![The yield strength and ductility data of SLM/DMLS-AlSi10Mg and cast counterpart alloy (A360) from literature [11] [14] [15] [24] [27] [31] [53] [54] [49] [50].](https://www.researchgate.net/profile/Amir-Hadadzadeh/publication/326692966/figure/fig2/AS:657419424260096@1533752413172/The-yield-strength-and-ductility-data-of-SLM-DMLS-AlSi10Mg-and-cast-counterpart-alloy_Q320.jpg)
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1
Strengthening mechanisms in direct metal laser sintered AlSi10Mg:
comparison between virgin and recycled powders
Amir Hadadzadeh1, 2
†
, Carter Baxter1, Babak Shalchi Amirkhiz2, Mohsen Mohammadi1
1Marine Additive Manufacturing Centre of Excellence (MAMCE), University of New Brunswick, Fredericton, NB,
E3B 5A1, Canada
2CanmetMATERIALS, Natural Resources Canada, 183 Longwood Road South, Hamilton, ON, L8P 0A5, Canada
Abstract
Rod shaped samples of AlSi10Mg additively manufactured using recycled powder through
direct metal laser sintering (DMLS) process showed higher quasi-static uniaxial tensile strength in
both horizontal and vertical build directions than those of cast counterpart alloy. In addition, they
offered mechanical properties within the range of other additively manufactured counterparts.
TEM showed that the microstructure of the as-built samples comprised of cell-like structures
featured by dislocation networks and Si precipitates. HRTEM studies revealed the semi-coherency
characteristics of the Si precipitates. After deformation, the dislocation density increased as a result
of generation of new dislocations due to dislocation motion. The dislocations bypassed the
precipitates by bowing around them and penetrating the semi-coherent precipitates. Strengthening
of recycled DMLS-AlSi10Mg alloys manufactured in both directions was attributed to Orowan
mechanism (due to existence of Si precipitates), Hall-Petch effect (due to eutectic cell walls), and
dislocation hardening (due to pre-existing dislocation networks). Due to the slightly different
microstructure, the contribution of each strengthening mechanism was slightly different in
identical samples made with virgin powder.
Keywords: Direct Metal Laser Sintering (DMLS); Additive manufacturing; Transmission electron
microscopy (TEM); Dislocation; Strengthening mechanism.
†
Corresponding author: Email: amir.hadadzadeh@unb.ca; Tel: +1 506 458 7104; Fax: +1 506 453 5025.
2
1. Introduction
Metal additive manufacturing (AM) is a process that uses metallic powder or wire to build
a part based on a 3D model, layer by layer until completion [1]. This way of manufacturing is very
attractive in comparison to other possibilities because of its ability to accommodate complex
design and cost saving [2] [3]. Amongst the available AM processes, powder bed fusion (PBF) is
the most commonly-used [4]. In PBF, a focused energy source selectively sinters or melts layers
of powder bed on top of each other [5]. PBF-laser based process, which is known as selective laser
melting (SLM) or direct metal laser sintering (DMLS), has specifically received significant
attention in recent years [1] [6] [7] [8] [9] [10] [11] [12].
SLM/DMLS of Al-Si and Al-Si-Mg alloys have been involved in multiple studies in recent
years, since these alloys possess light weight, superior specific strength, and outstanding corrosion
resistance [13] [14] [15] [16] [17]. Li et al. [18] and Suryawanshi et al. [19] studied SLM of Al-
12Si alloy and reported concurrent improvement of strength and ductility compared to cast
counterparts, due to very fine microstructure resulted from SLM. Evolution of very fine and unique
microstructure during SLM of aluminum alloys is a result of very high cooling rates in the range
of 103-108 °C/s [11] [20] [21] [22]. Similarly, higher strength and ductility obtained for
SLM/DMLS-AlSi10Mg under quasi-static loading conditions, reported in various studies [15] [23]
[24] [25]. Wu et al. [26] attributed the high strength of SLM-AlSi10Mg to the very fine cell-like
structure of the alloy where fine Si precipitates and eutectic Si boundaries inhibited dislocation
motion during plastic deformation. While Chen et al. [27] attributed the high strength of SLM-
AlSi10Mg mainly to the Orowan strengthening mechanism, Li et al. [28] described the strength
increment as a result of both Orowan and Hall-Petch mechanisms. Higher strength of SLM-
AlSi10Mg under high strain rate loading conditions compared to the same cast alloy was reported
by Zaretsky et al. [29]. Similar to the quasi-static loading conditions, both Si precipitates and
eutectic Si contributed to dislocation motion inhibition and hardening mechanism during dynamic
impact loading of DMLS-AlSi10Mg [30].
To make the SLM/DMLS process more affordable to produce aluminum parts, it is
essential to develop knowledge on the possibility of recycling the unused powder and reusing it
[31] [32]. Since a limited fraction of the powder is consumed during SLM process, the powder
outside the consolidation area can be recycled and reused [33] [34]. On the other hand, the quality
3
of the products in terms of microstructure and mechanical properties should not be compromised
using the recycled powder, specifically for sensitive applications such as aerospace industry [31].
In addition, growing sectors such as marine, off shore oil and gas, and shipbuilding industries have
asked for less expensive ways to adopt AM, where one sustainable way is to employ recycled
powder for additive manufacturing of parts regardless of specific AM technology. Therefore, it is
imperative to study the effect of recycled powder on the microstructure evolution, strengthening
mechanism, and mechanical properties of the products fabricated using SLM/DMLS process.
To do so, horizontal and vertical DMLS-AlSi10Mg samples were built using recycled
AlSi10Mg powder. The samples were printed using the same process parameters and then
subjected to uniaxial tensile test under quasi-static loading condition. The microstructure of the
samples and deformation mechanism were then studied in detail using advanced electron
microscopy characterization techniques. A comparison of mechanical properties, strengthening
mechanisms, and microstructures of the samples made using recycled and virgin AlSi10Mg
powder was finally presented.
2. Experimental procedure
2.1. AlSi10Mg powder
AlSi10Mg powder provided by EOS GmbH was the subject of study in this work. After
one build cycle of DMLS process and removing the parts, the unused powder was sieved using a
sieving equipment provided by the EOS Company with 60 µm mesh size to prepare the recycled
powder. Recycled AlSi10Mg powder (one time used), without mixing with the virgin powder, was
used to additively manufacture the samples. In addition, for comparison purposes complementary
samples were also made using virgin (unused) AlSi10Mg powder. The chemical composition of
the virgin powder is listed in Table 1. Analysis of the chemical composition of the recycled powder
showed that the level of the major alloying elements did not change [31], which is consistent with
other studies on the recycled AlSi10Mg powder [32] [35] [36]. The recycled and virgin powders
possessed spherical or near-spherical morphology with an average size of 10 ± 8 µm and 9 ± 7
µm, respectively [31]. Two types of powders (virgin and recycled) were very similar in terms of
powder size distribution (PSD), since they were sieved before using. Similarity of PSD was
4
reported for the recycled AlSi10Mg in the previous studies [32] [36]; however, a slight increase in
the average size of the recycled powder is due to agglomeration during heating cycles in the first
DMLS process.
Table 1- Chemical composition of virgin AlSi10Mg powder used in this study
Element
Si
Mg
Fe
Al
Weight (%)
10.8
0.35
0.55
Balance
2.2. DMLS process
The samples prepared for this study were additively manufactured using an EOS M290
machine through DMLS technique with the specifications shown in Table 2. The laser power,
scanning speed, hatching distance, and powder layer thickness values are the same as the ones
used for virgin powder and it yields the least porosity and the best mechanical properties, where
they are listed in Table 3. The samples were manufactured under Ar-0.1%O2 atmosphere using the
strip scanning strategy where the laser beam was rotated 67 degrees between consecutive layers.
Table 2- EOS M290 machine specifications
Laser Type
Beam Spot Size
(µm)
Building plate dimensions (mm ×
mm × mm)
Preheat temperature
(°C)
400 W Yb-fiber
laser
100
250 × 250 × 325
200
Table 3- Process parameters used to build the samples
Laser Power (W)
Scan Speed (mm/s)
Hatch Distance (µm)
Layer Thickness (µm)
370
1300
190
30
To investigate the impact of building direction on the microstructure evolution and
mechanical properties of DMLS-AlSi10Mg, rod shaped samples with 120 mm height and 12 mm
diameter were manufactured in both vertical and horizontal directions (three samples in each
direction) as shown in Figure 1. The longitudinal axes in the vertical and horizontal samples were
parallel and perpendicular to the building direction, respectively. In order to acquire a deeper
understanding of the microstructure of recycled DMLS-AlSi10Mg alloy built in horizontal and
5
vertical directions, similar rod shape samples with the same geometry were fabricated using virgin
AlSi10Mg powder with the same process parameters [31].
Figure 1- Schematic of horizontal and vertical samples.
2.3. Transmission electron microscopy
Details of the microstructures of as-built and deformed samples were studied using an FEI
Tecnai Osiris TEM. The TEM facility was equipped with a 200 keV X-FEG gun. The precipitates
were analyzed using the Super-EDS X-ray detection system combined with the high current
density electron beam in the scanning transmission electron microscopy (STEM) mode. Spatial 1
nm resolutions were obtained during EDS elemental mapping using a sub-nanometer electron
probe. Electron transparent samples for TEM characterization were prepared using ion milling
method. Initial slices of samples were cut with < 500 µm using a diamond wafering blade followed
by punching of 3 mm diameter disks using a Gatan puncher. The disks were then polished to a
thickness of 80-90 µm followed by dimpling using alumina suspension down to about 10 μm at
sample center. The final ion milling of the dimpled disk was done using a Gatan 691 PIPS with
liquid nitrogen cooling at 5, 3, and 1 keV and gun angle of 4° for 130, 10 and 30 minutes average
time at each step respectively, until the perforation [37].
Microstructural analyses were performed perpendicular to the longitudinal axes of the rods
and tensile specimens after deformation, from the necking area. Such a configuration was chosen
to systematically study the deformation mechanism in planes normal to the tensile direction.
6
Therefore, the microstructure of the horizontal sample was studied in the y-z plane, where for the
vertical sample the microstructure was analyzed in the x-y plane (see Figure 1 for planes).
2.4. Uniaxial tensile test
Mechanical properties of the vertical and horizontal samples were evaluated at room
temperature under quasi-static tensile loading. Cylindrical tensile samples (with a gauge length of
24 mm and a diameter of 6 mm) were machined from the as-built rods based on ASTM E8-15a
standard [38]. The uniaxial tensile tests were conducted using an Instron Model 1332 universal
testing machine at a strain rate of 9×10-4 s-1. Three tensile tests were performed for each building
direction and engineering stress-strain curves were recorded.
3. Results & Discussion
3.1. Microstructure of as-built DMLS-AlSi10Mg
Figure 2 shows the STEM bright field (STEM-BF) microstructure of the as-built samples,
taken from the inside of the melt pools, since the overall structure of DMLS-AlSi10Mg is
dominated by the features inside the melt pools [18] [19] [39]. Similar to typical SLM/DMLS-
AlSi10Mg, recycled AlSi10Mg microstructure consisted of fine cell-like primary aluminum (α-
Al) developed, which then bounded by a continuous network of eutectic Si [27] [40]. Referring to
EDS elemental maps of Fe and Mg, some intermetallic phases formed at the cell boundaries, which
are mainly Al8FeMg3Si6 and Al3FeSi [30].
Microstructure of horizontal sample is featured by a combination of equiaxed and columnar
cells as the columnar cells evolved along the building direction due to epitaxial growth [41]. On
the other hand, equiaxed cells observed in the microstructure of vertical sample are the cross-
sections of the columnar dendrites, since the microstructure is studied in the x-y plane. The average
cell size (eutectic Si wall to wall distance) in horizontal and vertical samples is 0.57 µm and 0.55
µm, respectively. This is close to the observations made earlier for SLM-AlSi10Mg fabricated
using virgin powder [27].
7
(a)
(b)
Figure 2- STEM-BF microstructure of (a) vertical and (b) horizontal samples along with the
corresponding EDS elemental maps.
Details of submicron characteristics of primary Al dendrites are shown in Figure 3. The
unique feature observed inside the cells is the entangled network of dislocations interacting with
both the eutectic Si and Si precipitates, referring to the EDS elemental maps of Si [30]. These
networks evolved due to rapid solidification and associated thermal stresses during DMLS process
[42] [43] [44]. The number of dislocations observed in these two samples was different, where
higher number of free dislocations along with dislocation networks was presented in the horizontal
sample. Several locations on the vertical and horizontal TEM samples were studied to confirm this
observation.
By measuring the TEM foils thickness using electron-energy loss spectroscopy (EELS),
log ratio technique [45], and the method reported by Pesicka et al. [46] and Rojas et al. [47], the
dislocation density ( in m-2) was calculated for both samples. By conducting the measurements
over various locations, the dislocation density was calculated as 2.15 × 1014 m-2 and 1.41 × 1014
8
m-2 in horizontal and vertical samples, respectively. Different dislocation density values in the
horizontal and vertical samples were due to various heating and cooling cycles applied during the
building process. In fact, by changing the building direction, the geometry of the sample with
respect to the building plate and the contact area between the sample and plate changed
significantly. Therefore, thermal boundary conditions and thermal gradients changed in a way that
the dislocation density varied between the horizontal and vertical samples [48].
(a)
(b)
(c)
(d)
Figure 3- STEM-BF microstructure of (a) vertical and (b) horizontal samples. The
corresponding EDS elemental maps of Si are shown in (c) and (d). All images were taken with
the beam direction along the low-index zone axis orientation of Al.
To further investigate the Si precipitates in the Al matrix, high resolution TEM (HRTEM)
of these precipitates were investigated as shown in Figure 4. Fast Fourier transform (FFT) patterns
of the matrix and precipitates are also shown in the same figure. Comparison between the FFT
patterns of the matrix and precipitates in both vertical and horizontal samples shows that there is
an orientation relationship between the precipitates and the matrix. However, it was observed that
9
the interface between the precipitates and the matrix was not fully coherent implying semi-
coherent precipitates for both vertical and horizontal recycled DMLS-AlSi10Mg alloys.
The size of precipitates in different locations on the TEM foils for both samples were
studied afterwards, where the average representative precipitate size in the horizontal and vertical
samples was 61 nm and 65 nm, respectively. This is in the range of coherent nano-Si precipitates
size reported by Li et al. [28] using virgin AlSi10Mg powder; however, the precipitates in the
current study were not coherent with the matrix for both vertical and horizontal samples. This is
mainly due to specific thermal boundary conditions applied during DMLS process in the current
study (which are different from those applied in [28]), where a semi-coherent interface between
the Si precipitates and the Al matrix evolved. By changing the thermal boundaries and heating-
cooling cycles, the precipitates could be exposed to high temperatures for longer time, which then
leads to evolution of semi-coherent interface.
(a)
(b)
Figure 4- HRTEM images of Si precipitates in (a) vertical and (b) horizontal samples along
with the corresponding FFT patterns of matrix (box 1) and precipitates (box 2).
10
3.2. Mechanical properties
Typical engineering stress-strain curves of recycled DMLS-AlSi10Mg built in horizontal
and vertical directions under uniaxial tensile loading are shown in Figure 5, where the two samples
exhibited consistent flow behavior. However, the main difference between them is the tensile
elongation. While the horizontal sample elongated up to ~7.2%, the vertical sample failed at a
strain of ~5.5%. Ductility of DMLS components is sensitive to defects such as voids and cracks
rather than microstructural characteristics [44]. Evolution of such defects is strongly dependent on
the processing conditions. Since both samples were fabricated using the same process parameters,
it seems that the configuration of defects was different in the two samples so that the vertical
sample was more prone to fail at lower fracture strains [31]. Yield strength (YS) and ultimate
tensile strength (UTS) of the horizontal sample was 235 MPa and 386 MPa, respectively. The
vertically built sample possessed YS and UTS of 210 MPa and 392 MPa, respectively.
Tensile tests of recycled DMLS-AlSi10Mg revealed that both YS and elongation of the
vertical and horizontal samples are higher than those of cast counterpart alloy (for mechanical
properties of die cast A360 from see [49] [50]), as shown in Figure 6. Such an enhancement in the
mechanical behavior is due to the very fine microstructure of the DMLS alloy (cell size of <1 µm
in DMLS parts in comparison to typical 100-300 µm in A360 [51] [52]). In addition, YS and
elongation of SLM/DMLS-AlSi10Mg produced from virgin powder were summarized from
literature and shown in the same graph. The uniaxial tensile properties of recycled DMLS-
AlSi10Mg are within the range of those of virgin SLM/DMLS-AlSi10Mg, which indicates the
feasibility of producing AM parts from recycled powder without compromising the mechanical
properties. It should be noted that, the mechanical properties from literature reported in Figure 6
were resulted from using various process parameters including different laser power, powder layer
thickness, scan speed, and powder bed fusion machine.
11
Figure 5- Engineering stress-strain curves of the horizontal and vertical recycled AlSi10Mg
samples.
Figure 6- The yield strength and ductility data of SLM/DMLS-AlSi10Mg and cast counterpart
alloy (A360) from literature [11] [14] [15] [24] [27] [31] [53] [54] [49] [50].
3.3. Deformation behavior
Figure 7 shows the deformed microstructure of horizontal and vertical samples in STEM-
BF mode. The microstructure of deformed DMLS-AlSi10Mg in both directions is featured with
heavily entangled dislocation networks, developed toward the cell walls (eutectic Si) and in
12
interaction with the Si precipitates [55]. Obviously, dislocation glide during plastic deformation
resulted in the generation of new dislocations and increase of dislocation density [27] [30].
Therefore, an entangled network was developed. Moreover, the cell walls acted as barrier against
dislocation motion and hindered their further dynamics [44]. Referring to the EDS elemantal maps
of the samples, morphology of the eutectic Si networks did not change during plastic deformation.
Moreover, the dislocation density is higher at the vicinity of the eutectic Si walls [55].
Using the method described earlier, the dislocaton density was calculted for the deformed
DMLS-AlSi10Mg alloys in both directions. The calculated dislocation density was 2.4 × 1014 m-2
in the horizontal sample and 2.7 × 1014 m-2 in the vertical one. While the dislocation density in the
as-built horizontal sample was ~1.5 times higher than of that in the vertical sample, the post-
deformed dislocation density in two samples was very close. Under quasi-static cold deformation
of DMLS-AlSi10Mg, the dislocation density increased up to a near-saturation level, followed by
final fracture. Since the initial dislocation density in the vertical sample was lower, the new
dislocations generated more rapidly than the horizontal sample. As the dislocation density was
increased in the material and some dislocations were piled-up [56], a “back stress” developed in
the matrix and inhibited further motion and generation of dislocations [57] [58].
13
(a)
(b)
Figure 7- STEM-BF images of deformed (a) vertical and (b) horizontal samples along with the
corresponding EDS elemental maps.
Despite the similarity in the amount of the alloying elements in the virgin and recycled
powders and low propensity of recycled AlSi10Mg powder to absorb high level of oxygen [32]
[35] [36], the deformed microstructure was analyzed in terms of possible oxides, and the
corresponding oxygen and magnesium EDS maps are shown in Figure 8. As seen, the overall
oxygen in the microstructure is not significant; however, some local oxygen concentration is
observed in both samples. In both vertical and horizontal samples, it appears that oxygen tended
to form magnesium oxide. Presence of oxygen in the recycled AlSi10Mg powder is in the form of
thin films of MgAl2O5 compounds on the surface of the powders [32]. During the DMLS process
and in the presence of heat, it appears that magnesium oxides formed and distributed mainly at the
14
vicinity or along the eutectic Si walls. Presence of oxides can have a detrimental effect on the
elongation of the samples [35], with no significant effect on the strength.
(a)
(b)
Figure 8- EDS elemental maps of oxygen and magnesium corresponding to STEM-BF images
shown in Figure 7, (a) vertical and (b) horizontal samples. Oxides are marked by white circles
in the O map.
3.4. Strengthening mechanism
Existence of particles and precipitates in the matrix of alloys is a common mechanism of
strengthening, since they act as obstacles against the motion of dislocations [59]. Dislocations
bypass precipitates by Orowan looping, cross-slip or particle shearing (cutting/penetrating) [60]
depending on the characteristics of the precipitates. Since the Si precipitates in recycled DMLS-
AlSi10Mg alloys both in the vertical and horizontal directions are semi-coherent (Figure 4), a
misfit exists at the precipitate-matrix interface. Therefore, a misfit strain field was shared between
the particle and the matrix [61], which affected the precipitate-dislocation interaction.
Details of dislocation-Si precipitate interaction and how dislocations bypass the particles
are shown in Figure 9 for the vertical and Figure 10 for the horizontal sample. In the vertical
15
sample where the plastic deformation (elongation) was less than the horizontal sample (~5.5%
versus ~7.2%), an evidence of the early stage of dislocation-precipitate interaction can be observed
(Figure 9). In this stage, the dislocation bypassed the Si precipitates by bowing around them.
Similarly, in the horizontal sample, bypassing of dislocation from the precipitates initiated with
the bowing procedure as seen in Figure 10 (a). As deformation continued, the dislocation passed
the precipitate and bowed toward it (Figure 10 (b)). In some cases, bowing away from the
precipitates after bypassing them is possible as well.
Bowing of dislocation toward or away the precipitate is due to the offset position of the
dislocation slip plane from the precipitate’s center [62]. If the slip plane of the dislocation is
slightly below the precipitate center, the dislocation bows toward the precipitate after bypassing
and if the slip plane of the dislocation is slightly above the precipitate center, the dislocation bows
away from the precipitate after bypassing [62]. In the case that the precipitate center is on the slip
plane of the dislocation, the dislocation remains straight after bypassing [62]. Since no evidence
of dislocation loop formation around the precipitates after bypassing was observed, it seems that
the mechanism of dislocation bypassing in the current study for recycled DMLS-AlSi10Mg is
particle penetrating [62]. Both horizontal and vertical recycled DMLS-AlSi10Mg TEM foils were
scanned in multiple locations for other strengthening mechanisms and dislocation-Si precipitates
interactions, where solely the same mechanism reported in Figure 9 and Figure 10 was observed
through out the foils. Based on the observation in many locations, a simplistic schematic of
dislocation-Si precipitate interaction in recycled DMLS-AlSi10Mg for both vertical and horizontal
samples is proposed and shown in Figure 11, for the cases that the slip plane of the dislocation is
slightly below or above the precipitate center. Based on the proposed mechanism, the dislocation
moves in the matrix as a result of applying stress on the material. Once the dislocation reaches the
vicinity of the precipitate, the interaction between the dislocation and the particle initiates. Due to
the misfit stress around the semi-coherent precipitate, the dislocation does not remain straight as
it bypasses the particle. After bypassing, the misfit stress modifies the glide force in a way that the
dislocation either bows away or toward the particle, depending on the relative position of particle
center and the dislocation slip plane.
16
Figure 9- STEM-BF images of dislocation-Si precipitate interaction (bowing) in deformed
vertical sample.
(a)
(b)
Figure 10- STEM-BF images of dislocation-Si precipitate interaction in deformed horizontal
sample in (a) early stage of interaction (bowing) and (b) after bypassing.
17
Figure 11- Schematic interaction of dislocation-Si precipitate in DMLS-AlSi10Mg where the
slip plane of the dislocation is slightly (a) below and (b) above the precipitate center.
Considering the characteristics of the deformed microstructure of recycled DMLS-
AlSi10Mg (Figure 7-Figure 10), the strengthening mechanism in the material can be attributed to
Orowan mechanism (due to existence of Si precipitates), Hall-Petch effect (due to eutectic cell
walls), and dislocation hardening (due to pre-existing dislocation network) [63]. Therefore, the
yield strength () of both alloys can be calculated as follows
(1)
where is an internal friction stress. The Orowan mechanism in Al-Si binary system can
be estimated using Eq. (2) [28] written as follows
(2)
where is a material constant, is the shear modulus, is the Burgers vector, is the
diameter of Si precipitates, and is the volume fraction of Si precipitates. The Hall-Petch
strengthening in AM parts (by considering the cellular structure) is estimated using Eq. (3) [64]
given as below
18
(3)
where is a material constant and is the aluminum cell size. The dislocation hardening
is estimated using Eq. (4) [65], which can be written as
(4)
where is a material constant, is the Taylor factor, and is the dislocation density.
The parameters used in Eqs. (1)-(4) are all listed in Table 4. In addition, the calculated values of
each strengthening component and predicted yield strength of recycled DMLS-AlSi10Mg in two
directions are summarized in Table 5. The percent error between the calculated and measured yield
strength in horizontal and vertical samples was 5.5% and 2.4%, respectively. The good agreement
between the calculated and measured yield strength values implies the reliability of proposed
strengthening mechanisms to calculate the strength of DMLS_AlSi10Mg.
Table 4- Parameters used in Eqs. (1)-(4)
Parameter
Units
Value
Reference
/Friction stress
MPa
72
[66]
/Material constant
-
0.4*
[67] [68]
/Shear modulus of
Al matrix
GPa
26.5
[27]
/Burgers vector of
Al
nm
0.286
[27]
/Diameter of Si
precipitates
nm
61 (horizontal)-65
(vertical)
Current study
/Volume fraction
of Si precipitates
vol.%
2.5
[28]
/Material constant
MPa.m1/2
0.04
[69]
/Cell size
µm
0.57 (horizontal)-0.55
(vertical)
Current study
/Material constant
-
0.16
[70]
/Taylor factor
-
3.06
[27]
/Dislocation
density
m-2
2.15 × 1014
(horizontal)-1.41 ×
1014 (vertical)
Current study
* Value for semi-coherent precipitates.
19
Table 5- Calculated strengthening components in recycled DMLS-AlSi10Mg
(MPa)
(MPa)
(MPa)
(MPa)
Measured YS
(MPa)
Error (%)
Horizontal
80
53
17
222
235
5.5
Vertical
75
54
14
215
210
2.4
3.5. Comparison with virgin DMLS-AlSi10Mg
Using the same advanced microscopy techniques as described before, the microstructure
of the virgin DMLS-AlSi10Mg in both directions were then characterized. The overall morphology
of aluminum cells in the virgin DMLS-AlSi10Mg was similar to that of recycled DMLS-
AlSi10Mg. The virgin DMLS-AlSi10Mg microstructure was consisted of fine cellular aluminum
(α-Al) bounded by a continuous network of eutectic Si. However, the average cell size in the virgin
DMLS-AlSi10Mg alloy was higher than those in the recycled samples, where it was 0.77 µm in
the horizontal sample and 0.86 µm in the vertical one.
Moreover, the submicron characteristics of the microstructure were different in the virgin
DMLS-AlSi10Mg samples. Figure 12 shows the STEM-BF images of virgin vertical and
horizontal DMLS-AlSi10Mg samples. Similar to the recycled samples, networks of dislocation
developed in the microstructure of virgin DMLS-AlSi10Mg. However, the dislocation density in
virgin vertical and horizontal samples was 3.05 × 1014 m-2 and 1.14 × 1014 m-2, respectively. In
contrasts to the recycled samples, the dislocation density in virgin vertical sample was higher than
that of virgin horizontal sample.
Figure 13, shows the HRTEM images of Si precipitates in virgin DMLS-AlSi10Mg
samples in vertical and horizontal directions along with the corresponding FFT patterns of matrix
and precipitates. The Si precipitates in the virgin samples developed with different characteristics
in comparison with the recycled ones. As shown in Figure 13, while fine and coherent Si
precipitates formed in the virgin vertical sample, coarse and semi-coherent ones developed in the
virgin horizontal sample. The average diameter of Si precipitates in vertical and horizontal virgin
samples was 25 nm and 62 nm, respectively.
The main reason of differences between the submicron characteristics of virgin and
recycled DMLS-AlSi10Mg can be related to the particle size. The recycled powder possessed
slightly larger particle size than the virgin powder since the particles absorbed energy in the
20
previous DMLS process and slightly enlarged [31]. Since finer particles provide a larger surface
area, they can absorb more laser energy and consequently their working temperature and sintering
kinetics are higher [71]. On the other hand, the effective thermal conductivity of the powder bed
is dependent on the amount of gas in the inter-particle pores, which is controlled by the particle
size [72]. Finer particles will provide less pores and higher effective thermal conductivity. The
complex interaction between the thermal boundaries, sample geometry, working temperature and
sintering kinetics led to differences in the submicron characteristics of virgin and recycled DMLS-
AlSi10Mg. Consequently, the variation in the thermal boundaries alters the thermal cycles
experienced by the virgin and recycled samples, which then leads to evolution of semi-coherent
precipitates in the recycled and coherent ones in the virgin vertical sample.
(a)
(b)
(c)
(d)
Figure 12- STEM-BF microstructure of virgin (a) vertical and (b) horizontal samples. The
corresponding EDS elemental maps of Si are shown in (c) and (d). All images were taken
with the beam direction along the low-index zone axis orientation of Al.
21
(a)
(b)
Figure 13- HRTEM images of Si precipitates in virgin (a) vertical and (b) horizontal samples
along with the corresponding FFT patterns of matrix (box 1) and precipitates (box 2).
Due to more pronounce difference in the microstructure and submicron characteristics of
virgin and recycled vertical samples, the mechanical properties of virgin vertical DMLS-
AlSi10Mg were measured for further discussion. Figure 14 shows the comparison between the
uniaxial tensile stress-strain curves of virgin and recycled DMLS-AlSi10Mg in vertical
configuration. The test was performed under the same quasi-static strain rate for consistency and
comparison. The yield strength of the two samples was very close, where, a slight difference
between the UTS of the samples was observed. Moreover, the elongation of the virgin sample was
slightly higher than that of the recycled one. The virgin samples were made using a slightly finer
powder (with an average particle size of ~9 µm in comparison with the average particle size of
~10 µm for the recycled powder), which can lead to better mechanical properties both in terms of
ultimate strength and ductility. Moreover, finer particles possess better densification properties
[71]; therefore, it is very likely that the porosity percentage in the recycled DMLS-AlSi10Mg was
slightly higher than the virgin one, which then resulted in lower elongation. In addition, presence
22
of magnesium oxide particles in the microstructure of recycled AlSi10Mg (Figure 8) is another
reason for lower elongation of the recycled sample.
Despite the dissimilarities in the microstructure of virgin and recycled samples, the yield
stresses were similar, in the vertical sample. Using the strengthening mechanism equations
developed earlier the similarities in terms of yield strength can be described. The yield strength of
vertical virgin DMLS-AlSi10Mg was calculated, where comparable results to the recycled one
were obtained, as shown in Table 6. Competing strengthening mechanisms in the virgin and
recycled samples contributed at different levels (Table 5 and Table 6). However, the overall
strength of the virgin and recycled samples, resulted from the overall contributions was
comparable.
Table 6- Calculated strengthening components in virgin DMLS-AlSi10Mg
(MPa)
(MPa)
(MPa)
(MPa)
Vertical
73*
43
20
209
* -constant in Eq. (2) for coherent precipitates is 0.15 [68].
Figure 14- Engineering stress-strain curves of the recycled and virgin vertical samples.
23
4. Conclusions
In the current study, feasibility of fabrication of DMLS-AlSi10Mg components using
recycled powder was investigated. Using both virgin and recycled powders, rod shaped samples
of AlSi10Mg were additively manufactured through DMLS process in vertical and horizontal
directions. Both recycled vertical and horizontal samples exhibited superior mechanical properties
compared to the cast counterpart. Moreover, the strength and elongation of the recycled DMLS-
AlSi10Mg were in the range of other SLM/DMLS-AlSi10Mg counterparts. The followings were
the main findings of the current study:
The microstructure of the as-built recycled DMLS-AlSi10Mg samples in both
directions comprised cell-like structure (primary α-Al bounded by eutectic Si walls)
and featured by pre-existing networks of dislocations and semi-coherent Si
precipitates.
After tensile tests, the dislocation density in both microstructures was increased and
saturated to almost the same order due to the motion of dislocations and generation
of new ones. Consequently, heavily entangled networks of dislocations evolved in
the microstructures.
It was observed that the dislocations interacted with the eutectic Si walls and Si
precipitates. Analysis of the dislocations-Si precipitates interaction in recycled
samples revealed that the dislocations bypassed the precipitates by bowing around
them and penetrating the precipitates due to the semi-coherency of the precipitates.
Strengthening of recycled DMLS-AlSi10Mg was attributed to Orowan mechanism
(due to existence of Si precipitates), Hall-Petch effect (due to eutectic cell walls),
and dislocation hardening (due to pre-existing dislocation network).
Modeled yield strength of the recycled DMLS-AlSi10Mg samples (considering
different strengthening mechanisms) was in good agreement with the actual
measured one in both directions.
The comparison between the microstructure of virgin and recycled DMLS-
AlSi10Mg samples showed differences in terms of cell size, Si precipitate
characteristics, and dislocation density. However, the calculated strength of the
samples was comparable as confirmed by tensile test of virgin samples.
24
Acknowledgement
The Authors would like to thank Natural Sciences and Engineering Research Council of
Canada (NSERC) grant number RGPIN-2016-04221 and New Brunswick Innovation Foundation
(NBIF) grant number RIF2017-071 for providing sufficient funding to execute this work. The
authors would also like to acknowledge AMM for fabricating the DMLS samples, Dr. Mark
Kozdras at CanmetMATERIALS for facilitating the research and Catherine Bibby for TEM
sample preparations.
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