A review of the wire arc additive manufacturing of metals 2018

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A review of the wire arc additive manufacturing of metals: properties, defects
and quality improvement
Article in Journal of Manufacturing Processes · August 2018
DOI: 10.1016/j.jmapro.2018.08.001
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Journal of Manufacturing Processes 35 (2018) 127–139
Contents lists available at ScienceDirect
Journal of Manufacturing Processes
journal homepage: www.elsevier.com/locate/manpro
A review of the wire arc additive manufacturing of metals: properties,
defects and quality improvement
T
⁎
Bintao Wua, Zengxi Pana, , Donghong Dingb, Dominic Cuiuria, Huijun Lia, Jing Xuc, John Norrisha
a
School of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of Wollongong, Wollongong NSW 2522, Australia
School of Mechatronic Engineering, Foshan University, Foshan, Guangdong 528000, China
c
School of Mechanical Engineering, Tsinghua University, Beijing 100084, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords:
Wire arc additive manufacturing (WAAM)
Materials
Defects
Quality improvement
Due to the feasibility of economically producing large-scale metal components with relatively high deposition
rates, significant progress has been made in the understanding of the Wire Arc Additive Manufacturing (WAAM)
process, as well as the microstructure and mechanical properties of the fabricated components. As WAAM has
evolved, a wide range of materials have become associated with the process and its applications.
This article reviews the emerging research on WAAM techniques and the commonly used metallic feedstock
materials, and also provides a comprehensive over view of the metallurgical and material properties of the
deposited parts. Common defects produced in WAAM components using different alloys are described, including
deformation, porosity, and cracking. Methods for improving the fabrication quality of the additively manufactured components are discussed, taking into account the requirements of the various alloys. This paper
concludes that the wide application of WAAM still presents many challenges, and these may need to be addressed in specific ways for different materials in order to achieve an operational system in an acceptable time
frame. The integration of materials and manufacturing process to produce defect-free and structurally-sound
deposited parts remains a crucial effort into the future.
1. Introduction
In recent years, wire arc additive manufacturing (WAAM) has increasingly attracted attention from the industrial manufacturing sector
due to its ability to create large metal components with high deposition
rate, low equipment cost, high material utilization, and consequent
environmental friendliness. The origin of the WAAM process can be
traced back to the 1925s when Baker [1] proposed to use an electric arc
as the heat source with filler wires as feedstock materials to deposit
metal ornaments. Since then, consistent progress has been made on the
development of this technology, particularly in the last 10 years. The
WAAM technique bears various nomenclatures by different research
institutions worldwide [2–24], as shown in Fig.1. Today, WAAM has
become a promising fabrication process for various engineering materials such as titanium, aluminium, nickel alloy and steel. Compared to
traditional subtractive manufacturing, the WAAM system can reduce
fabrication time by 40–60% and post-machining time by 15–20% depending on the component size [25]. For instance, recent breakthroughs in WAAM technology have made it possible to fabricate aircraft landing gear ribs with a saving of approximately 78% in raw
⁎
material when compared with the traditional subtractive machining
process [26].
Due to the highly complex nature of WAAM, many different aspects
of the process need to be studied, including process development, material quality and performance, path design and programming, process
modelling, process monitoring and online control [9]. Several WAAM
review papers have been published by leaders in the field, covering
state-of-the-art systems, design, usage, in-situ process monitoring, insitu metrology and in-process control and sensing [26–31]. Nevertheless, there is still a need for a systematic review of the properties of
various WAAM-processed materials, the defects associated with different alloys, and a summary of current and future research directions
that are aimed at quality improvements for the alloy classes of interest.
This paper reviews the microstructure and mechanical properties of
various metals, including titanium and its alloys, aluminum and its
alloys, Ni-based alloy, steel and other intermetallic materials fabricated
by the various WAAM processes. The common defects that have been
found to occur for different materials are also summarized. The current
methods for both in-process and post-process quality improvement and
defect reduction are introduced. Finally, a discussion is given on
Corresponding author.
E-mail address: zengxi@uow.edu.au (Z. Pan).
https://doi.org/10.1016/j.jmapro.2018.08.001
Received 23 February 2018; Received in revised form 1 August 2018; Accepted 1 August 2018
Available online 08 August 2018
1526-6125/ © 2018 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 35 (2018) 127–139
B. Wu et al.
Fig. 1. The distribution of main WAAM research groups.
first design uses an enclosed chamber to provide a good inert gas
shielding environment, similar to laser Power-Bed Fusion (PBF) systems. The second design uses existing or specially designed local gas
shielding mechanisms, with the robot positioned on a linear rail to
increase the overall working envelop. It is capable of fabricating very
large metal structures up to several a meters in dimension. Fig. 2 shows
an example of this design of WAAM system, used for the research and
development at the University of Wollongong (UOW).
Fabricating a part using WAAM involves three main steps: process
planning, deposition, and post processing. For a given CAD model, 3D
slicing and programming software generates the desired robot motions
and welding parameters for the deposition process, aimed at producing
defect-free fabrication with high geometrical accuracy [22,23,33].
Based on the welding deposition model for the specific material being
used to fabricate the component, the 3D slicing and programming
software offer automated path planning and process optimization to
avoid potential process-induced defects [34–37]. During fabrication,
the robot and external axis provide accurate motion for the welding
torch to build up the component in a layer-by-layer fashion. Advanced
WAAM systems can be equipped with various sensors to measure
welding signals [38], deposited bead geometry [39], metal transfer
behaviour [40] and interpass temperature [32,41,42], thereby supporting in-process monitoring and control to achieve higher product
quality. This is an area of current and future research interest, with the
potential for significantly improving WAAM process performance.
improving quality of WAAM fabricated parts through process selection,
feedstock optimization, process monitoring and control and post-process, including proposals for future research directions.
2. Wire arc additive manufacturing (WAAM) systems
2.1. Classification of WAAM process
Depending on the nature of the heat source, there are commonly
three types of WAAM processes: Gas Metal Arc Welding (GMAW)-based
[32], Gas Tungsten Arc Welding (GTAW)-based [2] and Plasma Arc
Welding (PAW)-based [3]. As listed in Table 1, specific class of WAAM
techniques exhibit specific features. The deposition rate of GMAWbased WAAM is 2–3 times higher than that of GTAW-based or PAWbased methods. However, the GMAW-based WAAM is less stable and
generates more weld fume and spatter due to the electric current acting
directly on the feedstock. The choice of WAAM technique directly influences the processing conditions and production rate for a target
component.
2.2. Robotic WAAM system
Most WAAM systems use an articulated industrial robot as the
motion mechanism. Two different system designs are available. The
Table 1
Comparison of various WAAM techniques.
WAAM
Energy source
Features
GTAW-based
GTAW
GMAW-based
GMAW
Non-consumable electrode; Separate wire feed
process;
Typical deposition rate: 1-2 kg/hour;
Wire and torch rotation are needed;
Consumable wire electrode;
Typical deposition rate 3-4 kg/hour;
Poor arc stability, spatter;
Reciprocating consumable wire electrode;
Typical deposition rate: 2-3 kg/hour;
Low heat input process with zero spatter, high
process tolerance;
Two consumable wires electrodes;
Typical deposition: 6-8 kg/hour;
Easy mixing to control composition for
intermetallic materials manufacturing ;
Non-consumable electrode; Separate wire feed
process;
Typical deposition rate 2-4 kg/hour;
Wire and torch rotation are needed;
Cold metal
transfer
(CMT)
Tandem GMAW
PAW-based
Plasma
3. Metals used in WAAM process
WAAM processes use commercially available wires which are produced for the welding industry and available in spooled form and in a
wide range of alloys as feedstock materials. Table 2 indicates the
commonly used alloys and their various applications in WAAM. Manufacture of a structurally sound, defect free, reliable part requires an
understanding of the available process options, their underlying physical processes, feedstock materials, process control methods and an
appreciation of the causes of the various common defects and their
remedies. This section reviews the metals that are commonly used in
WAAM, with a particular emphasis on the microstructure and mechanical properties of the additively manufactured alloys.
3.1. Titanium alloys
Titanium alloys have been widely studied for application of additive
manufacturing in aerospace components due to their high strength-toweight ratio and inherently high material cost. There are increasing
128
Journal of Manufacturing Processes 35 (2018) 127–139
B. Wu et al.
Fig. 2. WAAM system design concepts, University of Wollongong.
build direction owing to thermal gradient [66], commonly seen in additively manufactured titanium alloy components.
Table 3 summarizes the microstructure and mechanical property
data (tensile strength, yield strength and elongation) of Ti6Al4V samples fabricated using various WAAM technologies [62,64,67–73]. The
as-forged and as-cast minimum specifications from ASTM standards are
also listed for comparison. As shown in Fig.4, the tensile property of asfabricated Ti6Al4V samples is close to that of wrought Ti6Al4V and
exceeds that of cast Ti6Al4V as specified by ASTM standards. In addition, WAAM fabricated Ti6Al4V samples show anisotropic properties
with lower strength and higher elongation values in the build direction
(Z) compared to deposition direction (X), which is mainly attributed to
the grain size of α lamellae and the orientation of the elongated prior β
grains.
Table 2
Metals typically used with WAAM process.
Applications
Aerospace
Automotive
Marine
Corrosion resistance
High temperature
Tools and molds
Alloys
Ti-based
Al-based
Steel-based
Ni-based
Bimetal
[26]
–
[49,50]
[52,53]
[56]
–
[43]
[46]
–
–
–
–
–
[47]
[51]
–
–
[59,60]
[44]
–
–
[54]
[57]
–
[45]
[48]
–
[55]
[58]
–
demands for more efficient and lower cost alternatives to the conventional subtractive manufacturing methods, which suffer very low fly-tobuy ratios for many component designs. There exists many business
opportunities for the WAAM process, particularly for large-sized titanium components with complex structures [26].
It is well accepted that the microstructure of the product depends on
its thermal history during the fabrication process. The distinctive
WAAM thermal cycle, which involves repeated heating and cooling
[61,62], produces meta-stable microstructures and inhomogeneous
compositions in the fabricated part [63]. For example, Baufeld et al
[62] investigated the microstructures of Ti6Al4V fabricated using a
GTAW-based WAAM system, and found two distinctive regions on the
as-built wall. In the bottom region, where alternating bands are perpendicular to the build direction, a basketwave Widmanstätten structure with α phase lamellae is present, while in the top region, where no
such bands appear, needle-like precipitate is the main structure. Similar
microstructural evolution has also been observed in the PAW-based
process. Lin et al [64,65] reported a graded microstructure along the
build direction and identified the martensite α’ structure, Widmanstätten structure and basket-wave structure from the bottom to the top
region of the fabricated component, as shown in Fig.3. An epitaxial
growth of β grains with discrete direction is also observed along the
3.2. Aluminum alloys and steel
Although fabrication trials for many different series of aluminum
alloys, including Al-Cu (2xxx) [76], Al-Si (4xxx) [77] and Al-Mg (5xxx)
[40] have been successfully carried out, the commercial value of
WAAM is mainly justifiable for large and complex thin-walled structures, since cost of manufacturing small and simple aluminum alloy
components using conventional machining processes is low [78]. Using
WAAM to fabricate steel is unpopular for the same reason although it is
the most commonly used engineering material [79]. Another reason for
the poor commercial application of WAAM in aluminum is that some
series of aluminum alloys, such as Al 7xxx and 6xxx, are challenging to
weld due to turbulent melt pool and weld defects, which frequently
occur during the deposition process.
In general, as-deposited additively manufactured aluminum alloy
parts have inferior mechanical properties compared to those machined
from billet material. In order to achieve higher tensile strength, most of
the as-deposited aluminum parts undergo post-process heat treatment
to refine the microstructure. Table 4 lists the yield strength (YS), ultimate tensile strength (UTS), and elongation of WAAM-fabricated
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Journal of Manufacturing Processes 35 (2018) 127–139
B. Wu et al.
Fig. 3. Optical microstructure of the deposited wall (from Lin, et al. [64]): (a) the bottom region; (b) the middle region; (c) the top region.
Table 3
Mechanical properties of Ti6Al4V fabricated from various WAAM processes.
Process
Condition
Microstructure
YS [MPa]
UTS s [MPa]
EL [%]
Reported by
Cast
Wrought
GTAW
/
/
AF
/
/
Columnar prior β grains + Widmanstätten α/β
758 (min)
860 (min)
/
ASTM F1108
ASTM F1472
Baufeld et al. [13]
AF
α phase lamella
basket weave structures
lamellar structure
/
860
930
929
965
939
HT (600 °C/4 h/
FC)
HT (834 °C/2 h/
FC)
AF
Widmanstätten α/β + Columnar β grains
/
/
>8
> 10
9 ± 1.2a
9 ± 1b
16 ± 3a
7.8 ± 2.3b
12.5 ± 2.5a
6 ± 3b
21 ± 2a
14 ± 2b
14.8 a
11.7 b
16.5 ± 2.7a
7.8 ± 2b
11.6 ± 2.4a
6.6 ± 2.6b
20.4 ± 1.8a
13.5 ± 2b
/
Prior columnar β + Martensite α’
Prior columnar β + martensite α’+ fine basket-weave
structure
909 ± 13.6b
877 ± 18.5 b
988 ± 19.2b
968 ± 12.6 b
7 ± 0.5b
11.5 ± 0.5
b
/
972 ± 41
lamellar structure
/
Plasma
Pulsed-PAM
PAM
977 ± 14
a
931 ± 19
971 ± 28
b
Widmanstätten α + banded coarsened lamella α
803 ± 15
a
950 ± 21
/
861 ± 14
/
891 ± 16
918 ± 17 a 1033 ± 19
b
a
892 ± 31
b
HT (600 °C/2 h/
FC)
AN(834 °C/2 h/
FC)
AF
600 °C/840 °C
AF
AF
a
b
b
AF
(min)
(min)
± 41a
± 39b
± 24 a 1033 ± 32
937 ± 21
a
963 ± 22
b
a
915 ± 14
976 ± 35
a
b
981 ± 8
931 ± 17
a
962 ± 29
b
/
856 ± 21
a
893 ± 24
b
b
b
Baufeld et al. [67]
Wang et al. [74]
Brandl et al. [71]
Martina et al.
[75]
Lin et al. [65]
Lin et al. [64]
AF: as fabricated, HT: heat treated, AN: annealed.
a
In build direction.
b
Orthogonal to build direction.
The microstructure of WAAM fabricated Inconel 718 parts generally
consists of large columnar grains with interdendritic boundaries delineated by small Laves phase precipitates and MC carbides [81]. Xu
et al. [82] reported that columnar dendrite structures decorated with a
large amount of Laves phase, MC carbides and Ni3Nb are also present in
WAAM-fabricated Inconel 625 parts, as shown in Fig. 5. It is worth
noting that the microstructure can be refined to smaller dendritic arm
spacing, less niobium segregation and discontinuous Laves phase in the
interdendritic regions using post-process heat treatments, which are
beneficial to the mechanical properties.
Table 5 lists the mechanical properties of several Nickel-based superalloys fabricated using the WAAM process. For GMAW-based
WAAM-fabricated Inconel 718 alloy, the yield and ultimate tensile
strength is 473 ± 6 MPa and 828 ± 8 MPa respectively. These values
lie between the minimum values specified by ASTM for wrought and
cast materials, whereas the elongation is much lower than the standards
for both wrought and cast conditions. As for WAAM-fabricated Inconel
625 alloy, the YS, UTS and elongation all meet the requirement set by
2219aluminium alloy samples. Due to the uniform distribution of large
diamond particles within the microstructure, the sample exhibits lower
UTS and YS than that of the wrought part specified by ASTM standard.
However, after heat treatment, significant improvement beyond ASTM
standard can be observed in both strength and elongation as a consequence of the grain refinement [43].
3.3. Nickel-based superalloys
Nickel-based superalloys are the second most popular material
studied by the additive manufacturing research community after titanium alloys, mainly due to their high strengths at elevated temperatures and high fabrication cost using traditional methods. Nickel-based
superalloys are widely applied in aerospace, aeronautical, petrochemical, chemical and marine industries due to their outstanding strength
and oxidation resistance at temperatures above 550 °C. To date, various
Nickel-based superalloys, including Inconel 718 and Inconel 625 alloy
have been studied after WAAM processing.
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Journal of Manufacturing Processes 35 (2018) 127–139
B. Wu et al.
Fig. 4. Mechanical property comparison for Ti6Al4V parts fabricated using WAAM: (a) yield and ultimate strength, (b) elongation.
example, severe oxidization for titanium alloys, porosity for aluminum
alloys, poor surface roughness in steel as well as severe deformation and
cracks in bimetal components have been found to typically occur.
Table 7 lists the major defects that are commonly present in components fabricated using current WAAM techniques. The details of these
common defects and their relationship to the materials will be discussed
this section.
ASTM for cast materials, and are slightly lower than those for wrought
material.
3.4. Other metals
Other metals have also been investigated for potential fabrication
using WAAM, such as magnesium alloy AZ31 for automotive applications [87], Fe/Al intermetallic compounds [88,89] and Al/Ti [90,91]
compounds, as well as bimetallic steel/nickel [92] and steel/bronze
[93] parts for the aeronautic industry. The detailed mechanical properties of these materials fabricated using WAAM are listed in Table 6.
Most of this research has focused on determining the microstructural
and mechanical properties of samples taken from simple straight-walled
structures, rather than developing a process to fabricate functional
parts. Manufacturing intermetallic parts with accurate pre-designed
composition still poses major challenges for the WAAM process.
4.1. Deformation and residual stress
Like other additive manufacturing process, distortion and residual
stress are inherent to the WAAM process and it is impossible to completely avoid its generation. The residual stress can lead to distortion of
the part, loss of geometric tolerance, delamination of layers during
deposition, as well as deterioration of fatigue performance and fracture
resistance of the additively manufactured components. Therefore,
control and minimization of deformation and residual stress is a key
area if research.
Various types of deformation appear in WAAM fabricated parts,
including longitudinal and transvers shrinkage, bending distortion,
angular distortion and rotational distortion [103]. The distortions are
caused by thermal expansion and shrinkage of the part during repeated
melting and cooling processes, which is particularly an issue for large
thin walled structures [104]. Residual stress is the stress that remains in
the material when all external loading forces are removed. If the residual stress is sufficiently high, it can be a critical influential factor in
the mechanical properties and fatigue performance of the fabricated
component. If the residual stress exceeds the local UTS of the material,
cracking will take place, while if it is higher than the local YS but lower
than UTS, warping or plastic deformation will occur [105]. Ding et al.
[32] found that the residual stress uniformly distributes across the
4. Common defects in WAAM-fabricated component
Although the mechanical properties of components fabricated by
WAAM are in many cases comparable to those of their conventionally
processed counterparts, there are however some AM processing defects
that must be addressed for critical applications. Porosity, high residual
stress levels, and cracking, must be avoided, particularly for parts exposed to extreme environments where these defects lead to failure
modes such as high temperature fatigue. Defects in WAAM can occur
for various reasons, such as poor programming strategy, unstable weld
pool dynamics due to poor parameter setup, thermal deformation associated with heat accumulation [95], environmental influence (such as
gas contamination) and other machine malfunctions. As shown in
Fig. 6, certain materials tend to be vulnerable to specific defects. For
Table 4
Tensile properties of WAAM-fabricated aluminum alloy (2xxx).
Materials
Process
Condition
Microstructure
YS[MPa]
UTS [MPa]
EL [%]
Reported by
Al6.3Cu
Wrought(2219)
CMT
T851
AF
/
Fine dendrites + equiaxed grains
Homogeneous dispersed θ precipitates
390
262
264
458
466
> 4
15.8 ± 0.3a
18.6 ± 1.5b
13.6 ± 0.9a
14b
ASTMB211M [80]
Gu et al. [43]
HT(T6)
267
128
133
305
333
AF: as fabricated, HT: heat treated.
a
In build direction.
b
Orthogonal to build direction.
131
±
±
±
±
2a
5b
6a
6b
±
±
±
±
4a
2b
3a
3b
Journal of Manufacturing Processes 35 (2018) 127–139
B. Wu et al.
Fig. 5. The major phases appearing in the as-deposited microstructure of Inconel 625 alloy (from Xu et al. [83]).
4.2. Porosity
WAAM deposited wall, and the residual stress in the preceding layer has
little effect on the future layers. After release of clamping, however, the
internal stress is redistributed with a much lower value at the top of
integral part than at the interface to the substrate, resulting in bending
distortion of the component. Path planning also involves the distortion
and residual stress evolution in WAAM process [100]. If appropriate
deposition path designs, it will help in the significant improvement in
these defects, especially in large metal fabrication. A detailed overview
of the residual stress origin would exceed the scope of this article.
Among all WAAM engineering materials, bimetal components exhibit high levels of residual stress and deformation due to the material
thermal expansion difference. Hence, accurate interpass temperature
control is needed when bimetal materials are used. WAAM-fabricated
Inconel alloy has comparatively lower residual stresses levels, but it is
more susceptible to process defects such as delamination, buckling and
warping, since its residual stress is usually higher than the yield stress
[106]. Other comparatively softer materials, such as aluminum alloys,
easily respond to deformation defects due to their high thermal expansion coefficients. A better understanding pf the effect of material
characteristics in WAAM processing is needed for controlling residual
stress and deformation during deposition.
Deformation and residual stress are associated with many process
parameters, such as welding current, welding voltage, feeding speed,
ambient temperature, shielding gas flow rate, etc. There is still a lack of
systematic methods for controlling defects through optimized selection
or manipulation of these parameters. Fortunately, several post-process
treatments that have been proven to effectively mitigate residual stress
and deformation, and these will be discussed in Section 5.
Porosity is another common defect in WAAM processing that needs
to be minimized due to adverse effects on mechanical properties [107].
Firstly, porosity will lead to a component with low mechanical strength
by damage from micro-cracks, and secondly, it usually brings low fatigue property to deposition via spatially with different size and shape
distribution. In general, this type of defects are mainly classified as
either raw material-induced [108] or process-induced [109].
The WAAM raw material, including as-received wire and substrate,
often has a degree of surface contamination, such as moisture, grease
and other hydrocarbon compounds that may be difficult to completely
remove. These contaminants can be easily absorbed into the molten
pool and subsequently generate porosity after solidification. Among
common engineering materials, aluminum alloy is the most susceptible
to this defect as the solubility of hydrogen in solid and liquid (0.036
against 0.69 cm3/100 g at a melting point of 660℃, respectively) is
significantly different [110]. Even small amounts of dissolved hydrogen
in the liquid state may exceed the limit of solubility after solidification,
resulting in porosity [111]. Therefore, the cleanliness of raw materials
is critical, especially for aluminum alloys.
Process-induced porosity is usually non-spherical, and mainly
caused by poor path planning or an unstable deposition process. When
the deposition path is complex or the manufacturing process is
changeable, insufficient fusion or spatter ejection is easily produced,
creating gaps or voids in these influenced regions.
To control porosity, the following methods can be adopted: (1) an
AC GMAW-based process or CMT-PADV based process (cold metal
transfer pulsed advanced, a controlled short-circuiting GMAW transfer
process) is preferred, especially for aluminum; (2) the highest quality
Table 5
Mechanical properties of various Ni-based superalloys using different WAAM processes.
Materials
Process
Condition
Microstructure
YS [MPa]
UTS [MPa]
EL [%]
Reported by
IN 718
IN 625
GMAW
Cast
Wrought
PPAM
AF
/
/
AF
AF
IC
473 ± 6
350
490
438 ± 38a
423 ± 22b
449
480
828 ± 8
710
855
721 ± 32a
718 ± 19b
726
771
28 ± 2
48
50
48.6 a
49.2 b
43
50
Baufeld. [81]
ASM [83]
ASM [83]
Xu et al. [82]
PPAW
PPAW
GTAW
HT(980 °C /STA)
469
802
42
Wang et al. [85]
CMT
AF
Nb precipitates + dendritic structure
/
/
Laves phase + columnar dendrite
structure
Laves phase + MC carbides + δ-Ni3Nb
Laves phase +
NbC carbides
Coarser Laves particles + Nb
precipitates
Nb,Mo precipitates + dendrite structure
/
684 ± 23
722 ± 17
AF: as fabricated, HT: heat treated, IC: interpass cooling.
a
In build direction.
b
Orthogonal to build direction.
132
a
b
40.13 ± 3.7
42.27 ± 2.4
Xu et al. [84]
Xu et al. [84]
a
b
Ding et al. [86]
Journal of Manufacturing Processes 35 (2018) 127–139
B. Wu et al.
Table 6
Mechanical properties of other metallic materials fabricated using WAAM process.
Materials
Ti/Al
Process
GTAW
Condition
Microstructure
AF
Interdendritic γ structures + fully lamellar colonies
HT(1200 °C/24 h)
Full γ microstructure
425 ± 15 471 ± 14
HT(1060 °C/24 h)
Fully lamellar structure
copper twinning
Columnar Fe3Al grains
austenite, δ-ferrite in low part;
dendritic in up part
the mixture of α-Fe and ε-Cu
487 ± 14a
486 ± 12a
63 ± 2.1a
847 ± 2b
/
231 ± 2.5a
944 ± 19 b
565 ± 15 a
0.35
0.5b
1.1a
1.1b
0.30a
0.45b
63 ± 4.0a
3.2 ± 0.1 b
/
/
305a
48.5
Cu/Al
Fe/Al
Fe/Ni
GTAW
GTAW
GMAW
AF
AF
AF
Fe/Cu
GMAW
AF
YS[MPa]
UTS[MPa]
EL[%]
a
424 ± 30 474 ± 17
b
a
b
488 ± 50 549 ± 23
a
b
a
b
412 ± 11 472 ± 11
569 ± 12a 590 ± 4b
a
Reported by
Ma et al. [24]
Dong et al. [94]
Shen et al. [88]
Takeyuki
et al. [92]
Liu et al. [93]
AF: as fabricated, HT: heat treated.
a
In build direction.
b
Orthogonal to build direction.
solid between layers. Generally, this deficiency is visible and cannot be
repaired by post-process treatment. In order to prevent this defect, preprocess treatment such as preheating of the substrate needs to be considered.
Bimetal material combinations, such as Al/Cu, Al/Ti and Al/Fe, are
quite susceptible to cracking and delamination when fabricated with
the WAAM process. The dissimilar metals have large differences in their
mutual solubility and chemical reactivity so that the intermetallic
phase-equilibrium is freely broken, thus inducing crack growth along
grain boundaries. Also, Inconel alloy readily generates solidification
cracking issues due to the existence of liquid film at terminal solidification [101]. Both of these material types should receive particular
attention to avoid cracking and delamination.
To control crack defects, corresponding measures can be taken as
follows: (1) Mixed wires and optimization of their compositions; (2)
Decrease the cooling rate during the deposition process (3) Other
measures to improve strength rather than solution treatment.
shielding gas, tight gas seals, non-organic piping and short pipe lengths
are highly recommended; (3) the wire and substrate surfaces are as
clean as possible before fabrication; (4) high quality feedstock should
be used; (5) the deposited bead shape needs to be optimized; (6) the
thermal profile during processing should be monitored and controlled;
(7) post-deposition treatment, such as interpass rolling can be applied.
4.3. Crack and delamination
Similar to residual stress and deformation, cracking and delamination not only involves the thermal signature of the manufacturing
process, but also relates to the material characteristics of the deposit.
Ordinarily, the crack is categorised as either a solidification crack or
grain boundary crack within the WAAM component [105]. The former
type of crack depends mainly on the solidification nature of the material and is usually caused by the obstruction of solidified grain flow or
high strain in the melt pool [112]. Grain boundary cracking often
generates along the grain boundaries due to the differences between
boundary morphology and potential precipitate formation or dissolution [113]. Delamination or separation of adjacent layers takes place
due to incomplete melting or insufficient re-melting of the underlying
5. Current methods for quality improvement in the WAAM process
Generally, WAAM parts require post-process treatment to improve
Fig. 6. The correlation between materials and defects in WAAM processes.
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B. Wu et al.
Table 7
Tendency of various defects in WAAM fabricated parts.
Material
Ti6Al4V
H08Mn2Si steel
Copper-coated steel
ER4043 Al alloy
AA2319 Al alloy
5356 Al alloy
Inconel 625
Inconel 718
AZ31 Mg alloy
Nickel-Al-Cu
Steel–bronze bimetal
Steel- nickel bimetal
Intermetallic Fe/Al
Intermetallic Al/Ti
Intermetallic Al/Cu
Process
TIG
Plasma
CMT
DCEP-GMAW
DE-GMAW
GMAW
CMT
VP-GTAW
CMT
CMT-PADV
VP-GTAW
PPAD
GTAW
GMAW
PMIG
CMT
GMAW
GMAW
GTAW
GTAW
GTAW
Defect or feature
Ref.
Porosity
Cracking
Delamination
Oxidation
Substrate adherence
Surface finish
No
No
No
No
low
No
High
No
High
No
No
High
No
Medium
No
No
No
No
High
Low
No
No
No
No
No
No
No
No
No
No
No
Yes
Yes
No
Yes
No
No
No
No
Yes
Yes
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Yes
No
No
No
No
No
No
Yes
Light
No
Light
Light
No
Light
Light
No
No
No
No
No
No
No
Light
No
No
No
Serious
No
Light
Good
Good
Good
Medium
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Medium
Good
Good
Good
Medium
Good
Poor
Smooth
Smooth
Smooth
Poor
Waviness
Medium-rough
Smooth
Medium-rough
Smooth
Smooth
Smooth
Smooth
Smooth
Smooth
Medium-rough
Smooth
Smooth
Medium-rough
Medium-poor
Rough
Rough
[13]
[75]
[96]
[96]
[18]
[97]
[77]
[98]
[99]
[99]
[100]
[82]
[101]
[14]
[87]
[86]
[93]
[92]
[89]
[24]
[102]
Fig. 7. Schematic diagram of WAAM with cold rolling process [104].
respectively. In addition, post-process heat treatment plays an important role in grain refinement, especially for WAAM-fabricated aluminum and Inconel alloy [67].
The decision to use post-process heat treatment depends on the
material alloying system and also the pre-heat treatment state.
Generally, high carbon content materials must be heat treated, while a
few materials can be damaged by this technique. Therefore, the utilization of post heat treatment process to WAAM component needs to
consider the specific material and its application.
material properties, reduce surface roughness and porosity, and remove
residual stress and distortions. By appropriate application of post process, the majority of issues that influence deposition quality can be
mitigated or eliminated. Presently, several post-process treatment
technologies have been reported to improve part quality in the WAAM
process. This section will review these post-process techniques, both
their features and limitations.
5.1. Post-process heat treatment
5.2. Interpass cold rolling
Post-process heat treatment is widely used in WAAM to reduce residual stress, enhance material strength and as a method of hardness
control. The selection of a suitable heat treatment process depends on
the target material, additive manufacturing methods, working temperature and heat treatment conditions. If the heat treatment state is set
incorrectly, the probability of cracking will increase under mechanical
loading, as the combination of existing residual stress with load stress
exceeds the material’s design limitation.
As summarized Tables 3–5, after heat treatment, the mechanical
strength of WAAM-fabricated parts improved significantly, with increase of 4%, 78%, 5% and 17% being reported for titanium alloy,
aluminum alloy, Nickel-based superalloys and intermetallic Ti/Al alloy,
Rolling of the weld bead between each deposited layer has been
shown to reduce residual stresses and distortion [114].Interpass cold
rolling not only lowers residual stress, but also brings more homogeneous material properties. In the WAAM process, the thermal gradient with deposition layers and alternate re-heating and re-cooling
process result in the target part having anisotropic microstructural
evolution and mechanical properties. The cold rolling technique significantly reduces microstructural anisotropy through plastically deforming the deposition. Fig.7 shows the schematic diagram of an interpass cold rolling system developed at Cranfield University. A slotted
134
Journal of Manufacturing Processes 35 (2018) 127–139
B. Wu et al.
Table 8
Material properties of component fabricated using WAAM with interpass cold rolling.
Materials
Condition
Microstructure
YS[MPa]
UTS[MPa]
EL[%]
Hardness
[HV]
Ref.
Ti6Al4V
AF
Widmanstätten α + Martensite
22.8 ± 0.2a
13.8 ± 1.2b
14.2 ± 1.3a
10.0 ± 0.5b
13.0 ± 1.0a
13.1 ± 1.8b
15.5
13
10.3
7.3
54
23
6
3
2
15.8 ± 0.3a
18.6 ± 1.5b
14.6 ± 0.4a
15 ± 1.4b
12 ± 0.6a
13.2 ± 0.2b
7.4 ± 0.6a
8.6 ± 0.4b
[115,117,120,122]
Grain size decrease from average 200 μm to 100μm
903 ± 3a
945 ± 6b
1022 ± 3a
1006 ± 5b
1078 ± 3a
1077 ± 18b
260
274
288
304
697
1034
1321
1062
1925
262 ± 4a
264 ± 2b
270 ± 7a
271 ± 9b
286 ± 3a
293 ± 6b
312 ± 8a
323 ± 9b
/
RL:50
807 ± 2a
853 ± 10b
916 ± 22a
911 ± 11b
993 ± 31a
1028 ± 5b
132
145
190
244
292
750
1239
1529
1900
128 ± 2a
133 ± 5b
142 ± 4a
146 ± 3b
188 ± 4a
195 ± 2b
241 ± 2a
249 ± 1b
RL:75
Al 2319
AISI 301 L N
Al-6.3Cu
AF
RL:15
RL:30
RL:45
AF
CR:20
CR:40
CR:60
CR:80
AF
RL:15
/
Austenite phase
transformed to α'-rnartensite
+ deformed austenite
Fine dendrites + equiaxed grains
With the increase load, the grain size is decreased with freeform
deformation
RL:30
RL:45
/
/
/
/
/
/
220
345
456
522
610
68.3
[121]
[116]
[43]
77.4
90.1
102.3
AF: as-fabricated; RL: Rolling loads (KN); CR: Cold reduction (%).
a
In build direction.
b
Orthogonal to build direction.
5.3. Interpass cooling
roller is used to refine the microstructure and enhance tensile strength
in the longitudinal direction by supporting external force [115,116]. As
shown in Table 8, both ultimate tensile strength and yield strength in
the build direction were improved through interpass cold rolling, which
contributes to more homogeneous material properties in the target
component.
Interpass cold rolling also can play a critical role in the healing of
hydrogen porosity in WAAM-fabricated aluminum parts. Generally,
high dislocation density is produced by the rolling process. These dislocation can act as preferential sites for atomic hydrogen absorption
[117] and as well as ‘pipes’ for the hydrogen, allowing to diffuse to the
surface [118]. Table 9 summarizes the outcomes documented in the
literature, in terms of the pore incidence and size distribution in aluminum components fabricated using WAAM with interpass cold rolling.
The porosities existing in as-fabricated component can be reduced or
even eliminated when interpass cold rolling is applied [75].
This technique is only feasible for simple deposited parts, such as
straight walls, due to the geometrical limitation of the rolling process.
For more complex components with curves and corners, special flexible
tooling need to be developed to achieve an effective rolling process,
thus limiting the range of industrial application. Cold rolling technique
will also reduce residual stress, but the ability to reduce overall part
distortion is yet to be proven [26,119].
Recently, interpass cooling has been developed and evaluated at the
University of Wollongong. Fig. 8 presents the schematic diagram of a
WAAM system with interpass cooling. The moveable gas nozzle, which
supplies argon, nitrogen or CO2 gas, is used to provide active, or forced,
cooling on the fabricated part during and/or after deposition of each
layer. Using such rapid cooling, the in-situ layer temperature and heat
cycle can be controlled within a range to obtain the desired microstructure and mechanical properties. This process may also potentially
reduce residual stress and distortion, although this aspect has not been
investigated.
An initial feasibility study shows promising results when using
forced interpass cooling with compressed CO2 to fabricate Ti6Al4V
thin-walled structures, as shown in Fig. 9 [122]. It was found that interpass cooling produces less surface oxidation, refined microstructure,
improved hardness and enhanced strength. In addition, manufacturing
efficiency is significantly improved due to the reduction of dwell time
between deposited layers. More detailed research findings will be presented in future.
Table 9
The distribution of porosity in aluminum part using WAAM with cold rolling.
Materials
Condition
Layers
Height
(mm)
Deformation
(%)
Number of pores
(in area of 120 mm2)
Pores diameter (mm)
Reported by
Al 2319
AF
RL:15
RL:30
RL:45
AF
RL:15
RL:30
RL:45
21
25
30
38
30
35
45
55
49.4
50.6
49.4
49.8
49.5
49.6
51
49.5
/
13.9
30.0
44.2
/
14.1
31.3
45.4
614
192
5
Elimination
454
336
11
Elimination
13.5
12.5
8.8
Elimination
25.1
13
9.6
Elimination
Gu et al. [76]
Al 5087
AF: as-fabricated; RL: Rolling loads (KN).
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Journal of Manufacturing Processes 35 (2018) 127–139
B. Wu et al.
Fig. 8. Schematic diagram of the combined WAAM gas cooling process.
methods. An in-depth understanding of various materials, ideal process
setup, in-process parameter control and post processing is essential for
achieving such a goal. After a systematic review and analysis, a qualitybased framework aiming to achieve high-quality and defects-free
WAAM process is proposed, as shown in Fig. 10. Three main aspects are
considered: feedstock optimization, manufacturing process, and postprocess treatment.
Selection of the most suitable welding WAAM process for the deposition material can ensure manufacturing process stability and contribute to reduction of defects. For example, if the CMT-PADV process is
used for producing aluminum parts, porosity defects can be dramatically reduced when compared to other GMAW modes [98]. Moreover,
integrated and reliable process monitoring and control systems are
needed to maintain the stability of the process and ensure the quality of
production. Usually, the bead geometry, interpass temperature, arc
characteristics and metal transfer behaviour are included in process
monitoring and control. Controlling the interpass temperature within a
reasonable range is beneficial to microstructural evolution and the resulting mechanical properties. Further, regulating the arc characteristics and metal transfer behaviour in real time is helpful to process
stability and avoidance of defects. Based on the process monitoring data
that has been collected during deposition, one (or more) of several postprocess treatments can be selected to mitigate defects and improve
mechanical performance.
Considering the material characteristics, microstructural evolution
and mechanical properties can also be optimized through new feedstock
composition design. It is well known that different alloying elements
have specific effects on material properties. By referring to the phase
5.4. Peening and ultrasonic impact treatment
Peening and ultrasonic impact treatments (UIT) have been used in
welding applications to reduce local residual stress and improve weld
mechanical properties. Both techniques are cold mechanical treatments
that impact the weld surface using high energy media to release tensile
stress by imposing compressive stress at the treatment surface. Usually,
the mechanical peening process produces compressive stresses at a
limited depth below the component surface, such as around 1–2 mm in
carbon steels [123]. Ultrasonic impact treatment produces grain refinement and randomizes orientation, thus contributing to mechanical
strength improvement. It is reported that after ultrasonic impact
treatment, the surface residual stress of WAAM-fabricated Ti6Al4V part
can be reduced to 58% and the microhardness can be increased by 28%
compared to the as-fabricated sample. Also, the surface-modified layers
undergo plastic deformation with significant grain refinement and
dense dislocations [124]. The ultrasonic impact treatment is limited by
penetration depth, which is up to 60 μm below surface. Therefore, although both techniques are good post-process treatments, they have
negligible effect on internal residual stresses of large metal part fabricated using WAAM.
6. Discussion
Improving process stability, eliminating or decreasing deposition
defects and producing components with high quality and mechanical
performance have become major research focuses in making the WAAM
process more competitive against other additive manufacturing
Fig. 9. Mechanical properties of Ti6Al4V part produced by WAAM with interpass cooling using CO2 gas: (a) Hardness; (b) Tensile strength and elongation.
136
Journal of Manufacturing Processes 35 (2018) 127–139
B. Wu et al.
Fig. 10. A quality-based framework for the WAAM process.
chemical metallurgical mechanisms in WAAM process to provide a
guidance for the process optimization, improvement and control. The
defects generated in WAAM-produced part are closely related to the
target material characteristics and process parameters. The development of strategies or ancillary process to overcome defects generation
are of prime importance. With the requirement of high quality WAAM
part, the proposed quality-based framework will see a wide application
in the future years.
As WAAM matures as a commercial manufacturing process, development of a commercially available WAAM system for metal components is an interdisciplinary challenge, which integrates physical
welding process development, materials science and thermo-mechanical engineering, and mechatronic and control system design. Although
much research has been carried out over recent years in various areas,
such as process planning, programming, and material study, a general
purpose off-the-shelf WAAM system similar to commercially available
powder-bed fusion systems is yet to be developed. Due to the variety of
requirements of different engineering materials and the varying scale of
fabrication, many different WAAM system designs are expected to be
developed that will be optimized for particular applications, rather than
a single system that is capable of addressing all of the possible problems.
diagram, the desired deposition microstructures can be obtained via
adding specific alloying elements in the feedstock and subsequent mechanical properties improved. Moreover, availability of wire mixings
provides a potential possibility for building large functionally graded
products for special applications. For example, twin-wire GTAW-based
WAAM has been successfully developed to produce intermetallic graded
materials [24,89]. The development of new powder cored wires, will
offer exciting opportunities for fabricating target components with accurate metal composition. In summary, using new welding consumables
brings a cost effective solution, which supports high deposition quality
through obtaining the desired microstructures, lowering manufacturing
costs by reducing or eliminating pre-weld cleaning and re-work, and
providing safer working environments by reducing weld fumes.
Another essential part of WAAM processing for most materials is
post-process treatment, which is used to reduce residual stresses and
distortion, refine microstructures, improve microhardness and enhance
material strength. However, post-process technologies have their own
limitations, for instance, peening and ultrasonic impact treatment only
improve material property and reduce defects near the part surface,
while extended heat treatment of certain materials promotes grain
growth rather than grain refinement. Currently, most WAAM fabricated
parts need to be post-process treated with a selective combination of
technologies to reduce the defects and improve the product quality to
the greatest extent possible.
Acknowledgements
This research was carried out at the Welding Engineering Research
Group, University of Wollongong. The authors would like to acknowledge the National Natural Science Foundation of China for their financial support (Grant No. 51501034).
7. Conclusions
A detailed review of recent technological developments in WAAM
process has been presented, with a focus on microstructure, mechanical
properties, process defects and post-process treatment. Through
matching a knowledge of material characteristics with the performance
features of particular WAAM techniques, a quality-based framework is
proposed, for producing high-quality and defect-free components.
In WAAM of metallic materials, the fundamental interrelationships
between material composition and microstructure govern the material
properties and fabrication quality. Since the WAAM process is an inherently non-equilibrium thermal process, it is challenging to predicate
and control the microstructural evolution, which is responsible for the
variation of mechanical properties in the deposited part. Further research attention should be paid on the study of underlying physical and
References
[1] Ralph B. Method of making decorative articles, in. Google Patents 1925.
[2] Dickens P, Pridham M, Cobb R, Gibson I, Dixon G. Rapid prototyping using 3-D
welding, in. DTIC Document 1992.
[3] Spencer J, Dickens P, Wykes C. Rapid prototyping of metal parts by three-dimensional welding. Proc Inst Mech Eng Part B J Eng Manuf 1998;212:175–82.
[4] Kovacevic R, Beardsley H. Process control of 3D welding as a droplet-based Rapid
prototyping technique. Proc. Ofthe SFF Symposium, Univ. of Texas at Austin
1998:57–64.
[5] Dwivedi R, Kovacevic R. Automated torch path planning using polygon subdivision
for solid freeform fabrication based on welding. J Manuf Syst 2004;23:278–91.
137
Journal of Manufacturing Processes 35 (2018) 127–139
B. Wu et al.
[38] Sequeira Almeida P. Process control and development in wire and arc additive
manufacturing. Cranfield University; 2012.
[39] Xiong J, Yin Z, Zhang W. Closed-loop control of variable layer width for thinwalled parts in wire and arc additive manufacturing. J Mater Process Technol
2016;233:100–6.
[40] Geng H, Li J, Xiong J, Lin X, Zhang F. Optimization of wire feed for GTAW based
additive manufacturing. J Mater Process Technol 2017;243:40–7.
[41] Zhang S, Li J, Kou H, Yang J, Yang G, Wang J. Effects of thermal history on the
microstructure evolution of Ti-6Al-4V during solidification. J Mater Process
Technol 2016;227:281–7.
[42] Denlinger ER, Heigel JC, Michaleris P, Palmer TA. Effect of inter-layer dwell time
on distortion and residual stress in additive manufacturing of titanium and nickel
alloys. J Mater Process Technol 2015;215:123–31.
[43] Gu J, Ding J, Williams SW, Gu H, Bai J, Zhai Y, et al. The strengthening effect of
inter-layer cold working and post-deposition heat treatment on the additively
manufactured Al–6.3Cu alloy. Mater Sci Eng A 2016;651:18–26.
[44] Uriondo A, Esperon-Miguez M, Perinpanayagam S. The present and future of additive manufacturing in the aerospace sector: a review of important aspects. Proc
Inst Mech Eng G J Aerosp Eng 2015;229:2132–47.
[45] Sing SL, An J, Yeong WY, Wiria FE. Laser and electron‐beam powder‐bed additive
manufacturing of metallic implants: a review on processes, materials and designs.
J Orthop Res 2016;34:369–85.
[46] Murr LE, Gaytan S, Ceylan A, Martinez E, Martinez J, Hernandez D, et al.
Characterization of titanium aluminide alloy components fabricated by additive
manufacturing using electron beam melting. Acta Mater 2010;58:1887–94.
[47] Guo N, Leu MC. Additive manufacturing: technology, applications and research
needs. Front Mech Eng 2013;8:215–43.
[48] Kainer KU. Metal matrix composites: custom-made materials for automotive and
aerospace engineering. John Wiley & Sons; 2006.
[49] Leyens C, Peters M. Titanium and titanium alloys: fundamentals and applications.
John Wiley & Sons; 2003.
[50] Kim TB, Yue S, Zhang Z, Jones E, Jones JR, Lee PD. Additive manufactured porous
titanium structures: through-process quantification of pore and strut networks. J
Mater Process Technol 2014;214:2706–15.
[51] Wang R, Beck FH. New stainless steel without nickel or chromium for marine
applications. Met. Prog. 1983;123:72.
[52] Aziz-Kerrzo M, Conroy KG, Fenelon AM, Farrell ST, Breslin CB. Electrochemical
studies on the stability and corrosion resistance of titanium-based implant materials. Biomaterials 2001;22:1531–9.
[53] Wu B, Pan Z, Li S, Cuiuri D, Ding D, Li H. The anisotropic corrosion behaviour of
wire arc additive manufactured Ti-6Al-4V alloy in 3.5% NaCl solution. Corros Sci
2018;137:176–83.
[54] Lu G, Zangari G. Corrosion resistance of ternary Ni P based alloys in sulfuric acid
solutions. Electrochim Acta 2002;47:2969–79.
[55] Stoloff N, Liu C, Deevi S. Emerging applications of intermetallics. Intermetallics
2000;8:1313–20.
[56] Varghese OK, Gong D, Paulose M, Grimes CA, Dickey EC. Crystallization and hightemperature structural stability of titanium oxide nanotube arrays. J Mater Res
2003;18:156–65.
[57] Bewlay B, Jackson M, Subramanian P, Zhao J-C. A review of very-high-temperature Nb-silicide-based composites. Metall Mater Trans A 2003;34:2043–52.
[58] Jackson M, Bewlay B, Rowe R, Skelly D, Lipsitt H. High-temperature refractory
metal-intermetallic composites. JOM 1996;48:39–44.
[59] Bissacco G, Hansen HN, De Chiffre L. Micromilling of hardened tool steel for
mould making applications. J Mater Process Technol 2005;167:201–7.
[60] Yan L. Wire and arc addictive manufacture (WAAM) reusable tooling investigation. 2013.
[61] Thijs L, Verhaeghe F, Craeghs T, Humbeeck JV, Kruth J-P. A study of the microstructural evolution during selective laser melting of Ti–6Al–4V. Acta Mater
2010;58:3303–12.
[62] Baufeld B, Biest Ovd, Gault R. Microstructure of Ti-6Al-4V specimens produced by
shaped metal deposition. Int J Mater Res 2009;100:1536–42.
[63] Herzog D, Seyda V, Wycisk E, Emmelmann C. Additive manufacturing of metals.
Acta Mater 2016;117:371–92.
[64] Lin J, Lv Y, Liu Y, Sun Z, Wang K, Li Z, et al. Microstructural evolution and mechanical property of Ti-6Al-4V wall deposited by continuous plasma arc additive
manufacturing without post heat treatment. J Mech Behav Biomed Mater
2017;69:19–29.
[65] Lin JJ, Lv YH, Liu YX, Xu BS, Sun Z, Li ZG, et al. Microstructural evolution and
mechanical properties of Ti-6Al-4V wall deposited by pulsed plasma arc additive
manufacturing. Mater Des 2016;102:30–40.
[66] Hirata Y. Pulsed arc welding. Weld Int 2003;17:98–115.
[67] Baufeld B, Brandl E, Van der Biest O. Wire based additive layer manufacturing:
comparison of microstructure and mechanical properties of Ti–6Al–4V components fabricated by laser-beam deposition and shaped metal deposition. J Mater
Process Technol 2011;211:1146–58.
[68] Wang F, Williams SW, Rush M. Morphology investigation on direct current pulsed
gas tungsten arc welded additive layer manufactured Ti6Al4V alloy. 2011.
[69] Brandl E, Greitemeier D. Microstructure of additive layer manufactured Ti–6Al–4V
after exceptional post heat treatments. Mater Lett 2012;81:84–7.
[70] Szost BA, Terzi S, Martina F, Boisselier D, Prytuliak A, Pirling T, et al. A comparative study of additive manufacturing techniques: residual stress and microstructural analysis of CLAD and WAAM printed Ti–6Al–4V components. Mater Des
2016;89:559–67.
[71] Brandl E, Baufeld B, Leyens C, Gault R. Additive manufactured Ti-6Al-4V using
welding wire: comparison of laser and arc beam deposition and evaluation with
[6] Jandric Z, Labudovic M, Kovacevic R. Effect of heat sink on microstructure of
three-dimensional parts built by welding-based deposition. Int J Mach Tools Manuf
2004;44:785–96.
[7] Song YA, Park S, Choi D, Jee H. 3D welding and milling: part I–a direct approach
for freeform fabrication of metallic prototypes. Int J Mach Tools Manuf
2005;45:1057–62.
[8] Song YA, Park S, Chae S-W. 3D welding and milling: part II—optimization of the
3D welding process using an experimental design approach. Int J Mach Tools
Manuf 2005;45:1063–9.
[9] Zhang Y, Chen Y, Li P, Male AT. Weld deposition-based rapid prototyping: a preliminary study. J Mater Process Technol 2003;135:347–57.
[10] Zhang YM, Li P, Chen Y, Male AT. Automated system for welding-based rapid
prototyping. Mechatronics 2002;12:37–53.
[11] Kwak YM, Doumanidis CC. Geometry regulation of material deposition in near-net
shape manufacturing by thermally scanned welding. J Manuf Process
2002;4:28–41.
[12] Katou M, Oh J, Miyamoto Y, Matsuura K, Kudoh M. Freeform fabrication of titanium metal and intermetallic alloys by three-dimensional micro welding. Mater
Des 2007;28:2093–8.
[13] Baufeld B, Biest OVd, Gault R. Additive manufacturing of Ti–6Al–4V components
by shaped metal deposition: microstructure and mechanical properties. Mater Des
2010;31(Suppl 1):S106–11.
[14] Clark D, Bache MR, Whittaker MT. Shaped metal deposition of a nickel alloy for
aero engine applications. J Mater Process Technol 2008;203:439–48.
[15] Merz R, Prinz F, Ramaswami K, Terk M, Weiss L. Shape deposition manufacturing,
engineering design research center. Carnegie Mellon Univ. 1994.
[16] Xiong J, Zhang G, Gao H, Wu L. Modeling of bead section profile and overlapping
beads with experimental validation for robotic GMAW-based rapid manufacturing.
Robot Comput Integr Manuf 2013;29:417–23.
[17] Xiong J, Zhang G, Qiu Z, Li Y. Vision-sensing and bead width control of a singlebead multi-layer part: material and energy savings in GMAW-based rapid manufacturing. J Clean Prod 2013;41:82–8.
[18] Yang D, He C, Zhang G. Forming characteristics of thin-wall steel parts by double
electrode GMAW based additive manufacturing. J Mater Process Technol
2016;227:153–60.
[19] Aiyiti W, Zhao W, Lu B, Tang Y. Investigation of the overlapping parameters of
MPAW-based rapid prototyping. Rapid Prototyp J 2006;12:165–72.
[20] Suryakumar S, Karunakaran KP, Bernard A, Chandrasekhar U, Raghavender N,
Sharma D. Weld bead modeling and process optimization in Hybrid Layered
Manufacturing. Comput Des 2011;43:331–44.
[21] Ding D, Pan Z, Cuiuri D, Li H. A tool-path generation strategy for wire and arc
additive manufacturing. Int J Adv Manuf Technol 2014;73:173–83.
[22] Ding D, Pan Z, Cuiuri D, Li H. A practical path planning methodology for wire and
arc additive manufacturing of thin-walled structures. Robot Comput Integr Manuf
2015;34:8–19.
[23] Ding D, Pan Z, Cuiuri D, Li H, Larkin N. Adaptive path planning for wire-feed
additive manufacturing using medial axis transformation. J Clean Prod
2016;133:942–52.
[24] Ma Y, Cuiuri D, Li H, Pan Z, Shen C. The effect of postproduction heat treatment on
γ-TiAl alloys produced by the GTAW-based additive manufacturing process. Mater
Sci Eng A 2016;657:86–95.
[25] Reduce costs and increase output with robotic welding, in, https://www.scottautomation.com/applications/metal-fabrication/welding/.
[26] Williams SW, Martina F, Addison AC, Ding J, Pardal G, Colegrove P. Wire + arc
additive manufacturing. Mater Sci Technol 2016;32:641–7.
[27] Ding D, Pan Z, Cuiuri D, Li H. Wire-feed additive manufacturing of metal components: technologies, developments and future interests. Int J Adv Manuf Technol
2015;81:465–81.
[28] Levy GN, Schindel R, Kruth JP. Rapid manufacturing and rapid tooling with layer
manufacturing (LM) technologies, state of the art and tuture perspectives. CIRP
Ann Manuf Technol 2003;52:589–609.
[29] Frazier WE. Metal additive manufacturing: a review. J Mater Eng Perform
2014;23:1917–28.
[30] Lewandowski JJ, Seifi M. Metal additive manufacturing: a review of mechanical
properties. Annu Rev Mater Res 2016;46:151–86.
[31] Gao W, Zhang Y, Ramanujan D, Ramani K, Chen Y, Williams CB, et al. The status,
challenges, and future of additive manufacturing in engineering. Comput Des
2015;69:65–89.
[32] Ding J, Colegrove P, Mehnen J, Ganguly S, Sequeira Almeida PM, et al. Thermomechanical analysis of Wire and Arc Additive Layer Manufacturing process on
large multi-layer parts. Comput Mater Sci 2011;50:3315–22.
[33] Ding D, Pan Z, Cuiuri D, Li H. A multi-bead overlapping model for robotic wire and
arc additive manufacturing (WAAM). Robot Comput Integr Manuf
2015;31:101–10.
[34] Ding D, Pan Z, Cuiuri D, Li H, van Duin S, et al. Bead modelling and implementation of adaptive MAT path in wire and arc additive manufacturing. Robot
Comput Integr Manuf 2016;39:32–42.
[35] Ding D, Shen C, Pan Z, Cuiuri D, Li H, Larkin N, et al. Towards an automated
robotic arc-welding-based additive manufacturing system from CAD to finished
part. Comput Des 2016;73:66–75.
[36] Ding DH, Pan Z-X, Dominic C, Li H-J. Process planning strategy for wire and arc
additive manufacturing. Robotic welding, intelligence and automation. Springer;
2015. p. 437–50.
[37] Ding D, Pan Z, Cuiuri D, Li H, van Duin S. Advanced design for additive manufacturing: 3D slicing and 2D path planning. New trends in 3D printing. InTech;
2016.
138
Journal of Manufacturing Processes 35 (2018) 127–139
B. Wu et al.
[99] Cong B, Ding J, Williams S. Effect of arc mode in cold metal transfer process on
porosity of additively manufactured Al-6.3%Cu alloy. Int J Adv Manuf Technol
2015;76:1593–606.
[100] Wang H, Kovacevic R. Variable polarity GTAW in rapid prototyping of aluminum
parts. Proceedings of the 11th Annual Solid Freeform Fabrication Symposium
2000.
[101] Tian Y, Ouyang B, Gontcharov A, Gauvin R, Lowden P, Brochu M. Microstructure
evolution of Inconel 625 with 0.4 wt% boron modification during gas tungsten arc
deposition. J Alloys Compd 2017;694:429–38.
[102] Dong ZPB, Shen C, Ma Y, Li H. Fabrication of copper-rich Cu-Al alloy using the
wire-arc additive manufacturing process. Metallurgical and Materials Transactions
B: Process Metallurgy and Materials Processing Science 2017;48:3143–51.
[103] Masubuchi K. Analysis of welded structures: residual stresses, distortion, and their
consequences. Elsevier; 2013.
[104] Colegrove PA, Coules HE, Fairman J, Martina F, Kashoob T, Mamash H, et al.
Microstructure and residual stress improvement in wire and arc additively manufactured parts through high-pressure rolling. J Mater Process Technol
2013;213:1782–91.
[105] Sames WJ, List F, Pannala S, Dehoff RR, Babu SS. The metallurgy and processing
science of metal additive manufacturing. Int Mater Rev 2016;61:315–60.
[106] Mukherjee T, Zhang W, DebRoy T. An improved prediction of residual stresses and
distortion in additive manufacturing. Comput Mater Sci 2017;126:360–72.
[107] Edwards P, O’Conner A, Ramulu M. Electron beam additive manufacturing of titanium components: properties and performance. J Manuf Sci Eng
2013;135:061016.
[108] Busachi A, Erkoyuncu J, Colegrove P, Martina F, Ding J. Designing a WAAM based
manufacturing system for defence applications. Procedia Cirp 2015;37:48–53.
[109] Sames WJ, Medina F, Peter WH, Babu SS, Dehoff RR. Effect of process control and
powder quality on inconel 718 produced using electron beam melting. 8th international symposium on superalloy 718 and derivatives. John Wiley & Sons, Inc.;
2014. p. 409–23.
[110] Devletian JH, Wood WE. Factors affecting porosity in aluminum welds - a review.
Welding Research Council; 1983.
[111] Bai J, Ding H, Gu J, Wang X, Qiu H. Porosity evolution in additively manufactured
aluminium alloy during high temperature exposure. IOP conference series: materials science and engineering. IOP Publishing; 2017. p. 012045.
[112] What is hot cracking (solidification cracking), in, TWI Ltd, http://www.twi-global.
com/technical-knowledge/faqs/material-faqs/faq-what-is-hot-cracking-solidification-cracking/, 2017.
[113] Davis TA. The effect of process parameters on laser deposited Ti-6Al-4V, electronic
theses and dissertations. 2004. Paper 319.
[114] Colegrove PA, Donoghue J, Martina F, Gu J, Prangnell P, Hönnige J. Application of
bulk deformation methods for microstructural and material property improvement
and residual stress and distortion control in additively manufactured components.
Scr Mater 2017;135:111–8.
[115] Huang JX, Ye XN, Xu Z. Effect of cold rolling on microstructure and mechanical
properties of AISI 301LN metastable austenitic stainless steels. J Iron Steel Res Int
2012;19:59–63.
[116] Martina F, Colegrove PA, Williams SW, Meyer J. Microstructure of interpass rolled
wire+ arc additive manufacturing Ti-6Al-4V components. Metall Mater Trans A
2015;46:6103–18.
[117] Martina F, Roy MJ, Colegrove PA, Williams SW. Residual stress reduction in high
pressure interpass rolled wire+ arc additive manufacturing Ti-6Al-4V components. Proc. 25th Int. Solid Freeform Fabrication Symp. 2014. p. 89–94.
[118] Williams S. Wire + arc additive manufacture – current and future developments.
2017https://waammat.com/documents.
[119] Residual stress reduction in high pressure interpass rolled wire þ arc additive
manufacturing Ti–6Al–4V components.
[120] Williams S. Wire + arc additive manufacture. 2017https://waammat.com/
documents.
[121] Donoghue J, Antonysamy AA, Martina F, Colegrove PA, Williams SW, Prangnell
PB. The effectiveness of combining rolling deformation with Wire–Arc Additive
Manufacture on β-grain refinement and texture modification in Ti–6Al–4V. Mater
Charact 2016;114:103–14.
[122] Wu B, Pan Z, Ding D, Cuiuri D, Li H, Fei Z. The effects of forced interpass cooling
on the material properties of wire arc additively manufactured Ti6Al4V alloy. J
Mater Process Technol 2018;258:97–105.
[123] Cheng X, Fisher JW, Prask HJ, Gnäupel-Herold T, Yen BT, et al. Residual stress
modification by post-weld treatment and its beneficial effect on fatigue strength of
welded structures. Int J Fatigue 2003;25:1259–69.
[124] Li G, Qu S, Xie M, Li X. Effect of ultrasonic surface rolling at low temperatures on
surface layer microstructure and properties of HIP Ti-6Al-4V alloy. Surf Coat
Technol 2017;316:75–84.
respect to aerospace material specifications. Phys Procedia 2010;5:595–606.
[72] Zhang J, Zhang X, Wang X, Ding J, Traoré Y, Paddea S, et al. Crack path selection
at the interface of wrought and wire + arc additive manufactured Ti–6Al–4V.
Mater Des 2016;104:365–75.
[73] Brandl E, Schoberth A, Leyens C. Morphology, microstructure, and hardness of
titanium (Ti-6Al-4V) blocks deposited by wire-feed additive layer manufacturing
(ALM). Mater Sci Eng A 2012;532:295–307.
[74] Wang F, Williams S, Colegrove P, Antonysamy AA. Microstructure and mechanical
properties of wire and arc additive manufactured Ti-6Al-4V. Metall Mater Trans A
2013;44:968–77.
[75] Martina F, Mehnen J, Williams SW, Colegrove P, Wang F. Investigation of the
benefits of plasma deposition for the additive layer manufacture of Ti–6Al–4V. J
Mater Process Technol 2012;212:1377–86.
[76] Gu J, Ding J, Williams SW, Gu H, Ma P, Zhai Y. The effect of inter-layer cold
working and post-deposition heat treatment on porosity in additively manufactured aluminum alloys. J Mater Process Technol 2016;230:26–34.
[77] Wang P, Hu S, Shen J, Liang Y. Characterization the contribution and limitation of
the characteristic processing parameters in cold metal transfer deposition of an Al
alloy. J Mater Process Technol 2017;245:122–33.
[78] Brice C, Shenoy R, Kral M, Buchannan K. Precipitation behavior of aluminum alloy
2139 fabricated using additive manufacturing. Mater Sci Eng A 2015;648:9–14.
[79] Agrawal BK. Introduction to engineering materials. New Delhi: Tata McGraw-Hill;
2007.
[80] ASTM B. 221-standard specification for aluminum and aluminum-alloy extruded
bars, rods, wire, profiles, and tubes. 2005.
[81] Baufeld B. Mechanical properties of INCONEL 718 parts manufactured by shaped
metal deposition (SMD). J Mater Eng Perform 2012;21:1416–21.
[82] Xu F, Lv Y, Liu Y, Shu F, He P, Xu B. Microstructural evolution and mechanical
properties of inconel 625 alloy during pulsed plasma arc deposition process. J
Mater Sci Technol 2013;29:480–8.
[83] Bauccio M. ASM metals reference book. ASM international; 1993.
[84] Xu FJ, Lv YH, Xu BS, Liu YX, Shu FY, He P. Effect of deposition strategy on the
microstructure and mechanical properties of Inconel 625 superalloy fabricated by
pulsed plasma arc deposition. Mater Des 2013;45:446–55.
[85] Wang JF, Sun QJ, Wang H, Liu JP, Feng JC. Effect of location on microstructure
and mechanical properties of additive layer manufactured Inconel 625 using gas
tungsten arc welding. Mater Sci Eng A 2016;676:395–405.
[86] Ding D, Pan Z, van Duin S, Li H, Shen C. Fabricating superior NiAl bronze components through wire arc additive manufacturing. Materials 2016;9:652.
[87] Guo J, Zhou Y, Liu C, Wu Q, Chen X, Lu J. Wire arc additive manufacturing of
AZ31 magnesium alloy: grain refinement by adjusting pulse frequency. Materials
2016;9:823.
[88] Shen C, Pan Z, Ma Y, Cuiuri D, Li H. Fabrication of iron-rich Fe–Al intermetallics
using the wire-arc additive manufacturing process. Addit Manuf 2015;7:20–6.
[89] Shen C, Pan Z, Cuiuri D, Dong B, Li H. In-depth study of the mechanical properties
for Fe3Al based iron aluminide fabricated using the wire-arc additive manufacturing process. Mater Sci Eng A 2016;669:118–26.
[90] Ma Y, Cuiuri D, Hoye N, Li H, Pan Z. Characterization of in-situ alloyed and additively manufactured titanium aluminides. Metallurgical and Materials
Transactions B: Process Metallurgy and Materials Processing Science
2014;45:2299–303.
[91] Ma Y, Cuiuri D, Shen C, Li H, Pan Z. Effect of interpass temperature on in-situ
alloying and additive manufacturing of titanium aluminides using gas tungsten arc
welding. Addit Manuf 2015;8:71–7.
[92] Abe T, Sasahara H. Dissimilar metal deposition with a stainless steel and nickelbased alloy using wire and arc-based additive manufacturing. Precis Eng
2016;45:387–95.
[93] Liu L, Zhuang Z, Liu F, Zhu M. Additive manufacturing of steel–bronze bimetal by
shaped metal deposition: interface characteristics and tensile properties. Int J Adv
Manuf Technol 2013;69:2131–7.
[94] Dong B, Pan Z, Shen C, Ma Y, Li H. Fabrication of copper-rich Cu-Al alloy using the
wire-arc additive manufacturing process. Metall Mater Trans B 2017.
[95] Wu B, Ding D, Pan Z, Cuiuri D, Li H, Han J, et al. Effects of heat accumulation on
the arc characteristics and metal transfer behavior in Wire Arc Additive
Manufacturing of Ti6Al4V. J Mater Process Technol 2017;250:304–12.
[96] Almeida P, Williams S. Innovative process model of Ti–6Al–4V additive layer
manufacturing using cold metal transfer (CMT). Proceedings of the Twenty-First
Annual International Solid Freeform Fabrication Symposium 2010.
[97] Xiong J, Zhang G, Zhang W. Forming appearance analysis in multi-layer singlepass GMAW-based additive manufacturing. Int J Adv Manuf Technol
2015;80:1767–76.
[98] Wang H, Jiang W, Ouyang J, Kovacevic R. Rapid prototyping of 4043 Al-alloy
parts by VP-GTAW. J Mater Process Technol 2004;148:93–102.
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