Additive Manufacturing in Multidisciplinary Cooperation and Production (Springer Tracts in Additive Manufacturing) [1st ed. 2024] 3031376706, 9783031376702

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Springer Tracts in Additive Manufacturing

Igor Drstvensek · Snehashis Pal · Nataša Ihan Hren Editors

Additive Manufacturing in Multidisciplinary Cooperation and Production

Springer Tracts in Additive Manufacturing Series Editor Henrique de Amorim Almeida, Polytechnic Institute of Leiria, Leiria, Portugal Editorial Board Abdulsalam Abdulaziz Al-Tamimi, Riyadh, Saudi Arabia Alain Bernard, Ecole Centrale de Nantes, IRCCyN UMR CNRS 6597, Nantes Cedex 03, France Andrew Boydston, University of Washington, Seattle, USA Bahattin Koc, Maltepe, Sabanci University, Istanbul, Türkiye Brent Stucker, Louisville, KY, USA David W. Rosen, Atlanta, GA, USA Deon de Beer, Bloemfontein, South Africa Eujin Pei , College of Engineering, Design and Physical Sciences, Brunel University London, London, UK Ian Gibson, University of Twente, Enschede, Overijssel, The Netherlands Igor Drstvensek, University of Maribor, Maribor, Slovenia Joaquim de Ciurana, University of Girona, Girona, Spain Jorge Vicente Lopes da Silva, CTI Renato Archer, Campinas, São Paulo, Brazil Paulo Jorge da Silva Bártolo, Nanyang Technological University, Singapore, Singapore Richard Bibb, Loughborough University, Leicestershire, UK Rodrigo Alvarenga Rezende, Uniara, Araraquara, Brazil Ryan Wicker, University of Texas at El Paso, El Paso, TX, USA

The book series aims to recognise the innovative nature of additive manufacturing and all its related processes and materials and applications to present current and future developments. The book series will cover a wide scope, comprising new technologies, processes, methods, materials, hardware and software systems, and applications within the field of additive manufacturing and related topics ranging from data processing (design tools, data formats, numerical simulations), materials and multi-materials, new processes or combination of processes, new testing methods for AM parts, process monitoring, standardization, combination of digital and physical fabrication technologies and direct digital fabrication.

Igor Drstvensek · Snehashis Pal · Nataša Ihan Hren Editors

Additive Manufacturing in Multidisciplinary Cooperation and Production

Editors Igor Drstvensek Faculty of Mechanical Engineering University of Maribor Maribor, Slovenia

Snehashis Pal Faculty of Mechanical Engineering University of Maribor Maribor, Slovenia

Nataša Ihan Hren Medical Faculty, Department of Maxillofacial and Oral Surgery University of Ljubljana Ljubljana, Slovenia

ISSN 2730-9576 ISSN 2730-9584 (electronic) Springer Tracts in Additive Manufacturing ISBN 978-3-031-37670-2 ISBN 978-3-031-37671-9 (eBook) https://doi.org/10.1007/978-3-031-37671-9 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Foreword

Having been active in additive manufacturing (AM) for three decades, I have read many books on AM. Some have been written for the educational community, while others have been targeted at industry. This book has a role to play in both these domains. The four chapters in the book combine to give an important insight into the current AM ecosystem. The knowledge presented represents numerous disciplines and is largely centered on using AM in a production environment. Chapter 1 contains eight papers focusing on polymer AM. Although the oldest of the various AM technologies, the use of polymer AM is still very widespread, and it is being used increasingly for final production parts. For this trend to accelerate, a greater understanding of the processing characteristics is needed together with a wider range of materials. Most of the papers presented in this chapter deal with one or other of these requirements. The remaining papers report innovative applications of polymer AM. The five papers in Chap. 2 address the subject of metal AM. Metal AM technology is a more recent development than polymer technologies but its use has grown very quickly, especially in the aerospace and medical sectors. The papers presented here provide a better understanding of the properties of two specific metals as well as two interesting applications. The final paper in this chapter is a comprehensive review of the role and impact of nanocomposites in AM. This is a particularly exciting area since it has been demonstrated that relatively small proportions of different nanomaterials can have a significant impact on a range of material properties. Chapter 3 covers the rapidly developing area of AM simulation and optimization. Together with increased understanding of process characteristics and material properties, this topic promises to drive the further development of AM being used for production purposes. AM needs to become more “industrialized” and, for this to happen, simulation must be used to drive process optimization within a production environment. This chapter presents an eclectic range of papers that deal with various aspects of simulation and optimization. The last two papers in the chapter deal with optimization of application rather than process optimization. The final chapter of the book presents four papers on the topic of medical AM. Although a relatively niche area of AM application, the lessons learnt from medical v

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AM can often be applied to a wider domain. For example, one of these papers addresses ceramic materials for medical AM. Processing of ceramics is notoriously difficult. The need for medical ceramics has driven process development to a point where important industrial ceramics are now being processed using AM. Likewise, the development of porous structures for bone replacement is now finding other applications in the industrial domain. I recommend this book to undergraduate and graduate students who want to learn more about AM or to gain insight for advancing their own research. I also recommend it to people working in industry who want to keep abreast of important developments in AM. This includes those working in the medical devices industry, who will find the final chapter particularly valuable. The greatest benefit from this book will be when readers from different backgrounds sit down together to discuss what they have learnt and then use this knowledge to drive further multidisciplinary cooperation in the future. Prof. R. Ian Campbell Emeritus Professor of Computer Aided Product Design Loughborough University Loughborough, UK

Preface

Although they are far from mature, additive technologies are gaining attraction in industry, not only in research and development but also in everyday production, at least in areas where production volumes are relatively low. According to Wohlers Report, final part production is the fastest growing application of additive manufacturing (AM), which is also becoming increasingly important in series production. For example, BMW produced more than 300,000 series parts using additive manufacturing technologies in 2021 (Wohlers Report 2022). The year 2017 marked the beginning of a period in which large companies entered the AM market by overtaking some major players in the field. Perhaps most notable was the acquisition of Arcam and Concept Laser by GE, which demonstrated the company’s serious interest in additive manufacturing of sophisticated turbine parts. On the other hand, research interest in AM is also growing, and most importantly, it is shifting from finding applications to understanding the fundamental processes behind additive technologies. The tremendous research interest is reflected in a growing number of new scientific journals devoted to AM. For a long time, the Rapid Prototyping Journal (first published in 1995) was the only journal dedicated to additive manufacturing technologies, which at the time focused mainly on prototyping. Later, other journals followed, such as Virtual and Physical Prototyping in 2006, and Additive Manufacturing and 3D Printing and Additive Manufacturing saw the light of day in 2014. In the meantime, some more general journals started to accept papers with AM content, mainly because of the research goals that have since shifted from prototyping and design to material behavior, metallurgy, polymer chemistry, optics, and other areas. Some misconceptions that led to inflated expectations or rejection of the new technologies have also since been dispelled, promoting their acceptance in broader industrial sectors. Some authors believed that AM will replace other forms of production, which is highly unlikely due to the layered nature of AM; rather, it represents another tool in the toolbox that is very well suited for complex products that can be assembled or joined together with simpler parts produced in a conventional way. Many designers also believe that AM or 3D printing is a plug-and-play-push-the-button-and-forget process, which is not true. AM processes rely on many technological parameters vii

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that need to be set before production, and to make it even more complicated, these parameters depend on the shape of the product. This also disproves the myth that complexity is free in AM. Although the layered nature of AM makes it relatively easy to produce geometrically complex parts, it still comes at a significant cost. Even if the pre-production phase (topology optimization, etc.) is not taken into account, the process and especially the post-processing phase can be extended by the geometric complexity of the products (Brajlih et al., 2010). As for the application side of AM, medical devices and implants were one of the main promoters of the new technology from the beginning. This also triggered some exaggerated expectations in terms of solutions for everything, which led to patients seeking the help of engineers. The situation has stabilized over the years and, most importantly, has enabled interdisciplinary communication between physicians and engineers. This led to some significant breakthroughs that helped many patients have a better quality of life than was possible before, and opened up many new areas where technology is being used. Basic research on AM conducted at many universities, institutes, hospitals, and R&D departments around the world has helped overcome many barriers to the adoption of AM technologies. The process has been lengthy and is far from complete, but it is already evident in many new industries that, encouraged by the success of early adopters, have adopted the new technology at all stages of production. That’s why we asked researchers in the fields of medicine, materials science, design, and engineering to contribute their latest findings to this book. The result is a collection of current research projects, case studies, and new findings that shed light on the latest research trends and emerging issues in the broad field of additive manufacturing technologies. Maribor, Slovenia Maribor, Slovenia Ljubljana, Slovenia

Igor Drstvensek Snehashis Pal Nataša Ihan Hren

Contents

Part I 1

2

3

4

5

6

Polymers

Accelerated Non-Isothermal Powder Bed Fusion of Polypropylene Using Superposed Fractal Exposure Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Samuel Schlicht and Dietmar Drummer Understanding Geometry Dependent Temperature Fields in Laser Powder Bed Fusion of PA12 by Means of Infrared Thermal Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sandra Greiner, Samuel Schlicht, and Dietmar Drummer Additively Manufactured Glass Fiber Reinforced Polymeric (GFRP) Structures for High Performance Applications . . . . . . . . . . . Marius Rimašauskas, Tomas Kuncius, R¯uta Rimašauskien˙e, and Magdalena Mieloszyk

3

15

25

Effects of Printing Direction and Multi-material on Hardness of Additively Manufactured Thermoplastic Elastomers for Comfortable Orthoses and Prostheses . . . . . . . . . . . . . . . . . . . . . . . . Paweł Michalec, Sakine Deniz Varsavas, Florian Arbeiter, Robert Weidner, and Lisa-Marie Faller

33

Holistic Characterization of PBF-LB/P Powder Regarding Isothermal Crystallization, Rheology and Optical Properties Under Process Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Maximilian Marschall, Simon Cholewa, Sebastian-Paul Kopp, Dietmar Drummer, and Michael Schmidt

43

Mechanical Behaviour of Polyamide 12 Parts Manufactured by SLS Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Telma Ruivo, Mário S. Correia, Henrique A. Almeida, and Ana M. Amaro

53

ix

x

7

8

Contents

Effect of Automotive Fluids on Additive Manufactured Components for the Automotive Industry . . . . . . . . . . . . . . . . . . . . . . . . Rui Pedrosa, Maria Leopoldina Alves, and Henrique A. Almeida

65

3D Bioprinting of Cellulosic Structures for Versatile Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Özkan Yapar

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Part II 9

Metals

Comparison of Material Properties of Duplex Stainless Steel 1.4462 Processed by DED-LB/M and PBF-LB/M . . . . . . . . . . . . . . . . . 105 Andreas Maier, Manuel Rühr, Marcel Stephan, Stephan Roth, and Michael Schmidt

10 Influence of Carbon Content on the Material Properties of Low-Alloyed Steel Bainidur AM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Dominic Bartels, Tobias Novotny, and Michael Schmidt 11 Additive Technology Driven Integrated Thermoelectric and Photovoltaic Network Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Sohorab Hossain and Surajit Chattopadhyay 12 Fine Porous Structures Fabricated Using Laser Powder Bed Fusion of Ti–6Al–4 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Salome Sanchez, Ahmad Zafari, Ali Gökhan Demir, Leonardo Caprio, Barbara Previtali, Malgorzata Holynska, Ian Gibson, and Davoud Jafari 13 Role of Nano Composite in Additive Technologies: A State of Art Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Dheeraj Kumar, Amit Kumar, Kumar Amit, and Prabas Banerjee Part III Simulation and Optimization 14 A New Non Linear Fuzzy Approach (NLFA) for Performance Evaluation of FDM Based 3D Printing Materials . . . . . . . . . . . . . . . . . 157 Premangshu Mukhopadhyay and Bipradas Bairagi 15 Multimaterial 3D Printing of Programmable Architected Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 Mehrshad Mehrpouya, Jonne F. Postmes, Ava Ghalayaniesfahani, and Ian Gibson 16 Fabricating Lightweight Gear Using 3D Printing and Topology Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Riad Ramadani, Gül Okudan Kremer, Marko Kegl, and Jožef Predan

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17 Validation of Simplified Injection Molding Simulation Results for Conformal Cooling with a Hybrid Mold Insert Using Thermal Imaging Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Janez Gotlih, Timi Karner, Mirko Ficko, Igor Drstvensek, Tomaz Brajlih, and Miran Brezocnik 18 Do it Yourself Kits (DOK) for Additive Manufacturing (AM) for Educational Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Bernhard Heiden and Bianca Tonino-Heiden 19 Additive Manufacturing Technology: An Insight into Re-Shaping the Creative Industry . . . . . . . . . . . . . . . . . . . . . . . . . . 209 Raka Mitra Part IV Medical Application 20 Design and Analysis of Porous Structure Implant Based on Primitive Triply Periodic Minimal Surface . . . . . . . . . . . . . . . . . . . . 219 Mahdi Sarkari, Sadegh Rahmati, and Mohammad Nikkhoo 21 3D Printing of Ceramics for Modern Medical Engineering . . . . . . . . 235 Jessica Sohl and Joana Dias 22 Case Report: Preoperative Planning of Revision Total Hip Arthroplasty with a 3D Printed Hemipelvis Model in Juvenile Idiopathic Arthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 Rok Vojkovi´c and Borut Pompe 23 Additive Manufacturing of Functionalized Material Systems for Medical Applications: Potentials and Challenges in Additive Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 Dietmar Drummer, Samuel Schlicht, and Sandra Greiner

Part I

Polymers

Chapter 1

Accelerated Non-Isothermal Powder Bed Fusion of Polypropylene Using Superposed Fractal Exposure Strategies Samuel Schlicht

and Dietmar Drummer

Abstract Laser-based powder bed fusion of polymers (PBF-LB/P) is predominantly based on the quasi-isothermal processing of semi-crystalline polymers. To overcome existing limitations in the range of polymers suitable for PBF-LB/P, the non-isothermal, support-free powder bed fusion of polymers by means of superposed fractal, phase-shifted exposure strategies is proposed. Using polypropylene as a model material, thermographic monitoring allows for observing cooling rates exceeding 50 K s−1 , leading to temporary supercooling of the polymer melt. By varying the applied laser power, an interdependence of structural boundary conditions and applied exposure parameters on emerging temperature fields and corresponding crystallization temperatures can be derived. Resulting thermal process properties, assessed in situ, are characterized by the exposure-dependent crystallization of the polymer melt. Formed temperature fields implicitly influence the layer formation and formed crystalline modifications, yielding the exposure-dependent formation of α/β-PP. The presented results indicate the suitability of the proposed discretized exposure strategies for significantly increasing the build rate in support-free, nonisothermal powder bed fusion of polymers while significantly reducing the thermal exposure of applied materials compared to existing additive manufacturing processes. Allowing for the targeted modification of part properties, the proposed processing strategy represents the methodological foundation for the additive manufacturing of thermo-sensitive, functionalized multi-material systems and the integration of pharmaceuticals. Keywords Non-isothermal laser sintering · Powder bed fusion of polymers · Low temperature powder bed fusion · Mesomorphic polypropylene

S. Schlicht (B) · D. Drummer Institute of Polymer Technology, Friedrich-Alexander-Universität Erlangen-Nürnberg, Am Weichselgarten 10, 91058 Erlangen, Germany e-mail: [email protected] Collaborative Research Center 814, Friedrich-Alexander-Universität Erlangen-Nürnberg, Am Weichselgarten 10, 91058 Erlangen, Germany © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_1

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1.1 Introduction Laser-based additive manufacturing processes represent the well-established state of the art in rapid prototyping and small-series production, exhibiting an unrivaled geometric flexibility. PBF-LB/P is predominantly characterized by the quasiisothermal processing of semi-crystalline polymers, being inherently interlinked to the continuous heating of the build chamber and the powder bed surface, hence inhibiting the crystallization of molten polymer. However, recent research has queried the assumption of idealized isothermal processing, indicating the occurrence of isothermal crystallization within the build chamber [1–3]. In an attempt to overcome structural limitations of quasi-isothermal processing, specifically thermal material aging [4, 5], a limited range of material suitable for quasi-isothermal processing, thermal degradation of thermo-sensitive materials and economic constraints, Schlicht et al. [6] proposed the application of segmented, fractally sequenced, locally quasisimultaneous exposure strategies for significantly reducing build chamber temperatures in PBF-LB/P. However, quasi-simultaneous exposure strategies, characterized by the repetitive exposure of distinct segments, are inherently restricted due to a considerably reduced build rate. Aforementioned limitations of both quasiisothermal additive processing of polymers and quasi-simultaneous processing imply the requirement of novel processing strategies, combining the build rate of quasiisothermal processing with a significantly extended range of accessible materials, economic advantages and a considerably increased energy efficiency.

1.2 State of the Art 1.2.1 Interdependencies of Exposure Strategies and Layer Formation Mechanical and morphological properties of parts produces by quasi-isothermal and non-isothermal laser-based processing both depict a significant influence of the underlying exposure parameters. Extensive research has been applied regarding quasi-isothermal processing for investigating the influence of exposure parameters and geometric boundary conditions on emerging thermal process characteristics [7, 8] and corresponding mechanical properties [9–11]. Quasi-isothermal laserbased processing of polymers has been found to be considerably influenced by the consolidation phase following the exposure process, exhibiting an influence of the layer time on emerging mechanical properties [12] and on resulting morphological properties [13]. Optically observed consolidation kinetics depict a dependency on underlying processing parameters [13], implicitly influencing corresponding part densities. Consequently, the influence of varied exposure strategies and exposureinduced temperature fields is interlinked to the emerging consolidation phase in quasi-isothermal processing. In contrast, the occurrence of a consolidation phase

1 Accelerated Non-Isothermal Powder Bed Fusion of Polypropylene …

5

in non-isothermal PBF-LB/P is inherently restricted due to the rapid crystallization of the polymer melt [6], depending on the cooling rate [14]. Existing research on support-based non-isothermal PBF-LB/P is predominantly based on the parameterwise adaption of existing linear exposure strategies [15–19]. To allow for support-free non-isothermal processing, the proposed application of fractally sequenced, locally quasi-simultaneous exposure strategies have been described [6], representing the basis for extending the consolidation phase in non-isothermal processing based on the repetitive exposure of distinct segments. Underlying exposure strategies, applied for non-isothermal PBF-LB/P, rely on the spatial and temporal discretization of the melting and subsequent crystallization of semi-crystalline polymers. However, described exposure strategies are associated with considerably increased processing times, hence limiting the economic viability of existing non-isothermal processing strategies.

1.2.2 Crystallization Characteristics of Polypropylene Considering the aforementioned influence of the rapid crystallization of applied materials on the process characteristics of non-isothermal powder bed fusion, understanding isothermal and non-isothermal crystallization kinetics of polypropylene is essential. Since the occurrence of comparably high cooling rates has been shown to be correlated with supercooling of the polymer melt [20], the assessment of temperaturedependent crystallization rates is crucial for assessing the influence of crystallization kinetics on morphological properties of formed layers. Mubarak et al. [20] identified a quasi-Gaussian dependency of the crystallization rate and the corresponding temperature. Rapid cooling of the polymer melt, exceeding cooling rates of 150 K s−1 and a crystallization temperature below 85 °C, respectively, lead to the predominant formation of mesomorphic, metastable states [21]. Formed metastable crystalline structures are subject to phase changes at ambient temperatures [22–24], converting into the α-phase while maintaining ductile properties associated with the mesomorphic phase [25, 26]. These considerations imply a time–temperature-dependency of thermal and morphological part properties of quenched specimens. In contrast to the aforementioned predominant conversion-induced formation of the α-phase, slow cooling from the melt, occurring in quasi-isothermal PBF-LB/P predominantly yields the β-modification [27].

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1.3 Materials and Methods 1.3.1 Fractal Exposure Implementation The applied exposure strategy is characterized by the superposition of identical, phase-shifted exposure paths based on the fractal, space-filling Peano curve [28]. By applying the algorithm proposed by Yang et al. [29], the space-filling curve is trimmed according to the underlying geometry, representing a square cross-section of A = 22.5 × 22.5 mm2 . The applied exposure strategy is characterized by an increased hatch distance of distinct space-filling curves, schematically displayed in Fig. 1.1. Fractal exposure strategies are implemented applying iterative programming, allowing for the scale-invariant as well as local adaption of structural exposure properties. The applied exposure strategy aims to obtain a discrete exposure of particular exposure vectors, inhibiting the mechanical interaction of distinct, consecutively exposed vectors. Based on a successive phase-shifted exposure of a distinct layer, previously unconnected, re-solidified melt pools are connected, forming a closed layer. The underlying phase shift corresponds to a biaxial, equidistant shift of underlying space-filling curves, yielding an effective hatch distance, corresponding to the minimum distance of two exposure vectors, of deffective = dhatch /2. Therefore, the superposed, temporally separated exposure allows for obtaining a homogeneous layer formation while minimizing shrinkage-induced deflections due to the mesoscale compensation of crystallization shrinkage. Given the constant laser spot diameter, the applied hatch distance represents a structural influence on the desired discretization of distinct exposure vectors. Fig. 1.1 Schematic depiction of the applied exposure strategy, depicting the superposition of discrete phase-shifted, subsequently exposed color-coded fractal exposure structures

1 Accelerated Non-Isothermal Powder Bed Fusion of Polypropylene … Table 1.1 Processing parameters applied for the non-isothermal powder bed fusion of polymers

7

Parameter set

Hatch distance (mm)

Laser power (W)

1

0.3

11.00

2

0.3

12.25

3

0.3

13.50

4

0.3

14.75

5

0.3

16.00

1.3.2 Design of Experiments For investigating the non-isothermal layer formation, the applied laser power is varied in discrete steps, displayed in Table 1.1. An exposure speed of 1,500 mm s−1 , a part bed temperature of 25 °C and a laser focus diameter of 0.5 mm, exhibiting a Gaussian intensity distribution, are kept constant throughout the process. A hatch distance of 0.3 mm is applied for implementing a parameter-dependent discretization of the formation and crystallization of emerging melt pools under non-isothermal conditions. Polypropylene powder of type PP nat 01, BASF SE, Ludwigshafen, Germany, exhibiting a melting peak of 137.2 °C ± 0.29, is used as the underlying model material for the manufacturing of single layers.

1.3.3 Thermographic Process Monitoring Inline process monitoring is based on the thermographic characterization of emerging temperature fields using a thermographic camera of the type VELOX 1310 k SM, IRCAM GmbH, Erlangen, Germany, applying a frame rate of 355 Hz and an isotropic spatial resolution of 140 μm. Based on a preceding calibration, an emission coefficient of ε = 0.805 is applied. For determining the temporal decay of emerging temperatures, mean temperatures of a square region of 1 × 1 mm2 , located in the center of a particular square cross-section, are used for subsequent analysis.

1.3.4 Thermal and Morphological Analysis Thermal characterizations of manufactured specimens are based on DSC measurements for investigating process-specific thermal material properties, using a device of type TA Instruments DSC 2500 (TA Instruments, New Castle, DE, USA). A heating rate of dT/dt = 60 K min−1 is applied for optimizing the sensitivity of the measurement. Complementary morphological analysis is conducted using polarized light microscopy, applying thin cuts of 10 μm thickness. A microscope of type

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Axio Imager 2 (Carl Zeiss Microscopy GmbH, Oberkochen, Germany) is used for investigating the part morphology.

1.3.5 Infrared-Spectroscopic Material Characterization For identifying emerging process-dependent crystalline phase compositions, ATRFTIR spectroscopy is used, applying an ATR spectroscope of type Invenio FTIR, Bruker Corp. Billerica, USA, specifying a spectral range from ν = 400 cm−1 to 4,000 cm−1 .

1.4 Results and Discussion 1.4.1 Thermal Process Characteristics Using thermographic imaging, interdependencies of the applied laser powder and the peak melting temperature as well as the crystallization temperature can be derived. Given the fractal structure, an invariance of locally emerging temperature fields [30] towards the exposure sequence and, therefore, the underlying geometry can be observed. Evident in Fig. 1.2, an increased laser power is correlated with increased peak temperatures as well as varying cooling conditions, implicitly influencing the thermal material exposure. The influence of the applied laser power indicates a thermal interaction of distinct, consecutively exposed exposure vectors next to each other on emerging temperature fields. In this regard, Greiner et al. [8] discussed the geometry-dependency of heat built-up on resulting temperature fields, describing a considerable proportion of the observed thermal energy to rely on the thermal interaction of consecutively exposed scan paths. Displayed in Fig. 1.2b, the applied laser power has a significant effect on the extent of supercooling. The peak crystallization temperature is determined based on the crystallization-induced local minimum of the heating rate, being induced by the exothermic crystallization process. Based on the applied parameter variation, a quasi-linear relation of the emerging peak crystallization temperature and the underlying laser power can be observed for both exposure cycles. The second exposure step, applied subsequent to the initial exposure, depicts a significant increase of detected peak temperatures, correlated with reduced cooling rates. Elevated crystallization peak temperatures are assumed to rely on locally elevated powder bed temperatures, induced by the initial exposure cycle. Furthermore, an adapted laser-matter interaction of previously re-solidified and densified material, leading to an increased extent of superficial energy absorption [31] and a subsequent slowed cooling, may influence emerging crystallization peak temperatures. With regard to a reference crystallization temperature of 100.1 °C ± 0.18 (dT/

1 Accelerated Non-Isothermal Powder Bed Fusion of Polypropylene …

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Fig. 1.2 Overview of thermographically determined transient temperatures in dependence of the applied laser power and b corresponding parameter-dependent peak crystallization temperatures, n =5

dt = −10 K min−1 ) assessed by means of ex situ DSC measurements, observed deviations from the aforementioned linear relation indicate the emergence of a saturation level for applying increased laser power levels. Therefore, based on thermographic investigations of process-dependent supercooling, an interdependence of the applied processing parameters, material-specific crystallization kinetics and the thermal interaction of discrete exposure cycles can be derived, influencing the discretized crystallization of distinct exposure paths and the corresponding processing capability of the applied non-isothermal processing strategy.

1.4.2 Interdependence of Process Characteristics and Thermal Material Properties A considerable extent of supercooling, derived from thermographic investigations, is correlated with the occurrence of metastable crystalline modifications [21, 32]. Based on the initial heating run, displayed in Fig. 1.3, no occurrence of exothermic coldcrystallization [33] can be observed, indicating a neglectable extent of metastable mesomorphic modifications. Furthermore, no significant influence of the applied laser power on the emerging melting enthalpy and the peak melting temperature can be observed, indicating a similar degree of crystallinity independent of applied processing parameters.

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Fig. 1.3 Differential calorimetric measurements of samples in dependence of the applied laser power and the underlying hatch distance

1.4.3 Morphological Part Properties Considering the previously discussed rapid cooling in non-isothermal PBF-LB/ P, affecting emerging crystallization temperatures, effects on formed microstructural superstructures and corresponding crystalline properties are evident. Based on the qualitative comparison of emerging crystalline superstructures, a considerable influence of applied processing parameters on formed crystalline structures can be derived, evident in Fig. 1.4. The parameter-dependent formation of varying crystalline superstructures is consistent with previously discussed thermographic observations, indicating the cooling-rate dependent formation of morphological variances. In accordance with DSC measurements, showing no occurrence of metastable phases, infrared spectroscopic measurements exhibit a distinct peak at 841 cm−1 ,

Fig. 1.4 Polarization micrographs of emerging parameter-dependent crystalline superstructures

1 Accelerated Non-Isothermal Powder Bed Fusion of Polypropylene …

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Fig. 1.5 ATR-FTIR spectrum of manufactured specimens in dependence of the applied laser power

being associated with the formation of α/β-crystals, displayed in Fig. 1.5. In contrast, Qian et al. [32] identified a peak associated with the formation of a mesomorphic phase to occur at a wave number of 842.5 cm−1 , hence confirming the neglectable presence of metastable mesomorphic fractions. Considering significantly reduced crystallization peak temperatures observed during the initial crystallization process, thermographic findings indicate the temporary formation of the mesomorphic phase based on high supercooling, followed by a laser-induced conversion into the α-phase. The proposed intermediate formation of a mesomorphic phase is in accordance with both infrared spectroscopic investigations and corresponding aforementioned DSC characterizations, indicating no presence of metastable phase compositions. In addition, a merely insignificant correlation of the observed peak frequency and the underlying laser power indicates the facilitated formation of the mesomorphic phase using increased laser power levels. Given the two-step exposure process, an increased energy density is correlated with an elevated extent of molten polymer formed during the initial exposure cycle, depicting recrystallization peak temperatures below 75 °C. Although a reduced laser power may contribute to the initial formation of a mesomorphic phase due to increased cooling rates, the quantitative extent of the formed mesomorphic phase is assumed to prevail for increased energy density levels due to an increased overall mass of re-solidified polymer. Consequently, the applied non-isothermal processing strategy allows for obtaining α-phase fractions by means of laser-based in-process conversion of intermediately formed metastable material phases, not accessible by quasi-isothermal crystallization from the melt [27].

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1.5 Conclusion and Outlook Non-isothermal powder bed fusion of polymers offers the opportunity for manufacturing polymer-based components at reduced ambient temperatures, considerably reducing thermal material exposure and interlinked material aging. Within the present paper, a novel type of exposure strategies based on consecutively exposed, spatially superposed fractal space-filling curves for considerably accelerated non-isothermal processing has been demonstrated. Using the Peano curve as the underlying fractal structure for the processing of polypropylene, a homogeneous layer formation can be obtained while applying a powder bed temperature of TB = 25 °C. Thermographic in situ investigations reveal a significant influence of the applied laser power on emerging peak temperatures and resulting cooling kinetics, implicitly influencing the extent of supercooling. Applying microscopic and infrared spectroscopic investigations, thermographically observed varying crystallization temperatures are correlated with an adapted crystalline superstructure. Process-induced increased supercooling inhibits the formation of regular spherulites while influencing the extent of mesomorphic phases formed during the processing. Based on the dual-step exposure, a laser-induced in-process conversion of an intermediately formed mesomorphic phase into stable phase modifications. Consequently, the presented findings show the applicability of discretized exposure strategies for the considerably accelerated additive manufacturing of polymers by a factor of 25 relative to previously proposed quasisimultaneous processing. Furthermore, a route for the targeted adaption of material phases not accessible by means of traditional processing can be demonstrated, representing a foundation for locally varied material properties. Acknowledgements Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Project-ID 61375930 SFB 814–“Additive Manufacturing”, subproject B03.

References 1. Drummer, D., Greiner, S., Zhao, M., Wudy, K.: A novel approach for understanding laser sintering of polymers. Addit. Manuf. 27, 379–388 (2019) 2. Soldner, D., Greiner, S., Burkhardt, C., Drummer, D., Steinmann, P., Mergheim, J.: Numerical and experimental investigation of the isothermal assumption in selective laser sintering of PA12. Addit. Manuf., 101676 (2020) 3. Soldner, D., Steinmann, P., Mergheim, J.: Modeling crystallization kinetics for selective laser sintering of polyamide 12. GAMM-Mitteilungen 44, e202100011 (2021) 4. Wudy, K., Drummer, D.: Aging effects of polyamide 12 in selective laser sintering: Molecular weight distribution and thermal properties. Addit. Manuf. 25, 1–9 (2019) 5. Paolucci, F., van Mook, M.J.H., Govaert, L.E., Peters, G.W.M.: Influence of post-condensation on the crystallization kinetics of PA12: From virgin to reused powder. Polymer (Guildf). 175, 161–170 (2019) 6. Schlicht, S., Greiner, S., Drummer, D.: Low temperature powder bed fusion of polymers by means of fractal quasi-simultaneous exposure strategies. Polymers (Basel). 14, (2022)

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7. Wegner, A., Witt, G.: Process monitoring in laser sintering using thermal imaging. In: 22nd Annual international solid free. fabr. symp—an addit manuf. conf. SFF 2011. (2011) 8. Greiner, S., Wudy, K., Wörz, A., Drummer, D.: Thermographic investigation of laser-induced temperature fields in selective laser beam melting of polymers. Opt. Laser Technol. 109, 569– 576 (2019) 9. Jain, P.K., Pandey, P.M., Rao, P.V.M.: Effect of delay time on part strength in selective laser sintering. Int. J. Adv. Manuf. Technol. 43, 117 (2008) 10. Drexler, M., Lexow, M., Drummer, D.: Selective laser melting of polymer powder—part mechanics as function of exposure speed. Phys. Procedia. 78, 328–336 (2015) 11. Sindinger, S.-L., Kralovec, C., Tasch, D., Schagerl, M.: Thickness dependent anisotropy of mechanical properties and inhomogeneous porosity characteristics in laser-sintered polyamide 12 specimens. Addit. Manuf. 33, 101141 (2020) 12. Pavan, M., Faes, M., Strobbe, D., Van Hooreweder, B., Craeghs, T., Moens, D., Dewulf, W.: On the influence of inter-layer time and energy density on selected critical-to-quality properties of PA12 parts produced via laser sintering. Polym. Test. 61, 386–395 (2017) 13. Schlicht, S., Jaksch, A., Drummer, D.: Inline quality control through optical deep learning-based porosity determination for powder bed fusion of polymers. Polymers (Basel). 14, (2022) 14. Paolucci, F., Baeten, D., Roozemond, P.C., Goderis, B., Peters, G.W.M.: Quantification of isothermal crystallization of polyamide 12: Modelling of crystallization kinetics and phase composition. Polymer (Guildf). 155, 187–198 (2018) 15. Lv, Y., Thomas, W., Chalk, R., Hewitt, A., Singamneni, S.: Experimental evaluation of polyphenylsulfone (ppsf) powders as fire-retardant materials for processing by selective laser sintering. Polymers (Basel). 13, (2021) 16. Chatham, C.A., Long, T.E., Williams, C.B.: Powder bed fusion of poly(phenylene sulfide) at bed temperatures significantly below melting. Addit. Manuf. 28, 506–516 (2019) 17. Niino, T., Haraguchi, H., Itagaki, Y., Iguchi, S., Hagiwara, M.: Feasibility study on plastic laser sintering without powder bed preheating. In: 22nd Annual International Solid Free. Fabr. Symp—An Addit. Manuf. Conf. SFF 2011, pp.17–29 (2011) 18. Kigure, T., Yamauchi, Y., Niino, T.: Relationship between powder bed temperature and microstructure of laser sintered PA12 parts. In: 2019 International solid freeform fabrication symposium. University of Texas at Austin, (2019) 19. Niino, T., Haraguchi, H., Itagaki, Y., Hara, K., Morita, S.: Microstructural observation and mechanical property evaluation of plastic parts obtained by preheat free laser sintering. In: 23rd Annual international solid free fabr symp—an addit manuf. conf. SFF 2012, pp. 617–628 (2012) 20. Mubarak, Y., Harkin-Jones, E.M.A., Martin, P.J., Ahmad, M.: Modeling of non-isothermal crystallization kinetics of isotactic polypropylene. Polymer (Guildf). 42, 3171–3182 (2001) 21. Piccarolo, S., Saiu, M., Brucato, V., Titomanlio, G.: Crystallization of polymer melts under fast cooling. II. High-purity iPP. J. Appl. Polym. Sci. 46, 625–634 (1992) 22. Schael, G.W.: A study of the morphology and physical properties of polypropylene films. J. Appl. Polym. Sci. 10, 901–915 (1966) 23. Gezovich, D.M., Geil, P.H.: Morphology of quenched polypropylene. Polym. Eng. Sci. 8, 202–209 (1968) 24. Kapur, S., Rogers, C.E.: Aging of quenched polypropylene. J. Polym. Sci. Polym. Phys. Ed. 10, 2107–2124 (1972) 25. De Rosa, C., Di Girolamo, R., De Ballesteros, O.R., Pepe, M., Tarallo, O., Malafronte, A.: Morphology and mechanical properties of the mesomorphic form of isotactic polypropylene in stereodefective polypropylene. Macromolecules 46, 5202–5214 (2013) 26. De Rosa, C., Auriemma, F., Di Girolamo, R., de Ballesteros, O.R.: Crystallization of the mesomorphic form and control of the molecular structure for tailoring the mechanical properties of isotactic polypropylene. J. Polym. Sci. Part B Polym. Phys. 52, 677–699 (2014) 27. Mollova, A., Androsch, R., Mileva, D., Gahleitner, M., Funari, S.S.: Crystallization of isotactic polypropylene containing beta-phase nucleating agent at rapid cooling. Eur. Polym. J. 49, 1057–1065 (2013)

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28. Peano, G.: Sur une courbe, qui remplit toute une aire plane. Math. Ann. 36, 157–160 (1890) 29. Yang, J., Bin, H., Zhang, X., Liu, Z.: Fractal scanning path generation and control system for selective laser sintering (SLS). Int. J. Mach. Tools Manuf. 43, 293–300 (2003) 30. Greiner, S., Schlicht, S., Drummer, D.: Temperature field homogenization by fractal exposure strategies in laser sintering of polymers. In: Proceedings of the 17th Rapid Tech 3D Conference Erfurt, Germany, 22? 23 June 2021, pp. 188–197. Carl Hanser Verlag GmbH & Co. KG (2021). 31. Osmanlic, F., Wudy, K., Laumer, T., Schmidt, M., Drummer, D., Körner, C.: Modeling of laser beam absorption in a polymer powder bed. Polymers (Basel). 10, (2018) 32. Qian, C., Zhao, Y., Wang, Z., Liu, L., Wang, D.: Probing the difference of crystalline modifications and structural disorder of isotactic polypropylene via high-resolution FTIR spectroscopy. Polymer (Guildf). 224, 123722 (2021) 33. Schawe, J.E.K.: Analysis of non-isothermal crystallization during cooling and reorganization during heating of isotactic polypropylene by fast scanning DSC. Thermochim. Acta. 603, 85–93 (2015)

Chapter 2

Understanding Geometry Dependent Temperature Fields in Laser Powder Bed Fusion of PA12 by Means of Infrared Thermal Imaging Sandra Greiner, Samuel Schlicht, and Dietmar Drummer

Abstract Laser powder bed fusion of polymers (LPBF) is a promising additive manufacturing technology that allows for the generation of complexly shaped parts with high mechanical properties. However, enhancing the reproducibility of part properties is one of the main challenges on the technologies way to industrialization. Therefore, the process has to be robust, the resulting part properties must be predictable and reproducible. The basic material-beam-interactions have to be analyzed to assess the impact of geometry dependent temperature history on part properties. Within this contribution, the influence of part geometry at constant surface area on temperature fields and layer formation is investigated. Besides quadratic cross-sections, triangular, round, oval and hollow structured geometries are considered. Infrared thermal imaging measurements were conducted during exposure to identify variations of the transient temperature levels. The fabricated parts were fundamentally characterized to correlate the temperature values to part properties. It is clearly visible that the scan vector length is decisive for the resulting temperature level and an impact on the part properties is observable at monolayer level. The results indicate that exposure parameters should be optimized with regard to the particular layer geometry, especially for high variations of the cross-sectional area. Keywords Additive manufacturing · Laser powder bed fusion · Thermography

S. Greiner (B) · S. Schlicht · D. Drummer Institute of Polymer Technology, Friedrich-Alexander-Universität Erlangen-Nürnberg, Am Weichselgarten 10, 91058 Erlangen, Germany e-mail: [email protected] Collaborative Research Center 814—Additive Manufacturing, Am Weichselgarten 10, 91058 Erlangen, Germany © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_2

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2.1 Introduction and State of the Art Laser powder bed fusion of polymers is an additive manufacturing (AM) technology that allows for the generation of parts and assemblies with the highest possible degree of geometrical freedom. Compared to other AM technologies, end use parts with high dimensional accuracy and mechanical strength can be achieved without the need of support structures [1]. However, this geometrical freedom leads to certain drawbacks, such as limited predictability or reproducibility of part properties [2]. Both target values are majorly influenced by the thermal history during part generation [3– 5]. The parts temperature history is initially induced by laser energy input, affected by the build chamber heating system and the resulting heat flows. The heat flows are geometrically affected by multiple interacting parameters, such as the build chamber geometry [6], the packing density [7], the part geometry itself [8, 9] and the positioning of the part [10]. Importantly, those parameters cannot easily be varied without affecting each other. For that reason, fundamental research has to be performed in order to understand the basic thermal interrelations between exposure process and part geometry. Therefore, gaining insights on the basic material-beam-interactions is necessary to realize geometry invariant part processing that allows for predictable part generation with enhanced reproducibility. Regarding the state of the art, measurements of the temperature development in dependence on scan vector length were performed in [11], whereas in [2] the maximum temperature values could be correlated to the part density. The influence of laser exposure and impact time on thermal fields and part morphology was shown for different quadratic cross-sections in [12], whereas the variation of scan vector length and number of parallel hatches at different surface areas was part of [13]. Jaksch et al. [8] investigated thin-walled parts with different cross-sectional geometries and volume-surface ratios. It was found, that the part mechanics were drastically affected by the cross-sectional area, whereas the impact of the cross-sectional shape was not of statistical significance. However, the variation of the part geometry at equal surface area has not been studied, yet. This knowledge on the temperature fields, their geometric and parametric variations is crucial for deeply understanding and adapting the LPBF process.

2.2 Experimental The experiments were performed using the Polyamide 12 (PA12) powder PA 2200 from EOS GmbH (Krailling, Germany). A physical powder mixture of virgin powder and low thermally aged overflow powder was prepared in a lab mixer, being a stable process condition for the performed processing experiments. A self-build, almost freely parameterizable research LPBF system, which is equipped with an infrared thermal camera VELOX 1310 k SM (IRCAM, Erlangen), was used for the experiments. The research system is equipped with a 50 W CO2 -laser with a spot size of

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Fig. 2.1 Schematic depiction of the applied sample geometries: Dense geometries (left) and hollow geometries (right)

approximately 500 μm. The y-parallel meander hatching strategy was applied to the different dense and hollow geometries (see Fig. 2.1). The geometries are characterized by similar nominal cross-sectional areas (see Table 2.1), but varying scan vector lengths and number of scan vectors (see Table 2.2). Although a major influence from processing parameters, such as scan speed or laser power can be expected, the exposure parameters were kept constant at an optimum level [9]. The laser power was set to 16 W, the scan speed was 2 m/s and the hatch distance was 0.2 mm, resulting in an energy density of 40 mJ/mm2 . Table 2.1 Basic geometrical description of the manufactured single-layer test specimens Geometry

Equation

Dimensions in mm

Exposed area in mm2

Square

a2

a = 10.0

100.0

Triangle

½·a·b

a = 15.2, b = 13.2

100.3

Circle

π·r2

r = 5.6

Oval

π·a·b

a = 3.0, b = 10.6

100.0

Hollow square

a1 2 -a2 2

a1 = 15.0, a2 = 11.2

100.0

Hollow triangle

½·a1 ·b1 – ½·a2 ·b2

a1/ a2 = 20.0/13.0, b1 /b2 = 17.3/11.3

100.0

Hollow circle

π·r1

r1 = 15.0, r2 = 9.9

100.1

Hollow oval

π·a1 ·b1 – π·a2 ·b2

a1 /a2 =10.0/6.8, b1 /b2 = 6.0/4.1

100.3

2–

π·r2

2

99.9

Table 2.2 Resulting geometry dependent influences on the single-layer test specimens Geometry

Number of parallel scan vectors

Exposure time in s

Measured area in mm2

Square

51

0.35

94.7 ± 2.7

Triangle

72

0.37

96.5 ± 0.2

Circle

58

0.35

92.7 ± 1.6

Oval

110

0.37

89.9 ± 1.7

Hollow square

77

0.51

85.3 ± 5.9

Hollow triangle

87

0.45

77.2 ± 1.0

Hollow circle

77

0.46

95.6 ± 6.9

Hollow oval

103

0.49

87.8 ± 6.8

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In addition, the layer height was set to 100 μm. The build chamber temperature was 170 °C. During processing, thermographic videos with a framerate of 600 Hz are acquired, limiting the spatial resolution to 160 × 160 pix. The thermographs of the different geometries are evaluated for three different time steps: ten frames after the beginning of exposure (begin), at half exposure time (center) and ten frames prior to the end of exposure (end). To simplify the interpretation of the results, the emission coefficient is matched to the build chamber temperature and set to a value of 0.9. According to the number of acquired frames during exposure, the exposure time can be determined (see Table 2.2). In addition, the temperature profiles of the maximum temperature and the geometry integrated mean temperature are evaluated in dependence of time. Furthermore, the maximum temperatures of both graphs are compared with respect to the present cross-sections. For every geometry, three samples were prepared. After exposure, the fabricated monolayers were carefully extracted from the powder bed. Surrounding powder was gently removed with a brush and the sample mass was determined using precision scales. The layer height was determined by using microtome thin sections of the fabricated parts. Using the Zen core program, the layer height was microscopically (Axioplan, Zeiss) determined at ten points in the part center with equidistant spacing of 50 μm. The area of the parts was evaluated using microscopic projections of the parts.

2.3 Results and Discussion Within this paragraph, the results of the geometry dependent exposure will be discussed. In Fig. 2.2, the thermographs of the four dense geometries at three different time steps are shown. It is clearly visible that within the first few hatches the lowest temperatures can be measured. As the scan vectors of circular and oval scan patterns are the smallest, higher temperatures are present, due to a lower chance of intermediate material cooling. Within the center position, higher but scan vector length dependent maximum temperatures can be measured for all geometries. After a defined number of scan vectors, the maximum temperature of the quadratic geometry is stable, whereas for geometry dependent scan vector lengths, a temperature change is observable. Regarding the triangular geometry, the highest temperatures can be detected at the end of exposure, as the scan vector length is decreasing over time. Regarding hollow geometries (see Fig. 2.3), the temperature levels at the beginning of exposure are similar to those of dense parts as no significant geometry change is present, yet. As the part thickness of the different hollow structures is comparable, similar equivalent temperature levels are present in the center of the exposed areas. The temperature levels are 5–10 K higher as compared to the dense parts, as lower laser return times can be expected. At the end of exposure, again temperature levels, which are comparable to the temperatures of the dense geometries, are present.

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Fig. 2.2 Thermographic images of the exposure progress for dense testing geometries at different points in time

In Fig. 2.4, the temperature profiles during exposure of the different geometries are compared. For the quadratic area, the maximum temperature increases at first and remains stable after a certain number of scan vectors. The maximum temperature of the triangle increases with decreasing scan vector length and therefore the reduction of intermediate cooling time. The curves of the round and oval parts seem to be almost identical. Although the scan vectors are shorter than the scan vector lengths of the square or of the triangle geometry, only a slight initial increase in temperature is observable. Again, an initial temperature balance has to be established until the material-beam-interactions fully develop into geometry dependent behaviour. This can be seen at the end of exposure for the round shapes, as sharp temperature increases are noticeable. The mean temperature profiles of the dense geometries increasingly rise during monolayer exposure as the proportion of the exposed area increases over time. For hollow quadratic geometries, lower temperatures can be assigned to the higher scan vector length at the beginning and the end of exposure. The center temperatures are several degrees higher, due to shorter laser return times. The maximum temperatures are expected to be measured shortly after the change of direction. However, only a slighter impact on the maximum temperature profiles of the other geometries is observable. Nevertheless, the mean temperature curves are shifted significantly. Compared to the dense structures, the profiles of the hollow structures are characterized by a similar curve progression in the beginning, followed by a flatter temperature

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Fig. 2.3 Thermographic images of the exposure progress for hollow testing geometries at different points in time Square

Dense Hollow Mean Max

240 220 200 180 160

0

1

2

260

Temperature in °C

Temperature in °C

260

Dense Hollow Mean Max

220 200 180 160

3

Triangle

240

0

1

Time in s Circle

Dense Hollow Mean Max

240 220 200 180 160

0

1

Time in s

2

3

260

Temperature in °C

Temperature in °C

260

2

3

Time in s Oval

Dense Hollow Mean Max

240 220 200 180 160

0

1

Time in s

2

3

Fig. 2.4 Maximum and mean temperature profiles of exposed testing geometries: square (top left), triangle (top right), circle (bottom left) and oval (bottom right)

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Mean temperature in °C

200

TB = 170 °C 195 n = 3

Dense Hollow

190 185 180 175 170

vs = 2 m/s, PL = 16 W, ED = 40 mJ/mm²

Maximum temperature in °C

increase and a steep temperature increase at the end of exposure. The most notable difference lies in the higher exposure time that is present for the fabrication of all hollow structured geometries. This can be attributed to the higher number of scan vectors and of changeover procedures. In addition, the faster cooling of the hollow cross-sectional surfaces is remarkable. When considering monolayers, this can be attributed to the shorter paths for heat transfer and a higher surface to volume ratio. The maximum temperatures of the mean and maximum temperature profiles are quantified in Fig. 2.5. It is observable that the mean temperature peaks are comparable for dense and hollow geometries, respectively. However, higher temperatures are present for the dense geometries. This can be explained by the difference in measured exposure time. Within fractions of seconds, the temperature locally decreases to build chamber temperature, leading to a time dependency of mentioned mean temperature values Fig. 2.6.

square triangle circle oval Cross-sectional geometry

230

Dense Hollow

TB = 170 °C 225 n = 3 220 215 210 205 200

vs = 2 m/s, PL = 16 W, ED = 40 mJ/mm²

square triangle circle oval Cross-sectional geometry

Fig. 2.5 Maximum temperatures of the mean temperature profile (left) and the maximum temperature profile (right) during exposure

25

TB = 170 °C n=3

Dense Hollow

20 15 10

vs = 2 m/s, PL = 16 W, ED = 40 mJ/mm²

square triangle

circle

oval

Cross-sectional geometry

250 Layer height in µm

Sample mass in mg

30

TB = 170 °C 225 n = 3

Dense Hollow

200 175 150 125 100

vs = 2 m/s, PL = 16 W, ED = 40 mJ/mm²

square triangle

circle

oval

Cross-sectional geometry

Fig. 2.6 Sample mass (left) and layer height (right) of the fabricated monolayer parts

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Regarding maximum temperatures, the highest values can be measured for the hollow square parts. However, this value is afflicted with high standard deviations that are induced by time dependent temperature controls within the laser sintering system. Nevertheless, only slight differences regarding the maximum temperatures can be determined, which might be caused by an insufficient recording frequency. With regard to the monolayer properties, similar sample masses can be measured, whereas greater differences occur in the measured layer height, which ranges between 120 μm and 180 μm. Concerning the averaged values, a correlation with the maximum temperature levels seems to be present. The analysis of the measured area of the exposed part in Table 2.2 demonstrates high deviations, which can be explained by monolayer curling, which occurred due to an early part removal. Especially for hollow geometries, undersizing is more pronounced due to a lower curling resistance, which is propagated by a higher length of part edges, which allow free shrinkage.

2.4 Conclusion In the state of the art, major differences in resulting part properties could be identified by the variation of the cross-sectional area of quadratic parts. Within the present manuscript, the influence of variations of the scan vector length on thermal and monolayer properties was analyzed for similar cross-sections, highlighting locally variable scan vector lengths and different part geometries. It was found that a geometry dependent temperature history is resulting from geometrical variance and scan path design. However, for monolayers of identical cross-sectional area only a minor influence is visible on part properties, such as the sample mass or the layer height. Therefore, an adaption of process parameters is especially necessary for parts with a high variability in cross-sectional area. A subdivision of large cross-sectional areas into smaller surface elements could therefore contribute significantly to the unification of the property profile. Regarding this approach, a larger impact from exposure sequence can be expected. Future work will deal with the interacting influence of multilayer parts with uniform cross-section and the process monitoring of parts with various part cross-sections in build height. Furthermore, the impact of part segmentation and nearby exposure areas will be analyzed. Acknowledgements Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)-Project-ID 61375930-SFB 814 “Additive Manufacturing” TP B03 and T03.

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References 1. Schmid, M.: Laser sintering with plastics: technology, processes, and materials. Carl Hanser Verlag, München (2018) 2. Wegner, A., Witt, G.: Understanding the decisive thermal processes in laser sintering of polyamide 12. In: Proceedings of PPS-30, AIP Conference Proceedings 1664, pp. 160004– 1–160004–5. (2015) 3. Josupeit, S., Schmid, H.-J.: Temperature history within laser sintered part cakes and its influence on process quality. Rapid Prototyp. J. 22(5), 788–793 (2016) 4. Kiani, A., Khazaee, S., Badrossamay, M., Foroozmehr, E., Karevan, M.: An investigation into thermal history and its correlation with mechanical properties of PA12 parts produced by selective laser sintering process. J. Mater. Eng. Perform. 29, 832–840 (2020) 5. Taylor, S., Beaman, J., Fish, S.: Thermal history correlation with mechanical properties for polymer selective laser sintering (SLS). In: Proceedings of the 28th annual international solid freeform fabrication symposium, The University of Texas at Austin, vol. 28, pp. 1448–1463. (2017) 6. Li, X., Van Hooreweder, B., Lauwers, W., Follon, B., Witvrouw, A., Geebelen, K., Kruth, J.-P.: Thermal simulation of the cooling down of selective laser sintered parts in PA12. Rapid Prototyp. J. 24(7), 1117–1123 (2018) 7. Josupeit, S.: On the influence of thermal histories within part cakes on the polymer laser sintering process. Shaker Verlag, Düren (2019) 8. Jaksch, A., Cholewa, S., Drummer, D.: Understanding geometry dependent part behavior of thin walled structures in powder bed fusion of polymers. In: Procedia CIRP, 12th CIRP conference on photonic technologies, vol. 111, pp. 28–32. (2022) 9. Greiner, S., Jaksch, A., Cholewa, S., Drummer, D.: Development of material-adapted processing strategies for laser sintering of polyamide 12. Adv. Ind. Eng. Polym. Res. 4(4), 251–263 (2021) 10. Launhardt, M., Fischer, C., Drummer, D.: Research on the influence of geometry and positioning on laser sintered parts. Appl. Mech. Mater. J. 805, 105–114 (2015) 11. Wegner, A., Witt, G.: Process monitoring in laser sintering using thermal imaging. In: Proceedings of the 22nd annual international solid freeform fabrication symposium, The University of Texas at Austin, vol. 22, pp. 405–414. (2011) 12. Greiner, S., Wudy, K., Wörz, A., Drummer, D.: Thermographic investigation of laser-induced temperature fields in selective laser beam melting of polymers. Opt. Laser Technol. 109, 569– 576 (2019) 13. Greiner, S., Drummer, D.: Understanding aspect ratio effects in laser powder bed fusion of polyamide 12 by means of infrared thermal imaging. In: Procedia CIRP, 12th CIRP conference on photonic technologies, vol. 111, pp. 253–256. (2022)

Chapter 3

Additively Manufactured Glass Fiber Reinforced Polymeric (GFRP) Structures for High Performance Applications ¯ Rimašauskien˙e , Marius Rimašauskas , Tomas Kuncius , Ruta and Magdalena Mieloszyk

Abstract Recently, significantly increased demand for new lightweight products inspired by reduction of the production costs and other resources allows additive manufacturing (AM) to occupy an increasingly important place in the global manufacturing (GM) environment. Additive manufacturing technologies are important because can help to reduce delivery time, and cost, and improve the design. Technologies long time associated with the production of polymeric parts for visualization now are showing very huge potential in the manufacturing of prototypes and small series of products from ceramics, metals and, composite materials. Moreover, 3D printed plastic parts have relatively poor mechanical properties and rarely can be used as structural parts for high-performance applications. Therefore, additive manufacturing of composite structures reinforced with short and especially long fibers is increasingly important in the modern manufacturing environment. This research focuses on the additive manufacturing of continuous glass fiber-reinforced polymeric (GFRP) structures. As a matrix material, polylactic acid (PLA) was used. New developed method for the impregnation of glass fibers and 3D printing process of the glass fiber reinforced structures is introduced in the article. Material extrusion process as a base to design an efficient AM manufacturing method was used. Developed methods were validated and samples for mechanical performance investigation of GFRP structures were printed. Tensile and flexural tests showed significant improvement of the mechanical properties of composites compared with the pure plastic parts. For example, the average tensile and flexural strength of the samples were 263 MPa and 145 MPa, while pure PLA results are only 46.6 MPa and 85.1 MPa respectively.

M. Rimašauskas (B) · T. Kuncius · R. Rimašauskien˙e Kaunas University of Technology, Student˛u 56, 51424 Kaunas, Lithuania e-mail: [email protected] M. Mieloszyk Institute of Fluid Flow Machinery, Fiszera 17, 80-231 Gdansk, Poland © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_3

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Keywords Glass fiber reinforced composites · Mechanical testing · Fiber impregnation

3.1 Introduction Additive manufacturing also known as 3D printing is a group of technologies used for the production of complex parts and products from various materials. Layer-by-layer production allows to minimize wastes and delivery time and increase flexibility and profitability. Fused deposition modelling (FDM) or Fused filament fabrication (FFF) is an additive manufacturing process based on material extrusion. During printing process, the thermoplastic material filament is fed to the printing head where it is melted and extruded on a building plate according to CNC controlled toolpath. Most of thermoplastics can be 3D printed, however, polylactic acid (PLA), and acrylonitrile butadiene styrene (ABS) are most likely used. 3D printing of some thermoplastics can be very challenging because it requires higher temperatures of the printing head, printing bed and building chamber [1]. For example, printing head, bed and building chamber temperatures are 365, 160, 80, and 420 °C, 110 °C, 70 °C for PEI and PEEK materials respectively [2]. Such materials have very specific properties as high chemical resistance and can be used for functional applications. When 3D printed parts should be used for structural applications they should be with improved mechanical properties. Short carbon fiber reinforced composites are high-performance materials [3] however, they cannot compete with continuous fiber reinforced composites [4]. When short fibers are added into PLA, ABS, PA and other materials, the mechanical properties of composites are enhanced in some extent, however with some thermoplastics, like TPI mechanical properties become even worse [5]. Although most researchers are concentrating on the development of processes for continuous carbon fiber reinforced polymers (CFRP) printing it is also worth to mention that continuous glass fiber reinforced polymers (GFRP) have their own advantages [6]. For example, glass fiber is a dielectric material and has high tensile strength and good chemical resistance [7], moreover the most important is that price of glass fiber is significantly lower in comparison with carbon fiber. In this research, continuous glass fiber reinforced composites were produced by using a fused deposition modelling process. To define the mechanical properties of composite materials, tensile, flexural tests were performed, and the main mechanical properties were calculated. Moreover, to improve mechanical properties and adhesion between continuous glass fiber and matrix material impregnation of fiber was used [8]. The process was developed earlier and tested by impregnating continuous carbon fiber with different solution concentrations, however, it was never used with continuous glass fiber. Presented methodology and received results can be used for the production of low-cost and high-mechanical performance glass fiber reinforced composites. Meanwhile, the developed impregnation process and received mechanical properties can be used for the further development of the technology which could be used in the automotive or aerospace industry or medical sector.

3 Additively Manufactured Glass Fiber Reinforced Polymeric (GFRP) …

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3.2 Methodology 3.2.1 Materials The main thermoplastic material used in this study was polylactic acid (PLA) purchased from Polymaker (China). This material was used as matrix material in all produced samples. According to the producer tensile strength of the material is 46.6 ± 0.9 MPa, Young’s modulus is 2636 ± 330 MPa, flexural strength is 85.1 ± 2.9 MPa while the glass transition temperature is 61 °C and recommended nozzle temperature and printing speed are 190–230 °C and 40–60 mm/min respectively. The glass fiber strand EC11-300 used for the reinforcement was provided by SaintGobain Vetrotex (Check Republic). PLA biopolymer pellets 3D580 from NatureWorks (USA) were used as material for glass fiber impregnation. The tensile strength and Young’s modulus of this material are 51 MPa and 2315 MPa respectively.

3.2.2 Printing of GFRP Composites For the printing of glass fiber reinforced composites samples Mecreator 2 3D printer made by Geeetech (China) was used. However, mentioned 3D printer is not designed for printing of composite materials that is why significant changes of the printing head is needed in order to print composites. The research scheme is presented in Fig. 3.1, while the printing scheme is in Fig. 3.2. As can be seen in Fig. 2c, glass fiber is coming from the spool and is fed to the printing head directly by using an additional input channel. PLA material is fed to the printing head by using a traditional filament feeding system.

Fig. 3.1 Research scheme

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Fig. 3.2 3D printing scheme; a Experimental setup, b Photo of the printing head, c Scheme of the printing head

A mixture of both materials is performed in the heating element. The main problem related to the in situ—impregnation process is related to insufficient impregnation of glass fiber. The main reasons for poor impregnation are too fast movement of glass fiber in the printing head and too high viscosity of PLA material. To solve this problem, additional impregnation of glass fiber can be used. In this article additional impregnation of glass fiber with a solution made from PLA 3D850 and solvent methylene chloride (CH2 Cl2 ) from producer Eurochemicals (Lithuania) was used. The impregnation procedure was presented by the authors and allows to improve mechanical properties of the composite structure by increasing adhesion between the matrix and reinforcement [8]. In the printing head heated up to 220 °C matrix material and material used for impregnation are melted and mixed to create a strong bond between them. Later composite material is extruded through the Ø1.5 mm diameter nozzle on the heated printing bed. Simplify3D software was used in order to prepare the toolpath and adjust printing parameters. The most important printing parameters are presented in Table 3.1.

3.2.3 Characterization The main mechanical properties of the produced composite specimens were defined by using tensile and flexural tests. Specimens were printed according to the main requirements and recommendations of ASTM D3039 and ASTM D7264 standards. For each test, five specimens were printed with a constant rectangular cross-section.

3 Additively Manufactured Glass Fiber Reinforced Polymeric (GFRP) … Table 3.1 Printing parameters

Nozzle diameter, mm

29

1.5

Line width, mm

1.3

Layer height, mm

0.5

Extrusion multiplier, %

0.8

Print-head temperature, °C

220

Printing bed temperature, °C

80

Cooling, %

100

The nominal dimensions of the samples were 4 × 12 × 150 mm and 2 × 12 × 150 mm for flexural and tensile tests respectively. However, as it is very well known, fused deposition modelling is not the most accurate process from additive manufacturing technologies. Therefore, each specimen was individually measured and weighed. All measured results were used for the calculation of mechanical properties. Even though tabs are not required according to the standard, they were printed and glued to both ends of tensile specimens. This was done because, during the tensile test, specimens were clamped in a manual wedge action gripper which generates significant pressure on composite and can negatively influence mechanical performance. Tabs were printed from PLA material with the size 50 × 12 × 2.5 mm and glued to the specimens by using cyanoacrylate glues. Testing speed was 1 mm/min and 2 mm/min for flexural and tensile tests respectively. For both tests, the same universal machine Tinius Olsen H25KT (United Kingdom) was used. During tensile tests, strains were determined by using a video extensometer VEM 300 (United Kingdom). Finally, for data acquisition and analysis software Horizon was used.

3.3 Results After mechanical tests, results were collected, and analysis was done. In the Table 3.2, the main results are presented. As can be seen, the maximum breaking force during the tensile test was 7998 N while the maximum tensile strength was 268 MPa. The standard deviation of breaking force and tensile strength was 182.5 N and 3.3 MPa respectively. Meanwhile, the variation coefficient of before mentioned parameters not exceeding 2.3% and the results can be evaluated as reliable. A little bit higher deviation of results can be seen in data of Young’s modulus, where the average value is 12296 MPa and the variation coefficient is 4.6%. Stress–strain curves of tensile tests are presented in Fig. 3a. As can be seen from this graph, the behavior of all glass fiber-reinforced specimens is the same or very similar during tension. However, not consistent results can be seen comparing data after flexural tests. Results in Table 3.2, show that there is a quite high deviation of the bending force and flexural strength.

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Table 3.2 Results of mechanical testing Results of flexural tests

Results of tensile tests Specimen no.

Max. force, N

Tensile strength, MPa

Young modulus, MPa

Specimen no.

Max. force, N

Flexural strength, MPa

1

7672

258.7

12,663

1

180

140.2

Flexural modulus, MPa 140.2

2

7968

268.0

11,791

2

237

183.8

183.8

3

7998

262.7

11,740

3

166

130.0

130.0

4

7578

264.2

12,228

4

134

107.2

107.2

5

7818

263.7

13,061

5

210

165.3

165.3

Average

7806.8

263.4

12,296.6

Average

185.4

145.3

9959.8

Standard deviation

182.5

3.3

567.4

Standard deviation

39.7

30.0

345.2

Variation coefficient, %

2.3

1.3

4.6

Variation coefficient, %

21.41

20.64

3.47

Tensile stress, MPa

Maximum bending force and flexural strength were 237 N and 145 MPa, moreover, the variation coefficient was more than 20%. These results can be clearly seen in Fig. 4a, where the stress–strain curves of each sample are presented. This indicates significant problems in the production process. Comparing flexural modulus results can be stated that deviation is acceptable. Such differences in flexural tests results can be explained by inconsistent extrusion of plastic material. It is worth to remind that unidirectional 0° composites were used during experiments. The nature of samples indicates that samples can be loaded with higher loads in tension because the direction of fibers and tension direction are the same. Moreover, from Fig. 3.4b it can be seen that the samples didn’t break completely, instead of rupture, delamination occurred in the samples and this can be another reason for the fluctuation of the results. 300 200 100 0 0

1 1 sample

Strain, % 2 sample

4 sample

5 sample

2 3 sample

(a) Fig. 3.3 Results of tensile test; a stress–strain curves, b failure modes

(b)

Flexural stress, MPa

3 Additively Manufactured Glass Fiber Reinforced Polymeric (GFRP) …

31

200 150 100 50 0 0

1

Strain, %

1 sample

2 sample

4 sample

5 sample

2

3 3 sample

a)

b)

Fig. 3.4 Results of flexural test; a stress–strain curves, b failure modes

It can be assumed, that in order to improve flexural properties amount of polymer should be increased and controlled precisely in the meantime increased attention to interlayer adhesion also should be paid too. After performing mechanical tests failure modes were determined according to the used standards. For tensile specimens GAT (grip-at tab-top) and GAB (grip-at tab-bottom) failure modes were dominant. However, some samples (No. 2) showed SGM (long. Splitting- at gage-middle) failure mode. When failure modes of flexural samples are evaluated is worth mentioning, that samples did not break completely. It can be stated that delamination occurred instead of rupture of the sample. It could happen due to poor interlayer adhesion or insufficient bonding between reinforcing material and matrix. Analytically calculated reinforcement content showed that produced specimens had about 36% of glass fiber by weight. It is clear that the amount of reinforcement material has a significant influence on the mechanical performance of composites, therefore 3D printing parameters can be adjusted in order to increase weight ratio of glass fiber in the composite structure.

3.4 Conclusions In this research work, continuous fiber-reinforced composites were prepared, and the main mechanical properties were determined. The average tensile strength and Young’s modulus of the 3D printed composite material are 263 MPa and 12,296 MPs respectively. Meanwhile, flexural strength and flexural modulus are 145 MPa and 9959 respectively. During analysis of the results, it was determined that the results of tensile tests are very stable however results of flexural tests have very high deviation. Analytically calculated glass fiber content in the samples is about 36% by weight and can be increased by adjusting layer height and line width.

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Acknowledgements This research was funded by a grant (No. S-M-ERA.NET-20-1) (project: “Additive Manufactured Composite Smart Structures with Embedded Fibre Bragg Grating Sensors”, acronym: “AMCSS”) from the Research Council of Lithuania.

References 1. Geng, P., Zhao, J., Wu, W., Ye, W., Wang, Y., Wang, S., Zhang, S.: Effects of extrusion speed and printing speed on the 3D printing stability of extruded PEEK filament. J. Manuf. Process. 37, 266–273 (2019) 2. Li, W., Sang, L., Jian, X., Wang, J.: Influence of sanding and plasma treatment on shear bond strength of 3D-printed PEI, PEEK and PEEK/CF. Int. J. Adhes. Adhes. 100(102614), (2020) 3. Tanabi, H.: Investigation of the shear properties of 3D printed short carbon fiber-reinforced thermoplastic composites. J. Thermoplast. Compos. Mater. 35(11), 2177–2193 (2022) 4. Maqsood, N., Rimašauskas, M.: Characterization of carbon fiber reinforced PLA composites manufactured by fused deposition modeling. Compos. Part C: Open Access. 4(100112), (2021) 5. Ye, W., Lin, G., Wu, W., Geng, P., Hu, X., Gao, Z., Zhao, J.: Separated 3D printing of continuous carbon fiber reinforced thermoplastic polyimide. Compos. Part A: Appl. Sci. Manuf. 121, 457– 464 (2019) 6. Chen, K., Yu, L., Cui, Y., Jia, Pan, K.: Optimization of printing parameters of 3D-printed continuous glass fiber reinforced polylactic acid composites. Thin-Walled Struct. 164(107717), (2021) 7. Saidane, E.H., Arnold, G., Louis, P., Pac, M.-J.: 3D printed continuous glass fibre-reinforced polyamide composites: Fabrication and mechanical characterisation. J. Reinf. Plast. Compos. 41(7–8), 284–295 (2022) 8. Rimašauskas, M., Kuncius, T., Rimašauskien˙e, R.: Processing of carbon fiber for 3D printed continuous composite structures. Mater. Manuf. Processes 34(13), 1528–1536 (2019)

Chapter 4

Effects of Printing Direction and Multi-material on Hardness of Additively Manufactured Thermoplastic Elastomers for Comfortable Orthoses and Prostheses Paweł Michalec , Sakine Deniz Varsavas , Florian Arbeiter , Robert Weidner , and Lisa-Marie Faller Abstract Medical assistive devices such as orthoses and prostheses (O&P) are essential for people with physical limitations to provide them with more active, productive and independent lives. However, studies show that the rejection rate of O&P is high, mostly due to the lack of comfort caused by improper fitting and the usage of rigid materials. Additive manufacturing makes possible development of a fully customized O&P with soft materials what can result in better fitting and more homogeneous distribution of loads on the limb. Therefore, in this study, the effects of printing direction on the hardness, and the thickness ratio between soft materials in multi-material printed samples on the combined hardness are investigated. The following materials are used to produce samples with the extrusion-based 3D-printer Pollen New PAM Series P: PLA, TPE 45 Shore-A and TPE 30 Shore-00. FurtherThe research leading to these results has received funding from the Federal Ministry for Digital and Economic Affairs (BMDW) within the framework of COIN “Aufbau”, 8th call of the Austrian Research Promotion Agency (FFG)—project number 884136 (iLEAD). This work is a part of “Smarter Leichtbau-4.0” and “Smarter Leichtbau-4.1” project funded by the European Regional Development Fund Grant (KWF-5462-30088-43455) and (KWF-5462-34039-49275), respectively. P. Michalec (B) · S. D. Varsavas · L.-M. Faller ADMiRE Research Center, Carinthia University of Applied Sciences, Europastrasse 4, 9524 Villach, Austria e-mail: [email protected] P. Michalec Institute for Mechatronics, University of Innsbruck, Technikerstraße 13, 6020 Innsbruck, Austria S. D. Varsavas · F. Arbeiter Materials Science and Testing of Polymers, Montanuniversitaet Leoben, Otto Gloeckel-Straße 2, 8700 Leoben, Austria R. Weidner Chair for Production Technology, Institute for Mechatronics, University of Innsbruck, Technikerstraße 13, 6020 Innsbruck, Austria © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_4

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more, 12 specimens are printed and tested with the Shore hardness method. As a result, the printing direction does not cause a significant change in the final hardness of the soft materials. The increased TPE 30 Shore-00 layer’s height in combined materials changes hardness from 68.9 to 48.0 Shore-00. In addition, the increased TPE 45 Shore-A thickness increases combined hardness from 60.9 to 76.4 Shore-00. The change in thickness of both materials causes almost linear change in combined hardness. These results can be used to achieve the optimum comfort for individuals and a rise in the acceptance rate of O&P. Keywords Orthosis · Prosthesis · Additive manufacturing · Thermoplastic elastomers · Hardness

4.1 Introduction Orthoses and prostheses (O&P) are wearable medical assistive devices that enable people with physical limitations to improve their quality of life and support activities of daily living. These devices allow them to lead more active, productive and independent lives [1]. Although O&P are essential for people in need, studies indicate that the acceptance rate of them is noticeably low. It is reported that the rejection rate of upper limb prostheses is 44% [2] while the rejection rate of orthoses reaches up to 80% depending on the orthosis type [3]. Moreover, it is emphasized that the main reasons for the rejections are low comfort, limited functionality and high weight [2, 3]. Furthermore, comfort is strongly affected by the fit of the O&P devices to the limb where misfit can lead to sore skin and pressure marks. In this context, a more homogeneous spread of pressure over the limb could be achieved by employing softer materials, resulting in a better adaptation to impacts and a higher comfort perception [4]. Additive manufacturing is a method using computer-aided design to produce the 3-dimensional elements layer-by-layer [5]. Material extrusion-based additive manufacturing (MEX-AM) becomes more popular because of its wide range of materials offered with more reasonable prices compared to other techniques. Although the technologies for additive manufacturing of hard and stiff materials by MEX-AM are well-developed, MEX-AM of softer and flexible materials, i.e. thermoplastic elastomers (TPE) still possess significant challenges [6]. TPEs are materials with combined properties of melt-processability of thermoplastics and high elasticity and flexibility of elastomers, making them a strong candidate for usage in additive manufacturing of customized O&P [7]. Figure 4.1 presents the most widely used MEX-AM methods. First of all, a Bowden extrusion type of the filament printers (Fig. 4.1a) has a large build volume and clean movements, whereas for the printing of soft materials, they cause complications of buckling or bending of the filament in the Bowden tube. Direct extrusion filament printers (Fig. 4.1b) are capable of printing softer materials with more reliable extrusion performance compared to Bowden extrusion, but are limited to materials with a hardness down to around 84 Shore-A [8] which is not

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Fig. 4.1 MEX additive manufacturing methods for soft materials and an occurring buckling problem: a Bowden extrusion; b direct extrusion; c pellet extrusion

soft enough for flexible medical O&P, soft sensors and actuators [9]. In general, the low modulus and hardness of TPE filaments cause filament buckling, annular back-flow, poor inter-layer and bed adhesion in MEX-AM [10]. To solve the listed extrusion difficulties and to be able to print materials in Shore-00 hardness scale, the pellet extrusion method (Fig. 4.1c) is proposed. In this method, pellets in a hopper are gravity fed to the screw which has three heating zones. The material is melted and then pushed through the nozzle by the rotation of the screw. By overcoming the challenges of filament MEX-AM in soft materials printing, the pellet extrusion method creates the opportunity for the usage of a wide range of materials from soft and flexible thermoplastics to rigid ones. Furthermore, additive manufacturing is a highly relevant technology in order to provide customization to O&P, since it gives a high freedom of design, as well as a fast and economic production of custom-fit O&P [11]. However, the impact of printing with soft materials in MEX-AM on the final hardness of component is still unresearched. Therefore, the aim of this study is to measure the effect of different printing directions and thickness ratios on mechanical properties, as hardness of MEX-AM multi-material printed soft materials, which can be correlated with O&P comfort.

4.2 Experimental Setup 4.2.1 Materials MEX-AM experiments are conducted using the following materials from Pollen AM company (Ivry sur Seine, France): PLA Natural—Polylactic acid (PLA), TPE High Flexibility—45 Shore-A (TPE45ShA) and TPE Ultra Flex—30 Shore-00

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Table 4.1 Mechanical properties of the materials used in MEX-AM Mechanical property PLA TPE30Sh00 Hardness 83 Shore D [12] Tensile strength, Yield 60 MPa Elongation at break 6%

51 VLRH 1.5 MPa 800%

TPE45ShA 44 Shore A 9 MPa 850%

(TPE30Sh00). The mechanical properties of the materials provided by the supplier are given in Table 4.1.

4.2.2 Characterization For the characterization of the hardness of additively manufactured specimens, the Shore hardness method is chosen, since it provides accurate results for additively manufactured parts [13]. According to the ASTM D2240 Standard [14], rectangular prism-shaped specimens with min. 6 mm thickness and 40 mm width, which allows taking five measurements from the surface, are printed and the hardness values of specimens is measured with ZwickRoell Shore-00 digital durometer (ZwickRoell GmbH & Co.KG, Ulm, Germany). The surfaces used for the measurement are shown in Fig. 4.2a and are indicated by the red arrows. Firstly, the correct read-out of the durometer is validated with the calibration sample. Then, from each hardness specimen, five measurements are taken at a distance of 6 mm from each other and 12 mm from the edge. Next, the mean hardness is calculated. The measurements are conducted in standardized conditions (21 .◦ C, 50% RH).

4.2.3 Specimens Design and 3D-Printing In order to evaluate the effects of printing direction on the hardness and the effect of changing thickness of materials with different hardness values on the combined hardness of multi-material printed specimens, two groups of samples are prepared. The first group consists of four single-material samples made out of TPE30Sh00 and TPE45ShA printed horizontally (H)—laying flat on the printing bed, and vertically (V)—built in the z-direction (Fig. 4.2a). The second group consists of eight multimaterial samples which have a PLA base, then TPE30Sh00 layer of varied thickness T and finally a varied number of thin TPE45ShA layers on a top (Fig. 4.2b). The thickness of TPE30Sh00 (.TT P E30Sh00 ) changes by 1 mm, while the thickness of TPE45ShA (.TT P E45Sh A ) varies by 0.2 mm which is an equivalent of one printed layer. The design given in Fig. 4.2b mimics a cross-section of a comfortable wearable device. PLA is commonly used for a rigid structure of 3D-printed O&P in order

4 Effects of Printing Direction and Multi-material on Hardness of Additively … Table 4.2 Printing parameters with Pollen AM New Series P printer PLA TPE30Sh00 Parameter name [.◦ C]

Cold temperature 62 Extruder temperature 167 [.◦ C] Head temperature [.◦ C] 185 60 Bed temperature [.◦ C] Flow [%] 50 Printing speed [mm/s] 25

37

TPE45ShA

45 130

57 130

205 35 40 20

220 60 53 25

Fig. 4.2 Samples for the verification of the influence on hardness of: a print direction; b layer heights in combined materials

to fix joints and transfer loads. The function of TPE30Sh00 is to provide comfort. However, this material is sticky, leading to a quick dirt adhesion and complicated clean keeping. Therefore, a TPE45ShA layer on the top is introduced, due to the fact that it is still considered as a soft material while it does not have the previously named limitations. The thickness of the soft materials should be kept low, since it affects the final size of the O&P, leading to undesirable bulkiness. Pollen New PAM Series P (Pollen AM, Ivry sur Seine, France) is used to produce all the test specimens by MEX-AM. The Pollen New PAM Series P printer is a printer which uses the pellet extrusion method based on the micro-extrusion process. The printing parameters for all the materials are listed in Table 4.2. All samples are printed with 100% zig-zag infill, a 0.4 mm nozzle, 0.4 mm line width and 0.2 mm layer height. The specimens are listed in Table 4.3.

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Table 4.3 Geometry of specimens used in the study Designation . T P L A [mm] . TT P E30Sh00 [mm] TPE30Sh00(H) TPE30Sh00(V) TPE45ShA(H) TPE45ShA(V) Multi-5-1-02 Multi-5-2-02 Multi-5-3-02 Multi-5-4-02 Multi-5-2-04 Multi-5-2-06 Multi-5-2-08 Multi-5-2-1

– – – – 5 5 5 5 5 5 5 5

6 6 – – 1 2 3 4 2 2 2 2

. TT P E45Sh A

[mm]

– – 6 6 0.2 0.2 0.2 0.2 0.4 0.6 0.8 1

4.3 Results and Discussion In Fig. 4.3a, the vertically and horizontally printed materials are compared. As a result of the Shore-00 hardness measurements, there are no significant differences between the hardness of samples printed horizontally and vertically. The small difference observed in TPE30Sh00 hardness specimens can result from a lower quality sample when printing vertically due to the soft nature of the material. In a discussion of the results Multi-5-2-02 specimen is used as a reference. The TPE30Sh00 layer height has a significant impact on the final hardness (Fig. 4.3b). The Multi-5-1-02 specimen has 13% higher hardness while Multi-5-3-02 and Multi5-4-02 are lower by 6.5% and 21% respectively. The higher .TT P E30Sh00 leads to a lower contribution of PLA into the final hardness, which decreases overall measured hardness.

Fig. 4.3 Effect of different variables on a Shore-00 hardness, a printing direction; b TPE30Sh00 thickness; c TPE45ShA thickness

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Fig. 4.4 Mean results of layers thickness effect on combined Shore-00 hardness with standard deviation; a TPE30Sh00; b TPE45ShA

The higher .TT P E45Sh A results in a higher hardness (Fig. 4.3c). The first additional layer of TPE45ShA affects the overall hardness by 2%. The next layers increase the hardness by 11.5%, 19%, and 25.5% compared to the initial .TT P E45Sh A . This increment is the effect of a higher portion of the harder material in the sample. Furthermore, Fig. 4.4 presents that in both cases the changes in hardness and the layer thickness ratios are almost linear. On the market, there are variety of O&P with hardness in a wide range. For instance, in the case of liners for leg prostheses, Local EVA, EVA Zote, Thailand Pelite and ICRC Pelite liners that are available on the market have an average hardness of 10.96 Shore-A (approximately 55 Shore-00), 3.79 Shore-A (approximately 43 Shore-00), 42.78 Shore-A (approximately 87 Shore-00) and 40.11 Shore-A (approximately 86 Shore-00), respectively [15]. Therefore, all the material combinations used in this study have similar hardness compare to the ones used in commercial O&P, therefore they are suitable to be used as the media to provide a comfort. Furthermore, the research demonstrates that adjusting hardness of the O&P to specific needs can be simply done with MEX-AM technology by manipulating the number of soft layers, resulting in more homogeneous spread of pressure.

4.4 Conclusion This work focuses on the influence of the printing direction of soft materials and combined thickness in multi-material printing on hardness in MEX-AM with the pellet extrusion method. Two soft materials, TPE30Sh00 and TPE45ShA, and one rigid PLA are combined together in order to mimic comfortable O&P, and they are tested in different thickness ratios. In this work, 12 specimens are printed, and their hardness is measured. The results indicate no significant differences in the final hardness

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of the soft materials between printing them horizontally or vertically. Furthermore, there is an influence observed of both materials’ thicknesses on the final hardness of multi-material prints. The low thickness of O&P is desirable. However, the low thickness may result in harder devices, therefore lower wearing comfort. Nevertheless, in all the cases of multi-material prints, achieved hardness is comparable with hardness of available O&P on the market. In the future, further tests on the hardness and compressive stiffness of multi-material printing should be conducted in order to investigate fluctuations of hardness and stiffness values between prints. Usage of MEX-AM technology gives a possibility to produce customized, well-fitting O&P what can decrease a drop-rate. Additionally, usage of soft materials can further influence comfort of wearing. The findings of this work can give an important input for the future works on the customized comfortable O&P. The similarity in the horizontal and vertical printing, as well as simple adjustment of hardness due to a layer thickness, can be advantageous for customized soft O&P printing due to a higher freedom of design and more consistent behavior of produced parts. The possibility of achieving a soft structure within thin layer of the material is important for the future implementation, since O&P should not protrude from the body and should be convenient in activities of daily living.

References 1. WHO Standards for prosthetics and orthotics: World Health Organization. Switzerland, Geneva (2017) 2. Salminger, S., Stino, H., Pichler, L.H., Gstoettner, C., Sturma, A., Mayer, J.A., Szivak, M., Aszmann, O.C.: Current rates of prosthetic usage in upper-limb amputees—have innovations had an impact on device acceptance? Disabil. Rehabil. 44(14), 3708–3713 (2022). https://doi. org/10.1080/09638288.2020.1866684 3. Swinnen, E., Lafosse, C., van Nieuwenhoven, J., Ilsbroukx, S., Beckwée, D., Kerckhofs, E.: Neurological patients and their lower limb orthotics: an observational pilot study about acceptance and satisfaction. Prosthetics Orthot. Int. 41(1), 41–50 (2017) 4. Pabón-Carrasco, M., Reina-Bueno, M., Vilar-Palomo, S., Palomo-Toucedo, I.C., RamosOrtega, J., Juárez-Jiménez, J.M.: Analysis and assessment through mechanical static compression tests of damping capacity in a series of orthosis plantar materials used as supports. Int. J. Environ. Res. Public Health 18(1), 115 (2020) 5. Varsavas, S.D., Riemelmoser, F., Arbeiter, F., Faller, L.M.: A review of parameters affecting success of lower-limb prosthetic socket and liners and implementation of 3d printing technologies. Mater. Today: Proc. (2022) 6. Kumar, N., Jain, P.K., Tandon, P., Pandey, P.M.: Extrusion-based additive manufacturing process for producing flexible parts. J. Braz. Soc. Mech. Sci. Eng. 40(3), 1–12 (2018) 7. Faller, L.M., Deniz Varsavas, S., M.J. Ali, A., Michalec, P., Lakshmi Gidugu, S., Spintzyk, S., Riemelmoser, F.O.: iLEAD—intelligent lightweight functional and hybrid 3d-printing for medical assistive devices: current status focusing on the multi-material aspect. Mater. Today: Proc. (2022). 10.1016/j. matpr.2022.09.537, https://www.sciencedirect.com/science/article/ pii/S2214785322063775 8. Miron, V.M., Lämmermann, S., Çakmak, U., Major, Z.: Material characterization of 3d-printed silicone elastomers. Procedia Struct. Integr. 34, 65–70 (2021)

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9. Xavier, M.S., Fleming, A.J., Yong, Y.K.: Finite element modeling of soft fluidic actuators: overview and recent developments. Adv. Intell. Syst. 3(2), 2000187 (2021). https://doi.org/10. 1002/aisy.202000187 10. Shen, N., Liu, S., Kasbe, P., Khabaz, F., Kennedy, J.P., Xu, W.: Macromolecular engineering and additive manufacturing of poly(styrene- b -isobutylene- b -styrene). ACS Appl. Polym. Mater. 3(9), 4554–4562 (2021). https://doi.org/10.1021/acsapm.1c00616 11. Alqahtani, M.S., Al-Tamimi, A., Almeida, H., Cooper, G., Bartolo, P.: A review on the use of additive manufacturing to produce lower limb orthoses. Prog. Addit. Manuf. 5(2), 85–94 (2020). https://doi.org/10.1007/s40964-019-00104-7 12. Brzakalski, D., Sztorch, B., Frydrych, M., Pakula, D., Dydek, K., Kozera, R., Boczkowska, A., Marciniec, B., Przekop, R.E.: Limonene derivative of spherosilicate as a polylactide modifier for applications in 3d printing technology. Molecules (Basel, Switzerland) 25(24) (2020). 10.3390/molecules25245882 13. Sikora, P., Gnatowski, A., Golebski, R.: Tests of mechanical properties of semicrystalline and amorphous polymeric materials produced by 3d printing. MATEC Web Conf. 254, 06003 (2019). https://doi.org/10.1051/matecconf/201925406003 14. ASTM D2240-15(2021): Test method for rubber property durometer hardness. 10.1520/D224015R21 15. Sasaki, K., Guerra, G., Rattanakoch, J., Miyata, Y., Suntharalingam, S.: Sustainable development: a below-knee prostheses liner for resource limited environments. J. Med. Devices 14(1) (2020). https://doi.org/10.1115/1.4045835

Chapter 5

Holistic Characterization of PBF-LB/P Powder Regarding Isothermal Crystallization, Rheology and Optical Properties Under Process Conditions Maximilian Marschall, Simon Cholewa, Sebastian-Paul Kopp, Dietmar Drummer, and Michael Schmidt

Abstract Laser based powder bed fusion of Polymers is an additive manufacturing technology that meets requirements of established manufacturing processes; thus, it is essential to expand the original range of applications from prototype generation to functional components. Semi-crystalline materials, mainly polyamide 12 (PA 12), dominate the market. Prior to solidification, material crystallization takes place during the building phase, yielding changes in material properties including rheology and optical properties. To gain a deep process understanding for reliable part production, a process-adapted evaluation is required to ascertain how long the molten polymer remains amorphous with reduced absorption and viscous dominant behavior, to the point when crystallites are forming. In this contribution, the effect of solidification on optical characteristics during an isothermal process is discussed. The goal of this process adapted characterization method is to reduce feedback circles for powder development, discretize process parameter settings, and obtain a wide range of process relevant material characteristics for PBF-LB/P for the first time in such an integrated manner. This contribution compares temperature-dependent optical M. Marschall (B) · S.-P. Kopp · M. Schmidt Bayerisches Laserzentrum GmbH, Konrad-Zuse-Straße 2-6, 91052 Erlangen, Germany e-mail: [email protected] M. Marschall · S. Cholewa · S.-P. Kopp · D. Drummer · M. Schmidt Collaborative Research Center (CRC) 814 “Additive Manufacturing”, 91058 Erlangen, Germany M. Marschall · S.-P. Kopp · M. Schmidt Erlangen Graduate School in Advanced Optical Technologies (SAOT), Paul-Gordan-Straße 6, 91052 Erlangen, Germany S. Cholewa · D. Drummer Institute of Polymer Technology (LKT), Friedrich-Alexander-Universität Erlangen-Nuremberg, Am Weichselgarten 10, 91058 Erlangen, Germany M. Schmidt Institute of Photonic Technologies (LPT), Friedrich-Alexander-University Erlangen-Nuremberg, Konrad-Zuse-Straße 3-5, 91052 Erlangen, Germany © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_5

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properties of a polymer powder layer and melt to rheological behavior of the isothermally solidifying melt pool. To ensure this comparability, both measurement setups are equipped with IR spectroscopy, which allows to analyze the isothermal crystallization progress. A modified double integrating sphere setup with FTIR (Fouriertransform infrared spectroscopy) and a rheometer with ATR (attenuated total reflection) were used. It was found out, that the isothermal crystallization already occurs faster than thought, even within preheating temperatures significantly above crystallization temperature in the upper layers. Thus, curling and thickness variation can appear due to change in absorptance and rheology, and impact the manufacturing quality. This can be a failure criterium for development stage of tailored polymer powders for PBF-LB/P. Keywords Additive manufacturing · PBF-LB/P · Isothermal crystallization · Rheology · Optical properties

5.1 Introduction For more and more applications, additive manufacturing techniques are becoming a significant alternative to conventional forming processes [1]. The layer-by-layer generating process calls to a high degree of freedom, but it also requests a complex set of properties for the used powder material and process control features. As a result of some polymer materials meeting the requests for a stable process, those, like PA12, are used a lot for industrial laser-based powder bed fusion with polymers (PBF-LB/P); however, there is a growing need for more application-tailored materials [2]. Complex powders for additive manufacturing are subject to the undefined specifications of suitable material qualities. A combination of fundamental polymer material characteristics, such as melting or crystallization on-set temperature and molecular weight, and powder-specific characteristics, such as particle size distribution and flowability, are used to describe an industrially accessible powder [3]. But as people who work with novel materials have experienced, tailoring powder properties to achieve similar values as in data sheets of established powders will not guarantee a flawless process. Therefore, a new strategy for characterizing these materials is evaluated and added to the state of the art used parameters to describe the material. The new strategy offers potential to overcome these challenges, and allows material development to already qualify the polymer powder in low quantities. The need for parameter studies on a manufacturing system might be dramatically reduced, hence less waste material is produced during powder development. In addition, feed-back circles for material development will become faster. This contribution exceeds state of the art process knowledge by establishing deep fundamental research for polymer powder behavior in the most process near conditions of PBF-LB/P. Although it has long been believed that PBF-LB/P would be an isothermal process, this contribution will demonstrate that even in an ideal isothermal condition, during the process preheating temperature, the first few layers begin to crystallize. The viscous phase

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layer’s optical and mechanical properties have already been impacted by this crystallization. The knowledge of this behavior will be helpful for powder conception and these methods can be used to qualify these failure criteria during development.

5.2 Methods The investigated material is a PBF-LB/P powder PA12 (EOSINT P/PA2200, EOS. GmbH, Germany), which is a white basic powder that is commonly used for laser sintering processes. The methods used below are dived into two groups, the first is regarding optical characterization with double integrating spheres and FTIR (Fouriertransform infrared spectroscopy), and the second is a rheometer coupled with ATR (attenuated total reflection)-FTIR spectroscopy. The FTIR measurements serve as the bridge between mechanics and precise optical properties measurement, to enable chaining of the measurements results.

5.2.1 Double Integrating Sphere and Modular FTIR Setup The setup to characterize optical properties of a powder layer is split in two main parts. The part with the double integrating spheres is used to investigate optical properties as the interaction of a CO2 -laser beam (wavelength 10.6 μm) with the material layer within simulated process conditions hence temperature and defined layer. This method was already employed in previous publications [4–7]. The setup consists of two integrating spheres with internal highly diffuse reflective gold coating and open ports for detectors. Between the two spheres a movable heat stage (FTIR600, Linkam Scientific Instruments LTD., UK) is mounted, where a sample layer of a defined height is prepared with a doctor blading device on top of BaF2 glass sheets. The CO2 -laser (Synrad firestar v20) irradiates the layer while measuring with 2 W, beam diameter of 2 mm and a duration of 0.5 s. The silver heat block inside the chamber controls the temperature with accuracy of 0.1 K. With the measured values, after referencing, for total reflectance (R) and total transmittance (T ), absorptance (A) is calculated according to the following equation [8]. R + A + T =1

(5.1)

T is subdivided into diffuse and unscattered portions, latter has to be measured separated from total transmittance [9, 10] (Fig. 5.1). The second part of the setup is a modular FTIR (VIR-200, JASCO Deutschland GmbH) with DLATGS detector for wavenumber 7800–350 cm−1 , whose parabolic mirror focused beam path uses the same sample chamber (FTIR-600) as used for the integrating spheres system. In this configuration, the setup can analyze a sample in transmission mode at a circle area of 2 mm diameter. To ensure comparability

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Fig. 5.1 Schematic demonstration of the double integrating sphere system for thermo-optical material characterization with additional modular FTIR

to the ATR-FTIR, the settings on the FTIR were set to resolution of 4 cm−1 with 6 repetitions per measurement without automatic signal processing. The metastable fundamental vibration at 2920 cm−1 could not be measured in transmission mode through the molten polymer layer of 100–200 μm. Therefore, signal evaluation was done by calculating a relative degree of crystallization on the band of interest with the polymer melt as 0% and the last isothermal value for 100%. The data was processed afterwards with standard Boltzmann fit and normalized to 1. PA 12 tends to form a crystalline γ-phase under atmospheric normal pressure and low thermodynamic driving force, which is given in isothermal state above its crystallization temperature [11]. As built PBF-LB/PA12 parts are therefore dominated by γ-phase spherulites [12]. Literature assumes before forming the γ-phase, a high temperature α, -phase crystallizes, which transitions to γ-phase [13, 14]. Distinction of both phases is only possible via X-ray diffraction, as the α, -phase shows a diminished FTIR-peak at 936 cm−1 , collapsing into the 945 cm−1 of the γ-phase [13, 15]. Several bands in FTIR are assigned to describe γ-phase crystallization [15, 16]. From those, the CONH in plane vibration band provided the clearest measurable peaks in the transmission spectrum, therefore, the 945 cm−1 peak was chosen to describe the isothermal crystallization process. For the measurements, the prepared powder layer was heated for a short time at 200 °C and then cooled at 20 K/min to 5 °C above isothermal holding temperature, then cooled at 5 K/min to 166/168/170/172 °C. This temperature was hold up to 3 resp. 4 h while FTIR measurements endured. The isotherms were chosen according to

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the preheating temperature in the process window and its deviations to be considered in the top layers of the PBF process [3].

5.2.2 Rheometer and ATR-FTIR The measurements for describing crystallization hardening are structured as follows: To measure these characteristics–crystallization and solidification–simultaneously, the Rheonaut (Thermo Electron GmbH Germany) is presented in this study. The Rheonaut is a rheometer combined with a FTIR-module, where crystallization is evaluated using fourier-transform infrared spectroscopy (FTIR). In this technique, an infrared beam is directed through a crystal onto the sample surface to generate the attenuated total reflection, which is then analyzed with a deuterated triglycine sulfate (DTGS) detector. The performed FTIR (Thermo Fisher Scientific, Nicolet IS10) absorption measurements were recorded with a spectral resolution of 4 cm−1 , and six repeat scans ranging from 4000 to 600 cm−1 . Furthermore, the crystallization band (945 cm−1 ) was used to calculate the crystallization index by setting it in relation to a reference band. Concurrently, the material’s rheological properties were investigated via rheological experiments with a rheometer (HAAKE MARS 60). By using the rheometer in oscillating mode, the viscoelastic behavior was examined. For this purpose, a sinusoidal deformation γ, with an angular frequency ω, was applied to the sample. To describe whether elastic or viscous properties predominate, the storage and loss modulus were determined by measurement. A sensor received stress from an elastic body that was directly proportional to the deformation, and the phase delay, δ, between excitation and response signal was equal to zero. The viscoelastic sample had a phase difference of 0 < δ < π/2. In the molten state, the viscous component (loss modulus G,, ) dominated, and as a result of the crystallization, the elastic component (storage modulus G’) increased at a faster rate than the loss modulus. Additionally, the point of intersection–also cross-over at δ 45°–of the parameters identified the point at which both parameters were in equilibrium, yielding a behavioral transition from a viscoelastic fluid to a viscoelastic solid. The measuring program for the simultaneous measurement is as follows: The sample is melted at 190 °C and then cooled at a cooling rate of 5 K/min to the isothermal holding level of 168 °C. The oscillating measurements are carried out at a low frequency of 1 Hz and a deformation of 0.5% to accurately simulate the LS process without shearing. The measuring gap is 1 mm, and the plate diameter is 25 mm.

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5.2.3 Differential Scanning Calorimetry The DSC comparison measurements were carried out using the DSC instrument Q2000 from TA Instruments. The sample chamber was purged with nitrogen; the sample weights were approx. 3 mg. The material samples were heated at 60 K/min to 200 zC, held for 1 min to remove residual crystals, and then cooled at 60 K/min to 168 °C isotherm. To describe the temporal course of crystallization, the relative degree of crystallization was calculated.

5.3 Results and Discussion 5.3.1 Isothermal Crystallization Observation The samples were measured in transmission mode with modular FTIR. Figure 5.2 shows the progress of crystallization in isothermal state within the process window of PA12. The figure illustrates the temperature dependent crystallization above the crystallization temperature of PA12, which is given with 138 °C [3]. Within the FTIR spectrum, γ-phase crystallization referring to 945 cm−1 was evaluated. It is understood that for 166/168/170 °C iso-temperature the Boltzmann sigmoidal fit converges into saturation, which is interpreted as a thermo-dynamic equilibrium that describes the end of the isothermal crystallization process at this isotherm. As for 172 °C, the Boltzmann fit does not converge, which related to the fact, that crystallization processes still endure and are not yet in a thermo-dynamic equilibrium. Therefor a fifth order fit was used to describe the measurement data which is more suitable for an unfinished process compared to the shown raw data. Starting at 200 min on the 172 °C isotherm, a slow saturation expression occurred, indicating the slowing down of crystallization at this time. The isotherm of 172 °C expresses a stage before proceeding further. As this step is already distinct in raw data and not exceedingly pronounced by the fit, it can be assumed, that this step is formed by prevailing high temperature α’-spherulite crystallization [13, 14]. Those are not stable and are replaced by switching into γ-phase by advancing time. α’- and γ-spherulites peaks are close together in the FTIR spectrum and can interfere with each other taking into account the resolution of the interferometer. The results of the rel. degree of crystallization for ATR-FTIR in Fig. 5.2 show that a change over time is observed for the selected crystalline bands (945 cm−1 ) and the reference band for (2920 cm−1 ). This change can be explained by the formation of the molecular structures as a result of crystallization. All measurements show a typical S-shaped curve for the crystallization progress–expressed with a Boltzmann fit, according to the fundamental thermodynamic relation–, which is composed of the crystallization nucleation, an almost linear crystallization progress and a flattening at the end–as the crystal growth is hindered by already existing crystals. The crystallization progress by ATR-FTIR, as well as FTIR from the modular setup, is reached

5 Holistic Characterization of PBF-LB/P Powder Regarding Isothermal … PA12 isothermal crystalization at 945 cm-1 rel. degree of isothermal crystallization

Fig. 5.2 Relative degree of isothermal crystallization of PA12 on FTIR evaluation peak 945 cm−1 over isothermal state duration per temperature; compared to other measurement techniques

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at later times compared to the DSC measurements. This deviation can result from the fact that heat flow measurements at the beginning and end of the measurement are less sensitive than the optical data. This is particularly the case when the melt subcooling is low, as the crystallization rate is lower here, which means that the lower heat flows can be lost in the noise. The FTIR curve from the modular setup at 168 °C shows a distinct deviation to the DSC and ATR measurements. This is conditioned by the number of observable events of crystallization within the layer, as the observed sample volume is larger, measuring in transmission mode, therefor also closer to process conditions as on the ATR-FTIR. Additionally, due to the different sample volume and the low thermal conductivity, the sample core could have a slightly higher temperature. As shown in Fig. 5.2, assuming a temperature deviation on the FTIR setup sample core of >1 K, the curve would be between the 168 and 166 °C graph, and therefore similar to the ATR-FTIR curve. But this also shows, that this setup is able to resolve substantially lower amounts of crystallization, compared to the DCS measurement in Fig. 5.2, under process near conditions.

5.3.2 Rheological Material Behavior Due to Isothermal Crystallization In the following Fig. 5.3, the results of the simultaneously to ATR-FTIR conducted rheological experiments are shown. The figure on the left shows the development of the torque required to apply the deformation. After a short initial time, an almost linear logarithmic progression can be observed. The increasing torque necessary to realize the deflection is due to the crystallization progress. The figure on the right gives an exact representation of the composition of the viscoelastic melt. At the beginning

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Fig. 5.3 Rheological measurements, development of torque (left) and viscoelastic characteristics due to crystallization

of the measurement, the viscous parts dominate (G,, > G, ). Due to crystallization, the storage modulus shows a larger increase than the loss modulus, resulting in a transition to an elastically dominated melt. Thus, this employing solidification due to crystallization will already have influence to warping in delicate geometric areas. Dependent of the real temperature in the powder bed this will occur in the upper building layers and not only in the solidified depth of the powder bed.

5.3.3 Isothermal Crystallization Influence on Optical Properties Thermo-optical properties have been investigated with the double integrating sphere setup. The isothermal FTIR measurement in Fig. 5.2. was taken at 168 °C between the two datapoints in Fig. 5.4 at this temperature. This clearly demonstrates, that already isothermal crystallization impacts on the absorptance of the 10,6 μm CO2 laser radiation with around 20%-point total. It can be concluded, that this deviation on absorptance is already significant to take into account for built jobs with high layer downtime. In dependance of the isothermal crystallization degree, exposure parameters have to be adapted to prevent adhesion and necking of particles in the powder bed outside of the desired contours, as absorptance increases significantly over time already in the upper layers. Also, penetration depth of the incident beam is shorter the longer the isotherm endures.

5 Holistic Characterization of PBF-LB/P Powder Regarding Isothermal … Fig. 5.4 Optical properties of a 200 μm PA12 (2200) powder layer depending on temperature resp. to the process window

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5.4 Conclusion It was shown, that with FTIR as a linking measurement method, optical and rheological properties can be chained to express the most recent fundamental knowledge about isothermal crystallization processes during the building process of PBF-LB/P. Also, this process starts earlier than presumed and already affects the first upper layers in the powder bed. It could be proved, that absorptance increases and transmission directions change with advancing crystallization already at an early isothermal state within the preheating temperature of the building chamber. All investigated temperatures were significantly above the defined crystallization temperature of PA12, but even though, marginal temperature variations in the powder layer already cause a distinct time dependent isothermal crystallization. This likewise causes a change in the concept-duality of coexisting preheated powder and viscous polymer melt pool in the upper layers, proofed by the rheological change from a viscous phase to viscoelastic behavior. These effects, which start earlier than yet suspected, are reasons why some newly developed polymer powder for PBF do not perform as expected, regarding the powder-specific characteristics and DSC measurements which are used to develop suiting parameters within the given process window. Curling, warping and thickness deviations at fine structures can appear. Considering, that PA12 is a polymer which suits good for PBF-LB/P, these effects are expected to be worse on other, modified, polymer powders, to be proofed in future publications. This introduced chained characterization method aims to enhance powder development by elaborating deep fundamental process knowledge and direct the spotlight of material properties, which have also to be taken into account, towards rheological and optical properties. Acknowledgements The authors gratefully acknowledge funding of the Collaborative Research Center 814 (CRC 814), sub-project A3, by the German Research Foundation (DFG)-Project No.

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61375930- and of the Erlangen Graduate School in Advanced Optical Technologies (SAOT) by the Bavarian State Ministry for Science and Art.

References 1. Ehrhardt, M.: Editorial: 3D-druck: Vom hype zum alltagseinsatz. Kunststoffe, (2022) 2. Christ, M.: Methoden zur Funktionalisierung und beschichtung von polymerpartikeln für die additive Fertigung. Friedrich-Alexander-Universität Erlangen-Nürnberg, (2021) 3. EOS GmbH.: Produktinformation EOSINT P/PA2200-Pulver 2022 4. Marschall, M., Heintges, C., Schmidt, M.: Influence of flow aid additives on optical properties of polyamide for laser-based powder bed fusion. Procedia CIRP 111, 51–54 (2022) 5. Schuffenhauer, T., Stichel, T., Schmidt, M.: Employment of an extended double-integratingsphere system to investigate thermo-optical material properties for powder bed fusion. J. Mater. Eng. Perform. 30, 5013–5019 (2021) 6. Schuffenhauer, T., Stichel, T., Schmidt, M.: Experimental determination of scattering processes in the interaction of laser radiation with PA12 powder. Procedia CIRP 94, 85–88 (2020) 7. Heinl, M., Laumer, T., Bayer, F., Hausotte, T.: Temperature-dependent optical material properties of polymer powders regarding in-situ measurement techniques in additive manufacturing. Polym Test 71, 378–383 (2018) 8. Poprawe, R.: Lasertechnik für die Fertigung: Grundlagen, Perspektiven und Beispiele für den innovativen Ingenieur; mit 26 Tabellen. Springer, Berlin Heidelberg (2005) 9. Pickering, J.W., Moes, C.J.M., Sterenborg, H.J.C.M., Prahl, S.A., van Gemert, M.J.C.: Two integrating spheres with an intervening scattering sample. J Opt Soc Am A 9, 621 (1992) 10. Pickering, J.W., Prahl, S.A., van Wieringen, N., Beek, J.F., Sterenborg, H.J.C.M., van Gemert, M.J.C.: Double-integrating-sphere system for measuring the optical properties of tissue. Appl Opt 32, 399 (1993) 11. Hiramatsu, N., Haraguchi, K., Hirakawa, S.: Study of Transformations among α, γ and γ’ forms in nylon 12 by X-ray and DSC. Jpn. J. Appl. Phys 22, 335–339 (1983) 12. Salmoria, G.V., Leite, J.L., Paggi, R.A.: The microstructural characterization of PA6/PA12 blend specimens fabricated by selective laser sintering. Polym Test 28, 746–751 (2009) 13. Ramesh, C.: New crystalline transitions in Nylons 4,6, 6,10, and 6,12 Using high temperature x-ray diffraction studies. Macromolecules 32, 3721–3726 (1999) 14. Paolucci, F., Baeten, D., Roozemond, P.C., Goderis, B., Peters, G.W.M.: Quantification of isothermal crystallization of polyamide 12: Modelling of crystallization kinetics and phase composition. Polymer 155, 187–198 (2018) 15. Han, J., Cao, Z., Gao, W.: Remarkable sorption properties of polyamide 12 microspheres for a broad-spectrum antibacterial (triclosan) in water. J. Mater. Chem. A 1, 4941 (2013) 16. Rhee, S., White, J.L.: Investigation of structure development in polyamide 11 and polyamide 12 tubular film extrusion. Polym. Eng. Sci 42, 134–145 (2002)

Chapter 6

Mechanical Behaviour of Polyamide 12 Parts Manufactured by SLS Process Telma Ruivo, Mário S. Correia , Henrique A. Almeida , and Ana M. Amaro

Abstract This study was elaborated in order to study the mechanical behaviour of specimens built by additive manufacturing, namely Selective Laser Sintering (SLS). SLS is a technique that consists of orienting a laser beam over powdered raw material and this incision causes the material to heat up, melting it into the desired parts. Unprocessed powdered raw material acts as a support structure during the construction process, which is one of the advantages of process optimization. Other advantages are the construction of parts or prototypes with complex geometries in a short time and the diversity of materials that can be used in this technology as raw material. In this study, the equipment was kindly provided by a company from Marinha Grande, Centimfe, where the specimens necessary for the execution of the tests were made. The test pieces were constructed in 4 different orientations, with a zigzag construction path. The mechanical tests considered in this study were: tensile, stress relaxation and hardness. These tests allowed to analyse the mechanical of the specimens obtained after the sintering of the raw material under study, namely polyamide 12. The main objective of this study was to evaluate the influence of the construction orientation, considering the mechanical, where orientation 4 (Edge 0°, horizontal XX axis) was the one that obtained the best results, having been built according to the XX and ZZ axes, which gives it a higher mechanical resistance. Keywords Additive manufacturing · Selective laser sintering · Polyamide 12 · Mechanical testing

T. Ruivo · M. S. Correia · H. A. Almeida (B) School of Technology and Management, Polytechnic Institute of Leiria, Leiria, Portugal e-mail: [email protected] M. S. Correia · A. M. Amaro Centre for Mechanical Engineering, Materials and Processes, University of Coimbra, Coimbra, Portugal H. A. Almeida Computer Science and Communication Research Centre, Polytechnic Institute of Leiria, Leiria, Portugal © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_6

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6.1 Introduction Selective laser sintering (SLS), developed and patented by Dr. Carl Deckard and Dr. Joe Beaman at the University of Texas at Austin in the mid-1980s (USA patent US4863538A), is an additive manufacturing (AM) technique that uses an infrared laser beam as a power and heat source to sinter powdered raw material (typically nylon or polyamide). The laser selectively fuses the powdered raw material by scanning cross-sections generated from a 3D CAD model on the surface of the powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness and a new layer of powdered raw material is applied on top, and the laser sintering process is repeated until the part is completed [1–4]. It is possible to observe in Fig. 6.1 a schematic representation of a SLS printer. According to the “ASTM F42−Additive Manufacturing” group, the SLS process falls within the Powder Bed Fusion category. Several factors and processing parameters directly influence the quality of the part during the sintering process [5–7]. Some of the processing parameters that affect the sintering process are: the laser power [8], the laser diameter (beam spot size) [9], the laser energy density (the laser power divided by the product of scan speed and laser diameter) [7, 10], the laser wavelength [7], the hatching space (the distance between adjacent scan lines) [11], the scanning speed [12], the layer thickness [12–14] and particle size and composition. The preheating temperature, powder bed temperature, and the sintering temperature affect the porosity of the sintered samples [15]. During the process, if the melt flows through the layers, the porosity of specimens will decrease [16]. The common practice of mixing fresh and reused powders can also have a significant influence on the degree of powder consolidation [6, 17–20]. When

Fig. 6.1 Schematic representation of a SLS printer

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one or more of the above variables are changed, the degree of material densification should be addressed before part production for optimum part quality. Because the mechanical performance of parts made by additive manufacturing is an important factor, several studies reporting mechanical testing results have been published [6, 19, 21–23]. Depending on the type of part application, these properties may include tensile testing, hardness, fatigue endurance, flexural testing, impact resistance, etc. The mechanical tests considered in this study were: tensile, stress relaxation and hardness. These tests allowed to analyse the mechanical of the specimens obtained after the sintering of the raw material under study, namely polyamide 12. The main objective of this study was to evaluate the influence of the construction orientation, considering the mechanical properties. In this paper, the tensile and hardness results will be addressed.

6.2 Materials and Methods 6.2.1 Material A variety of materials can be used in the SLS process. In this research, polyamide 12 (PA12) powder with the commercial name DuraForm ProX PA [25] was used to build the samples. This powder is a common raw material used in SLS printers. According to EN ISO 10993–1, the parts produced from PA12 are chemically and physically durable and biocompatible. These parts are suitable for various applications due to their high strength and stiffness [24]. The average particle size of PA12 powder was 50 μm.

6.2.2 Methods Test Samples The 3D Systems sPro 60 HD (Fig. 6.2) was used for building the test samples. The building chamber of this equipment is: 381 × 330 × 460 mm. For each test it was necessary to use test samples of different dimensions. For the tensile, creep-relaxation and hardness tests the same type of test samples were used and the test sample’s dimensions were based on the ASTM D638 standards (Fig. 6.3). For the two different mechanical tests, the test samples were built in four different orientations. For each mechanical test there were five tests for each different orientation. One of the objectives of this study is to analyse the existence of variation in

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Fig. 6.2 sPro™ SLS® center−3D systems Fig. 6.3 Test samples according to the ASTM D638 standards

mechanical behaviour due to the building orientation. For this purpose, four different orientations were chosen (Fig. 6.4): • • • •

Orientation nr. 1−Flat 0°, horizontal XX axis; Orientation nr. 2−Flat 90°, horizontal YY axis; Orientation nr. 3−Vertical, ZZ axis; Orientation nr. 4−Edge 0°, horizontal XX axis.

Tensile and Relaxation Tests The tensile and relaxation test were performed on ZwickRoell Z100 (Fig. 6.5).

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Fig. 6.4 Schematic representation of the specimens’ construction orientation 1–Flat 0°, horizontal XX axis; 2−Flat 90°, horizontal YY axis; 3–Vertical, ZZ axis; 4–Edge 0°, horizontal XX axis

Fig. 6.5 ZwickRoell Z100 equipment

The ASTM D638 testing for the DuraForm ProX PA material according to its data sheet PA12 can be observed in Fig. 6.6.

6.2.3 Shore a Hardness The Instron Shore equipment automatic operating stand Model 903 for Shore testing according to the ASTM-D2240 standards was used (Fig. 6.7).

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Fig. 6.6 DuraForm ProX PA plastic per ASTM D638 testing [25] Fig. 6.7 Model 903 for Shore testing from instron

6.3 Results and Discussions 6.3.1 Tensile E Relaxation Results In Table 6.1 it is possible to observe the maximum values for each building orientation. The test samples with the orientation 4 Edge 0°, horizontal XX axis presents the highest values for the tensile behaviour (force and strength). For the building orientations other than the XX-axis the tensile strength is always lower than the other building orientations. The lowest value is presented by test sample 3 Vertical ZZ axis, could be addressed for vertical building orientation and the building layers

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Table 6.1 Tensile test maximum force observed Building orientation

1–Flat 0°, horizontal XX axis

Maximum force [N]

917,9

782,5

784,8

1099,4

Tensile strength [MPa]

25,9

23,3

21,9

29,2

Young modulus [MPa]

904,9

1108,9

1032,8

1012,2

2–Flat 90°, horizontal YY axis

3–Vertical ZZ axis

4–Edge 0°, horizontal XX axis

of the test sample that are perpendicular to the deformation on the tensile test. In the case of test sample 2 Flat 90°, horizontal YY axis, the roller moves forward and drags the powdered raw material in a sideways movement regarding the position of the test sample. In the case of this type of building orientation, smaller construction areas in a perpendicular position regarding the powder roller, the powder repositioning is inefficient. The sintering process of these layers isn’t as efficient as predicted, thus affecting the mechanical behaviour. The tensile tests were performed five times for each building orientation. The curves presented in the Fig. 6.8, refer to the average of the tests for each orientation and not to a single test, representing, however, the typical behaviour observed for each building orientation, through the stress–strain curves. The relaxation tests were performed with the duration of 2000s for all building orientations. An initial displacement of 3 mm was applied, where the maximum force is reached in this displacement interval. After this, the load stabilisation curve was

Fig. 6.8 Stress versus Strain plot (average) for each building orientation

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observed during the defined time. In Table 6.2 it is possible to observe the force evolution during the relaxation tests for each building orientation. The test samples with the orientation 4 Edge 0°, horizontal XX axis presents the lowest values for tensile force and the final force after the relaxation time. But in the relaxation analysis, in average, the influence of the force decrease is in the order of 30% for all building orientations. As a result, for the relaxation, it isn’t possible to address any influence on the building orientation of the test samples. Similar as the tensile test, it is possible to observe the average force relaxation curve concerning the building orientation (Fig. 6.9). The relaxation behaviour between the test samples is similar to the behaviour verified between the test samples in the tensile testing. This is also verified by observing Table 6.2. Table 6.2 Relaxation test–force evaluation Maximum force [N]

Final force [N]

Force variation [N]

% Force decrease

1–Flat 0°, horizontal XX axis

765,49

522,58

242,90

31,73

2–Flat 90°, horizontal YY axis

827,31

577,67

249,64

30,18

3–Vertical ZZ axis

741,58

503,08

238,51

32,16

4–Edge 0°, horizontal XX axis

627,40

441,26

186,14

29,67

Fig. 6.9 Average relaxation behaviour for each building orientation

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Table 6.3 Hardness shore a test results 1–Flat 0°, horizontal XX axis

Shore A (average)

Standard deviation

80,5

5,4

2–Flat 90°, horizontal YY axis

78,2

1,2

3–Vertical ZZ axis

90,8

0,8

4–Edge 0°, horizontal XX axis

87,3

1,1

Table 6.3 describes the average of the Shore hardness tests for each test sample. The tests were carried out in 5 different zones of the surface of the test sample. The global average is shown in Table 6.3, grouping the general results obtained in each orientation. Although the supplier’s description presents theoretical values for the hardness in the Shore D test, it was not possible to perform the same test in the laboratory, because the values obtained were higher than the expected ones, therefore the Shore A test was used. In the analysis of the previous table, it is possible to observe that the hardness is influenced by the building orientation and the area associated to each builder layer. Thus, is possible to observe that in the case of test sample 3 Vertical ZZ axis construction as smaller construction area per layer and the hardness is higher and with a lower standard deviation. Thus, in this case, the dragging of the raw material does not influence the hardness of the processed material. On the other hand, in the case of test sample 1 Flat 0°, horizontal XX axis, due to the larger area to be processed (larger amount of material dragged and more laser time for sintering), the hardness value as well as its standard deviation is influenced.

6.4 Conclusions Several factors and processing parameters directly influence the quality of the parts during the sintering process. In this study, only the building orientation of the produced parts was considered. The mechanical tests considered in this study were: tensile, stress relaxation and hardness. Regarding the tensile test, it was found that the specimen with building orientation 4 Edge 0°, horizontal XX axis presents the highest stress values. In this case the building orientation is similar to the load orientation, so there is a preferential alignment that favours the mechanical resistance. In the stress relaxation test, it was verified that the test samples with building orientation Flat 90°, horizontal YY axis presents the highest values of initial force and relaxation of 30% until the end of the 2000s. This 30% relaxation is similar for all the building orientations. Finally, in the analysis of the Hardness Shore A results, it is possible to verify that the hardness is influenced by the building orientation and the area associated to each build layer. In the case of sample 3 Vertical ZZ axis, a smaller construction area per

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layer presents the highest value for hardness with a lower standard deviation. The dragging of the raw material in this case does not have influence on the hardness of the processed material. On the other hand, for larger areas to be processed (larger amount of material dragged and more laser time to sinter), the hardness values as well as its standard deviation is influenced. Acknowledgements This research is sponsored by FEDER funds through the program COMPETE–Programa Operacional Factores de Competitividade–and by national funds through FCT –Fundação para a Ciência e a Tecnologia–, under the project UID/EMS/00285/2020 and LA/ P/0112/2020.

References 1. Razaviye, M., Tafti, R., Khajehmohammadi, M.: An investigation on mechanical properties of PA12 parts produced by a SLS 3D printer: An experimental approach. CIRP J. Manuf. Sci. Technol., 38, 760-768 (2022). https://doi.org/10.1016/j.cirpj.2022.06.016 2. Lekurwale, S., Karanwad, T., Banerjee, S.: Selective laser sintering (SLS) of 3D printlets using a 3D printer comprised of IR/red-diode laser. Ann. 3D Print. Med. 6, 100054 (2022). https:// doi.org/10.1016/j.stlm.2022.100054. ISSN 2666–9641. S2CID 247040011 3. Awad, A., Fina, F., Goyanes, A., Gaisford, S. and Basit, A.W.: Advances in powder bed fusion 3D printing in drug delivery and healthcare. Adv. Drug Deliv. Rev. 174, 406–424 (2021). https:// doi.org/10.1016/j.addr.2021.04.025. ISSN 0169–409X. PMID 33951489. S2CID 233869672. 4. Charoo, N.A., Barakh Ali, S.F., Mohamed, E.M., Kuttolamadom, M.A., Ozkan, T., Khan, M.A., Rahman, Z.: Selective laser sintering 3D printing—an overview of the technology and pharmaceutical applications. Drug Dev. Ind. Pharm. 46 (6), 869–877 (2021). https://doi.org/ 10.1080/03639045.2020.1764027. ISSN 0363–9045. PMID 32364418. S2CID 218490148. 5. Gibson, I., Shi, D.: Material properties and fabrication parameters in selective laser sintering process. Rapid Prototyp. J. 3(4), 129–136 (1997). https://doi.org/10.1108/13552549710191836 6. Zarringhalam, H., Hopkinson, N., Kamperman, N.F., De Vlieger, J.J.: Effects of processing on microstructure and properties of SLS nylon 12. Mater. Sci. Eng., A 435, 172–180 (2006). https://doi.org/10.1016/j.msea.2006.07.084 7. Charoo, N.A., Barakh Ali, S.F., Mohamed, E.M., Kuttolamadom, M.A., Ozkan, T., Khan, M.A., Rahman, Z.: Selective laser sintering 3D printing–an overview of the technology and pharmaceutical applications. Drug Dev. Ind. Pharm. 46(6), 869–877 (2020). https://doi.org/10. 1080/03639045.2020.1764027 8. Morgan, R., Sutcliffe, C.J., O’Neill, W.: Experimental investigation of nanosecond pulsed Nd: YAG Laser re-melted pre-placed powder beds. Rapid Prototyp. J. 7(3), 159–172 (2001). https:// doi.org/10.1108/13552540110395565 9. Francis, Z.R.: The effects of laser and electron beam spot size in additive manufacturing processes. Dissertation, Carnegie Mellon University. (2017). https://kilthub.cmu.edu/articles/ thesis/The_Effects_of_Laser_and_Electron_Beam_Spot_Size_in_Additive_Manufacturing_ Processes/6723563/1/files/12258530.pdf 10. Yang, Z., Peng, H., Wang, W., Liu, T.: Crystallization behavior of poly (ε-caprolactone)/layered double hydroxide nanocomposites. J. Appl. Polym. Sci. 116(5), 2658–2667 (2010). https://doi. org/10.1002/app.31787 11. Pilipovi´c, A., Brajlih, T., Drstvenšek, I.: Influence of processing parameters on tensile properties of SLS polymer product. Polymers, 10/11, 1208 (2018). https://doi.org/10.3390/polym1011 1208

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12. Wang, R.J., Wang, L., Zhao, L., Liu, Z.: Influence of process parameters on part shrinkage in SLS. Int. J. Adv. Manuf. Technol. 33(5), 498–504 (2007). https://doi.org/10.1007/s00170-0060490-x 13. Starr, T.L., Gornet, T.J., Usher, J.S.: The effect of process conditions on mechanical properties of laser-sintered nylon. Rapid Prototyp. J. (2011). https://doi.org/10.1108/13552541111184143 14. Dingal, S., Pradhan, T.R., Sundar, J.K., Choudhury, A.R., Roy, S.K.: The Application of Taguchi’s method in the experimental investigation of the laser sintering process. Int. J. Adv. Manuf. Technol. 38(9), 904–914 (2008). https://doi.org/10.1007/s00170-007-1154-1 15. Hofland, E.C., Baran, I., Wismeijer, D.A.: Correlation of process parameters with mechanical properties of laser sintered PA12 parts. Adv. Mater. Sci. Eng. 2017, 4953173 (2017). https:// doi.org/10.1155/2017/4953173 16. Ling, Z., Wu, J., Wang, X., Li, X., Zheng, J.: Experimental study on the variance of mechanical properties of polyamide 6 during multi-layer sintering process in selective laser sintering. Int. J. Adv. Manuf. Technol. 101(5), 1227–1234 (2019). https://doi.org/10.1007/s00170-018-3004-8 17. Tontowi, A.E., Childs, T.H.C.: Density prediction of crystalline polymer sintered parts at various powder bed temperatures. Rapid Prototyp. J. 7(3), 180–184 (2001) 18. Goodridge, R.D., Tuck, C.J., Hague, R.J.M.: Laser sintering of polyamides and other polymers. Prog. Mater Sci. 57(2), 229–267 (2012) 19. Beal, V.E., Paggi, R.A., Salmoria, G.V., Lago, A.: Statistical evaluation of laser energy density effect on mechanical properties of polyamide parts manufactured by selective laser sintering. J. Appl. Polym. Sci. 113(5), 2910–2919 (2009) 20. Kruth, J.-P., Levy, G., Klocke, F., Childs, T.H.C.: “Consolidation phenomena in laser and powder-bed based layered manufacturing. CIRP Annals—Manufacturing Technol., 56(2), 730– 759 (2007) 21. Kim, G.D., Oh, Y.T.: A benchmark study on rapid prototyping processes and machines: quantitative comparisons of mechanical properties, accuracy, roughness, speed, and material cost. Proc. Inst. Mech. Eng., Part B: J. Eng. Manuf. 222(2), 201–215 (2008) 22. Chunze, Y., Yusheng, S., Jinsong, Y., Jinhui, L.: A nanosilica/Nylon-12 composite powder for selective laser sintering. J. Reinf. Plast. Compos. 28(23), 2889–2902 (2009) 23. Yan, C., Hao, L., Xu, L., Shi, Y.: Preparation, characterisation and processing of carbon fibre/ polyamide-12 composites for selective laser sintering. Compos. Sci. Technol. 71(16), 1834– 1841 (2011) 24. Giordano, C.M., de Senzi Zancul, E., Rodrigues, V.P.: Análise dos custos da produção por manufatura aditiva em comparação a métodos convencionais. Rev. Científica Eletrónica Eng. Produção E Correl. 16(2), (2016). ISSN 1676–1901 25. DuraForm ProX PA Datasheet A4: https://www.3dsystems.com/sites/default/files/2022-02/3dsystems-duraform-prox-pa-sls-datasheet-usa4-2022-02-11-a-print.pdf

Chapter 7

Effect of Automotive Fluids on Additive Manufactured Components for the Automotive Industry Rui Pedrosa, Maria Leopoldina Alves, and Henrique A. Almeida

Abstract The additive manufacturing faces nowadays a large development at all the methods included. One of this method´s is FDM (Fused Deposition Modelling). An identified limitation of FDM is associated to the printing process itself, which brings porosity to the components. The present work presents a study of permeability changes in components produced by FDM when in contact with automotive fluids, both diesel fuel and coolant fluids. To achieve this goal, laboratorial tests were carried out in components produced in different polymeric materials, namely, ABS, PC and PC-ABS. The components were coated by a layer of epoxydic resin to reduce the fluid absorption, and were merged in diesel fuel during different periods, at the regular temperatures observed on a running automotive. Both mass measurements and some mechanical laboratorial tests were performed, before and after the immersion, in coated and non-coated components. Keywords FDM · Polymeric materials · Automotive fluids · Tensile strength

R. Pedrosa · M. L. Alves · H. A. Almeida (B) School of Technology and Management, Polytechnic Institute of Leiria, Leiria, Portugal e-mail: [email protected] M. L. Alves INESC Coimbra–Institute for Systems Engineering and Computers at Coimbra, Polytechnic Institute of Leiria, 2411-901 Leiria, Portugal H. A. Almeida Computer Science and Communication Research Centre, Polytechnic Institute of Leiria, Leiria, Portugal © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_7

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7.1 Introduction Additive manufacturing (AM) technology is widely used in several industries and sectors, namely the aerospace, machinery manufacturing, automotive, medical and other fields [1]. With the arrival of the Industry 4.0, additive manufacturing technology has gained more and more importance by all industrial sectors worldwide [2]. The development of the AM industry provides a valuable opportunity for the rapid development of modern manufacturing industry and the transformation and upgrading of traditional manufacturing industry, alongside with the optimum designs that may occur from the design capabilities of the AM technologies and the sustainability benefits [3–6]. The automotive industry, similar to other industries, has highly strict regulations on the components, namely, the material properties, the design rules (geometric and dimensional tolerancing), usage of material and the component’s performance. Several studies have been presented regarding the applicability of several AM materials for the fabrication of automotive components along with the capability of using AM technologies to produce lightweight components with the aid of generative design or topological optimization [7–13]. However, when in the presence of automotive fluids and considering the polymer automotive components, this industrial sector is still faithful to plastic components produced through plastic injection moulding. The reasons of this selection are the non-homogeneous properties, the porosity levels and the surface roughness of the components produced by the existing AM technologies [14, 15]. This research presents a study of permeability changes in components produced by FDM when in contact with automotive fluids, both diesel fuel and coolant fluids. To achieve this goal, laboratorial tests were carried out in components produced in different polymeric materials, namely, ABS and PC. To increase the permeability of the components, they were coated by a layer of epoxydic resin to reduce the fluid absorption, and then were merged in diesel fuel during different periods, at the regular temperatures observed on a running automotive. Both mass measurements and some mechanical laboratorial tests were performed, before and after the immersion, in coated and non-coated components to determine the mechanical properties of the parts produced by FDM.

7.2 Materials and Methods During the laboratory procedure it was necessary to use several equipment available in the mechanical engineering laboratories of the School of Technology and Management of Polytechnic of Leiria. For the test specimens printing a Prusa I3 MK3S printer was used, which incorporates a MK52 heated table (Fig. 7.1a), a self-levelling heated table and an integrated filament calibration system.

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Fig. 7.1 a Printer Prusa I3 MK3S; b Chamber HEK-GMBH MCP-Vacuum; c Chamber P Selecta

To replicate the temperatures verified in a motor vehicle, the test samples immersed in the different fluids were placed in two chambers. The diesel fuel containers were stored in one of the chambers and the oil containers in the other, in order to avoid fluids contamination. A HEK-GMBH MCP-Vacuum-Casting-System (Fig. 7.1b) was used to dry the specimens after being coated with resin (at 60 ºC for 18 h) and to store the oil-immersed samples (at 110 ºC for the various periods established). The other chamber, a P Selecta (Fig. 7.1c), was used to store the specimens immersed in diesel fuel during the periods pre-established, at room temperature. After printing all the specimens, coating them with resin and immersing them in the during pre-defined periods, uniaxial tensile tests were performed according to ASTM D638 standard. The tensile tests were performed at room temperature in a Zwick Z100 universal testing machine (Fig. 7.2). The adopted methodology followed to carry out this work is illustrated in Fig. 7.3. Fig. 7.2 Zwick Z100 universal testing machine

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Fig. 7.3 Methodology followed to carry out the laboratorial tests

Table 7.1 Main material processing features and characteristics

Filament thickness [mm]

ABS (brand Filo Alfa)

PC (brand RepRap)

1,75

1,75

Extruding nozzle temperature [ºC]

250–290

270–290

Platform temperature [ºC]

70–110

100–110

The materials used to print the laboratorial samples were ABS and PC (Table 7.1). The printed specimens were coated by means of dipping in a mixture of Sicomin SR1500 resin and SD2503 hardener, procedure performed at room temperature. The resin, which main applications are the aeronautic, naval, and automotive industries, were submitted to a curing process that was carried out at 60 ºC for 18 h.

7.2.1 Laboratorial Procedure The printed samples used in the laboratorial tests were modelled in SolidWorks software, with a geometry according to ASTM D638 standard (type IV samples). After the modelling phase, the CAD file was imported to the printing equipment in STL format, and the definition of several printing parameters were carried out. After importing the model from SolidWorks it is necessary to define in the printer software the material to be used, the main processing parameters and the filling features. In a first stage, the specimens were printed according to a vertical orientation (Fig. 7.4a) of the layers, which leads to lower temperature variations along the specimens when compared to those obtained by printing in a horizontal orientation

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Fig. 7.4 a Specimens produced in the vertical position, with construction supports; b Specimens produced in the horizontal position, with no construction supports; c set of 4 specimens printed at a time.

(Fig. 7.4b). This lower temperature variation is a consequence of the shorter time spent creating for each layer, which leads to lower warping in the final piece, allowing the production of better quality specimens. This vertical construction methodology also allows a decrease in the amount of material used, a reduction in the manufacturing time of about 30% and an improvement in the surface finish of the printed geometry. During the printing phase, the specimen’s orientation in the horizontal position was chosen since after performing some tests with the vertical positioning initially defined, it became clear an increase of material used in each specimen. In addition, the horizontal positioning did not bring about significant increases in warping, as expected. To optimize the printing process, 4 specimens were printed at a time, with a duration of 1 h and 58 m for each set (Fig. 7.4c). For all the materials to be printed a layer thickness of 0.15 mm was used in the printing of each specimen and the “QUALITY” strategy was chosen, which provides the best relation between the quality of the produced part and the printing time required. The characteristics of the layers and of the brim were defined. The brim is the first printed layer and was defined with 5 mm length, with a higher perimeter than the specimen, which ensures the adhesion of the specimen to the printing table, reducing warping. The brim also helps the removal of the specimen from the table after printing, reducing the chances of damaging the specimen in this process.

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Although the layer thickness was set to 0.15 mm, the thickness used for the first layer was 0.2 mm, in order to increase the adhesion of the specimen to the table, so as to reduce possible specimens warping. The specimens were printed with a layer density of 1 and a rectilinear pattern to minimize voids between deposited layers. Finally, the printing temperatures were defined. For the processing temperatures definition, the reference temperatures contained in the respective datasheet of each material were used. However, it was necessary to make some adjustments to optimize the printing process. To isolate the printing environment an acrylic box was placed around the printer so that the printing temperatures could be more easily reached and maintained. The test specimens were immersed in the fluids under study, diesel fuel and oil, for periods of 7, 14, 21 and 28 consecutive days. For each dipping process, 3 specimens of each type were used (with or without coating, of each material) to ensure the results repeatability. A total of 96 test specimens were used, which correspond to 2 different materials (ABS, PC), with and without coating, 2 different types of fluids and 4 different dipping periods, as well as 3 samples for each one of the laboratorial tests. The specimens were coated with Sicomin SR1500 resin and the corresponding hardener. After determining the mass of each specimen before coating, the specimens were dipped in the mentioned mixture, in a mass fraction of 100 g of resin for each 33 g of hardener. The immersion of the specimens was performed manually, dipping each specimen in the solution, draining and placing them on a tray for subsequent drying in the HEKGMBH MCP-Vacuum chamber, for a period of 18 h at 60 ºC (Fig. 7.5). A total of 21 specimens of each material were submerged, in a total of 63 specimens. The immersion of the test specimens in the fluids under study required 12 recipients, of which 6 were used to place the specimens immersed in oil and the remaining

Fig. 7.5 Specimens in the chamber HEK-GMBH MCP-Vacuum

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Fig. 7.6 a Specimens in the chamber HEK-GMBH MCP-Vacuum during the dipping period in oil and b in the chamber P Selecta during the dipping period in diesel fuel Fig. 7.7 a Stress–Strain curve resulted from the uniaxial tensile tests of virgin ABS; b Tested specimens of virgin PC and ABS

a)

b)

Fig. 7.8 PC test specimens immersed in multigrade oil a uncoated and b coated

6 to place the specimens immersed in diesel fuel (both coated and uncoated specimens). The containers with oil were made of ceramic to avoid contamination of the oil and to resist the permanence at high temperatures during the predefined period in

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the chamber. The containers with diesel fuel were made of polymer, given the characteristics of the fluid and the temperatures to which the specimens were subjected (room temperature). In a motor vehicle, multigrade oil usually reaches its optimum lubrication characteristics at around 60 ºC, since below this temperature an excessive friction between the moving components of an internal combustion engine occurs. On the other hand, at high temperatures, around 135 ºC, the oil loses its lubrication characteristics. For racing vehicles, where performance levels are crucial, higher temperatures, around 150 ºC, can be reached. A conventional engine that has been operating for some time can easily reach 100–110 ºC, which is the normal operating temperature for a vehicle on the road (Davis, 2022). Therefore, a temperature of 110 ºC was used in the chamber during the oil immersion of the test specimens. Considering the average temperatures felt in Portugal and considering the most frequently used oil in automotive vehicles, a multigrade 5W30 oil was used. Diesel fuel does not undergo major temperature variations throughout the fuel supply circuit, so that the room temperature may be considered for the laboratorial tests. Therefore, the containers with oil were placed in the chamber at 110 ºC (ceramic plates on the top of containers ensured that the specimens did not float and remained totally submerged in oil). In the case of diesel fuel, there was no need to adopt any strategy to ensure that the specimens remained submerged given the geometry of the containers, and they were placed inside another chamber at room temperature (Fig. 7.6). The mass of all specimens was determined before and after their coating with resin and after the period of dipping in the respective fluids. The AG204 balance, available at the Materials Laboratory of the School of Technology and Management, was used for that purpose. Once the dipping process was concluded, the specimens were subjected to tensile tests. For this purpose, at the end of each period (7, 14, 21 and 28 consecutive days) under defined temperature conditions (110 ºC for oil and room temperature for diesel fuel), the specimens were removed from the containers, dried with absorbent paper to eliminate any excess fluid, and then subjected to unidirectional tensile tests (according to ASTM D638). Figure 7.7 presents the evolution of stress with strain for the specimens produced from ABS, before coating and dipping. Hereafter, these specimens type will be denominated virgin specimens. The tests were performed in a Zwick 100 equipment, with a displacement velocity of the experimental apparatus upper beam of 5 mm/min and a distance between the fixing zone of the specimens of 65 mm. Figure 7 presents the tested specimens of virgin PC and ABS, and the Stress–Strain curve resulted from the uniaxial tensile tests of virgin ABS.

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7.3 Results and Discussion 7.3.1 ABS Test Specimens Table 7.2 presents the results obtained from the uniaxial tensile tests performed on the ABS specimens. The results of the displacements and maximum tensile stress can be observed for the different immersion periods. The values identified with the immersion period of “0 days” correspond to virgin ABS specimens, which were neither coated nor immersed in any fluid. For the diesel immersed specimens, a slight increase in displacement was observed after 14 to 28 days, indicating an increase in flexibility and ductility of the coated specimens. For the uncoated specimens, the variations are less significant in any of the immersed periods, compared to the displacement resulting from the uncoated specimens. On the other hand, the uncoated specimens immersed in oil suffered a very pronounced variation of displacement, in all the immersion periods. This variation indicates an increase in flexibility and ductility of the material, which was minimized by the application of the coating, meaning that the coated specimens presented a minimal variation in displacement, except for the longest dipping period (28 days). Table 7.2 Mean displacement values and maximum tensile strength for ABS specimens Days

Medium

Coating

Displacement [mm]

Maximum Strain [MPa]

Average

Average

Standard deviation

Standard deviation

0

None

No Coating

1,544

0,019

35,269

1,963

7

Oil

Coating

1,563

0,215

29,235

4,066

No Coating

2,795

0,335

45,703

2,103

Diesel Fuel 14

21

28

Oil

Coating

1,547

0,729

31,754

0,477

No Coating

1,507

0,054

33,004

1,463

Coating

1,505

0,316

28,673

9,342

No Coating

2,602

0,562

33,682

1,994

Diesel Fuel

Coating

1,722

0,819

31,166

4,094

No Coating

1,402

0,055

29,045

1,842

Oil

Coating

1,438

0,106

25,546

4,103

No Coating

2,381

0,396

45,549

3,58

Diesel Fuel

Coating

1,624

0,768

32,028

1,257

No Coating

1,491

0,048

31,378

0,934

Oil

Coating

1,155

0,398

18,897

12,511

No Coating

2,648

1,266

45,87

0,197

Diesel Fuel

Coating

1,632

0,141

32,23

2,802

No Coating

1,507

0,093

29,117

0,672

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The large difference between the displacements observed in the uncoated specimens immersed in oil and in diesel fuel is remarkable. This indicates that the properties of oil and the higher temperatures at which the immersion were made make the uncoated specimens more ductile, and with greater ability to deform without breaking. For the uncoated specimens immersed in oil, tensile strength values increased significantly for all periods, which was not observed for the coated specimens. This indicates that the multigrade oil immersion increases the ductility of the material, widening its elastic regime. On the other hand, the application of the coating decreased the tensile strength of the specimens, so it can be concluded that the resin layer makes the specimens more fragile. As for the specimens immersed in diesel, few differences were found for both coated and uncoated specimens. The properties of diesel and the immersion at room temperature did not significantly change the tensile strength of the ABS specimens.

7.3.2 PC Test Specimens Among the specimens that were subjected to the immersion process, the ones that demonstrated the greatest degradation by contact with the multigrade oil were the PC specimens. The uncoated specimens fragmented while still in the oil container during the first 7 days of the immersion process (Fig. 8a). The coated specimens broke as soon as they were fixed in the tensile testing machine (Fig. 8b). Thus, for the uniaxial tensile tests of the PC specimens, only the results for the specimens immersed in diesel are presented, and only 3 immersion periods (7, 14 and 21 days) were carried out. The values of displacement and maximum tensile stress obtained from the uniaxial tensile tests performed on the specimens immersed in diesel fuel, coated and uncoated, in the different periods, are shown in Fig. 7.9. The first bar of each graph refers to the period of 0 days, which corresponds to the values for virgin material, uncoated and not immersed in any fluid. The coated PC specimens, immersed in diesel fuel (Fig. 7.9a), suffered an average displacement of 0.802 ± 0.392 mm at the end of 7 days and 0.653 ± 0.309 mm after 21 days. For the uncoated specimens the average values of 1.148 ± 0.046 mm at 7 days and 0.99 ± 0.096 mm after 28 days were observed. According to Fig. 7.9a, the overall displacement of the immersed specimens (with and without coating) decreased when compared to the displacement observed in specimens in virgin material. This indicates a loss of ductility in the specimens that were immersed in diesel fuel, whether coated or uncoated. It should be noted that this decrease in displacement is more pronounced in coated specimens, which indicates that the resin layer covering the specimens makes them more fragile. The average maximum tensile strength values (Fig. 7.9b) for the coated PC specimens were 25.049 ± 12.039 MPa at the end of 7 days and 19.265 ± 9.106 MPa at day 28. The uncoated specimens presented values of 36.934 ± 2.679 MPa at the end

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Fig. 7.9 a Displacement and b Maximum tensile stress values of the PC test specimens immersed in diesel

of 7 days and 31.813 ± 2.145 MPa at 28 days. The value for the virgin PC specimens was 49.229 ± 0.701 MPa. These values indicate a decrease in mechanical strength for both coated and uncoated specimens after diesel immersion. This decrease is, however, more accentuated in the coated specimens. It can be concluded that diesel fuel imposes a degradation on PC specimens, making them more fragile, and the resin coating accentuates this effect.

7.4 Conclusion The several advantages presented by additive manufacturing processes captured the attention of multiple industries, which allowed them to increase the offer of products and equipment.

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The aim of this work was to understand the variation of permeability in components manufactured by FDM, when in contact with fluids present in a car with an internal combustion engine, by applying an epoxy resin coating. For this purpose, several FDM specimens were produced, then coated with an epoxy resin and immersed in the selected fluids, namely multigrade oil and diesel, for different periods. After the immersion periods, the specimens were tested in the laboratory using uniaxial tensile tests. From the analysis carried out, it was concluded that PC is not a suitable material to be in contact with multigrade oil at normal operating temperatures (110 ºC) in a motor vehicle with an internal combustion engine. Its low ductility and high fragility caused the uncoated specimens to fragment while still in the containers where they were immersed in the fluids, and the coated specimens to break as soon as they were subjected to the uniaxial traction test. ABS proved to be the most suitable material for use in contact with multigrade oil at high temperatures. Oil immersion of ABS manufactured components tends to improve their ductility. Acknowledgements This work was partially supported by project UIDB/00308/2020.

References 1. Almeida, H.A., Vasco, J. C.: Expectations of Additive Manufacturing for the Decade 2020– 2030”, Progress in Digital and Physical Manufacturing - Proceedings of ProDPM’19, Almeida et al. (Eds.), Lecture Notes in Mechanical Engineering, Springer (ISBN: 978–3–030–29040–5 (Print) 978–3–030–29041–2 (eBook)), Cham, 10–19 (2020). https://doi.org/10.1007/978-3030-29041-2_2 2. Lee, J., Chua, P.C., Chen, L., Ng, P.H.N., Kim, Y., Wu., Qiong, Jeon, S., Jung, J., Chang, S., Moon, S.K.: Key Enabling Technologies for Smart Factory in Automotive Industry: Status and Applications. Int. J. Precis. Eng. Manuf. 1, 93–105 (2023). https://doi.org/10.57062/ijpem-st. 2022.0017 3. Almeida, H.A., Pei, E., Vitorino, L.: “Sustainability for 3D printing”, Sustainability for 3D Printing, K. Sandhu et al. (Eds.), Springer (ISBN: 978–3–030–75235–4), Chapter 1: 1–13 (2022). https://doi.org/10.1007/978-3-030-75235-4_1 4. Charles, A., Hofer, A., Elkaseer, A., Scholz, S.G.: Additive Manufacturing in the Automotive Industry and the Potential for Driving the Green and Electric Transition” in: Scholz, S.G., Howlett, R.J., Setchi, R. (eds) Sustainable Design and Manufacturing. KES-SDM 2021. Smart Innovation, Systems and Technologies, vol. 262. Springer, Singapore (2022). https://doi.org/ 10.1007/978-981-16-6128-0_32 5. Bockin, D., Tillman, A.-M.: Environmental assessment of additive manufacturing in the automotive industry. J. Clean. Prod. 226, 977–987 (2019). https://doi.org/10.1016/j.jclepro.2019. 04.086 6. Priarone, P.C., Catalano, A.R., Settineri, L.: Additive manufacturing for the automotive industry: on the life-cycle environmental implications of material substitution and lightweighting through re-design. Prog Addit Manuf (2023). https://doi.org/10.1007/s40964023-00395-x 7. Jankovics, D., Barari, A.: Customization of Automotive Structural Components using Additive Manufacturing and Topology Optimization. IFAC PapersOnLine 52–10, 212–217 (2019)

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8. Jothi Basu, R., Abdulrahman, M.D., Yuvaraj, M.: Improving agility and resilience of automotive spares supply chain: The additive manufacturing enabled truck model. Socioecon. Plann. Sci. 85, 101401 (2023). https://doi.org/10.1016/j.seps.2022.101401 9. Sargini, M.I.M., Masood, S.H., Palanisamy, S., Jayamani, E., Kapoor, A.: Additive manufacturing of an automotive brake pedal by metal fused deposition modelling. Materials Today: Proceedings 45, 4601–4605 (2021). https://doi.org/10.1016/j.matpr.2021.01.010 10. Delic, M., Eyers, D.R.: The effect of additive manufacturing adoption on supply chain flexibility and performance: An empirical analysis from the automotive industry. Int. J. Production Economics 228, 107689 (2020). https://doi.org/10.1016/j.ijpe.2020.107689 11. Schmitt, M., Mehta, R.M., Kim, I.Y.: Additive manufacturing infill optimization for automotive 3D-printed ABS components. Rapid Prototyping Journal 26(1), 89–99 (2020). https://doi.org/ 10.1108/RPJ-01-2019-0007 12. Shanmugam, S., Naik, A., Sujan, T., Desai, S.: Developing Robust 3D Printed Parts For Automotive Application Using Design For Additive Manufacturing And Optimization Techniques. INCOSE International Symposium 29, 394–407 (2019). https://doi.org/10.1002/j.2334-5837. 2019.00694.x 13. Yu, L., Wu, S., Feng, Y., Zhao, C.: Development and Prospect of Additive Manufacturing Technology in Automobile Field. Academic Journal of Science and Technology 3(3), 243–246 (2022). https://doi.org/10.54097/ajst.v3i3.2991 14. Ramalho, F.Q., Alves, M.L., Correia, M.S., Vilhena, L.M. and Ramalho, A.: Study of Laser Metal Deposition (LMD) as a Manufacturing Technique in Automotive Industry, Progress in Digital and Physical Manufacturing, Lectures Notes in Mechanical Engineering, pp. 225–239 (2020) Springer, https://doi.org/10.1007/978-3-030-29041-2_29 15. Leal, R., Barreiros, F.M., Alves, L., Romeiro, F., Vasco, J.C., Santos, M., Marto, C.: Additive manufacturing tooling for the automotive industry. Int. J. Advanc. Manufact. Technol. ISSN 0268–3768, 92, pp. 1671–1676 (2017). https://doi.org/10.1007/s00170-017-0239-8

Chapter 8

3D Bioprinting of Cellulosic Structures for Versatile Applications Özkan Yapar

Abstract Utilizing renewable and biodegradable feedstocks have been researched extensively to develop higher value-ended products, due to increasing awareness on environmental issues associated with use of fossil-based resources. Among them, cellulose is the most abundant natural biopolymer which exhibits an excellent source of raw material. Interestingly, cellulose can be converted into cellulose derivatives i.e., ethers and esters and be produced its micro/nano forms (e.g., nano-fibrillated, and nano-crystalline cellulose, bacterial (nano) cellulose), and can be regenerated for development of aerogels, filaments, or bio (nano) composites, which represent its potential applications for textiles, pharmaceutical and packaging industries etc. Recent studies indicated, those cellulosic materials have been investigated in additive manufacturing to acquire three-dimensional (3D) bio-printed patterns and tailored design objects such as filaments. This paper focused on 3D printable materials, structures, and techniques, specifically to elucidate data for their novel regenerated cellulose forms using ‘green’ solvent systems. Herein, dissolving wood pulp was used as main cellulosic substrate, which was dissolved in a non-derivatizing solvent, an ionic liquid, that was used to acquire a bioink which was subsequently printed in 3D filament forms via air-gap spinning technique of a configured syringepump set-up. Subsequently, exploited sample was coagulated and regenerated in a bath, consisting of deionized water, and eventually air-dried at room temperature (RT) in lab. Resultant filament showed remarkable tensile property. Printing results confirmed efficiency of the applied method to fabricate interesting prototypes. In this context, utilization of only DI water at RT contributes to design and enhance an environmentally and economically sustainable 3D bioprinting system. Keywords 3D bioprinting · Regenerated cellulose filaments · Ionic liquids

Ö. Yapar (B) Faculty of Mechanical Engineering, University of Maribor, Maribor, Slovenia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_8

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8.1 Introduction The global population is expected to reach 9.7 billion in 2050 and 10.4 billion in 2100, according to the medium scenario of the United Nations [1]. Due to the fact that, intense research activities have been carried out to deal with the manufacturing demands and needs [2]. Therefore, unsustainable production from non-renewable resources and their disposed waste are major problem to be (bio) degraded and/ or coped, which causes a substantial pollution globally as well as climate changes derived from their green gas emissions [2, 3]. Because, petroleum-based synthetic polymers are extensively utilized and play an undeniable role in human and animal health. In the past decade, plastic waste has been also classified as hazardous, and its global pollution needs to be solved crucially [3]. Just because, less than 10% of plastic waste gets recycled, 12% goes for incineration, and the remaining amount of 78% goes to landfills or leaks into the natural environment [4]. Approximately, 11% of the global plastic waste reaches oceans and drastically impacts marine ecosystems [5]. Enormous reports were already published that non-biodegradable plastic waste became pollutants in rivers and the oceans through input from the land [2, 3]. If that current figure will remain same by 2050, the plastic industry may need 20% of the crude oil supply to assure the desired amount of plastic production [6]. Each year, 8 million tons of plastic is dropped at sea. There would be more amount of plastic than fish in the oceans by 2050 if that circumstance will remain as unchanged [7]. Consequently, scientific investigations for research and product development acts of biobased materials have increased extensively due to the awareness of those ecological and health issues related to traditional petroleum-based polymers. So that, have led to an inclination toward the researchers, governments, and corporations to focus their interest on bio-degradable, cheap, and natural polymer alternatives [8–10].

8.1.1 Cellulose Cellulose is a polysaccharide, which is the most abundant natural biopolymer in nature and is considered an almost inexhaustible source of raw material to be exploited for the increasing demand for environmentally friendly and biocompatible products due to its bio-based, biodegradable and renewable property, suitability of its surface for chemical modifications, exhibiting mechanically robust performance and thermally and chemically stable and environmentally benign character, thus represents approximately 1.5 × 1012 tons of the total annual production of biomass [11–13]. Main sources of the cellulose are woods [14], plants [15, 16] or agricultural residues [17–19], tunicate [20, 21], algae [22–24], and bacteria [25–27]. In this context, wood pulp remains the most important raw material source for the processing of cellulose, most of which is extensively utilized for the production of paper and cardboards [12]. Softwoods (e.g., Norway spruce wood [28]) and hardwoods (e.g., birch wood [29] possess about 40–47% of cellulose by its weight percentage in their

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composition. Non-wood biomasses have cellulose content from 30 to 95% such as sugar-cane, cotton, bagasse, coir, sisal, hemp, flax and so forth [30, 31].

8.1.2 Cellulose Structure and Morphology in a Nutshell Woods are primarily consisting of cellulose, lignin, and hemicellulose. In that composition of hemicellulose and lignin matrix, cellulose microfibrils are embedded and aligned in their oriented layers [32, 33]. Cellulose is a polydisperse and syndiotactic natural polymer, consisting of a linear chain of ringed glucose molecules with repeat units of two anhydro glucose rings ((C6 H10 O5 ) n, where n depending on the source of the cellulose, linked through oxygen covalently bonded to C1 of one glucose ring and C4 of the adjoining ring, the so-called β 1–4 glycosidic bond [34–36]. These single cellulose chain units constitute the elementary fibrils which are organized in crystalline and amorphous regions where are composed of cellulose nanocrystals and disordered parts, respectively [32]. The crystalline cellulose is polymorphic and convertible into its I, II, III and IV structures. The crystalline phase of cellulose I is consisting of two metastable structures, i.e., triclinic (Iα ) and monoclinic (Iβ ). The ratio of Iα to Iβ structures differs and depends on the source of the cellulose. The Iα structure is the allomorph which is typically known for most algal and bacterial cellulose with a triclinic unit cell [22]. The Iβ allomorph exists in plantbased and tunicate-based cellulose sources with a monoclinic unit cell including two parallel chains [21]. Moreover, depending on the applied processes cellulose I, cellulose II and III types can be obtained. In this point, cellulose II is considered to have the most stable owing its monoclinic structure and produced by structural and irreversible transformation of cellulose I type. That (trans) formation is carried out either via regeneration process, which is basically consisting of dissolution with several solvent systems following its recrystallization or by mercerization process by means of aqueous sodium hydroxide treatments of cellulose I, which will be briefly described in next sub-chapters of this paper [32, 37].

8.1.3 Cellulose Sources and Its Derivatives Cellulose could be classified basically according to its sources of origin i.e., wood, plant, animal, algae, and bacteria. They can be categorized simply based on their structure and morphology that will be briefly elucidated in next chapters. In this context, regenerated cellulose is apart from that classification and categorization, but highly important be added as a particular type due to its different sources of origin and derivation methods. In consequence, different acronyms are used for each cellulose kind. In this regard, the woods are composed of two kinds i.e., softwood and hardwood. The plants and their residuals are comprised of (ligno) cellulosic fibers such as cotton,

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flax, hemp, jute, kenaf, kapok, ramie, sisal, etc. The plant residuals are consisting of bagasse, rice husk, potato/carrot peel waste, oil palm biomass, cereal straw, etc. Cellulose molecules in either wood or plant have a complex, multi-level structure where are linked to other biopolymers like hemicellulose and lignin, but they exhibit a different microstructural organization and also differ in cellulose, hemicellulose, and lignin content, which makes each of them is unique [38]. Particularly, it has to be stated that cotton fibers does not have considerable amount of lignin i.e. 0–5% or hemicellulose i.e. 2.0–6.4% in compare with the wood based cellulose fibers, just because it has high cellulose content i.e. 82–96% [39].

Bacterial, Algal, and Tunicate (Nano) Celluloses The type of cellulose generated via bacteria is called as bacterial cellulose (BC) or bacterial nanocellulose (BNC), exhibiting an almost pure form of cellulose around/ above 90% without embedded into lignin or hemicellulose or others like wood and plant based cellulose fibers. The BC or BNC is primarily produced extracellularly only one genus of Gram-positive bacteria namely Sarcina and by Gram negative bacteria such as Agrobacterium, which is the most widely used type in its kind [40]. The BNC comprises with randomly assembled, ribbon-shaped fibrils ( 90% ferrite) by supressing the solid-state transformation into austenite. The correspondingly finegrained PBF-LB/M microstructure leads to high strength. However, due to the lack of austenite, the material also possesses low ductility. Therefore, substrate preheating is required during PBF-LB/M to reduce the crack susceptibility. By subsequent solution annealing at temperatures slightly above 1,000 °C the ferrite–austenite phase ratio can be adjusted to ensure a sufficient ductility and toughness [12]. Due to the higher energy density compared to PBF-LB/M, DED processes characteristically have lower cooling rates of 103 –104 K/s [13], which result in an enhanced austenite formation. In addition, the cyclic reheating of subsequent layers during PBF-LB/M and DED promotes the precipitation of secondary austenite. At least, depending on the energy source and process management applied, microstructures with an austenite percentage of 30–70% can be achieved by DED [5, 11, 14]. It has been demonstrated that almost defect-free specimens with a balanced ferrite– austenite phase ratio can be generated by DED-LB/M when adjusting the process conditions [14]. Iams et al. [11] showed that the intrinsic reheating cycles in DEDLB/M lead to three different morphologies of austenite. In addition to the primarily formed grain boundary austenite, Widmanstätten-like austenite grows from grain boundaries into the ferritic grains and finely distributed intragranular austenite is secondarily precipitated. However, there is still a lack of data on the relations between microstructural and mechanical properties of AM-processed DSS in as-built (AB) and heat-treated (HT) conditions. The purpose of this study is to examine these correlations on the example of a 1.4462 being processed by DED- and PBF-LB/M,

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and after a post-process heat treatment. Based on the knowledge gained, it should be possible to adjust the mechanical properties according to the specification of the base material 1.4462 (DIN EN 10088–3) and to qualify AM for processing of DSS.

9.2 Experimental Setup 9.2.1 Materials and Processes DSS 1.4462 (Thyssenkrupp Materials Trading GmbH) was used as the powder feedstock material for both AM processes. The DSS was atomized under a nitrogen atmosphere and then sieved into two process-specific particle size fractions. The chemical composition was determined by X-ray fluorescence spectroscopy and carrier gas hot extraction. Particle size distribution and sphericity (SPHT) were measured using a Camsizer X2®. The powder particle size ranges from 20 μm–125 μm (SPHT = 0.85) for DED-LB/M and from 20 μm–53 μm (SPHT = 0.82) for PBF-LB/M. Table 9.1 lists the measured powder compositions and the alloy specification of 1.4462 according to DIN EN 10088–3. A DED-system of type TLC 3008 (TRUMPF SE + Co. KG) was used to generate the DED-LB/M specimens. The laser-processing cell is equipped with a five-axis motion system, a 1 kW cw Yb:YAG disk laser (λ = 1,030 nm), a gravimetric powder feeder, and a YC52 (Precitec KG) laser processing head with a four-jet powder nozzle. The working distance between powder nozzle and substrate plane was set to z = 13.5 mm to work within the focusing plane of powder jets. The laser spot diameter was kept constant at dL = 1.5 mm. Nitrogen was used as shielding gas (15 l/min) to minimize oxidation and to prevent a nitrogen loss from the DSS during the melting process. Helium was used as powder carrier gas with a flow rate of 7 l/ min. The process parameter set for generating the DED-LB/M specimens is listed in Table 9.2 and was developed in previous studies [14]. The PBF-LB/M specimens were generated using an EOS M 290 equipped with a 400 W cw Yb-fibre laser (λ = 1,070 nm; dL = 0.1 mm) under a nitrogen inert Table 9.1 Alloy specification of 1.4462 and composition of DED- and PBF-LB/M powder Chemical composition of 1.4462 in wt.-% Fe

C

Si

Mn

Cr

Ni

Mo

N

O

DIN EN 10088–3

Bal

≤ 0.03

≤ 1.0

≤ 2.0

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Table 9.2 Process parameters for DED- and PBF-LB/M of DSS 1.4462 Process

Laser power

Scanning speed

Powder mass flow 5.0 g/min

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Hatch distance

Layer height

Substrate preheating

0.64 mm

0.45 mm

/

0.12 mm

0.04 mm

80 °C

gas atmosphere. The substrate was made of 1.4404 and preheated to 80 °C. A bidirectional scanning strategy (67° rotation per layer) and an optimized parameter set (Table 9.2) was used for generating structures with a maximum relative part density. Cubic specimens with geometries of 15 × 15 × 60 mm3 (for porosity analysis, microstructure analysis, ferrite, and hardness measurements, and Charpy impact tests) and cylindrical specimens of 15 × 15 × 65 mm3 (for tensile tests) were built using both PBF-LB/M and DED-LB/M. The specimens were oriented vertically to the substrate. For subsequent adjustment of the phase ratio and homogenization of the AB microstructure, specimens of both AM processes were heat treated in a muffle furnace (Nabertherm N60/HR). The specimens were annealed at 1,050 °C for 2 h in an argon gas atmosphere prior to water quenching.

9.2.2 Characterization Methods For metallographic investigations, vertical cross-sections of cubic specimens (15 × 15 × 60 mm3 ) were prepared for both the different AM processes (DED-LB/ M and PBF-LB/M) and material conditions (AB and HT). The cross-sections were grinded and polished down to 0.25 μm. Optical microscopy (OM) was performed to evaluate the build quality in terms of relative porosity. After etching the polished cross-sections according to Beraha II (H2 O, HCl, (NH4 )HF2 , K2 S2 O5 ) for 30 s – 1 min, the contrasted microstructure was examined by OM using a Zeiss Axioplan 2. The resulting ferrite–austenite phase ratio was studied by measuring the ferrite content magnetic-inductively in dependence of the build height. For this, a Feritscope FMP30® (Helmut Fischer GmbH) was used. Hardness profiles (3 × 10 measurement grids; 3 mm distance in x-direction, 5 mm distance in z-direction) of specimen crosssections were generated by means of an Innovatest Falcon 603 hardness indenter to investigate the Vickers hardness (HV10) as a function of build height. For quasistatic tensile tests, the cylindrical specimens were milled to standardized tensile specimens of type B6 × 30 (DIN 50125). Tensile tests were performed at room temperature according to DIN EN ISO 6892–1 using a servo-hydraulic universal testing system (Galdabini Quasar 200) with a maximum force of 200 kN and a strain rate of 0.00025 s−1 (A224). Furthermore, the impact toughness of the milled cubic specimens (10 × 10 × 55 mm3 ) was determined at room temperature using Charpy impact tests according to DIN EN ISO 148–1. For this, a Galdabini IMPACT 450

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Ø 6

Ø6

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R2x2

45°

Fig. 9.2 Schematic diagrams of a tensile test specimen (DIN 50125) and b Charpy impact test specimen (DIN EN ISO 148–1)

with a maximum impact energy of 450 J was used. Schematic diagrams of both specimen types are provided in Fig. 9.2.

9.3 Results A cubic specimen was taken from each process to evaluate the build quality in terms of relative porosity. The relative part densities of the DED-LB/M and PBF-LB/M specimens were around 99.80% and 99.99%, respectively. Furthermore, no cracks were observed within both specimens. Metallographic analyses revealed only small amounts of gas pores both for DED-LB/M and PBF-LB/M. Figures 9.3 and 9.4 show the AB and HT microstructures of the DED-LB/M and PBF-LB/M specimens at 25 × and 500 × magnification, respectively. The AB microstructure (Fig. 9.3 left) of the DED-LB/M specimen is very heterogenous and is characterized by a periodic alternation of ferrite-rich (dark) and austenite-rich (bright) zones. These austenite-rich zones are a consequence of the

IA

top

top

IA

GA

1 mm

50 µm

1 mm

bottom

bottom GA

1 mm

50 µm

50 µm

IA

WA

GA WA

1 mm

50 µm

Fig. 9.3 Optical micrographs of DSS 1.4462 processed by DED-LB/M in AB (left) and HT (right)

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top

GA

top WA

111

IA WA

GA 1 mm

50 µm

bottom

1 mm

1 mm

50 µm

bottom

50 µm

1 mm

50 µm

Fig. 9.4 Optical micrographs of 1.4462 processed by PBF-LB/M in AB (left) and HT (right)

overlapping and cyclic heat treatment of deposited tracks and layers due to the layerby-layer build-up. Higher magnifications reveal the growth of Widmanstätten-like austenite (WA) from the austenitic grain boundaries (allotriomorphic grain boundary austenite, GA) into the ferritic grains. As the build height increases, a higher amount of WA is formed and the precipitation of intragranular austenite (IA) is promoted. This effect is due to heat accumulation and reduced cooling rates that support the austenite formation due to the altered transformation behavior. Post-process heat treatment by solution annealing and quenching homogenized the microstructure due to recrystallization (Fig. 9.3 right). AM-related heat-affected zones were largely dissolved, resulting in a ferritic matrix interspersed with finely distributed austenite structures and a still balanced phase ratio. No detrimental intermetallic phases (e.g. sigma-phase) were found. Optical micrographs of the PBF-LB/M specimens in AB (Fig. 9.4 left) display an almost completely ferritic microstructure with small fractions of austenite at the ferrite-ferrite grain boundaries. These austenite fractions increase with increasing number of deposited layers. At higher magnification, thin contours of melt pool boundaries and a faint substructure of elongated grains traversing several melt pools can be seen. This is an indicator for an epitaxial grain growth. By subsequent heat treatment of the microstructure, fine austenite structures were formed within the ferrite matrix (Fig. 9.4 right). As for the heat-treated DED-LB/M specimens, grain boundary austenite, small amounts of Widmanstätten-like austenite, and finely precipitated intragranular austenite can be observed. Figure 9.5 shows the ferrite content and Vickers hardness of all cross-sections along the build direction. The DED-LB/M DSS has a ferrite content of 54 ± 1% in the bottom area (10 mm) in the as-built condition. The ferrite content decreases along build direction in the first 20 mm and stagnates at a ferrite content of 42 ± 1% until the top region of the specimen is reached (approx. 50 mm). This progression can be explained by the effect of heat accumulation, which occurs in particular within the first few

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80 60 40 20

n=3 build height

n=3

hardness in HV10

ferrite content in %

a) 100

350 300 250

substrate (1.4462)

200

0 0

1

2 3 4 5 build height in cm

6

0

1

2 3 4 build height in cm

5

6

Fig. 9.5 Ferrite content (a) and hardness (b) of DED- and PBF-LB/M processed 1.4462

millimeters of the part. In the area close to the substrate, high cooling rates are predominant and promote a fine-grained ferritic microstructure. Heat accumulation within the subsequent layers increases the average melt pool size, which results in a decreased cooling rate. Hence, the decreased cooling rates promote austenite formation and result in grain coarsening. The resulting hardness profile correlates with the ferrite profile and shows a drop from 270 ± 5 HV10 at the bottom to 245 ± 4 HV10 at the top. Heat treatment of the specimens homogenized the phase ratio and the material hardness to about 45–48% ferrite and 238–245 HV10, respectively. The PBF-LB/M specimens offered a high ferrite content of 83–86% in the as-built condition. This phase fraction is mainly unaffected by the build height. However, due to substrate preheating and the use of a process parameter set with relatively high energy input for PBF-LB/M, a small amount of austenite is formed. In contrast to DED-LB/M, the smaller austenite fraction in AB is due to a suppressed solid-state transformation as a result of low interpass temperatures and extremely high cooling rates. The hardness is decreasing from 355 ± 4 HV10 to 324 ± 5 HV10 along the build direction. As for DED-LB/M, this effect can be explained by grain coarsening due to the reduced cooling rates. Solution annealing and quenching increased the austenite percentage of the PBF-LB/M specimens to a balanced phase ratio with 42– 44% ferrite along the entire cross-section. This causes a significant hardness decrease (241–248 HV10) compared to the as-built condition. Furthermore, tensile tests and Charpy impact tests were carried out on the DEDLB/M and the PBF-LB/M specimens for both conditions. Figure 9.6 summarizes the results. The heat-treated DED-LB/M specimens possess a slightly reduced strength (yield strength (YS) = 455 ± 3 MPa; ultimate tensile strength (UTS) = 730 ± 3 MPa) compared to the DED-LB/M specimens in as-built condition (YS = 468 ± 1 MPa; UTS = 758 ± 1 MPa). However, elongation at break is slightly increased after heat treatment from A = 31 ± 1% to 35 ± 1% indicating a sufficient ductility in both conditions and a typical comparable load extension behavior. Impact toughness (absorbed impact energy KV ) also increases from KV = 83 ± 10 J to 91 ± 12 J.

9 Comparison of Material Properties of Duplex Stainless Steel 1.4462 …

800 UTS min 600 YS min 400

960

n=3 758

730 468

31

100

877 720 465 39

455 35 15

200 0

80 60

A in %

1000

40 A min 20 0

b)

impact energy KV in J

UTS / YS in MPa

a)

200

113

n=3

160

125

120 83 80 KV, min 40

91 48

0

DED-AB DED-HT PBF-AB PBF-HT UTS YS A

Fig. 9.6 UTS, YS, A (a) and impact energy KV (b) of DED- and PBF-LB/M processed 1.4462

Referring to Fig. 9.6, the DED-LB/M specimens in both conditions (AB and HT) fulfill the specifications of 1.4462 according to DIN EN 10088–3 (YS ≥ 450 MPa; UTS = 640–840 MPa; A ≥ 25%; KV ≥ 60 J). PBF-LB/M of DSS 1.4462 resulted in high strength at YS = 877 ± 5 MPa and UTS = 960 ± 12 MPa, A = 15 ± 1% elongation at break and an impact toughness of KV = 48 ± 12 J. Elongation at break and impact toughness in AB condition are significantly below the specification of 1.4462. This proves that a post-process heat treatment is indispensable to increase the ductility. After solution annealing and quenching of the AB material, the minimum limits of the specification were exceeded and especially the impact toughness was significantly increased to KV = 125 ± 15 J.

9.4 Conclusion Correlations of microstructural and mechanical properties of DSS 1.4462 generated by DED- and PBF-LB/M are provided. Both processes allow for a defect-free generation of specimens. Hardness, tensile properties, and impact toughness are strongly influenced by the phase ratio and microstructural features. Thereby, DED-LB/M specimens are characterized by an almost balanced ferrite–austenite phase ratio with grain boundary, Widmanstätten, and intragranular austenite morphologies observed by microstructure analysis. DED-LB/M resulted in a reduced strength (YS = 468 ± 1 MPa) but high elongation at break (A = 31 ± 1%) and high impact toughness (KV = 83 ± 10 J). Subsequent heat treatment by solution annealing and quenching had a minor effect on the phase ratio. Correspondingly, the tensile properties were similar to the as-built condition. In both conditions (AB and HT), the DED-LB/M specimens exceeded the minimum specification limits of DSS 1.4462. PBF-LB/M specimens, in contrast, possess a significantly lower austenite content (14–17%) in the as-built condition due to the higher process-specific cooling rates. Consequently, a higher

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hardness was determined, which explains the higher tensile strength (YS = 877 ± 5 MPa) but lower ductility (A = 15 ± 1%). Applying a post-process heat treatment was beneficial to meet the specifications of DSS 1.4462 in terms of strength (YS = 720 ± 1 MPa) and ductility (A = 39 ± 1%) by balancing the phase ratio. Moreover, the impact toughness of the PBF-LB/M was increased significantly from KV = 48 ± 12 J to 156 ± 19 J. Further investigations will focus on the investigation of adapted process strategies for PBF-LB/M so that an increased austenite content and sufficient ductility can be achieved already in as-built condition and a post-process heat treatment will not be required. Additionally, wear and corrosion resistance will be studied, both for PBF-LB/M and DED-LB/M of DSS 1.4462 to investigate the correlations between the respective mechanism and the underlying microstructure. Acknowledgements The authors would like to thank the Bayerische Forschungsstiftung for funding the project AZ-1465-20.

References 1. Örnek, C., Engelberg, D.L.: Correlative EBSD and SKPFM characterisation of microstructure development to assist determination of corrosion propensity in grade 2205 duplex stainless steel. J Mater Sci 51, 1931–1948 (2016) 2. Pickering, F.: Physical metallurgy of stainless steel developments. Int. Met. Rev. 21, 227–268 (1976) 3. Gunn, R.N.: Duplex Stainless Steels: Microstructure, Properties and Applications (1997) 4. Chen, T.H., Yang, Y.J.: Effects of solution treatment and continuous cooling on sigma-phase precipitation in a 2205 duplex stainless steel. Mater. Sci. Eng. A 311, 28–41 (2001) 5. Haghdadi, N., et. al.: Additive manufacturing of steels: a review of achievements and challenges. J. Mater. Sci. 56, 64–107 (2021) 6. Niendorf, T., et al.: Highly anisotropic steel processed by selective laser melting. Metall. Mater. Trans., B 44(4), 794–796 (2013) 7. Saedi, K., Gao, X., Zhong, Y., et al.: Hardened austenite steel with columnar sub-grain structure formed by laser melting. Mater. Sci. Eng. A 625, 221–229 (2015) 8. Iams, A., Keist, J., Giannuzi, L., Palmer, T.: The evolution of oxygen-based inclusions in an additively manufactured SDSS. Metall. Mater. Trans. A 52, 3401–3412 (2021) 9. Jiang, Z., Chen, X., Huang, H., et al.: Grain refinement of Cr25Ni5Mo1.5 DSS by heat treatment. Mater. Sci. Eng. A 363, 263–267 (2003) 10. Kain, V.: Stress corrosion cracking (SCC) in stainless steels. In: Woodhead Publishing Series in Metals and Surface Engineering, 199–244 (2011) 11. Iams, A., Keist, J., Palmer, T.: Formation of austenite in additively manufactured and postprocessed duplex stainless steel alloys. Metall. Mater. Trans. 51, 982–999 (2020) 12. Hengsbach, F., Koppa, P., Duschik, K., et al.: Duplex stainless steel fabricated by selective laser melting-Microstructural and mechanical properties. Mater. Des. 133, 136–142 (2017) 13. DebRoy, T., et al.: Additive manufacturing of metallic components-process, structure and properties. Prog. Mater Sci. 92, 112–224 (2018) 14. Maier, A., et al.: Influence of process parameters on mechanical and microstructural properties of DSS 2205 (1.4462) processed by DED-LB/M. Proc CIRP 111, 241–246 (2022)

Chapter 10

Influence of Carbon Content on the Material Properties of Low-Alloyed Steel Bainidur AM Dominic Bartels , Tobias Novotny, and Michael Schmidt

Abstract Low-alloyed steels are used in various application fields like gearing and bearing technology. The hardenability of these steels is mainly determined by the carbon content, which increases the strength of the martensitic phase. Since lowalloyed steels typically possess a low carbon content below 0.2 wt.%, an additional post-process heat treatment in a carbon-rich atmosphere is necessary to increase the carbon concentration in the product’s case. Another potential approach for improving the strength of the part’s surface is provided by applying additive manufacturing processes like laser-based directed energy deposition (DED-LB/M). Powder-based DED-LB/M supports in-situ alloying since multiple powder hoppers can be used for supplying the different powder materials. By adding e.g., carbon or hard particles, the hardness and wear resistance of the part can be tailored to the needs of the final application. However, to exploit the potentials of in-situ alloying for the deposition of optimized structures, the influence of the chemical composition, especially the carbon content, on the resulting material properties needs to be known. Within this work, the low-alloyed steel Bainidur AM (0.23 wt.% C) is processed by means of DED-LB/M. Furthermore, elemental carbon nanoparticles are added to increase the total carbon concentration up to 0.3 wt.%, 0.35 wt.%, and 0.4 wt.% within the powder. Multiple layers are manufactured to investigate the underlying material properties. The relative part density is only barely affected by the different carbon contents. Furthermore, the increased carbon content did not result in an increased crack tendency. Light optical microscopy reveals a primarily martensitic microstructure for all carbon contents. The material hardness increases linearly with increasing carbon concentration. Whereas the hardness of the unmodified Bainidur AM falls D. Bartels (B) · T. Novotny · M. Schmidt Institute of Photonic Technologies (LPT), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Konrad-Zuse-Straße 3/5, 91052 Erlangen, Germany e-mail: [email protected] D. Bartels · M. Schmidt Erlangen Graduate School in Advanced Optical Technologies (SAOT), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Paul-Gordan-Straße 6, 91052 Erlangen, Germany © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_10

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in the range of just below 400 HV1, a maximum hardness of around 560 HV1 was observed for a carbon content of 0.4 wt.%. Keywords DED-LB/M · Low-alloyed steels · Carbon concentration · Coating · Bainidur AM

10.1 Introduction Low-alloyed case-hardening steels like 16MnCr5, 20MnCr5, or 18CrNi8 are used in different fields of application due to their material properties [1]. Their alloy design results in good processability across different manufacturing technologies at the expense of material hardness. This can be attributed to a low con-centration of carbon and nitrogen, which are considered major drivers of hardenability in steels. The chemical composition of this class of steels supports the carbon and nitrogen diffusion at elevated temperatures, which is beneficiary for hardening these materials in a subsequent step [2]. Carburizing and nitrating heat treatments require long holding times at high temperatures. Correspondingly, these processes are mostly used for large batches of e.g., gears, shafts, or bearings. The fabrication of smaller lot sizes, however, is not necessarily economically efficient. Furthermore, the conventional case-hardening process is limited regarding the spatially-resolved carburizing of the parts. To tailor the carbon diffusion, the specimens need to be e.g., covered to avoid excessive carbon concentrations in the shell/case of the product [3]. Additive manufacturing technologies like directed energy deposition (DED-LB/ M) are mainly used for the fabrication of small lot sizes since the long processing times are associated with high costs-per-part. However, the flexibility regarding material use and local processing allows for the tailoring of surfaces by e.g., selectively increasing the carbon content [4] or adding hard phase particles [5]. In the past, DED-LB/M has been used for the processing of different materials, ranging from aluminum alloys [6] over titanium alloys [7] to high-strength steels [8] and even duplex stainless steels [9]. Low-alloyed steels, however, have received comparatively little research attention. Bartels et al. [10] studied the influence of different process parameters on the resulting material properties of the lowalloyed steel Bainidur AM. A wide parameter window could be identified for the processing of these steels by means of DED-LB/M. Furthermore, first investigations on increasing the hardness by adding hard phase particles have been performed [11]. In contrast to DED-LB/M, case-hardening steels like 16MnCr5 or 20MnCr5 have already been researched more thoroughly in laser powder bed fusion (PBF-LB/M) [12–14]. The additively manufactured steels possess a high relative part density and do not possess cracks when processed by means of PBF-LB/M. Furthermore, the underlying microstructure can be identified as at least partially bainitic. First studies also investigated the in-situ modification of the material properties by carbon [15, 16] and tungsten carbide [16] addition.

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Hentschel et al. [4] added carbon black nanoparticle concentrations of up to 0.5 wt. % to tool steel AISI H11 prior to processing the powder blends by means of DEDLB/M. They found that crack-free specimens could be manufactured for total carbon contents of up to approximately 0.6 wt.%. The material hardness increased continuously for higher carbon content, which was attributed to the higher strength of the martensitic phase. This approach appears feasible for experimentally determining the limits of carbon addition to low alloy steels for the DED-LB/M process. Goal of this work is to study the influence of different carbon concentrations on the resulting material properties of the low-alloyed steel Bainidur AM. The carbon content is increased continuously to analyze the resulting microstructure and the corresponding material hardness.

10.2 Materials and Methods Low-alloyed steel Bainidur AM with a particle size range from 45 to 90 µm was used for performing the experiments. A Camsizer was further used for determining the particle size distribution of the steel powder. D10 , D50 , and D90 values of the Bainidur AM powder are 58.8 µm, 80.8 µm, and 100.4 µm, respectively. For generating the powder blends, carbon black nanoparticles (Type: N550 carbon black, Harald Scholz GmbH, Germany) with a carbon content of approximately 99% were used. The particle size varies between 50 and 100 nm according to the supplier. All powder materials (Bainidur AM and carbon nanoparticles) were dried in a vacuum furnace at 110 °C for 12 h prior to mixing. Powder mixtures with a total mass of 300 g were prepared. The carbon concentration of the base powder was determined using an elemental analyzer of type CS (ELTRA GmbH, Germany). A carbon content of 0.23 wt.% was determined for Bainidur AM. Three additional powder mixtures were generated with total carbon contents of 0.3 wt.% (+0.07 wt.%), 0.35 wt.% (+0.12 wt.%), and 0.4 wt.% (+0.17 wt.%) were generated. First, the base material Bainidur AM was filled into a glass container. Next, the required amount of carbon black was added until the desired carbon content was achieved. The powder blend was stirred using a spoon to avoid undesired agglomerations at the surface of the glass. After that, the powder blend was mixed for two hours using a turbula mixing unit. DED-LB/M experiments were performed on an ERLAS 50,237 DED machine (ERLAS GmbH, Germany). The machine is equipped with a 4 kW diode laser with a characteristic wavelength ranging from 940 to 963 nm (Type: LDF 4000–4, Laserline GmbH, Germany). Laser spot size can be adjusted between 1 and 3 mm using a zoom optic. 16MnCr5 steel plates (200 × 200 x 15.3 mm3 ) were used as substrates for all experiments (Abrams Steel GmbH, Germany). The process parameters were selected based on previous investigations using the low-alloyed steel Bainidur AM [10]. Throughout the experiments, all parameters, except for the chemical composition of the powder material, were kept constant. The process parameters are listed in Table 10.1.

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Process parameter

Value

Laser Power [W]

600

Feed Rate [mm/min]

400

Laser Spot Size [mm]

1.5

Weld Track Width [mm]

1.5

Powder Mass Flow [g/min]

2.98 ± 0.02

Avg. layer height [mm]

0.45

Shielding and carrier gas flow were set to 20 L/min and 4 L/min, respectively. Argon of type 4.6 was used for both gas flows. The powder material was supplied using a three-jet nozzle. Four-layered specimens with an edge-length of 10 mm were manufactured. The core of the specimen was generated using a meander-shape strategy. An overlap of 50% was chosen between the single weld tracks that form the final geometry. Afterwards, the four contour tracks were deposited along each edge of the specimen. The build direction was rotated by 90 °C after the deposition of each layer. After fabrication, all samples were analyzed in a metallographic laboratory. The specimens were cut in half before cold mounting. Samples were further prepared by grinding and polishing with a 1 µm diamond suspension. Images of the polished cross-sections were generated by means of optical light microscopy to assess the relative part density and crack formation. Furthermore, the material hardness was determined on these cross-sections using an indentation tester. The hardness was measured every 300 µm and at least five times per layer. After the hardness measurements were performed, the samples were etched using a 3%-Nital solution to reveal the microstructure. Again, optical light microscopy was used to analyze the cross-sections within the different regions of the specimens.

10.3 Results and Discussion The results section is divided into three sub-sections. First, the macroscopic relative part density and etching behavior is addressed. The microstructural properties in different regions of the specimen are presented in a second step. Finally, the underlying material hardness is shown for the different carbon concentrations.

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Fig. 10.1 Cross-sections of the additively manufactured structures for a unmodified Bainidur AM, b 0.3 wt.% C, c 0.35 wt.% C, and d 0.4 wt.% C

10.3.1 Macroscopic Structure of the Specimens In the first step, the cross-sections of the specimens were analyzed regarding relative part density and internal defects like cracks and larger pores. The images of the etched cross-sections are presented in Fig. 10.1. All specimens possess a high relative part density exceeding 99.9%. Noticeable pores were only found in the contour track. Furthermore, no cracks were formed even for the higher carbon concentrations. This shows that a good part quality can be achieved despite the high cooling rates and the increased carbon content. Furthermore, the geometrical part properties like part height or weld track geometry did not change significantly. Each specimen can be divided into three main regions: core, contour, and substrate. The microstructure of the substrate is mostly unaffected, except for the heat-affected zone. Within the core region, the DED-LB/M-specific weld tracks can be identified. The boundary of these weld tracks appears mostly whitish after etching, which might indicate the presence of retained austenitic. Finally, the contour tracks are characterized by a bright etching. This region was exposed to a faster cooling during build-up due to the ambient atmosphere. Since the contour was always manufactured at the end of one layer, no in-situ heat treatment from adjacent weld tracks is present.

10.3.2 Microstructural Part Properties The microstructure was analyzed in two different regions of each specimen. This includes the dilution zone between substrate and cladding (see Fig. 10.2) as well as the core region close to the surface (see Fig. 10.3) of the additively manufactured structure. Figure 10.2 shows the microstructure that is formed in the dilution zone of the specimens. A similar microstructure can be observed in the dilution zone for all carbon concentrations. The structure appears predominantly lath-like with some shares of

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Fig. 10.2 Etched cross-section of the dilution zone for a unmodified Bainidur AM, b 0.3 wt.% C, c 0.35 wt.% C, and d 0.4 wt.% C

white blocks. These blocks are most likely austenite that was not completely transformed during cooling. Furthermore, the weld boundaries possess a slightly different structure than the fusion zone of the weld tracks. The weld track boundaries appear brighter due to the white and blueish structures. In contrast, the fusion zone of the specimens is characterized by browner etching response. Moving towards the top region of the specimen, a change in microstructure can be observed. Figure 10.3 shows the images for the different carbon concentrations. Here, a change from the lath-like structure towards a more granular-like structure can be observed. This granular structure is permeated with fine needles, which are characteristic for martensite. Furthermore, the microstructure tends to be refined with increasing carbon concentration. This might be attributable to the carbon black nanoparticles, which could act as nucleation agents during solidification.

10.3.3 Material Hardness Finally, the material hardness was determined for the different carbon concentrations. The hardness was measured within each layer as well as within the substrate and the dilution zone. Three samples were analyzed per parameter combination. Figure 10.4

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Fig. 10.3 Etched cross-section of the top regions for a unmodified Bainidur AM, b 0.3 wt.% C, c 0.35 wt.% C, and d 0.4 wt.% C

shows the material hardness for the different carbon concentrations. The unmodified base material Bainidur AM is characterized by the lowest material hardness. At the surface of the specimen, an average hardness of around 390 HV1 was obtained. Increasing the carbon content to 0.3 wt.% C leads to an increase in hardness up to 460 HV1. An even higher carbon content of 0.35 wt.% C results in a hardness of around 510 HV1. The highest hardness of around 560 HV1 was determined for the highest carbon concentration (0.4 wt.% C). This hardness rise can be explained by the higher hardness of the martensite for larger carbon contents. Furthermore, the material hardness also increases in the dilution zone for higher carbon concentrations. This is due to both mixing effects and the energy input in this region. Since the carbon content of the base material is lower than that of the coating in all cases, the mixing results in a higher hardness of the dilution zone. Furthermore, the penetration depth of the laser results in a local melting of the material as well as the formation of the corresponding heat-affected zone. This also results in an increased material hardness in this region, even though the carbon concentration is most likely the same as in the substrate. Finally, the experimentally obtained hardness is compared with the theoretically achievable one. The as-built hardness as well as the maximum theoretical hardness are shown in Table 10.2. The hardness values of the martensitic-hardened specimens were extracted from literature [17, 18]. It can be seen that the material hardness falls short of what is

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Fig. 10.4 Material hardness of the coatings deposited with different carbon concentrations

Table 10.2 Experimentally determined hardness for Bainidur AM and its powder blends with different carbon concentrations. The theoretical material hardness was approximated based on the work of Gerber and Wyss using a 95% and 99% transformation [17, 18] Powder Blend

Hardness as-built [HV1]

Hardness, 95% transformed [HV1]

Hardness, 99% transformed [HV1]

Bainidur AM

390

420 to 430

450 to 470

0.30 wt.% C

460

480 to 490

500 to 520

0.35 wt.% C

510

520 to 530

550 to 570

0.40 wt.% C

560

560 to 570

600 to 620

theoretically achievable. The hardness of Bainidur AM in the as-built state should be approximately 470 HV1 for a hardened specimen. This can be attributed to several effects. First, and most likely, the microstructure is not transformed completely into martensite. This is associated with a decrease of the hardness due to retained austenite and other low-strength phases. Second, the in-situ heat treatment during DED-LB/ M reduces the hardness by tempering the initially martensitic microstructure. This tempering is caused by adjacent weld tracks and by the processing of subsequent layers. Third, the heat accumulation during build-up increases the lower transformation temperature to which the molten material is cooled. The transformation is therefore performed differently in e.g., lower and higher regions of the specimens. This assumption is supported by the different etching behaviors in the top regions and the ones close to the substrate.

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10.4 Conclusion This work shows the influence of different carbon concentrations on the material properties of the low-alloyed steel Bainidur AM processed by means of DED-LB/ M. Defect-free specimens could be manufactured both for the base material and for carbon concentrations of up to 0.4 wt.%. Different microstructures were observed within the dilution zone and the coated structure due to the different chemical compositions and cooling conditions. The microstructure analysis is supported by hardness measurements. The material hardness rises almost linearly with increasing carbon concentrations. Depending on the carbon content, structures with a material hardness of as much as 560 HV1 could be generated successfully. The hardness of the DEDLB/M specimens approximates the theoretically achievable hardness for martensite when roughly 95% of the austenite was transformed. Future work will focus on the processing of powder blends with even higher carbon ratios and the simultaneous admixture of hard phase particles like tungsten carbide to further improve the hardness and wear resistance. Acknowledgements The authors would like to thank the German Federal Ministry for Economic Affairs and Energy for funding the joint project HyConnect (FKZ: 03LB3010*), within the framework of which the present investigations were carried out. We also thank the Deutsche Forschungsgemeinschaft DFG for the grant 359757317. Furthermore, the authors would like to thank the Deutsche Edelstahlwerke Specialty Steel GmbH & Co. KG and Schaeffler Technologies AG & Co. KG for their support. The authors gratefully acknowledge the support provided by the Erlangen Graduate School in Advanced Optical Technologies. Funding This work was funded by the German Federal Ministry for Economic Affairs and Energy for funding the joint project HyConnect (FKZ: 03LB3010*). Data Availability Statement The data is available upon on request.

References 1. Schatt, W. (ed): Konstruktionswerkstoffe des Maschinen- und Anlagenbaues: 131 Tabellen (Dt. Verl. für Grundstoffindustrie; Wiley-VCH, Stuttgart, [Weinheim], 2001) 2. Grosch, J., Bartz, W.J.: Einsatzhärten: Grundlagen - Verfahren - Anwendung - Eigenschaften einsatzgehärteter Gefüge und Bauteile: Grundlagen - Verfahren - Anwendung - Eigenschaften einsatzgehärteter Gefüge und Bauteile (Expert, Tübingen, 2019) 3. Davis, J.R.: Surface hardening of steels: Understanding the basics: understanding the basics (ASM International, Materials Park, OH, 2002) 4. Hentschel, O., Siegel, L., Scheitler, C., Huber, F., Junker, D., Gorunow, A., Schmidt, M.: Metals 8, 659 (2018) 5. Benarji, K., Ravi kumar, Y., Jinoop, A.N., Paul, C.P., Bindra, K.S.: J. Mater. Eng. Perform. 30, 6732 (2021) 6. Javidani, M., Arreguin-Zavala, J., Danovitch, J., Tian, Y., Brochu, M.: J Therm Spray Tech 26, 587 (2017) 7. Azarniya, A., Colera, X.G., Mirzaali, M.J., Sovizi, S., Bartolomeu, F., St Weglowski, M., Wits, W.W., Yap, C.Y., Ahn, J., Miranda, G., Silva, F.S., Madaah, H.R., Hosseini, Ramakrishna, S., Zadpoor, A.A.: J. Alloys Compounds 804, 163 (2019)

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8. Hentschel, O., Scheitler, C., Fedorov, A., Junker, D., Gorunov, A., Haimerl, A., Merklein, M., Schmidt, M.: J. Laser Appl. 29, 22307 (2017) 9. Maier, A., Munk, A., Kühl, A., Rühr, M., Hentschel, O., Kaufmann, F., Schrauder, J., Roth, S., Schmidt, M.: Procedia CIRP 111, 241 (2022) 10. Bartels, D., Hentschel, O., Dauer, J., Burgmayr, W., Schmidt, M.: Lasers in Manufacturing (LiM) 2021 (2021) 11. Bartels, D., Rohdenburg, J., Mohr, A., Rothfelder, R., Schmidt, M.: Procedia CIRP 111, 237 (2022) 12. Schmitt, M., Schlick, G., Seidel, C., Reinhart, G.: Procedia CIRP 74, 76 (2018) 13. Robatto, L., Rego, R., Mascheroni, J., Kretzer, A., Criscuolo, I., Borille, A.: Procedia CIRP 108, 873 (2022) 14. Aumayr, C., Platl, J., Zunko, H., Turk, C.: Berg Huettenmaenn Monatsh 165, 137 (2020) 15. Schmitt, M., Gottwalt, A., Winkler, J., Tobie, T., Schlick, G., Stahl, K., Tetzlaff, U., Schilp, J., Reinhart, G.: Metals 11, 896 (2021) 16. Bartels, D., Novotny, T., Hentschel, O., Huber, F., Mys, R., Merklein, C., Schmidt, M.: Int J Adv Manuf Technol 120, 1729 (2022) 17. Liedtke, D.: Mat.-wiss. u. Werkstofftech. 34, 86 (2003) 18. Gerber, W., Wyss, U.: Von Roll Mitteilungen 7, 13 (1948)

Chapter 11

Additive Technology Driven Integrated Thermoelectric and Photovoltaic Network Modeling Sohorab Hossain

and Surajit Chattopadhyay

Abstract To reduce carbon emissions for sustainable environmental development, professionals are looking for alternate and renewable type energy resources for electricity generation. Photovoltaic-based electricity generation has gained lots of popularity. However, this method suffers from low efficiency and non-availability of solar light at night time and during very cloudy environments. To cope with this situation, this work focuses on the advancement of a thermoelectric generation that does not depend on light intensity. At first, a small thermoelectric generator (TEG) has been modeled using semiconductor material with the help of additive manufacturing. Then several TEGs have been combined to form modules to enhance the power generation capacity. The module is then integrated with a photovoltaic (PV) unit. This integration increases overall efficiency and makes it suitable for a greater span of time. Keywords Integration · Module · Photovoltaic · Thermoelectric generation

11.1 Introduction Modern civilization wants to give more importance to sustainable development than other matters. However, achieving this goal is not so easy and therefore all leading professionals, educationists, social workers, etc. are searching for better and easier methods to achieve this using their own respective thought processes. Technologists are also putting in their best effort, but they are facing a lot of technical challenges to cope with pollution while harvesting electricity from conventional resources. Researchers are trying to find solutions through some alternate natural resources

S. Hossain (B) MCKV Institute of Engineering, Howrah, West Bengal, India e-mail: [email protected] S. Chattopadhyay Ghani Khan Choudhury Institute of Engineering and Technology, Malda, West Bengal, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_11

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like solar, tidal, wind, etc. that produce electricity without or with less carbon emission. Among them, solar photo voltaic-based power generation is gaining popularity very rapidly. However, it has some limitations of low efficiency, non-availability of solar light 24/7, etc. Wind energy is another source of energy; however it suffers from conversion difficulties, high cost of installation, manufacturing difficulties, lack of sufficient wind thrust in all places, etc. Tidal energy has a limitation in its area of installation and application. Some, but still comparatively fewer attempts are found to achieve the above advantages through the proper use of thermal energy for electricity generation by advancing the concept of thermoelectric generation (TEG). It refers production of electricity from thermal energy by creating temperature differences. Thus, a thermo-electric generator works on the principle of proper utilization of thermal imbalance towards the production of electric energy. Material science has developed many new metal alloy combinations that are now used in TEG [1, 2]. Their thermoelectric features are often found attractive in terms of power generation. However, the performance of a TEG also depends on temperature difference [3]. Semiconducting materials have now become alternatives to metal alloys [4]. These materials reduce the cost of raw materials involved in the manufacturing process. Also, variation of thermoelectric features was found possible. This variation can be achieved by changing the semiconductor type and the percentage of impurities doped in them. Many studies and tests were carried out to focus on the performance of TEG in power generation [5–8]. Optimum operating conditions are being evaluated by different working conditions and material combinations. Some researchers are trying to use TEG technology to increase power generation capacity mainly through the photovoltaic approach [9, 10]. To establish TEG as a reliable method for electricity harvesting, it should be integrated with conventional electrical bus and network systems and should gain the capability to feed power along with other energy resources [11, 12]. The concept of very small grids used in PV technology can be utilized for TEG also. Also, a literature study suggests that the performance of a generation unit depends not only on the power generation capacity but also on the quality of the power [13]. The quality of generated output is normally judged by measuring different quality parameters involving shapes of the voltage and current in time and frequency domains. TEG efficiency is very much dependent on the temperature difference applied across the TEG [2, 14]. This temperature difference can be achieved by either increasing the hot junction temperature, decreasing the cold plate junction, or both. Room temperature was used as cold plate temperature in [2] and optimum power generation is reached. On the other hand, TEG semiconductorbased has many similarities with photovoltaic generation producing DC power. In the present scenario, PV is extensively used in microgrid applications [15]. On the other hand, besides thermoelectric elements [16], additive technology is being used for the development of different types of semiconductor layers [17–19]. It indicates that TEG has also great potential to be used as one energy resource in microgrids.

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Along with the development, additive technology has brought tremendous advancement in manufacturing. Using this technology, suitable semiconductor materials can be formed to develop TEG having suitable electrical features. However, comparatively very few works have been observed that deal with the integration of photo voltaic electricity generation with thermoelectric-based electricity generation. This has motivated authors to study the challenging area and to propose an effective solution for this purpose. The paper has been designed in six sections. After a brief review in Sect. 11.1, the modeling of TEG has been presented in Sect. 11.2. Module formation has been described in Sect. 11.3. TEG integration with PV has been described in Sect. 11.4. Simulated results have been presented along with a discussion in Sect. 11.5. In the end, the conclusion has been provided in Sect. 11.6.

11.2 TEG Modeling Thermoelectric generation is implanted using Seebeck’s principle. This needs a minimum of two dissimilar metals. Based on this, many metal combinations were tested to generate electricity from thermal energy-driven temperature differences. Then, with the advancement of material technology, different metal alloy combinations were tested to generate electricity. In the last few decades, semiconductor technology has progressed a lot and has brought a revolution in electronics as well as power generation. Semiconductor-made photovoltaic cells have become very popular for electric energy harvesting from solar light. Also, the material cost of semiconductors is very low. Their properties can easily be varied by changing the percentage of impurity or doping concentration. Considering all these, in this work, semiconductors were used to model thermoelectric generators (TEG). A small unit of the generator has been shown in Fig. 11.1. This generator is mainly made of two types of semiconductor materials, namely, P–type, and N-type materials. Three-dimensional intrinsic P-type material can be made using silicon or germanium materials doped with impurities like boron, aluminum, etc. that belong to the IIIA column in the periodic table and p-type is characterized by the availability of holes due to doping impurities. Similarly, threedimensional intrinsic N-type material can be made using silicon or germanium materials doped with impurities like phosphorus (P), arsenic (As), or antimony (Sb), etc. that belong to the V column in the periodic table and p-type is characterized by the availability of free electrons due to doping impurities. The PN combinations is placed between two junctions having different temperatures: low temperature or cold terminal and high temperature or hot terminal. Semiconductors are thermally less conductive and doped semiconductors show good electrical conductivity under certain conditions. In presence of temperature difference, the unit will produce electric energy following Seebeck’s principle. At first, a voltage difference or gradient will be formed across two terminals. If a load is connected across the terminal, current will flow extracting electric energy from the

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Fig. 11.1 Small single unit of a thermoelectric generator

thermal source. The simple circuit using resistive load with the TEG unit has been shown in Fig. 11.2. In this circuit, the TEG unit will behave like a source of electric energy i.e. an electric generator in presence of temperature difference between the hot plate and cold plate. It will cause a flow of direct current through the resistive load till the temperature difference is maintained. Thus, a TEG extracts thermal energy from the heat source and converts it to electric energy to drive the circuit. Fig. 11.2 Circuit diagram of a TEG connected with resistive load

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11.3 TEG Module Formation Each TEG unit has its own power generation capacity constrained by its voltage and current limits. To overcome this limitation, multiple units were coupled electrically forming a TEG module to enhance output voltage and current capacity. Module formed by multiple cells has been shown in Fig. 11.3. At first, individual series combination of cells was formed and then their parallel combination was made. Thus, the module consists of the parallel combination of many series of connected cells. The output voltage depends on the number of cells connected in one series combination. Thus, if the number of cells in one series combination increases, then output voltage increases. Mathematically, V =

M 

Vi

i=1

where, M is the number of TEG units connected in one series path. Vi is the voltage output of a TEG unit. V is the output voltage of the module. On the other hand, output current delivering capacity depends on the number of parallel paths in one module. Thus, if the number of parallel paths in one module combination increases, then output current delivering capacity increases. Mathematically,

Fig. 11.3 Module formation using many TEG cell units connected on series and parallel combinations

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I =

N 

Ii

i=1

where, N is the number of parallel paths. Ii is the current in each parallel. I is the total output current of the module.

11.4 Integration with PV However, researchers are not making effort in thermoelectric technology to operate them in isolated or standalone mode only. Therefore, research is going on to generate power using TEG simultaneously with other existing methods of electric energy harvesting. For this intention, photovoltaic generation unit becomes an easy and nearest option to be integrated with as they are based on semiconductor materials and also, produce DC output. The common output form makes the process of integration easier. The electrical integration of TEG and PV modules have been shown in Fig. 11.4. Modules of both energy sources are connected with a DC bus. DC bus is connected with the DC load. Additional DC to DC voltage controllers may be connected between generator modules and the DC bus.

Fig. 11.4 Integration of TEG modules with PV modules through a DC bus

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Table 11.1 Observations made from the isolated and integrated operations of TEG SN

Operating mode

Load type Observations

1

TEG operating in isolated mode

Resistive

The smooth operation was To increase the voltage and found current capacity of the module

Challenges found

2

TEG operating in integrated mode with PV

Resistive

Overall efficiency and generation capacity were improved The possibility of back currents was observed

To control voltage in synchronism with PV To protect the system from back currents

11.5 Observations from Simulated Results and Discussion The integrated module has been tested in a simulation environment. The major observations have been presented in Table 11.1. The performance was tested in two different conditions: (a) isolate run where PV was disconnected and (b) integrated run where both TEG and PV were connected. A temperature difference of 100 K was maintained between hot plate and cold plate. The output of TEG module and/or PV module is connected with a resistive load through a DC bus. For particular temperature differences, the load current was found constant giving constant power. Voltage and current magnitudes increase with the rise of temperature difference. However, in the integrated run, it was found difficult to generate the same voltage at both modules. Though the overall current and power generation capacity were increased, circulating current was also noticed during unequal PV and TEG module voltages. Thus, this study opens a new scope for research to overcome these circulating or backward currents.

11.6 Conclusion This paper presents the use of the thermoelectric principle for harvesting electric energy from thermal energy. Based on Seebeck’s principle, the generator was modeled using semiconductor materials. P-type and N-type semiconductors were used in modeling the generator unit. To increase power generation capacity, several generator units were combined to model modules of thermoelectric generators. Then, this module was integrated with photovoltaic modules to develop electric network models. The integration was found to have enhanced operating time span and power generation capacity. The additive manufacturing process can be utilized in a better way to produce TEG having better and more flexible thermoelectric characteristics. However, unequal voltages of two different types of modules were identified as a big challenge to be overcome that undoubtedly opens future scope for research.

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References 1. Liu, W., Jie, Q., Kim, H.S., Ren, Z.: Current progress and future challenges in thermoelectric power generation: from materials to devices. Acta Mater. 87, 357–376 (2015) 2. Zulkepli, N., Yunas, J., Mohamed, M.A., Hamzah, A.A.: Review of thermoelectric generators at low operating temperatures: working principles and materials. Micromachines (Basel). 12(7), 734 (2021). https://doi.org/10.3390/mi12070734.PMID:34206662;PMCID:PMC8303398 3. Sun, D., Shen, L., Yao, Y., Chen, H., Jin, S., He, H.: The real-time study of solar thermoelectric generator, Appl. Thermal Eng. 119, 347–359, ISSN 1359–4311 (2017). https://doi.org/10.1016/ j.applthermaleng.2017.03.075 4. Ahiska, R., Mamur, H.: A review: thermoelectric generators in renewable energy. Int. J. Renew. Energy Res. 4(1) (2014) 5. Wang, H., Xie, W., Yu, B., Qi, B., Liu, R., Zhuang, X., Liu, S., Liu, P., Duan, J., Zhou, J.: Simultaneous solar steam and electricity generation from synergistic salinity-temperature gradient. Adv. Energy Mater. 11, 2100481 (2021). https://doi.org/10.1002/aenm.202100481 6. Liu, A.T., Zhang, G., Cottrill, A.L., Kunai, Y., Kaplan, A., Liu, P., Koman, V.B., Strano, M.S.: Adv. Energy Mater. 8, 1802212 (2018). https://doi.org/10.1002/aenm.201802212 7. Wolf, M., Abt, M., Hoffmann, G., Overmeyer, L., Feldhoff, A.: Ceramic-based thermoelectric generator processed via spray-coating and laser structuring, Open Ceramics, 1, 100002, ISSN 2666–5395 (2020). https://doi.org/10.1016/j.oceram.2020.100002 8. Jaziri, N., Boughamoura, A., Müller, J., et al.: A comprehensive review of thermoelectric generators: technologies and common applications. Energy Rep. (2019). https://doi.org/10. 1016/j.egyr.2019.12.011 9. Toberer, E.: Solar thermoelectric generators: Pushing the efficiency up. Nat Energy 1, 16172 (2016). https://doi.org/10.1038/nenergy.2016.172 10. Lv, Y., Chen, J., Zheng, R.K., et al.: Photo-induced enhancement of the power factor of Cu2 S thermoelectric films. Sci Rep 5, 16291 (2015). https://doi.org/10.1038/srep16291 11. Chattopadhyay, S., Das, A.: Overhead Electric Power Lines: Theory and practice, IET, London, ISBN: 9781839533112 (2021) 12. Chattopadhyay, S.: Nanogrids and Picogrods and their integration with electric vehicles, IET, London, ISBN: 978–1–83953–482–9 (2022) 13. Chattopadhyay, S., Mitra, M., Sengupta, S.: Electric Power Quality, Springer, Netharland, ISBN 978–94–007–0635–4 (2011) 14. Champier, D.: Thermoelectric generators: a review of applications, Energy Conversion and Management, 140: 167–181, ISSN 0196–8904 (2017), https://doi.org/10.1016/j.enconman. 2017.02.070 15. Das, T.K., Banik, A., Chattopadhyay, S., Das, A.: FFT based Classification of Solar Photo Voltaic Microgrid System. In: 2019 Second International Conference on Advanced Computational and Communication Paradigms (ICACCP), Gangtok, India, pp. 1–5 (2019). https://doi. org/10.1109/ICACCP.2019.8882995 16. Tian, Y., Loskoski, K., Meyers, S., Van Hooreweder, B., Molina-Lopez, F.: Selective Laser Sintering of Blade-Coated Thermoelectric Materials with Tunable Thickness. In: 2020 IEEE SENSORS, Rotterdam, Netherlands, pp. 1–4 (2020). https://doi.org/10.1109/SENSORS47125. 2020.9278769 17. Chang, J. Ge, T., Lin, T.: Co-Design between Semiconductor, Low-Variation Fully-Additive Printed/Flexible Printing and Variation-Tolerant Digital Circuit Design: (Invited Paper). In: 2020 4th IEEE Electron Devices Technology & Manufacturing Conference (EDTM), Penang, Malaysia, pp. 1–4 (2020). https://doi.org/10.1109/EDTM47692.2020.9118043 18. Kumar, A., Lee, W.H., Wang, Y.L.: Optimizing the Isotropic Etching Nature and Etch Profile of Si, Ge and Si0.8Ge0.2 by Controlling CF4 Atmosphere With Ar and O2 Additives in ICP. IEEE Trans. Semicond. Manuf. 34(2), 177–184 (2021). https://doi.org/10.1109/TSM.2021.3057100 19. Ueoka, H., Yodogawa, M.: Ceramic Manufacturing Technology for the High Performance PTC Thermistor. IEEE Trans. Manufact. Technol. 3(2), 77–82 (1974). https://doi.org/10.1109/ TMFT.1974.1135679

Chapter 12

Fine Porous Structures Fabricated Using Laser Powder Bed Fusion of Ti–6Al–4 V Salome Sanchez , Ahmad Zafari , Ali Gökhan Demir , Leonardo Caprio , Barbara Previtali , Malgorzata Holynska , Ian Gibson , and Davoud Jafari

Abstract This study explores the capability of Laser Powder Bed Fusion (LPBF) to fabricate fine porous Ti–6Al–4V structures for electrochemical applications, specifically for Regenerative Fuel Cells electrodes. High porosity and high surface area are crucial characteristics to maximise electrochemical properties. Therefore, the focus of this study was to produce electrodes by utilising a pulsed wave LPBF system to fabricate thin walls with process-induced stochastic porosity by manipulating process parameters—i.e., duty cycle, laser power and scanning speed. Although the highest porosity was obtained from a duty cycle of 50%, laser power of 100 W and scanning speed of 1500 mm/s, the specimen lacked structural integrity. An increase in duty cycle to 87.5% improved the structural integrity of the thin wall and led to regularly spaced pores. The latter result is promising as it paves the way to controlling the formation and location of pores in LPBF-processed specimens. Overall, this study highlights the capability of LPBF in producing fine porous structures for electrochemical applications. Keywords Laser powder bed fusion · Ti–6Al–4V · Porous electrode · Feature resolution

S. Sanchez · A. Zafari · I. Gibson · D. Jafari (B) Department of Design Production and Management, Faculty of Engineering Technology, University of Twente, Enschede, Netherlands e-mail: [email protected] A. G. Demir · L. Caprio · B. Previtali Department of Mechanical Engineering, Politecnico Di Milano, Via La Masa 1, 20156 Milan, Italy M. Holynska TEC-QEE, ESA-ESTEC, Noordwijk, The Netherlands © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_12

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12.1 Introduction Laser Powder Bed Fusion (LPBF) is one of the most commonly used Additive Manufacturing (AM) techniques to fabricate near-end-use metallic components in various industries, such as biomedicine, aerospace and energy [1]. One of the major challenges associated with LPBF is the high level of porosity that is inherent in the process [2]. Higher levels of porosity in metal components can negatively impact mechanical properties, especially fatigue [3] and creep [4]. Consequently, significant effort has been devoted to optimise LPBF process parameters to minimise porosity and produce near-fully-dense specimens. However, certain applications benefit from components with high porosity and high surface area, such as medical implants and prosthetics [5], filter elements [6] and electrodes in Regenerative Fuel Cells (RFCs). Indeed, RFC electrodes are porous thin-walled components [7] which require active surfaces for electrochemical reactions and good permeability. In electrodes, the ideal pore size for electrolysis performance was found to be 10 µm [7, 8]. Compared to Direct Energy Deposition and Electron Beam Melting, LPBF offers higher feature resolution, with a minimum feature size of around 100 µm [9, 10]. This makes LPBF more suitable to create thin structures with high surface areas. Despite this, limited research has been performed on the use of LPBF for fine porous components. Yadroitsev et al. manufactured 140 µm, thin walls with stainless steel grade 904L [11] while Su et al. fabricated a 100 µm stainless steel 316L thin wall [12]. Abele et al. varied LPBF process parameters to fabricate stainless steel 316L thin walled (minimum 125 µm) porous elements which attained a maximum porosity of 17.5% [13]. As a result of the limited number of studies, the LPBF of porous RFC electrodes was targeted for space applications. Ti–6Al–4V was selected for this research for its LPBF manufacturability and the fact that numerous studies have focused on process optimisation of this alloy [14], which are helpful to find right parameters for obtaining high porosity and high feature resolution. Laser type, laser power, scanning speed and hatch spacing are among the LPBF parameters with the most effects on feature size and porosity [14]. Indeed, a review of the optimisation of process parameters for Ti–6Al–4V has shown that using low laser powers (below 100 W) is more likely to lead to porosity, regardless of the scanning speed [14]. Caprio et al. further showed that the use of a Pulsed Wave (PW) laser results in a smaller feature size with the same energy input, as compared to a Continuous Wave (CW) laser, leading to a decrease in laser track width [15]. This makes PW useful when thin struts or precise features are required [16]. Temporal control of the energy input through pulsation can also be beneficial for sintering or partially melting the particles on the powder bed. Scanning speed is also noted as one of the critical process parameters affecting feature resolution and part density for Ti–6Al–4V [17]. For a constant laser power, using lower scanning speed results in a wider meltpool [18, 19] (i.e. a larger feature size) for both PW and CW lasers [20]. Overall, a PW laser is more likely to result in thinner scan tracks and higher porosity, making PW key for fabricating porous thin electrodes. As laser power and

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scanning speed also greatly affect porosity and feature size, they will be studied as well. Overall, a limited number of studies report the advantages of LPBF-induced porosity for obtaining improved permeability and wettability properties [21]–[23], key for energy applications. Electrochemical properties require significant amounts of porosity, as well as ultrafine pores, which were not obtained in previous studies. Therefore, this paper aims to sinter thin Ti–6Al–4V walls to minimise meltpool width, increase the amount of porosity through lack-of-fusion and create fine pores from spaces between partially fused particles.

12.2 Experimental Methods This section will describe the experimental methods used to sinter thin Ti–6Al–4V walls, to minimise meltpool width and to increase the amount of porosity. First, the LPBF parameters used will be presented, followed by the characterisation methods.

12.2.1 Laser Powder Bed Fusion Thin walls (15 mm × 17 mm) were printed as a single laser track on each layer using a Renishaw (Wotton-under-Edge, United Kingdom) AM250 LPBF system and TEKNA (Mâcon, France) Ti–6Al–4V powder with particle sizes of 15–45 µm and mean particle diameter of 38.8 µm (measured according to ASTM B822 [24]). The elemental composition of the powder is reported in Table 12.1. As using PW can yield significant feature resolution and higher porosity, it was used to print the specimens. In a PW system, the laser is pulsed on for a selected amount of time, then off for the rest of the cycle, as illustrated in Fig. 12.1. The scan Table 12.1 Elemental composition of TEKNA Ti–6Al–4V powder

Element

Concentration (wt.%)

Al

6.50

Fe

0.20

V

4.01

Y

P1 > P3 > P5 > P4. The ranking orders of the alternatives are also graphically represented in Fig. 14.2. It implies that P2: Polylactic Acid (PLA) is the best alternative FDM material for 3D printing technology with respect to the criteria under consideration as per the Table 14.10 Aggregate Weighted Performance Rating (AWPR), Performance Index and Rank

AWPR

PI

Rank

P1

1.9540

0.6700

3

P2

2.1669

1.0000

1

P3

1.5445

0.0354

4

P4

1.5217

0.0000

6

P5

1.5222

0.0008

5

P6

1.9728

0.6991

2

max

2.1669

min

1.5217

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Aggregate Weighted Performance Rating (AWPR)

2.5000

AWPR

2.0000

167

2.1669 1.9728

1.9540 1.5445

1.5217

1.5222

P3

P4 Alternative

P5

1.5000 1.0000 0.5000 0.0000

P1

P2

P6

Fig. 14.1 Aggregate Weighted performance Ratings of the Alternatives

proposed algorithm. P6: Composites (carbon fiber, kevlar, fiber glass) is the second best alternative FDM material for the purpose. Comparison of the results by proposed method with existing simple additive weighted (SAW) method has been shown in Table 14.11 and graphically presented in Fig. 14.3. It is easily observed that the first, second and third ranking order of the alternative by the proposed and the existing method are absolutely identical. Though the ranking orders of the other three deviate from each other slightly. The reason is that the proposed method uses non-linear assessment of performance indices and the existing method linear one. The Spearman’s Rank correlation coefficient is 0.82857 which is highly positive correlation between the proposed and existing method. The Ranking Order

7

Ranking Order

6

6

5

5

4

4

3

3

2

2

1 0

1 P1

P2

P3

P4

Alternative

Fig. 14.2 Ranking order of the Alternatives

P5

P6

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Table 14.11 Comparison of the results by proposed method with existing method Alternatives

Composite score

Rank by SAW method

Rank by Proposed method

P1

1.7487

3

3

P2

1.9437

1

1

P3

1.3112

5

4

P4

1.3562

4

6

P5

1.2987

6

5

P6

1.8312

2

2

7

Comparison of Ranking Orders by Proposed Method and Existing Method

Ranking Order

6

6

6

5

5

4

4

3

5 4

3

2

2

1 0

1 P1

P2

P3 P4 Alternatives SAW

P5

P6

Proposed Method

Fig. 14.3 Comparison of ranking orders by proposed Method and existing method

result obviously validates the proposed algorithm for the purpose of FDM material selection for printing technology.

14.5 Conclusion Evolution and selection of FDM materials for 3D printing is one of the most crucial tasks for industrial decision makers in industry 4.0. This task becomes harder when decision is to be made on the basis of intangible factors. Due to presence of multiple conflicting new criteria application of fuzzy MCDM techniques is inevitable for finding the best solution. This investigation attempts to introduce a new fuzzy MCDM technique which has been applied to a real life problem on the evolution of FDM materials for 3D printing technology. The results of the proposed method and the comparison with the existing method ensure the applicability, validity, effectiveness and usefulness in the domain of FDM material selection. This algorithm may also be

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useful for similar decision making purpose in diverse field. The main contribution of the current investigation may be summarized as follows: • A novel MCDM technique is introduced for performance measure of different alternative materials. • Group decision making is incorporated for more accurate evaluation and decision. • Non-linear assessment of alternatives has been carried out. • The concept of fuzzy set theory is applied in capturing uncertainty in industrial decision making. • Trigonometric function is applied in computation of performance index. In this current investigation, all the criteria have been assumed as mutually independent and purely intangible. The consideration of dependent criteria a mixture of both tangible and intangible factors under heterogeneous decision making procedure may be the direction of future research in this aspect.

References 1. Adrian I.Y., ,Carlos, J.S., Esther, L., Irune, I., Elena, B., Daniel, C., Alejandro, A., Sara, A., Carlos, R., Fouad, B., Mohamed, B., Nassim, S., Alice, H., Sam, G., Roger, H., Richard, S., Bianca, R., Pieter, V., Oscar, B., Hugo, S., João, N.F., Emanuel, A., Maria, T.B., Isabel, S., Miriam, T., Jorge, L.: Artificial reefs built by 3D printing: Systematisation in the design, material selection and fabrication. Constr. Build. Mater. 362, 129766 (2023) 2. Aksel, R., Haris, S., Subrata, S., Izabela, E.: Supplier selection for aerospace & defense industry through MCDM methods. Clean. Eng. Technol. 12, 100590 (2023) 3. Sadaf, N., Aliyeh, K., Mohammad-Hossein, J., Sara, A.: Selecting suitable wave energy technology for sustainable development, an MCDM approach. Renew. Energy 202, 756–772 (2023) 4. Kui., W., Guoquan, X., Jiangyang, X., Tao, L., Yong, P., Jin, W., Honghao, Z.: Materials selection of 3D printed polyamide-based composites at different strain rates: A case study of automobile front bumpers. J. Manuf. Process. 84, 1449–1462 (2022) 5. Shouzhen, Z., Jiamin, Z., Chonghui, Z., José, M.M.: Intuitionistic fuzzy social network hybrid MCDM model for an assessment of digital reforms of manufacturing industry in China. Technol. Forecast. Soc. Chang. 176, 121435 (2022) 6. A., H., Abishini, K.M.B., Karthikeyan: Application of MCDM and Taguchi super ranking concept for materials selection problem. Mater. Today: Proc. 72, 2480–2487 (2022) 7. Pratiksha, L., Anand, B., Ravinder, K., Nejla, M., Mohsen, S.: Benchmark using multi criteria decision making (MCDM) technique to optimally select piston material. Eng. Anal. Boundary Elem. 142, 52–60 (2022) 8. Rupinderpreet, S., Chandan, D., Dharmpal, D.: Analyzing performance indicators of advanced manufacturing technology implementation using MCDM. Mater. Today: Proc. 47(13), 3750– 3753 (2021) 9. Rohit, A.: Sustainable material selection for additive manufacturing technologies: A critical analysis of rank reversal approach. J. Clean. Prod. 296, 126500 (2021) 10. Saikat, C., Shankar, C.: Material selection of a mechanical component based on criteria relationship evaluation and MCDM approach. Mater. Today: Proc. 44(1), 1621–1626 (2021) 11. Prateek, S., Emanuele, P., Konstantinos, S., Mark R.J.: Sustainability metrics for rapid manufacturing of the sand casting moulds: A multi-criteria decision-making algorithm-based approach. J. Clean. Prod. 311, 127506 (2021)

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12. Vishwas, D., Angappa, G., Milind, A., Priyanka, V.: An integrated Delphi-MCDM-Bayesian Network framework for production system selection. Int. J. Prod. Econ. 242, 108296 (2021) 13. Honghao, Z., Yiyun, W., Kui, Wang., Yong, P., Danqi, W., Song, Y., Jin, W.: Materials selection of 3D-printed continuous carbon fiber reinforced composites considering multiple criteria. Mater. & Des. 196, 109140 (2020) 14. Yuchu, Q., Qunfen, Q., Paul, J.S., Xiangqian, J.: An additive manufacturing process selection approach based on fuzzy Archimedean weighted power Bonferroni aggregation operators. Robot. Comput.-Integr. Manuf. 64, 101926 (2020) 15. Massimo, B., Giovanni, E., Giovanni, R.: A TOPSIS-based approach for the best match between manufacturing technologies and product specifications. Expert Syst. Appl. 159, 113610 (2020) 16. Ikuobase, E., Okpako, S.: Oghenenyerovwho application of MCDM method in material selection for optimal design: A review. Results Mater. 7, 100115 (2020) 17. Sucheta, A., Vivek, A., Jitendra, K., Dixit.: Green manufacturing: A MCDM approach. Mater. Today: Proc. 26(2), 2869–2874, (2020) 18. Ching-Chiang, Y., Yi-Fan, C.: Critical success factors for adoption of 3D printing. Technol. Forecast. Soc. Chang. 132, 209–216 (2018) 19. Ren, D., Fun., K., Choi, K.S.: A multicriteria decision making method for additive manufacturing process selection. Rapid Prototyp. J. 28(11), 77–91 (2022)

Chapter 15

Multimaterial 3D Printing of Programmable Architected Structures Mehrshad Mehrpouya, Jonne F. Postmes, Ava Ghalayaniesfahani, and Ian Gibson

Abstract Research on architected structures has been developed extensively in recent years. This is because of their unique capabilities, offering a wide range of properties from enhanced mechanical strength to energy and vibration absorption. Thanks to advances in 3D printing technologies, there is the possibility to design and fabricate complex structures using multiple materials with diverse properties. However, strong adhesion between dissimilar materials is required to ensure integrity in the fabricated multimaterial structures. This study investigates the role of the applied designs and materials on the functional characteristics of the architected structures. Different multimaterial configurations are evaluated based on energy absorption performance. This study also realizes how to manipulate the local structure and composition, with a combination of soft and hard materials, to achieve a broad range of mechanical responses. Therefore, it provides a unique opportunity to help program the architected metamaterial based on the required functionality. Keywords Additive manufacturing · 3D printing · Architected structures · Multimaterial · Mechanical metamaterial

15.1 Introduction Additive manufacturing (AM), also known as 3D printing technology, provides a unique capability in the design and fabrication of highly complex geometries and structures that cannot be easily created by more conventional methods [1, 2]. Architected structures are one of the promising design methods using AM, offering lightweight parts with engineered deformability, intrinsic energy absorption, and shock damping that can have many applications in aerospace, medical, automotive, M. Mehrpouya (B) · J. F. Postmes · A. Ghalayaniesfahani · I. Gibson Faculty of Engineering Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_15

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and construction fields [3–5]. AM technology enables the fabrication of such architected structures, like cellular lattices or sandwich structures, with different shapes and scales so that the AMed structures can exhibit a wide range of mechanical performances [6–9]. Multimaterial 3D printing technology enables the advancement of complex structures using different materials of diverse properties. This enables designers to tune the mechanical properties of the structures, such as strength, stiffness, and energy absorption, based on the desired function [10, 11]. Accordingly, the final structure can be programmed, based on different material design strategies where both materials distribution and architectures can play roles in the functionality of the AMed parts [12]. This study investigates the impact of design and multimaterial configurations on the functional characteristics and energy absorption performance of the AMed architected structures. In the design section, one architected structure with antichiral topology is selected. This is because such structures exhibit high compression strength and energy absorption based on the results of the previous study [13]. For printing multimaterial structures, Fused Filament Fabrication (FFF) technology is applied with the capability of multimaterial 3D printing using two deposition nozzles. For that, two materials with different mechanical performances, one soft and one hard, are utilized. In the next step, the placement of these two materials in the core of the structures has been studied. This can help to discover how the design and combination of hard and soft materials in an architected structure may change the mechanical performance of the overall structures, and finally, learn how to tune it for a particular functionality.

15.2 Materials and Methods 15.2.1 Materials In this study, two types of polymer, Polylactic acid, (PLA, MakerPoint, The Netherlands) and Polycaprolactone (PCL100, 3D4Makers, The Netherlands) filaments with a diameter of 2.85 ± 0.1 mm, are used to print the multimaterial structures. Table 15.1 presents an overview of the thermomechanical properties of PLA and PCL filaments used. Table 15.1 Material properties of PLA and PCL Materials

Density

Tensile strength

Young modulus

Melting temperature

Glass temperature

gr/cm3

MPa

MPa

°C

°C

PLA

1.24

66

3027

145–165

57

PCL

1.1

45

350

58–60

−60

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Table 15.2 The applied operational parameters in this study Materials

Bed temperature

Printing velocity

Nozzle temperature

Layer height

°C

mm/s

°C

mm

PLA

60

40

200

0.2

PCL

30

10

130

0.2

15.2.2 Printing and Testing Setup The samples are printed by an FFF 3D printer (Ultimaker 3, Ultimaker B.V., The Netherlands). The structures are designed by SolidWorks 2020 (Dassault Systèmes SolidWorks Corporation, USA) and the STL files are used as input to Ultimaker Cura 4.11.0 (Ultimaker BV, The Netherlands) for slicing the structures and choosing the proper printing parameters. Adhesion spray and blue mask tape are applied to the printing bed, providing perfect contact between the printing bed and the molten layer. All printing setups, including the nozzle speed and temperature, are applied according to the recommendations from the filament manufacturers (see Table 15.2). To measure the mechanical performance of the printed structures, a ZwickiLine universal testing machine (ZwickRoell, The Netherlands), equipped with a 5 kN load cell, is used. The deformation speed is set at 8 mm/min. The mechanical compression tests are all performed under ambient conditions and the testing for each sample is repeated three times to check the reproducibility of the results.

15.2.3 Material Design Strategy Mechanical performance and printability of the multimaterial structure are considered as two main factors for choosing a proper design for this study. Accordingly, an anti-chiral structure with a negative Poisson ratio, which offers unique mechanical deformation properties, high impact resistance, and promising energy absorption abilities, is selected. The final structure, including the size and number of cells, is designed based on the result of a previous study [13]. Figure 15.1 shows the front and isometric views of the anti-chiral structure used in this study. Out of many possible ways to place the PLA (black) and PCL (white) materials in the anti-chiral structures, four designs are created as shown in Fig. 15.2. It is done based on a proportional transition between hard to soft materials in the structure. For that, in the transition part, half of the cylinders are fabricated in white, and the other half in black materials. Accordingly, the name of each structure is made based on its material distribution. It is notable to mention that the center of all cylinders is fabricated using only PLA material as this prevents any deformation in the shape of the cylinders during the loading step. Therefore, all displacements only occur in the ligaments in the structures.

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Fig. 15.1 The geometry and dimension of the design architected structure in this study. All the dimensions are in mm

(a) HSH

(b) SHS

(c) FOU

(d) DIA

Fig. 15.2 Four multimaterial designs, including PLA (black) and PCL (white) materials; a HardSoft-Hard, b Soft-Hard-Soft, c Flexible OUtside, and d DIAgonal

15.3 Results and Discussion For the mechanical analysis, all four samples are compressed till a certain pre-set displacement of 18 mm which is in the densification range of the force–displacement diagram. Figure 15.3 illustrates the compression performance in all multimaterial structures. As expected, PCL unit cells are compressed faster compared to PLA unit cells. This is mainly due to the significant difference in the mechanical strength of PLA and PCL as shown in Table 15.1. This trend can be observed in all structures through different steps of the deformation process. Figure 15.4 shows the results of the mechanical compression test for four multimaterial samples including SHS, HSH, FOU, DIA structures. The results reveal a significant difference in the mechanical response of these structures. The SHS samples present the highest compression strength of 460.4 N, while the HSH samples have the lowest value of 232.2 N. The other two FUO and DIA samples are somewhere in between with values of 388.4 N and 336.4 N, respectively. The values are calculated based on the average of three replications for each sample.

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DIA

FOU

SHS

HSH

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D= 0 mm

D= 6 mm

D= 12 mm

D= 18 mm

Fig. 15.3 Performance of all multimaterial anti-chiral structures in the compression test

Also, the mechanical performance of the structures is evaluated based on the long plateau stress regions in the force–displacement graphs. This region indicates the energy absorption (EA) of the cellular structure as the result of the full compaction of the structure under the compressive load [14]. Equation (15.1) gives the amount of EA where δd is the maximum compression displacement in the densification region and F is the compression force [15].  EA =

δd

Fdδ

(15.1)

0

The mass of the structures has a direct impact on their functional performance, in particular, when there is a combination of two or more materials with different properties in a multimaterial structure. For this purpose, the specific energy absorption (SEA) of all structures, which is defined by the ratio of the absorbed energy to the total mass (m) of the structure, is determined in this study. Equation (15.2) provides information about the mechanical performance of the structures based on their mass which is a critical limitation in some applications [15].

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(a) HSH

(b) SHS

(c) FOU

(d) DIA

Fig. 15.4 The compression performance of four multimaterial anti-chiral structures

SE A =

EA m

(15.2)

Figure 15.5 illustrates the results of compressive strength and SEA for all four multimaterial structures in one graph. As shown, the HSH design has the highest SEA value of 317.24 J/Kg while SHS shows the lowest SEA value of 179.45 J/Kg. It reveals that the trend of the SEA results is similar to the compressive strength of the structures. This can be explained according to the number of hard and soft unit cells in the anti-chiral structures. To count the number of unit cells, as some cells are shared (half-half) between PLA and PCL materials, half-unit cells are counted. It is notable that the two half layers attached to the flat surfaces at the top and bottom are neglected as they do not play any role in the energy absorption during the compression test. As a result, the number of half-unit cells, fabricated from hard material, in HSH, SHS, FOU, and DIA are 16, 8, 12, and 12, respectively. This can obviously justify the achieved results for the mechanical performance of these structures. This graph also shows although the results for FOU and DIA designs are very similar, somewhere between HSH and SHS structures, the FOU structure has slightly higher mechanical strength compared to the DIA one. This also can be explained due

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Fig. 15.5 The results of compressive strength and specific energy absorption for four multimaterial structures

to the difference in design configurations in these two structures and their performance under the loading process. As shown in Fig. 15.3, the compression of the DIA sample is diagonal and the part made of soft material (PCL) buckled and moved to the right. Figure 15.4d demonstrates a continuous and smooth deformation in the force–displacement diagram. However, in the FOU structure, the soft material is placed outside and on top of the hard, so the structure is compressed from the top to reach the hard layers made of PLA. This also can be seen in Fig. 15.4c where sequential collapses in different layers of the FOU structure have occurred, in the direction from soft to hard layers. As a result, the maximum force is obtained in the last layer of the structure where it is fully PLA material.

15.4 Summary and Conclusion This study has investigated the role of materials and design configurations on the mechanical response of architected structures and in particular energy absorption performance. For this purpose, PLA and PCL are used in multimaterial 3D printing of anti-chiral structures in the concept of hard and soft materials. These two materials then are placed in different configurations (HSH, SHS, FOU, and DIA) in structures. In the end, they are evaluated based on the compressive strength and energy absorption properties. The result shows this approach can provide a wider range of mechanical performance that cannot be achieved in single material structures. Therefore, it can be used for programming adaptive structures, using a combination of soft and hard materials, to achieve the required mechanical and functional performance. This can be accomplished through an inverse design method based on the set of requirements and constraints for the target applications. This approach has great potential

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to be employed in a research direction with a wide range of engineering applications such as protective gear, sports equipment, soft robotics, etc. However, further research is needed to investigate the impact of a variety of designs and materials on the functionality of such structures.

References 1. Gibson, I., et al.: Additive Manufacturing Technologies, vol. 17. Springer (2021) 2. Mehrpouya, M., et al.: The benefits of additive manufacturing for sustainable design and production. In: Sustainable Manufacturing, pp. 29–59. Elsevier (2021) 3. Benedetti, M., et al.: Architected cellular materials: A review on their mechanical properties towards fatigue-tolerant design and fabrication. Mater. Sci. Eng. R. Rep. 144, 100606 (2021) 4. Yin, H., et al.: Review on lattice structures for energy absorption properties. Compos. Struct., 116397 (2022) 5. Niknam, H., Akbarzadeh, A.H.: Graded lattice structures: Simultaneous enhancement in stiffness and energy absorption. Mater. & Des. 196 (2020) 6. Zhao, W., et al.: Mechanical behavior analyses of 4D printed metamaterials structures with excellent energy absorption ability. Compos. Struct. 304, 116360 (2023) 7. Najafi, M., Ahmadi, H., Liaghat, G.: Evaluation of the mechanical properties of fully integrated 3D printed polymeric sandwich structures with auxetic cores: experimental and numerical assessment. Int. J. Adv. Manuf. Technol. 122(9–10), 4079–4098 (2022) 8. Serjouei, A., et al.: 4D printed shape memory sandwich structures: Experimental analysis and numerical modeling. Smart Mater. Struct. 31(5), 055014 (2022) 9. Barletta, M., Gisario, A., Mehrpouya, M.: 4D printing of shape memory polylactic acid (PLA) components: Investigating the role of the operational parameters in fused deposition modelling (FDM). J. Manuf. Process. 61, 473–480 (2021) 10. Boley, J.W., et al.: Shape-shifting structured lattices via multimaterial 4D printing. Proc. Natl. Acad. Sci. 116(42), 20856–20862 (2019) 11. Yavas, D., et al.: Design and fabrication of architected multi-material lattices with tunable stiffness, strength, and energy absorption. Mater. Des. 217, 110613 (2022) 12. Mueller, J., Lewis, J.A., Bertoldi, K.: Architected multimaterial lattices with thermally programmable mechanical response. Adv. Func. Mater. 32(1), 2105128 (2022) 13. Mehrpouya, M., et al.: Functional behavior and energy absorption characteristics of additively manufactured smart sandwich structures. Adv. Eng. Mater. 24(9), (2022) 14. Ashby, M.F., Gibson, L.J.: Cellular solids: structure and properties, pp. 175–231. Press Syndicate of the University of Cambridge, Cambridge, UK (1997) 15. Zeng, C., et al.: Compression behavior and energy absorption of 3D printed continuous fiber reinforced composite honeycomb structures with shape memory effects. Addit. Manuf. 38, 101842 (2021)

Chapter 16

Fabricating Lightweight Gear Using 3D Printing and Topology Optimization Riad Ramadani, Gül Okudan Kremer, Marko Kegl, and Jožef Predan

Abstract This research deals with the design and fabrication of a lightweight gear having honeycomb body structure. A honeycomb design is a complex structure that cannot be produced using traditional manufacturing methods. Selective Laser Melting (SLM) technology, on the other hand, enables the direct production of such innovative lightweight structures from their CAD models. The gear body structure was designed and optimized by employing topology optimization, and it was fabricated by AlSi10Mg alloy utilizing SLM. Numerical results showed that the honeycomb structure of the gear body has sufficient strength to carry the loads, while the practical example demonstrates the benefit of using 3D printing for lightweight products with intricate designs. Keywords 3D printing · Honeycomb structure · Topology optimization · Gears

16.1 Introduction 3D printing, aka Additive Manufacturing (AM) process can replicate the computeraided design (CAD) and other digital model into a real part [1]. This process uses the layer-by-layer building up method by slicing the digital model into micron level [2]. Most of AM process uses powder or liquid materials to fabricate the part accordingly. Therefore, a complex designed part such as topology optimized, honeycomb structure can be manufactured using AM process. Additionally, depending on the material R. Ramadani (B) University of Prishtina “Hasan Prishtina”, Faculty of Mechanical Engineering, Rr. Agim Ramadani, Ndërtesa E Fakulteteve Teknike, 10000 Prishtina, Kosovo e-mail: [email protected] G. Okudan Kremer School of Engineering, University of Dayton, 300 College Park, Dayton, OH 45469, USA M. Kegl · J. Predan Faculty of Mechanical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_16

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selection, the product can be manufactured. However, several manufacturing parameters are involved in AM process that determine the metallurgical properties of the product [3]–[5]. Fabricating the polymer products might be simpler comparing to the metallic/alloy products. Therefore, fabricating a gear body using AlSi10Mg alloy has a bigger challenge [6]. Since the gears are primary components in many transmission applications, both, the weight reduction and smooth operation are critical. Because of repeated stiffness changes during tooth meshing, each toothed gear ring becomes an internal generator of vibrations [7]–[9]. Therefore, many research work have been carried out to reduce gear weight and vibration by introducing the function of the gear body [10, 11]. A suitable powder material was used to fill the bores in the gear body [12, 13]. To a certain extent, this was successful because this hinders the pressure wave propagation and induces some damping. Another goal of gear body modification was to reduce the weight of the gear. Weight reduction resulted in thinner walls and the introduction of holes. These measures reduced the weight of the gear but did not have any significant impact on vibration generation and emission. From the dynamic point of view, a weaker and slimmer gear body might even make the situation worse [14]. In this study, a spur gear has been fabricated with AlSi10Mg alloy by using the SLM process. The purpose was to fabricate a lighter gear with a honeycomb body structure that also had lower gear vibration. It is supposed that honeycomb structure would increase the flexibility of the gear body and accumulate the dynamic load transmitted from the gear ring to the hub. Obviously, the honeycomb structure must be strong enough to carry the load. For that reason, topology optimization was carried out by using ProTOp software to maximize the strength of the gear after having material removed by the imposed honeycomb structure. The gear with the optimized honeycomb body structure was fabricated by the SLM process by overcoming some difficulties of the production process due to unwanted thermal and physical behavior of the product segment during manufacturing. It should be noted that this study will only focus on the optimization and 3D printing of the practical example of a lightweight gear.

16.2 Design and Optimization of the Gear Body Structure 16.2.1 Design of the Gear Body Structure In this work a spur gear with module 2.5 mm, number of teeth z = 34, pressure angle αn = 200, and gear width b = 10 mm was addressed. The numerical model of the gear was prepared in Simulia Abaqus [15]. The gear model was partitioned to specify the gear body to be optimized; the gear hub diameter is 35 mm and the gear ring diameter is 68.750 mm. The whole gear is meshed with 2,918,412 hexahedral elements of type C3D8, which are the Eight-node linear brick element. Because topology optimization necessitates a fine mesh, the number of

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Pressure (one side) Design domain Fixed domain

Pressure (other side) Constrained area

Fixed domain

Fig. 16.1 A spur gear with free and fixed domains

elements is large. Additionally, 68 load cases were created for the model, in which a pressure load of 618 MPa was applied on both sides of the gear teeth. Each load case is valid in one step. A boundary condition constraining the inner surface of the gear hub was applied as well. The first step in developing an appropriate optimization model was to designate subdomains as free and fix domain of a part. In our case the hub ring and the toothed ring were defined as fixed domains, while the gear body was defined as a design domain, free for optimization. A spur gear with designated fixed and free domains is presented in Fig. 16.1. In the second step of developing the optimization model, the gear body had to be configured in order to replace the full solid design with a honeycomb design. The gear body was configured by using honeycomb cells defined in a cylindrical coordinate system. The radial dimension of the honeycomb cell was chosen to be 3.2 mm and the angle was 5.294 degrees. The width of the cube diagonal lattice cell was 3.333 mm. The diameter of the cell diagonals was allowed to change within the range from 0.5376 to 1.2 mm. Another solid ring was added to increase the thickness of the gear ring. The radius of the ring was chosen to be 34.375 mm. Consequently, the thickness of the gear ring becomes 5 mm. At this stage the volume part of the body was 41.6% of the full solid design. This initial configured design is presented in Fig. 16.2.

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Fig. 16.2 Initial design of the gear body, configured as a honeycomb structure

16.2.2 Optimization of the Gear Body Structure The optimization procedure is an iterative process carried out in ProTOp. Each iteration requires one finite element analysis and one computation of topology parameters. ProTOp performs structural topology optimization by implementing an efficient evolutionary method based on heuristic principles and structural strain energy density [16]. The stresses of the initial design obtained for the volume part of 41.6% were around 850 MPa (Fig. 16.3; gear tooth region), while the stresses within the honeycomb structure peaked at around 700 MPa (Fig. 16.3; detail view; orange colour). The honeycomb structure was then optimized by employing topology optimization, the volume part of the free domain has to be increased from 41.6% to 44.8%. Consequently, this also reduced stresses within the tooth region, which has now reached approximately 567 MPa. However, more important are the stress levels

Fig. 16.3 Von Misses stress of the initial honeycomb structure

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Fig. 16.4 Von Misses stress of the optimized honeycomb structure

within the optimized structure which now dropped considerably to only about 190 MPa, Fig. 16.4. It might be worth noting that the optimized honeycomb structure has significantly reduced gear mass. Consequently, the volume of the gear with a solid body is about 51,130 mm3 , whereas the volume of the gear with the optimized honeycomb structure is about 30,850 mm3 . Therefore, the total volume is reduced by around one third. Additionally, the mass of the gear with a solid body is 0.40137 kg, whereas the mass of the gear with the optimized honeycomb structure is 0.24217 kg.

16.3 Manufacturing the Gear with Honeycomb Body Structure The topology optimized gear having honeycomb structure is manufactured with AlSi10Mg alloy using Selective Laser Melting (SLM) AM process. The gear has been built up keeping its thickness in Z direction using 150 W laser power, 450 mm/ s scanning speed, 0.035 mm hatch spacing (laser beam diameter is 0.05 mm) and 0.025 mm layer thickness. These manufacturing parameters are considered as one of the best combinations of these manufacturing parameters in Arrow LMP200 selective laser machine with a Yb:glass fiber laser from Dentas, LLC, Maribor, Slovenia. The results are obtained by the studies done by the authors. As aluminum and magnesium show highly flammable characteristics while contacting with oxygen, the oxygen level of the SLM building chamber was kept less than 10 ppm. Several complications occurred during the manufacturing process which was required to solve to obtain a desired shape and design of the gear. The complications are separation of gear part form the supportive bars, separations between the layers, bending and stopping of manufacturing run due to bending and sticking of re-coater. The manufacturing complications occurred due to high thermal

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gradient and thermal stress accumulation. However, finally a well-structured gear was manufactured for the further testing.

16.4 Conclusion A honeycomb structure was used to generate lightweight gears. The structure was designed and optimized by employing topology optimization. Therefore, the initial stresses of the honeycomb structure were reduced and as a result, the structure was strong enough to carry the loads. The optimized gear was produced by AlSi10Mg in SLM process. The manufacturing challenges included bending, separation due to big volume of the part to be manufactured and finding out the best manufacturing parameters. The main disadvantage of a gear with honeycomb structure is that the teeth flank of a printed gear must be grinded which requires an extra production operation.

References 1. Pal, S., et al.: Tensile properties of selective laser melting products affected by building orientation and energy density. Mater. Sci. Eng. A 743, 637–647 (2018). https://doi.org/10.1016/J. MSEA.2018.11.130 2. Limbasiya, N., Jain, A., Soni, H., Wankhede, V., Krolczyk, G., Sahlot, P.: A comprehensive review on the effect of process parameters and post-process treatments on microstructure and mechanical properties of selective laser melting of AlSi10Mg. J. Mater. Res. Technol. 21, 1141–1176 (2022). https://doi.org/10.1016/j.jmrt.2022.09.092 3. Pal, S., Drstvensek, I.: Physical behaviors of materials in selective laser melting process. DAAAM Int. Sci. B. 239–256 (2018). doi: https://doi.org/10.2507/daaam.scibook.2018.21 4. Zhou, S., et al.: Impacts of defocusing amount and molten pool boundaries on mechanical properties and microstructure of selective laser melted AlSi10Mg. Materials (Basel) 12(1) (2018), doi: https://doi.org/10.3390/ma12010073 5. Pal, S., Tiyyagura, H.R., Drstvenšek, I., Kumar, C.S.: The effect of post-processing and machining process parameters on properties of stainless steel PH1 product produced by direct metal laser sintering. Procedia Eng. 149, 359–365 (2016). https://doi.org/10.1016/j.proeng. 2016.06.679 6. Dong, Z., et al.: Microstructural evolution and characterization of AlSi10Mg alloy manufactured by selective laser melting. J. Mater. Res. Technol. 17, 2343–2354 (2022). https://doi.org/ 10.1016/j.jmrt.2022.01.129 7. Smith, J.D.: Gear noise and vibration, 2nd edn. CRC Press, New York (2003) 8. Fakhfakh, T., Chaari, F., Haddar, M.: Numerical and experimental analysis of a gear system with teeth defects. Int. J. Adv. Manuf. Technol. (2005). https://doi.org/10.1007/s00170-0031830-8 9. Fakhfakh, T., Walha, L., Louati, J., Haddar, M.: Effect of manufacturing and assembly defects on two-stage gear systems vibration. Int. J. Adv. Manuf. Technol. (2006). https://doi.org/10. 1007/s00170-005-2602-4 10. Ramadani, R., Belsak, A., Kegl, M., Predan, J., Pehan, S.: Topology optimization based design of lightweight and low vibration gear bodies. Int. J. Simul. Model. 17, 92–104 (2018)

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11. Ramadani, R., Kegl, M., Predan, J., Belšak, A., Pehan, S.: Influence of cellular lattice body structure on gear vibration induced by meshing. J. Mech. Eng. 64(10), 611–620 (2018). https:// doi.org/10.5545/sv-jme.2018.5349 12. Xiao, W., Li, J., Wang, S., Fang, X.: Study on vibration suppression based on particle damping in centrifugal field of gear transmission. J. Sound Vib. (2016). https://doi.org/10.1016/j.jsv. 2015.12.014 13. Xiao, W., Huang, Y., Jiang, H., Jin, L.: Effect of powder material on vibration reduction of gear system in centrifugal field. Powder Technol. (2016). https://doi.org/10.1016/j.powtec.2016. 01.038 14. Li, S.: Experimental investigation and FEM analysis of resonance frequency behavior of threedimensional, thin-walled spur gears with a power-circulating test rig. Mech. Mach. Theory (2008). https://doi.org/10.1016/j.mechmachtheory.2007.07.009 15. “Abaqus/CAE 6.14 User‘s Guide (2014). Dassault Systemes Crop. Providence”. 16. “CAESS ProTOp. www.caess.eu”.

Chapter 17

Validation of Simplified Injection Molding Simulation Results for Conformal Cooling with a Hybrid Mold Insert Using Thermal Imaging Technology Janez Gotlih, Timi Karner, Mirko Ficko, Igor Drstvensek, Tomaz Brajlih, and Miran Brezocnik

Abstract Conformal cooling in injection molding of thermoplastics enabled by the additive manufacturing technology is a breakthrough solution to the problem of temperature control at critical locations in the mold. This paper discusses the design and manufacturing of a hybrid mold insert, focusing on evaluating the predictive quality of a simplified simulation model that does not require modeling of the hybrid mold and does not account for the different material properties of the mold components. The hybrid mold was fabricated in a two-step process. The lower part of the insert was fabricated by milling and the upper part of the insert, including the conformal cooling channels, was fabricated using the additive manufacturing technology of selective laser melting. The hybrid mold insert consists of two materials: maraging steel 1.2709 for the upper part of the mold insert and tool steel 1.2343 for all other parts of the mold. To validate the simulation results, experimental measurements were performed after ejection of the product during the injection molding process using a thermal imaging camera. It was confirmed that the simplified simulation model agrees well with the measurement results and that it can be used to simulate the injection molding process when a hybrid mold consists of materials with similar material properties. Keywords Injection molding simulation · Conformal cooling · Hybrid mold

J. Gotlih (B) · T. Karner · M. Ficko · I. Drstvensek · T. Brajlih · M. Brezocnik University of Maribor, Smetanova Ulica 17, 2000 Maribor, Slovenia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. Drstvensek et al. (eds.), Additive Manufacturing in Multidisciplinary Cooperation and Production, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-37671-9_17

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17.1 Introduction Polymer processing in injection molding is based on heating and cooling of the polymer. One of the main tasks of the mold is to take the molten polymer and cool it as quickly as possible to the temperature at which the plastic product can be ejected. The success of the process depends on optimizing the difference between the melt temperature and the mold temperature. From the point of view of product quality, the temperature difference should be as small as possible; from the point of view of production, it should be as large as possible. The mold temperature can be controlled by a cooling system integrated into the mold, and the cooling medium is usually water. The optimal design of the cooling system is a complex task. On the one hand, the shape of the product dictates the design of the cooling system; on the other hand, there are manufacturing constraints. For shallow engravings, the temperature is controlled by drilled channels [1, 2], while baffles or bubblers are used for tall cores and box shapes [3, 4]. Thin cores are more problematic because they offer little space for cooling circuits and are difficult to cool due to their slenderness [5]. Tool steels with high thermal conductivity or alloys of copper and beryllium are used to cool molded parts that cannot support a drilled cooling circuit [6]. An advanced solution is conformal cooling, where the temperature control channels follow the shape of the product and ideally always run at the same distance from the product surface [7]. Molds with conformal cooling channels are possible through additive manufacturing (AM) technology. Various AM techniques are suitable for mold making. It has been shown that low-cost techniques such as fused filament fabrication (FFF) and selective laser sintering (SLS) for manufacturing injection molds result in significant losses in mold durability and cooling efficiency, making low-cost molds unsuitable for mass production [8, 9]. Molds produced by a powder bed fusion (PBF), a metal AM process, are more suitable for high-performance mold production. Selective laser melting (SLM) is one of the most used methods to build metal products layer by layer [10–12]. SLM enables the fabrication of complicated cooling channel layouts and cross-sectional designs even with integrated cellular structures [13, 14]. On the other hand, smooth outer surfaces cannot be achieved with SLM, which reduces cooling performance and mold life, and mold components made with AM also significantly increase the price of the mold [15]. When designing the mold, the advantages and disadvantages of possible cooling system designs must be evaluated. The concept of a hybrid mold was introduced to combine the advantages of conventional and additive manufacturing. A hybrid mold is a mold that is assembled from components that are partially manufactured by conventional machining, and partially by additive manufacturing [15, 16]. In addition to process quality criteria such as cycle time and achieving uniform temperature distribution over the entire cavity surface [17–20], product quality criteria must also be considered [21]. Due to the complexity of the IM process, the design of cooling systems is supported by numerical simulations. Either the boundary element method (BEM), which is easier to implement since it requires less detailed modeling, or finite element method (FEM), which is more detailed, is

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used [22]. Measurements of process characteristics, such as cycle times, temperature distribution, and product deformation, are performed to verify the simulation results. Papadakis et al. [23] used a thermal imaging camera to compare the temperature of the product when it was produced with conventional and conformal cooling, while Kirchheim et al. [24] used a thermal imaging camera to evaluate the temperature of a hybrid mold insert. In this paper, a thermal imaging camera measurement method is used to verify that a complex mold with a hybrid insert with internal openings can be accurately simulated using a simplified BEM simulation without accurate modeling of the mold components consisting of different materials.

17.2 Materials and Methods The product for injection molding is a car fog lamp housing (see Fig. 17.1). The critical area is the box-shaped part in the upper part of the product, as it cannot be cooled by conventional methods. The cooling system is designed to improve the cooling of the critical area by conformal cooling, while most of the cooling system remains conventional. The magenta cooling channel consists of the conventional inlet and outlet channels with a diameter of 6 mm, which are in the cylindrical part of the product, and the conformal cooling channel with a diameter of 5 mm, which is connected to the inlet and outlet channels and runs laterally along the upper part of the product through the critical area. The arrangement of the conformal cooling channels is designed to follow the shape of the product and to be evenly spaced from the surface of the product. The design allows for easy maintenance and offers the possibility of surface treatment to improve heat transfer and mold durability. All other five channels were intended for conventional machining by drilling and milling (see Fig. 17.1). Overall, only the conformal part of the magenta cooling channel was designed to be manufactured by additive manufacturing.

Fig. 17.1 The cooling concept of the fog lamp housing and detail of the critical area

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17.2.1 Material Properties An ABS-PC material Lupoy HR5007AB from the manufacturer LG Chemical was selected for the product material. Materials with compatible mechanical and thermal properties were selected for the hybrid tool. Tool steel 1.2343 was selected for the base of the insert, and maraging steel 1.2709 was selected for the top of the insert. Water was selected as the cooling medium. The chemical compositions of the of tool steel 1.2343 and maraging steel 1.2709 are given in Table 17.1. The thermal properties of the materials are listed in Table 17.2.

17.2.2 Numerical Simulation of the Injection Molding Process Using Autodesk MoldFlow software, a study was made to simulate the injection molding process. Figure 17.2 on the left-hand side shows the mesh model with the injection site. The injection site is represented by a yellow cone. The most thickened areas of the polymer product are all accessible to standard cooling and can be easily cooled. But even then, thick areas are a problem due to product shrinkage and intrusion, even when cooling is possible. This is especially a problem when the thickened areas are at the end of the polymer flow, where pressure is lower, and filling is worse than near the injection point. The cooling channels are modelled as beam mesh elements. Figure 17.2 on the right-hand side shows the mesh of the conformal cooling circuit with inlet boundaries. The inlets to the channels are shown by light blue arrows. The details of the mesh model are given in Table 17.3. A dual-domain BEM analysis was performed to simulate the IM process. The settings are summarized in Table 17.4.

17.2.3 The Injection Molding Process and Temperature Measurements A Battenfeld 6500 injection molding machine was used for injection molding production. During production, thermal conditions in the mold were measured using a FLIR E6 thermal imaging camera. During each ejection, the entire surface of the mold was scanned with the thermal imaging camera to obtain information about the temperature level and temperature difference between each cavity and to check for overheating zones. The critical area of each cavity was checked individually.

Co

Tool steel / 1.2343

/

Maraging 17.00–19.00 8.50–9.00 steel 1.2709

Ni

Ti

Al

1.10–1.40

/

/

4.50–5.20 0.60–0.80 0.05–0.10

Mo