat treatment postprocessing steps are usually taken. Figure 1.12B shows the DED process performed on different metallic parts by AddUp/BeaAM Inc.

1.3.7 Sheet lamination Sheet lamination is an AM process which is significantly different from the other AM processes since the material feedstock is not a liquid resin, a filament, or powder. However, as the name implies, sheets of material are laminated or bonded together building up a 3D component. The process can be applied to a variety of different material types including paper, PVC polymer, metal, or ceramic. As shown in Figure 1.13A, material sheets or foils are bonded together initially and 2D outline of the desired part is obtained by cutting the laminate using a laser or blade. Alternatively, material can be machined away using conventional CNC milling. After cutting (or machining)

1.3 Classification of AM technologies

(A)

Cutting laser

Mirror

17

(B)

(C)

Roller

Platform

Sheet material Figure 1.13: (A) Schematic of sheet lamination technology, (B) aluminum heat exchanger fabricated via sheet lamination technology, (C) x-ray image of the aluminum heat exchanger showing internal channels. Images were reprinted with permission from [20].

process, the next sheet is placed on the top of the existing piece. Sheet placement, bonding, and cutting process continue until the desired 3D object is created. If paper sheets are used as a feedstock, bonding is achieved with an adhesive or glue, whereas for PVC sheets, a thermoplastic polymer melting is used to bond the sheets together. In metal lamination, a localized energy source, laser or ultrasonic waves are used to bond a stack of precision cut metal sheets to form a 3D object. Ultrasonic bonding which is also known as ultrasonic AM or ultrasonic consolidation is the most commonly used metal sheet lamination technique. By applying ultrasonic wave and mechanical pressure on sheet metal stacks at room temperature, the contacting interface surfaces of stacked sheets are bonded by diffusion in atomic scale rather than melting. This process is great for making low cost, full color prints which does not require high geometrical complexity. The process can also be used to fabricate parts with internal structures without using a support structure. Additionally, this process is extremely useful for metal printing processes where the thermal stress of melting metal powder would be problematic such as powder bed fusion methods. Thermal stress is minimal in sheet lamination process; however, in order to further reduce thermal residual stress that might take place during bonding, an external cooling procedure is applied for a short period time between the lamination of each layer. Compared to other AM technologies, sheet lamination is less commonly utilized due to the high cost of the sheet lamination systems, very specialized applications, and limited geometrical freedom of parts fabricated with this technology. An aluminum heat exchanger block with an internal channel is shown in Figure 1.13B. Internal channels can be fabricated in any layer and at different complexity as shown in Figure 1.13C. This figure represents an X-ray image of the sheet laminated

18

1 Introduction

Table 1.1: Comparison of AM technologies. AM method

Materials

Resolution

Advantages

Disadvantages

Vat UV Curable – µm polymerization Photopolymers (Acrylates/Epoxides)

Excellent surface quality, high resolution, no porosity, isotropic properties

Limited mechanical properties, aging

Material jetting

UV Curable – µm Photopolymers (Acrylates/Epoxides)

Fast, allows multimaterial, multicolor printing

Low viscosity ink required

Binder jetting

Starch PLA Metals Ceramics

Powder bed fusion

Thermoplastics (PA, – µm PA, PEEK) Metals (stainless steel, titanium)

FFF

Thermoplastic polymers (ABS, PLA, nylon, PC, PETG, PEEK)

– µm

Direct write

Thermosets (epoxy, cyanate ester, bismaleimide) Composites Hydrogels Biomaterials

 µm– cm Broad range of materials,

Low surface quality, room temperature printing

Sheet lamination

PVC Paper Sheet metals

– µm

Low cost, low thermal stress

Limited geometrical freedom

Directed energy deposition

Metals (aluminum, copper, titanium, tantalum, copper, nickel, and steel alloys)

– µm

Fastest AM technology

Poor surface finish, requires postprocessing, thermal stresses in the parts

– µm

Fast, allows Limited mechanical multimaterial printing properties, rough surfaces

Best mechanical properties, less anisotropy, applicable to broad range of materials

Rough surfaces, thermal stress in the printed part, poor powder reusability

Compact, inexpensive Limited materials, D printers, good high temperature, resolution porosity, and anisotropy

19

1.4 Timeline/history of AM

component illustrating the ability for complex internal flow paths, which are impossible with traditional manufacturing methods. As described earlier, each of the seven AM technologies is unique in terms of the underlying technology to fabricate 3D objects layer by layer. Each method offers certain benefits and limitations in terms of applicable materials, cost, resolution, speed, build volume, and so on. Table 1.1 summarizes each seven categories comparing and contrasting the major properties of each AM techniques.

1.4 Timeline/history of AM To understand the future of AM technology, it is essential to look back at the history of AM and how it transformed our lives through its development. Although it all started 40 years ago, this technology has already revolutionized the manufacturing industry, biomedicine, architecture, automotive, and aerospace industry, in short, everything. Figure 1.14 summarizes some of the important milestones of the AM technology since the first patent was filed in 1984. Description of these major events is given and also briefly described as follows:

History of additive manufacturing SLA is invented

84

First kidney printed

First SLM printer 88

FDM is invented

95

98 First inkjet printer

02

First UAV printed

First leg prosthesis

04

Rep-rap project

08

10

First car printed

11

First meat is printed 14

18

Future

First print in space

Figure 1.14: Timeline of additive manufacturing technology development.

– The idea of AM was conceived in 1970s with the development of computers, CAD systems, laser technology, and micron-resolution motors. However, timeline of AM really starts with the first patent filed by Charles W. Hull in 1984 on SLA process. This technology was then commercialized when he founded 3D Systems company in 1986. – In 1988, 3D Systems developed the STL file format for CAD models for slicing off the 3D models into 2D layers. Since then, STL file format has been considered to

20

–

–

–

–

–

–

– –

1 Introduction

be the main file format for layer-by-layer manufacturing (AM) users and greatly assisted for the merge of AM technologies. In 1988, Scott Crump invented FFF technology using wax and a hot glue gun. This extrusion technique was named as fused deposition modeling (FDM) and patented by the company Stratasys founded by Scott Crump, which later became one of the largest AM companies in the world. In 1989, initial patent was awarded for powder bed fusion technologies of SLS, SLM, and electron beam melting (EBM) based on the work at the University of Texas at Austin. Although the patent was awarded in 1989, SLM and DMLS were developed in Germany as a part of a project between the Fraunhofer Institute, EOS, and others in 1995 [21]. These technologies dramatically changed the AM especially for the metallic materials. First commercial binder jetting machines came out after 1994. MJP process was introduced by 3D Systems and in 1996. In 1998, polyjet technology was developed by Objet [22]. These binder jet technologies lead to multicolor, very realistic prints at a high resolution and made a significant impact on AM industry. In 2002, AM was applied on biomedicine by 3D printing a miniature kidney model. This functional kidney was capable of filtering blood and producing urine in an animal model. After 17 years, in 2018, 3D-printed kidney saved the life of a 2-year-old boy as the surgeons used the 3D-printed kidney model to perform the organ transplant [23]. Implementation of AM on biomedical field expanded dramatically after 2000s. In 2003, Thomas Boland from Clemson University patented the use of inkjet printing for cells and cell constructs [24]. This process allowed the deposition of cells into organized 3D matrices placed on a substrate. In 2005, Adrian Bowyer at the University of Bath started the replicating rapidly (RepRap) project to make the FFF technique available to everyone, and created an open-source 3D printer that was capable of RepRap itself, at least partially [25]. Because of this open-source design along with the expired patents, new AM companies emerged reducing the machine cost and rapidly increasing the progress in the AM technologies. In 2008, the first prosthetic leg was used by a person. All parts of the prosthetic (leg, knee, foot, and socket) fabricated additively without any assembly needed. In 2010, entire body of a car, including its glass panel prototypes, was fabricated with AM processes. This first additively manufactured car, named Urbee, was the result of a collaboration between Winnipeg engineering group, Kor Ecologic, and Stratasys [26]. In 2014, Oak Ridge National Laboratories (ORNL) designed and additively manufactured Shelby Cobra electric car using FFF technology. The car was manufactured using a big area AM 3D printer developed by ORNL and Cincinnati Inc.

1.4 Timeline/history of AM

21

– In 2011, the University of Southampton engineers additively fabricated the first unmanned air vehicle (UAV). Entire structure of this UAV including wings, control surfaces, and access hatches was additively manufactured. – In 2014, the first object was printed in space. The International Space Station’s newly installed 3D printer made history by manufacturing the first object ever additively manufactured in space. NASA’s FFF 3D printer was developed under a contract with the Made In Space startup company, which was founded in 2010. The purpose of the 3D printer was to experiment with the possibility of manufacturing crucial replacement parts on the station, foregoing the expense of shipping them via rocket. – In 2018, Italian bioengineer Giuseppe Scionti developed a technology to generate fibrous plant-based meat analogs using a custom AM system based on DW method. Additively manufactured samples had matching texture and nutritional values to those of natural meat [27]. This was a breakthrough application for the implementation of AM on food industry and a big step toward resolving hunger in the world. Looking back the timeline of AM and what has been accomplished in the past 40 years is overwhelming. It has changed the manufacturing industry and transformed our lives. So, what lies ahead of us? What is the future of AM technology? It is difficult to predict the future but based on the trend line of this technology, it is certain to say that it will grow bigger, further replace the traditional manufacturing systems, and change our world for better.

2 Additive manufacturing of polymers 2.1 Classification of polymers Polymers are a class of engineering materials that are composed of chains of repeating chemical units called monomers. In fact, the word “Polymer” is derived from two Greek words, “Poly” and “Mer,” which mean “many” and “units,” respectively [28]. Repeating units or monomers can be just a few atoms or they might be complicated ring-shaped structures containing many more molecules. Natural polymers such as proteins, cellulose, enzymes, starches, and nucleic acids are found in living organisms and they perform important biological functions. Other natural polymers are derived from plants and animals and these natural polymer materials include wood, rubber, cotton, wool, leather, and silk. It was, however, a different type of polymers, nonnatural, or synthetic ones, which revolutionized our world in the past 50 years. Synthetic polymers such as plastics, rubbers, and epoxies are synthesized from petroleumbased organic molecules. These materials can be produced inexpensively, and their properties can be engineered to be superior to their natural counterparts. In many applications, metal and wood parts have been successfully replaced by synthetic polymers that provided lower cost, lightweight, and corrosion/chemical resistance advantages. Polymers can be classified into three major groups according to their molecular orientation as shown in Figure 2.1. As a result of their unique microstructure, each group of polymers, thermoplastics, thermosets, and elastomers possesses different physical behaviors. Details of each category and the AM technologies to fabricate these materials are described in the next section.

Polymers

Thermosets

Heat sensitive

Thermoplastics

Light sensitive

Figure 2.1: Classification of polymeric materials.

https://doi.org/10.1515/9781501518782-002

Elastomers

Thermoplastic elastomers

Thermoset elastomers

2.1 Classification of polymers

23

2.1.1 Thermoplastics Thermoplastic polymers have linear or slightly branched molecular structures as shown in Figure 2.1. Molecules in a thermoplastic polymer are held together by relatively weak intermolecular forces. As a result of these weak interactions, thermoplastic materials soften when exposed to heat and then return to their original condition when they are cooled down. Thermoplastics have a wide range of applications since they can be shaped easily and repeatedly by simply a heating process. Some applications of these polymers include toys, food packaging, thermal insulation, machine parts, and credit cards.

2.1.2 Thermosets Unlike thermoplastics that are held together by weak intermolecular forces, thermoset polymers are formed by cross-linked monomer chains. During the curing process, thermoset resin (usually in liquid form) is mixed with a catalyst, or an external energy is applied to start a chemical reaction cross-linking the monomer chains together irreversibly. Three-dimensional cross-linking network between the polymer chains restricts the motion of the chains and leads to a rigid material that does not softens or melts under heat-like thermoplastics. There are two main types of thermosetting polymers: heat-sensitive and light-sensitive thermosets. For heatsensitive thermosets, thermal (heat) energy is applied to cross-link the polymer chains. For light-sensitive resins, however, light (UV, visible, etc.) is used for the external cross-linking energy as described in Vat polymerization section in Chapter 1. Since cross-linked or cured thermosets do not melt when heat is applied, these materials are ideal for applications where structural rigidity at high temperatures is required. Thermoset polymers are usually stronger than the thermoplastics and more resistant to elevated temperatures. Thermosets usually have excellent aesthetic appearance and are cost-effective materials compared to thermoplastics. However, thermosets are not recyclable since they cannot be reformed back to their liquid form unlike thermoplastics, which can be melted and reformed to the new configurations for recycling purposes.

2.1.3 Elastomers Elastomers are cross-linked polymers. However, unlike thermoset polymers, crosslinking density is very low (Figure 2.1). Therefore, the polymer chains still have some freedom to move, but are prevented from permanently moving relative to each other by the strong bonds (cross-links) between the molecules. As a result of

24

2 Additive manufacturing of polymers

this microstructure, elastomers can be stretched easily and they can rapidly return to their original dimensions when the applied stress is released. As shown in Figure 2.1, elastomers can also be subclassified as thermoplastic and thermoset elastomers. Thermoplastic elastomers melt when heated similar to thermoplastics and therefore, they are commonly used in thermal manufacturing processes, such as injection molding, since heat can be used to shape these materials. Thermoplastic polyurethanes (TPUs) are major group of thermoplastic elastomers and used for various applications, including the production of foam seating, seals, and gaskets. Thermosetting elastomers do not show melting or softening under heat. Rubbers are the most commonly used materials for this elastomer type due to their flexibility and durability. Rubbers are widely used for the manufacture of tires, tubes, hoses, window profiles, gloves, balloons, conveyor belts, and adhesives.

2.2 Selection of polymers for AM The ability of melting and shaping polymers under heat is an important selection criterion for AM of polymers as described in the previous section. However, in addition to melting ability and response to heat, there are different parameters influencing the material selection process as shown in Figure 2.2.

Ease of printing Elongation at break

Visual quality

Maximum stress

Selection criteria

Material cost

Melting temperature

Layer adhesion Glass transition temperature

Figure 2.2: Physical properties of polymers affecting their selection process.

– Printability: The polymer selected for AM should have consistent flow during AM. In addition, it should adhere strongly to the printbed and the previously printed layers for structural strength and geometrical accuracy during the printing process.

2.3 AM of thermoplastic polymers

25

– Mechanical performance: Mechanical strength, stiffness, elongation at break, and impact resistance are important parameters defining the usage of additively manufactured part. Different polymers have different mechanical responses to different loading conditions. Selecting the correct type and classification of polymers will have significant effect on the functionality of the manufactured part. – Visual quality: Polymers can have variety of aesthetic appearances. Although surface finish of the additively manufactured part strongly depends on the fabrication method and process parameters, selection of the polymer type, color, and microstructure will also alter the visual quality of the part. – Resistance to moisture absorption: Unlike metals and ceramics, polymer materials absorb moisture in humid environments. Absorption of moisture can lead to defects during the AM process. Some polymer materials (i.e., nylon) may require special storing requirements due to their high affinity to moisture absorption. In addition to manufacturing defects, moisture can lead to warpage, hygromechanical stresses, and cracking on the printed parts. – Material toxicity: Toxicity of polymers used in AM is a growing concern. Some polymers, especially thermosetting resins, may contain toxic compounds. During curing process, these chemicals may create toxic fumes and also leach from the additively fabricated part into aqueous media. If the additively fabricated part is not fully cured, these may compromise the part biocompatibility. Nontoxic, biodegradable polymers are preferred for their safety and environmental benefits. Polylactic acid (PLA) is a biodegradable thermoplastic polymer, and it is the most commonly used polymer material for AM.

2.3 AM of thermoplastic polymers PLA, acrylonitrile butadiene styrene (ABS), polyethylene terephthalate glycol, highdensity polyethylene, polycarbonate (PC), and nylon are the most commonly used thermoplastic polymers in AM. These polymers have low-melting temperatures, and therefore, they can be additively fabricated using FFF technique as discussed in Chapter 1. For higher temperature applications, special thermoplastic materials having high-melting temperatures such as polyetherimide (ULTEM), polyether ether ketone (PEEK), and polyetherketoneketone (PEKK) are preferred. As shown in Table 2.1, melting temperatures of these polymers would reach up to 400 °C. In addition, these polymers mark high glass transition temperatures compared to the conventional polymeric materials. Glass transition temperature is an important physical property for the end use of polymers. Above this temperature, polymer physical behavior changes from glassy (or crystalline) state to a rubbery state. Therefore, maximum operation temperatures of polymers are designed not to exceed the glass transition temperatures. High-performance polymers described earlier require special printing systems to melt and extrude them on the printbed.

26

2 Additive manufacturing of polymers

Table 2.1: Thermal properties of common thermoplastic materials for AM. Polymer type

AM method

Extruder temperature

Max. service temperature

PLA ABS PC PET PVA ULTEM PEEK PEKK Nylon Polyimide Epoxy Cyanate ester

FFF FFF FFF FFF FFF FFF FFF FFF FFF/SLS SLA/DW DW DW

– – – – – – – – – – – –

     .   – – – 

Cost ($/kg) – – – – – – – – – > – >

CTE (µm/m°C)

Reference

     . – – – . – –

[] [] [] [] [] [] [–] [, ] [, ] [–] [] [, ]

In addition to FFF extrusion, thermoplastics can also be additively manufactured using powder bed fusion methods. Nylon is the most commonly used polymer material for selective laser sintering (SLS) powder bed fusion technique dominating nearly 90% of the SLS manufacturing [29]. Different materials including polystyrenes and polyaryletherketones are also used in small amounts in SLS. Unlike FFF, only a limited number of thermoplastic materials can be fabricated using SLS method. Particle shape, powder distribution, thermal, rheological, and optical requirements must be considered and only a particularly controlled property combination leads to successful SLS implementation [30]. The extent of the material properties required for successful SLS process limits the available materials to only a few successful thermoplastics so far. Table 2.1 shows the glass transition and melting (printing) temperatures of the common thermoplastic materials used for FFF and SLS additive manufacturing processes. Ultimate tensile strength and elastic modulus are the major properties regarding the mechanical performance of polymers. Figure 2.3 summarizes these properties for the additively manufactured thermoplastic polymers. As shown in this figure, there is wide range of variation in the previously published results, which indicates the effects of printing parameters, material composition, and environmental parameters on the mechanical properties. High temperature thermoplastic materials such as ULTEM and PEEK provide higher strength than the other additively manufactured materials. However, the material cost and the need of special printing systems to melt them at higher temperatures are the disadvantages of these material systems. In addition to tensile strength and elastic modulus, elongation at break and fracture toughness are also important mechanical properties for certain applications.

2.4 AM of thermosets

27

120

Strength (MPa)

100

PEEK

80 ULTEM 60 PC

Nylon 40 ABS

20 0

PLA

0

1

3 2 Elastic modulus (GPa)

4

5

Figure 2.3: Tensile mechanical properties of additively fabricated thermoplastic polymers.

Polyamides such as nylon and PC are reported to show high elongation at break among thermoplastics materials and reach over 25% [41]. Despite their superior fracture resistance, ease of fabrication and low cost, nylon is susceptible to moisture absorption and special care must be taken to keep the material feedstock in dry condition prior to AM. Manufacturing with moist nylon filament or powder feedstock can lead to defects in manufactured parts. In addition to nylon and PC, ABS has also been reported to have high elongation (~20%) at fracture in certain studies [42, 43]. However, tensile strength of ABS is lower than the other commonly used thermoplastic polymers.

2.4 AM of thermosets Thermosets are highly cross-linked polymers. Cross-linking can be carried out by applying ultraviolet (UV) light on photopolymer resins or by heating thermally curing resins. Photopolymers are generally fabricated via Vat polymerization techniques or material jetting where UV light is used for material curing. Thermally activated/cured thermosets are generally printed using direct write extrusion process.

2.4.1 AM of photosensitive thermosets Typical photopolymer materials used in Vat polymerization are composed of monomers, oligomers, photoinitiators, epoxies, and a variety of other additives including inhibitors, dyes, toughening agents that adjust the printability and physical properties of photopolymers [6]. Photosensitive resins have been reported to

28

2 Additive manufacturing of polymers

have two primary drawbacks that have prevented their more widespread usage as production materials: sensitivity to water/humidity and tendency to age. Several studies have shown that mechanical properties of additively manufactured parts fabricated via Vat polymerization change over time under a variety of environmental parameters such as temperature, moisture, and UV exposure [44–46]. Similarly, researchers have also characterized mechanical properties of photocurable thermosets fabricated via material jetting process. Results indicate that parts have considerable variability in tensile and compressive properties [47] and exhibit anisotropy. Similar to the parts fabricated via Vat polymerization, material jetted parts have also been reported to have time dependent mechanical properties due to the effects of aging [48]. This indicates that aging is independent of fabrication method but takes place due to the change of polymer molecular structure and the degree of polymer cross-linking. Considering the complexity of the photopolymer chemistry and the variety of these resins for different manufacturers, it is difficult to make a generic comparison for mechanical performance of these systems. However, tensile mechanical properties of selected photosensitive thermosets are given in Figure 2.4 to show the distribution of these properties in the previously published studies. This figure also shows that mechanical properties of the additively manufactured parts depend not only on the additive manufacturing method but also on the chemical structure of the polymer. Same AM technology such as material jetting can be used to fabricate both soft (Durus, Stratasys) and stiff (High temp., Stratasys) photopolymers using the same printer.

120 Direct write

Cyanate ester [1]

Strength (MPa)

100

Material jetting SLA

Polyimide [1]

80

Epoxy [2]

60

High temp. [2] Epoxy [2] Epoxy [3]

Epoxy [2]

40

Epoxy [2] Epoxy [2]

Durus [2]

20

Tango black [2]

0 0

1

2 3 Elastic modulus (GPa)

4

5

Figure 2.4: Tensile mechanical properties of additively fabricated thermoset polymers.

2.5 AM of elastomers

29

2.4.2 AM of heat-sensitive thermosets Heat-sensitive thermosets can be additively fabricated via direct write extrusion without the assistance of UV light exposure if the viscosity of the material is sufficiently high. To adjust viscosity, liquid thermoset resin is mixed with rheology modifiers such as clay or silica nanoparticles. In addition, UV light can be assisted to further enhance the solidification process during direct write printing [49]. In this case, photosensitive compounds must be added into the thermoset resin. Direct write manufacturing of different types of thermoset resins such as epoxies [14, 16], cyanate esters [40], and polyimides (Kapton) [50] have been reported in literature. Tensile mechanical properties of some of these notable studies are shown in Figure 2.4.

2.5 AM of elastomers Elastomers are weakly cross-linked polymers that can be stretched extensively under mechanical loads. AM of these material systems finds growing applications in various fields due to their unique deformability, fracture resilience, electrical, and thermal insulation properties. As shown in Figure 2.1, there are two classes of elastomer polymers: thermoset elastomers and thermoplastic elastomers. As the name indicates, thermoplastic elastomers behave similar to thermoplastic polymers as they are softened and melted under heat. Thermoset elastomers behave like thermosets since these polymers can be cured/solidified by applying light or heat that irreversibly cross-link the polymer chains. Photocurable elastomers therefore can be manufactured via Vat polymerization (SLA, DLP, CLIP) and material jetting processes similar to photosensitive thermosets. Various commercially available thermoset elastomer resins have been developed, including PDMS [51], Carbon EPU40, Stratasys TangoPlus, Formlabs Flexible, and Spot-A-Elastic [52]. Despite the high resolution printing with these UVcurable resins and Vat polymerization technology, maximum elongation at break has been limited to 170–220% in these AM techniques, currently [52]. As an alternative to Vat polymerization and material jetting, direct write techniques were also used for different types of elastomers without the need of UV curing. Direct write AM can be applied to any highly stretchable elastomer such as ecoflex by which 900% elongation at break has been reported in literature [53]. Although this technique offers flexibility in terms of the variety of applicable elastomeric material, part complexity and geometric resolution in this method are reduced compared to photopolymerization methods as discussed in Chapter1. Figure 2.5 presents examples of flexible elastomers fabricated by Formlabs Inc. via Vat polymerization showing the high flexibility and high-dimensional accuracy. The second class of elastomers, thermoplastic elastomers, are also commonly used for AM applications. These materials are easily melted within the print head and fabricated with FFF method. TPU is the most commonly used elastomer for this

30

2 Additive manufacturing of polymers

Figure 2.5: Three-dimensional-printed structures made with Formlabs elastic resin using Vat polymerization technology. Figures were reprinted with permission from Formlabs Inc.

process and high (700%) elongation at break for this material was reported in literature [54]. In addition to the high degree of stretchability, elastomers fabricated with this technique possess high dimensional accuracy (0.1–0.2 mm) as dictated by the FFF technology.

3 Additive manufacturing of polymer composites For rapid prototyping of an actual model or for applications where the components are under low level of mechanical loading, neat thermoplastic and thermoset polymers are ideal material choices. However, if higher mechanical strength is necessary for structural applications or multifunctionality of the additively manufactured part is required, polymer composites are preferred choice. Polymer matrix can be infused with powders, short fibers, and continuous fibers for this purpose. The classification of additively manufactured polymer composites according to the dopant or reinforcement material type is shown in Figure 3.1. Each category will be examined in the following section and comparison will be made regarding the additive manufacturing methods and the physical properties for each composite group.

3.1 Additive manufacturing of powder-doped polymer composites Infusing polymers with materials in powder form is commonly used for composite manufacturing. These doping materials can be in nanoscale such as carbon nanotubes (CNTs), quantum dots, and graphene platelets. It can also be low aspect ratio materials in microlength scale such as carbon black (CB) and metallic and ceramic powders. Powder doping of polymers allow tailoring of various material properties of printed parts including electrical conduction, shape memory, dielectricity, piezoelectricity, optical properties, and thermal conduction. Powder-doped composites can be fabricated by a wide range of additive manufacturing methods including extrusion methods (FFF, direct write), Vat polymerization, binder jetting, material jetting, and powder bed fusion due to the small size of the powder-based additives and the ease of mixing these materials with polymer matrix. Arguably, the most common and beneficial usage of powder doping on polymer composites is adjusting the electrical properties of polymer composites. As a result, many researchers have investigated the effects of different dopants on electrical properties of additively manufactured polymer structures. CNTs have been reported to have high electrical conductivities similar to those of metals. Recently, Chizari et al. [55] fabricated highly conductive CNTs/PLA (polylactic acid) nanocomposites used for additive fabrication of conductive scaffold structures. A ball milling mixing method was used to disperse the multiwalled CNTs (MWCNTs) at high concentrations (up to 40 wt.) in PLA. Postiglione et al. also used PLA/MWCNT nanocomposite for additive manufacturing of conductive 3D structures using a direct write method [56]. They reported a percolation threshold concentration of 0.67% CNTs with a conductivity of 10 S/m, and the highest conductivity was obtained with 5 wt.% MWCNT with 100 S/m. Graphene is a 2D nanomaterial that attracts a growing interest in advanced manufacturing. Graphene has low resistivity, high thermal and electrical conductivity, https://doi.org/10.1515/9781501518782-003

32

3 Additive manufacturing of polymer composites

Polymer composites

Powder reinforced composites

Short fiber reinforced composites

Continuous fiber reinforced composites

Figure 3.1: Classification of additively manufactured polymer composite materials.

and optical transparency. Multiple studies have used graphene to fabricate conductive FFF filaments, and highly conductive (166 S/m) graphene-PLA FFF filaments were successfully fabricated [57]. High cost of graphene, however, is the biggest concern for wider application of these feedstock materials. CB is another conductive filler material used for the additive fabrication of polymer composites. CB is produced from the incomplete combustion of heavy petroleum products such as coal tar and therefore, it is readily available and inexpensive [58]. Low cost, chemical stability, and high conductivity make CB one of the most popular conductive additives. Kwok et al. prepared conductive polypropylene (PP)-based thermoplastic composites suitable for electrical circuit printing using FFF-based 3D printing. High conductivity (~200 S/m) was achieved with composites containing a high percentage loading of CB filler (≥30 wt%) [57]. The authors also showed that additively manufactured composites containing over 25% CB by weight were suitable for fabrication and repair of the practical size electrical circuits. Similar to electrical conductivity, thermal conductivity of polymer composites can also be enhanced significantly by addition of conductive micro/nanomaterials. This is due to the fact that within the material, heat can be transferred by electrons that are also the carriers of electric current. k = ke + k L

(3:1)

where κ is total thermal conductivity, κe is the electronic contribution, and κL the lattice contribution on heat conduction within a material. As electrical conductivity of a material increases, electronic contribution in eq. (3.1) and therefore, thermal conductivity increases as well. Recent studies have shown that conductive dopants (CNTs, graphene, CB, copper, bronze, magnetic iron, and stainless steel) in powder form can significantly enhance thermal conductivity of polymer composites [59, 60]. Therefore, conductive material doped polymer composites can transport heat more effectively compared to undoped polymers and provide uniform temperature distribution between the print layers. As a result, thermal stresses, warpage, and spatial inconsistencies that

3.1 Additive manufacturing of powder-doped polymer composites

33

occur due to the high temperature gradients can be minimized in components fabricated with these high thermal conductivity materials. Piezo electric materials can convert compressive and tensile stresses into an electric charge, or vice versa. These materials find various applications ranging from speakers and acoustic imaging to energy harvesting and electrical actuators. Piezoelectric materials can be fabricated in different geometries by machining and/ or mechanical dicing using saws. These conventional fabrication methodologies limit the size and shape complexity of piezoelectric elements. In addition, brittle piezoelectric materials would be extremely difficult to shape into 3D geometries using these fabrication techniques. To address these issues, Kim et al. [61] have demonstrated a novel tool for fabricating 3D piezoelectric materials that relies on piezoelectric nanoparticles embedded in a photocurable Poly(ethylene glycol) diacrylate (PEGDA) polymer. Barium titanate nanoparticles (BTO) were chemically modified with acrylate surface groups, which formed direct covalent linkages with the polymer matrix under light exposure. In this study, piezoelectric polymer composites were fabricated in complex 3D geometries (mushroom, cross, tapered cantilever, and microtubule structures) using SLA Vat polymerization technique. Doping with 10 wt% loading of the chemically modified BTO nanoparticles resulted in the piezoelectric coefficient of ∼40 pC/N, which was found to be higher than the samples fabricated using unmodified BTO nanoparticles and CNTs. Nanomaterials can also be used to adjust optical properties of the additively manufactured polymers. Depending on their size and environment, zero-dimensional nanoparticles absorb light at different frequencies. As a result, these nanomaterials have found common use for their rich-coloring effects by different industries including cosmetics, food, and clothing. Recently, carbon quantum dots (CQDs) having 2–3 nm size were used to alter optical response of photocurable additive manufacturing resins [62]. As shown in Figure 3.2, statue of liberty figures were fabricated using SLA method by mixing CQD nanoparticles with clear SLA resin and laser solidification into specified geometries. Optical response of the nanoparticle doped and undoped (neat) and control samples to UV light were measured as shown in the figure. As it will be discussed in the next section in more detail, doping materials in powder form either do not have high aspect ratios required for mechanical strength enhancement or do not possess reinforcing capability due to their nanoscale sizes (i.e., CNTs). Therefore, instead of these nano/microscale doping materials, short and continuous fiber reinforcements in larger scale are preferred to enhance strength and stiffness (modulus) of polymer composites. Although enhancing tensile strength and stiffness of polymers using micro/nanoscale reinforcements directly is not a viable option, CNTs can be used indirectly to enhance the mechanical strength of additively manufactured polymers. Sweeney at al. [63] described the use of CNT-coated thermoplastic filaments to increase the strength of the printed parts in FFF process. In this method, thermoplastic filaments are coated with a thin CNT layer by dipping these filaments into MWCNT ink prior to the 3D printing process. These CNT-coated

34

3 Additive manufacturing of polymer composites

(A)

(B)

Figure 3.2: Change of optical properties in additively manufactured photopolymer: (A) SLA printed the Statue of Liberty of CQD doped and undoped (right) photopolymer (the scale bar is 5 cm) and (B) comparison of the specimen with the US dime. Figure was reprinted from [62].

filaments are then extruded via FFF to fabricate the components. Since the CNTs are at the surface of the filament, during the extrusion process, they are deposited at the interface of the printed roads. As the microwave heating is applied on the printed materials, CNTs respond to these microwaves within polymers that cause local melting at the interface region between the printed layers. This facilitates material diffusion in these areas and strengthens the adhesion between the layers. Since these regions are the weakest sections in components fabricated via FFF method, local melting at these areas has been shown to increase fracture and tensile strength of the 3D-printed materials. This method is also named as locally induced RF welding.

3.2 Additive manufacturing of short fiber-doped composites Additive manufacturing of short fiber reinforced polymer composites (SFRPCs) are gaining growing interest in composite manufacturing since the complex structural parts can be manufactured without the need of special manufacturing tools. SFRPCs can be manufactured with the same experimental procedures used for the fabrication of neat and/or powder-infused polymers. In addition to the ease of manufacturing, short fiber reinforcement can significantly increase mechanical properties of polymers such as tensile strength, modulus of elasticity, and fracture toughness unlike the powder-based reinforcements. Processing and additive manufacturing of short fiber reinforced composites depend highly on the types of

3.2 Additive manufacturing of short fiber-doped composites

35

polymer matrix: thermoplastic and thermoset polymers. Therefore, each composite type will be investigated separately.

3.2.1 Short fiber reinforced thermoplastic composites Additive manufacturing of thermoplastic materials using FFF manufacturing is a well-established technology, and it offers high-resolution and low-cost investment benefits as discussed in Chapter 1. Multiple studies exist in literature where chopped polymer [64, 65], glass [66, 67], and carbon [68, 69] fibers were mixed with thermoplastic polymer resins such as PLA and ABS to prepare composite filament feedstock for FFF process. Short fiber reinforced composites were then fabricated by melting these filaments and extruding them on the printbed using FFF. Carbon fiber is the most commonly used fiber reinforcement type due to its high strength, chemical and temperature resistance, and low density. Previous studies performed on short carbon fiber reinforcement have shown significant increase in tensile strength and elastic modulus of composites compared to the neat, unreinforced thermoplastic matrix. Enhanced stiffness (modulus) is especially important since the material distortion and warping is significantly lowered in short fiber reinforced stiff composites during 3D printing process [70]. Although short fibers could be well oriented in the matrix as shown in Figure 3.3, and high fiber volume ratio (~40%) could be achieved, maximum tensile strength of the composites fabricated in these studies is below 100 MPa range (less than the unreinforced, thermoplastic PEEK polymer, see Figure 2.3). The major reasons behind the low strength observed in short fiber reinforced thermoplastic composites are the porosity between the printlines (unavoidable in FFF process) and poor fiber-matrix interfacial adhesion as evidenced by the protruding carbon fibers shown in Figure 3.3. The comparison of FFF-printed specimens to the compression molded samples by Tekinalp et al. [69] shows that tensile strength and modulus of additively manufactured composites reinforced with well-aligned fibers enhance as a function of fiber content; however, these mechanical properties do not exceed those fabricated via compression molding where fibers are randomly aligned (Fig. 3.4). Short fiber reinforced thermoplastic filaments are commercially available today. These filaments can be used in any FFF type 3D printing system provided that the nozzle size and the fiber volume fraction are selected properly to prevent clogging of the extruder with the short fibers during the printing process.

3.2.2 Short fiber reinforced thermoset composites As described in the previous section, weak adhesion between the short fiber reinforcements and thermoplastic polymer is the main cause of low strength in these

36

3 Additive manufacturing of polymer composites

(A)

(B)

500μm

(C)

150μm

(D) Triangular gap among beads

50μm

50μm

Figure 3.3: Fracture surface SEM micrographs of FFF fabricated materials: (A)–(B) neat ABS and (C)–(D) carbon fiber reinforced composite (figure was reprinted with permission).

composites. To enhance adhesion between fiber reinforcement and the matrix, fibers are often coated with a few manometers of thick surfactant layer that is commonly known as sizing. This coating, usually polymer, chemically couples the matrix and the fiber creating a strong adhesion between these two components. Sizing chemistry is well developed for coupling carbon fibers and thermoset resins where liquid resin can wet the fiber surface and facilitate the chemical adhesion process. Additive manufacturing of liquid thermoset polymers can be achieved using direct write

37

3.2 Additive manufacturing of short fiber-doped composites

(A)

(B) 80 70 60 50 40

Compression molded

30

FDM printed

20 0

10

20

30

Fiber loading (wt%)

40

Tensile modulus (GPa)

Tensile strength (MPa)

90

20 18 16 14 12 10 8 6 4 2 0

Compression molded FDM printed 0

10

20

30

40

Fiber loading (wt%)

Figure 3.4: Comparison of FFF fabricated, carbon reinforced thermoplastic against compression molding: (A) tensile strength comparison and (B) tensile modulus comparison. Figure was reprinted with permission from [69].

method as described in Chapter 1. Direct write additive manufacturing of carbon fiber reinforced epoxy thermoset composites were introduced in 2014 by Compton et al. [16] for the first time. However, in this study, short fibers did not enhance the tensile strength of the fabricated composites, which was probably due to the weak adhesion between the thermoset matrix and the unsized fibers used in the composite. Recently, Pierson et al. performed direct write manufacturing to fabricate carbon fiber/epoxy composites where carbon fibers were sized for epoxy resin [71]. In this study, tensile strength of 127 MPa was reached by using only 5.5% carbon fiber by volume as reinforcement. This was 236% increase in strength and 259% increase in elastic modulus compared to the unreinforced epoxy. In addition, comparison of the same material type fabricated via compression molding showed that additively manufactured samples showed 30% higher tensile strength and 47% higher elastic modulus. As shown by the SEM images in Figure 3.5, direct write process leads to the alignment of carbon fibers along the printing direction unlike compression molding where fibers are randomly aligned. In this study, the authors also observed that fiber aspect ratio, fiber volume fraction, and orientation significantly affect the mechanical properties (strength and modulus). The relationship between the fiber morphology and the mechanical properties are described in the next section in more detail. In addition to carbon fiber, short Kevlar fibers were also used as reinforcements to additively fabricate thermoset composites by direct write method [64]. This study showed that composite strength and modulus increased as a function of Kevlar fiber volume fraction. In addition to the strength and modulus, elongation at break of the composites was also increased due to the high flexibility of Kevlar (aramid) fibers. Therefore, additively manufactured Kevlar reinforced composites have high potential for applications where higher fracture toughness and impact resilience are needed along with high strength and stiffness.

38

3 Additive manufacturing of polymer composites

Figure 3.5: SEM micrographs of the fractures surfaces: (A-C) random fiber alignment within compression molding sample and (D-F) uniformly aligned short carbon fibers in 3D-printed composite. Image was reprinted with permission from [71].

Maximum fiber loading in all of these additively manufactured thermoset composite studies described earlier were limited to maximum of 5.5% by volume. It was reported that exceeding carbon fiber over 5.5% results in discontinuous flow, nozzle clogging, and inability of fabrication. Developing novel additive manufacturing technologies to fabricate short fiber reinforced composites with high fiber loading will make a tremendous impact for the adaptation of these materials at wider scales.

3.3 Prediction of mechanical properties of short fiber reinforced composites For composite systems where short fibers are perfectly aligned, elastic modulus can be predicted by the well-known Halpin–Tsai analytical model [14, 72, 73] as follows: EL =

ð1 + 2sηL f ÞEm 1 − ηL f

and

ηL =

ðEr =Em − 1Þ ðEr =Em + 2sÞ

(3:2)

3.3 Prediction of mechanical properties of short fiber reinforced composites

39

where s is the aspect ratio of the fibers, f is the fiber volume ratio, and Er and Em are the elastic moduli of the reinforcement and matrix, respectively. The elastic modulus (ER) of a material with randomly oriented fibers can be obtained by the modulus in longitudinal and transverse directions as follows: ER =

3 5 EL + ET 8 8

(3:3)

where ET is the elastic moduli in longitudinal and transverse directions and obtained by ET =

ð1 + 2ηT f ÞEm 1 − ηT f

and

ηT =

ðEr =Em − 1Þ ðEr =Em + 2Þ

(3:4)

According to these models, modulus in longitudinal direction increases by increasing the aspect ratio of the fiber and the fiber volume ratio within the composite. In transverse direction, however, fiber aspect ratio has no effect on the composite modulus. Strength predictions can also be made using models described in [72, 73]. The ultimate strength of a material with aligned fibers is obtained as follows: 8 > >
σ 3 Þ > : f σr 1 − r + ð1 − f Þσm , s ≥ sc 4sσm

(3:5)

where sC is the critical aspect ratio of the fibers, f is the fiber volume ratio, and σr and σm are the ultimate strengths of the fiber reinforcement and the matrix, respectively. For strength calculations, critical aspect ratio (sC ) plays an important role. Critical aspect ratio (sC ) is defined as follows: pffiffiffi σr 3 sc = (3:6) 2σm The strength of a material (such as that fabricated by compression molding) with randomly oriented fibers is calculated using the directional strengths as follows: σR =

3 5 σL + σT 8 8

where σT is the strength in transverse direction and is obtained by   f σT = σm pffiffiffi − f + 1 3

(3:7)

(3:8)

These analytical formulations indicate that if a fiber aspect ratio is below a critical value (sc), fiber strength does not contribute to the composite strength. This agrees well with the concept that the increase in aspect ratio results in an increase in strength

40

3 Additive manufacturing of polymer composites

and explains why tensile strength cannot be enhanced using powder reinforcements with low aspect ratio as described in the previous section. Experimental validation of the predictions above was performed recently by Pierson et al. [14] as shown in Figure 3.6 where the experimental results for composites were both fabricated by additive manufacturing and compression molding techniques. Experimental test results for the compression molded composites where fibers are randomly aligned closely match the random fiber orientation predictions as shown in this figure. The authors also observed that strength and modulus of the composites fabricated with additive manufacturing was lower than those predicted according to the analytical models described earlier. This mismatch can be due to the fact that analytical models given in eqs. (3.2)–(3.8) assume perfect alignment of fibers within the composite. As shown in SEM images, high degree of fiber alignment was observed in the additively manufactured composites; however, fibers were not perfectly aligned in these material systems. Fiber misalignment as well as printing defects such as voids could lower the strength and stiffness of the additively manufactured thermoset composites. (A)

(B) 250

AM experiment Alighned fiber prediction CM experiment Random fiber prediction

10

Tensile strength (MPa)

Elastic modulus (GPa)

12

8 6 4 2 0

0

1

2

3

4

5

Carbon fiber volume fraction (%)

6

AM experiment Aligned fiber prediction CM experiment Random fiber prediction

200 150 100 50 0

0

1 2 3 4 5 Carbon fiber volume fraction (%)

6

Figure 3.6: Experimental and analytical model comparison of additively manufactured and compression molded epoxy-carbon fiber thermoset composites for (A) elastic modulus and (B) ultimate tensile stress. Image was reprinted with permission from [71].

3.4 Alignment of short fibers within additively manufactured composites Mechanical performance of SFRPCs strongly depends on the fiber length (or aspect ratio) and the alignment of these short fibers within the fabricated composite structure. Theoretical [74, 75] and the experimental [14] studies have shown recently that if the short fibers are well aligned, mechanical performance can be enhanced significantly in the alignment direction. In addition to the mechanical properties, thermal and moisture absorption properties of the composites can also be manipulated/

3.5 Additive manufacturing of continuous fiber reinforced composites

41

enhanced by aligning fibers in specified orientations. Therefore, controlling the fiber alignment has been investigated by many researchers due to its significant impact on the physical properties of the additively fabricated materials. Different techniques can be used to perform the alignment of short fibers in 3D printing technologies such as applying electrical/magnetic field or shear induced fiber alignment. Each of these methods is briefly explained later. External magnetic and electric fields have been applied to manipulate fiber alignment in the liquid resin during stereolithography process. Nakatomo and Kojima used magnetic field to align ferromagnetic γ-Fe2O3 fibers within a polymer [76]. For electrical field alignment, conductive whiskers were used instead of ferromagnetic fibers. When the electric field is applied on the photopolymer filled with the whiskers, a moment is created on the whiskers which align them along the electric field in the liquid photopolymer [77]. Fiber alignment by electrical and magnetic field application during additive manufacturing requires advance mathematical understanding of the forces generated from external forces, and these methods are limited to certain type of materials (i.e., magnetic or conductive materials). In direct write additive manufacturing of fiber reinforced polymer composites, fibers are suspended in a viscous polymer medium and are forced through a converging nozzle aligning along the fluid flow direction due to shear forces between fibers and nozzle walls. Simulation studies carried out by Lewicki et al. [78] and Yang et al. [79] suggest that internal surface-to-volume ratio of the nozzle and rheology properties of the continuous phase fluid are two important factors to achieve efficient shear alignment in extrusion. In other words, fiber alignment takes place naturally in direct write fabrication if these parameters are optimized. Significant effort is ongoing to optimize process parameters and develop better strategies to enhance fiber alignment in additive manufacturing. Manipulating fiber orientation in the desired directions and achieving high degree of fiber alignment (>90%) will dramatically alter mechanical, thermal, and electrical properties of polymer composites and hence make a great impact in additive manufacturing of these materials.

3.5 Additive manufacturing of continuous fiber reinforced composites Reinforcing composites using short fibers is capable of enhancing strength, stiffness as well as fracture resistance provided that the fibers are well aligned in the printing direction and high fiber volume ratios are achieved. The enhancement in strength and stiffness increases as the aspect ratio (s) of the fiber reinforcement increases as shown in eq. (3.5). Therefore, if high strength is needed as that of metallic parts, continuous fiber reinforcement should be considered. FFF technique has been applied by multiple researchers to fabricate continuous fiber reinforced polymer composites, recently. In these studies, different types of

42

3 Additive manufacturing of polymer composites

fibers (glass, carbon, and Kevlar) were used to reinforce thermoplastic nylon polymer using commercially available MarkForged 3D printer [80, 81]. Strength and modulus of composite samples significantly increase as the fiber loading amount is increased in composite. In these studies, maximum strength and modulus enhancement was obtained in the longitudinal (0°) direction along which fibers are deposited. Cross-hatch deposition (±45°) of fibers resulted maximum elongation at break for the same amount of fiber loading [80]. Figure 3.7 represents four types of dogbone specimens fabricated for mechanical characterization: unreinforced nylon, carbon fiber composite reinforced in uniaxial print direction, Kevlar reinforcement in uniaxial print direction, and Kevlar reinforcement in cross-hatch (±45°) array.

Nylon

Nylon/C-fiber unidirectional

Nylon/Kevlar unidirectional

Nylon/Kevlar +45–45

Figure 3.7: Three-dimensional-printed continuous fiber reinforced composites. Unreinforced nylon and composites reinforced with uniaxial carbon fibers, uniaxial Kevlar fiber, and cross-hatched Kevlar fiber. Matrix material is nylon in all composites.

In order to estimate the elastic modulus of the continuous fiber reinforced composites along the fiber direction, the rule of mixtures is commonly utilized. Prediction of elastic moduli for the rule of mixture is reported as follows: Ec = Er f + ð1 − f ÞEm

(3:9)

where E and V represent elastic moduli and fiber volume fraction, respectively. Indices r, m, and c correspond to fiber reinforcement, matrix, and composite, respectively. Similarly, composite strength (σc ) can also be predicted from the rule of mixture equation as follows: σc = σr f + ð1 − f Þσm

(3:10)

43

3.5 Additive manufacturing of continuous fiber reinforced composites

The rule of mixtures model predicts that the strength and modulus of additively manufactured composites increase significantly as the fiber volume fraction increases. High volume fractions (~40%) can be achieved with dual nozzle commercial FFF systems where the first nozzle is used for fiber deposition while the second nozzle is used to extrude thermoplastic polymer, which solidifies and encapsulates fiber reinforcement right after deposition. In-nozzle impregnation methods have also been used recently where fiber bundle and thermoplastic melt are mixed within the extruder and deposited on the surface using a single nozzle. The schematic of in-nozzle impregnation process is shown in Figure 3.8. Similar to the dual nozzle extrusion, high fiber volume fraction was also reported with this method [82, 83].

(A)

(B)

Thermoplastic resin filament Reinforcing fibers

Twisted jute yarn Drive gear Printer head Carbon-fiber tow

Preheater

Heater z

y x Continuous fiber composites

(c)

Nozzle Hot table

10 mm

Carbon FRTP

Printer nozzle

Figure 3.8: FFF printing of continuous fiber composites by in-nozzle impregnation: (A) schematic and (B) fiber bundles used in FFF; and (C) photograph of the 3D printing process. Figure was reprinted with permission from [83].

Continuous fiber reinforcement offers significant improvement in mechanical properties compared to discontinuous fibers. However, compared to the additive manufacturing of powder and short fiber reinforced composites, additive manufacturing of continuous fiber composite systems possesses additional difficulties such as controlling the dual material feeding, slicing for multimaterial system, and weak bonding between the fibers and the thermoplastic polymer. Surface modification of carbon fiber bundle with methylene dichloride and PLA particles [82] improved adhesion and increased tensile and flexural strength. Oztan et al. [80] also reported that microstructural defects in FFF process can significantly reduce the strength and stiffness of the composites. It has been also shown that optimizing the additive manufacturing process parameters (i.e., print temperature, layer thickness, hatch spacing, number of fiber layers, etc.), thermal treatment of the printed samples and postpressure treatment could reduce manufacturing defects and improve mechanical performance of these composite materials [81, 84].

44

3 Additive manufacturing of polymer composites

As discussed in short fiber composite section, thermoplastic polymers weakly adhere on reinforcing fibers. Recently, additive manufacturing of continuous fiber reinforced epoxy (thermoset) composites has been introduced by Hao et al. [85]. In this study, a carbon fiber bundle is pulled through an epoxy resin pool and this wetted fiber tow is then extruded on the printbed using a 2-mm printing nozzle. Tensile strength of 792.87 MPa and elastic modulus of 161.4 GPa could be achieved in this study. Ming et al. [86] used a similar system with a difference of initial impregnation step as shown in Figure 3.9A. In this work, a 3K carbon fiber tow was impregnated with E-20 epoxy resin at 130 °C. Lowered resin viscosity at high temperature and fiber tow pulling process with multiple rollers helped the penetration of the resin into the fibers. This fabricated filament was then fed into a heated printer head where the fiber filament moves through a viscous epoxy resin at 130 °C (similar to the wetting step) and printed on the sample surface (Figure 3.9B) where the resin cools down becoming more viscous which results in enhanced shape stability. After curing at 160 °C, unprecedentedly high tensile strength and modulus of 1,476.11 MPa and 100.28 GPa were achieved in this study.

Figure 3.9: Additive manufacturing of continuous carbon fiber reinforced thermoset composites. Figure was reprinted from [86].

Additive manufacturing of continuous fiber reinforced thermoset composites is at its infancy and more work is necessary for fabrication of composite structures with high mechanical performance. The proof-of-concept studies described earlier show the challenges and limitations of this additive manufacturing technology. It is however clear that this active area of research will gain more popularity in the near future due to superior mechanical properties of additively manufactured continuous fiber reinforced composites reported in these recent studies.

3.6 Mechanical performance comparison

45

3.6 Mechanical performance comparison of additively manufactured polymer composites Polymer composites can be additively manufactured using different types of reinforcements. Micro/nanoscaled dopants in powder form can be utilized to alter especially thermal and electrical properties of polymers. Polymer additive manufacturing methods such as Vat polymerization, extrusion, and material jetting can be applied for fabrication of these composites. Similar techniques can be used for fabrication of SFRPC. Short fibers provide enhanced strength, stiffness, impact resistance, and reduced warpage. Continuous fiber reinforced composites however are preferred for higher strength applications. Currently, extrusion based additive manufacturing is the only suitable method for the additive fabrication of these systems. Although the fabrication of these composites is more challenging compared to discontinuous reinforced composites, they offer significantly higher mechanical performance. Figure 3.10 shows the comparison of the strength and elastic modulus properties of selected studies on additively manufactured, short, and continuous fiber composites.

1,600 1,400

Strength (MPa)

1,200

Continuous fiber thermosets

1,000

Continuous fiber 800 thermoplastic 600 400 200 Short fiber thermoset 0 0

50

100

150

200

Elastic modulus (GPa) Figure 3.10: Strength versus modulus comparison of additively manufactured fiber reinforced composites.

As shown in this figure, continuous fiber reinforced composites are significantly stronger and stiffer compared to short fiber composites. In addition, mechanical performance of thermoset composites exceeds those of thermoplastic matrix composites. However, there are only a limited number of studies regarding the additive manufacturing of thermoset-based composite materials. Figure 3.11 shows how

46

3 Additive manufacturing of polymer composites

1,600 1,400

Strength (MPa)

1,200

Thermoset composites

1,000 800 600 400 200 Thermoplastic composites 0 0

10

20

30

40

50

60

Fiber volume fraction (%) Figure 3.11: Variation of strength as a function of fiber volume fraction.

tensile strength polymer composites vary as a function of fiber volume fraction and polymer matrix type. In this chart, short and continuous composites are grouped together under two main categories of thermoset and thermoplastic composites. It is clear from the figure that thermoset composites provide higher strength for the same amount of fiber volume content compared to thermoplastic composites. The main reason behind this result is the weak adhesion between the fibers and the thermoplastic matrix as described previously. In addition, direct write additive manufacturing of thermoset composites results less defects and porosity compared to FFF process used for thermoplastic manufacturing. As discussed in the previous section and implied in Figure 3.11, short fiber reinforced thermosets have high potential for strength enhancement. The fiber volume fraction however is limited to 5–6% in the state-of-the-art studies. Therefore, future efforts for composite additive manufacturing must focus on increasing the fiber volume fraction in short fiber reinforced thermoset composites, enhancing the fiber–matrix adhesion in thermoplastic composites and optimization of the process parameters in continuous fiber reinforced thermoset composites.

4 Additive manufacturing of metals Polymers and polymer composites form the majority of the feedstock materials used in additive manufacturing (AM) today. AM of metals, however, is gaining an increased popularity due to the unmatched mechanical performance of metals and their higher service temperatures over polymers. In addition, manufacturing of metal feedstock materials is based on well-established technologies providing lowcost benefits. The sale of metal AM systems marked a remarkable (~80%) increase in 2017 compared to 2016 as reported by Wohlers Associates Inc. [87], the company offering strategic advice on AM. Rapid increase on AM of metals is driven by the improved metal processing technologies such as higher part quality, enhanced fabrication speed, and lowered cost of metal additive machines. Additively manufactured metals can be used in numerous fields including dental, aerospace, biomedical, and automotive industries. Additively manufactured, biocompatible metals such as Ti-6Al-4V are preferred in orthopedics where patient-specific implants can be fabricated. In addition, the capability of fabricating these implants with controlled porosity has benefits of matching the mechanical properties of implants to those of the native joints and bone ingrowth allowance to improve their functionality. Aerospace is also one of the leading industries promoting the new technologies for additively manufactured metals due to the possibility of fabricating lightweight components in complex geometries leading to significant amounts of weight reduction. In automotive industry, AM is mainly preferred for rapid prototyping and the concept design stages rather than the end part manufacturing. AM, however, is preferred for manufacturing of numerous components in luxury vehicles and the automobiles that are customized for the user with the limited number of productions. In addition, for highly complex parts justified by a substantial vehicle improvement, AM can be the preferred choice over the traditional manufacturing. The number applications and the users of additively manufactured metal components are on the rise in various fields due to the recent developments of this technology. As described in Chapter 1, different methods can be used for metal AM. Powder bed fusion, direct energy deposition (DED), binder jetting, and extrusion are some of these major metal AM techniques. The market breakdowns for the metal printing technologies used during 2019 are shown in Figure 4.1 as reported by Aniwaa [88]. DED uses powder or wire metal feedstock and this technique is preferred to fabricate large parts due to its high deposition speed. DED using wire is faster than the powder feedstock and large components can be fabricated. Low resolution is the downside of this metal fabrication technique. Extrusion involves mixing metal powders (up to 60–70%) with thermoplastic material and extrusion of the prepared filaments using FFF method. Although simple and cost-effective, high-grade metallic properties (mechanical, thermal, and electrical) cannot be achieved with these metal–polymer composites fabricated via FFF. Binder jetting can be used to fabricate metal parts with high https://doi.org/10.1515/9781501518782-004

48

4 Additive manufacturing of metals

Extrusion 10%

Others 4%

Material/binder jetting 16%

Direct energy deposition 16% Powder bed fusion 54%

Figure 4.1: Market breakdown in metal additive manufacturing technologies [89].

resolution. This technique, however, is a multistep process, including printing, binder removal, and sintering process. Porosity and shrinkage of the component after sintering are also other disadvantages of this process in metal printing. Powder bed fusion is by far the most commonly used metal AM technology covering more than half of the metal manufacturing industry as shown in Figure 4.1. Powder bed fusion is preferred over the other metal additive methods due to its versatility, repeatability, usability of the fabricated parts without postprocessing, and the availability of large range of powder feedstock. This book will therefore focus on the powder bed fusion in the remainder of this chapter, and the current status of this technology will be described. Metal powder bed fusion technologies can be classified into two main groups according to the type of the energy source for coalescing metal powders: laser-assisted manufacturing and electron beam melting. In laser-assisted manufacturing, high power laser sinter is used to partially or fully melt the powder feedstock for part forming. Electron beam melting technology uses electron beam energy source instead to melt the metallic powders in vacuum to form layer-by-layer deposition. Whether a laser or an electron beam source is used for the manufacturing process, powder bed fusion requires high-quality powder feedstock to manufacture metallic components having the desired performance. The next section describes the common technologies used for preparing metallic powder feedstock for powder bed fusion process.

4.1 Feedstock material fabrication for powder bed fusion The quality and the performance of additively manufactured components depend strongly on the quality and the cost of the metal powders used in powder bed fusion.

4.1 Feedstock material fabrication for powder bed fusion

49

The powders must meet the rigorous requirements of chemical composition and morphology. In addition, microstructural composition makes a great impact on the physical properties of the manufactured parts. In general, metallic powders must be free of any contamination to maximize the quality. Upper limits of the contaminants of the metallic powders used for AM are specified in ASTM standards F42.05 on Materials and Processes [90]. As an example, ASTM standard mandates that the oxygen and nitrogen content of finished Ti-6Al-4V products to be lower than 0.2 and 0.05 wt%, respectively [91]. In addition, powder bed fusion methods require the powder to have excellent flowability, which dictates that the powder must have spherical shape and the particle sizes must be reasonably large. The flowability of the powder decreases with a decrease in particle size. However, geometric resolution and surface quality diminish as the powder size is increased. Therefore, spherical metallic powders with low contamination and good flowability are the desired feedstock materials for metal AM. To fabricate metallic powders in spherical shapes to be used for powder bed fusion feedstock, four major processing routes are preferred. These are water atomization, gas atomization (GA), plasma atomization, and plasma rotating electrode process (PREP). Atomization in powder manufacturing refers to a separation process where bulk liquid is broken down into small droplets. This concept is used in all of the spherical powder fabrication techniques where solid metal in wire or powder form is melted and converted to metal droplets that solidify into a spherical shape as it cools down. In water atomization processes, the liquid alloy free falls through the atomization chamber. It is then rapidly cooled down and atomized by the water jets around the chamber. As the metal droplets solidify, fabricated powder is collected at the bottom of the chamber and dried. Due to the high cooling rates, metal powder produced with this method is typically coarse, highly irregular in morphology that reduces both packing and flowability properties [92]. In addition, metal oxidation is a great concern that does not only influence the powder flow behavior but also impacts the melt pool, and consequently, changes the bulk material composition and the mechanical properties of the fabricated parts. Therefore, water atomization is not suitable for reactive materials such as titanium, and it is mostly applied in steel powder production. To overcome the limitations of water atomization that yields low spheroidicity and applied only to nonreactive materials, GA is preferred as the feedstock fabrication method for powder bed fusion. GA process is similar to water atomization; however, liquid metal is atomized by high pressure jets of gas instead of water during the atomization process (Figure 4.2A). Inert gas (nitrogen or argon) is generally used to reduce oxidation and contamination of the metal. Due to the lower heat capacity of the gas (compared to water), the metal droplets cool down more slowly and therefore more spherical powder particles are obtained. Although interstitial elements can be well controlled in gas-atomized powders, there are still potential contamination risks that can originate from the ceramic crucibles and atomizing nozzles used in this technique [92]. Electrode induction melting GA (EIGA) can lower this contamination issue. In EIGA process, metal bar is melted by an induction coil

50

(A)

4 Additive manufacturing of metals

Melt liquid

Ar

(B)

(C)

Electrode

Melt powder

Plasma torches

Titanium spool

Motor

Argon

Melt powder

Fixed tungsten electrode Vacuum

Powder Powder

(A)

Hopper

Vacuum pump

(B)

200 μm

(C)

200 μm

50 μm

Figure 4.2: Schematics of the atomizing processes and SEM micrographs of Ti-6Al-4 V spherical powders fabricated by (A) gas atomization, (B) plasma rotating electrode process and (C) plasma atomization. Reprinted with permission from [94].

prior to entering the atomization chamber and a film of molten metal flows downward into a gas stream for atomization. Since metal feedstock does not come in contact with either crucible or electrode during atomization process, contamination is minimized. Internal voids within individual particles that contain trapped argon are another concern for gas-atomized metal powders [93]. Plasma atomization resembles the EIGA process since a metal wire is used as the metal feedstock as shown in Figure 4.2C. However, this metal wire is fed into a hot zone and melted by plasma torches unlike induction coils used in EIGA process. Molted wire is broken into metal droplets that would cool rapidly. Similar to the EIGA process, plasma-atomized metal powders have high purity since the liquid metal does not contact any other crucible or electrode that may contaminate the powder before solidification. In general, the yield of fine powder using the plasma wire atomization technique is significantly higher than that of conventional GA processes. The plasma-atomized powder has very good sphericity and fewer satellites particles than the gas-atomized powder. However, porosity due to trapped gas during atomization still exists in plasma-atomized powders [93]. In addition, the feedstock must be in wire form, which would increase the fabrication cost and reduce applicability of this method on wider range of metals and alloys. Recently, a new plasma atomization technology (a.k.a. plasma spheroidization) has been developed where metal powders are used as feedstocks rather than metal wires. In this technique, the particle sizes do

4.2 Feedstock materials used in metal AM

51

not change during powder plasma atomization but rather irregular particles are converted into spherical powders. Plasma-atomized particles typically have high spheroidicity as the other atomized powders shown in Figure 4.2, and the impurity level of the plasma spheroidized powders is largely determined by the quality of the feedstock powder. Using powder feedstock instead of wire and minimal amount of satellite in the fabricated powder are advantages of this technique. High investment cost of plasma atomization is the main drawback in this method. In order to fabricate spherical metal powders with the utmost purity, PREP, is preferred. In this method, as its name indicates, a metal rod electrode rotating at a high speed is melted by the tungsten-tipped cathode (Figure 4.2B). The molten liquid is then spun off from the electrode surface to form droplets because of the generated centrifugal force. These droplets solidify during the flight of these particles and form spherical metal powders. Metal powders fabricated with PREP technique has high purity as the liquid metal has no contact with other metals or crucibles. In addition, these powders have minimal gas pores compared with the other production methods, which use the high pressure gas. In addition, liquid metal droplets fly radially away from the metal surface under the effect of a centrifugal force, and therefore the chance of collisions of droplets and particles to form satellites is very low [93]. High cost and low yield of powder fabrication are the downsizes of this fabrication technique. Yield is low since the fabricated powder is usually too coarse for powder bed fusion AM applications. Increasing the rotation speeds of the metal electrode reduces the particle size but may create dynamic forces and balancing issues as reported in [95]. Comparison of the powder manufacturing technologies is given in Table 4.1, where major properties of these techniques are summarized. New powder production methods are currently under development to obtain, high purity and low-cost feedstock fabrication. Electrolytic methods, metallothermic processes, and the hydride–dehydride process are some of these techniques with high potential to be considered as cost-effective alternatives for metal powder production [96].

4.2 Feedstock materials used in metal AM Common materials used in AM of metals are steel (stainless and tool), aluminum, titanium, and its alloys as well as Ni-based alloys (Inconel) and other metallic materials. Figure 4.3 shows the breakout percentages for the current usage of these metals in AM industry. Stainless steel and titanium-based materials mark the majority of the metallic material used by AM applications currently. Steel is the most commonly used engineering material as of today. Similar to the conventional manufacturing, steel is also the most commonly used metallic material in AM as shown in Figure 4.3. Excellent mechanical properties make these materials the

52

4 Additive manufacturing of metals

Table 4.1: Summary of metal powder processing technologies (modified from [92]). Manufacturing process

Particle size, Advantages µm

Disadvantages

Metal types

Water atomization

– High throughput Range of particle sizes Feedstock in ingot form

Low spheroidicity Satellites present Wide PSD Low yield in – µm

Nonreactive

Gas atomization

– Wide range of materials Feedstock in ingot form High throughput High spheroidicity

Satellites present Wide PSD Low yield in – µm

Ni, Co, Fe, Al Ti (EIGA)

Plasma atomization

– High purity High spheroidicity

Feedstock in wire or powder form High cost

Ti, Ti-Al-V

PREP

– Highest purity High spheroidicity

Very low yield High cost

Ti, Exotics

Tool steel 4% Aluminum 15% Stainless steel 50% Inconel 8%

Titanium 23%

Figure 4.3: Commonly used metals in powder bed fusion AM [97].

4.2 Feedstock materials used in metal AM

53

desired material choice for AM applications. In addition, metal additive technologies are suitable for the fabrication of various types of steels. Austenitic stainless steels (i.e., 316 L) are the most commonly used steel type in AM. However, maraging steel, precipitation hardenable stainless steels, martensitic cutlery grade, and tool steel (H13) have also been investigated. Mechanical properties of steels depend strongly on their microstructure, which can be tailored by controlling the process parameters such as the cooling rate, the temperature gradients, and the elemental composition. Therefore, metal AM offers producing metallic parts with unique microstructures and mechanical properties by the precise control of the process parameters during manufacturing.

4.2.1 Titanium and titanium alloys Titanium (Ti) and its alloys are gaining popularity in AM due to their excellent strength, lightweight, and biocompatibility. As a result, these metals have already found numerous applications in automotive, aerospace, and healthcare industry. In addition, high material costs, difficulty in manufacturing these materials via conventional processing make these materials excellent candidates for specialized AM applications. Pure Ti and Ti-6Al-4V alloy have been widely used in commercially available powder bed fusion technologies. Although to a lesser degree, other titanium alloys have also been investigated including Ti-24Nb-4Zr-8Sn and Ti-6Al-7Nb for biomedical applications, and Ti6.5Al-3.5Mo-1.5Zr-0.3Si for aerospace applications [96].

4.2.2 Aluminum alloys Unlike steel and titanium, conventional manufacturing of aluminum (i.e., casting, machining etc.) is relatively easy and cost-effective. Therefore, AM of aluminum parts has lower commercial advantage compared to steel and titanium. In addition, aluminum alloys are known to be challenging materials for AM due to their poor flowability during recoating process in powder bed fusion. These materials also reflect laser highly during laser based fusion process [96]. All of these manufacturing difficulties and the commercial disadvantages limit the use of aluminum in this field. However, low cost and lightweight of aluminum make it and its alloys an attractive option for AM. High thermal conductivity of Al reduces thermally induced stresses, thus also reducing the need for extensive support structures. In addition, the high thermal conductivity allows for higher processing speeds. By the development of new AM technologies overcoming the difficulties of aluminum AM and with more aluminum powder suppliers, it is expected that aluminum will be among the key AM materials in the shift toward larger batch production of mass goods [98].

54

4 Additive manufacturing of metals

4.2.3 Other metals High performance metals other than steels, aluminum, and titanium have also been under investigation by AM community. Ni-based superalloys are the major metals among these used for high-temperature applications, such as Inconel 625 and Inconel 718, and NiCoCr for biomedical applications [96]. In addition, invar (nickel–iron) alloys have been used for applications requiring a low coefficient of thermal expansion in powder bed fusion. High entropy alloys are defined as metallic alloys that contain three or more multiprincipal elements, with equal, or near equal atomic percentages [99]. High entropy alloys are solid solution alloys with a single crystal structure and present special mechanical and chemical properties such as combination of high yield strength and ductility, microstructural stability, and high mechanical strength at elevated temperatures, high resistance to corrosion, and oxidation. As a result, these materials have found significant interest in AM community. Powder bed fusion has been used to manufacture various different types of high entropy alloys including FeCoCrNi, AlCoCrFeNi, and CoCrFeMnNi [99–101]. In these studies, superior strength and ductility of these materials have been reported due to the formation of single-phase solid-solution state.

4.3 Design considerations in metal AM Unlike conventional manufacturing, several process-related conditions must be taken into account in AM of metals. These are void formation, residual stresses, surface roughness, and the postprocessing. Each of these parameters can significantly alter the mechanical performance of the manufactured parts, and therefore they must be carefully assessed and optimized. Following section describes these process parameters and their effects on the performance of the additively fabricated metal parts.

4.3.1 Void formation Spatter ejection (powder jump) takes place when a metal powder flies out of the laser’s path, lands back on certain parts of the fabricated material contaminating the powder bed, and therefore affecting the build quality of a layer. This is more pronounced if the laser intensity is very high. In addition to the spatter, using high laser intensity will lead to higher melt pool dynamics and reduced density originating from pores formed due to entrapped gas [96]. These pores have generally spherical shape, as they are formed during evaporation of material. On the other hand, if the laser intensity (power) is too low, the metal powders may not fuse adequately, and these loose powders can create voids within the printed part. These pores forming as a result of unmolten material have irregular-shaped voids. Irregular-shaped

4.3 Design considerations in metal AM

55

voids lead to higher stress concentrations than the spherical voids and act as crack initiation sites during mechanical testing. Therefore, irregular-shaped voids are considered to be more deteriorating on mechanical properties than the spherical ones as these voids can lead to premature material failure.

4.3.2 Residual thermal stresses During metal 3D printing, pristine alloy feedstock is exposed to a series of processing steps, including rapid melting and fast solidification, which can lead to thermal stress in the finished part [102]. When this residual stress exceeds the strength of the printing material or substrate, defects, such as cracking in the part or warpage of the substrate, can occur. Residual stress can be minimized by postheat treatment and optimizing the printing process parameters. Thermal stresses are also material dependent, and therefore, different metals (aluminum, steel, titanium) can show different levels of sensitivity to volumetric changes under the same thermal conditions leading to different amounts of residual thermal stresses.

4.3.3 Surface roughness Surface roughness is a defect affecting the quality, aesthetics as well as the mechanical performance of printed parts. Although surface roughness may not affect the static mechanical performance of the additively manufactured parts, it can be detrimental in fatigue type of loading conditions as it will be explained later. It is therefore a common practice to mechanically or chemically reduce the surface roughness in additively manufactured components. Powder size, cooling rate, and spatter can affect the surface roughness.

4.3.4 Postprocessing Postprocessing may be applied on the fabricated metallic components to reduce thermal stresses, reduce porosity, optimize grain microstructure, and reduce surface roughness. Therefore, unlike polymer AM, postprocessing procedures are commonly performed on additively manufactured metals while keeping the parts connected to the build platform. ASTM has recently reported the postprocessing conditions to standardize the postprocessing for metals manufactured via powder bed fusion in ASTM F3301 standard [103].

56

4 Additive manufacturing of metals

4.3.4.1 Stress relief As described earlier, residual stresses can be detrimental for static and dynamic mechanical properties of additively manufactured metallic components. These stresses must be relieved before the part is removed from the build plate to avoid the warpage and/or cracking of the parts. Stress relieving is achieved by inserting the entire build plate in an oven. Temperature and the duration of the stress relief process are specified for different metals in the ASTM F3301 standard. 4.3.4.2 Heat treatment In addition to the stress relieving, heat treatment such as aging and solution annealing are performed on the fabricated to optimize the grain microstructure and improve the mechanical properties of the parts. Heat treatment may distort the dimensions of the parts, and therefore, it is usually applied prior to the final surface finishing and machining. 4.3.4.3 Hot isostatic pressing Hot isostatic pressing (HIP) is defined as the simultaneous application of high temperature and pressure on metallic components to reduce porosity within these parts. Temperature control is provided by a furnace, which is located within a pressure vessel. Inert gas is used during the HIP process to apply uniform isostatic pressure that forces internal voids to collapse and densify the material. Eliminating defects and voids in HIP process significantly improve mechanical properties of the metallic components. HIP process is a well-established process for a wide variety of materials such as titanium, steel, aluminum, and superalloys. HIP process is commonly used on additively manufactured components and near theoretical densities can be achieved by complete elimination of porosity within the fabricated parts. 4.3.4.4 Machining and surface treatments Mechanical machining of the fabricated metal parts may be required to ensure dimensional accuracy of the finished part. In addition, surface finishing might also be required to improve the surface quality, reduce surface roughness, clean internal channels, or remove partially melted particles on a part. As discussed earlier, surface roughness is not only detrimental for the aesthetics of the additively fabricated part but also significantly lowers the fatigue mechanical performance of the component. Surfaces of the fabricated parts can also be treated chemically or coated to improve physical appearance and functional properties such as reactivity, biocompatibility, corrosion, and wear resistance.

4.4 Mechanical properties of additively manufactured metals

57

4.4 Mechanical properties of additively manufactured metals As described earlier, there is a strong correlation between the process-dependent microstructure and the mechanical properties of the additively fabricated metals. Figure 4.4 summarizes the relationship between the AM process parameters, microstructure, and the mechanical performance of the fabricated parts. Process parameters such as laser intensity, laser scan speed and direction, powder type, and morphology and postprocessing operations all determine the final product microstructure. Grain size and orientation, porosity, roughness, and density are the microstructural properties characterized for additively manufactured metals. As a result of these microstructural characteristics under the selected process parameters, mechanical properties are determined. Strength (tensile, yield), hardness, elongation at break, and fatigue limit are the major mechanical properties identifying the structural performance of the fabricated parts.

‒ Powder properties (purity, moisture level, size, shape) ‒ Print properties (pressure, laser power, scan speed) ‒ Postprocessing (HIP, heat treatment, surface finish) Process parameters

‒ ‒ ‒ ‒ ‒

Grain size Grain orientation Pore size Pore shape Roughness

‒ ‒ ‒ ‒ ‒

Microstructure

Mechanical properties

Yield strength Ultimate strength Elongation Fatigue strength Toughness

Figure 4.4: Relationship between the process parameters, microstructure, and mechanical properties of additively manufactured metals.

Several metallurgical processes play important roles in forming the ideal microstructure in additively manufactured parts for the intended application. Grain refinement, precipitation hardening, solid-solution strengthening, and age hardening are some of these processes defining the final microstructure, and therefore, the mechanical properties of the fabricated metal. Selecting the right elemental composition of the metal feedstock and controlling process and postprocess conditions are the active areas of research, and it is out of the scope of this book. Excellent studies exist to further understand the strengthening mechanisms in metals [104, 105] and the relationship between the microstructure and the mechanical properties in additively manufactured metallic components [96]. Porosity is a key parameter affecting the mechanical performance of metal structures. As described previously, pores (or voids) facilitate crack initiation and propagation and may significantly deteriorate the mechanical performance of additively manufactured components. Not only the total porosity but also the pore size distribution and pore shape determine the final mechanical properties. Therefore,

58

4 Additive manufacturing of metals

minimizing the porosity is one of the primer goals in metal AM. Recent advancement in metal AM technology allows fabrication of metallic parts with a density over 99.5% for different metals. This can be further increased and thermal stresses forming during the fabrication process are minimized by HIP and thermal annealing postprocessing operations. As a result of minimizing porosity and optimizing the AM process parameters, fabricating metallic parts having comparable static mechanical strength to the conventional manufacturing methods is achievable for different metals. In fact, static strength can be even higher in additively manufactured parts compared to those fabricated via conventional means (i.e., casting) since finer grains are obtained in AM [96]. Strength, however, is highly anisotropic in AM as the grains are preferentially oriented and elongated in the printing direction. In general, higher strengths are reported in the printing direction compared to the orthogonal directions. Static mechanical properties of selected metals are given in Table 4.2. As shown in this table, the static strength for additively manufactured metals can match or even exceed their conventionally manufactured counterparts. Elongation at break and ductility of the additively manufactured metals have usually high variation and strongly depend on the process conditions and postprocessing operations. Residual stresses, oxygen content, and porosity are reported to be major factors reducing ductility in additively manufactured metals [96]. Ductility, however, can be enhanced, if needed, by heat treatment that reduces the thermal stresses and help reorganization of the grain microstructure. Fatigue strength of metals is an important parameter determining their durability under dynamic loads. Similar to the static strength, fatigue performance of metallic components is affected by the microstructure including the grain size and orientation, defects, and porosity. However, unlike static strength, fatigue strength depends on the surface finish and roughness of these components. Surface defects, unmelted metal particles on the surface and high surface roughness can increase stress concentration and initiate crack formation at the surface significantly reducing the fatigue performance of the fabricated component. Reducing surface roughness by machining, sand blasting, and polishing can significantly improve the fatigue properties. In addition, HIP and thermal treatment processes can further enhance the fatigue performance of the fabricated parts by minimizing the porosity and tailoring the grain microstructure, respectively. It is shown in Table 4.3 that performing postprocessing operations on the fabricated specimens can enhance the fatigue strengths of additively manufactured metals to match cast and wrought materials. In summary, metal AM is gaining a rapid popularity due to the recent advancement in this technology and better control of the manufacturing process parameters. As a result, mechanical performance of these materials can approach, and sometimes exceed the static mechanical properties obtained for similar materials processed conventionally. Weak fatigue performance, lower ductility, high variability of results

59

4.4 Mechanical properties of additively manufactured metals

Table 4.2: Static mechanical properties selected metals fabricated via powder bed fusion (PBF) or conventional (cast, wrought) manufacturing (redrawn according to [96]). Metal type

Process

Postprocess

Yield strength

Ultimate Elongation tensile (%) strength

Ref.

L-Stainless steel Wrought Annealed PBF No DED No

  

  

  

 Stainless steel

 

 

 [] . []

,

,



[]



,



[]

Wrought Annealed LBM No

-PH precipitation Wrought Sol. treated and aged

[] [] []

Hardening steel

PBF

Sol. treated

Ni- Maraging steel

PBF

No

,

,

H High-speed steel

DED

No

,

,



[]

AlSi

Cast PBF

No No

 

 

 

[] []

AlSiMg

Cast PBF

No No

 

 



[] []

AlMgSiCu

PBF

HIP

–



–

[]

. []

ALMg.Sc.MnZr PBF

Aged







[]

Ti

Sheet

No







[]

Ti-Al-V

PBF No Cast No Wrought Sol. treated and aged PBF No PBF Heat treated (in situ) DED No

  ,

 , ,

. []  []  []

, ,

, ,

. [] . []



,

. []

PBF

,

,

. []

Ti-.Al-.Mo.Zr-.Si

No

60

4 Additive manufacturing of metals

Table 4.3: Fatigue strengths of selected metals fabricated via powder bed fusion (PBF) or conventional (cast, wrought) manufacturing (redrawn according to [96]). Metal type

Process

Postprocess

Surface treatment

Fatigue strength at  (MPa)

Ref.

 L-Stainless steel

PBF

No Heat treated HIP

No Machined Machined

  

[] [] []

-PH Precipitationhardening steel

PBF

No

Polished



[]

AlSi

Cast PBF

No Stress relieved

Polished Polished

– 

[] []

AlSi Mg

Cast PBF PBF

No No No

Polished No Polished

  

[] [] []

ALMg.Sc.MnZr

PBF

Aged

Polished



[]

Ti-Al- V

Wrought PBF PBF PBF PBF

No Stress relieved Stress relieved Stress relieved Stress relieved

Polished No Polished No Polished

    

[] [] [] [] []

depending on the manufacturing process, instrument, feedstock type, and other process parameters limit wider use of these processing techniques for fracture-critical applications. Future work is needed to understand the sources beneath these issues and resolve them to make the metal AM a stronger alternative for the conventional metal manufacturing technologies.

5 Additive manufacturing of ceramics Ceramic materials possess unique mechanical, thermal, and electrical properties that cannot be achieved with polymers and metals. These properties include high level of hardness, excellent thermal and electrical insulation, chemical and wear resistance, and extremely high melting point. These properties make ceramics preferred materials for applications involving harsh environments (high wear, temperature, pressure, etc.). In addition, ceramics are earth abundant, natural materials providing excellent biocompatibility, and low costs. As a result, traditional ceramics derived from common, naturally occurring raw materials such as clay and quartz sand are as old as human race. These ceramic materials have been extensively used as foodware, clay brick and tile, industrial abrasives, refractory linings, and Portland cement. In addition to these traditional ceramics, high performance ceramics have found great usage in the recent years. High performance ceramic materials such as oxides, carbides, and nitrides exhibit superior mechanical properties including high strength, high resistivity against high temperature, corrosion, and oxidation. Therefore, these ceramic materials find growing interest in the fields of aerospace, automotive, and energy conversion where components such as the gas turbines, engines, batteries, and heat exchangers undergo high operation temperatures. Fabrication of ceramic materials in complex geometries using conventional manufacturing methods, however, is extremely difficult due to the low ductility, high melting temperature, and high crack damage sensitivity of these materials. AM offers new opportunities for forming ceramic materials into complex geometries and allows fabrication of customized and lightweight ceramic parts via topology optimization. This design flexibility has attracted various applications of additively manufactured ceramics. Due to the biocompatibility of ceramics and the ability of fabricating patient-specific parts, biomedical applications dominate the current usage of additively manufactured ceramics. Biocompatible-bioinert ceramics such as alumina and zirconia are commonly used in dental applications. Biocompatible–bioactive materials such as hydroxyapatite or bioactive glasses are preferred in orthopedical implants (knee, hip, spinal fusion, etc.), where bone tissue repair and growth within or around the implant are desired. Additively manufactured ceramics used for aerospace applications include high temperature resistant, tough, and strong ceramics that are very difficult or impossible to fabricate using conventional methods. Turbine components, spark plugs, hypersonic engines are some of these aerospace applications. Figure 5.1 represents some examples of additively manufactured ceramics used in mechanical, aerospace and medical applications. Ceramic additive manufacturing can be classified into three major categories according to the type of ceramic feedstock: powder-based ceramic manufacturing,

https://doi.org/10.1515/9781501518782-005

62

5 Additive manufacturing of ceramics

(A)

(B)

(C)

5 mm

2 mm

2 mm

(E)

(D)

1 cm Figure 5.1: Examples of additively manufactured ceramic parts used for different applications: (A) alumina gear parts fabricated using the LCM technique; (B) alumina turbine blade; (C) alumina cellular cube. (A)—(C) were reprinted with permission from [136]. (D) Cranial segment fabricated via powder bed fusion. Figure was reprinted with permission from [137], (E) SiOC-based turbine wheel. Figure was published with permission from [138].

slurry-based ceramic manufacturing, and bulk solid-based ceramic manufacturing as shown in Figure 5.2. Processing approaches, advantages, and disadvantages of these fabrication methods are explained in the following section.

Ceramics feedstock for AM

Powder based

Slurry based

Bulk solid based

– Powder-bed fusion – Binder jetting

– Vat polymerization – Direct ink writing

– Laminated object manufacturing – FFF

Figure 5.2: Classification of additive manufacturing techniques based on the ceramic feedstock type.

5.1 Powder-based ceramic additive manufacturing In powder-based ceramic additive manufacturing, loose ceramic powders are used as feedstock material. Ceramic powders are bonded together via spraying liquid binder on them (binder jetting) or they are melted (partially or fully) using a laser beam (powder bed fusion). Binder jetting and powder bed fusion are commonly

5.1 Powder-based ceramic additive manufacturing

63

adopted technologies for ceramic additive fabrication due to their design flexibility without the need of any support structures.

5.1.1 Binder jetting of ceramics Binder jetting is the most commonly used additive manufacturing technology for ceramic manufacturing. As described in Chapter 1, in binder jetting process, an inkjet printing head jets a binding liquid onto ceramic powder giving it a 3D shape. At the end of the process, binder is removed, and the remaining ceramic particles are sintered to enhance the stability and the strength of the fabricated part. Multiple nozzles can be used to fabricate large (meters long) objects [139] or deposit simultaneous layers of smaller objects for mass manufacturing. Binder jetting is an additive manufacturing technology with the benefits of high speed, low cost, and design flexibility. This technique is used to fabricate ceramics with high porosity. Porous biomedical ceramic materials find various applications, most notably in biomedical field. Components used for tissue engineering such as tissue scaffolds, biomedical implants, and cages are commonly preferred to be porous than solid since the porous network allows tissue and cell ingrowth in these biomedical systems. Hydroxyapatite and calcium phosphates are such biocompatible ceramic materials and commonly used in binder jetting technique to print scaffolds and implants for bone replacement. Although porosity within additively fabricated ceramics is advantageous in scaffolding applications, it adversely affects the mechanical performance of ceramic materials. Voids form within the fabricated ceramic specimens due to the imperfect packing of ceramic particles and also due to the requirement of the sacrificial binder material that is removed during the sintering process. Porosity of 40–60% is commonly observed in binder jetting process. Compared to metals, porosity is a greater concern for mechanical performance of ceramics that show significantly higher sensitivity to defects and possess very low fracture toughness. In order to achieve high density and, therefore, high structural stability, postprocessing technologies have been developed for ceramic additive manufacturing. These techniques involve infiltration of the porous parts, applying isostatic compaction prior to sintering and liquid sintering of the binder jetted parts. In the infiltration method, a secondary material in liquid form such as molten metals or ceramic/metal slurries penetrates into and fill the pores of the ceramic part. Filling the voids and densification of the ceramic component lead to significant enhancement of mechanical performance due to the reduction of porosity. Infiltration method has also been used to fabricate secondary ceramic materials by the reaction between the porous ceramic preform and the infiltrated material. Liquid silicon is one of the most commonly used infiltration materials to fabricate reaction bonded silicon carbide ceramics. In this method, liquid silicon is infiltrated

64

5 Additive manufacturing of ceramics

into binder jetted preforms and heated to temperatures giving rise to the interaction between the preform and the silicon and the formation of new ceramic phases such as silicon carbide [140, 141] and titanium silicon carbide (Ti3SiC2) [142] depending on the selected preform material. In addition to infiltration, isostatic pressure (or compaction) is commonly used in ceramic additive manufacturing to reduce porosity. Pressure is applied on “green” samples prior to sintering step. It was reported that cold isostatic pressure application performed on binder jet processed Ti3SiC2 structures before sintering resulted highly dense (99%) ceramic structures [143]. Similarly, heat and pressure can be applied simultaneously in warm isostatic pressure technique. Yoo et al. [144] applied warm isostatic pressure on alumina parts to achieve 99.2% relative density. The main limitation of the performing isostatic pressures is the difficulty of application of this technique on complex geometries, such as parts with internal cavities where application of pressure can lead to undesirable deformations or material failure. Liquid sintering has also been shown to be an effective postprocessing method to reduce porosity within the additively manufactured ceramic structures. In liquid sintering method, secondary phases/materials are added into the ceramic mix prior to printing. During the sintering process at elevated temperatures, these additives/dopants melt and take a viscous liquid form, enhance sintering, powder bonding processes, and minimize porosity by filling the cavities. This method has been applied by multiple researchers to fabricate highly dense (95–98%) ceramic structures using ZnO/SiO2 additives into tricalcium phosphate (TCP) [145] and hydroxyapatite (HA)/wollastonite additives in glass ceramic [146]. In summary, binder jetting is a flexible and a highly adopted technology for additive fabrication of porous ceramic structures. Ceramic particles can be fabricated into complex geometries using small amounts of organic or inorganic binders sprayed by a printing head. Porous ceramics fabricated with technique may find immediate applications without the need of any postprocessing. Achieving highly dense ceramic components, however, requires performing postprocessing operations that can result in achieving mass densities of ceramic components similar to the theoretical bulk ceramic values.

5.1.2 Powder bed fusion of ceramics Powder bed fusion has similarity to binder jetting since it does not require any support material for printing process as the powder bed acts as the support for the successive layers. Powder adhesion and the additive manufacturing are, however, performed by melting the loose powders using a laser beam source rather than spraying an adhesive material. Direct sintering of ceramic powders via laser beam is difficult due to the low thermal shock resistance of ceramics, poor sintering, and high cost due to the extremely

5.1 Powder-based ceramic additive manufacturing

65

high melting temperatures of these materials. As a result, direct melting of ceramic powders has been used only in limited number of studies. Using direct sintering, ceramic parts were fabricated out of alumina [147], zirconia [148], and zirconia/yttrium oxide (ZrO2–Y2O3) [149] powders. Large porosity, crack formation, and high surface roughness were observed in these fabricated parts. To mitigate the issues of direct sintering, laser microsintering method was developed recently [150], and this method allows melting and sintering of ceramic powders in submicron length scale with a laser. Al2O3- and SiC-based ceramics were successfully fabricated with low roughness and at high resolution. However, this method is only applicable to small parts (micro-millimeter scale) and not practical for fabrication of larger parts. Due to the difficulty of direct melting or sintering of ceramic powders, an alternative powder bed fusion methodology is developed where ceramic powder is mixed with a lower temperature melting material that is melted by the laser during the additive manufacturing process and allows the formation of 3D shape. These binder materials are also selected to be temperature shock tolerant to minimize the risk of residual stresses and cracking during sintering. Polymers, metals, and ceramics (i.e., glass) can be used as the liquid-forming binder phase. If polymer (organic) binder is utilized as the binder, this binder material is removed after the laser-sintering process by heat, and the remaining ceramic powders are sintered in a furnace at elevated temperatures. Inorganic binders, however, cannot be removed by firing at elevated temperatures. These binders remain in the fabricated ceramic material or interact with the ceramic powder and form secondary phases. If metallic binder is utilized, an inert gas is used during the fabrication to prevent possible oxidation of the binder material. During the preparation of multimaterial powder feedstock, the binder can be either mixed with the ceramic powder or it can be coated directly on the surface of the powder. It has been shown that the latter is advantageous since it results homogenous distribution, better sintering, and mechanical performance. Various binders have been used in powder bed fusion fabrication including polyetheretherketone, polyamide, ammonium phosphate, TCP, polycarbonate, glass, and boron oxide. As expected, using the sacrificial binder material results in significant porosity and these porous ceramics find wide range of biomedical applications. For structural applications, however, denser ceramics are required and therefore densifications methods such as infiltration and isostatic compaction described in the previous section are commonly performed on porous ceramic preforms for these applications. Organic binder-based powder bed fusion process cannot be applied to enclosed geometries since the binder cannot be drained out of the fabricated component. In addition to porosity, low geometrical resolution and high degree of shrinkage (during sintering and also isostatic compaction processes) are limitations for powder bed fusion fabricated ceramics.

66

5 Additive manufacturing of ceramics

5.2 Slurry-based ceramic additive manufacturing Slurry-based ceramic additive manufacturing techniques involves the preparation of liquid feedstock in the form of ink or paste in which ceramic particles are dispersed. This ceramic-based liquid slurry can be additively manufactured via Vat polymerization, material jetting, or paste extrusion techniques.

5.2.1 Vat polymerization of ceramics In Vat polymerization, ceramic particles are contained within a photopolymer. As the photopolymer is cured via a UV light source, ceramic material remains surrounded by the crosslinked polymer network. The fabricated part usually goes under additional heat treatment process for the removal of the polymer phase and sintering of the remaining ceramic particles. In this method, homogeneity, stability, and viscosity of the mixture are essential to obtain the desired part quality. Ceramic particles must be well dispersed within the photopolymer and this condition must be stable during the printing process. Segregation of ceramic particles results in homogeneous part fabrication and poor material performance. Low volume content of ceramic particles within the composite mix increases dispersion efficiency. However, this leads to porous ceramic network and high shrinkage that are not desirable for structural applications. Another process parameter that needs to be carefully controlled is the mixture viscosity. Ceramic/photopolymer mixture is desired to have low viscosity and good flow characteristics that are optimized for the Vat polymerization process. In addition to viscosity, high volume of ceramic powders may negatively affect the photopolymerization process since the UV light may be absorbed or scattered by the ceramic particles leading to insufficient curing. Different types of ceramics such as SiO2, Al2O3, ZrO2, and SiC can be fabricated with Vat polymerization technology. SLA and DLP are the major Vat polymerization technologies used for ceramic additive manufacturing. As described in Chapter 1, DLP involves the projection of light as a 2D layer instead of raster scanning of a single-point-based light interaction, and therefore, DLP process significantly enhances the printing speed. DLP ceramic printing technique is commercialized in the name of lithography-based ceramic manufacturing (LCM) by founding company of Lithoz GmbH [151]. Figure 5.3 shows examples of ceramic components fabricated via LCM polymerization. Due to the speed and the resolution of the process, fine ceramic structures including lattice structures [153], heat exchangers [154], and negative Poisson’s ratio metamaterial structures [155] have been fabricated with this ceramic-additive fabrication method.

67

5.2 Slurry-based ceramic additive manufacturing

B

A

1 cm D

C

1 cm F

E

5 mm

1 cm

5 mm

5 mm

Figure 5.3: Images of silicon nitride-based ceramic parts fabricated via LCM additive manufacturing: (A) gyroids, (B) impeller, (C) spinal implant (posterior lumbar interbody fusion cage), (D) cutting tools, (E) dental two-piece implants with M1.6 inner thread, and (F) de Laval nozzle. Figure was reprinted from [152].

5.2.2 Direct writing of ceramics Direct ink writing is defined as the extrusion of viscous liquids (or pastes) layer by layer to construct 3D objects. Direct write (DW) technique enables a cheaper and faster alternative compared to the photopolymerization. In this technique, ceramic suspensions compose of a ceramic powder, dispersant, polymer binder (i.e., polyvinyl pyrrolidone and methyl cellulose) and solvent such as water. Alumina has been used commonly in this method due to its low cost, availability, and ease of densification. Highly loaded viscous inks can be prepared over 60% ceramic content that helps minimizing the shrinkage after the sintering step [156]. Similar to all other ceramic additive manufacturing technologies, biomedical implants have been the most common applications for ceramics fabricated with DW method. Porous ceramic structures can be fabricated, which show the structural similarity to native bone. In terms of material choice for ceramics printed via direct ink writing, calcium phosphate glasses, and HA are the most common ceramics used to fabricate artificial bone scaffolds due to their excellent biocompatibilities. Low resolution and high level of porosity of the fabricated part are the major issues of the direct ink writing processed ceramics. Infiltration of metals and molten glasses can be performed as the postprocessing operations to increase the density and improve structural properties of these materials. Figure 5.4 represents low and high magnification images of a lead zirconate titanate (PZT) ceramic component

68

5 Additive manufacturing of ceramics

fabricated via direct ink writing [157]. PZT is a piezoelectric ceramic, which has the ability to respond to electric field and change its shape.

(A)

(B) y

x z

5 mm

STMesh A2

200 μm

Figure 5.4: PZT structure fabricated via direct ink writing; (A) low magnification image of the PZT structure and (B) SEM image showing detailed microstructure. Images were reprinted from [157].

A recent study [158] used direct ink writing to fabricate bioinspired ceramic materials that could not be obtained via any other additive manufacturing technologies. In this study, hexagonal alumina platelets (~5 μm diameter and 0.5 μm thickness) and submicron alumina powder were mixed together and 3D printed in different geometries. High shear stresses during the DW process yielded the alignment of the alumina platelets perpendicular to the nozzle wall giving concentric circles of platelets as shown in Figure 5.5. As the shear forces are stronger close to the nozzle wall, platelets are well aligned at the wall vicinity and more random at the core where yield stress is minimum. In this study, the nozzle length was found to be the most important parameter affecting the platelet alignment. As shown in this figure, longer nozzles allowed more time for the alignment of the platelets and the core size having randomly oriented platelets was minimized. In addition to the control of alumina nanoplatelets, bioinspired, Bouligand mesostructures were fabricated in this study. Bouligand structure is a helical arrangement of successive layers as shown in Figure 5.6. And this structural arrangement is used in many natural materials such as plywood and chitin-protein fibers in insects due to their ability to enhance impact resistance and toughness [159]. By using this natural observation, alumina-based ceramics described earlier in the form of Bouligand structures were fabricated via direct ink writing from alumina-based ceramics by rotating the print direction 30 ° at each layer. Fabricating ceramic layers in the form of Bouligand configuration enhances the fracture toughness of additively manufactured ceramics compared to both transfilament and interfilament configurations (Figure 5.6).

69

5.3 Bulk solid-based technologies

(B) 100,000

(A)

Storage modulus (G′) Loss modulus (G″)

Modulus (Pa)

10,000

Yield point

1,000 Gel flowing 100 Gel at rest 10

100

1,000 Shear stress (Pa)

(C)

1 mm (D) length

12.5 mm length

(E)

25 mm length

Figure 5.5: (A) SEM image of a single alumina platelet, (B) dynamic mechanical analysis of the printing paste showing shear thinning (C)–(E) SEM images of filaments printed using different nozzle lengths. Partial images (A)–(E) were reprinted from [158], https://doi.org/10.1038/ s41598-017-14236-9.

This was mainly due to the enhanced resistance of the material to crack propagation as the cracks are forced to propagate helically instead of following a straight path in Bouligand structures as shown in this figure. Figure 5.6 also shows the additively manufactured ceramic gears to demonstrate the flexibility and the resolution of the DW technology to fabricate ceramic components. Direct ink writing is well suited for the fabrication of porous ceramic structures with periodic features where high surface quality is not a priority. Alignment of micro/nanoscale constituents in direct ink writing process is a unique capability of this technique, and this can be used to fabricate mesostructures with novel properties. However, dense engineering ceramics are difficult to process using direct ink writing, thus limiting the applications of these ceramics.

5.3 Bulk solid-based technologies Unlike the powder or liquid-based techniques, bulk solid-based ceramic additive manufacturing involves the feedstock materials to be in bulk solid form such as a

70

5 Additive manufacturing of ceramics

(A)

(B)

Transfilament (D)

(G)

(C)

Interfilament (E)

Bouligand (F)

(H)

Figure 5.6: Bioinspired, Bouligand structure: microstructures of bioinspired platelet-epoxy composites produced via robocasting of platelet pastes and comparison to natural analogues. Schematic diagrams of the three morphologies (A) trans-filament, (B) interfilament, (C) bioinspired, Bouligand structure, (D)–(F) SEM images of fracture surfaces of the three composites, (G) Printed parts to demonstrate the flexibility of the technique and (H) SEM images of the microstructure of the printed part. Scale bars 10 and 500 μm. Image was reprinted from [158], https://doi.org/ 10.1038/s41598-017-14236-9.

thin layer of ceramic sheet or a filament. Sheet lamination and fused filament fabrication are the manufacturing techniques using these solid feedstocks, respectively.

5.3.1 Sheet lamination Sheet lamination uses ceramic sheet preforms as the feedstock materials. Ceramic sheets are prepared by tape-casting method as illustrated in Figure 5.7. In tape casting, ceramic powder is blended with a solvent, binder material, and other additives to prepare a homogenized mix. This mixture is then deposited on a surface and thinned with blades. Due to the usage of these flattening blades, this method is also known as doctor

5.3 Bulk solid-based technologies

Binder

71

Additives Ceramic powder

Pump

Homogenizer

Mixer

Slurry

Solvent

Drying

Reservior

Doctor blade region

Peeling belt

Figure 5.7: Schematic of tape casting process. Figure was reprinted with permission from [160].

blading method. At the final drying step, the solvent is evaporated from the film material. Tape casting technique is widely used for the fabrication of thin sheets not only from ceramics but also from polymers, metals, and paper. A few micron-thin films can be fabricated with this technique. After completing the green ceramic sheet preparation, these sheets are rolled onto the working platform and the excess material is removed. The next layer is then deposited until the final 3D part is obtained as described in Chapter 1 in detail. After the final layer deposition, the part is sent for binder removal and sintering at elevated temperatures. Different types of ceramics have been reported to be fabricated with this technique including Al2O3, zirconia, SiC [161], piezoelectric materials [162], HA for bone implant preparation [163], and ceramic composites including Si/SiC [164] and ZrO2/ Al2O3 [165]. High speed, applicability to different ceramic materials, and low fabrication temperature (prior to sintering) are the advantages of sheet lamination. However, the requirement of sheet ceramic feedstock, high porosity, and poor surface adhesion between the layers that may cause delamination are the drawbacks of this method.

5.3.2 Fused filament fabrication Fused filament fabrication (FFF) technique is based on melting of a thermoplastic filament and solidifying upon deposition on a printbed as described in Chapter 1. Brittle ceramics cannot be shaped into flexible filaments as the FFF feedstock material; however, ceramic powders can be mixed with thermoplastic polymers to prepare ceramic-

72

5 Additive manufacturing of ceramics

polymer composite filaments. These highly ceramic loaded (up to 60%) filaments can then be used to fabricate 3D structures using a conventional FFF instrument. Therefore, the major benefit of fabricating ceramics with the FFF method is the ability of using commonly used, low cost FFF printers rather than more expensive printing systems. After the printing process is completed, the part is subjected to binder polymer removal and sintering of the remaining ceramic powders to achieve densification. Similar to other ceramic additive manufacturing technologies, ceramic FFF method has been applied for the fabrication of bioceramic scaffolds due to high porosity left behind the removed polymer material. In addition, alumina, silicon nitride [166], and piezoelectric [167] ceramics have also been successfully fabricated via FFF method. Well dispersion of ceramic particles within the polymer is essential for the part quality and porosity. In addition, size and shape of these particles dictate the quality of the printed part along with the standard FFF process parameters such as rod width, layer thickness, build orientation, raster angle, and porosity. Up to this section in this book, we have investigated the additive manufacturing technologies of ceramic materials under the three main classification of feedstock type: powder-based, slurry-based, and bulk solid-based ceramic additive manufacturing. Each of these technologies have certain drawbacks and benefits to be considered during when selecting the ideal technology for the certain application. Table 5.1 summarizes these commonly used additive manufacturing technologies listing the main characteristic properties, benefits, drawbacks, and the main applications for each fabrication technique in comparison. Table 5.1: Comparison of ceramic additive manufacturing technologies (redrawn from [168]). Feedstock type

AM method

Resolution Speed

Powder based

Binder jetting

µm–mm

Powder bed fusion

Feedstock cost

Applications

Medium Medium

Medium

Structural/ bio

µm–mm

Medium Low

Low

Structural/ bio

Stereolithography

µm

Slow

High

High

Structural/ bio

Digital light processing

µm

Medium High

High

Structural/ bio

Direct ink writing

µm–mm

Medium Low

Low

Structural/ bio

Solid based Sheet lamination

µm–mm

High

Medium

Structural

mm

Medium Low

Medium

Functional

Slurry based

Fused filament fabrication

Surface quality

Medium

5.4 Additive manufacturing of polymer-derived ceramics

73

5.4 Additive manufacturing of polymer-derived ceramics Polymer-derived ceramics, as their name indicates, are obtained by transforming preceramic polymers into ceramics. Precursor materials used for the process are organosilicon polymers such as poly(organocarbosilanes), poly(organosiloxanes), poly(organosilazanes), and poly(organosilylcarbodiimides [169]. Polymer-to-ceramic transformation takes place by heating silicon-based polymers (orgonosilicon) at about 1,000 °C under inert atmospheres and breaking of the C–H bonds. The release of H2, CH4, or other volatile compounds results in the formation of an inorganic ceramic material [170]. Figure 5.8 shows that different ceramic compounds can be obtained by altering the precursor polymer. As the figure indicates, SiC can be obtained from poly(organocarbosilanes). SixCyOz can be derived from poly(organosiloxanes) and SixCyNz ceramic compounds can be derived from either poly(organosilazanes) or poly(organosilylcarbodiimides).

R1

Silicon-based polymers:

Si R2

R1 R3

R1

Si S C

Si S O

R2 R4 n

R2

Poly(organocarbosilanes) ganocarbosilanes)

ΔT SiC

n

Poly(organosiloxanes)

ΔT SixCyOz

X n

R1

R1

Si S

Si N

N R2 R3 n

C

N n

R2

Poly(organosilazanes) Poly(organosilylcarbodiimides) Poly(organosilylcarbo

ΔT

ΔT SixCyNz

Figure 5.8: Molecular structures and the types of polymer precursors transforming into ceramic materials. Figure was reprinted with permission from [169].

Polymer-derived ceramics have unique properties such as high chemical durability, high creep resistance, semiconducting behavior and stability at ultrahigh temperatures (up to 2,000 °C) [169]. Therefore, these materials find applications as temperatureresistant materials or functional materials in micro/nanoelectronics fields. Although polymer-derived ceramics materials were discovered over 50 years ago, their popularity has increased dramatically after the recent developments of their additive manufacturing technologies. Since the precursor materials for these materials are polymers, these preceramic polymers can be additively manufactured into complex geometries and then pyrolyzed to form ceramic parts. Especially Vat polymerization has been successfully

74

5 Additive manufacturing of ceramics

applied to additively fabricate these materials since photoactive thiol, vinyl, acrylate, methacrylate, or epoxy groups can be easily attached to organosilazanes [171]. In addition, metallic precursors can be added into organosiloxane preceramic polymers to fabricate metal-based ceramics as introduced by [172]. Figure 5.9 shows the additive manufacturing methodology of ceramic structures in this study where the preparation of the photocurable preceramic polymer, DLP printing of this polymer and pyrolysis steps are described to fabricate octet truss structure shaped polymer derived ceramics.

(A)

(C)

(D)

(E)

Print platform Resin trough

Synthesis of preceramic polymers

N2

(B) LED source Modeling

DLP 3D printing

Pyrolysis of preceramic polymers

Final ceramic component

Figure 5.9: Additive manufacturing of polymer derived ceramics. (A) Precursors used in the printing mix, (B) octet truss structure was designed using CAD software, (C) DLP 3D printing process, (D) pyrolysis process performed on 3D-printed pre-ceramic polymer, (E) ceramic material derived after the pyrolysis process. Figure was reprinted with permission from [172].

Figure 5.10 shows the designed and fabricated polymer-derived ceramic part in this study before and after pyrolysis steps. As shown in this figure, complex lattice structures can be successfully fabricated via this technology. The main benefits of using preceramic polymers in additive manufacturing is taking advantage of using well established, high reposition technologies to fabricate ceramic components. Additively manufactured, polymer-derived, and metal-doped ceramics can also find novel applications in magnetics, catalysis, MEMS, and so on. Main limitation of additive manufacturing of polymer-derived ceramics is the high level of shrinkage (>20%) of the printed structure during the polymer to ceramic transformation as shown in Figure 5.10.

5.5 Mechanical properties of AM ceramics As described throughout this chapter, high porosity is a common characteristic property of additively manufactured ceramic parts. Depending on the manufacturing process, fabricated parts can have the porosity range in between 20% and 60%. This high level of porosity adversely affects the mechanical properties of ceramic parts and limit

5.5 Mechanical properties of AM ceramics

(A)

75

(B)

10 mm

(C)

(D)

10 mm Figure 5.10: (A) Octet lattice structure designed as CAD model, (B)–(C) 3D-printed polymer-derived ceramic component before and after pyrolysis. (D) Picture of ceramic component after pyrolyzed at 1,000 °C. Figure was reprinted with permission from [172].

their usage except the applications where high porosity is desired such as biomedical scaffolding. As a result of high porosity level, bending strength lower than 30 MPa is commonly observed in these highly porous ceramics regardless of the ceramic type or the fabrication method. Figure 5.11 shows a summary of the compressive strength as a function of porosity reported by Zocca et al. [173]. It is clear that the porosity in the ceramic parts significantly reduces the strength in these materials. Additively manufactured ceramics used for structural applications are almost always postprocessed and densification steps take place such as infiltration, isostatic compaction and heat treatment of the fabricated parts as described previously. As a result, high density (>90%) can be achieved in these ceramic parts and therefore, mechanical properties can be significantly increased. Table 5.2 summarizes mechanical properties of selected ceramic materials after densification process is applied as reported by Wang et al. [174]. It is clear from the table that strength of the ceramic parts can be dramatically enhanced after applying the densification process and the mechanical properties of these parts become similar to those fabricated via conventional ceramic fabrication technologies. Nevertheless, in general, mechanical properties of additively manufactured ceramics are still lower than those fabricated via conventional methods. Both fracture strength and fracture

76

5 Additive manufacturing of ceramics

Compressive strength (MPa)

100

Farzadi 2014 Calcium sulfate P-3DP Fielding 2012 TCP P-3DP Fielding 2012 TCP + ZnO P-3DP

10

Kolan 2014 bioglass P-SLS Chu 2002 HA SL

Genet 2013 HA/TCP DIW Fu 2011 bioglass DIW

1

Franco 2011 HA/TCP DIW Miranda 2007 HA DIW Houmard HA/TCP DIW Non-AM processing 0.1 20

30

40

50

60

70

80

Porosity (%) Figure 5.11: Compressive strength versus porosity of porous ceramics fabricated via additive manufacturing. Figure was reprinted with permission from [173].

Table 5.2: Comparison of the mechanical properties of additively and conventionally fabricated ceramic materials redrawn according to [174]. Fracture toughness (MPa*m/)

Ref.

–

.

[]

Additive



–

[]

Additive



–

[]

Additive

–

.–.

[]

Conventional





[]

Additive



–

[]

Additive



.

[]

–

.–.

[]

Additive

.

–

[]

Conventional





[]

Additive



–

[]

Conventional





[]

.–.

.

[]

Ceramic type

Manufacturing type

Aluminum oxide (AlO)

Conventional

Zirconium oxide (ZrO)

Silica (SiO)

Silicon carbide

Boron carbide

Conventional

Additive

Fracture strength (MPa)

5.5 Mechanical properties of AM ceramics

77

Table 5.2 (continued ) Ceramic type

Manufacturing type

Silicon nitride (SiN)

Conventional

Aluminum nitride (AlN)

Fracture strength (MPa)

Fracture toughness (MPa*m/)

Ref.

–

–

[]

Additive



.–.

[]

Conventional



.

[]

Additive



–

[]

toughness of additively manufactured ceramics are lowered due to the unavoidable defects and porosity even after performing postprocessing operations described earlier. Additive manufacturing of ceramic materials is an active area of research due to the superior properties of these materials over metals and polymers such as thermal and chemical resistivity, unique mechanical properties, low cost, and biocompatibility. However, additively manufactured ceramic parts find limited applications currently since these components possess high porosity stemming from the low powder packing density and the pore formation due to the removal of sacrificial organic materials used during the printing process. One straightforward approach to increase the density of the powder bed is the deposition of ceramic slurries instead of dry powders. Although slurry-based ceramic AM is advantageous over powder-based fabrication, subsequent densification steps are necessary to obtain nearly pore-free parts. Therefore, compared to polymer and metal additive manufacturing, ceramic additive manufacturing remains as an expensive, time-consuming, multistep process which limits the adoption of this technology by a wider AM community.

6 Bioprinting Bioprinting can be defined as a procedure where living or nonliving biomaterials are additively manufactured to imitate natural tissues or organs. In this regard, bioprinting is a very promising research field for tissue engineering and regenerative medicine, which aims at replacing a pathological or necrotic body tissue with an artificial one. Biomaterials printed in three-dimensional (3D) geometries can also be used to understand an underlying mechanism of a biological function (or a disease) and test the efficacy of a drug on 3D-printed biomaterials that mimic those in vivo. Therefore, bioprinting finds a wide range of applications today in basic research, biomedicine, and pharmaceutical industry. Bioprinting process starts with a computer-aided design (CAD) for the natural tissue/organ of interest. This process is followed by the preparation of the printing feedstock material, or bioink, which may consist of a natural or synthetic material for the desired application. Bioink is then deposited on the printing platform or on the existing biomaterial in a layer-by-layer fashion through a computer-controlled algorithm. To fully understand the benefits and limitations of the bioprinting process, it is essential to describe available bioink materials used for bioprinting, additive manufacturing procedures commonly used to deposit bioink materials, and the functions of these biomaterials after printing for in vivo or in vitro applications. Therefore, this chapter is structured accordingly to inform the readers the common bioprinting methodologies, available bioinks used as feedstock material today, and the major successful applications of this technology in biomedicine. This chapter concludes with the limitations and challenges of the current bioprinting technology. Reviewing the recent studies in this emerging field will guide biomedical researchers to comprehend the limits of 3D printing technology and foster new collaboration opportunities for manufacturing engineers and scientists to broaden the extent of their research.

6.1 Bioprinting methods Bioprinting technologies can be classified into four major categories: inkjet printing, extrusion, Vat polymerization, and laser-assisted printing. Schematic representation of these techniques and the comparison of their major properties such as printing speed, cell viability, and investment costs is shown in Figure 6.1. Brief description of each method is given later. Extrusion is a common bioprinting technique in which biomaterials are extruded through a nozzle of the extrusion system by a pneumatic or piston-controlled pressure system (Figure 6.1A). In the deposition process, the bioink is deposited onto a platform bed layer by layer forming a 3D structure. To obtain the structural integrity of https://doi.org/10.1515/9781501518782-006

6.1 Bioprinting methods

79

Figure 6.1: Commonly used bioprinting technologies: (A) extrusion bioprinting, (B) inkjet bioprinting, (C) laser-assisted bioprinting, and (D) Vat polymerization bioprinting.

the extruded material, the bioink viscosity must be high enough to overcome the gravitational forces and avoid sagging. Viscosity can be enhanced by adding a curing agent in the bioink and initiation of the chemical cross-linking of the biopolymer within the bioink by applying an external stimulation (thermal, light, etc.). Cross-linking process can also take place at room temperature naturally without any intervention depending on the curing reaction type. Extrusion process is a simple bioprinting technology that is capable of printing highly viscous bioinks with high cell densities. Multimaterial printing is also possible using multiple extrusion channels integrated on a single printing setup. However, shear stress induced during the extrusion process may cause cell deformation and damage. Therefore, cell viability may be lowered if the process parameters such as biomaterial concentration, nozzle pressure, and nozzle diameter are not successfully optimized. In addition, the material choice is currently limited since the bioink material must be able to encapsulate cells through hydrogels. Inkjet bioprinting is another bioprinting methodology which can be considered under material jetting additive manufacturing classification described in Chapter 1. Therefore, the bioink is sprayed (or jetted) onto a surface in a drop-on-demand fashion. In this type of bioprinting, bioink deposition can be regulated by thermal or piezoelectric actuation as shown in Figure 6.1B. In thermal actuation, heating element near the nozzle increases the bioink temperature forming a bubble. This bubble forces the bioink to move out of the nozzle orifice. In piezoelectric actuation, however, inkjet printer utilizes piezoelectric element to generate pressure pulse, which leads to dispensing of liquid through the nozzle orifice. Similar to the extrusion

80

6 Bioprinting

bioprinters, inkjet bioprinters are relatively cheap and simple manufacturing systems. However, the speed of inkjet printing is much faster since multiple printheads can work in parallel (rather than in series in extrusion bioprinting), which allows deposition of multiple cell types at high speed. Due to the precise control of the dropwise bioink deposition, inkjet printing has high resolution (~30 µm) [184]. In addition, inkjet printing results in relatively high cell viability for especially piezoelectric actuation systems. High temperature applied on bioinks in the process of thermal actuation may cause damage and influence the activity of cells after printing. The main limitation of the inkjet printing is the low-viscosity requirement of bioink (~0.1 Pa.s) [185]. This requirement makes the printing of viscous hydrogels and extracellular matrix (ECM) rather difficult. Cell aggregation and nozzle clogging are also commonly observed issues when high cell densities are used. Therefore, the printing is usually operated with low-cell density bioinks to assure printability. Laser-assisted bioprinting is a high-resolution printing technique in which cells are deposited from the source platform to the substrate platform (Figure 6.1C). In this technique, the laser pulse is applied on the bioink-containing cells and this laser pulse creates a bubble. This bubble forces cells to move from the source to the substrate platform where the cells are collected. This technique can allow deposition of individual cells in a precise way; therefore, it has superior resolution compared to other bioprinting techniques. In addition, it is capable of printing highly viscous inks without the issue of clogging since it is a nozzle-free deposition technology. Cell viability is also very high in laser-assisted printing (>95%). The main limitation of this technology, however, is the low volume of the deposited ink, and therefore, manufacturing process is relatively slower than the other bioprinting methods. As a result, laser-assisted printing is preferred for printing smaller samples. High instrument cost compared to other bioprinting technologies is also another drawback of this technique. Vat polymerization bioprinting is based on the photopolymerization of a bioink consisting of light-sensitive polymer. In this technique, photopolymer is selectively cured layer by layer to create 3D biomaterial as shown in Figure 6.1D. Light can be projected in terms of two-dimensional (2D) pattern (DLP) instead of raster scanning (SLA) of the laser line by line as described in Chapter 1 in detail. This feature results in a significant increase in printing speed compared to other bioprinting technologies. As described earlier, Vat polymerization results high resolution and hence, biomaterials with high dimensional accuracy can be fabricated with this methodology. In conventional Vat polymerization technologies, UV light source is preferred due to its high energy. UV light, however, can damage DNA of cells in bioink, and therefore, visible light is gaining popularity in stereolithography bioprinting process. Vat polymerization-based bioprinters provide high cell viability (>85%) since the selective cross-linking of bioink by light does not involve shear stress to cells or high thermal loading. Bioink, however, must be transparent to allow light to pass the material without significant scattering. To minimize light

6.2 Bioink types used in bioprinting

81

scattering and homogenous cross-linking, cell density is usually kept at low level, which is also another limiting factor for this biofabrication technique.

6.2 Bioink types used in bioprinting Bioink preparation is the most important step for bioprinting. Properties of the bioink such as the ink type, the concentration of the constituents, gelation time, and viscosity can greatly affect the printability and the properties of the fabricated samples. In general, bioinks should be printable into stable 3D constructs with high structural integrity. Bioinks must also have high level of biocompatibility supporting cell adhesion, proliferation, and spreading [186]. Therefore, the choice of the suitable bioink is essential for the success of bioprinting. High level of biodegradability is another property sought for the prepared bioinks. Hydrogels are biopolymers that satisfy these bioink requirements. Hydrogels can be naturally derived such as collagen, gelatin, agarose, and chitosan or synthetic such as polyethylene glycol (PEG) and polyurethane (PU). These biopolymers are cross-linked by controlling the external stimuli, including temperature, pH, light, enzymes, and ions. Cross-linked hydrogels are designed to be highly biocompatible and allow the cellular activities described earlier. Hydrogels can be naturally derived such as alginate, gelatin, and chitosan. They can also be synthetic such as PEG and PU based. Commonly utilized natural and synthetic bioinks are described briefly later. Alginate is one of the most commonly used bioink material due to its low cost, and fast gelation process using ionic calcium compounds (CaCl2, CaCo3, and CaSO4). High viscosity of alginate is another benefit of this bioink material. Since alginate itself does not allow cell adhesion, it is usually mixed with natural polymers such as collagen and fibrinogen to facilitate adhesion process. Gelatin is a natural polymer obtained from the hydrolysis of collagen; hence, it has excellent biocompatibility and biodegradability properties. Gelatin is easily cross-linkable, soluble in water, and nonimmunogenic [187]. Despite these advantages, gelatin hydrogel exhibits a relatively rapid degradation and poor mechanical strength, restricting its use as bioink. GelMA hydrogel, a gelatin-based hydrogel functionalized with methacrylic anhydride, has been proven to control degradation rate and improve the mechanical strength of gelatin [188]. However, GelMA requires photoinitiator for the cross-linking process. High concentration of the photoinitiator and the UV emission to activate the photopolymerization can be toxic to cells [189] and therefore their effects must be limited to maximize the cell viability. Chitosan is a natural polysaccharide biomaterial obtained from the outer skeleton of shellfish or fungal fermentation [190]. It is highly biocompatible, and has antibacterial properties. Chitosan is widely used in bioprinting of bone, skin, and cartilage tissues. This bioink type, however, suffers from slow gelation time and

82

6 Bioprinting

poor mechanical properties. It can be mixed with gelatin to improve printability and 3D construct shape fidelity [191]. Chitosan hydrogels are widely used in 3D bioprinting, especially for the applications of bone and skin repair/replacement and microflow channel fabrication. Naturally derived hydrogels have been used extensively as bioinks since they can support cell functions and have biodegradable properties. However, due to their low viscosity, these hydrogels have generally poor mechanical properties to support tissues; therefore, synthetic polymers such as PEG are in use for applications requiring high mechanical performance. PEG is a synthetic polymer and it has tailorable mechanical properties required for bioprinting. It is nontoxic and bioinert. Therefore, cell adhesion is poor in PEG-based hydrogels. Similar to the alginate, it is commonly mixed with other active biopolymers to improve cell adhesion properties. Pluronic is another synthetic polymer widely used in bioprinting. Pluronic has similar properties to PEG. The ECM is a 3D network of extracellular proteins (collagen, fibrinogen, glycoproteins, enzymes, etc.), and it involves numerous biological functions including cellular growth, tissue repair, and remodeling. In bioinks, one or a mix of these proteins are used to support the cellular functions during and after the printing process. These bioinks, however, can hardly mimic the native ECM properties due to the complexity of the native ECM structure. Decellularized ECM (dECM) is a promising candidate as a bioink precursor since it has the appropriate structure and inductive cues to enable cellular growth and differentiation processes [192]. Decellularization agents (ionic, nonionic zwitterionic detergents, and enzymatic and physical agents) are capable to remove cells from the native ECM to prepare dECM compound. dECM can then be mixed with the preferred cells and used as the bioink material mimicking the natural environments for these cells. As described earlier, bioinks have different mechanical, biocompatibility, and biodegradability properties and they can be either natural or synthetic biopolymers. Native dECM can also be used as a precursor for bioinks to better resemble the microenvironment in vivo. Preparation of cost-efficient bioinks with suitable mechanical and biological properties is an active area of research and it has the utmost importance for the future of bioprinting process and tissue engineering. Table 6.1 summarizes major bioink types that are currently in use and the major properties for each bioink type.

6.3 Bioprinting applications Bioprinting has been used in various applications including the biofabrication of functional artificial tissues and organs for repair or replacement. It has also been used to additively fabricate cancer tissues with the aim of understanding the underlying biophysical mechanisms of the cancer-related diseases. Major bioprinting

6.3 Bioprinting applications

83

Table 6.1: Bioink types and properties (redrawn according to [193]). Bioink type

Cross-linking mechanism

Bioactiveness Bioprinting technique

Alginate

Ionic cross-linking with divalent ions; covalent cross-linking with PEG photocross-linking with methacrylate

Bioinert

Extrusion, laser-assisted and inkjet

Collagen

Physical pH- and temperature-mediated cross-linking, chemical cross-linking with genipin/transglutaminase/lysyl oxidase

Bioactive

Laser-assisted and microwave-based

Gelatin

Physical cross-linking via temperaturedependent/opposite charged polymers; chemical cross-linking with horseradish and hydrogen peroxide after modification with phenolic hydroxyl group

Bioactive

Extrusion and laserassisted

Hyaluronic Chemical cross-linking via coupling of acid tyramine/NHS/Cu(I)-catalyzed cycloaddition reaction/host–guest interactions/HA-based copolymer hydrogels/oxidation of thiol-modified HA/thiol-modified HA using PEG derivatives/furan-modified HA derivatives with dimaleimide PEG/gold nanoparticles/photo-cross-linking of tyramine-substituted HA

Bioinert

Extrusion, laser-assisted, microwave-based, and Vat polymerization

PEG

Functionalized through hydroxyl end groups to form PEGDA, PEGDMA, n-PEG photo-cross-linkable hydrogels with different photoinitiators: Irgacure  ( nm), LAP (visible light), VA ( nm), eosin Y (visible light), riboflavin with triethanolamine

Bioinert

Vat polymerization, extrusion, and inkjet.

GelMA

Chemical photo-cross-linking with different photoinitiators: Irgacure  ( nm), LAP (– nm), VA- ( nm), eosin Y (– nm), ruthenium and sodium persulfate (– nm)

Bioactive

Vat polymerization, extrusion, and inkjet.

Pluronic

Temperature-dependent physical cross-linking, modification of terminal hydroxyl moieties to form photo-cross-linkable hydrogels

Bioinert

Extrusion

84

6 Bioprinting

applications regarding the functional tissues involve vascular structures and blood vessels, neuronal tissues, cornea, cardiac tissues, skin, cartilage, and kidney as shown in Figure 6.2. Brief explanation of the bioprinting procedures for each functional tissue is described later.

Figure 6.2: Current bioprinting applications for tissue engineering in human body. Figure was edited and reprinted with permission from [193].

6.3.1 Bioprinting of blood vessels Blood vessels are essential components of the circulatory system allowing transportation of oxygen and nutrients to organs and tissues throughout the human body via blood stream. Design and fabrication of artificial blood vessels are long-term studies of tissue engineering, and it has only been successful for large artery blood vessels due to the difficulty of fabrication of small capillaries and the mismatch in elastic properties [194]. Recently developed bioprinting technologies such as sacrificial, embedded, hollow tube, and microtissue bioprinting have great potential to fabricate microvascular networks with interconnected microchannels in arbitrary geometries.

6.3 Bioprinting applications

85

Sacrificial bioprinting involves the deposition of sacrificial hydrogel material in the core of the bioink surrounded by the endothelial cells. After printing, sacrificial core layer is selectively removed and the remaining cells in the shell establish the endothelium creating the vascular pattern. This technique is the most established bioprinting technique for blood vessel printing. However, it is limited to simple geometries and cannot be used to generate sophisticated vasculature due to the lack of support in 3D volume. In embedded bioprinting technique, printing is performed with a relatively long syringe tip dipped into a viscous hydrogel material rather than on a flat printbed. Since the bioink is supported by this surrounding viscous hydrogel material as it is extruded from the syringe, this technique can produce freeform structures in 3D and complex blood vessels can be fabricated. Hollow tube bioprinting is based on the deposition of the hollow blood vessel in as single step using wet-spinning technology and, therefore, simplifies the bioprinting process. Blood vessel printing is important not only to replace the necrotic vascular tissue, and is also essential to make organ bioprinting possible with enclosed vascularate with food transport for these organs. Therefore, development of new bioinks with high biocompatibility and bioprinting of blood vessels mimicking the native tissue will make a significant impact in tissue engineering.

6.3.2 Skin bioprinting Skin is the largest organ in the human body, playing important functions, including protection, temperature regulation, and sensation. Skin bioprinting has the utmost importance for wound healing, replacement of the diseased skin tissue, and testing of medicinal and cosmetic products in vitro. Skin has complex, nonuniform 3D architecture including proteins (collagen, elastin), cells (keratinocyte, melanocyte), and other structures (hair follicles, sweat glands, fatty tissues, etc.). Inkjet bioprinting is commonly used for skin printing using the mixture of collagen and the skin cells. Artificial skin has been successfully fabricated by numerous researchers [195–198] with matching morphology to that of native skin to facilitate the protection and barrier functions. However, native skin serves several other key functions such as temperature regulation (hair follicles and sweat glands), touch sensation (sensory nerves), and skin hydration (sebaceous glands) [193]. Biofabrication of multifunctional, smart skin tissues serving these functions is under current investigation.

6.3.3 Cartilage printing Cartilage is an elastic tissue located at the joints of the skeletal system protecting bones by providing shock absorption and cushioning. Cartilage is also the structural component of ear and nose. Compared to the other connective tissues, cartilage has

86

6 Bioprinting

limited and slow self-repairing capability [199]. Therefore, the use of bioprinting technology has a great potential for cartilage repair and replacement. Chondrocyte cells secrete the ECM to maintain and sustain native cartilage. Therefore, it is the main cell type used in bioinks for the cartilage bioprinting process. However, limited availability of this cell type resulted in the search for alternative cells including mesenchymal stem cells (MSCs) that can be isolated from bone marrow. MSCs can differentiate into cells of the specialized connective tissues, including bone, adipose tissue, and cartilage [200]. Unlike other cell types, these stem cells also reported to have less immune rejection and therefore they have high potential in bioprinting of connective tissues including cartilage. One of the notable studies on bioprinting of cartilage tissue using MSCs is shown in Figure 6.3 [201]. In this study, human MSCs (hMSCs) were added into pluronic-alginate bioink mixture and printed in the shape of an ear and nose onto a heated stage, resulting in instantaneous solidification via the sol–gel transition of the pluronic, and the structures were then stabilized through alginate crosslinking using CaCl2 immersion.

A

D

B

C

E

F

Figure 6.3: Three-dimensional printing of cartilage tissue using alginate hybrid gel. (A) and (B) Post-cross-linking images of a full-sized ear and nose (scale bars are 1 cm). Wide-field fluorescence microscopy of (C) cross-hatch pattern, (D) a single fiber printed through a pipette tip, and (E) and (F) a single fiber printed through a 30-gauge needle (scale bars are 200 μm). Figure was reprinted from [201].

6.3 Bioprinting applications

87

6.3.4 Cardiac tissue bioprinting Due to the high rate of mortality, heart-related diseases remain to be a leading cause of death worldwide. Treatment of heart diseases is rather difficult since the muscular heart tissue (myocardium) has very limited regeneration capability in mammalians including human [202]. Heart transplantation is the only effective treatment against these heart diseases. However, organ availability, immune rejection, high cost, and surgical complications are the issues of this treatment method. Therefore, bioprinting of cardiac tissue is essential to solve existing limitations of the heart transplantation process and therefore it has compelling benefits to the patients suffering from heart diseases. In addition, bioprinting of in vitro heart models mimicking the structural and functional characteristics of native heart will make tremendous impact on drug discovery and development in this field. Unlike the other tissues considered for bioprinting, the bioprinting of a functional myocardium requires alignment of cells and the contraction capability of these cells; therefore, bioprinting of heart tissues remains challenging. Significant progress has been made in lab-scale bioprinting of cardiac cells (cardiomyocytes) loaded within bioinks. In a recent study by Noor [203], stem cells taken from a patient are differentiated into cardiomyocytes. In this study, cardiomyocytes are mixed with the hydrogel prepared from the patient’s own dECM. Endothelial cells, cells forming blood vessels, are also added to this mixture to create artificial heart with vascular channels similar to the native heart. The prepared bioink is printed into heart shape with the surrounding sacrificial matrix, which provides structural support for the printed heart. Postprocessing of the printed heart in a bioreactor results in the cardiomyocytes to self-organize and form solid cardiac tissue. The surrounding support material is then dissolved, leaving the fully printed 3D heart model infused with major blood vessels with blood-carrying capability as shown in Figure 6.4. The structure and functionality of the bioprinted cardiac patches are studied in vitro analyses, and cardiac cell morphology is assessed after transplantation,

(A)

(B)

(C)

(D)

Figure 6.4: Bioprinting of vascularized heart in small scale. (A) The human heart CAD model. (B) and (C) A printed heart within a support bath. (D) Blue and red dye injection into ventricles after extraction to demonstrate hollow chambers and the septum in between them. Partial figure was reprinted from [203].

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6 Bioprinting

revealing elongated cardiomyocytes with massive actinin striation. Figure 6.4 shows the CAD design (Figure 6.4A) and additive manufacturing process (6.4B-C) of an artificial heart integrated with the vascular structure (lumen). Embedded bioprinting was used in this work in which the viscous hydrogel acts as a support material. Figure 6.4D shows the fabricated heart with different colors representing the blood flow within different heart sections. This proof-of-concept study as well as the other recent research studies performed on bioprinting of cardiac tissues [204– 207] have demonstrated the great potential of the additive manufacturing technologies for personalized tissues and organs, and for the drug screening in an appropriate anatomical structure and patient-specific biochemical microenvironment.

6.3.5 Kidney tissue bioprinting Kidneys perform important functions in human body including blood filtration and regulating pH and fluid balance. Similar to the heart bioprinting process, fabrication of artificial kidney is an area of active research due to the high demand of kidney transplant and shortage of these organs. Kidney has complex microarchitecture, which also makes bioprinting the suitable technique for artificial fabrication of this organ. Threedimensional printing has been used in clinical kidney transplant surgery as an educative tool to help surgeons to determine the feasibility of transplantation and foresee the possible complications. This technology has been applied especially during the transplantation of adult kidneys to kid recipients (% High >%

Stereolithography Inkjet Extrusion Extrusion

[] [] [] []

>% % N/A

Extrusion Extrusion Extrusion

[] [] []

High >% >% >% N/A

Extrusion Extrusion Extrusion Extrusion Stereolithography

[] [] [] [] []

Breast Breast Breast Breast Brain Brain Brain Brain Brain Cervical Cervical Cervical

Reference

In these studies, 3D bioprinting techniques have opened a great range of opportunities, especially for in vitro monitoring of cancer progression with increased control on the ECM. Bioprinting has significantly improved biomimicry of cancer modeling compared to conventional tissue engineering, thanks to the enhanced accuracy and composition of tumor environment coupled with the availability of improved vascularization [230, 231]. The major advantage of additive manufacturing is that it allows more realistic, accurate, and facile modeling compared to wellestablished 2D techniques. Recent applications of bioprinting for this purpose have concluded appreciable results to further illuminate the cancer biology and explore new fields such as progression of cancer, cell interactions, drug efficiency, and

6.5 Bioprinting in cancer research

91

treatment methods. Bioprinting has been applied to only a limited variety of cancers: breast, brain, cervical, and ovarian as shown in Table 6.2. Implementing bioprinting on different types of tumors and understanding the biophysical cues on cancer initiation and progression mechanisms can significantly improve the drug efficacy and interaction systems in future cancer studies.

7 Topology optimization Optimizing the geometry of a structure is essential to reduce weight, material, and the cost of the manufacturing process; therefore, it provides significant economic and environmental benefits. Fabricating components with reduced weights without adversely affecting their functionality is especially important for aerospace applications. Recent advancements in additive manufacturing have made the optimization concept more important than ever since this manufacturing process does not have the design constraints of the conventional manufacturing methods and it is not bound to the geometries optimized in a limited design space. Geometrical optimization process involves the search of optimum design parameters to meet the design objectives such as mass, volume, and stress reduction while satisfying the design constraints. There are three major concepts where geometry of the design can be optimized. These are size, shape, and topology optimization techniques as shown in Figure 7.1. In a size optimization process, length, width, or thickness of the components are the design parameters and these geometrical parameters are optimized to achieve the optimized configuration using an iterative procedure. Similarly, shape optimization focuses on the modification of the component shape by which the ideal component function can be obtained such as homogenously distributed stress with minimum stress concentration. Topology optimization, however, is the process of determining the connectivity, shape, and location of internal and external structure of a solid domain by determining the best material distribution [233]. Therefore, topology optimization allows the structures to attain any shape within the design space with greater design freedom when compared to size and shape optimization methods that deal with only geometrical variables such as length, thicknesses, and shape of predefined components. Topology optimization compliments the manufacturability freedom of additive manufacturing process with this design freedom capability; hence, topology optimization finds a wide range of applications in early conceptual and preliminary design phases where changes made on the component topology have a significant impact on final part performance. It is also a common practice that all of these optimization methods are used together in a multistep optimization process. In the first phase, topology optimization is used due to its robustness and high flexibility as described earlier as an optimal configuration of the structure. It is then further optimized using the size and shape optimization process in detail to achieve the final optimal design under realistic loading configurations. The success of this design process, therefore, highly relies on the accuracy of the initially performed topology optimization process. In this chapter, the details and the importance of the topology optimization process are described, major methods used in topology optimization are shown, and the topology optimization integrated design process is explained using a finite element analysis software. https://doi.org/10.1515/9781501518782-007

7.1 Topology optimization for additive manufacturing

Size optimization

Shape optimization

93

Topology optimization

Figure 7.1: Structural optimization techniques. Figure was reprinted from [232].

7.1 Topology optimization for additive manufacturing Mathematical concept of topology optimization concept was initially applied for simple truss structures in 1904 and then applied to more complicated solid systems such as beams [234, 235], porous, and composite structures [236]. The mathematical concept was then integrated into numerical algorithms that lead to the development of commercial topology optimization simulation packages in 1990s which were suited for subtractive manufacturing. Although the mathematical principles of topology optimization were established more than a century ago and the numerical simulation softwares have become available in the past 30 years, the true potential and benefit of topology optimization have been revealed with the advent of additive manufacturing technologies, recently. AM technologies make the uncompromised fabrication of topologically optimized components possible with their unmatched design freedom. The general workflow of the integration of topology optimization is given in Figure 7.2. The procedures start with setting up a CAD model of the original design and importing the model into a topology optimization platform (software or an algorithm). A suitable topology optimization methodology is then selected to optimize the geometry of the design satisfying the constraints in the optimization study. The optimized domain can be smoothed out for the manufacturability and aesthetic purposes. The smoothed model is then reanalyzed to check if it satisfies the design requirements. If it fails this satisfaction, topology optimization algorithm is modified, and the optimization process is repeated until the desired design is achieved. Successful optimization process results in the optimized models that surpass the performance of the original model. At the final stage, topology-optimized model can be additively manufactured at high level of complexity and high performance. Since the conventional solid component fabrication is no longer required in additive manufacturing, components with highly porous topology and complex shape

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CAD model import

Postprocessing and smoothing

Finite element analysis

Selecting topology optimization method

Optimized geometry satisfying the constraints

Finite element analysis Homogenization SIMP Level set ESO/BESO

No Satisfactory? Yes

Figure 7.2: Workflow of topology optimization process for additive manufacturing.

can be achieved. In other words, topologically optimized structures with high strength and good energy absorption properties can be obtained at significantly low density. Therefore, mass reduction-based topology optimization finds wide range of applications especially in automotive and aerospace in which weight reduction is a highly desired phenomenon. An example problem aiming to reduce component mass using topology optimization is described in Section 7.3. In this case study problem, significant mass reduction is achieved without compromising the functionality and the strength of the designed part. Another important area that topology optimization has found applicability is in additive manufacturing of the design of support structures. Support structures are sacrificial elements, which are used during the additive manufacturing to support lowangled and overhanging sections of the fabricated parts with respect to the build plane. Although some additive manufacturing technologies such as powder bed fusion and binder jetting do not require support structures, supports are needed in most other additive techniques. Printing support structures can slow down the process and requires additional postprocessing steps for their removal. It is estimated that 40–70% of an AM product cost could be expended for removal of support structures as reported by Liu and coworkers [237]. In addition, the support material may not be removed completely in designs where these supports are inaccessible such as self-contained cavities; therefore, extra weight will be added to the final AM part. Therefore, it is important

7.1 Topology optimization for additive manufacturing

95

to design supports with minimum mass or optimize the part geometry in a way to completely eliminate the need for supports. Figure 7.3 shows some examples regarding the support design to minimize support weight. Figure 7.3A (reported by Huang et al. [238]) shows the comparison of a straight wall support and a slim support in which the support structure is slimmed down to save material and enhance the printing speed. Figure 7.3B shows an example of a tree-like support pattern, where significant support material can be saved compared to solid, straight wall support design. (A)

(B)

Figure 7.3: Topology-optimized support structures proposed for mass reduction. (A) Comparison of the slimmed and straight wall support structures. Partial image was reprinted from [238] with permission. (B) Tree-like support structure. Partial image was reprinted from [239] with permission.

Considering the mass reduction and maximizing the fabrication speed, overhang-free topology optimization for completely removing the need for support structure is a more appealing concept. However, in overhang-free topology optimization, the structural mechanical performance and even functionality of the fabricated part may be compromised [237]. In addition, support structures may be beneficial to assist heat transfer toward the base plate, thereby creating a uniform temperature distribution and minimizing the occurrence of thermal-related stresses and warping. Therefore, it would be more practical to strategically place sacrificial support material in regions where the placement would generate significant improvement in part performance rather than redesigning the parts to removed them completely. Selection of minimizing the support structure, complete elimination, or combining the both methods depend strongly on the part design and its functionality. Regardless of the selected methods, topology optimization plays an important role in the design process on additively manufactured components where support structures are considered. Although topology optimization in additive manufacturing has been mainly applied on strength-based weight minimization and the support design of the fabricated parts, other design objectives have been implemented in topology optimization as well. Topology optimization methods have been developed for optimizing additively manufactured thermal systems such as heat exchangers and heat sinks based on

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conductive, convective, and conjugate heat transfer concepts. In these applications, topology optimization is used to design geometries by modifying geometry-dependent heat transfer coefficients (conductive and convective) in the system to optimize the heat transfer in heat sinks and heat exchangers. Additively manufactured thermal systems designed via topology optimization have shown lightweight and increased thermal performance compared to the conventional heat transfer systems [240]. Lange et al. used topology optimization to design a heat sink component by maximizing the thermal conductivity throughout the part and minimizing its weight. It was shown that the topology-optimized heat sink was highly complex in terms of geometry, making it impossible to fabricate via traditional fabrication technologies. It was, however, possible to successfully fabricate the optimized design via powder bed fusion additive manufacturing evidencing the mutual importance of topology optimization and additive manufacturing. It was also experimentally validated in this study that the heat transfer coefficient with topology-optimized design was improved compared to the unoptimized heat sink component. In summary, topology optimization has been applied in numerous applications in additive manufacturing, including lightweight component design, minimizing or completely eliminating support structures, and maximizing thermal conductivity to achieve effective heat transfer process minimizing the thermal stresses and warping. More applications will emerge as the additive manufacturing technologies will continue to develop and more robust topology optimization concepts and numerical simulations will be available in the future. The major topology optimization methods currently in use are described in the next section.

7.2 Topology optimization methods Topology optimization is a mathematical method in which the material topology is modified to maximize the performance of the system. There are various topology optimization methods such as homogenization, SIMP (solid isotropic microstructure with penalty), ESO/BESO (evolutionary/bidirectional evolutionary structural optimization), and level set method. The main difference among methodologies lies in the parameterization of the design space. Some of the topology optimization methods explicitly define the design directly on the finite element domain while the others define a design implicitly using a separate function from which the structure is interpreted. Regardless of the methodologies selected, there is a generalized mathematical concept behind these optimization methods which can be described as follows: 8 > < Objective function min or max f ðxÞ (7:1) Constraints g ð xÞ ≤ 0 > : Variable range xmin ≤ x ≤ xmax

7.2 Topology optimization methods

97

In this definition, design variable (x) is the vector of independent variables that describes the design. It may represent parameters such as geometry, type of material, and distribution of material. Objective function f(x) is a function or many functions that return values, which indicate the suitability of the design. Objective function f(x) may represent weight, displacements in a given direction, stress, or manufacturing cost. In structural analysis problems (stress-based optimization), maximizing the overall stiffness of a structure or minimizing its compliance under a specific amount of mass removal is the most commonly utilized objective function. Constraints in the design problem are defined by the g(x) ≤ 0 function, which could be limits on mass, volume, stresses, displacements, eigenfrequencies, heat flux, and so on, and the design space is determined by the range of the independent variable range (xmin, xmax). The solution is sought using different governing equations that can be solved using a generic form as follows: K ðxÞu = F ðxÞ

(7:2)

where K(x) is the stiffness matrix and F(x) is the force vector. Type of the problem determines the governing equations. If the stress-based problem is solved in the design problem, stress–strain constitutive relationships are utilized. In a thermal conductivity design problem, heat equation is used as the governing equation. There are different topology optimization methods using these common optimization concepts. The main goal in these methods is to determine the existence or absence of material within a given region of a design domain. Solid-void concept is commonly used in topology optimization methods where solid represents the existence of a material, whereas void implies the absence of material. The main difference between the different topology optimization methods is the type of parameterization of the design space. Traditional methods such as homogenization and SIMP (solid isotropic material with penalization) explicitly define the design directly on the finite element domain while in the other methods (i.e., level set method) structure domain and boundaries are represented based on the implicit functions rather than an explicit parameterization of the design domain. The most commonly used SIMP and level set methods are briefly described later. Currently, the most widely used methods for structural topology optimization are explicit parameterizations that are broadly classified as density-based methods. Among all the density-based methods, SIMP is by far the most popular technique due to its conceptual simplicity and ease in implementation. As a result, nearly all commercial topology optimization tools utilize a density-based method [241]. The SIMP method predicts an optimal material distribution within a given design space, for given load cases, boundary conditions, manufacturing constraints, and performance requirements. In SIMP method, the design variables are defined for each element and normalized value for pseudodensity parameter ranging from 0 to 1.

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Density variables are penalized with a basic power law as shown in the following equation:   (7:3) f ρe = ρpe f0 where the objective function, f, is selected as the physical quantities such as material stiffness, cost, or conductivity depending on the problem type. ρe is the density of the finite elements used in the optimization and p is a finite penalty parameter commonly used in the range of 1–3. For each element, the assigned relative density can vary between a minimum value ρmin and 1, which allows the assignment of intermediate densities for elements (characterized as porous elements). The penalty factor p diminishes the contribution of elements with intermediate densities (gray elements). The penalty factor steers the optimization solution to elements that are either solid black ( ρe = 1) or void white ( ρe = ρmin). Numerical experiments indicate that a penalty factor value of p = 3 is suitable. Level set topology optimization method is a recently developed, shape-based topology optimization technique where implicit functions are used to define structural boundaries rather than an explicit parameterization of the design domain. As shown in Figure 7.4A–C, level set function is defined to represent boundaries when the level set function is zero and it matches the initial shape contour. In this 2D domain, optimization algorithm is applied to satisfy the objective function under the constraints. Following the optimization process, the shape will evolve (Figure 7.4B–D).

(A)

(B) Ω

𝜕Ω

Φ (x) > 0

D

Φ (x) = 0 Φ (x) < 0

(C)

(D)

Figure 7.4: Level set topology optimization method. (A) and (C) Two-dimensional topologies and (B) and (D) corresponding level set functions. Image was reprinted from [244] with permission.

7.3 Solution of topology optimization problem using ANSYS finite element software

99

Level set optimization produces smoother boundaries compared to SIMP method. In addition, the level set method does not use intermediate density material (gray zone) in the design domain resulting in clear, unambiguous geometries. However, level set method is a time-dependent (as the level set function moves along the domain with a controlled velocity) initial value problem and therefore its accuracy is strongly dependent upon the initial design, which is a major drawback of this methodology. However, new tools are under development to reduce this dependency [242]. Level set methods also require reinitialization during the process when the level set function is not satisfactory (too flat or too steep), which adds additional computational complexity; therefore, algorithms not requiring this process is under development to reduce the computational cost [243].

7.3 Solution of topology optimization problem using ANSYS finite element software As a demonstration example for topology optimization integration in additive manufacturing, the design process of a bracket under load is considered as shown in Figure 7.5A. The bracket is pulled with 5,000 N from the pinholes located at the upper right ears and the other pinholes are used as the cylindrical supports fixed in all directions (radial, tangential, and axial). The goal of this bracket design example is to reduce the mass of the bracket as much as possible without adversely affecting the structural stability of the part. ANSYS finite element analysis software (R19) was used to solve for the topology-optimized design problem where the bracket model was imported as shown in Figure 7.5B. Static structural finite element analysis under the loading conditions described earlier results in maximum stress of 100 MPa in the bracket (Figure 7.5C). It is clear from this figure that the majority of the bracket is under minimal stress (blue zones), which evidences the feasibility of topology optimization implementation. In the initial topology optimization problem, as the design constraint, mass is set to be reduced by 60%. Topology-optimized model for 60% mass reduction is given in Figure 7.5D, where extra material not contributing the structural stability in blue areas was removed by the topology optimization software. Density-based (SIMP method) topology optimization algorithm was selected as the optimization option, which achieved the optimized solution by maximizing the stiffness (or minimizing the compliance) as described earlier. Optimized model was then redrawn smoothing out the optimized geometry (Figure 7.5E) and structural finite element analysis was performed on this model under the same loading conditions as shown in Figure 7.5F. This figure indicates that maximum stress in the bracket remained the same (100 MPa) despite the removal of 60% mass in the original design. Therefore, in the second design iteration, design constraint was increased to 75% reduction in mass, which resulted in the topology shown in Figure 7.5G. Similar to the first design, the optimized design was smoothed out for the improved

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Figure 7.5: Step-by-step design process for the topology-optimized bracket under mechanical loading. (A) Loading and boundary conditions for the bracket, (B) imported bracket CAD design into ANSYS finite element software, (C) structural analysis of the original design, (D) 40% mass-reduced model obtained by the topology optimization algorithm, (E) smoothed CAD design based on the topology-optimized model, (F) structural analysis of the topology-optimized design, (G) 75% massreduced model obtained by the topology optimization algorithm, (H) smoothed CAD design based on the 75% mass-reduced model, and (I) structural analysis of the final design.

manufacturability and aesthetics purposes. Final design is shown in Figure 7.5H. Finite element analysis (Figure 7.5I) performed on this design under the same loading conditions showed only a slight increase in the maximum stress (112 MPa) in the bracket despite the removal of large mass from the original design. Therefore, this final design was found to be satisfactory for the amount of mass reduction, structural stability, and manufacturability properties. In summary, topology optimization is a unique tool to design additively manufactured parts with improved performance. This improvement can be reduction of mass, redesigning the support structure and/or optimizing the mechanical, thermal, and electrical properties. Development of more accurate and computationally efficient topology optimization methods will further enhance their integration in manufacturing of additively fabricated components.

8 Advanced concepts in additive manufacturing 8.1 Hybrid additive manufacturing Hybrid additive manufacturing (hybrid-AM) can be defined as a multimaterial layer-bylayer manufacturing process or integration of other manufacturing technologies with additive manufacturing (AM). Hybrid-AM extends the design flexibility of the AM process by adding a new dimension into the manufacturing paradigm. In other words, hybrid-AM uniquely combines the strength of multiple manufacturing processes or multiple materials to enhance the part performance or functionality. Therefore, some of the existing limitations of the conventional AM processes can be eliminated with this approach. In general, hybrid-AM processes follow a serial order and these are not simultaneous processes. Therefore, adverse interactions and influence of individual events on each other are minimal. In this book, hybrid-AM technologies are considered in terms of three major application fields: additive/subtractive hybrid-AM, hybrid-AM of multimaterial electronic components, and hybrid-AM in tissue engineering. The benefits and limitations of these methods are described in the following sections.

8.1.1 Additive/subtractive hybrid manufacturing As described throughout this book, AM has significantly extended the design flexibility of traditional subtractive manufacturing technologies. However, subtractive manufacturing has unique benefits over AM such as better surface finish and high speed. As a result of these distinct benefits of each manufacturing methodology, the concept of integrating additive and subtractive manufacturing technologies has been developed. Computer numerical control (CNC)-based digital subtractive manufacturing techniques are well suited for this integration since both AM and CNC manufacturing use the similar digital control platforms. Figure 8.1 shows the concept of hybrid-AM/subtractive manufacturing where CNC milling is performed on the surfaces of an additively manufactured part to create features with improved surface quality and precision at high speed [245]. In addition to milling, other subtractive manufacturing (machining) techniques can be integrated to AM such as turning and drilling further extending the capabilities of hybrid-AM. The hybrid-AM/subtractive manufacturing has unique advantages over traditional AM. As described, hybrid method combines the design flexibility and the excellent surface finish of subtractive manufacturing. Stair effect, which is a common geometrical artifact in additively manufactured components, can be completely eliminated by machining these surfaces in hybrid manufacturing. In addition, rare materials or materials that are difficult to machine can be initially fabricated via AM to near-net shape, and subsequent step of machining creates parts economically and https://doi.org/10.1515/9781501518782-008

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Substrate

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(1) Start Z X Y

(2) Additive process

(3) Milling

(4) Additive process Spindle

Laser

Workbench (5) Milling

(6) Additive process

(7) Milling

(8) Finish

This cycle is repeated...

Figure 8.1: Schematic of additive/subtractive hybrid manufacturing. Image was reprinted from [245] with permission.

with minimum waste. This is especially beneficial for rare materials with high costs. As described in Chapter 4, dynamic strength of metals is significantly affected by the surface roughness. Therefore, hybrid-AM/subtractive manufacturing allows fabrication of metallic components with higher dynamic strength compared to AM parts due to the improved surface quality [246].

8.1.2 Additive/additive hybrid manufacturing The main goal of integrating subtractive and additive technologies together is to achieve robust and high-speed manufacturing of metals with complex geometries and superior surface finish. Different AM methodologies can also be combined to unify the benefits of different AM technologies in the same manufacturing system. Considering the benefits and the limitations of the major AM technologies, an obvious choice for this integration is combining the high-resolution, single-material AM techniques with those having low-resolution but multimaterial capabilities. Therefore, the hybrid-AM system would have the desired surface quality and the capability of additively fabricating multiple materials in the same setup. As an example, researchers at the University of Texas el Paso [247] developed direct write (DW) integrated hybrid manufacturing technology, which enabled printing of electrical circuitries onto the structure surfaces fabricated via stereolithography (SLA). This concept was then further expanded to fused filament fabrication (FFF) to build 3D circuit boards for a CubeSat satellite [248]. Instead of DW process, material jetting was also integrated on the FFF system to fabricate electronic structures at high resolution (~10 µm) [249]. Therefore, additive/additive hybrid-AM technologies focus on integrating multimaterial AM

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technologies such as direct ink writing and material jetting into the robust, high-resolution AM methods such as FFF and SLA. The most common needs and uses of the hybrid-AM/AM technologies are in electronic applications. Electronic systems usually involve the combination of metal conductors, ceramic insulators, dielectric materials, polymers, and semiconductors. The hybrid-AM enables embedding of these multimaterial systems in electronic components with high degree of complexity realizing functional systems at reduced cost and weight. High geometrical complexity is demanded in many automotive, medical, and aerospace fields, which creates challenges in the production of traditional PCB-based electronics. Figure 8.2 shows an example hybrid process combining SLA and direct ink writing developed by Jo et al. [250] to fabricate a functional 3D-printed circuit board (PCB) device in a complex geometry. Circuit diagram of the electronic system is given in Figure 8.2A. As shown in this figure, the exterior wall is fabricated by

(A)

(B) Aduino MCU

6V Via hole 330 Ω

Resistor

Lower layer

Upper layer

LED

1 kΩ

M C U

Battery Sensor Resistor

Upper layer 10 mm

(C)

(D) LED

Sensor

LED

Figure 8.2: Additively fabricated printed circuit board (PCB) system. (A) Circuit diagram of a 3D-printed (B) bottom view of the PCB with an Arduino-programmed MCU, a photosensor, a battery, resistors, via holes, and LEDs, (C) top view of the printed PCB, (D) image of the 3D PCB operation, showing that the LED turns on when the photosensor is covered. Figure was reprinted with permission from [251].

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the SLA method photopolymerizing the liquid polymer material around the electronic components initially. The process is then paused to clean the partially printed part and dispense interconnects using direct ink writing. Other electronic components such as battery resistors and diodes are also laid down on the printed surfaces at this step. The SLA process then resumes to build photopolymer around the electronic components embedding them within the polymer as shown in Figure 8.2B–D. The hybrid-AM/AM technologies developed for manufacturing of electronic components have tremendous potential due to their design flexibility and their capability of multimaterial fabrication with minimal material waste. The main limitation of these methods is the intermittent process between the different manufacturing steps leading to significant reduction of the manufacturing speed. The development of new hybrid-AM technologies that are capable of simultaneous integration of multimaterial AM techniques will significantly enhance the adoption of hybrid-AM technology for various novel applications in the future.

8.1.3 Hybrid additive manufacturing/scaffolding technologies AM plays an important role in tissue engineering since it allows successful fabrication of patient-specific 3D implant structures and scaffolds. However, the lack of surface texture mimicking that in native tissues adversely affects the cell adhesion and proliferation of the fabricated structures limiting the implementation of these AM methods for certain tissue engineering applications [252]. In addition, low resolution of AM technologies prevents the manufacturing of submicrometer structures mimicking those observed in natural extracellular matrix (ECM) and hierarchical porous architectures with multimodal pore size distributions [253]. Hybrid-AM technologies combining standard AM with cell scaffolding techniques can be used to optimize the surface morphology of the printed structure for cell adhesion and proliferation. In addition, multimaterial, multiscale (micro-nano-macro) materials can be deposited on the printed structures further tailoring the biophysical properties of these 3D-printed biomaterials. Electrospinning and freeze-drying are among these scaffolding techniques that are readily applicable to AM integration. Freeze-drying methods can be integrated into AM technologies to fabricate structures consisting of multiscale (micro and nano) pores. Fabrication of custom 3D architectures with multilevel pore arrangement ensures a larger surface area for cell adhesion and proliferation [254]. Freeze-drying is a commonly used technology, which involves freezing of the material, lowering the pressure, and finally removing the solvent by sublimation. This technology can be easily integrated into DW AM to fabricate tissue scaffolding materials in 3D geometry. In this hybrid-AM methodology, a polymer fiber solution is dispensed at low temperature using DW AM, and then the solvent is freeze-dried leaving the porous fiber network behind. The porosity level of the

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fabricated scaffold can be controlled by simply adjusting the process parameters such as temperature. Natural biopolymers such as 3D collagen can be used to create hierarchical 3D scaffolds with highly porous surface that act as cell entrapment system. As an alternative route to freeze-drying, wet-spinning system can be used to manufacture porous fiber networks in 3D. This technique relies on collecting a solidifying filament of polymer solution into a coagulation bath with predefined layer-by-layer patterns [255, 256]. The produced fibers possess an enhanced biological response due to their “spongy” morphologies. This hybrid manufacturing technique is advantageous since it is a single-step process compared to the freeze-drying-based hybrid manufacturing. However, applicability of this method to only specific biomaterials limits its applications. Electrospinning is one of the most common polymer fiber production techniques that uses electric force to draw charged polymer solutions to produce fibers in the range of nano- and microlength scales. AM and electrospinning can be integrated together in different configurations resulting in hybrid-AM technologies. Electrospun fibers can be deposited using sacrificial targets fabricated by AM. This allows micropatterned scaffolds consisting of random fibers with a defined 3D surface microtopography. Alternatively, melt electrospun fibers can be directly printed on the surface similar to the FFF process. Melt electrospinning involves melting of thermoplastic polymers during the electrospinning process rather than liquid solution of polymer materials in a solvent. Direct printing of fibers instead of using sacrificial targets allows the controlled single-step deposition of the fibers on the printbed at high resolution. Recently, submicron range, uniform thermoplastic fibers were additively fabricated by Hochleitner et al. [257] using melt electrospinning-based hybrid-AM. Despite its high resolution and controlled microstructure, this technique requires a sophisticated experimental setup integrated with heating and electrical insulation, which is a major limiting factor. In addition, high heat applied on the polymer material may adversely affect the temperature-sensitive materials (i.e., collagen, growth factors, etc.) To overcome these challenges, electrohydrodynamic jet printing (E-jetting) technique was developed to fabricate polycaprolactone (PCL) scaffolds recently. Ethanol targets used in this technique allow the fabrication of highly porous 3D scaffolds with controlled filament orientation at room temperature [258]. It was reported that 3D fibrous scaffolds consisting of entangled micro/nanofibers improved cell seeding efficiency and cell adhesion compared to the scaffolds fabricated via extrusion-based AM [252]. In summary, integration of fiber scaffolding technologies with AM techniques such as FFF or DW creates unique scaffold structures that act as ideal cell adhesion and proliferation locations for tissue engineering. Fiber-based structures fabricated in 3D geometries can mimic the native ECM topographically and spatially. The porosity, fiber size, and orientation can be controlled by using different scaffolding technique and

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controlling the process parameters. Table 8.1 presents the major hybrid-AM scaffold fabrication technologies summarizing benefits and limitations. Table 8.1: Summary of hybrid-AM technologies used in tissue engineering (redrawn according to [252]). Hybrid-AM method

Advantages

Limitations

Materials

Applications

Freeze-drying

High porosity with smaller pore size Control of porosity level

Two-step process

PLGA Chitosan PLLA/TCP Type-I collagen PLLA/chitosan

Bone Cartilage Nerve

Wet spinning

Spongy fiber morphology One-step process

Limited set of biomaterials

PCL PCL/HA Star poly (ε-caprolactone)/HA

Bone

Melt Submicron fiber size electrospinning and micron size fiberto-fiber distance Microscale threads consisting randomly interwoven micro/ nanofibers E-Jetting

Complex PCL experimental setup (heating + electrical insulation)

Different strut Difficult control of morphology depending micro/nanosized on the viscosity of strut size target solution

PCL

Skin Neural Vascular TE

Bone

8.2 Additive manufacturing of thermoelectric materials Thermoelectric effect is an exciting concept dealing with the direct conversion of heat energy into electricity and it has great potential to be the ultimate remedy for waste energy recovery in general. Considering the significant amount of energy around us ultimately goes unused in terms of waste heat, and the importance of the waste heat recovery can be better recognized. Thermoelectricity refers to a phenomenon by which thermal energy is converted directly into electrical energy (and vice versa) without any moving parts or working fluids. Thermoelectric materials are able to generate electricity under a temperature differential, which renders them capable materials in reduction of the waste heat. If a thermal gradient is introduced (e.g., engine heat on one side with ambient temperature on the

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107

opposing side) along these materials, an electric potential, ΔV, is created according to the following equation: ΔV = SΔT

(8:1)

where S and ΔT are Seebeck coefficient and relative temperature, respectively. Heavily doped semiconductors are the best thermoelectric materials that possess high Seebeck coefficients, high electrical conductivities, and low thermal conductivities which are required to avoid the loss of temperature gradient along the material. Based on the doping type, thermoelectric material can be either p- or n-type. If a thermoelectric material is kept under thermal gradient, in p-type material, positive charges (holes) move from the hot side to the cold side. In n-type material, negative charges (electrons) move from the hot side to the cold side. If both materials are connected in serial electrically and parallel thermally, electric current flows in the system and temperature gradient is converted into electrical energy as shown in Figure 8.3A. A single module shown in this figure is capable of limited amount of electrical energy. To produce higher level of energy, multiple thermoelectric modules are arranged in a serial connection as shown in Figure 8.3B. Thousands of modules may be required for applications demanding large power needs such as NASA’s radioisotope thermoelectric generators used for multiple space missions producing kilowatt range of electrical power.

Hot surfa

(A)

ce

(B)

p-Type n-Type +

+ + + + + + Holes

–

– – – – – – Electrons

Cold surf ace

Electric fl

ow

Figure 8.3: Principle of thermoelectric energy generation. (A) Thermoelectric effect in p- and n-type thermoelectric materials and (B) assembled modules in a thermoelectric generator device.

Since its discovery over 100 years ago, thermoelectricity has attracted great scientific interest and it has been extensively investigated by numerous researchers; however, it has found only niche applications such as deep space exploration systems by NASA as described earlier. Fundamental challenges of thermoelectric systems limiting its use in our daily lives are twofold: low-energy conversion efficiency and difficult multistep process of their fabrication even for simple geometries. Conventional fabrication of thermoelectric device is a multistep process, including powder synthesis, ingot preparation, dicing, metallization, leg dicing, and assembly (Figure 8.4) [259].

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(A)

(B)

Insulating substrate Electrical shunts

n-Type

p-Type

Powder synthesis/ alloying

Ingot sintering

Ingot dicing

Module assembly

Leg dicing

Metallization

Figure 8.4: (A) Schematic of thermoelectric unicouple system. (B) Processing steps for the fabrication of a conventional thermoelectric device. Image was reprinted from [259] with permission.

The fabrication and the assembly of hundreds or even thousands of modules for the fabrication of a single device are challenging processes used in the conventional manufacturing of these systems. In addition, conventional manufacturing is only limited to flat thermoelectric module geometries, and therefore, complex geometries such as recently developed annular thermoelectric systems with higher efficiencies cannot be fabricated via these conventional manufacturing techniques. AM offers a transformative potential for the manufacturing of next-generation thermoelectric systems and overcomes some of these challenges. Extrusion, Vat polymerization, and powder bed fusion technologies have been implemented for the AM of thermoelectric materials recently. In addition to the flexibility in device size and shape, the cost and time for the fabrication of thermoelectric systems can be reduced significantly using AM technologies since the same printing device can be used to make electrical contacts and deposit thermoelectric materials. Therefore, the entire manufacturing process can be automated. Some of these notable works developed recently are described in more detail. SLA technique was used by He and coworkers [260] in 2015 where p-type bismuth antimony telluride (Bi0.5Sb1.5Te3, BST) powder was mixed with photocurable resin in a Vat, and 3D structures were additively manufactured by curing the photopolymer surrounding the thermoelectric powder. Fabrication steps are shown in Figure 8.5. Custom manufacturing setup in this study allowed preparation of components highly loaded with thermoelectric powders (up to 60 wt%). Printed components were then thermally annealed at 350 °C up to 6 h enhancing the thermoelectric properties. Thermoelectric figure of merit (ZT) is the most important parameter showing the conversion efficiency of thermoelectric materials. In this study, maximum ZT of 0.12 was achieved for the samples fabricated via SLA and annealed for 6 h. In 2019, Oztan et al. [261] fabricated bismuth telluride (Bi2Te3) in complex geometries using FFF technique in which thermoelectric material could be additively manufactured with the assistance of a sacrificial polymer matrix. The process starts with

Composite resins

Laser

Formlabs form 1+ SLA 3D printer

5 wt% < BST ≤ 60 wt%

3D fabrication

BST ≤ 5 wt%

Amorphous carbon and residual photoresins

Thermal annealing

Pore

BST

Figure 8.5: Stereolithography additive manufacturing of thermoelectric materials. Image was reprinted from [260] with permission.

(b)

(a)

Photoresins

Composite resins

BST

3D printing

8.2 Additive manufacturing of thermoelectric materials

109

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8 Advanced concepts in additive manufacturing

mixing acrylonitrile butadiene styrene (ABS) and Bi2Te3 powders and preparing a thermoelectric filament precursor using a filament extruder as shown in Figure 8.6. A desktop FFF printer can then be used to 3D print this filament feedstock into any custom geometry. Similar to the SLA process, 3D-printed samples need heat treatment postprocessing steps to maximize the thermoelectric performance. During postprocessing, the sacrificial ABS polymer in the printed composite is initially removed and then the remaining thermoelectric material (Bi2Te3) is sintered below its melting temperature (585 °C). In this study, higher ZT of 0.54 was achieved at room temperature testing. Higher efficiency of FFF compared to the SLA method may be due to higher amount of the thermoelectric material within the printed composites (80 vs 60 wt%) and the heat treatment process performed at elevated temperatures (up to 550 °C), where stronger bonding between the powders could be obtained enhancing the electrical conductivity and the thermoelectric efficiency. Similar to the SLA fabrication, high porosity was observed in the fabricated samples, which need to be minimized to improve the thermoelectric conversion efficiency.

Polymer / TE powder

Mixture

Extrusion

Printing

Figure 8.6: Schematic of FFF additive manufacturing of thermoelectric materials [261].

DW (paste extrusion) AM technique was also utilized to fabricate thermoelectric materials in viscous ink form in 2018 [262]. Instead of using an organic printing aide material, this study introduced Sb2Te3 chalcogenidometallate (ChaM) ions as inorganic binders to prepare printable Bi2Te3-based inks. Inorganic binder-based thermoelectric inks showed high level of shear thinning, and therefore, successful paste extrusion of these materials was possible as described in Chapter 1. Paste-extruded specimens need heat treatment to facilitate the bonding between the thermoelectric powders and maximize the thermoelectric conversion efficiency. Homogenous thermoelectric properties were achieved in paste-extruded materials, and their dimensionless figure-of-merit (ZT) values were 0.9 (p-type) and 0.6 (n-type), which were comparable to the bulk material values. Main limitations in the paste extrusion of thermoelectric materials are the requirement of postprocessing (heat treatment) of the printed parts, and high level of shrinkage (~20%) after the heat treatment process is implemented.

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111

The above challenges (high porosity, low efficiency, and requirement of postprocessing and filament fabrication) can be overcome using powder bed fusion technologies, including selective laser sintering (SLS) and selective laser melting (SLM). Laser-based powder bed fusion of thermoelectric materials was first introduced in 2015 by Leblanc et al., where the proof-of-concept study investigated the feasibility of rapid prototyping of thermoelectric compounds using SLM technologies [263]. Optimizing the process parameters (i.e., laser power) allowed them to reach the figure of merit of 0.11 in a follow-up study. A schematic of the SLM technology used by the Leblanc group along with the fabricated Bi2Te3 components is shown in Figure 8.7. In 2017, SLM was used to fabricate fine (~3.4 µm) n-type Bi2Te2.7Se0.3 powder achieving maximum ZT of 0.84 [264]. This study evidenced the capability of laser bed fusion techniques to fabricate thermoelectric materials with conversion efficiencies similar to those fabricated via conventional techniques. Qiu et al. [265] prepared Bi0.4Sb1.6Te3 bulk materials using slurry-based SLM technology. Slurry-based SLM process starts with the fabrication of thermoelectric powder by thermal explosion followed by the ball milling procedures. Some of the powder is spark plasma sintered to fabricate bulk substrate material for laser sintering. The remainder of the powder is mixed with alcohol and spread over the substrate to obtain a thin slurry layer (~50 µm). The alcohol is then evaporated by a heater volatilizing the alcohol, and the powder layer was laser sintered. In this study, high ZT of 1.1 at 316 K was achieved along with the high mechanical strength of 91 MPa. Reduced grain size as well as high density of dislocation defect resulting from rapid solidification lead to greatly

(A)

Laser

Galvo and F-theta lens

(B) 5 mm

Fabricated part Roller

Loose powder Spacer (C) 5 mm

Build plate (D) 5 mm

Figure 8.7: (A) Schematic of SLM additive manufacturing procedure of thermoelectric materials (B) disk-shape-printed Bi2Te3 part with 16 W laser power (relative density ∼81%), (C) disk-shapeprinted Bi2Te3 part with 25 W of laser power (relative density ∼88%), and (D) rectangular-shapeprinted Bi2Te3 part. Figure was reprinted with permission from [266].

112

8 Advanced concepts in additive manufacturing

strengthening the mechanical properties. This work provides an effective solution for preparation of Bi2Te3-based materials with high texture, robust mechanical properties, and excellent TE performance and would be instructive for other layered structure material systems. Similar to SLM, SLS method was also investigated for the AM of thermoelectric components. Unlike SLM, material is partially melted in SLS as described in Chapter 1. The fabrication of the porous TE samples of Bi0.5Sb1.5Te3 (BST) using SLS was performed by Shi et al. [267] recently. Since the powders are partially melted in SLS AM, this process results in porous thermoelectric structures. SLS-fabricated porous Bi0.5Sb1.5Te3 was reported to have high ZT value of 1.29 at 54 °C. Since the thermal conductivity is reduced significantly by the porosity as well as boundaries and defects forming in the SLS process, higher figure of merit was achieved compared to the bulk BST materials fabricated via conventional manufacturing methods. In summary, AM has a significant potential as an alternative method for the thermoelectric device manufacturing reducing the manufacturing difficulties while minimizing material losses and providing geometric flexibility. Table 8.2 summarizes the notable studies in which AM technologies of thermoelectric materials are reported. Bismuth telluride-based thermoelectric materials have been the first choice in these studies since, arguably, this material type is the best performing thermoelectric material at room temperature, is cost-effective, and is well investigated previously. Nevertheless, thermoelectric AM technology is at its infancy and novel materials, enhanced properties, and process optimization are well sought in the near future to facilitate the functional thermoelectric device fabrication and the wide range of use of this technology in energy conversion applications.

Table 8.2: The comparison of research studies reported on additive manufacturing of thermoelectric materials. Maximum ZT

AM method

Printer type

Material

SLA

Custom and commercial (Form ) D printer Commercial D printer (Ultimaker ) Custom D printer

Bi. Sb. Te .

[]

BiTe

.

[]

Bi.Sb.Te (p-type) BiTe BiTe.Se. Bi.Sb.Te

. (p-type) . (n-type) . . .

[] [] [] []

Bi.Sb.Te

.

[]

FFF Paste extrusion SLM SLM Slurry based SLM SLS

Custom D printer Custom D printer Custom D printer Commercial D printer (XJRP SLS)

Reference

8.3 Four-dimensional printing with smart materials

113

8.3 Four-dimensional printing with smart materials The concept of 4D printing was initially introduced by Tibbits in 2013 TED talk [268]. Four-dimensional printing is defined as adding time variable to the 3D printing (additively manufacturing) where the shape, property, or functionality of a 3D-printed structure can change as a function of time [269, 270]. In this regard, 4D printing is an extension of 3D printing and it includes all the process steps of 3D printing: CAD modeling, layer-by-layer fabrication, and postprocessing. In addition to these steps, an external stimulation is applied on the 3D-printed structure to obtain predictable changes on this structure as a function of time. Therefore, the main difference in 4D printing compared to 3D printing is the usage of smart materials responding to the external stimuli and achieving unique time-dependent response. A key point in 4D printing is not achieving time-dependent changes but rather obtaining these changes in a predictable manner. Therefore, 4D printing requires extensive mathematical modeling to predict the precise responses of the 3D-printed smart materials. As a result of external stimulation, these smart materials used in 4D printing transform into another stable state. Different types of stimuli can be applied on smart materials giving rise to 4D printing. As shown in Figure 8.8, external stimulation can be applied in terms of heat, light, electricity, moisture, pH, and magnetic force. Different states of the 4D-printed structures can be achieved as a result of the selected smart material type, the configuration of the 3D-printed part, and the external stimulation type. Accordingly, the structure can shift from one state to another under the corresponding stimulus in a predicted way. Therefore, the fundamental building blocks of 4D printing are the smart material type, 3D printing configuration determined by the mathematical models and stimulus.

Smart materials –

Hydrogels

–

Ceramics

–

Alloys

–

Composites

–

Polymers

External stimulation

4D printing

Math model

3D printing

Heat pH Moisture Light

Electricity

Figure 8.8: Schematic representation of the 4D printing procedure including the smart material and external stimulation configurations.

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8 Advanced concepts in additive manufacturing

The application of external stimulus is necessary to trigger the changes on smart materials and create altered shapes, properties, and functionalities. Heat (temperature), moisture (water), light, electricity, magnetic force, and pH are the most commonly used stimuli to fabricate 4D-printed structures. These stimuli can also be used in combination such as simultaneous application of heat and water to achieve the desired configurations. The selection of the stimulus is performed according to the specific application, which determines the type of smart material employed in the 4D-printed structure. As shown in Figure 8.8, the materials used in 4D printing process are categorized similar to those in 3D printing: hydrogels, ceramics, alloys, polymers, and composites. However, these unique materials respond to external stimuli and therefore they are “smart” materials. Material selection is one of the most critical steps in 4D printing process. Smart materials used in 4D printing have different capabilities defined by the following characteristics: self-sensing, decision making, responsiveness, shape memory, self-adaptability, multifunctionality, and self-repair. Major types of smart materials investigated in 4D printing are described in the next section.

8.3.1 Four-dimensional printing materials As described earlier, 4D printing materials are able to change their properties under external stimulus. Although there are only limited number of materials available for 4D printing compared to those available for conventional 3D printing, significant research progress has been made recently to extend the list of stimuli-responsive smart materials. Major materials used in 4D applications are hydrogels, shape-memory polymers (SMP), liquid crystal elastomers (LCE), and active composites. 8.3.1.1 Four-dimensional-printed hydrogels Smart hydrogels used in 4D printing are based on the concept of using inactive rigid and active soft polymers in the printed system. When a stimulus is applied on this bilayer material, soft hydrogel swells or shrinks in response to the stimuli, and the rigid polymer keeps its shape. As a result, bending deformation is obtained in the structure. If the series of these materials are connected in a predesignated way, folding and unfolding motion can be created. There are mainly two different stimuli applied on hydrogel-based smart materials: moisture (water immersion) and temperature (heat). Hydrophilic hydrogel materials are able to swell when immersed in water; therefore, these rubber-like materials can induce significant changes in the 3D-printed bilayer structure in the concept described earlier. Figure 8.9A shows an example of the shapechanging capabilities of water-responsive, 4D-printed hydrogel structures presented in a pioneering work by Tibbits [270]. In this study, hydrogel-based smart materials were fabricated in the form of a series of hinges that transformed a linear structure into a 3D cube when immersed in water. Water-responsive hydrogel smart materials were also

115

8.3 Four-dimensional printing with smart materials

(A)

(B)

t=0

t = 25 min

Figure 8.9: Examples of 4D printing of hydrogel materials: (A) line-to-cube transformation of a water-immersed hydrogel structure. Partial image was reprinted from [270] with permission. (B) Biomimetic 4D printing of a flower with multiple petals. Partial image was reprinted from [271] with permission.

used recently to fabricate 4D-printed parts mimicking the plant cell wall structure [271] as shown in Figure 8.9B. In this work, bilayer structure consisted of soft acrylamide material (hydrogel) and cellulose fibrils with high stiffness (rigid layer). The composite structure was printed via DW extrusion technique in which bilayers were oriented in different angles with respect to the long axis of each petal (90°/0° and −45°/45°). Water immersion created swollen form of the soft material resulting in the complex geometries as shown in this figure. Temperature-responsive hydrogels have also been developed as a smart material feedstock of 4D printing. Poly(N-isopropylacrylamide), or PNIPAm, is a common thermoresponsive material used for 4D printing applications. When immersed in aqueous solution, PNIPAm hydrogel becomes hydrophilic and swells at temperatures below 32 °C. Above this temperature, this hydrogel starts to dehydrate, leading to shrinkage and shape transformation. Therefore, similar to the water-responsive hydrogel composites, if a temperature-responsive hydrogel is 3D printed along with a nonresponsive polymer, bilayered composite hinge system can be fabricated. Wu and coworkers fabricated temperature-responsive hydrogels with disproportional swelling property [272]. In other words, these composite hydrogels in aqueous medium showed bending deformation at temperatures above 32 °C since the different components of the composite result in different amounts of swelling at this temperature level. Four-dimensionalprinted composite hydrogels are also capable of reversible shape deformation in response to both hydration and temperature-responsive hydrogels consisting of PNIPAm as reported by Naficy et al. [273]. 8.3.1.2 Shape-memory polymers SMPs have attracted growing interest in 4D printing due to their unique ability to transform their shapes when exposed to an external stimulus and to recover a permanent

116

8 Advanced concepts in additive manufacturing

shape when the stimulus is removed. SMPs differ from hydrogels since these materials show gradual controlled transformation under stimuli making these preferred materials for morphing structure applications [274]. An example of shape change of PCL macromethacrylate SMPs by varying their temperature is shown in Figure 8.10A [275]. Eiffel tower and bird figurines were 3D printed using SLA technique using a heated Vat of 90 °C. As the temperature of the printed structure varied, different geometries were obtained resulting in reversible shape change under thermal stimuli. Multiple SMPs and/or SMPs along with materials not responding to external stimuli can be 3D printed together in a precisely designed configuration (shape, placement, and mixing ratio) to obtain complex geometries in these composite systems. If SMPs with different properties are used in this multi-SMP system, SMPs respond differently to the external stimuli under the stimuli and therefore selective shape change can be obtained in a precise way. Digital SMPs are the main class of these multi-SMP systems, where SMPs with different glass transition temperatures are used to obtain temperature-controlled shape change. An example of this transformation is shown in Figure 8.10B, where a mailbox was designed with digital SMPs responding to thermal stimulus [276]. In this application, digital SMPs were used along with the thermally nonresponsive materials to obtain the desired configuration. Multistep programming of digital SMPs is also possible, where changing the temperature in multiple steps results in multiple different configuration of the printed structures.

A

2 cm

10 mm

B

t=2.5 s

t=6.3 s

t=7.2 s

t=11.0 s

Figure 8.10: (A) Thermally activated single material shape-memory polymers. Eiffel tower and bird figurines respond to temperature stimulus changing their shapes. Partial image was reprinted from [275] with permission. (B) Time lapse of the folding process of a mailbox-shaped digital SMPs. Image was reprinted from [276].

Although thermally responsive SMPs are the most common SMP type, other SMPs responding to different stimuli also exist such as those responding to pH change, solvent, moisture (chemoresponsive materials), electrical (electroresponsive materials), stress/ pressure (mechanoresponsive materials), magnetic field (magnetoresponsive materials), and light (photoresponsive materials) [277].

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117

8.3.1.3 Elastomer actuators SMPs are the commonly preferred 4D printing materials due to their high elastic deformation, low cost, low density, and potential biocompatibility. However, most SMPs only exhibit a one-way response, while further active responses require additional programming steps. Reversible shape change can be obtained in hydrogels due to swelling and shrinking when the stimuli are applied and reversed. However, the response time of hydrogels to an external stimulus is long and shape change is a relatively slow process. LCEs are unique materials that exhibit rapid and reversible shape changes; therefore, these materials are becoming increasingly popular as a 4D-printed smart material. Since LCEs combine the properties of highly stretchable elastomers and self-assembly of liquid crystals, they are preferred in fabrication of smart stretchable structures such as soft robots, implantable biomedical devices, and systems based on artificial muscles. Shape change in LCEs is through a transition between the liquid crystal (nematic) state and the isotropic state in response to stimuli such as light, heat, and electrical or magnetic fields. By thermal cycling above and below its nematic to isotropic transition temperature (TNI), the LCE can alternate between its nonaligned (isotropic) and aligned (anisotropic) states, causing large and reversible shape change [278]. Reversibility of 4D-printed LCEs under temperature stimulus was extensively investigated recently by Kotikian et al. [279]. Figure 8.11 reported in this study represents the shape shifting process from 2D to 3D, where 3D-printed LCE polymers in 2D geometries transform into 3D cone (Figure 8.11A) and saddle (Figure 8.11A) configurations upon increasing the temperatures over TNI. Tremendously high out-of-plane deformation (~1628%) was observed in these structures leading to these 3D transformations. Similarly, Figure 8.11C and D shows isotropic shrinkage and 3D conical array expansion of LCEs under thermal stimulus, respectively. It was also shown in this study that upon cooling, reversible shape morphing was observed in these 4D-printed LCE materials.

8.3.2 Applications of 4D-printed structures Predicting the exact state of the material as a result of these applied stimuli leads to unique properties of 4D-printed structures such as self-assembly, multifunctionality, and self-repair. Therefore, these structures possess great potential in terms of reduced volume and transportation benefits. Smart structures can be printed in low-volume forms such as 2D sheets or compressed structures. They can then be transformed into larger 3D forms prior to the actual usage. Three-dimensional fabrication of these structures at low volumes creates tremendous advantages of packageability and transportation. In addition, 4D-printed structures can find various applications in biomedical, electronics, and robotic fields. Biomedical devices such as stents and adaptive scaffolds can be fabricated via these structures. These biomedical systems can be preformed into a temporary shape, inserted into the body through a smaller

118

(A)

8 Advanced concepts in additive manufacturing

(C)

(D) (B)

Figure 8.11: Shape morphing in LCEs under external stimuli. Two-dimensional to 3D transformation of disk-shaped LCEs (≈0.4 mm thick) into (A) cone and (B) saddle configurations. (C) Top-down images of mesh-shaped LCEAs (≈0.5 mm thick) after printing (left) and shrinking into an isotropic form (right) upon heating above TNI (scale bars = 5 mm). (D) Top and side views of an LCEA sheet after printing (left) and morphing (right) into a conical array upon heating above TNI (scale bars = 5 mm). Figure was reprinted with permission from [279].

surgical incision, and deployed into its desired configuration under an external bodyinduced stimulation. Targeted drug delivery is another application where the drugs encapsulated into 3D-printed structures can only be released when reached to the targeted locations in which the interaction with the targeted tissue/organ can provide the required stimulation. Self-assembly of 4D-printed parts can also be used in aerospace structures, antennas, and satellites in which the smart parts are unfolded when they are in use and folded back to minimize their volumes. Another potential application of 4D-printed parts includes self-assembling of buildings, especially in remote locations such as those at war zones or in outer space where minimum human involvement is preferred [280].

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119

In soft robotics, 4D-printed structures can also find unique applications. Figure 8.12 shows an example of gripping motion using a 4D-printed structure as reported by Ge et al. [281]. Since no wiring or mechanical hinges are necessary in these actuators, reliability and simplicity are greatly enhanced. As a result, material cost is reduced and if precisely controlled, these devices can be used for gentle interaction with fragile objects [282].

I

II

III

IV

Figure 8.12: Snapshots of a 4D-printed SMP gripping a bolt. Image was reprinted from [281].

Self-repair is also a notable property investigated in 4D-printed structures. Since these structures are capable of reorganizing their structures, external stimulation can be applied to repair the damaged zones and the functionality is retained. It was demonstrated in numerous studies that materials used in 4D printing are capable for structural restoration of damages and can be applied in various applications such as restoration of damaged pipes in a plumbing system as well as self-healing vascular models for tissue engineering. In this chapter, the concept of 4D printing, material types, and major applications are described. Despite its remarkable potential due to these unique properties, 4D printing is still under development stage since its introduction in 2013, and extensive research work is needed to make it an established technology desired for groundbreaking applications.

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Index 4D printing 113 active composites 114 additive manufacturing of ceramics 61 additive manufacturing of metals 47 additive manufacturing of polymer composites 31 additive manufacturing of polymers 22 Additive Manufacturing of Thermoplastic Polymers 25 Additive Manufacturing of Thermosets 27 additive/additive hybrid manufacturing 102 additive/subtractive hybrid additive manufacturing 101 Additive/Subtractive Hybrid Manufacturing 101 Advantages of AM 2 age hardening 57 aging 28 alginate 81 alumina 65, 66, 71 aluminum 53 ammonium phosphate 65 analytical model 38, 42 annular thermoelectric systems 108 ASTM 11, 55 BESO 96 big area additive manufacturing 20 Binder Jetting 10, 47, 63, 94 bioactive 61 bioactive glasses 61 bioceramic 72 biocompatibility 56, 61, 67, 81, 82, 89, 117 biocompatible 47 biodegradability 81, 82 biofabrication 82 bioinert 61 bioink 78, 79, 81 biomaterial 78 biomedical implants 63 bioprinting 78, 82, 90 bismuth telluride 108, 112 blood vessels bioprinting 84 boron oxide 65 Bouligand structure 68

https://doi.org/10.1515/9781501518782-010

CAD 2, 19, 78, 93, 113 cages 63 cancer bioprinting 89 carbon fiber 42 carbon nanotube 32, 33 carbon quantum dot 33 cardiac tissue bioprinting 87 cartilage bioprinting 85 casting 53, 58 cell adhesion 104 cell viability 78 ceramic 10, 15, 16, 61 chemical composition 49 chemical resistance 61 chemo-responsive materials 116 chitosan 81 CNC 16, 101 constraints 97 contamination 49 continuous fiber reinforced composites 41, 45 Continuous Liquid Interface Production 7 corrosion resistance 56 cost 15, 17, 26, 47, 48, 61, 64, 72, 77, 78, 92, 117 creep resistance 73 decellularized extracellular matrix 82, 87 densification 75 density-based optimization 97 design 3 Digital Light Processing 7 direct energy deposition 47 direct write 12, 13, 14, 29, 37, 41, 67, 104, 110 Directed Energy Deposition 15 displacement-controlled extrusion 12 doctor blading method 71 ductility 54, 58, 61 elastic modulus 37, 45 elastomers 22, 23, 29 electrical aalignment 41 electrical insulation 61 electrode induction melting gas atomization 49 electrohydrodynamic jet printing 105 electrolytic method 51

136

Index

electron beam assisted manufacturing 48 electron beam melting 48 electro-responsive materials 116 electrospinning 104, 105 elongation at break 57 epoxy 44, 74 ESO 96 external stimulus 114, 115, 117 extracellular matrix 82 extrusion 47, 78

Inconel 54 infiltration 63, 67, 75 inkjet bioprinting 79 inkjet printing 78 ionic photopolymerization 6 isostatic compaction 63, 75 isotropic transition temperature 117

fatigue 55, 57, 58 feedstock 57, 61, 72 feedstock materials 51 FFF 21 fiber alignment 38, 39, 40 fiber-matrix adhesion 46 figure of merit 108, 110, 111, 112 finite element analysis 92 flowability 49, 53 fracture toughness 26, 77 free-radical polymerization 6 freeze-drying 104 fused filament fabrication 11, 12, 26, 29, 34, 35, 43, 46, 70, 71

laser assisted bioprinting 80 laser assisted manufacturing 48 laser based fusion 53 laser micro sintering 65 laser-assisted printing 78 lead zirconate titanate 67 Level Set Method 96 Level set topology optimization 98 liquid crystal elastomers 114 liquid deposition modeling 12 liquid sintering 63, 64 lithography-based ceramic manufacturing 66 locally induced RF welding 34 loss modulus 13

gas atomization 49 gelatin 81 gelation 81 GelMA 81 glass fiber 42 Glass transition temperature 25 grain refinement 57 grain size 58 graphene 32

machining 53, 56 magnetic alignment 41 magneto-responsive materials 116 manufacturability 92 maraging steel 53 martensitic steel 53 mass reduction 95 Material Extrusion 11 Material Jetting 9, 66 Material toxicity 25 material waste 4 mechanical performance 25, 26, 29, 45, 47, 54, 55, 57, 58, 74 mechano-responsive materials 116 melt electrospinning 105 Metal 10, 15, 16 metal oxidation 49 metallothermic process 51 microstructural composition 49 microstructure 57, 58 morphology 49 multi-functionality 117

hardness 57 health implications 8 heat sensitive thermosets 29 heat treatment 56, 58, 75 high entropy alloys 54 history of Additive manufacturing 19 homogenization 96 hot isostatic pressing 56, 58 Hybrid Additive Manufacturing 101, 104 hydride-dehydride process 51 hydrogels 79, 81, 85, 114, 117 hydroxyapatite 61, 63, 67

Kevlar fiber 37, 42 kidney bioprinting 88

Index

Multijet printing 9 multiple parts 11 natural polymers 22 Newtonian fluid 13 Ni-based superalloys 54 No Tooling 3 nozzle impregnation 43 objective function 97 optimization 92 organosilicon polymers 73 orthopaedical implants 61 overhang-free topology optimization 95 oxidation 54 paper 16 paste extrusion 12, 13, 66 PCL 116 photocurable preceramic polymer 74 photocurable resins 5 photoinitiator 5, 27 photopolymer 80 photo-responsive materials 116 photosensitive thermosets 27 piezoelectric 33, 71, 72 piezoelectric actuation 79 plasma atomization 49 plasma rotating electrode process 49 pluronic 82 PNIPAm 115 Polyjet 9 polymer 15, 22 polymer derived ceramics 73 polyurethane 81 porosity 57, 58, 63, 64, 71, 75, 77 post process 59 post processing 54, 64, 77, 94, 111 Powder Bed Fusion 14, 47, 64, 94 powder bed fusion technologies 108 powder doped polymer composites 31 powder feedstock 48 powder-based ceramic AM 62 precipitation hardening 57 pressure controlled extrusion 12 Printability 24

randomly aligned fibers 39 rapid prototyping 47 recoating 53 residual stresses 54 Resistance to moisture absorption 25 resolution 19, 47, 48, 65, 67, 72, 102 robocasting 12 satellite 51 seebeck coefficient 107 Selection of Polymers for Additive Manufacturing 24 selective laser melting 15, 111 selective laser sintering 15, 26, 111 self-assembly 117, 118 self-repair 117 shape memory polymers 114 shear alignment 41, 68 Shear rate 13 shear thinning 12, 13 Sheet Lamination 16, 70 short fiber reinforced composites 34, 35, 38, 45 shrinkage 65, 74, 110 silicon carbide 71 SIMP 96, 97, 99 sintering 48, 63, 64 sizing 36 skin bioprinting 85 slim support 95 slurry based ceramic additive manufacturing 66 smart material 113 solid solution strengthening 57 spatter ejection 54 speed 47, 72 spheroidicity 49, 51 spheroidization 50 stainless steel 51 stair effect 101 stereolithography 33, 102 storage modulus 13 straight wall support 95 strength 45 streolithography 7, 19 stress relief 56 subtractive manufacturing 1, 101 support 94

137

138

Index

support structures 11 surface adhesion 71 surface roughness 54, 55, 58, 65, 102 synthetic polymers 22 tape casting 70 temperature responsive hydrogels 115 tensile strength 37 thermal actuation 79 thermal conductivity 53 thermal insulation 61 thermal shock resistance 64 thermal stress 17, 55 thermal stresses 15, 16, 55 thermoelectric generator 107 thermoelectric materials 106, 108 thermoplastic elastomers 29 thermoplastic polymer 47 thermoplastics 22 thermoset composites 46 thermoset elastomers 29 thermosets 22, 23 tissue engineering 63, 78, 104 tissue scaffolds 63

titanium 51, 53 tool steel 53 topology optimization 61, 93 Tree-like support 95 tricalcium phosphate 64, 65 ultrasonic additive manufacturing 17 Ultrasonic bonding 17 Vat Photopolymerization 5 Vat polymerization 23, 27, 66, 78, 80, 108 viscosity 79, 81 Visual quality 25 void formation 54 Volumetric Vat Manufacturing 8 warm isostatic pressure 64 warpage 15, 45 water atomization 49 water responsive hydrogel 114 wear resistance 56, 61 zirconia 66, 71