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Metal Powder—Based Additive Manufacturing
Highly comprehensive resource covering all key aspects of the current developments of metal powder—based additive manufacturing
Metal Powder—Based Additive Manufacturing provides valuable knowledge and critical insights regarding the recent advances in various metal powder—based additive manufacturing techniques. This book also reviews typical powder preparation processes and highlights the significance of metal powder—based additive manufacturing for various industrial applications.
The key features covered in this book:
- A rigorous overview of the underlying theories and practical applications of metal powder—based additive manufacturing techniques, including laser powder bed fusion, electron beam melting, laser-based directed energy deposition, and metal binder jetting.
- An expansive introduction of each technique and its significance pertaining to the printing processes, metallurgical defects, powder materials, equipment, and the microstructures and mechanical properties of the printed parts.
- A deep exploration of the preparation processes of metal powders for additive manufacturing and the effects of different processes on the powder properties.
- Comprehensive case studies of parts printed by metal powder—based additive manufacturing for various industrial applications.
By providing extensive coverage of relevant concepts in the field of metal powder—based additive manufacturing, this book highlights its essential role in Industry 4.0 and serves as a valuable resource for scientists, engineers, and students in materials science, powder metallurgy, physics, and chemistry. The rich research experience of the authors in additive manufacturing ensures that the readers are provided with both an in-depth understanding and informative technical guidance of metal powder—based additive manufacturing.
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Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Title Page
Copyright
Biography
Preface
1 Introduction
1.1 History and Fundamentals of AM
1.2 AM Techniques
1.3 Metal Powder–Based AM
1.4 Post‐Processing
1.5 Powder Properties and Characterization Methods
1.6 Challenges and Future Trends of Metal Powder–Based AM
1.7 Summary
References
2 Metal Powder Preparation Processes
2.1 Atomization
2.2 Mechanical Mixing
2.3 Reduction Process
2.4 Powder Modification
2.5 Summary
References
3 Laser Powder Bed Fusion
3.1 History
3.2 Fundamentals
3.3 Printing Process
3.4 Metallurgical Defects
3.5 Powder Materials
3.6 Equipment
3.7 Typical Materials Used in LPBF
3.8 Mechanical Metamaterials for LPBF
3.9 Summary
References
4 Electron Beam Melting
4.1 History
4.2 Fundamentals
4.3 Preheating and Melting Processes
4.4 Metallurgical Defects
4.5 Powder Materials
4.6 Equipment
4.7 Microstructures and Mechanical Properties
4.8 Summary
References
5 Laser‐Based Directed Energy Deposition
5.1 History
5.2 Fundamentals
5.3 Deposition Process
5.4 Metallurgical Defects
5.5 Powder Materials
5.6 Equipment
5.7 Microstructure and Mechanical Properties
5.8 Summary
References
6 Metal Binder Jetting
6.1 History
6.2 Fundamentals
6.3 Printing Process
6.4 Raw Materials
6.5 Equipment
6.6 Microstructure and Mechanical Properties
6.7 Summary
References
7 Applications
7.1 Aerospace
7.2 Biomedical
7.3 Automotive
7.4 Molding and Tooling
7.5 Energy
7.6 Jewelry
7.7 Marine, Oil, and Gas
7.8 Challenges and Risks
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 AM categories, and their representative processes and commercial c...
Table 1.2 Post‐processing for metal powder–based AM.
Table 1.3 Summary of particle size indices and particle shape indices and th...
Chapter 2
Table 2.1 Summary of processing parameters for gas atomization.
Table 2.2 Characteristics of Fe–Si–B metallic glass alloy powders produced v...
Table 2.3 Comparison of the plasma rotating electrode process and gas atomiz...
Table 2.4 Comparison of mechanical alloying and mechanical blending for the ...
Table 2.5 Summary of the advantages and disadvantages of mechanical alloying...
Table 2.6 Comparison of titanium powders prepared by the hydride–dehydride a...
Table 2.7 Processing parameters of plasma spheroidization for different meta...
Table 2.8 Summary of representative AM metal powders developed by various pr...
Table 2.9 The preparation processes used by representative commercial compan...
Chapter 3
Table 3.1 Summary of metal powder materials amenable to LPBF.
Table 3.2 Effects of powder granulometry on the performance of LPBF‐printed ...
Table 3.3 Summary of typical commercialized equipment for LPBF.
Table 3.4 Summary of available powders for the commercialized LPBF equipment...
Chapter 4
Table 4.1 Summary of commercialized and experimental powder materials for EB...
Table 4.2 Summary of the specifications of EBM machine models produced by Ar...
Table 4.3 Summary of the mechanical properties of EBM‐printed Ti–6Al–4V allo...
Chapter 5
Table 5.1 Summary of powder materials for L‐DED, including commercialized ma...
Table 5.2 Summary of representative commercialized equipment for L‐DED.
Table 5.3 Summary of available powders for commercialized L‐DED equipment.
Table 5.4 Summary of the tensile properties of various metal materials print...
Chapter 6
Table 6.1. Summary of the most investigated powder materials for MBJ.
Table 6.2 Summary of standard commercialized equipment for MBJ.
Table 6.3 Summary of the tensile properties of MBJ‐printed metallic material...
List of Illustrations
Chapter 1
Figure 1.1 Schematics of the seven broad categories of AM technologies accor...
Figure 1.2 Micrographs of powder particles exhibiting various surface featur...
Figure 1.3 Particle size distribution curve showing the size gauges
D
10
,
D
50
Figure 1.4 Variation of the packing density with the proportion of large pow...
Figure 1.5 Schematic of a fixed funnel method for measuring the angle of rep...
Chapter 2
Figure 2.1 Overview of the preparation processes for metal powders.
Figure 2.2 SEM images showing the morphology of Ti–6Al–4V powder particles p...
Figure 2.3 Schematic diagram of gas atomization. The parameter
α
is the...
Figure 2.4 EBSD and EDS analysis of CoCrFeMnNi HEA powder particles. (a) The...
Figure 2.5 Schematic illustrations of different gas atomization techniques. ...
Figure 2.6 Powder morphology produced by different gas atomization processes...
Figure 2.7 Schematic diagram of water atomization. The parameter
α
is t...
Figure 2.8 SEM images showing the morphology of Fe–Si–B–C–P magnetic powder ...
Figure 2.9 Morphology of 17‐4 PH powders prepared by (a) gas atomization and...
Figure 2.10 X‐ray microtomography and SEM images of Inconel 625 powders prep...
Figure 2.11 Schematic illustration of plasma atomization. Source: Yolton and...
Figure 2.12 Schematic illustration of the plasma rotating electrode process....
Figure 2.13 Representative centrifugal spray models of direct drop formation...
Figure 2.14 Morphology of Ti–6Al–4V powders produced by different atomizatio...
Figure 2.15 Schematic of mechanical alloying. (a) Process, and (b) mechanism...
Figure 2.16 SEM images displaying the morphology of Al6061–TiO
2
composite po...
Figure 2.17 SEM micrographs of mechanically blended powders. (a) Titanium–2 ...
Figure 2.18 Schematic of the hydride–dehydride process in producing titanium...
Figure 2.19 SEM images displaying the morphology of hydride–dehydrided titan...
Figure 2.20 (a) Schematic of a hydride–milling–dehydride system. Resultant m...
Figure 2.21 Schematic illustration of the Fray–Farthing–Chan Cambridge elect...
Figure 2.22 SEM images showing the morphology of titanium powder produced by...
Figure 2.23 Schematic illustration of induction plasma spheroidization. RF s...
Figure 2.24 SEM images showing the morphology of tungsten powder before and ...
Figure 2.25 Property analysis of raw and spheroidized tungsten powders. (a) ...
Figure 2.26 Schematic illustration of the granulation–sintering–deoxygenatio...
Figure 2.27 SEM images showing the morphology of Ti–6Al–4V powder particles ...
Figure 2.28 (a) Schematic illustration of the fluidized‐bed treating process...
Chapter 3
Figure 3.1 (a) Schematic illustration of an LPBF machine. Source: Reproduced...
Figure 3.2 (a) Laser absorptivity of Al, Ag, Au, Cu, Mo, Fe, and steel with ...
Figure 3.3 Effects of laser power and scanning speed on the process outcomes...
Figure 3.4 Four main basic scanning patterns in a single layer. (a) Unidirec...
Figure 3.5 Scanning strategies in multiple layers. (a) Rotation of bidirecti...
Figure 3.6 A melt pool produced by LPBF from the 316L stainless steel powder...
Figure 3.7 Representative melt pool morphology of LPBF‐printed 316L stainles...
Figure 3.8 Representative melt pool morphology displaying the transition fro...
Figure 3.9 Time snapshots presenting the surface temperature evolution in a ...
Figure 3.10 Effect of melt flow on the melt track formation of tungsten prin...
Figure 3.11 Balling phenomenon in LPBF resulting from different mechanisms d...
Figure 3.12 Spattering behaviors during the LPBF printing of 316L under diff...
Figure 3.13 Time‐series radiographs acquired during the LPBF printing of an ...
Figure 3.14 Typical morphology of gas pores in: (a) a Ti–6Al–4V powder parti...
Figure 3.15 Formation of keyhole pores harnessed by
in situ
X‐ray imaging an...
Figure 3.16 Secondary electron images for lack‐of‐fusion pores in LPBF‐print...
Figure 3.17 (a) Schematic illustration of the dynamics of pore motion and me...
Figure 3.18 Basic mechanisms of stress and plastic deformation development d...
Figure 3.19 Cracks induced by residual stresses in LPBF‐printed metal parts....
Figure 3.20 Stress distribution in a thin‐walled part. (a)
X
‐component stres...
Figure 3.21 (a) Schematic illustration of the sample orientation for residua...
Figure 3.22 Schematic illustration of the solidification crack initiation in...
Figure 3.23 Fundamental mechanisms of powder spreading in LPBF revealed thro...
Figure 3.24 Microstructures of pure Ti dependent on LPBF process parameters....
Figure 3.25 Microstructure of a Ti–6Al–4V alloy printed by LPBF. (a) Fine‐gr...
Figure 3.26 Microstructures of a Ti–6Al–4V alloy. (a, b) Stress‐relieved con...
Figure 3.27 Effect of layer thickness on the microstructure of a Ti–6Al–4V a...
Figure 3.28 SEM images of the cellular microstructure of (TiB + TiC)/Ti comp...
Figure 3.29 Schematic illustration of the titanium–tantalum gyroid scaffold ...
Figure 3.30 Isometric views of the microstructure of an Al–Si10–Mg alloy. (a...
Figure 3.31 (a)
Electron backscatter diffraction
(
EBSD
) image of the grains ...
Figure 3.32 Lattice‐matched nanoparticles were added to high‐strength Al all...
Figure 3.33 Contour pole figures and inverse pole figures of LPBF‐printed Al...
Figure 3.34 (a) Relationship between temperature and the fraction solid
f
s
, ...
Figure 3.35 (a) 3D TEM image composite view for an as‐fabricated cylinder al...
Figure 3.36 Microstructure morphology of an LPBF‐printed Inconel 625 superal...
Figure 3.37 A typical microstructure of an LPBF‐printed 316L stainless steel...
Figure 3.38 Re‐austenitization of LPBF‐printed maraging steel during aging a...
Figure 3.39 Schematic of the preparation procedure of a micro‐ and nanoporou...
Figure 3.40 Characterization of the catalyst. (a) Photographs of the structu...
Figure 3.41 Classification of metamaterials according to their functionaliti...
Figure 3.42 Classification of mechanical metamaterials. LPBF‐printed mechani...
Figure 3.43 Strut‐based lattice structures. (a) BCC, (b) BCCZ, (c) FCC, (d) ...
Figure 3.44 Representative topologies of the unit cells for shell lattice st...
Figure 3.45 Schematic drawings and skeletal graphs of TPMSs. (a) Primitive, ...
Figure 3.46 Continuous functionally graded porous titanium scaffolds manufac...
Figure 3.47 Fundamentals of origami‐inspired metamaterials. (a) Schematic of...
Figure 3.48 (a) Parameterization of a unit cell of the stacked Miura‐ori des...
Figure 3.49 LPBF‐fabricated mechanical metamaterials with vanishing shear mo...
Figure 3.50 Geometrical designs for auxetic metamaterials. (a) 2D re‐entrant...
Figure 3.51 (a) Comparison of LPBF‐printed auxetic metamaterials with 316L u...
Chapter 4
Figure 4.1 (a) Schematic illustration of an Arcam EBM system. Source: Reprod...
Figure 4.2 Typical scanning patterns used in EBM. (a) Zigzag pattern, (b) co...
Figure 4.3 Schematic of various phenomena that occur during the EBM process....
Figure 4.4 Pre‐sintering and melting processes in EBM. (a) Pre‐sintering by ...
Figure 4.5 (a) Exemplary melt surface and poor melt surface caused by materi...
Figure 4.6 (a) EBM‐printed HEA samples and (b) SEM image of a rough sample s...
Figure 4.7 (a) Electron backscatter diffraction map indicating cracks propag...
Figure 4.8 SEM images of the microstructures of EBM‐printed Ti–6Al–4V sample...
Figure 4.9 (a) OM image and (b) schematic representing the equiaxed‐to‐colum...
Figure 4.10 Microstructure of an EBM‐printed Ti2448 alloy. (a) OM and (b) SE...
Figure 4.11 (a) Representative spherical pore defect in an EBM‐printed γ‐TiA...
Figure 4.12 OM images of the microstructure of an EBM‐printed Inconel 625 al...
Figure 4.13 Microstructure of a nickel‐based alloy CMSX‐4 fabricated by EBM ...
Figure 4.14 Microstructure of an EBM‐printed Co–Cr–Mo alloy along the build ...
Figure 4.15 (a) SEM image of columnar grains with dendrites in an EBM‐printe...
Figure 4.16 Microstructure of an EBM‐printed 316L alloy. (a) 3D micrograph u...
Chapter 5
Figure 5.1 Schematic diagrams illustrating the attributes and mechanisms per...
Figure 5.2 (a) Variation in the melt pool size and deposited layer thickness...
Figure 5.3 Porosity in a 316L sample printed by L‐DED measured with
X‐ray co
...
Figure 5.4 Surface quality of 316L samples printed by L‐DED. (a) Side surfac...
Figure 5.5 System components of an L‐DED machine. Source: Kim et al. (2017)/...
Figure 5.6 Microstructure of Ti–6Al–4V parts fabricated by L‐DED. (a) In the...
Figure 5.7 Optical images showing microstructures of commercially pure Ti pr...
Figure 5.8 Stress–strain curves of a Ti–6Al–4V alloy printed by L‐DED for di...
Figure 5.9 Microstructures of Inconel 718 parts fabricated by L‐DED under (a...
Figure 5.10
Electron backscatter diffraction
(
EBSD
) results displaying the t...
Figure 5.11 Different locations of the microstructure of a single‐layer Inco...
Figure 5.12 Different positions of the microstructure of a 316L part printed...
Figure 5.13 (a) Overview of a compositionally graded Fe–19Ni–
x
Al structure, ...
Figure 5.14 (a) Schematic of an L‐DED process, including a simple sketch of ...
Figure 5.15 (a) SEM image of deposited Al–Si10–Mg with a heat input of 480 W...
Figure 5.16 Hierarchical microstructure of a printed Cu–9Al–5Fe–5Ni alloy, i...
Figure 5.17 Multi‐material samples manufactured by L‐DED. (a) Schematic of t...
Figure 5.18 (a) Bimetallic Cu–Ni structure fabricated via L‐DED; the element...
Figure 5.19 Microstructure of compositionally graded Al
x
CuCrFeNi
2
HEA produc...
Chapter 6
Figure 6.1 Schematic illustration of the MBJ process. (a) Preparing a powder...
Figure 6.2 Schematic illustration of the binder–powder interactions in MBJ. ...
Figure 6.3 Schematic illustration of different granule formation mechanisms....
Figure 6.4 Schematic illustration of the main scanning strategies in MBJ. (a...
Figure 6.5 Illustration of three sintering stages, revealing the details in ...
Figure 6.6 Schematic illustration of drop‐on‐demand and continuous‐jet print...
Figure 6.7 HP thermal inkjet printhead in HP Metal Jet printers. (a) Image o...
Figure 6.8 Pores of sintered samples printed from (a) 316L, (b) powder agglo...
Figure 6.9 Schematic illustration of the microstructure evolution of an MBJ‐...
Figure 6.10 Schematic illustration showing (a) the interaction between the b...
Figure 6.11 (a) Schematic of the printing and curing stages with a MOD ink a...
Chapter 7
Figure 7.1 Repair of a T700 blisk using the laser engineering net‐shaping pr...
Figure 7.2 (a) IN718 helicopter engine combustion chamber. Source: Reproduce...
Figure 7.3 (a) GE LEAP engine fuel nozzle fabricated through LPBF. (b) Compr...
Figure 7.4 Typical LPBF‐printed components for the aerospace sector. (a) Cop...
Figure 7.5 Implants fabricated by metal powder–based AM. Applications of LPB...
Figure 7.6 (a) Design of an implant model, (b) printed Ti–6Al–4 V implant, (...
Figure 7.7 Dental implants fabricated by LPBF. (a) Co–Cr removable partial d...
Figure 7.8 (a) Race car steering knuckle produced by LPBF. Source: Reproduce...
Figure 7.9 Mold products printed through LPBF by commercial companies. (a) M...
Figure 7.10 Mold inserts with conformal cooling channels printed via LPBF. (...
Figure 7.11 Tool inserts printed via LPBF. (a) Printed inserts for die casti...
Figure 7.12 Heat exchangers printed through LPBF by commercial companies. (a...
Figure 7.13 Heat sinks fabricated through LPBF. (a) Pinfin‐Al6061, (b) Diamo...
Figure 7.14 Chemical reactor components printed through EBM. (a) Reactor tub...
Figure 7.15 Fuel cell components printed through LPBF. (a) Ti–6Al–4V BPP and...
Figure 7.16 (a) Cufflinks and (b) a ring are made of 18‐karat yellow gold. S...
Figure 7.17 Jewelry samples printed by LPBF using 18‐karat yellow gold with ...
Figure 7.18 EBM‐printed Pelton turbine system including a Pelton wheel, pens...
Figure 7.19 Crankshaft repaired by L‐DED. (a) Oil‐bore region and (b) its bo...
Guide
Cover Page
Title Page
Copyright
Biography
Preface
Table of Contents
Begin Reading
Index
Wiley End User License Agreement
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Metal Powder–Based Additive Manufacturing
Kun Zhou and Changjun Han
Authors
Prof. Kun Zhou
Nanyang Technological University
50 Nanyang Avenue
Singapore ‐ 639798
Prof. Changjun Han
South China University of Technology
381 Wushan Road
Tianhe District
Guangzhou
China ‐ 510641
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The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2023 WILEY‐VCH GmbH, Boschstr. 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978‐3‐527‐34631‐8
ePDF ISBN: 978‐3‐527‐82222‐5
ePub ISBN: 978‐3‐527‐82223‐2
oBook ISBN: 978‐3‐527‐82224‐9
Cover Image © Christian Bay/Getty Images
Cover Design Wiley
Biography
Kun Zhou is a Professor of Mechanical Engineering in the School of Mechanical and Aerospace Engineering at Nanyang Technological University, Singapore. His research interests focus on mechanics of materials and additive manufacturing. He has founded the journal Smart Manufacturing and serves as its Editor‐in‐Chief. He is also a co‐founder of Journal of Micromechanics and Molecular Physics and serves as its Co‐Editor‐in‐Chief. He was elected Fellow of Institution of Mechanical Engineers, Royal Aeronautical Society, Royal Society of Chemistry, Institute of Physics, and Institute of Materials, Minerals & Mining.
Changjun Han is an Associate Professor in the School of Mechanical and Automotive Engineering at South China University of Technology, China. He achieved his Ph.D. from Huazhong University of Science and Technology in 2018 and subsequently worked as a postdoctoral fellow in the School of Mechanical and Aerospace Engineering at Nanyang Technological University, Singapore for over two years. He has rich experience in additive manufacturing of metallic materials. He serves as an Associate Editor of Smart Manufacturing.
Preface
Additive manufacturing (AM), commonly known as three‐dimensional (3D) printing, has emerged as a revolutionary technology capable of fabricating parts with complex geometries in a layer‐by‐layer manner, thereby granting unprecedented design freedom. It has evolved from a tool primarily employed for rapid prototyping into a well‐recognized technology for producing functional parts with highly desirable properties. This technology provides a cost‐effective option for the low‐volume production of highly specialized parts for a wide variety of applications in the aerospace, art, biomedical, automotive, electronics, manufacturing, energy, building and construction, marine, “oil and gas industries”.
Powder–based AM is a major subset of the AM family, in which parts are using powders as feedstock materials. Within this category, powder bed fusion (PBF), directed energy deposition (DED), and binder jetting are presently the predominant techniques. In particular, laser powder bed fusion (LPBF), electron beam melting (EBM), laser‐based directed energy deposition (L‐DED), and metal binder jetting (MBJ) are commonly adopted powder–based AM processes for printing metal parts.
The production of defect‐free and structurally sound parts through metal powder–based AM requires an in‐depth understanding of the feedstock materials, the printing processes, and the underlying mechanisms of defect formation in the printed parts. Notably, metal powder–based AM is multidisciplinary in nature and entails key concepts from different subjects, including materials science, engineering, powder metallurgy, physics, and chemistry. Therefore, understanding the fundamentals of metal powder–based AM may constitute a daunting challenge for novice researchers.
This book introduces the fundamentals of powder preparation processes and metal powder–based AM techniques while emphasizing their significance in various industrial applications. For more experienced AM practitioners, the recent progress of metal powder–based AM techniques is reviewed along with noteworthy case studies, which feature the wide applicability of metal powder–based AM techniques.
Chapter 1 presents a brief introduction to metal powder–based AM. The history and fundamentals of AM are first highlighted, followed by seven categories of AM techniques with their representative processes and commercial companies. Next, different types of post‐processing treatment techniques for metal AM parts are illustrated. Powder properties and characterization methods are also discussed comprehensively. Finally, the challenges and future trends of metal powder–based AM are outlined. Researchers who are unfamiliar with metal powder–based AM techniques are encouraged to peruse this chapter to attain a solid understanding of these principles.
Chapter 2 presents the fundamentals of the preparation processes of metal powders for AM, specifically, atomization, mechanical mixing, the reduction process, and powder modification. A comprehensive review of the relationships between the preparation processes and powder properties is provided. For more experienced researchers, this chapter serves as a concise handbook in which the key concepts underlying AM powder metallurgy are succinctly explained.
Chapters 3–6 delve into various metal powder–based AM techniques, namely, LPBF, EBM, L‐DED, and MBJ. Chapter 3 focuses on the LPBF technique, with regard to its history, fundamentals, printing process, metallurgical defects, powder materials, equipment, and the microstructures and mechanical properties of the representative materials investigated for the printed parts. In particular, the characteristics of popular LPBF‐printed mechanical metamaterials, which typically correspond to a high Young's modulus, high shear and bulk moduli, and zero or negative Poisson's ratio, are comprehensively evaluated.
In Chapter 4, the EBM technique is introduced with an overview of its history, fundamentals, processing characteristics, powder materials, equipment, and the microstructures and mechanical properties of the most extensively studied materials for the printed parts. The preheating and melting processes during EBM printing are also described, and their influence on the surface quality of the printed parts is discussed.
Chapter 5 expounds on the L‐DED technique in terms of its history, fundamentals, deposition process, metallurgical defects, powder materials, equipment, and the microstructures and mechanical properties of the representative materials for the printed parts. Additionally, the unique capability of L‐DED to fabricate multi‐materials and functionally graded materials is highlighted.
Chapter 6 is devoted to the MBJ technique, with respect to its history, fundamentals, printing process, powder and binder materials, equipment, and the microstructures and mechanical properties of the representative materials for the printed parts. The unique ability of MBJ to print parts using refractory metals, magnesium and its alloys, and magnetic alloys, which are difficult to print using the abovementioned three techniques, is also examined.
Chapter 7 reports on the latest advances in metal powder–based AM products in the aerospace, biomedical, automotive, molding and tooling, energy, marine, oil and gas, and jewelry industries. This chapter focuses on providing insights of both academic and industrial relevance to readers and keeping them abreast of recent applications involving metal AM.
The completion of this book could not have been possible without the excellent teamwork displayed by the members of our research group, with whom our discussions have always been fruitful. We greatly appreciate the efforts of Liming You, Chengcheng Wang, Haiyang Fan, Yujia Tian, Jiazhao Huang, Boyuan Li, Zhuohong Zeng, Devesh Kripalani, Priyanka Vivegananthan, Asker Jarlöv, Feng Liu, Bin Liu, Liming Tan, Yung Zhen Lek, Kiran Raj, Pengfei Tan, Ming Gao, Ran An, Terence Zhi Xiang Hong, Yang Qi, Weiming Ji, Wei Fan, Meixin Zhou, Chao Cai, and Wei Zhu, who contributed valuable feedback during the preparation of this book. We are also deeply grateful to our collaborators, who have rendered us tremendous support in conducting research pertaining to metal powder–based AM.
Singapore, December 2022
Kun ZhouChangjun Han
1Introduction
This chapter presents a brief introduction to metal powder–based additive manufacturing (AM). The history and fundamentals of AM are first highlighted. Seven categories of AM techniques with their representative processes and commercial companies are then presented, followed by an overview of the types of metal powder–based AM techniques. Next, the types of post‐processing treatments for metal AM parts are illustrated. Powder properties and characterization methods are also discussed comprehensively. Finally, the challenges and future trends of metal powder–based AM are outlined.
1.1 History and Fundamentals of AM
Additive manufacturing (AM), commonly known as three‐dimensional (3D) printing, has emerged as a revolutionary technology capable of fabricating parts with complex geometries in a layer‐by‐layer manner, thereby granting extensive freedom of design (Martin et al. 2017; Kelly et al. 2019; Zhang et al. 2019). The term “additive manufacturing” describes “the process of joining materials to make parts from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing and formative manufacturing methodologies”, according to the ISO/ASTM 52900 standard (ISO and ASTM International 2015). ISO and ASTM are the abbreviations for the International Standards Organization and the American Society for Testing and Materials, respectively. The most commonly used term before AM was “rapid prototyping”, which is currently considered outdated because of the increasingly extensive applications of the process.
The advent of AM marks an important milestone in the history of manufacturing. In 1987, Stereolithography Apparatus‐1, the first commercial AM system, was launched by 3D Systems in the United States. The system enabled the printing of 3D parts from a computer‐aided design (CAD) model for the first time. Other AM techniques were introduced for commercial purposes soon thereafter, which include fused deposition modeling, selective deposition lamination, selective laser sintering, and laminated object manufacturing. Machines operating based on the inkjet printing process began appearing on the market in 1996. In 1998, Optomec delivered the first laser engineered net shaping metal powder system capable of producing near‐fully dense metal parts. Meanwhile, in 1999, a selective laser melting system was initiated by Fockele & Schwarze of Germany.
The expiry of old AM patents, such as the fused deposition modeling technique (which expired in 2009), has led to a surge in the number of low‐cost personal AM machines on the consumer market, some of which can even be assembled by hand. Moreover, the rise of online platforms, such as open‐source communities where AM design files are shared freely, further improves the accessibility of 3D printing technology. In 2014, the number of AM patents that expired has reached a peak, which has led to a greater variety of low‐cost machines flooding the consumer market.
Over the years, AM has evolved from a tool primarily used for visualization (i.e. rapid prototyping) into a well‐recognized technology for producing functional parts with desirable properties. Before printing, a 3D model is constructed via CAD and mathematically sliced into ultrathin printed layers along the build direction. Subsequently, the layers are printed according to their predefined shapes, with consecutive layers bonding to each other.
AM also provides a cost‐effective option for low‐volume customized production, which differs from conventional mold jetting methods. Parts comprising multiple components can be redesigned as single units and fabricated efficiently without assembly (Chua and Leong 2017). The distinct ability of this process to manufacture complex shapes and structures has already rendered it invaluable for producing prototypes or parts in industries including aerospace, military, biomedical, automotive, electronics, energy, molding, building and construction, marine and offshore, education, art, robotics, environment, and social culture.
AM is also perceived as an environmentally sustainable manufacturing technology, as it can potentially reduce up to 525.5 Mt of total carbon dioxide emissions by 2025 compared to conventional processes (Gebler et al. 2014; Ribeiro et al. 2020). Furthermore, in terms of product development, AM can reduce up to 70% of development costs and time‐to‐market by up to 90% compared to conventional processes (Gibson et al. 2014). With such remarkable benefits, the global market for AM is projected to reach nearly US$23 billion by 2023, with a compound annual growth rate of 22% (Tan et al. 2020).
1.2 AM Techniques
AM technologies have been classified into seven broad categories according to the ISO/ASTM 52900 standard, namely, vat photopolymerization, material jetting, material extrusion, powder bed fusion (PBF), directed energy deposition (DED), binder jetting, and sheet lamination techniques (ISO and ASTM International 2015), as shown in Figure 1.1. Table 1.1 provides an overview of the representative processes and commercial companies of these AM categories.
The vat photopolymerization technique utilizes radiation (e.g. ultraviolet [UV] and visible light) to selectively polymerize liquid photosensitive resins in a vat to form high‐resolution solid 3D structures. It includes various processes such as stereolithography (Hull 1986), digital light processing (Kuang et al. 2019), continuous liquid interfase production (Tumbleston et al. 2015), two‐photon photopolymerization (Saha et al. 2019), high‐area rapid printing (Walker et al. 2019), and computed axial lithography (Kelly et al. 2019). Representative companies for vat photopolymerization include 3D Systems (USA), Carbon (USA), Envisiontec (Germany), Lithoz (Australia), and UnionTech (China).
Figure 1.1 Schematics of the seven broad categories of AM technologies according to the ISO/ASTM 52900 standard. Source: ISO and ASTM International (2015)/ISO.
Table 1.1 AM categories, and their representative processes and commercial companies.
AM categories
Representative AM processes
Representative commercial companies
Vat photopolymerization
Stereolithography Digital light processing Continuous liquid interfase production Computed axial lithography Two‐photon photopolymerization High‐area rapid printing
USA
3D Systems, Stratasys, Carbon, Formlabs
Germany
Envisiontec
China
UnionTech, ZRapid
France
Prodways
Australia
Lithoz
Netherlands
Admatec
Italy
Sisma, DWS
Denmark
AddiFab
Material jetting
PolyJet MultiJet Aerosol jet NanoParticle jetting Electrohydrodynamic jetting
USA
3D Systems, Stratasys, Optomec, Solidscape, nScrypt
Israel
Xjet, Nano Dimension
Japan
Mimaki
Material extrusion
Fused deposition modeling (Fused filament fabrication) Direct ink writing 3D dispensing (3D plotting)
USA
Stratasys, Markforged, Essentium, Robocasting, Cincinnati
Germany
Arburg, BigRep
Netherlands
Ultimaker
Powder bed fusion
Laser powder bed fusion (Selective laser melting) (Direct metal laser sintering) Electron beam melting Selective laser sintering Multi jet fusion High speed sintering
USA
3D Systems, HP Inc., GE Additive, Velo3D
Germany
SLM Solutions, EOS, Trumpf, Voxeljet
China
Bright Laser Technologies, Farsoon Technologies, Eplus 3D, Laseradd
UK
Renishaw
Italy
Sisma
Directed energy deposition
Laser‐based directed energy deposition (Laser engineered net shaping) (Laser metal deposition) Laser wire additive manufacturing Wire arc additive manufacturing Electron beam freeform fabrication (Electron beam additive manufacturing)
USA
Optomec, Sciaky, Addere, Formalloy
Germany
Trumpf, DMG Mori
China
Bright Laser Technologies
France
BeAM
Australia
AML3D
Netherlands
MX3D
Japan
Sodick
South Korea
InssTek
Spain
Addilan
Binder jetting
Binder jetting
USA
3D Systems, HP Inc., Exone, Desktop Metal, Digital Metal
Germany
Voxeljet
China
Long Yuan
UK
Raplas
Sheet lamination
Laminated object manufacturing Ultrasonic additive manufacturing Selective deposition lamination Composite‐based additive manufacturing
USA
Fabrisonic, Evolve Additive, Impossible Objects
Figure 1.1a illustrates the top‐down stereolithography process, a typical vat photopolymerization technique. Stereolithography is a common vat photopolymerization technique that utilizes an UV laser source positioned above or underneath a resin vat to selectively cure the exposed layer.
The material jetting technique is analogous to traditional two‐dimensional (2D) inkjet printing, in which liquid materials (e.g. photosensitive resins, thermoplastics, wax, and reactive materials) are deposited from inkjet printheads onto a build platform through either a drop‐on‐demand or a continuous approach and subsequently solidified through photopolymerization, cooling, etc. This technique is employed in commercialized systems such as PolyJet, MultiJet, Aerosol Jet, and NanoParticle Jet, as well as non‐commercialized systems based on electrohydrodynamic jetting.
Figure 1.1b shows a schematic of the material jetting technique. For example, in the PolyJet system, multiple depositions of photosensitive build materials and support materials (to form the support structures within the printed parts) in the form of droplets are made by nozzles in the printhead before undergoing UV curing. During post‐processing, the support structures are removed through mechanical (cutting with a water jet system) and/or chemical (dissolving with a solvent) procedures. Representative companies of material jetting include Stratasys (USA), 3D Systems (USA), Optomec (USA), Xjet (Israel), and Nano Dimension (Israel).
The material extrusion technique typically involves a continuous extrusion of polymer filaments, viscous inks, or even polymer pellets through a heated nozzle onto a build platform, on which the extruded molten material subsequently solidifies. Fused deposition modeling (also known as fused filament fabrication) and direct ink writing are the key representative processes in this category. The former utilizes thermoplastic filaments and pellets as feedstock materials, while the latter employs viscous inks such as pastes and concentrated polymer solutions (Truby and Lewis 2016). Recent studies have also established the significant potential of fused deposition modeling in printing metal parts. For example, in one such study, printable filaments were prepared through the extrusion of a polyolefin‐based binder mixed with 59 vol.% Ti–6Al–4 V powder and then fabricated into parts using the Renkforce 1000 printer (Zhang et al. 2020). In addition, solvent debinding, thermal debinding, and sintering were also conducted to densify the printed parts. Representative companies of material extrusion include Stratasys (USA), Markforged (USA), Essentium (USA), Robocasting (USA), and Ultimaker (Netherlands).
Figure 1.1c illustrates the fused deposition modeling process, a representative material extrusion technique. The build material and the support material, in the form of thermoplastic filaments, are heated to the molten state in the nozzles, extruded, and solidified on the build platform. The support structures are removed through mechanical and/or chemical procedures during post‐processing.
The PBF technique utilizes a heat source (e.g. electron beam or laser beam) to coalesce metal, polymer, or ceramic powder particles in a powder bed to build 3D objects, and it can theoretically process any powder–based materials on the condition that the corresponding powder particles can be fused or melted through heating. The technique can be categorized into laser powder bed fusion (LPBF) (also widely known as selective laser melting), electron beam melting (EBM), selective laser sintering, multi jet fusion, and high speed sintering processes. LPBF and EBM are mainly employed to print pure metals, alloys, and metal matrix composites (Han et al. 2020a, 2020b), while the selective laser sintering, multi jet fusion, and high speed sintering processes are typically implemented to treat polymers and their composites. Representative commercial companies of PBF include SLM Solutions (Germany), EOS (Germany), Trumpf (Germany), 3D Systems (USA), GE Additive (USA), Renishaw (UK), Bright Laser Technologies (China), and Farsoon Technologies (China).
Figure 1.1d presents a typical PBF process, LPBF, which utilizes a laser beam to selectively melt metal powder particles in the powder bed. The molten metal particles coalesce and solidify to form the printed part. While unmelted powder particles can serve as support, support structures are generally required to be printed to ensure the good printability of overhanging structures of a part.
The DED technique utilizes a laser beam, an electron beam, or an electric arc to melt metal powders or wires upon their deposition along the printing paths (Bermingham et al. 2015; Li et al. 2021). The technique includes processes such as laser‐based directed energy deposition (L‐DED) (also widely known as laser engineered net shaping and laser metal deposition), laser wire additive manufacturing (LWAM), wire arc additive manufacturing (WAAM), and electron beam freeform fabrication (EBF3, also known as electron beam AM). The L‐DED process is commonly employed to print parts using powders, while the other four processes employ wires for printing. Examples of representative companies for DED include Optomec (USA), Sciaky (USA), DMG Mori (Germany), Trumpf (Germany), BeAM (France), and Bright Laser Technologies (China).
Figure 1.1e illustrates a typical DED process, L‐DED, in which metal powder particles are delivered through channels in a nozzle and melted by a laser beam upon deposition onto a freeform substrate. The molten metal particles subsequently solidify to form the printed part.
The binder jetting technique utilizes one or more inkjet printheads to deposit droplets of a liquid polymer binder onto ceramic or metal powder particles in a powder bed and selectively glue them together to build 3D objects (Mostafaei et al. 2017). The droplets can be deposited through a single‐pass or multi‐pass printing strategy. Post‐processing procedures, such as curing, de‐powdering, infiltration, and sintering, are often required for green parts printed through this technique. Examples of commercialized companies specializing in binder jetting include Exone (USA), Hewlett–Packard (USA), Desktop Metal (USA), Digital Metal (USA), 3D Systems (USA), Voxeljet (Germany), and Long Yuan (China).
Figure 1.1f demonstrates a metal binder jetting (MBJ) process, in which a layer of metal powder particles is spread by a roller across the build platform, and a liquid binder is then selectively deposited to bond specific regions of the powder layer.
The sheet lamination technique can be employed to fabricate 3D objects by stacking and laminating thin sheets of materials (e.g. paper, metal sheets, ceramic tapes, woven fiber composite sheets, and thermoplastic foils) through different bonding (e.g. adhesive bonding, thermal bonding, and ultrasonic welding) and cutting (e.g. computer numerical control [CNC] milling, laser cutting, and water jet cutting) strategies. According to the ISO/ASTM standard, representative processes of the sheet lamination technique include laminated object manufacturing and ultrasonic AM. Laminated object manufacturing was the first commercialized sheet lamination process and was initially implemented to bond kraft paper. Subsequently, the applicability of this process for bonding plastic/metal tapes and foils has been investigated. Meanwhile, the ultrasonic AM process is commonly utilized for printing parts from metal foils and sheets (Dehoff and Babu 2010). In addition to the two aforementioned processes, a novel sheet lamination process, friction stir AM, is currently in development. In this process, metal plates are utilized as feedstock materials. A stirring pin is inserted into a newly added metal plate at high rotation rates, and the resultant friction between the stirring pin and the plate produces heat that softens and bonds the plate to the previous plate. Representative companies for sheet lamination include Fabrisonic (USA), Evolve Additive (USA), and Impossible Objects (USA).
Figure 1.1g presents a diagram of the sheet lamination process, ultrasonic AM, in which each layer of a metal foil is laid and bonded to the previous layer through ultrasonic welding. This process involves the actuation of a cylindrical sonotrode with an ultrasonic transducer to induce a scrubbing motion of the sonotrode. At the same time, a downward force is applied to the metal foils. The sonotrode rolls along the length of the foils due to the downward force, and ultrasonic vibrations are applied along the width of the foils through the sonotrode. Friction can be produced between the metal foils through the interaction of the downward force and ultrasonic vibrations. Bonding is achieved through disruption of the oxide layers between the metal foils utilizing friction, thereby promoting nascent metal‐to‐metal contact. The metal layer is subsequently cut through CNC milling to obtain the desired geometry, and the cycle is repeated until the metal part is completed.
1.3 Metal Powder–Based AM
Powder–based AM is an important subset of the AM family for manufacturing parts using powders as feedstock materials, and it employs the PBF, DED, and binder jetting techniques. In particular, LPBF, EBM, L‐DED, and are commonly adopted powder–based AM processes for printing metal parts. LPBF, EBM, and L‐DED share similar features, such as the usage of high‐energy heat sources, localized melting, and microstructural evolution upon solidification. Applying metal powder–based AM in producing structurally sound, defect‐free, and reliable parts requires an in‐depth understanding of existing printing techniques, the physical and chemical processes involved during printing, feedstock materials, process control methods, and underlying mechanisms of common defects and their prevention.
Metal powder–based AM excels in the following aspects: (i) its ability to recycle feedstock materials, (ii) its relatively high manufacturing accuracy as compared to wire‐ and sheet‐printing, and (iii) the capacity of certain powder–based AM processes (MBJ and EBM) to print parts without requiring support structures because of the support provided by the surrounding unfused or partially melted powder particles. Notably, metal powder–based AM plays an indispensable role in various domains, including the aerospace, biomedical, automotive, molding and tooling, energy, jewelry, marine, oil and gas, and repair and remanufacturing industries.
1.4 Post‐Processing
It is challenging to employ metal powder–based AM to directly fabricate parts with properties and surface characteristics that satisfy application requirements. Most metal powder–based AM techniques require post‐processing treatment to obtain the desired properties in the printed parts (ISO and ASTM International 2015). Post‐processing is crucial for addressing the main issues of AM parts, such as high surface roughness, high porosity, dimensional deviations with respect to the models, and substandard mechanical properties for industrial applications.
In certain metal powder–based AM processes (PBF and DED), a metal substrate where parts can be printed onto is often required. Therefore, the post‐processing process of wire cutting, typically using electric discharge machining (EDM) which generates a pulse discharge between a tool electrode and a target object (comprising printed parts and the substrate) for cutting, is required to separate the printed parts from the substrate. This process is also commonly employed to assign desired geometries to printed parts for various purposes (e.g. tensile, fatigue, or fracture toughness testing).
Table 1.2 Post‐processing for metal powder–based AM.
Categories
Representative techniques
Surface quality improvement
Manual grinding Machining Sandblasting Shot peening Mechanical polishing Chemical polishing Chemical etching Laser shock peening Laser polishing
Residual stress relief and defect reduction
Stress relief annealing Hot isostatic pressing
Aesthetic improvement
Spray painting Electroplating
Table 1.2 exhibits representative post‐processing techniques for metal powder–based AM. In accordance with their respective objectives, these techniques can be classified into the categories of surface quality improvement, residual stress relief and defect reduction, and aesthetic improvement.
1.4.1 Surface Quality Improvement
Printed metal components typically undergo post‐processing treatment to improve their surface quality (i.e. reduction in surface roughness), which includes manual grinding, machining, sandblasting, shot peening, mechanical and chemical polishing, chemical etching, laser shock peening, and laser polishing.
Manual grinding typically involves abrasive sandpapers with different grit sizes. However, such treatment is only applicable for prototypes or small batches of parts due to its low repeatability and high dependence on the skill of the operator. In addition, any support structures attached to the parts (e.g. overhangs) should first be removed before the grinding process.
Machining utilizes power‐driven machines and cutting tools to reduce the surface roughness of printed parts. In particular, CNC is a commonly adopted precision machining process characterized by stable machining quality and high flexibility, machining accuracy, and productivity. Notably, CNC allows for the customization of control programs in each working task to establish control over various tools (e.g. lathes, milling machines, and grinders) in reducing the surface roughness of printed parts.
Sandblasting is the process of removing rust, oxides, and oil contaminants from a surface with high‐speed sand (e.g. copper ore sand, emery sand, quartz sand, or iron sand) propelled by compressed air. Sandblasting operations can be categorized into dry‐type sandblasting and liquid blasting processes. The dry‐type sandblasting involves pure abrasive propellants capable of removing large amounts of surface material without contamination. In contrast, liquid blasting utilizes a mixture of abrasives and liquid, which removes only small amounts of material but introduces contamination to the parts.
Shot peening is a cold working process that bombards the surface of a material with a stream of small shots (i.e. spherical particles of metal or ceramic) with controlled intensity and coverage. It can increase surface hardness and extend the service life of a part by creating an induced compressive stress layer to enhance its fatigue resistance. Additionally, the surface roughness of printed parts can be reduced and their surface grains can be refined. The shot peening process is commonly performed using air blast systems or centrifugal blast wheels.
Polishing is the process of creating a smooth and specular surface through mechanical or chemical methods. Mechanical polishing methods include magnetically driven abrasive polishing, hydrodynamic cavitation abrasive finishing, and ultrasonic cavitation abrasive finishing.
Magnetically driven abrasive polishing utilizes a slurry comprising magnetic and abrasive materials in a viscous liquid to polish a surface. Hydrodynamic cavitation abrasive finishing is a novel surface modification process employing hydrodynamic cavitation along with abrasives to remove surface irregularities and decrease the surface roughness of a workpiece. In ultrasonic cavitation abrasive finishing, the application of ultrasound and micro‐abrasives produces a synergistic effect, in which the former induces the cavitation effect to remove partially melted powder particles on a surface, while the latter serves as bubble nucleation sites to increase the overall cavitation intensity, which further contributes to gradual surface erosion through high‐velocity abrasive collisions.
In chemical polishing, a ground sample is immersed in a polishing agent or swabbed with a chemical solution until a clean surface is obtained. Electropolishing is a representative electrochemical polishing process for producing smooth surfaces, and it is accomplished by creating an electrochemical cell in which the printed part is charged anodically. A varying current density is established across the material surface, and it is higher at the peaks and lower at the valleys of the surface topography. The relatively high current density at the protruding points on the surface causes these sites to rapidly dissolve, which levels the surface. However, the effectiveness of electropolishing is limited by the accessibility of counter electrodes in printed parts containing tight spaces and exhibiting complex geometries.
Chemical etching is a post‐processing treatment in which chemical reactions occur at the interface between a printed part and a chemical solution, leading to changes in the former's surface roughness. This approach is particularly applicable for treating printed parts with open porous structures.
Laser‐based treatment methods utilize laser sources to remove materials and improve the surface accuracy of AM metal parts. Laser‐based treatment includes laser shock peening and laser polishing.
In laser shock peening, a pulsating laser beam is directed onto the surface of a metal part, generating shock waves induced by the ablation of a sacrificial layer on the surface. These shock waves travel throughout the surface layer of the part to cause surface grain refinement and plastic deformation and induce compressive residual stress, which improves the resistance of a material toward fatigue crack initiation and propagation. The underlying mechanism of residual stress generation is similar to that of conventional shot peening, and plastic compression is achieved by the passage of shock waves.
Laser polishing, also known as laser remelting, can enhance the surface accuracy of AM parts while avoiding ablation. In this process, a high‐power laser source irradiates the material surface with low‐frequency pulses at high scanning speeds to induce local surface melting on the order of nanometers to micrometers. Laser remelting is an eco‐friendly process that can improve surface accuracy and reduce surface porosity without incurring any loss of surface materials.
1.4.2 Residual Stress Relief and Defect Reduction
Parts printed by metal powder–based AM processes such as PBF and DED are often subjected to residual stress, which must be relieved through heat treatment before the parts are suitable for industrial usage. The most commonly adopted heat treatment is stress relief annealing, which is a special annealing process that minimizes the residual stress within printed metal parts. This treatment is conducted by heating the parts to a specific temperature below their recrystallization temperature followed by air‐cooling. Low‐temperature stress relief annealing has a low impact on the microstructure and mechanical properties of a material (Wang et al. 2016), while high‐temperature stress relief annealing may refine grains, produce a low dislocation density, and alter the mechanical properties of a material (Xiong et al. 2017). Therefore, low‐temperature stress relief annealing is preferable to high‐temperature stress relief annealing because it is desirable not to change the microstructure of printed parts during the annealing process.
Hot isostatic pressing (HIP) heat treatment, a thermomechanical treatment process involving the simultaneous implementation of a high temperature (up to 2000 °C) and high isostatic pressure (up to 200 MPa) in a specially constructed vessel with gas as the pressure‐transmitting medium, is employed to reduce the porosity of printed parts and improve their densification. Argon gas is the most common pressure‐transmitting medium in HIP. Under a high temperature and pressure, the internal pores within a printed part tend to collapse, thereby leading to its densification.
The principal factor distinguishing HIP from other heat treatment techniques is its use of an inert gas as a pressure‐transmitting medium to produce uniform microstructural changes on the part surface. The isostatic pressure in HIP arises from the gas atoms colliding with the surface of the part, during which each gas atom is akin to the hammer in a forge. These atomic “hammers” reliably and consistently reach the entire part surface, which corresponds to a uniform pressure.
1.4.3 Aesthetic Improvement
Aesthetics is an indispensable aspect of printed parts. Therefore, improving the aesthetic quality of printed products can further increase their value. Generally, the “appearance, anti‐corrosion, anti‐aging, and anti‐slip” properties of parts should be considered. Commonly employed methods for achieving aesthetic improvement include spray painting and electroplating.
Spray painting is a painting technique in which paint particles are atomized and sprayed onto a surface. It includes various methods such as air gun spraying, electrostatic spray painting, airless gun spraying, automated linear spraying, and automated flatline spraying. Spray painting is often utilized to improve the appearance of a printed part by applying a smooth and flat coating of the desired color(s) on its surface.
Electroplating is a process that applies a metal coating on a part by external electric fields. In a salt solution containing the metal, the cations of the metal are reduced to atoms through the electrode reaction, and the atoms are subsequently deposited on the surface of the part (that acts as the cathode) to form the coating. Aside from enhancing the appearance of printed parts, the electroplating process can improve their resistance to oxidation, wear, and corrosion, as well as their electrical conductivity. Metals such as chromium, zinc, copper, and nickel are applicable for electroplating.
1.5 Powder Properties and Characterization Methods
Powders form the basis of powder–based AM, and their quality determines the printability and performance of the final parts. While the Technical Committee 119 of ISO has developed and published numerous standards for powder metallurgy since 1967, no standard defined specifically for powder–based AM exists, and thus previously defined metallurgy standards on powder characterization methods are adopted.
Powder sampling must be conducted before any powder characterization can be performed. Sampling methods include scoop sampling, conical pouring and quartering, and chute splitting. In scoop sampling, a scoop of powder is obtained for sampling. Conical pouring and quartering involve pouring the powder onto a flat horizontal surface and dividing the heap into four samples by a cross‐shaped cutter. In chute splitting, a chute splitter is employed for sample division. After sampling, a powder can be characterized in terms of its particle morphology, particle size, particle size distribution, density, flowability, chemical composition, and microstructure.
1.5.1 Particle Morphology
The morphology of powder particles can be determined via a standard glossary developed by the British Standards Institute (British Standards 2955 Glossary of Terms Relating to Powders). Particle morphology, which includes shapes and surface features, is dependent on the preparation processes during powder production. The shape of a particle can be described using the following terms: spherical (globular‐shaped), acicular (needle‐shaped), angular (sharp‐edged or roughly polyhedral‐shaped), crystalline (a geometric shape freely developed in liquid), dendritic (branched crystalline‐shaped), fibrous (regularly or irregularly thread‐like), lamellar (plate‐like), granular (equidimensional but irregularly shaped), irregular (lacking symmetry), or modular (round but irregularly shaped).
Table 1.3 Summary of particle size indices and particle shape indices and their descriptions.
Particle size index
Description
Martin's diameter
Length of the line that bisects the area of the particle image (all particles are measured in the same direction).
Feret's diameter
Maximum length of a particle measured in a fixed orientation.
Projected area diameter
Diameter of a circle with the same area as the 2D image of a particle.
Longest diameter
Maximum diameter of a particle.
Perimeter diameter
Diameter of a circle having the same circumference as the perimeter of a particle.
Maximum horizontal intercept
Length of the longest line that can be drawn through a particle in a fixed direction.
Particle shape index
Description
Elongation factor
Aspect ratio that is the ratio of the side lengths of an enveloping rectangle that has the minimum area around the cross‐section of a particle.
Bulkiness factor
Ratio of the area of a projected particle to the area of the enveloping rectangle.
Surface factor
Sphericity that is used to compare the surface of a particle and the surface of a sphere of equivalent volume.
Table 1.3 summarizes the particle size indices and particle shape indices that can be used to quantitatively describe the morphology of a particle (Allen 1997). Figure 1.2 presents representative surface features and defects of powder particles, which include “splat caps”, pores, elongated shapes, breakage, agglomeration, irregular shapes, and small satellites (Mostafaei et al. 2021).
The morphology of powder particles can be characterized by optical microscopy and scanning electron microscopy (SEM). Optical microscopy allows for the counting and measurement of particles at maximum magnification values ranging from 500 to 1500, depending on the model of the apparatus used. Meanwhile, a typical SEM is applicable for imaging surface features with a magnification of up to 3 × 106 and at a resolution of tens of nanometers, thereby enabling the evaluation of grain sizes and second phases on unetched surfaces. Furthermore, when equipped with a backscatter electron detector, an SEM device can facilitate the observation of microstructures on unetched surfaces.
Figure 1.2 Micrographs of powder particles exhibiting various surface features and defects. Source: Mostafaei et al. (2021)/reproduced with permission from Elsevier.
1.5.2 Particle Size Distribution
The particle size distribution of an AM powder is one of its most important characteristics. A volume distribution with respect to particle size can be plotted, as shown in Figure 1.3. The area under the distribution curve to the left of the vertical line x = Di corresponds to the percentage of the total powder particles that are of sizes smaller than or equal to a specific size gauge Di. For example, Di = 20 μm indicates that i% of all particles are smaller than or equal to 20 μm. The size gauges D10, D50, and D90 are the most commonly used indicators. The span, given by (D90 − D10)/D50, is sometimes selected to represent the width of a Gaussian particle size distribution. Additionally, the mean, median, and mode of a particle size distribution, which correspond to the average particle size, D50, and the peak of the distribution curve, respectively, can be determined.
Figure 1.3 Particle size distribution curve showing the size gauges D10, D50, and D90.
Particle size distribution is typically measured by the laser diffraction method. This method serves as a convenient and rapid analysis for a broad range of particle sizes. A typical laser diffraction measuring instrument contains a laser source, a particle dispersion module, a particle delivery module, and a detector. When the laser beam is blocked by a particle, part of the light scatters. An angle between the propagation direction of the original and scattered light is formed. A larger particle size results in a smaller angle. Upon being illuminated by parallel laser beams, particles of the same size in a sample deflect light at an identical angle. The scattered light is directed onto a sensor located on its focal plane, forming a series of concentric rings. The intensity of each ring, which corresponds to a specific angle of scattering, is measured to evaluate the volume size distribution of the constituent particles of the sample.
The particle size distribution of a sample can also be quantitatively measured from images captured by optical microscopy or SEM. The images of the constituent particles are usually post‐processed before their equivalent diameters are determined, and the entire procedure may be time‐consuming. An SEM machine developed by ASPEX Corporation is equipped with an automated feature analysis module, which is capable of quantifying the sizes of thousands of particles within several hours.
Sieve analysis, which involves the separation of particles according to their sizes, is also commonly employed to obtain the particle size distribution of a sample owing to its simplicity and low cost. A typical sieving unit comprises a series of sieves stacked on top of a shaker exhibiting rotary and tapping motions, with each sieve incorporating a phosphor bronze or stainless steel wire mesh cloth woven in a square mesh pattern. The sieves are stacked in the order of decreasing mesh sizes, with the sieve possessing the largest mesh size on top. Standard sieve sizes are specified in ISO standards 565 and 3310/1, ASTM standard E 11, and CIS standard GOST 3584.
1.5.3 Density
The packing density of a powder bed is defined as the ratio of the volume of its constituent powder particles to its total volume. It is a key parameter for assessing the packing efficiency of a powder bed in AM. A powder bed with a large packing density minimizes the porosity of the printed parts, thereby improving their mechanical properties. The packing density of a powder bed is influenced by a variety of factors such as the characteristics of the powder particles (size, size distribution, and shape), bulk properties (e.g. Young's modulus and hardness), and powder spreading parameters (powder layer thickness and spreading velocity). For bimodal powder mixtures containing both small and large particles, the maximum packing density can be achieved with specific ratios of the two types of powder particles, as shown in Figure 1.4.
The apparent density of a powder, with units of g/cm3, is defined as the mass per unit volume of its loose powder particles. The Hall funnel method is the most prevalent method for measuring the apparent density of a powder, and the powder is poured through a funnel to fill a 25 cm3 container. The apparent density is obtained by dividing the mass of the contents in the container by its volume. Generally, the apparent density of a powder decreases with its particle size and increases with its particle surface roughness. A powder with a wide size distribution possesses a relatively high apparent density since the space between its coarse powder particles is filled by smaller powder particles.
Figure 1.4 Variation of the packing density with the proportion of large powder particles for bimodal powder mixtures. Source: Adapted from German (1992).
The tap density of a powder is defined as its density when its container is tapped or vibrated under specified conditions. The tap density (unit: g/cm3
