Journal of Mechanical Science and Technology 33 (12) (2019) 5731~5737
www.springerlink.com/content/1738-494x(Print)/1976-3824(Online)
DOI 10.1007/s12206-019-1116-1
An experimental study on microstructural characteristics and mechanical properties
of stainless-steel 316L parts using directed energy deposition (DED) process†
Jung Sub Kim1, Byoung Joo Kang1 and Sang Won Lee2,*
1
Department of Mechanical Engineering, Graduate School, Sungkyunkwan University, Suwon 16419, Korea
2
School of Mechanical Engineering, Sungkyunkwan University, Suwon 16419, Korea
(Manuscript Received July 10, 2019; Revised August 17, 2019; Accepted September 4, 2019)
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Abstract
We investigated the microstructural characteristics and mechanical properties of stainless-steel 316L parts fabricated by directed energy deposition (DED) process, which is one of the additive manufacturing (AM) technologies. In this research, the 316L parts were
fabricated by DED process by varying three process parameters: Laser power, scanning speed and mass flow rate of powder. A total of
eight experimental cases were sorted out, and the DED parts from each experimental case were characterized in views of composition,
defects, geometrical height, micro-hardness, friction and modulus. The analysis showed that the mechanical properties – micro-hardness,
friction and modulus – of the 316L parts can be maximized in the case of the low laser power (400 W), high scanning speed (10 mm/s)
and low mass flow rate of powder (10 g/min). In addition, the defects such as blowholes and cracks can be minimized under the condition of the low laser power (400 W) and low mass flow rate (10 g/min), respectively.
Keywords: Additive manufacturing; Directed energy deposition; Mechanical property; Microstructure; Stainless steel powder
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1. Introduction
Among various advanced manufacturing technologies, additive manufacturing (AM), which can also be referred to as 3D
printing, is one of popular technologies for fabricating components in a layer-by-layer fashion from computer-aided design (CAD) model data. It has the ability to build components
with geometric and material complexity that could not be
produced by subtractive manufacturing processes. Thus, AM
technology has recently attracted much attention in academia
and industry for challenging possibilities of manufacture of
functional parts with required properties and complexity as
well as an increase in application to which the parts are applied [1-5].
Since the 1980s, a variety of AM technologies have been
developed commercially. They include stereo lithography
(SLA), fused deposition modeling (FDM), selective laser sintering (SLS), laminated objective manufacturing (LOM),
powder bed fusion (PBF), directed energy deposition (DED),
and so on [6, 7]. These technologies use a wide range of materials, such as polymer, metals, ceramics and composites.
Technologies like SLA, SLS, FDM and LOM use various
*
Corresponding author. Tel.: +82 31 290 7467, Fax.: +82 31 299 4690
E-mail address: sangwonl@skku.edu
This paper was presented at ICMDT 2019, Shiroyama Hotel, Kagoshima, Japan,
April 24-27, 2019. Recommended by Guest Editor Haedo Jeong.
© KSME & Springer 2019
†
types of polymers and composites for producing prototypes
for product design and orthopedic applications in dental, cardiovascular and tissue engineering. PBF and DED are such
AM technologies which utilize metals, ceramics and composites for aerospace and automobile industries related to high
value-added business [8, 9].
Among them, DED employs a high intensity energy source
(such as laser, plasma, and electron beam) to create a melt
pool into which metal powder or wire is injected. The melt
pool follows a path to fill each layer to deposit a thin metallic
layer on a substrate. Several processes are categorized in DED,
including laser engineered net shaping (LENS), direct metal
deposition (DMD), laser-based metal deposition (LBMD) and
so on. In these layer-by-layer deposition processes, metallic
components, smart structures, and functionally graded parts
can be manufactured. Thus, DED has recently received significant attention in various industrial fields [10-14].
However, it is difficult to directly apply the DED process to
industrial fields at present due to a significant degradation in
dimensional accuracy and mechanical properties in produced
parts and components. While producing a component with
DED process, each region of a component being built is affected by an intricate thermal history according to process
parameters during melting and rapid solidification. This thermal history can create anisotropic and heterogeneous microstructure unlike components produced by formative and sub-
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tractive manufacturing processes. In addition, defects are frequently formed during DED process, which can be detrimental to mechanical properties and microstructures of a component. Therefore, many studies have been conducted to understand characteristics of the DED process associated with process parameters [15-17].
Zhang et al. performed orthogonal DED experiments combining with the ideal overlapping model using stainless steel
316L powder to ascertain the optimal processing parameters
[16]. They also studied the cooling and solidification rates
during the DED process, and the microstructure, composition
and phase of the fabricated components based on the comprehensive experiments. Wang et al. studied the effect of processing parameters on the microstructure and tensile mechanical
properties of the DED part using stainless steel 304L powder
[17]. In their study, the linear heat input could result in the
anisotropic and heterogeneous microstructure and tensile mechanical properties within the component. Dinda et al. investigated the microstructural evolution, thermal stability and mechanical response of the as-deposited Inconel 625 component
by DED process [18]. They showed that the DED process
could produce directionally solidified components when the
appropriate processing strategy is applied. Fujishima et al.
found the optimal depositing condition where fewer pores
were generated during the DED fabrication of the Inconel 625
component [19]. Sun et al. conducted research to find dominant DED process parameters that could have a great effect on
the geometry and dilution of the part [20]. Their study found
that the central level of a laser power and the low levels of a
laser scanning speed and a powder feed rate can create parts of
good quality. Cottam et al. studied the DED process using Ti6Al-4V powder and found that dendritic and martensitic microstructures could be formed at the slowest travel speed and
highest laser power [21]. Chua et al. conducted thermomechanical finite element analyses (FEAs) to study the influence of process parameters on distributions of temperature and
residual stress of the deposited bead and the substrate in case
of a single layer deposition using a Ti-6Al-4V wire feeding
type DED process [22]. Lee et al. investigated the effects of
the process parameters on the geometry of a single track that
was deposited by DED process using AISI M4 powder, and
they performed bead geometry prediction using response surface method (RSM) to provide optimal processing conditions
for manufacturing DED components with enhanced quality
[23].
In the above-mentioned previous researches on the DED
process, the microstructures and mechanical/physical properties of the parts according to various process parameters were
studied. In addition, it was found that the parts are strongly
affected by the thermal history, especially, heating and cooling
rates. However, it is still a major challenge to find the relationship between process parameters and various engineering
properties of DED parts and components for them to be used
in many industrial fields.
Thus, in this study, the characterization on parts fabricated
Table 1. The material compositions of powder and substrate.
Element
(wt.%)
Fe
Ni
Cr
Mo
Si
Mn
C
Powder
Bal.
11.76
16.61
4.71
2.73
0.38
0.01
Substrate
Bal.
-
-
-
-
0.7
0.45
Fig. 1. SEM images of gas atomized stainless steel 316L powders.
by the DED process using stainless steel 316L powder was
conducted in terms of microstructures and mechanical properties. To vary properties of the parts, several DED process parameters such as a laser power, a scanning speed and a mass
flow rate of power were changed during the design of experiments. After making several parts via the DED process, their
characteristics were analyzed in terms of the microstructure,
composition, height, micro-hardness, friction and modulus. In
addition, their relationships with the DED process parameters
were discussed.
2. DED experiments
2.1 Material
Stainless steel has been very popular in various industrial
fields including petroleum, nuclear and chemical industries
because of its high strength and good corrosion and wear resistance. Among various components used in the abovementioned industries, some components have complicated
features such as inner cavities, cooling channels, mesh structures and coating that are difficult to fabricate using conventional formative and subtractive manufacturing processes. In
this context, stainless steel 316L powder has been used in the
DED process for manufacture of relatively large and complex
components [24].
In this study, a gas atomized stainless steel 316L powder
(Metcoclad 316L, Oerlikon Metco) was used for the DED
experiments as a depositing material. It has spherical morphology with a particle size of approximately from 44 m m to
106 m m, as given Fig. 1. For a substrate, carbon steel was
used. The compositions of the powder and substrate are summarized in Table 1.
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Table 2. The experimental design.
Fig. 2. Photo of the entire experimental system for the DED process.
Case
Laser power
(W)
Scanning speed
(mm/s)
Mass flow rate of powder
(g/min)
1
400
2
10
2
400
2
30
3
400
10
10
4
400
10
30
5
700
2
10
6
700
2
30
7
700
10
10
8
700
10
30
Table 3. The experimental conditions.
Laser power
2.2 Experimental setup
A fiber laser having a maximum power of 750 W (750 W
CW Single Mode Fiber Laser, Wuhan Raycus Fiber Laser
Technologies) was used for the DED experiments. Its wavelength and beam diameter are 1080 nm and 3 mm, respectively. The laser source was connected to the coaxial DED
head through the optic head having a QBH fiber connector.
The DED head that was adjusted by the optic head to focus a
beam with a diameter of 300 m m was attached to the z-axis of
the CNC machine (DH-400-2Z, Harim Machinery), and as a
result, a continuous supply of energy source and powder to a
molten pool on the substrate was realized.
The powder was delivered into a molten pool through the
powder feeder (GTV-Powder Feeder RF, GTV). A delivery
gas carrying the powder was argon, and a shielding gas to
protect the parts from oxidation during the DED process was
nitrogen, respectively.
A dust collector (NB-20, Nanos) was also installed to collect the dispersed powders. In addition, the chiller (YRC-2000,
Yescool) was assembled to cool the laser and the DED head
that could be overheated. A photo of the entire experimental
setup is shown in Fig. 2.
2.3 Experimental design and conditions
In each experiment, the parts were fabricated by depositing
five layers on the substrate, and each layer had a rectangular
shape with an area of 10 x 20 mm. Two levels of each of three
independent process parameters including a laser power
(400 W, 700 W), a scanning speed (2 mm/s, 10 mm/s) and a
mass flow rate of powder (10 g/min, 30 g/min) were considered, and therefore, a total of eight experimental cases were
sorted out, as summarized in Table 2. The delivery (argon)
and shielding (nitrogen) gas flow rates were 14 and 6 l/min,
respectively. The nozzle distance from the DED head tip to
the substrate was 10 mm, and the overlapping percentage between two single tracks was 31 %. These conditions, summarized in Table 3, were determined by several preliminary experiments. In each experimental case, one part was made and
400, 700 W
Scanning speed
2, 10 mm/s
Mass flow rate of powder
10, 30 g/min
Powder
Stainless steel 316L
Powder diameter
44 ~ 106 m m
Substrate
Carbon steel
Flow rate of delivery gas
(argon)
14 l/min
Flow rate of shielding gas
(nitrogen)
6 l/min
Nozzle distance
10 mm
Overlap
31 %
the measurements of the above-mentioned properties were
repeated five times for each part.
2.4 Experimental analysis method
To measure various properties of the parts fabricated by the
DED process, the parts should be properly prepared. First, the
part for each experimental case was cut along the as-deposited
direction using a diamond grinding wheel (Mecatome-T-210,
Price), and then its sample was mounted, polished and etched.
The compositions of the samples were investigated using
the SEM and EDS (JSM-7001F and Jeol) with a voltage of
5 KV. Before using the SEM and EDS, a thin coating of Au
was applied to the samples to minimize surface charging. The
height and microstructure of the samples were investigated by
using a laser optical microscope (VK-X200, Keyence). It can
intensively observe defects and grains in the cross section.
Finally, the hardness, friction and modulus of the samples
were investigated using a nano-indenter (Nano Test Vantage
Platform, Micro Materials). The hardness and modulus were
measured with a standard Berkovich tip with a constant load
of 50 mN and an indenting rate of 20 m N / S, respectively.
The tip was applied to the cross section of each deposited
layer and total of five indents were repeated. In addition, the
friction tests were repeated five times for each sample with a
load of 0.2 mN and a length of 10 m m.
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Fig. 5. Measured heights of the DED parts for each experimental case.
Fig. 3. Measured composition ratios of the sample DED part.
tios of an Fe element in each layer of the DED parts and the
substrate did not change considerably despite the different
DED process conditions (eight experimental cases). This observation could be explained based on the rule of mixture.
3.2 Height
Fig. 4. Measured composition ratios of Fe element for each experimental case.
3. Experimental results and discussions
3.1 Composition
The composition, including types of elements and their proportions, is an important factor because it could significantly
influence the properties of the DED parts. Fig. 3 shows the
composition ratios of the elements that were shown in Table 1
along the as-deposited direction. In addition, the composition
ratios of an Fe element are shown in Fig. 4 for each experimental case.
As can be seen in Fig. 3, the elements of the stainless steel
316L powder were stabilized from the third layer of the sample DED part. At the first and second layers of the sample
DED part and the sub-surface region of the substrate, there
could be significant dilution between the stainless steel (DED
part) and carbon steel (substrate), and this region could be
defined by the intermixed area. Therefore, to obtain the DED
part having the same composition ratio of stainless steel 316L
powder, the portion included in the intermixed area should be
eliminated.
The graphs given in Fig. 4 indicate that the composition ra-
The height of the DED part is a physical property that can
be changed according to the different process conditions. The
measured heights of the parts fabricated by the DED process
for eight experimental cases are shown in Fig. 5.
As can be seen in Fig. 5, the part in case 6 was the highest
among eight parts. In this case, the numerical values of laser
power, scanning speed and powder flow rate were 700 W, 2
mm/s and 30 g/min, respectively. On the other hand, the
height of the part was the smallest in the case 3 with 400 W,
10 mm/s and 10 g/min, respectively. It means that the high
laser power, low scanning speed and high powder flow rate
could be advantageous to increase the deposition speed of the
DED part. In this process condition, the energy density could
be significantly increased, and the sufficient powder could be
provided. As a result, the DED process speed could be considerably increased.
3.3 Defects
It is generally required to produce high-density 3D printed
parts without defects such as blowholes and cracks during
DED process. These defects can be minimized by taking optimal process parameters. Fig. 6 shows the cross-sectioned
images of the DED parts that were cut along the as-deposited
direction for the eight experimental cases.
The photos of the healthy DED parts are given in Figs. 6(a)
and (c), and they do not have any blowholes and cracks. On
the other hand, blowholes are observed in Figs. 6(e)-(h),
which are the cases for a laser power of 700 W. In particular,
many more blowholes are observed in Figs. 6(e) and (f).
These cases had the high laser power (700 W) and the low
scanning speed (2 mm/s), which could result in a very high
energy density. This condition might result in an excessive
melting of powders and their rapid vaporization. Thus, more
J. S. Kim et al. / Journal of Mechanical Science and Technology 33 (12) (2019) 5731~5737
(a) 400 W, 2 mm/s, 10 g/min
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(b) 400 W, 2 mm/s, 30 g/min
Fig. 7. Measured micro-hardness of the DED parts for each experimental case.
(c) 400 W, 10 mm/s, 10 g/min
(d) 400 W, 10 mm/s, 30 g/min
(e) 700 W, 2 mm/s, 10 g/min
(f) 700 W, 2 mm/s, 30 g/min
Fig. 8. Measured friction of the DED parts for each experimental case.
(g) 700 W, 10 mm/s, 10 g/min
(h) 700 W, 10 mm/s, 30 g/min
Fig. 6. Cross-section images of the DED parts for each experimental
case.
gas could be generated and more blowholes could be trapped
in the deposited layers. On the contrary, the cases having the
high laser power (700 W) and the high scanning speed (10
mm/s) – Figs. 6(g) and (h) – might result in a reduced energy
density, and less gas could be trapped. Thus, the number of
blowholes could be reduced. Meanwhile, as can be seen in
Figs. 6(b), (d), (f) and (h), cracks are observed. These cases
had the high mass flow rate of powders (30 g/min), and therefore, entire powders were not melted, which could result in
lack of fusion. Thus, cracks could be generated. More severe
cracks are observed in Fig. 6(b), which was the case of high
mass flow rate, low laser power (400 W) and low scanning
speed (2 mm/s), respectively.
3.4 Micro-hardness, friction and modulus
The measured micro-hardness, friction and modulus of the
DED parts for each experimental case are given in Figs. 7-9,
respectively. As can be seen in the graphs given in those figures, it is known that they showed a similar pattern according
to the experimental cases. The maximum values of microhardness, friction and modulus could be obtained in the experimental case 3. In this case, the laser power, scanning
Fig. 9. Measured modulus of the DED parts for each experimental
case.
speed and mass flow rate of powder was 400 W, 10 mm/s, and
10 g/min, respectively. It could be assumed that these process
parameters are most appropriate to fabricate the DED part
with the best mechanical properties among the eight experimental cases.
While comparing such results with those given in previous
sections, case 3 showed the smallest height and no defects of
the DED part. Thus, it is concluded that sufficient melting and
fusion of the powders occurred in this case, and that physical
and mechanical properties of the DED part could become best.
4. Conclusion
The effects on DED process parameters on the microstructure and mechanical properties of the stainless-steel (316L)
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parts were experimentally studied. The DED experimental
system was developed by retrofitting the conventional 3-axis
CNC machine, and a series of DED experiments were conducted by sorting out eight experimental cases. For these experimental cases, two levels of a laser power, a scanning speed
and a mass flow rate of powders were considered.
In the analysis, the microstructural characteristics, such as
composition and defects, were studied, and it was found that
the elements of stainless-steel powder were stabilized from the
third layer of the DED parts. This means that dilution between
the substrate and the deposited layers disappear from the third
layer.
Meanwhile, it was found that low laser power (400 W) and
low mass flow rate of powder (10 g/min) could minimize
blowholes and cracks and that such process conditions could
produce healthy parts from the DED process. However, many
defects were observed in the cases of the high mass flow rate
of powder (30 g/min) and the low scanning speed (2 mm/s). In
particular, cracks could be generated at the low laser power
(400 W) due to lack of fusion, and the blowholes could result
from rapid vaporization of the powders and subsequent gas
generation.
The mechanical properties of the DED parts were also investigated in terms of micro-hardness, friction and modulus. It
was demonstrated that such mechanical properties could be
maximized in the case of low laser power (400 W), high scanning speed (10 mm/s) and low mass flow rate of powder (10
g/min). On the other hand, they could be decreased at high
laser power (700 W), low scanning speed (2 mm/s) and high
mass flow rate of powder (30 g/min). The analysis results
could be used for setting up the initial process conditions to
produce healthy stainless-steel parts for the DED process.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government
(MSIP) (NRF-2018R1A2A1A05079477, NRF-2015R1A2A1
A10055948).
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Jung Sub Kim received his B.S. in
Mechanical Engineering, Catholic University of Daegu, Korea, in 2014. He is
currently a Ph.D. candidate in Mechanical Engineering, Sungkyunkwan University, Suwon, Korea. His research
interests include metal and polymer
additive manufacturing and environmentally-friendly mechanical machining technologies.
5737
Byoung Joo Kang received his B.S. in
Mechanical Engineering, Gachon University, Korea, in 2017. He also received
his M.S. from Mechanical Engineering,
Sungkyunkwan University, Suwon,
Korea in 2019. His research interests
include smartization of manufacturing
processes and equipment. He recently
joined the Smart Factory Control Team at LG CNS in 2019 as
an Application Development Associate.
Sang Won Lee received his B.S. and
M.S. in Mechanical Design and Production Engineering from Seoul National
University, Korea, in 1995 and 1997. He
received the Ph.D. in Mechanical Engineering from University of Michigan in
2004. Dr. Lee joined the School of Mechanical Engineering at Sungkyunkwan
University in 2006 and is currently a Professor. His research
interests include additive manufacturing, environmentallyfriendly mechanical machining, smart manufacturing and
data-driven design.