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Additive manufacturing using fine wire-based laser metal deposition

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Purpose Using wire as feedstock has several advantages for additive manufacturing (AM) of metal components, which include high deposition rates, efficient material use and low material costs. While the feasibility of wire-feed AM has been demonstrated, the accuracy and surface finish of the produced parts is generally lower than those obtained using powder-bed/-feed AM. The purpose of this study was to develop and investigate the feasibility of a fine wire-based laser metal deposition (FW-LMD) process for producing high-precision metal components with improved resolution, dimensional accuracy and surface finish. Design/methodology/approach The proposed FW-LMD AM process uses a fine stainless steel wire with a diameter of 100 µm as the additive material and a pulsed Nd:YAG laser as the heat source. The pulsed laser beam generates a melt pool on the substrate into which the fine wire is fed, and upon moving the X–Y stage, a single-pass weld bead is created during solidification that can be laterally and vertically stacked to create a 3D metal component. Process parameters including laser power, pulse duration and stage speed were optimized for the single-pass weld bead. The effect of lateral overlap was studied to ensure low surface roughness of the first layer onto which subsequent layers can be deposited. Multi-layer deposition was also performed and the resulting cross-sectional morphology, microhardness, phase formation, grain growth and tensile strength have been investigated. Findings An optimized lateral overlap of about 60-70% results in an average surface roughness of 8-16 µm along all printed directions of the X–Y stage. The single-layer thickness and dimensional accuracy of the proposed FW-LMD process was about 40-80 µm and ±30 µm, respectively. A dense cross-sectional morphology was observed for the multilayer stacking without any visible voids, pores or defects present between the layers. X-ray diffraction confirmed a majority austenite phase with small ferrite phase formation that occurs at the junction of the vertically stacked beads, as confirmed by the electron backscatter diffraction (EBSD) analysis. Tensile tests were performed and an ultimate tensile strength of about 700-750 MPa was observed for all samples. Furthermore, multilayer printing of different shapes with improved surface finish and thin-walled and inclined metal structures with a minimum achievable resolution of about 500 µm was presented. Originality/value To the best of the authors’ knowledge, this is the first study to report a directed energy deposition process using a fine metal wire with a diameter of 100 µm and can be a possible solution to improving surface finish and reducing the “stair-stepping” effect that is generally observed for wires with a larger diameter. The AM process proposed in this study can be an attractive alternative for 3D printing of high-precision metal components and can find application for rapid prototyping in a range of industries such as medical and automotive, among others.
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Additive manufacturing using ne wire-based
laser metal deposition
Muhammad Omar Shaikh
Institute of Medical Science and Technology, National Sun Yat-sen University, Kaohsiung, Taiwan
Ching-Chia Chen, Hua-Cheng Chiang and Ji-Rong Chen
Department of Mechanical Engineering, Southern Taiwan University of Science and Technology, Tainan, Taiwan
Yi-Chin Chou
Kuang Tai Metal Industrial Co. Ltd., Kaohsiung, Taiwan
Tsung-Yuan Kuo
Department of Mechanical Engineering, Southern Taiwan University of Science and Technology, Tainan, Taiwan
Kei Ameyama
Department of Mechanical Engineering, School of Science and Engineering, College of Science and Engineering Graduate,
Ritsumeikan University, Kusatsu, Japan, and
Cheng-Hsin Chuang
Institute of Medical Science and Technology, National Sun Yat-sen University, Kaohsiung, Taiwan
Abstract
Purpose Using wire as feedstock has several advantages for additive manufacturing (AM) of metal components, which include high deposition
rates, efcient material use and low material costs. While the feasibility of wire-feed AM has been demonstrated, the accuracy and surface nish of
the produced parts is generally lower than those obtained using powder-bed/-feed AM. The purpose of this study was to develop and investigate the
feasibility of a ne wire-based laser metal deposition (FW-LMD) process for producing high-precision metal components with improved resolution,
dimensional accuracy and surface nish.
Design/methodology/approach The proposed FW-LMD AM process uses a ne stainless steel wire with a diameter of 100
m
m as the additive
material and a pulsed Nd:YAG laser as the heat source. The pulsed laser beam generates a melt pool on the substrate into which the ne wire is fed,
and upon moving the XY stage, a single-pass weld bead is created during solidication that can be laterally and vertically stacked to create a 3D
metal component. Process parameters including laser power, pulse duration and stage speed were optimized for the single-pass weld bead. The
effect of lateral overlap was studied to ensure low surface roughness of the rst layer onto which subsequent layers can be deposited. Multi-layer
deposition was also performed and the resulting cross-sectional morphology, microhardness, phase formation, grain growth and tensile strength
have been investigated.
Findings An optimized lateral overlap of about 60-70% results in an average surface roughness of 8-16
m
m along all printed directions of the
XY stage. The single-layer thickness and dimensional accuracy of the proposed FW-LMD process was about 40-80
m
m and 630
m
m, respectively.
A dense cross-sectional morphology was observed for the multilayer stacking without any visible voids, pores or defects present between the layers.
X-ray diffraction conrmed a majority austenite phase with small ferrite phase formation that occurs at the junction of the vertically stacked beads,
as conrmed by the electron backscatter diffraction (EBSD) analysis. Tensile tests were performed and an ultimate tensile strength of about
700-750 MPa was observed for all samples. Furthermore, multilayer printing of different shapes with improved surface nish and thin-walled and
inclined metal structures with a minimum achievable resolution of about 500
m
m was presented.
Originality/value To the best of the authorsknowledge, this is the rst study to report a directed energy deposition process using a ne metal
wire with a diameter of 100
m
m and can be a possible solution to improving surface nish and reducing the stair-steppingeffect that is generally
observed for wires with a larger diameter. The AM process proposed in this study can be an attractive alternative for 3D printing of high-precision
metal components and can nd application for rapid prototyping in a range of industries such as medical and automotive, among others.
Keywords Additive manufacturing, Directed energy deposition, Fine wire, Pulsed laser
Paper type Research paper
The current issue and full text archive of this journal is available on
Emerald Insight at: www.emeraldinsight.com/1355-2546.htm
Rapid Prototyping Journal
© Emerald Publishing Limited [ISSN 1355-2546]
[DOI 10.1108/RPJ-04-2019-0110]
The authors are grateful for the nancial support provided by Kuang Tai
Metal Industrial Co., Ltd. for this research work.
Received 21 April 2019
Revised 6 July 2019
1 September 2019
Accepted 22 October 2019
1. Introduction
Additive manufacturing (AM), also popularly known as 3D
printing, involves building up complex structures using layer-
by-layer material deposition, and the history of AM has been
comprehensively reviewed by Wohlers and Gornet (2014).
Metal AM offers the possibility to fabricate single-component
metallic structures with complex shapes, which is generally not
possible using conventional machining techniques. This makes
AM attractive, especially when dealing with metals and alloys
that are expensive or have a high hardness. Most research
efforts in the area of metal AM have traditionally focused on
powder bed processes such as selective laser melting (SLM)
and electron beam melting (EBM), where a laser or electron
beam is used to selectively melt metal powder as in the works of
Heinl et al. (2008);Mumtaz and Hopkinson (2009,2010); and
Yap et al. (2017), among others. Using powder as feedstock
material has advantages because of the ability to fabricate
functionally graded materials with high dimensional accuracy
and improved surface nish. For example, Zhu et al. (2003)
fabricated metal parts using a powder bed process which
achieved dimensional accuracy and surface roughness of
60.05 mm and 14-16
m
m, respectively. However, there are
drawbacks of this technology that include low deposition rates,
higher material costs and porosity-related issues. In addition,
degradation of powders containing metals such as titanium and
aluminum via oxidation reduces manufacturing quality as
shown by Slotwinski et al. (2014).
An alternate approach for metal AM is the use of metal wire
as opposed to powder as the feedstock material. Wire-based
metal AM falls under the category of directed energy deposition
(DED) processes, where a heat source is used to melt metallic
wire resulting in bead formation upon solidication. These
weld beads can then be stacked laterally and vertically to
fabricate a 3D metal component. Using wire instead of powder
has several advantages, which include higher deposition rates
[Taminger and Haey (2006) reported values up to 330 g/min
for stainless steel], efcient material use (because nearly 100
per cent of the wire can be potentially used) and ease of wire
production resulting in relatively lower material costs.
However, there is often a trade-off between deposition rate and
dimensional accuracy, and parts produced using wire-feed AM
generally have lower accuracy as compared to powder bed
processes that limit applicability to fabricating large parts with
moderate complexity. Further issues include a poor surface
nish because of stair-steppingeffects and an additional
machining process that is generally required for high-accuracy
parts. In addition, their achievable geometrical resolution is low
as compared to powder bed processes that take advantage of
small powder sizes (typically less than 50
m
m). Ding et al.
(2015) have comprehensively reviewed wire-feed AM
technologies and have discussed this trade-off issue between
metal parts fabricated by powder and wire-feed processes.
To achieve dimensional capabilities similar to powder bed
processes, it is important to control the diameter of the feed
wire because deposition dimensions are generally about 5-15
times larger than the wire diameter. Generally, wire diameters
around 1 mm have been utilized as in the works of Oliari et al.
(2017) and Hussein et al. (2008), which limits the achievable
geometrical resolution. While most research in the area of
wire-feed DED focusses on achieving higher deposition rates,
only a few works have been reported that use microwire-based
DED processes. Jhavar et al. (2014) proposed microplasma
transferred arc (
m
PTA) process using P20 tool steel wire with
diameter of 0.3 mm that can achieve a layer height of
0.5-0.7 mm and a wall width of about 2 mm. However, there
are issues with using arcs as the heat source and lasers provide a
better alternative in terms of control over the heat extension.
Recently, Demir (2018) developed a micro wire-based DED
process using a 0.5mm 301 stainless steel wire as feedstock and
pulsed laser (millisecond long pulses) as the heat source. The
use of pulsed wave as opposed to continuous wave lasers allows
better heat input regulation and the proposed process could
successfully manufacture thin-walled structures with aspect
ratios of 20 and width between 700 and 800
m
m. These results
highlight the feasibility of using thinner wires to achieve
improved geometrical resolution. Furthermore, microwire-
based DED processes do not require high-intensity lasers and
large-beam diameters, owing to the relative ease of melting
thinner wires.
Herein, we have developed a ne wire-based laser metal
deposition (FW-LMD) process using a 304 stainless steel wire
with a diameter of 100
m
m as the feedstock material and a
pulsed laser as the heat source. Our aim in utilizing a ne wire
was to further improve accuracy, resolution and surface nish
of the produced metal parts. The outline of the paper is as
follows:
The materials used, system specications and
experimental plan are summarized. The size dimensions
and morphology of single-pass weld beads using varied
laser power, pulse duration and stage speed were
investigated.
Lateral overlap studies were conducted to ensure low
surface roughness of the rst layer before deposition of
successive vertical layers.
Feasibility for printing along different directions was
investigated by moving the XY stage at angles ranging
from 0° to 90°, while the wire-feed direction was kept
constant parallel to the x-axis.
Multilayer deposition was performed and detailed
material characterizations including cross-sectional
morphology, microhardness, phase formation and tensile
strength analysis were performed.
Finally, 3D structures with different geometries (e.g. thin
and inclined walls) were fabricated using the proposed
FW-LMD process.
2. Materials and methods
2.1 Materials
The substrate used in this study was made of 316 L stainless
steel while the ne wire with a diameter of 100
m
m was made of
304 stainless steel and was provided by Kuang Tai Metal
Industrial Co., Ltd. (Taiwan). The chemical composition of
the substrate and the wire in wt. percentage is shown in Tables I
and II, respectively:
2.2 Fine wire-based laser metal deposition system
The FW-LMD system primarily consists of an automatic wire
feeder, a pulsed Nd:YAG laser heat source and a numerically
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Muhammad Omar Shaikh et al.
Rapid Prototyping Journal
controlled XY stage as shown schematically in Figure 1(a),
and the system specications are summarized in Table III. The
range of pulse duration and pulse repetition rate of the Nd:
YAG source was 0.1-20 ms and 1-100 Hz, respectively. The
maximum peak power available to the pulses was 6 kW with a
resulting maximum average power and pulse energy of 100 W
and 50 J, respectively. The maximum wire feed rate was
45 mm/s, and argon was used as the shielding gas. The system
architecture for the control software is shown in Figure 1(b)
and is composed of a servo control and laser control module
linked to the software through the integrated system. The
dedicated software controls all information regarding process
parameters and can be modied using open-source 3D printing
software such as Slic3r. Finally, the systematic protocol for
fabricating a 3D metal structure using the proposed FW-LMD
process is shown in Figure 1(c).
2.3 Experimental plan
Wire-based laser metal deposition processes are sensitive to
various process parameters, which include laser power, pulse
duration and frequency, wire position and incidence angle and
stage speed, among others. The combined effect of these
parameters controls the energy input and affects the deposition
rate, morphology and dimensions of the weld beads. Because
Table I Chemical composition of the 316 L stainless steel substrate
Element C Si Mn P S Ni Cr Mo N Cu
Wt.% 0.05 0.4 1.75 0.035 0.02 8.02 18.06 0.22 0.026 0.32
Table II Chemical composition of the ne 304 stainless steel wire
Element C Si Mn P S Ni Cr Mo N
Min. Wt.% 0.030 1.00 2.00 0.045 0.015 10.00 16.50 2.00 0.100
Max. 13.00 18.50 2.50
Figure 1 (a) Schematic of the FW-LMD system; (b) control software architecture; (c) schematic of the systematic protocol to fabricate a 3D metal
structure using the proposed system
Table III Main characteristics of the FW-LMD system
Parameter Value
Laser type Nd:YAG
Emission wavelength (k)1064nm
Max average power 100W
Max peak power 6kW
Max pulse energy 50J
Pulse voltage 100-450 V
Pulse duration 0.1-20ms
Pulse repetition rate 1-100Hz
Spot size 200-3000
m
m
Maximum wire feed rate 45mm/s
Wire diameter 0.1mm
Shielding gas Ar
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Muhammad Omar Shaikh et al.
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they inuence metal transfer between the solid wire and the
melt pool, careful tuning of the parameters is necessary for
stable deposition with good accuracy and surface nish. Laser
power, pulse duration and stage speed were the focus of this
study while all the other parameters were kept constant. To
prevent variations and improve stability, the wire was fed at an
angle of 20° to the x-axis and its position was regularly
monitored.
For the single-layer deposition studies, we printed single-
pass weld beads with varying laser power, pulse duration and
stage speed while all the other parameters were kept constant as
summarized in Table IV. The dimensions and morphology of
the resulting beads because of variations in the three chosen
parameters were characterized using a laser confocal
microscope (Keyence, VK-X200K). We have also deposited
single-pass weld beads next to each other, and the lateral
overlap between each bead was optimized by testing six
different overlap values ranging from 30 to 80 per cent.
Furthermore, we have also tested the feasibility of fabricating
the single-pass welding beads in different directions by moving
the XY stage between angles of 0° and 90°.
For multilayer deposition studies, we have fabricated ve
layers vertically stacked on top of each other, where each layer
consists of 18 single-pass weld beads stacked next to each.
Laser power and pulse duration were used as the variable
parameters while all other parameters were kept constant as
shown in Table V. The cross-sectional morphology of the ve-
layer coating was observed using an optical microscope
(Olympus, BX51M). Microhardness was analyzed using a
Vickers microhardness tester (Mitutoyo, HM-113) with a load
of 100 g (HV0.1) and measurements were made along the
cross-sectional plane at six points where each point was
separated by a distance of 70
m
m. X-ray diffraction (XRD,
Bruker, D8 Advance) was performed to identify phase
formation while electron backscatter diffraction (EBSD)
analysis was carried out using a scanning electron microscope
(JEOL 7000 FE SEM) to investigate the grain growth and
orientation. We have also performed tensile testing (Shimadzu,
AGS-10kND) using a test piece which was fabricated by rst
printing multiple layers stacked on top of each other followed
by laser cutting to obtain the required shape and dimensions.
Finally, the feasibility of the proposed system to fabricate
different shapes and geometries, including thin and inclined
wall structures, was tested.
3. Results and discussion
3.1 Single-layer deposition
3.1.1 Single-pass weld bead
The dimensions (width and height) and morphology of the
single-pass weld bead for the three varied parameters, namely,
laser power, laser pulse duration and the XY stage speed are
shown in Figure 2. It was observed that the weld bead width
increases by increasing the laser power from 43.5 to 51.4W,
while the height decreases. Increasing laser power produces a
larger melt-pool area that ows outwards before cooling, thus
resulting in the increased width. A laser power of 48 W
produces a weld bead that has a width and height of about 500
and 65
m
m, respectively. In addition, the observed morphology
of the weld bead shows a progressive decrease in the ripple
density with increasing laser power, thus resulting in increased
surface roughness.
As the laser pulse duration increases from 1 to 2.4 ms, the
weld bead width increases, while the height decreases. The
effect on the width and height of the weld bead by increasing
pulse duration is similar to that observed when increasing the
laser power. The increased pulse duration also causes stronger
melting of the substrate and a larger melt-pool area, which
results in increased width and reduced height. The surface
morphology of the weld bead also changes from smooth to
rough as the pulse duration increases, owing to a decrease in
ripple density.
As the XY stage speed increases from 1.5 to 5 mm/s, the
width of the weld bead decreases from about 520 to 480
m
m,
while the decrease in height is more pronounced from about
110 to 40
m
m. The relatively lower change in width as
compared to height can be related to the xed 0.2 mm laser
spot size, which limits the minimum melt-pool area and width
reduction at high stage speeds. Using a stage speed of 3 mm/s,
we observed continuous bead formation and relatively higher
ripple density as compared to faster stage speeds.
Consequently, for the lateral overlap and XY stage direction
studies, we used laser power, pulse duration and stage speed of
48 W, 1.6 ms and 3 mm/s, respectively, for which we obtain
single-pass weld beads with improved surface nish.
3.1.2 Lateral overlap
We have studied the effects of the lateral overlap of the single-
pass weld beads that are stacked next to each other to deposit
the rst layer. The lateral overlap refers to the width of the
single-pass weld bead that is overlapped during the deposition
of the next bead. The topography of the previous layer plays an
Table IV Fixed and varied parameters for single pass weld bead
deposition
Fixed parameters Level
Spot diameter, d
s
(mm) 0.2
Pulse repetition rate, PRR (Hz) 40
Wire feeding angle, a(°) 20
Wire feeding direction Front
Wire feed rate, WFR (mm/s) 7.85
Varied parameters
(single-layer deposition)
Levels
Average power (W) 43.5 48.0 49.0 49.9 51.4
Pulse duration (ms) 1.0 1.4 1.6 1.8 2.4
Stage speed (mm/s) 1.5 2.5 3.0 3.5 5.0
Table V Fixed and varied parameters for multilayer deposition
Fixed parameters Level
Spot diameter, d
s
(mm) 0.2
Pulse repetition rate, PRR (Hz) 40
Wire feeding angle, a(°) 20
Wire feeding direction Front
Wire feed rate, WFR (mm/s) 7.85
Stage speed (mm/s) 3.0
Varied parameters (multi-layer deposition) Levels
Average power (W) 43.5 48.0 49.0 49.9 51.4
Pulse duration (ms) 1.0 1.4 1.6 1.8 2.4
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important role in determining the quality of the subsequent
deposited layers. Optimization of the lateral overlap is critical in
achieving a smooth surface and minimizing surface roughness
of the deposited layer to prevent presence of gaps and defects
during multilayer stacking to create the 3D metal component.
The morphology of the laterally stacked weld beads using
different overlap percentages from 30 to 80 per cent is
shown in the confocal and optical microscopy images in
Figure 3. The average surface roughness (R
a
) value observed
at each overlap was measured by the confocal microscope by
scanning over a xed area (1.4 mm 1mm). The
relatively higher average surface roughness (R
a
)valuesof
23.29 and 43.28
m
m were observed for the lowest (30 per
cent) and highest (80 per cent) lateral overlap, respectively.
When an overlap of 50 per cent or less was used, two distinct
single-pass weld beads with a valley in the middle were
observed in the 3D image, resulting in R
a
values of 17.15
and 11.31
m
m at an overlap of 40 and 50 per cent,
respectively. However, as the lateral overlap increased to 60
and 70 per cent, the height difference between laterally
stacked beads reduced signicantly, thus resulting in a
smoother surface prole with a R
a
of 8.05 and 7.83
m
m,
respectively. Increasing the lateral overlap increases the
height of the deposited rst layer as shown in the cross-
sectional optical microscopy images. In this study, we have
chosen a lateral overlap of 70 per cent to minimize surface
roughness and presence of voids or defects as subsequent
layers are deposited.
Figure 2 The three varied parameters (a) laser power (b) pulse duration and (c) XY stage speed used for single-pass weld bead deposition and the
resulting (i) bead width and height and (ii) morphology as shown in the confocal microscopy images
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3.1.3 XY stage direction
We have also tested the feasibility of printing in different
directions by moving the XY stage at angles ranging from 0° to
90° while the wire-feed angle was kept constant along the
x-axis. Four single-pass weld beads, laterally stacked at an
overlap of 70 per cent, were printed for each tested angle (0°,
30°, 60° and 90°), as shown in the confocal and optical
microscopy images in Figure 4. It was observed from the
confocal microscopy images that the weld bead surface
morphology was not signicantly affected and the maximum
variation in the measured R
a
values for the four different angles
was about 8
m
m. Furthermore, the height and cross-sectional
morphology of the deposited layers were relatively similar as
shown in the optical microscopy images, thus conrming
feasibility for printing along angles ranging from 0° to 690°.
However, if the angle is larger than 90°, the deposition
efciency will be lower because the wire-feed direction is
opposite to the stage movement. A rotating wire-feed head or
rotating stage will be required to address this issue and enable
printing along all XY stage directions.
3.2 Multilayer deposition
3.2.1 Cross-sectional morphology and microhardness
During deposition of the ve-layer coating (1.2 0.4 cm), the
laser power and pulse duration were varied while all other
parameters including the stage speed were kept constant. The
image and cross-sectional morphology of the ve-layer coating
and the resulting microhardness of the substrate, heat-affected
zone (HAZ) and coating is shown in Figure 5. It can be seen from
the optical microscopy image in Figure 5(b) that the ve layers
are densely stacked on top of each other without any visible voids,
pores or defects present between the layers. The microhardness
value obtained for the substrate, HAZ and coating using varied
laser power and pulse duration is relatively similar as shown in
Figure 5(c) and (d). The substrate microhardness is in the range
of 160-170 HV0.1 while the microhardness of the HAZ between
Figure 3 (i) Top-view and 3D confocal microscopy images and (ii) cross-sectional optical microscopy images of laterally stacked weld beads using
different overlap percentages of (a) 30 per cent, (b) 40 per cent, (c) 50 per cent, (d) 60 per cent, (e) 70 per cent and (f) 80 per cent. A lateral overlap of 70
per cent results in the lowest average surface roughness (R
a
) of 7.83
m
m
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Figure 5 (a) (i) Cross-sectional and (ii) top-view image of the stainless steel substrate with the ve-layer coating that has a length and width of 1.2 and
0.4 cm, respectively; (b) the optical microscopy image of the cross section of the ve-layer coating. The microhardness analysis of the substrate,
heat-affected zone and the coated layers for two variable parameters, namely, (c) laser power and (d) pulse duration
Figure 4 (i) Top-view and 3D confocal microscopy images and (ii) cross-sectional optical microscopy images of four laterally stacked weld beads at
deposited angles of (i) 0°, (ii) 30°, (iii) 60° and (iv) 90°
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the substrate and the rst layer increases to a value of about
230-240 HV0.1. In a previous study, Dong et al. (2016) created a
308-316 L stainless steel weld joint and observed that presence of
residual stresses in the HAZ results in higher microhardness as
compared to the substrate. In addition, Cui et al. (2013)
performed laser welding of stainless steel, where the fast
solidication of the melted zone produced a HAZ with higher
microhardness as compared to the substrate. The results
obtained in this study are similar to those in the literature where
the rapid absorption of heat by the substrate results in increased
microstructural hardness as compared to the substrate. Among
the coated layers, the third layer has a slightly lower hardness
value of about 230 HV0.1 as compared to the outermost layers
which have a microhardness value of about 240 HV0.1. The
third layer, which is the intermediate layer, is heated up during
deposition of the fourth and fth layers. Because it is not directly
linked to the substrate, it cools down slower, which results in
coarsening of grains and a decrease in the observed
microhardness.
3.2.2 X-ray diffraction and electron backscatter diffraction
analyses
The XRD spectra of the cross section of the ve-layer
coating for variable laser power and pulse duration conrm
the presence of two primary phases of iron, namely, the
austenite (
g
-Fe) and ferrite (
a
-Fe) phase as shown in
Figure 6(a). The main phase is austenite as highlighted by
the peaks at 2
u
values of about 43.5°, 51.2°, 74.7° and
90.6°. Ferrite is the minor phase with peaks present at 2
u
values of about 44.5°, 64.5° and 82.0°. Similar XRD peak
positions for austenite and ferrite are reported in the work of
Wang et al. (2017) who studied the microstructure evolution
and performed EBSD analysis of graded steel fabricated
using laser-based AM. For EBSD analysis, we have included
austenite and ferrite as the constituent phases in all
microscopy imaging analysis. The processed EBSD images
shown in Figure 6(b) and (c) conrm the presence of a
majority austenite phase with a minority ferrite phase that
begins to develop at the junction between two vertically
stacked layers. Moreover, the ferrite phase content increases
with increasing laser power and pulse duration. Based on the
phase diagram of steel, the ferrite phase forms at
temperatures lower than about 900°C and has a body-
centered cubic (BCC) crystal structure, whereas the
austenite phase exists at higher temperatures going up to
1300°C and has a face-centered cubic (FCC) structure.
Because the top and bottom surface of the weld bead cools
faster than the center, ferrite phase formation occurs at the
junction of the vertical beads while the bulk of the bead has
an austenite phase as conrmed by the EBSD analysis.
During formation, ferrite phase rejects gamma-stabilizing
elements such as carbon that diffuse into the core of the bead
Figure 6 (a) The XRD spectra observed for varying values of (i) laser power and (ii) pulse duration. EBSD images of the cross section of the ve-layer
coating for (i) lower and (ii) upper values of (b) laser power and (c) pulse duration. The lower and upper values for laser power are 46.1 and 48.5 W and
that for pulse duration are 1.2 and 2.0 ms, respectively
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and thus promotes austenite phase formation. Increasing
the laser power and pulse duration increases the melt-pool
area, resulting in slower cooling and solidication. This
provides more time for diffusion of gamma-stabilizing
elements and results in increased ferrite formation at the
interlayer junctions.
3.2.3 Tensile test
To fabricate test samples for tensile testing, we rst stacked
multiple layers to form a cuboid (13 98mm
3
)using
varied laser power and pulse duration. Then each cuboid
was laser cut to form the test sample for tensile testing as
shown in Figure 7(a). The stressstrain curve obtained
during the tensile test for the two varied parameters is shown
in Figure 7(b). The stressstrain curve is relatively similar
for all tested samples and an ultimate tensile strength (UTS)
of about 750 MPa was observed, which is relatively high
compared to the UTS values for bulk 304 stainless steel that
commonly range from 540 to 750 MPa. The phase content
and grain size play an important role in determining the
strength and fracture toughness of the nal printed metal
object. It is known that carbon dissolves poorly in ferrite
phase (no more than 0.021 per cent by mass at 723°C) while
the austenite phase can dissolve considerably more carbon
(asmuchas2.04percentbymassat1,146°C).Thelower
carbon content in ferrite could result in lower hardness and
strength of the deposited layers. However, since only a very
small amount of ferrite is present and the majority phase is
austenite as previously shown in the XRD and EBSD
analyses, the tensile strength and elongation to failure of all
the samples is relatively similar.
3.3 3D printed structures
Some examples of 3D printed metal structures using the
proposed FW-LMD process are presented in Figure 8.
Different shapes and geometries could be achieved with an
improved surface nish and a resulting dimensional
accuracy of about 630
m
m. Furthermore, the use of a ne
wire enabled printing of thin-walled and inclined structures
with a minimum achievable geometrical resolution of about
500
m
m.
4. Conclusions
The main conclusions of this research work can be summarized
as follows:
The proposed FW-LMD process used in this study shows
feasibility for metal AM with improved dimensional
accuracy and surface nish. 3D printed metal structures
with different geometries, including thin and inclined wall
structures with minimum achievable resolution of about
500
m
m, were successfully presented.
Optimizing the lateral overlap resulted in low surface
roughness of the rst layer and enabled multilayer stacking
without presence of any visible voids, pores or defects
between the layers.
The observed layer thickness, dimensional accuracy and
surface roughness were 40-80, 630 and 8-16
m
m,
respectively, which are comparable to powder bed
processes while being improved as compared to existing
DED-based wire-feed processes, as shown in Table VI.
Owing to the ne dimensions of the wire, the maximum
deposition rate achievable was 2 g/min, which is lower
than that reported for wires with a larger diameter. This
Figure 7 (a) (i) Geometrical dimensions of the test piece. (ii) Image of the multilayer cuboid. (iii) Image of the test piece obtained by laser cutting of the
cuboid. (b) The stressstrain curves observed for the test piece fabricatedusing different (i) laser power and (ii) pulse duration
Additive manufacturing
Muhammad Omar Shaikh et al.
Rapid Prototyping Journal
issue can be addressed by using a programmable and
automated wire feeding system that feeds wires of
varying diameters depending on the accuracy and
surface nish required for different areas of the printed
metal object. This would enable optimization of both
dimensional accuracy and deposition rate.
The AM process proposed in this study shows feasibility
for fabricating high-resolution metal components and
promise for rapid prototyping applications in a range of
industries.
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Additive materials Process
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Figure 8 (a, b) Multilayer deposition of different shapes with improved surface nish; (c) thin vertical wall structure that tapers toward the tip. The
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Additive manufacturing
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Rapid Prototyping Journal
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Corresponding author
Cheng-Hsin Chuang can be contacted at: chchuang@imst.
nsysu.edu.tw
For instructions on how to order reprints of this article, please visit our website:
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Additive manufacturing
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Rapid Prototyping Journal
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Fabricating geometrical-feature parts using gas metal arc welding (GMAW) additive manufacturing with flat position deposition free from a turn table are proposed through deposition of inclined thin-walled components. During the fabrication process, the torch direction was always maintained perpendicular to the substrate. The definition of inclination angle was presented. Forces of a force model for a pending molten pool were analyzed. Influences of offset distance, wire feed rate, and travel speed on the inclination angle were revealed and discussed. The maximum inclination angle increases along with the travel speed and decreases along with the wire feed rate. A 104-layered cylindrical thin-walled part with geometrical features was fabricated by the proposed approach. Possible sources of fabrication error were presented.
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A series of Ti-6Al-4V wall structures were additively manufactured (AM) using directed energy deposition (DED) with similar processing parameters and build paths to investigate the role of geometry on the resulting as-deposited microstructure and mechanical properties.While the aggregated tensile strengths (1049 ± 37 MPa), yield strengths (936 ± 43 MPa), and elongations (18 ± 4%) were relatively consistent, a more in-depth statistical analysis revealed statistically significant relationships between the resulting mechanical properties and the orientation with respect to the build direction.Tensile samples with the long dimension parallel to the substrate exhibited a higher average tensile strength than samples with the long dimension perpendicular to the substrate.In addition, the tensile strengths from thick multi pass wall structures were significantly higher than thin single pass wall structures.Finally, the tensile strengths decreased with increasing height above the substrate within the wall structures.Most of the observed differences in mechanical behavior can be attributed to differences observed in the average prior β grain sizes and shapes that impact the amounts of boundary strengthening within the structures.Qualitative differences within the microstructure were observed at different locations within individual and correlated with changes in tensile strength.
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Stress corrosion cracking (SCC) in the heat affected zone (HAZ) of a stainless steel 308L-316L weld joint in primary water of pressurized water reactor was investigated. Stress corrosion crack growth in the HAZ was observed in off-normal primary water chemistry with dissolved oxygen, but not in normal primary water chemistry with dissolved hydrogen. This suggests that it is unlikely a stress corrosion crack propagating in the HAZ could reach the fusion boundary and penetrate into the weld metal under normal primary water chemistry conditions. Microstructure analysis of the crack tip suggests that the SCC follows the slip-oxidation mechanism.