Artic le Materials Science
Selective laser melting 3D printing of Ni-based superalloy:understanding thermodynamic mechanisms
Mujian Xia • Dongdong Gu • Guanqun Yu •
Donghua Dai • Hongyu Chen • Qimin Shi
Received: 6 March 2016 / Revised: 21 April 2016 / Accepted: 27 April 2016 / Published online: 20 May 2016
� Science China Press and Springer-Verlag Berlin Heidelberg 2016
Abstract A mesoscopic model has been established to
investigate the thermodynamic mechanisms and densifica-
tion behavior of nickel-based superalloy during additive
manufacturing/three-dimensional (3D) printing (AM/3DP)
by numerical simulation, using a finite volume method
(FVM). The influence of the applied linear energy density
(LED) on dimensions of the molten pool, thermodynamic
mechanisms within the pool, bubbles migration and
resultant densification behavior of AM/3DP-processed
superalloy has been discussed. It reveals that the center of
the molten pool slightly shifts with a lagging of 4 lmtowards the center of the moving laser beam. The Mar-
angoni convection, which has various flow patterns, plays a
crucial role in intensifying the convective heat and mass
transfer, which is responsible for the bubbles migration and
densification behavior of AM/3DP-processed parts. At an
optimized LED of 221.5 J/m, the outward convection
favors the numerous bubbles to escape from the molten
pool easily and the resultant considerably high relative
density of 98.9 % is achieved. However, as the applied
LED further increases over 249.5 J/m, the convection
pattern is apparently intensified with the formation of
vortexes and the bubbles tend to be entrapped by the
rotating flow within the molten pool, resulting in a large
amount of residual porosity and a sharp reduction in
densification of the superalloy. The change rules of the
relative density and the corresponding distribution of
porosity obtained by experiments are in accordance with
the simulation results.
Keywords Selective laser melting � 3D printing �Mesoscopic simulation � Thermodynamics �Densification � Porosity
1 Introduction
Nickel-based superalloys are known as an attractive can-
didate for many industrial applications, e.g., gas turbine
disks, rocket motors, spacecraft, etc., due to their excellent
hot resistance to oxidation, high strength and wear resis-
tance [1–4]. However, since superalloys have the high
abilities of self-hardening and retaining superior mechani-
cal properties at high temperatures, they are difficult to be
manufactured by conventional processing methods, caused
by severe tool wear damage, low material removal rate,
poor thermal property and surface integrity [5–8].
The newly developed selective laser melting (SLM)
additive manufacturing/3D printing (AM/3DP) technology,
due to the possibility to fabricate the geometrically complex
components by user-defined computer aided design (CAD)
data files without tools or molds, has been proved to be an
effective and economical method for processing Ni-based
superalloys [9–11]. The as-built components having high
dimensional precision, perfect surface quality and out-
standing performance can be achieved precisely byAM/3DP
[12–15]. A number of previous attempts concerning thewear
resistance, high-temperature oxidation and mechanical
properties of AM/3DP Ni-based superalloys have been
investigated systematically [16–18]. Some typical defects
M. Xia � D. Gu (&) � G. Yu � D. Dai � H. Chen � Q. ShiCollege of Materials Science and Technology,
Nanjing University of Aeronautics and Astronautics,
Nanjing 210016, China
e-mail: [email protected]
M. Xia � D. Gu � G. Yu � D. Dai � H. Chen � Q. ShiInstitute of Additive Manufacturing (3D Printing),
Nanjing University of Aeronautics and Astronautics,
Nanjing 210016, China
123
Sci. Bull. (2016) 61(13):1013–1022 www.scibull.com
DOI 10.1007/s11434-016-1098-7 www.springer.com/scp
such as pore [19] and residual stress [20] have been observed
in as-fabricated components, limiting its further applications
in the practical industrial fields. Porosity is regarded as a
commonly existed processing defect that is usually observed
in amajority of themetallic parts processed byAM/3DP [21,
22], which results in the poor densification level and/or other
mechanical properties [19, 23]. Many previous studies have
ascribed the porosity formation to the following phenomena
including powder denudation [24], collapse of key holes
[25], gaseous bubbles entrapment [26], incomplete re-melt-
ing of some local sites [27] and splash [28]. Actually, for the
bubbles migration and porosity formation, it is difficult to be
monitored or characterized within the dynamic molten pool
during AM/3DP. So far, only few previous researches have
been reported on this issue. For instance, Dai and Gu [29]
have established a newmodel to investigate the densification
behavior of Cu-based composites by considering the
migration and escaping of bubbles. Nevertheless, there still
lacks of a relatively clear and comprehensive understanding
of the physical mechanisms for AM/3DP of Ni-based
superalloys. It is therefore necessary to give a thermody-
namic investigation on porosity evolution and densification
behavior of AM/3DP-processed superalloys.
In this paper, the influence of linear energy density
(LED) of laser on the thermodynamic behavior, bubbles
migration behavior and densification mechanism of AM/
3DP processed Ni-based superalloy was numerically
studied by applying commercially computational fluid
dynamics (CFD) software. To further validate the accuracy
of the newly developed mesoscopic model and to acquire
the optimal SLM AM/3DP processing parameters to fab-
ricate components with higher densification, the relative
density of the parts predicted by the numerical simulation
was compared with the experimental results.
2 Modeling approach and experimental procedures
2.1 Physical model
During SLM processing, the interaction between laser
beam and powder in the shield gas ambient is extremely
complex [30, 31]. Regarding the migration of gaseous
bubbles and attendant densification mechanism of powder,
a schematic of SLM physical model is depicted in Fig. 1a.
Recently, the powder-scale model, i.e., the mesoscopic
model, has been established to study the porosity evolution
and surface morphology of titanium alloy during SLM
[23]. In our present study, the powder system, processed in
3D size of (400 9 180 9 50) lm3 (X-, Y-, Z-axis), contains
a mixture of Ni-based superalloy powder particles and
argon gaseous bubbles, as shown in Fig. 1b. A number of
powder particles (average diameter of 30 lm) were closely
packed in powder bed with a relative packed density of
powder up to 47 %, which is close to the theoretical packed
density of 55 % of the spherical powder particles [32].
2.2 Governing equations
Generally, the movement of melt fluid is followed by the
mass, momentum and energy conservation, which can be
written as follows [33]:
oqot
þ oðquÞoX
þ oðqvÞoY
þ oðqwÞoZ
¼ 0; ð1Þ
qoV~
otþ V~ � rV~
!¼ lr2V~�rpV~þMs � V~ þ F, ð2Þ
qoT
otþ V~ � rT
� �¼ r � ðj rTÞ þ SH; ð3Þ
where q, j, l and p represent density, thermal conductivity,
dynamic viscosity and pressure, respectively. rT is the
temperature gradient distributed along the 3D coordinates,V~
is the motion velocity of the melt,Ms is the mass source, and
F~ refers to the body force. SH is the source term of the energy
equation in the X, Y and Z axis and can be defined by
SH ¼ �qo
otDH þr � ðV~DHÞ
� �; ð4Þ
where DH is the latent heat of phase transformation.
Based on the volume of fluid (VOF) model applied in
this simulation, the volume fraction equation for i phase is
[34]
oaiot
þ v~ � rai ¼Saiqi
; ð5Þ
wherePn
i¼1 ai ¼ 1, ai represents the volume fraction of
i phase, n is the total number of phases.
2.3 Boundary conditions
The transient spatial temperature distribution T(x, y, z, t) is
time-dependent. Prior to SLM process, the initial condition
of the uniform temperature distribution throughout the
powder bed at t = 0 can be settled as
Fig. 1 (Color online) Schematic of SLM physical model (a) and the
3D model established for simulating SLM process (b)
1014 Sci. Bull. (2016) 61(13):1013–1022
123
T x; y; z; tð Þ=t¼0 ¼ T0; x; y; zð Þ 2 Sn; ð6Þ
where T0, which represents the ambient temperature, is
293 K.
The natural thermal conductivity of the boundaries sat-
isfies the following equation [35]:
KoT
onþ hcðT � T0Þ þ reðT4 � T4
0 Þ ¼ q, ð7Þ
where n is the normal vector of the surface, hc is the heat
transfer coefficient of natural thermal convection, e is the
emissivity and r is the Stefan–Boltzmann constant.
A laser source with a volumetric Gaussian distribution
can be written as [36]
q ¼ 9PA
R2pH 1� 1e3
� � exp �9ðx2 þ y2ÞR2 log H
z
� �0@
1A; ð8Þ
where P is the laser power, R represents the diameter of
laser beam, H is the penetrated depth of heat source, and
A is the effective laser energy absorption of the material
[37]. The value of A is settled as 0.8 in the earlier study
[38].
The nonlinear behavior of thermal conductivity and
specific heat due to temperature change and phase trans-
formation should be taken into consideration, as shown in
Fig. 2.
The numerical simulation is carried out by the Fluent
commercial finite volume method (FVM) package and the
applied parameters are given in Table 1.
2.4 Experimental procedures
The gas atomized pre-alloy powder of Inconel 718 Ni-
based superalloy, with a purity of 99.7 %, was used as the
raw material. The SLM AM/3DP apparatus consisted of an
YLR-500-SM Ytterbium fiber laser with a power of
*500 W and a spot size of 70 lm (IPG Laser GmbH). The
processing parameters used in the experiments were in
accordance with that applied in the numerical simulation. V
was settled as a constant of 400 mm/s, and P were preset at
77.5, 88.6, 99.8 and 110 W, respectively. Thus, four dif-
ferent LEDs of 193.7, 221.5, 249.5 and 275 J/m were
changed to vary the SLM processing conditions, and were
defined by
LED ¼ P=v. ð9Þ
The SLM-processed samples for metallographic
examinations were cut, ground and polished according to
the standard procedures, and then etched with an acid
solution containing H2O2 (3 mL) and HCl (10 mL) for 10 s.
An Olympus PMG3 optical microscope (OM) was used to
observe the microstructural features of the SLM-processed
specimens. The relative density was defined as the
ratio between the density of SLM-processed Inconel 718
Ni-based alloy and the one of bulk-form Inconel 718 alloy
(8,200 kg/m3). Based on the Archimedes’ principle, the
density of SLM-processed Inconel 718 Ni-based alloy could
be calculated by the ratio between its qualitymeasured by the
electronic balance and volume of displacing water. To
ensure the accuracy of the measurement, each test was
repeated at least three times and the average value was taken
as the final value of the relative density.
3 Results and discussion
3.1 Temperature distribution and molten pool
dimensions
The temperature evolutions along X-axis (Y = 0,
Z = 20 lm) at various LEDs are shown in Fig. 3a. The
profiles of temperature distributions demonstrated that the
temperature generally increased with enhancing LEDs, but
the magnitude of temperature enhancement relied on the
input LEDs. Especially, the temperatures, which were
Fig. 2 (Color online) Thermal conductivity and special heat capacity
of Ni-based superalloy (Inconel 718) at various operating
temperatures
Table 1 The as-used material properties and SLM processing
parameters [38]
Parameters Value
Density, q (kg/m3) 8200
Absorption of superalloy powder, A 0.8
Ambient temperature, T0 (K) 293
Powder layer thickness, d (lm) 50
Radius of laser beam, D (lm) 35
Hatch spacing, s (lm) 50
Laser power, P (W) 77.5, 88.6, 99.8, 110
Scanning speed, v (mm/s) 400
Sci. Bull. (2016) 61(13):1013–1022 1015
123
distributed along the X-axis value ranging from 150 to
290 lm in the vicinity of the laser beam center, elevated
obviously with LEDs. The higher temperature gradients in
the front side of the moving laser beam at a larger LED
could be clearly observed. This was attributed to the fact
that the powder particles absorbed more energy under the
action of a higher LED, leading to more thermal accumu-
lation accordingly. It was noted that the temperature evo-
lutions of all applied LEDs presented a sharp reduction
(dashed line ellipse marked in Fig. 3a) at the location in X-
axis of 262 lm. This position was filled with gas phase
without the sufficient feeding of the molten material
between the adjacent melted powder particles, resulting in
the abrupt temperature difference due to the variation of
their absorption and thermal conductivity.
The enlarged characterization is carried out to reveal the
maximum temperature along X-axis on the specific position
ranging from 180 to 300 lm, as displayed in Fig. 3b. It was
found that the maximum temperature at various LEDs
presented at the location of 276 lm, i.e., the center of the
molten pool. This position was not consistent with the real
center of the laser beam (X = 280 lm), and shifted slightly
towards the negative direction of X-axis with a lagging of
4 lm. The similar phenomenon has also been found in
SLM of Cu-based composites [29], which resulted from the
combinational effect of thermal accumulation and variation
of thermal conductivity caused by the rapid transition from
powder to melt [39]. In this instance, the as-irradiated
zones located in the rear of the moving laser beam were
subjected to heat frequently. The slopes of the temperature
distribution curves (i.e., representing the temperature gra-
dients) in the rear side of the moving laser beam were much
steeper than those in the front side of the laser beam, owing
to the more significant thermal accumulation of the pre-
viously solidified regions of the track compared to that of
the powder particles. Meanwhile, in the front side of the
scanning laser beam, the maximum temperature and tem-
perature gradient were enhanced with increasing the LED,
which eventually increased to the maximum values at an
elevated LED of 275 J/m. The commensurate thermal
stresses and thermal cracks tended to be formed in the as-
built parts due to the release of stresses [13]. Therefore, the
SLM processing with a reasonable LED favors the pres-
ence of the more homogeneous temperature gradients and
the lower residual thermal stress.
The distance above the liquid line along scanning
direction, which was considered as the length of the molten
pool, was enhanced with the increase of LEDs (varying
from 12.2 lm at 193.7 J/m to 94.5 lm at 275 J/m)
(Fig. 3b). This was ascribed to the fact that an increase of
LEDs resulted in a higher temperature during AM/3DP
process and accordingly an enlarged molten pool length,
demonstrating the significance of high laser power for
thermal accumulation effect [40].
With respect to the width of the molten pool, it is
defined by the temperature counters of the pool. The width
of the molten pool is strengthened by increasing the LED,
as depicted in Fig. 4. The minimum and maximum of
width of the molten pool were 31.2 lm (at 193.7 J/m) and
67.9 lm (at 275 J/m), respectively, caused by the thermo-
capillary flow (Marangoni convection) induced by the
temperature gradient [41]. The applied laser source with a
Guassian energy distribution is responsible for the
appearance of the large thermal gradients in the molten
pool. The surface tension gradient and Marangoni con-
vection are accordingly generated, which tend to pull the
molten materials from the center to the periphery of the
pool. At the edge of the pool, the molten materials sink
down under the gravity force and, subsequently, the fluid at
the bottom floats up to the surface again owing to the
buoyancy force caused by density difference [40, 42].
Therefore, the outward Marangoni convection presents in
the molten pool, which favors the transfer of the melt from
Fig. 3 (Color online) a Temperature distributions at various LEDs on
the scanning path along X-axis (Y = 0, Z = 20 lm), t = 700 ls.b The local enlargement of (a) from 180 to 300 lm
1016 Sci. Bull. (2016) 61(13):1013–1022
123
the center to the edge of the pool. The higher input LED is
applied, the more intensive outward Marangoni convection
is obtained. In this situation, the width of the molten pool is
enhanced with the increase of LEDs. Therefore, it is
apparent that the applied LEDs play a crucial role in
determining the width of the molten pool that is elevated
with increasing the LEDs.
As shown in Fig. 4, it is further observed that the
surface morphology of the molten pool varied with the
different LEDs. The temperature gradient exerted from
the surface (with a maximum temperature) to the inner
region (with a minimum temperature) of powder particles
at a relative low LED of 193.7 J/m. Meanwhile, due to the
high viscosity of the molten pool, it was difficult for the
melt to spread outwards fully, thereby producing the
rough surface with a limited integrity [40]. In this case,
although the maximum temperature of the particle surface
reached 1,905 K that was higher than the melting point of
Inconel 718 Ni-based alloy (1,609 K), the core zones of
powder particles might still melt insufficiently, producing
a limited amount of liquid and a discontinuous liquid
track (marked by dashed rectangle). As the LED increased
to 221.5 J/m, the powder particles melted completely and
the resultant lower viscosity was beneficial to the com-
plete spreading of the melt flow. Thus, the surface quality
was significantly enhanced. However, as the LED further
exceeded 249.5 J/m, the melt spread easily owing to the
sharply reduced viscosity. In this situation, the instability
of the molten pool was enhanced and the irregular-shaped
tracks tended to be developed on the top surface, thus
reducing the surface integrity significantly. On the other
hand, when a relatively higher LED was applied, the
material evaporation companying with melt splash would
occur. Under this condition, the distinct recoil pressure
tended to be generated due to material evaporation and
gas expansion, which also resulted in a poor surface
integrity [23].
Concerning the depth of the molten pool, it was ana-
lyzed quantitatively by the temperature counters on the
cross-sections of the molten pools as well as the definition
of molten pool width. From Fig. 5, it was observed that the
depth of the molten pool decreased with increasing the
LED. The outward Marangoni convection, as discussed in
the above section, favored to the transfer of the melt and
heat from the bottom to the top of the molten pool at a
higher LED. As a result, the depth of molten pool was
reduced from 28.3 lm at 193.7 J/m successively to the
minimum value of 23.6 lm at 275 J/m.
3.2 Thermodynamics behavior and bubble migration
The previous publications reported that the bubble
migration was significantly influenced by the melt flow
[29]. In the present study, the thermodynamic behavior
within the molten pool, which was caused by the tem-
perature gradient, is introduced to investigate the bubble
migration and the velocity vector plots of convection
under different LEDs are given in Fig. 6. The intensities of
convection were significantly enhanced on increasing the
LEDs, and the obvious thermo-capillary convection pat-
terns were present at the relatively high LEDs above
193.7 J/m. For a relative low LED of 193.7 J/m, the
temperature gradient was insufficient, accordingly weak-
ening the resultant intensity of convection with a speed of
1.2 m/s (Fig. 6a). In this situation, the bubble migration
was slowed down. A majority of the bubbles were retained
within the molten pool, caused by the lower speed of the
bubbles (Fig. 7a). The bubble migration and melt flow at a
relatively low LED were similar to those during SLM of
Cu-based composites in our previous study [29]. As the
LED increased to 221.5 J/m, the present larger tempera-
ture gradient and surface tension gradient were favorable
to the formation of Marangoni convection with a larger
velocity of 2.6 m/s (Fig. 6b). The outward Marangoni
convection not only accelerated the heat and mass transfer
within the molten pool [43], but also provided the possi-
bility for bubbles to arise from the bottom to the top
surface of the molten pool. In this case, most of the bub-
bles moved and escaped from the molten pool easily at a
higher migration speed. Therefore, a more remarkable
Fig. 4 (Color online) Temperature contours at different LEDs.
a 193.7 J/m, p = 77.5 W, v = 400 mm/s; b 221.5 J/m, p =
88.6 W, v = 400 mm/s; c 249.5 J/m, p = 99.8 W, v = 400 mm/s;
d 275 J/m, p = 110 W, v = 400 mm/s
Sci. Bull. (2016) 61(13):1013–1022 1017
123
densification response of the molten pool was achieved in
this situation.
On increasing LED to 249.5 J/m, the molten pool
absorbed more laser energy, and the resultant temperature
and surface tension gradient increased rapidly. The out-
ward Marangoni convection was presented in conjunction
with the small-scaled vortexes in the upper region of the
molten pool (Fig. 6c). Even though the speed of melt flow
was enhanced, the behavior of bubbles still tended to be
suppressed by the small-scaled vortexes in the upper
regions of the molten pool [44]. Consequently, a fraction of
bubbles would be entrapped in the pool (Fig. 7c), caused
by the rotational flow vortexes. The densification showed a
certain extent of decrease as compared with that obtained
at 221.5 J/m. Interestingly, as the LED further increased to
275 J/m, the higher temperature and surface tension gra-
dient at the bottom of laser-induced molten pool tended to
be formed with considerable laser energy input, leading to
the formation of the outward Marangoni convection toge-
ther with the outward and inward patterns of scaling-down
vortexes in the upper and bottom zones of the molten pool,
respectively (Fig. 6d). In this situation, most of gaseous
bubbles were entrapped in the center of the upper and
bottom zones of the molten pool by the outward scaling-
down vortexes and inward scaling-down vortexes
(Fig. 7d). On the other hand, there was a high possibility
that the gaseous bubbles in the initial powder bed were
trapped and dragged into the molten pool, thereby pro-
ducing the residual porosity in the ultimately SLM-pro-
cessed parts, if the dynamics of melt flow were enhanced at
a higher level [38]. Thus, the resultant density after solid-
ification was relatively low. It was inferred that under an
appropriate LED, the reasonable Marangoni convection
could be formed in the molten pool, which favored the
Fig. 5 (Color online) Depths of SLM-processed molten pools at different LEDs. a 193.7 J/m, b 221.5 J/m, c 249.5 J/m, d 275 J/m
Fig. 6 (Color online) Velocity vector plots and patterns of convec-
tion in the molten pool in X–Z cross-sectional view at various LEDs.
a 193.7 J/m, b 221.5 J/m, c 249.5 J/m, d 275 J/m
1018 Sci. Bull. (2016) 61(13):1013–1022
123
sufficient escape of most gaseous bubbles in the finally
solidified components.
3.3 Densification mechanism
In the two-phase VOF model, when the bubbles escaped
from the molten pool, the volume would be occupied by the
metallic phase and the resultant densification would be
accordingly enhanced. As depicted in Fig. 8, a lower tem-
perature (Fig. 3a) and attendant higher melt viscosity l,were present in the molten pool at a lower LED of
193.7 J/m. The bubbles were difficult to be removed from
the molten pool and thereafter a number of spherical or
ellipse-like porosity was formed in the bottom regions of
the molten pool (Fig. 8a), inducing a lower relative density
of 95.2 % (Fig. 9a). As the LED increased to 221.5 J/m, the
significantly decreased viscosity was beneficial to the for-
mation of the lower viscous drag forces, favoring the bub-
bles to escape from the molten pool easily. Therefore, the
molten pool was nearly free of porosity (Fig. 8b), achieving
a higher relative density up to 98.9 % (Fig. 9b). As the LED
further increased to 249.5 J/m, the bubbles tended to arise
easily and then trapped by the present vortexes. In this
situation, the bubbles collided with each other within the
vortexes. A fraction of small bubbles, which were trapped
by vortexes with a lower speed, was caught and entrapped
Fig. 7 (Color online) Schematics of gaseous bubbles movement within the molten pool at various LEDs. a 193.7 J/m, b 221.5 J/m, c 249.5 J/m,
d 275 J/m
Fig. 8 Residual porosity within the molten pool in the cross-sectional
view of X–Z at various LEDs. a 193.7 J/m, b 221.5 J/m, c 249.5 J/m,
d 275 J/m
Sci. Bull. (2016) 61(13):1013–1022 1019
123
by the larger bubbles (Fig. 8c), resulting in a lower relative
density of 93.8 % (Fig. 9c). When the LED reached
275 J/m, the bubbles were entrapped by vortexes at the
bottom and upper of the molten pool (Fig. 8d), respectively,
and the resultant densification was reduced sharply to
89.3 % (Fig. 9d), caused by the lower surface tension of
melt flow in this instance [45].
3.4 Experiment verification
The cross-sectional surface morphologies of AM/3DP
components under different LEDs were characterized by
OM, as illustrated in Fig. 10. It was obvious that the
microstructural features of the SLM-processed samples
exhibited distinct variations with the applied LEDs. At the
relative lower LED of 193.7 J/m, a huge amount of nearly
sphere-shaped porosity presented in the ultimate SLM-
processed Inconel 718 Ni-base alloy. The observed
porosities, which possessed with an average size of 25 lm,
were distributed uniformly in the most regions of the part
(Fig. 10a). Instead, as the LED increased to 221.5 J/m,
little porosity was observed from the SLM-processed part
(Fig. 10b). However, at an evaluated LED of 221.5 J/m,
the considerable irregular porosity with the average size of
140 lm clustered in some local regions (Fig. 10c). At an
even higher of LED of 275 J/m, the larger-sized porosity
with the average size of 300 lm was formed in the SLM-
processed component, indicating a clustering and growing
of the porosities during SLM processing (Fig. 10d).
The formation of the residual porosity is generally
ascribed to the convection pattern and velocity of the melt
flow within the pool. On increasing the applied LED from
193.7 to 221.5 J/m, the temperature gradient is
successively enhanced in the molten pool and the resultant
surface tension and associated Marangoni convection are
formed, favoring the bubbles to escape from the pool.
Thus, the SLM-processed part presents the better
microstructural features free of the residual porosity.
However, as the considerably high LED exceeding
221.5 J/m is used, the small-scale vortexes are formed in
some local regions of the pool, which tend to entrap the
bubbles in the pool. In this condition, the considerably
entrapped bubbles experience clustering, colliding, merg-
ing and coarsening within the vortexes. Accordingly, the
residual porosity presents with a larger size of 300 lmunder the elevated LED of 275 J/m.
The quantitative analysis of densification showed that
the relative density calculated by simulation had a good
agreement with that obtained by experiments, as depicted
in Fig. 11. As the LEDs increased from 193.7 to 221.5 J/m,
the relative densities of Inconel 718 Ni-based alloy
obtained by experiments and simulations were successively
strengthened to the maximum of 98.9 % ± 0.5 % and
96.0 % ± 0.5 %, respectively. Instead, a significant
decrease of both the relative densities was obtained as the
LED exceeded 221.5 J/m. Hence, it was reasonable to
conclude that the appropriate LED of 221.5 J/m was
favorable for SLM-processed Inconel 718 Ni-based alloy to
obtain a preferable relative density, owing to the escaping
of the bubbles. The further observation was that the relative
densities obtained from experiments were in accordance
with those of simulations.
4 Conclusions
In the present study, an effective 3D FVM physical model
was performed to investigate the thermodynamic behavior
Fig. 9 (Color online) Relative density contour plots of molten pool in
X–Z cross-sectional view at variable LEDs. a 193.7 J/m, b 221.5 J/m,
c 249.5 J/m, d 275 J/m
Fig. 10 Micrographs of SLM-processed nickel-based superalloy at
different LEDs. a 193.7 J/m, b 221.5 J/m, c 249.5 J/m, d 275 J/m
1020 Sci. Bull. (2016) 61(13):1013–1022
123
and bubble migration within the molten pool under various
LEDs during SLM of Ni-based superalloy. The densifica-
tion mechanism was discussed in detail. The simulation
results were validated by the experimental results that were
conducted under the same laser processing parameters as
those used in the simulation. The conclusions regarding the
temperature distributions, migration of bubbles and densi-
fication mechanism of the molten pool with different LEDs
were drawn as follows.
(1) The LEDs played a crucial role in determining the
temperature evolutions, dimensions and surface mor-
phologies of the SLM-processed Inconel 718 Ni-based
alloy. The temperature evolutions, length, width and sur-
face integrity of the molten pool were enhanced with the
increase of the LEDs. Instead, the depth of the molten pool
was decreased with enhancing LEDs. As the LED of
221.5 J/m was properly settled, the considerably high
surface integrity was obtained. (2) The bubble migration
behavior was significantly influenced by the applied LEDs.
Employing an insufficient LED of 193.7 J/m and attendant
high viscosity resulted in a low motion speed of melt flow,
which was unfavorable for the bubbles to remove from the
pool. In contrast, the bubbles floated up with a higher speed
and escaped from the molten pool at an optimal LED of
221.5 J/m. Unfortunately, although the bubbles floated up
at an even higher velocity with the LEDs exceeding
249.5 J/m, the considerable bubbles still were entrapped
within the molten pool by the present vortexes. (3) Under a
reasonable LED of 221.5 J/m, less residual porosity and a
considerably high relative density were obtained of the
SLM-processed Inconel 718 Ni-based alloy, due to the
escaping of numerous bubbles. (4) The evolution rules of
the relative density and the corresponding distribution of
residual porosity obtained by experiments were in accor-
dance with the simulation results.
Acknowledgments This work was supported by the National Natural
Science Foundation of China (51575267, 51322509), the Top-Notch
Young Talents Program of China, the Outstanding Youth Foundation
of Jiangsu Province of China (BK20130035), the Program for New
Century Excellent Talents in University (NCET-13-0854), the Sci-
ence and Technology Support Program (the Industrial Part), Jiangsu
Provincial Department of Science and Technology of China
(BE2014009-2), the 333 high-level talents training project
(BRA2015368), the Science and Technology Foundation for Selected
Overseas Chinese Scholar, Ministry of Human Resources and Social
Security of China, the Aeronautical Science Foundation of China
(2015ZE52051), the Shanghai Aerospace Science and Technology
Innovation Fund (SAST2015053), the Fundamental Research Funds
for the Central Universities (NE2013103, NP2015206 and
NZ2016108), and the Priority Academic Program Development of
Jiangsu Higher Education Institutions.
Conflict of interest The authors declare that they have no conflict of
interest.
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