SIST-TP CEN/TR/ISO/ASTM 52912:2020

SLOVENSKI STANDARD
01-december-2020
Dodajalna izdelava - Konstruiranje - Proizvodnja delov s funkcijsko porazdeljenimi
lastnostmi (ISO/ASTM/TR 52912:2020)
Additive manufacturing - Design - Functionally graded additive manufacturing
(ISO/ASTM/TR 52912:2020)
Technischer Bericht für die Gestaltung von additiv gefertigten, gradierten Bauteilen
(ISO/ASTM/TR 52912:2020)
Fabrication additive - Conception - Fabrication additive à gradient fonctionnel
(ISO/ASTM/TR 52912:2020)
Ta slovenski standard je istoveten z: CEN ISO/ASTM/TR 52912:2020
ICS:
25.030 3D-tiskanje Additive manufacturing
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN/TR/ISO/ASTM
TECHNICAL REPORT
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
October 2020
ICS 25.030
English Version
Additive manufacturing - Design - Functionally graded
additive manufacturing (ISO/ASTM/TR 52912:2020)
Fabrication additive - Conception - Fabrication additive Technischer Bericht für die Gestaltung von additiv
à gradient fonctionnel (ISO/ASTM/TR 52912:2020) gefertigten, gradierten Bauteilen (ISO/ASTM/TR
52912:2020)
This Technical Report was approved by CEN on 31 August 2020. It has been drawn up by the Technical Committee CEN/TC 438.

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© 2020 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR/ISO/ASTM 52912:2020 E
worldwide for CEN national Members.

CEN/TR/ISO/ASTM 52912:2020 (E)
Contents Page
European foreword . 3

CEN/TR/ISO/ASTM 52912:2020 (E)
European foreword
This document (CEN/TR/ISO/ASTM 52912:2020) has been prepared by Technical Committee ISO/TC
261 "Additive manufacturing" in collaboration with Technical Committee CEN/TC 438 “Additive
Manufacturing” the secretariat of which is held by AFNOR.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Endorsement notice
The text of ISO/ASTM/TR 52912:2020 has been approved by CEN as CEN/TR/ISO/ASTM 52912:2020
without any modification.
TECHNICAL ISO/ASTM TR
REPORT 52912
First edition
2020-09
Additive manufacturing — Design
— Functionally graded additive
manufacturing
Fabrication additive — Conception — Fabrication additive à gradient
fonctionnel
Reference number
ISO/ASTM TR 52912:2020(E)
©
ISO/ASTM International 2020
ISO/ASTM TR 52912:2020(E)
© ISO/ASTM International 2020
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ii © ISO/ASTM International 2020 – All rights reserved

ISO/ASTM TR 52912:2020(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abreviations . 1
5 Concept of Functionally Graded Additive Manufacturing (FGAM) .3
5.1 General . 3
5.2 Homogeneous compositions — Single Material FGAM. 3
5.3 Heterogeneous compositions — Multi-material FGAM . 4
6 Advances of functionally graded additive manufacturing . 8
6.1 General . 8
6.2 AM and FGAM process . 8
6.3 Material extrusion . 9
6.4 Powder bed fusion .12
6.5 Directed energy deposition .13
6.6 Sheet lamination .14
7 Current limitations of FGAM .16
7.1 General .16
7.2 Material limitations . .16
7.2.1 General.16
7.2.2 Defining the optimum material property distribution .17
7.2.3 Predicting the material properties of manufactured components .17
7.2.4 Material selection .17
7.2.5 Understanding differences and defining tolerances .17
7.3 Limitations of current additive manufacturing technologies .17
7.4 CAD Software limitations .18
7.4.1 General.18
7.4.2 Data exchange formats .19
8 Potential applications of FGAM .20
8.1 General .20
8.2 Biomedical applications .21
8.3 Aerospace applications .21
8.4 Consumer markets.21
9 Summary .22
Bibliography .23
© ISO/ASTM International 2020 – All rights reserved iii

ISO/ASTM TR 52912:2020(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
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electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
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For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www .iso .org/ iso/ foreword .html.
This document was prepared by ISO/TC 261, Additive manufacturing, in cooperation with ASTM F 42,
Additive manufacturing technologies, on the basis of a partnership agreement between ISO and ASTM
International with the aim to create a common set of ISO/ASTM standards on additive manufacturing
and in collaboration with the European Committee for Standardization (CEN) Technical Committee
CEN/TC 438, Additive manufacturing, in accordance with the agreement on technical cooperation
between ISO and CEN (Vienna Agreement).
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO/ASTM International 2020 – All rights reserved

ISO/ASTM TR 52912:2020(E)
Introduction
Functionally Graded Materials (FGMs) were developed in 1984 for a space plane project to sustain high
thermal barriers to overcome the shortcomings of traditional composite materials (AZO Materials, 2002).
Traditional composites [Figure 1 a)] are homogeneous mixtures, therefore involving a compromise
between the desirable properties of the component materials. Functionally Graded Materials (FGMs)
are a class of advanced materials with spatially varying composition over a changing dimension, with
[56]
corresponding changes in material properties built-in . FGMs attain their multifunctional status by
mapping performance requirements to strategies of material structuring and allocation [Figure 1 b)].
The manufacturing processes of conventional FGMs include shot peening, ion implantation, thermal
spraying, electrophoretic deposition and chemical vapour deposition. Since additive manufacturing
processes builds parts by successive addition of material, they provide the possibility to produce
products with Functionally Graded properties, thereby introducing the concept often known as
Functionally Graded Additive Manufacturing (FGAM). As this area of work is new, driven by academic
research, and lacks available standardisation, there have been multiple different names proposed by
different researchers in different publications as terms for this area, for example, functionally graded
[56] [57]
rapid prototyping (FGRP) , varied property rapid prototyping (VPRP) and site-specific properties
[72]
additive manufacturing . However, even if there clearly is a great need for clarification of key terms
associated with FGAM, this document does not include any attempts of alignment in terminology.
This document is an overview of state of the art and the possibilities for FGAM enabled by present AM
process technology and thus a purely informative document. Since this overview is based on available
publications, and in order to facilitate cross referencing from these publications, this document has
used the terms concerning FGAM as they are used in the original publications.
a)  Traditional composite b)  FGM composite
Figure 1 — Allocation of materials in a traditional composite and an FGM composite
© ISO/ASTM International 2020 – All rights reserved v

TECHNICAL REPORT ISO/ASTM TR 52912:2020(E)
Additive manufacturing — Design — Functionally graded
additive manufacturing
1 Scope
The use of Additive Manufacturing (AM) enables the fabrication of geometrically complex components
by accurately depositing materials in a controlled way. Technological progress in AM hardware,
software, as well as the opening of new markets demand for higher flexibility and greater efficiency
in today’s products, encouraging research into novel materials with functionally graded and high-
performance capabilities. This has been termed as Functionally Graded Additive Manufacturing
(FGAM), a layer-by-layer fabrication technique that involves gradationally varying the ratio of the
material organization within a component to meet an intended function. As research in this field has
gained worldwide interest, the interpretations of the FGAM concept requires greater clarification.
The objective of this document is to present a conceptual understanding of FGAM. The current-state of
art and capabilities of FGAM technology will be reviewed alongside with its challenging technological
obstacles and limitations. Here, data exchange formats and some of the recent application is evaluated,
followed with recommendations on possible strategies in overcoming barriers and future directions
for FGAM to take off.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Abreviations
AM Additive Manufacturing (see ISO/ASTM 52900)
AMF Additive Manufacturing Format, see 8.4.2.1 (see ISO/ASTM 52900)
[48]
CAD Computer Aided Design
[14]
CAE Computer Aided Engineering
DED Directed Energy Deposition, see Clause 6 (see ISO/ASTM 52900)
DMLS Direct Metal Laser Sintering, the name for laser-based metal powder bed fusion process
[40]
by EOS Gmbh
EBM Electron Beam Melting, the name for electron beam based metal powder bed fusion
[40]
process by Arcam AB
[19]
FAV Fabricatable Voxel, see 8.4.2.2
© ISO/ASTM International 2020 – All rights reserved 1

ISO/ASTM TR 52912:2020(E)
[48]
FEA Finite Element Analysis
FEF Freeze-form Extrusion Fabrication, a material extrusion process based on the extrusion
of feedstock in the form of pastes and application of freeze drying to form a green body
which can be consolidated to the desired material properties by sintering. Presently
[34]
only used for research and development projects.
[18]
FEM Finite Element Method
[39]
FDM Fused Deposition Modelling, name for material extrusion processes by Stratasys Ltd.
[61]
FGAM Functionally Graded Additive Manufacturing
[61]
FGMs Functionally Graded Materials
FGRP Functionally Graded Rapid Prototyping, name for FGAM used by Neri Oxman in some
[56]
publications.
LMD Laser Metal Deposition, a common name for directed energy deposition processes that
uses laser as the source of energy to melt and fuse metallic materials as they are being
[21]
deposited, see Clause 6.
LOM Laminated Object Manufacturing, name of sheet lamination processes originally
[42]
developed by Helisys Inc.
MMAM Multi-Material Additive Manufacturing, name used for AM when using more than one
[61]
material in the same process.
MM FGAM Multi-Material Functionally Graded Additive Manufacturing, name for FGAM when the
functional grading is based on building parts using more than one material in the same
process, and the composition of the different material components is controlled by the
[43]
computer program.
PBF Powder Bed Fusion (ISO/ASTM 52900)
SHS Selective Heat Sintering, name of a powder bed fusion process that fuse polymer
powder by means of a thermal printhead instead of the more common laser. The
process was originally developed by Blueprinter but has been withdrawn from the
[40]
market following the bankruptcy of this company.
SLM Selective Laser Melting, name for laser-based metal powder bed fusion process orig-
inally developed in collaboration between F&S Stereolithographietechnik GmbH (Fock-
ele & Schwarze) and Fraunhofer Institute for Laser Technology. This name is a regis-
[40]
tered trademark of SLM Solutions Group AG and Realizer GmbH.
SLS Selective Laser Sintering, name for powder bed fusion process originally developed by
DTM Corp, but which has been assumed by 3D Systems by the acquisition of this com-
pany. Since this was the first powder bed fusion process to be commercialized, it has
[40]
sometimes been used synonymously for all powder bed fusion processes.
STL Stereolithography, name for a digital file format for three dimensional solid models
originally developed for the Stereolithography process by 3D Systems, hence the name.
Since this conversion to this format has been commonly available in several CAD
programs this file format has until present times effectively been functioning as a
de-facto standard for AM processes. (see ISO/ASTM 52900)
UAM Ultrasonic Additive Manufacturing, name for a metal sheet lamination process by
Fabrisonic LLC. The process fuses thin sheets (or ribbons) of metal by ultrasonic vibra-
[43]
tions.
2 © ISO/ASTM International 2020 – All rights reserved

ISO/ASTM TR 52912:2020(E)
[8]
VDM Vague Discrete Modelling
VPRP Variable Property Rapid Prototyping, name for FGAM used by Neri Oxman in some
[57]
publications.
3MF 3D Manufacturing Format, a digital file format for three dimensional solid models in
[3]
additive manufacturing, developed by the 3MF consortium, see 8.4.2.3.
5 Concept of Functionally Graded Additive Manufacturing (FGAM)
5.1 General
Additive Manufacturing (AM) is 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 (ISO/ASTM 52900). AM enables the direct fabrication of fine detailed bespoke
components by accurately placing material(s) at set positions within a design domain as a single
[76]
unit . The use of AM has given opportunity to produce parts using FGM, through a process known as
Functionally Graded Additive Manufacturing (FGAM). AM technologies suitable for the fabrication of
[43]
FGMs include Material Extrusion, Direct-Energy Deposition, Powder Bed Fusion, Sheet Lamination
and PolyJet technology.
Functionally Graded Additive Manufacturing (FGAM) is a layer-by-layer fabrication technique that
intentionally modify process parameters and gradationally varies the spatial of material(s) organization
within one component to meet intended function.
FGAM offers a streamlined path from idea to reality. The emergence of FGAM has the potential to
achieve more efficiently engineered structures. The aim of using FGAM is to fabricate performance-
based freeform components driven by their graduated material(s) behaviour. In contrast to conventional
single-material and multi-material AM which focuses mainly on shape-centric prototyping, FGAM is
a material-centric fabrication process that signifies a shift from contour modelling to performance
modelling. Having the performance-driven functionality built-in directly into the material is a
fundamental advantage and a significant improvement to AM technologies. An example includes highly
customizable internal features with integrated functionalities that would be impossible to produce
[5]
using conventional manufacturing . The amount, volume, shape and location of the reinforcement
in the material matrix can be precisely controlled to achieve the desired mechanical properties for a
[18]
specific application .
Reference [57] describes the concept of FGAM as a Variable Property Rapid Prototyping (VPRP) method
with the ability to strategically control the density and directionality of material substance in a complex
3D distribution to produce a high level of seamless integration of monolithic structure using the same
machine. The material characteristics and properties are altered by changing the composition, phase
or microstructure with a pre-determined location. The potential material composition achievable by
FGAM can be characterised into 3 types:
a) variable densification within a homogeneous composition;
b) heterogeneous composition through simultaneously combining two or more materials through
gradual transition;
c) using a combination of variable densification within a heterogeneous composition.
These three types of characteristics are described in 5.2 and 5.3.
5.2 Homogeneous compositions — Single Material FGAM
FGAM can produce efficiently engineered structures by strategically modulating the spatial position (e.g.
[43]
density and porosity) and morphology of lattice structures across the volume of the bulk material .
We term this as varied densification FGAM (also known as porosity-graded FGAM). Reference [56]
proposed this as a biological-inspired rapid fabrication that occurs in nature such as the radial density
© ISO/ASTM International 2020 – All rights reserved 3

ISO/ASTM TR 52912:2020(E)
gradients in palm trees, spongy trabecular structure of bone and tissue variation in muscle which is
heterogeneous in elasticity and stiffness. The directionality, magnitude and density concentration of
material substance in a monolithic anisotropic composite structure contribute to functional deviations
[54]
to modulate the physical properties, and to create functional shapes through structural hierarchy .
[27]
Man-made structures such as concrete pillars are typically volumetrically homogeneous . Varied
densification single-material FGAM was demonstrated through Steven Keating’s work on functionally
graded concrete being fabricated by a MakerBot machine with a modified material extruder. The
concrete piece showed a functional gradient of density to mimic the cellular structures of a palm tree,
from a solid exterior to a porous core. The porosity gradient was achieved by varying the powder
particle sizes that were assigned in different locations during the gradation process or by varying the
[43]
production process parameters . For Reference [27], the density was controlled by aggregating the
water ratio of the concrete at a given position, which led to excellent strength-to-weight ratio, making it
lighter and yet more efficient and stronger than a solid piece of concrete.
5.3 Heterogeneous compositions — Multi-material FGAM
Multiple-material Additive Manufacturing (MMAM) is achievable using conventional 3D printers
[77]
with multiple nozzles to deliver different materials to the platform . In powder bed fusion, MMAM
can be realized by utilizing a conventional delivery device in combination with a suction module to
[7]
remove powder after the solidifying process-step . As sharp interfaces exist in most conventional
[72]
MMAM composites where two materials meet and interact, this creates a brittle phase . Failure is
commonly initiated between discrete change of materials properties, such as delamination or cracks
[17]
caused by the surface tension between two materials . Multi-material (MM) FGAM seeks to improve
the interfacial bond by removing the distinct boundaries between dissimilar or incompatible materials.
The mechanical stress concentrations and thermal stress caused by different expansion coefficients
[72]
will be largely reduced . Figures 2, a) and b) explain the approach of voxellization of Multi-material
Additive Manufacturing according to Reference [7].
4 © ISO/ASTM International 2020 – All rights reserved

ISO/ASTM TR 52912:2020(E)
a) Conceptual diagram showing voxels arranged in 3D form (Fraunhofer IGCV and
Reference [7])
b) Illustration of MMAM (Fraunhofer IGCV and Reference [7])
Key
1 building direction
2 mono-material
3 2D hybrid
4 2D multi-material
5 3D multi-material
6 substrate
Figure 2 — Voxellization of multi-material additive manufacturing
© ISO/ASTM International 2020 – All rights reserved 5

ISO/ASTM TR 52912:2020(E)
a)  Multi-material AM b)  Functionally graded AM
Key
1 discrete change of material properties 3 pillar to reinforce shape
2 hard material for reinforcement 4 smooth variation in material change
[ ]
Figure 3 — Example of a part with multi-materials 73
Reference [10] addressed the coupling effect of materials through sandwich configurations to
achieve an optimum combination of component properties such as weight, surface hardness, wear
resistance, impact resistance or toughness; or to produce material gradients to change the physical,
[22][28]
chemical, biochemical or mechanical properties through complex morphology . As the geometric
arrangement of the two phases influences the overall material properties, the accuracy of the AM
process is properly managed to ensure that the final component fulfils the expected functional
[72]
requirements . The difference between a Multi-material AM and a Functionally Graded AM
part is illustrated in Figure 3 by Reference [73], Figure 4 further describes the continuous graded
microstructure of FGAM using 2 materials.
Key
1 phase 1 (particles with phase 2 as matrix)
2 transition phase
3 phase 2 (particles with phase 1 as matrix)
Figure 4 — Continuous graded microstructure of FGAM — 2 materials
The continuous variation within the 3D space can be produced by controlling the ratios in which two
[43]
or more materials that are mixed prior to the deposition and curing of the substances . According to
6 © ISO/ASTM International 2020 – All rights reserved

ISO/ASTM TR 52912:2020(E)
Reference [75], the compositional variation is controlled by the computer program to be considered
as FGAM. Raw materials that are pre-mixed or composed prior to deposition or solidification are not
considered to be FGAM. FGAM multi-layer composites can be divided into 4 types: transition between
2 materials [Figure 5 b)], 3 materials or above [Figure 5 c)], switched composition between different
locations [Figure 5 d)] and heterogeneous compositions with density variation [Figure 5 e)]
a)  Conventional MMAM b)  MM FGAM (2 materials) c)  MM FGAM
(3 materials)
d)  Switched composition e)  Varied density heterogeneous
Figure 5 — Various classes of multi-material arrangement
The variation of material within a heterogeneous component can be classed as 1D, 2D and 3D
[48]
gradient . Key parameters include the dimension of the gradient vector, the geometric shape and
the repartition of the equipotential surfaces. Figure 6 shows a diagram that classifies the different
gradients of FGAM parts that can be assigned.
Key
1 one-dimensional gradient
2 two-dimensional gradient
3 three-dimensional gradient
Figure 6 — Representation of classifying FGAM gradients
© ISO/ASTM International 2020 – All rights reserved 7

ISO/ASTM TR 52912:2020(E)
6 Advances of functionally graded additive manufacturing
6.1 General
AM has provided benefits including design freedom, reduced time to market in product development,
[5]
service and increased R&D efficiency . The emergence of FGAM expands the potential of prototyping
of more efficient engineering structure that restore better function and structural performance with
[61]
no tooling costs .
FGAM presents a new production paradigm in terms of industrial machinery, assembly processes, and
[20]
supply chains . It provides a vast range of opportunities for design, performance, cost and lifecycle
management. For instance, light-weight designs can be achieved by adjusting the lattice structures to
retain the structural strength and to achieve reduction in weight. The material matrix, reinforcement,
volume, shape and location of reinforcement and the fabrication method can all be tailored to achieve a
[18]
particular desired property for a specific application . Ground-breaking innovation can be achieved
through material substitution, especially in the medical implants industry, aerospace requirements
[50]
and for creative industries .
FGAM optimises the exploitation of materials and expands the design toolbox available in AM processes
[5][63]
in terms of multi-materiality . FGAM advances material process-ability and contributes to more
[57]
efficient material use . By simplifying the assembly of complex parts using dynamic gradients,
some of the known disadvantages of traditional composites can be avoided such as lowering the in-
plane and transverse stresses at critical locations, improving residual stress distribution, enhancing
breakage resistance and thermal properties, attaining higher fracture toughness, and reducing stress
[10][13]
intensity . FGAM can also provide desired properties at specific sections being site-specific or
[75]
strategic locations around the parts . Although part forming via AM requires a longer time than
conventional manufacturing, having the capability to consolidate several machining processes into a
single manufacturing sequence that could vastly reduce the overall manufacturing time-to-market.
The amount of support material can also be potentially reduced as FGAM components can be designed
to self-stabilize during the build process with minimum use of support structures. FGMs also offers
variable property supports where sacrificial areas could be designed to break away. FGAM has the
potential to meet future requirements in environmental sustainability such as reducing material using
and energy consumption.
6.2 AM and FGAM process
The key manufacturing process of Additive Manufacturing consists of 5 main stages, from 1) the
receiving of the CAD file into an AM system, 2) the conversion of CAD file into .STL (or AMF) File, 3)
3D CAD data is sliced into 2D layers, 4) layer-by-layer fabrication of the AM part 5) post processing or
end part finishing (e.g. support-structure removal, cleaning and polishing) (see Figure 7).
Key
1 CAD-based 3D model
2 .STL file
3 sliced layers
4 AM fabrication
5 end part finishing
Figure 7 — Additive manufacturing (AM) process flow
8 © ISO/ASTM International 2020 – All rights reserved

ISO/ASTM TR 52912:2020(E)
The methodology of FGAM introduces the importance of toolpaths. Path planning is the key influence on
the material distribution of the manufactured parts. The feature of toolpath planning by Reference [48]
and [47] is described as four steps in Table 1.
Table 1 — Feature of toolpath
Step 1: Defining the mechanical function of the part by describing the geometry, the
material distribution, the gradient dimension or vector, the shape of
Description of the Part
equi-composition or equi-property surfaces.
Geometry and Material
Distribution
Step 2: Material data that concerns the chemical composition and characteristics of the
material(s) used is gathered. The material distribution and orientation of slices are
Determination of manu-
defined. The toolpaths are evaluated and calculated. The mathematical data is used
facturing strategies
to find the most appropriate manufacturing strategy and printer.
Step 3: Numerical Control (NC) programming, involving paths and process parameters is
generated such as; but not limited to G programming language (ISO 6983) from the
Numerical Control (NC)
toolpath route. A 3D grid with machine data and the material distribution is
programming
generated to the defined paths.
Step 4: The NC program is used by; but not limited to CNC controller. The operation involves
fabricating slices in order to build 3-D cross-section profiles to construct the
Manufacturing
component layer by layer with pre-determined specific material deposition. The file
is sent to the AM machine for the production sequence to commence.
The main AM processes for FGMs including material extrusion, powder bed fusion, directed-energy
deposition and sheet lamination are discussed in the next section. Other technologies mainly used for
producing metal–metal or metal–ceramic FGM include Selective Laser Melting, laser cladding-based
techniques and Ultrasonic Consolidation (UC). For fabricating polymer–polymer or polymer–ceramic,
[15][16][17]
ceramic–ceramic FGM, selective laser sintering and inkjet printing have been mainly used ).
6.3 Material extrusion
Material extrusion is an AM process category in which material is selectively dispensed through a
nozzle or orifice (ISO/ASTM 52900) (see Figure 8). In this type of process, polymers such as ABS, PLA,
Nylon, etc. is drawn through a nozzle, where it is heated and deposited layer by layer onto the cross-
sectional area of object slice. The key process parameters in Material Extrusion are the filament width,
[43]
the raster-fill angle and the raster-fill pattern . When using the process for components where a high
tolerance is achieved, gravity and surface tension is accounted for Reference [21].
© ISO/ASTM International 2020 – All rights reserved 9

ISO/ASTM TR 52912:2020(E)
Key
1 material spool 4 object
2 heated element 5 support material
3 nozzle 6 build platform
[ ]
Figure 8 — Material extrusion 39
Freeze-form Extrusion Fabrication (FEF) is another material extrusion process of building FGAM parts
layer-by-layer through computer-based controlled extrusion and deposition. It uses a triple-extruder
[45]
mechanism, each carrying a paste of the material . The different material pastes are subsequently
sent to a static mixer to be mixed into a homogeneous paste (known as green part) as shown in
Figure 9. In Reference [34] investigation, the green part made up of alumina (Al O ) and zirconia (ZrO )
2 3 2
is freeze-dried at a below-freezing temperature of –25 °C and with a pressure of 3 000 Pa for 24 h.
Consequently, sintered at a high temperature of 1 °C/min up to 600 °C at the first heating to burn out the
organic binder. The second heating is 10 °C/min up to 1 550 °C for another 90 min, then brought to cool
back to room temperature at 25 °C/min. The heating temperature usually does not exceed the melting
[34]
temperature of the constituent materials . Energy-dispersive spectroscopy (EDS) is used to analyse
the material composition of the sintered FGM components. The results produced by Reference [34]
using the FEF system presented a positive compositional change across the graded samples.
Key
1 static mixer
2 FGM Green part
Figure 9 — Schematic diagram of static mixer and triple extruder
10 © ISO/ASTM International 2020 – All rights reserved

ISO/ASTM TR 52912:2020(E)
Continuous control over the material compositions and their gradients during the part building process
can be achieved by planning (with time delay taken into consideration) and controlling the relative
flow rates of the different pastes. For example, assuming that the three cylinders containing the three
different pastes have the same cross-sectional area, a desired paste mixture consisting of 20 % paste A,
30 % paste B, and 50 % paste C can be achieved by controlling the three plunger velocities with the
ratios of v :v :v = 2:3:5, where v , v , and v are the plunger velocities for pastes A, B, and C, respectively.
1 2 3 1 2 3
Material extrusion is a widespread and low-cost type of process with the advantages of readily available
materials such as ABS, which has good structural properties identical to a final production component.
Conversely, the accuracy and speed are low when compared to other AM processes. The nozzle radius
[32]
and thickness limits and reduces the final quality . Many factors (e.g. constant pressure of material)
are taken into account in order to increase quality of finish. As with most heat related post processing
processes, shrinkage is likely to occur and is taken into account if a high tolerance is required.
At present, the control of material mixing and extrusion need to be split into two separate systems, then
coordinated with the toolpath movement to produce specified gradients. Reference [54] noted that the
spindle output channels communicate directly with the extrusion system controllers.
Material extrusion has the potential to fabricate parts with locally controlled properties by changing
deposition density and deposition orientation. Two conceptual examples with locally controlled
properties are shown in Figure 10 a) and b). Sharing the identical geometries, different deposition
strategies are allocated to sections of the four parts to achieve locally controlled properties. The part’s
stiffness property can be locally controlled by depositing filament in various orientations and densities.
[35]
As a result, stiffness variations along the horizontal axis can be tailored .
a) Unidirectional deposition (θ = 0) with different deposition densities
b) [0/0], [0/90/0] s, and [±45] s deposition orientations respectively, with different deposition
densities for each portion
Figure 10 — An example showing the direction of deposition and densities represented from
the top view
Reference [68] developed a framework to fabricate ABS FGMs using material extrusion by using
variable properties in different regions such as tailoring the material properties. This work can be
extended for modelling and simulating the components for different loading conditions. Reference [68]
stressed that the fundamental step is to identify the process control parameters that were likely to
affect the properties of those parts. Based upon the previously built models for model volume which
is one of the main parameters that affect the material density and hence elastic modulus, the selected
build parameter considerations are: the raster width, contour width, air gap and raster angle.
© ISO/ASTM International 2020 – All rights reserved 11

ISO/ASTM TR 52912:2020(E)
6.4 Powder bed fusion
Powder bed fusion methods comprise of Direct Metal Laser Sintering (DMLS), Electron Beam Melting
(EBM), Selective Heat Sintering (SHS), Selective Laser Melting (SLM) and Selective Laser Sintering
(SLS) where both involve the spreading and sintering of 0,1 mm thick of powder material layer-by-
layer with a roller in between fusion of layers, selectively melt and fused together using either a laser
[40]
or electron beam (see Figure 11). PBF is a relatively inexpensive process with a larger range of
material options. The common powder based materials used for SHS is Nylon; Stainless Steel, Titanium,
[40]
Aluminium, Cobalt Chrome, Steel for DMLS, SLS, SLM, with addition of copper for EBM . Apart from
higher resolution with hierarchical and functional complexity, PBF advances from feedstock fluidity
and reusability an
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