University of Sydney An Analysis of 3D Printing for Metals Analysis Paper Metal additive manufacturing has gained traction in recent years, especially in t

University of Sydney An Analysis of 3D Printing for Metals Analysis Paper Metal additive manufacturing has gained traction in recent years, especially in the high value industries such as automotive, biomedical and aerospace. This has sparked huge research interest in the additive manufacturing processes that can be used to process metals. The following reviews provide good overview of the various processes.

Frazier (2014), Metal Additive Manufacturing: A Review, Journal of Materials Engineering and Performance 23, 1917–1928 (https://link.springer.com/article/10.1007/s11665-014-0958-z)
Sing et al. (2016), Laser and Electron?Beam Powder?Bed Additive Manufacturing of Metallic Implants: A Review on Processes, Materials and Designs, Journal of Orthopaedic Research 34 (3), 369-385 (https://onlinelibrary.wiley.com/doi/full/10.1002/jor.23075)
Zhang et al. (2018), Additive Manufacturing of Metallic Materials: A Review, Journal of Materials Engineering and Performance 27, 1-13 (https://link.springer.com/article/10.1007/s11665-017-2747-y)

Using (but not limited to) these three references (attached), discuss the development of the 3D printing processes for metals. In addition, suggest reasons why they are not widely adopted in industries yet and the possible solutions to overcome these obstacles.

Requirements:

1500 words
No limit to number of figures
No plagiarize JMEPEG (2018) 27:1–13
https://doi.org/10.1007/s11665-017-2747-y
ASM International
1059-9495/$19.00
Additive Manufacturing of Metallic Materials: A Review
Yi Zhang, Linmin Wu, Xingye Guo, Stephen Kane, Yifan Deng, Yeon-Gil Jung, Je-Hyun Lee, and Jing Zhang
(Submitted October 31, 2016; in revised form April 26, 2017; published online May 24, 2017)
In this review article, the latest developments of the four most common additive manufacturing methods for
metallic materials are reviewed, including powder bed fusion, direct energy deposition, binder jetting, and
sheet lamination. In addition to the process principles, the microstructures and mechanical properties of
AM-fabricated parts are comprehensively compared and evaluated. Finally, several future research
directions are suggested.
Keywords
additive manufacturing, mechanical property, metal,
microstructure, process
1. Introduction
In the 1980s, rapid prototyping (RP) was ?rst introduced to
produce a 3D prototype layer-by-layer from a computer-aided
design (CAD) (Ref 1). With the advancement of RP technique
and the need of high-ef?ciency manufacturing with the ability
to produce complex parts, the ?rst additive manufacturing
(AM) technique was brought on stage by researchers at
University of Texas Austin in 1986. In the past 30 years,
many new AM processes have been developed. These processes show several signi?cant advantages, including versatile
geometric capability, minimum human interaction requirement,
and reduced design cycle time (Ref 2). Since then, AM has
been successfully applied in numerous ?elds. Functional AM
parts with complex geometries have been used as aircraft
engine components (Ref 3, 4), automobile parts (Ref 5), and
space components (Ref 6, 7). According to the ASTM standard
published in 2009 (Ref 8), the AM techniques can be classi?ed
into the following categories, as listed in Table 1.
AM processes of metallic materials generally include (1)
powder bed fusion (PBF), (2) direct energy deposition (DED),
(3) binder jetting (BJ), and (4) sheet lamination (SL). Vat
polymerization is only capable of fabricating polymer materials. Other processes have been experimentally tested for metal
fabrication, e.g., liquid metal extrusion (Ref 10) and material
jetting (Ref 11, 12). However, they are still in early stages of
development, and there are no commercial systems yet.
This article is an invited paper selected from presentations at ‘‘Recent
Development in Additive Manufacturing: Process and Equipment
Development and Applications,’’ held during MS&TÕ16, October 23-27,
2016, in Salt Lake City, UT, and has been expanded from the original
presentation.
Yi Zhang, Linmin Wu, Xingye Guo, Stephen Kane, and Jing Zhang,
Indiana University – Purdue University Indianapolis, Indianapolis, IN
46202; Yifan Deng, University of California at San Diego, La Jolla,
CA 92093; and Yeon-Gil Jung and Je-Hyun Lee, Changwon National
University, Changwon, Gyeongnam 641-773, Republic of Korea.
Contact e-mail: jz29@iupui.edu.
Journal of Materials Engineering and Performance
The currently available commercial metal AM systems with
their manufacturers are listed in Table 2. The systems are
classi?ed based on the ASTM standard. The processing
information, including layer thickness range and laser beam
diameter, along with system energy sources, is also listed.
Laser-based powder bed fusion, including selective laser
melting (SLM), selective laser sintering (SLS), and direct metal
laser sintering (DMLS), is the most popular AM processes. In
these processes, laser power is usually in the range of 1001000 W depending on the manufacturer. The thickness of each
build layer of laser-based PBF can be as small as 20 lm, which
shows the advantage in terms of resolution over other AM
processes. Arcam is the manufacturer for electron beam-based
PBF. The power of an e-beam is much higher than a laser
source, and a thicker layer of metallic powder can be built in
each scan. Trumpf provides both powder feed DED- and laserbased PBF. ExOne and Fabrisonic are the manufacturers for BJ
and SL systems that are suitable for AM fabrication of metallic
materials.
Kaufui et al. conducted a review in 2012 on the
development and application of rapid prototyping (Ref 25).
In the review, two aspects limiting the application of AM
from industrial applications were discussed, these being
material capability and parts accuracy. Another review on the
microstructures of laser-/electron beam-based rapid prototypes
was conducted by Murr (Ref 26). The review paper
discussed how the material microstructure architectures can
be controlled by AM processes. Tapia et al. (Ref 27)
reviewed the process monitoring and control of metal AM
systems. The rationale and importance of research on realtime control of AM were identi?ed, in terms of improving
the product accuracy and material/time ef?ciency. Also in
2014, Frazier discussed AM processes, material properties,
and business considerations (Ref 28). The AM-processed
metallic materials were analyzed in terms of their microstructure evolution and static/dynamic properties. The paper
discussed the mechanical properties of AM parts to show
the process-microstructure-properties relationship, which was
further discussed by other researchers (Ref 29-31). Lewandowski compared the tensile properties and fatigue crack
behaviors of Ti6Al4V fabricated through PBF- and DEDbased AM processes (Ref 32). The results showed that the
mechanical properties may vary with AM process and AM
machine. Additional review articles include the material
properties and quali?cations, as well as the economic or
environmental impacts of AM processes (Ref 33, 34).
Volume 27(1) January 2018—1
Table 1 Summary of AM processes classi?ed by ASTM F42 (Ref 9) and their typical applications
Process
Application
Material extrusion
Vat polymerization
Binder jetting
Material jetting
Powder bed fusion
Sheet lamination
Direct energy deposition
Plastic prototyping
Prototyping, high surface ?nish parts
Prototyping, investment casting
Visual prototyping
Functional prototyping, engineering functional parts
Prototyping
Prototyping, functional parts, repairing metal parts and ?xtures
Table 2 Commercial AM systems for metallic materials
Manufacturer
Concept laser (Ref 13)
Sisma (Ref 14)
SLM Solutions (Ref 15)
Realizer (Ref 16)
Farsoon (Ref 17)
EOS (Ref 18)
Arcam AB (Ref 19)
Optomec (Ref 20)
Sciaky (Ref 21)
Trumpf (Ref 22)
ExOne (Ref 23)
Fabrisonic (Ref 24)
Layer thickness, lm Laser focus diameter, lm
System
Process
M1 cusing
MYSINT300
SLM500
SLM300i
FS271 M
M 400
Arcam Q20plus
LENS Print Engine
EBAM 300
TruLaser
Cell Series 7000
M print
SonicLayer 7200
PBF(SLM)
PBF(SLM)
PBF(SLM)
PBF(SLM)
PBF(SLS)
PBF(DMLS)
PBF(EBM)
DED(LENS)
DED (wire feed)
DED (powder feed)
20-80
20-50
20-74
20-100
20-80
N/A
140
25
N/A
N/A
50
100-500
80-115
N/A
40-100
90
…
…
…
…
BJ
SL(UAM)
150
150
…
…
The objective of this review article is twofold. The ?rst is to
provide the latest information regarding the AM metallic
material microstructures and mechanical properties. The second
is to cover the process-microstructure-property correlation of
binder jetting, sheet lamination, powder bed diffusion, and
direct energy deposition processes, thus providing a comprehensive review of all major AM processes for metallic
materials. The structure of this review article is arranged into
four major sections from section 2 to 5, based on the four major
AM processes. Each section is further divided into sub-sections
of process description, typical microstructures, and a compilation of mechanical properties. Section 6 provides the conclusion and suggested future research directions.
2. Powder Bed Fusion
Powder bed fusion (PBF) uses a high-energy power source
to selectively melt or sinter a metallic powder bed. Depending
on the type of power source, PBF can be further divided into
two major techniques: selective laser melting (SLM) which
uses a high-intensity laser, and electron beam melting (EBM),
which uses an electron beam. Both processes need a building
platform to hold the powder.
2.1 Powder Bed Fusion Equipment and Process
Even though the principles of these two processes are
similar, the processing steps are quite different. The schematics
of the SLM setup are shown in Fig. 1(a) (Ref 35). In the SLM
process, the laser beam passes through a system of lenses and
re?ected by a mirror onto the platform surface. The mirrors are
2—Volume 27(1) January 2018
Energy source
Fiber laser 200-400 W
Fiber laser 500 W
Quad ?ber lasers 4 9 700 W
Fiber laser 400-1000 W
Yb-?ber laser, 200 W
Yb-?ber laser, 1000 W
Electron beam 3000 W
IPG ?ber laser 1-2 kW
Electron beam
CO2 laser (15,000 W) or
YAG laser (6600 W)
…
20 kHz ultrasonic
vibration sonotrode
used to control the laser beam spot movement on the planar (X
and Y) directions on the designed paths. After a layer of
powder is selectively melted, the platform moves downward, a
recoating blade or brush pushes another layer of fresh powder
from the powder tank to the top of the previously built surface,
and the laser scan process repeats. The building chamber of an
SLM machine is ?lled with an inert gas, argon in most cases, to
avoid oxidization of metallic powders at high temperatures.
The EBM process is essentially developed from the
scanning electron microscope (SEM) technique (Ref 29). It
utilizes a much higher-power electron beam to selectively melt
the powder. Vacuum condition is required for the EBM process.
As shown in Fig. 1(b) (Ref 36), the electron beam source is
located on the top of the powder bed. The movement of the
electron beam is directly controlled by a lens system. A powder
hopper pours fresh powder onto the side of the platform, and
then, a layer of powder is coated by a rake on the top of
previously melted layer.
2.2 Microstructures and Mechanical Properties of Powder
Bed Fusion Fabricated Parts
Several studies have been focused on relating the PBF
process parameters to the resulting microstructure (Ref 35, 37).
Although at high processing temperatures, most of the scanned
powder is melted and densi?ed, the PBF fabricated parts still
contain some porosities (Ref 36). Figure 2 shows the
microstructure of an SLM processed Ti-6Al-4V part (Ref 38).
From the top view (Fig. 2a), one can easily observe the
parallelly orientated grains with /// or \ band-shaped patterns.
Each of these patterns has a width of the scan hatch space, and
it follows the laser scan direction. The highly orientated grains
are created by a high temperature gradient during fast heating
Journal of Materials Engineering and Performance
Fig. 1
Schematics of powder bed fusion equipment. (a) Selective laser melting and (b) electron beam melting
Fig. 2 (a) Microstructures of the SLM built Ti-6Al-4V object with (a) top view, (b) side view (Ref 38), (c) pore due to trapped gas, and (d)
pore due to insuf?cient heating (Ref 39)
and cooling process. From the side view (Fig. 2b), the grains
are mostly vertical with elongated shapes. The vertically
columnar grains are tilted according to the scan direction.
Horizontally dark bands can be observed due to the layer-wise
AM process. Two types of pore can be found in the PBF parts:
the pores due to trapped gas in the powder bed (Fig. 2c) and the
pores caused by insuf?cient melting (Fig. 2d), which are
mostly seen near the edge regions (Ref 39).
The grain microstructures of PBF parts are mostly affected
by two factors: the temperature gradient and the solidi?cation
interface velocity. Columnar grains develop when the temperature gradient is large and the interface velocity is small. In
Journal of Materials Engineering and Performance
contrast, small temperature gradient and large interface velocity
will form equiaxed grains. This grain transformation can be
calculated by the dendrite growth model by Hunt (Ref 40).
Based on this model, Nastac et al. (Ref 41) investigated several
nickel alloys and generated the solidi?cation maps for Inconel
718 and RS5 alloys. Sames et al. (Ref 42) developed a
processing window for the EBM process. Their works show
that Arcam fabricated Inconel 718 grain growth can be
speci?cally controlled by these two factors.
Both temperature gradient and interface velocity can be
affected by processing parameters like scan speed and laser/ebeam power. Using process design to control the microstructure
Volume 27(1) January 2018—3
has been mentioned in many recent works. Dehoff et al. (Ref
43) developed an EBM processing strategy that was able to
produce ?ne grained Inconel 718. Later, Helmer et al. (Ref 44)
studied the processing window, and they also obtained ?ne
epitaxial grains from columnar grains.
The mechanical properties of both SLM and EBM processed
materials are crucial to their applications. Important mechanical
properties such as elastic modulus, ductility, and fatigue of PBF
parts were reported (Ref 30, 45-49). Kruth et al. (Ref 50)
presented the binding mechanisms that affect the mechanical
properties of AM parts. The binding mechanisms can be
divided into four categories based on the degree of melting: (1)
solid-state sintering, (2) chemically induced binding, (3) partial
melting, and (4) full melting (Ref 50). PBF parts show
anisotropic properties including elastic modulus, yield stress,
and ultimate stress (Ref 47). This anisotropy is mainly caused
by insuf?cient heat energy which induces a lack of fusion at the
interface between each layer, so that the building direction is
weaker than the scanned planar direction.
For both PBF and DED processes, the properties of
Ti6Al4V alloy have been extensively investigated. This is due
to the high demand of this material for aerospace and medical
implant applications. Also, as suggested by Yang et al. (Ref
51), Ti6Al4V is dif?cult to fabricate using conventional
manufacturing methods. This problem can be easily solved
by AM, since only powder will be used. The tensile properties
of PBF fabricated Ti6Al4V parts were tested by many
researchers, and the resulting data are listed in Table 3. The
mechanical properties including YoungÕs modulus, yield
strength, ultimate strength, and strain at failure are compared
to the traditional wrought Ti6Al4V. It should be noted that the
orientation in Table 3 shows the tensile direction, where
horizontal refers the in-plane direction of each deposited layer,
and vertical refers the direction of accumulation. As shown in
the table, the YoungÕs moduli of both SLM and EBM processed
parts show similar values to the wrought one. Approximately a
10% difference can be observed when comparing the horizontal
and vertical orientations. For the SLM processed parts, the
yield and ultimate strengths are even better than that of the
conventional wrought material. This is mainly because PBF
uses very ?ne powders as raw material. The as-fabricated parts
behave more brittle with very limited failure strains. Effective
post-treatment, for example, hot isostatic pressing (HIP)
doubles the elongation, but HIP process decreases the yield
and ultimate strengths. The EBM processed parts show that the
vertical orientation has 30% less elongation than the horizontal
orientation, but no obvious difference is found in the yield and
ultimate strengths. A machining treatment for the EBM parts
can increase the YoungÕs modulus, yield strength, and ultimate
strength, but the elongation at failure is not changed.
The mechanical properties of other materials, including
aluminum alloys and stainless steels, were also studied.
However, the available data are not as abundant as for
Ti6Al4V. The effect of heat treatment on the tensile properties
of AlSi10Mg was studied by Krishnan (Ref 60). Tensile
properties of 15-5 stainless steel and fatigue properties of 316L
were presented in Ref 61 and 62, respectively.
It is noted that PBF processed parts are prone to several
issues, due to the weak bonding between layers and the
complicated thermal history. High temperature gradients cause
thermal residual stress that accumulates as the layers are built
up, resulting in distortion and wrapping of the product. Layer
delamination and cracking are also common due to thermal
stress and the weak bonding between layers.
Table 3 Mechanical properties of metallic materials fabricated by powder bed fusion technologies
Process
Wrought
(Ref 52)
SLM
Condition
Orientation
YoungÕs
modulus,
GPa
N/A
As fabricated
Longitudinal
113
945
979
0.100
EOSINT
M270 (Ref 52)
As fabricated
Horizontal
Vertical
Hor. and vert.
Horizontal
Vertical
Horizontal
Vertical
Vertical
109
115
112
105
102
112
110
119
972
1096
862
1070
1050
1000
920
967
1034
1130
931
1250
1180
1060
1000
117
0.055
0.012
0.240
0.060
0.080
0.125
0.160
0.089
Horizontal
Vertical
Horizontal
Vertical
Horizontal
Vertical
Horizontal
Vertical
Horizontal
Vertical
Horizontal
Vertical
N/A
N/A
105
102
103
98
NA
NA
NA
NA
104
101
114
115
118
117
1137
962
944
925
1006
1001
983
984
844
782
899
869
830
795
1206
1166
1036
1040
1066
1073
1030
1033
917
842
978
928
915
870
0.076
0.017
0.085
0.075
0.150
0.108
0.122
0.090
0.088
0.099
0.095
0.099
0.131
0.137
Equipment
Concept Laser M2
(Ref 53)
HIP
As fabricated
HIP
Realizer (SLM300i)
(Ref 54)
Trumpf (LF250)
(Ref 55)
As fabricated
As fabricated
Heat treated
EBM
Arcam A2
(Ref 56)
Arcam S12
(Ref 57)
Arcam S400
(Ref 58)
As fabricated
As fabricated
As fabricated
machined
Arcam (Ref 59)
4—Volume 27(1) January 2018
As fabricated
HIP
Yield
strength,
MPa
Ultimate
strength,
MPa
Failure
strain
Journal of Materials Engineering and Performance
3. Direct Energy Deposition
3.1 Direct Energy Deposition Equipment and Process
Another well-developed manufacturing technique is direct
energy deposition (DED). Instead of using a powder bed, DED
process uses injected metal powder ?ow or metal wire as
feedstocks, along with an energy source such as laser or
electron beam, to melt and deposit the material on the top of a
substrate. DED techniques can be divided into two major
categories based on the feedstocks. The ?rst category includes
methods developed from traditional welding technique using
metal wire as a feedstock. The second method named Laser
Engineered Net Shaping (LENS) (Ref 63) was developed by
Sandia National Laboratory in 1996, which uses powder ?ow
as a feedstock.
The schematic of a LENS machine is shown in Fig. 3. In a
building chamber, a Nd:YAG laser beam focuses on a point on
the building platform using a lens system, and at the same time,
metal powder is injected to the point through a powder nozzle.
The powder ?ows into the melt pool at the same time as the
laser source or the building platform moves. The melted
powder and the materials beneath solidify quickly, thus forming
a layer of material. After one layer is built, the laser lenses and
powder nozzle move up, and the laser heating and powder
injection processes repeat for the next layer (Ref 63).
Electron beam is another power source for the DED system
due to its high energy density. By using an electron beam, high
accuracy and good surface ?nishing can be achieved with low
deposition rates. The Electron Beam Freeform Fabrication
(EBF3) process was developed by NASA (Ref 64). It is
primarily used for space-based applications. The EBF3 process
uses a metal wire ?lament instead of powder injection. With
electron beam or laser source, the front end of the metal wire is
melted and selectively sprayed on the top of a substrate to form
a material layer.
3.2 Microstructures and Mechanical Properties of Direct
Energy Deposition Fabricated Parts
A comprehensive study on the microstructure of LENS
fabricated parts was ?rst reported by Grif?th et al. (Ref 65, 66).
In their study, the tensile properties of wrought materials were
used as a reference for comparison. They found that the yield
Fig. 3
Schematic of LENS process
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