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J. Laser Appl. 32, 022059 (2020); https://doi.org/10.2351/7.0000100 32, 022059
© 2020 Author(s).
Influence of laser beam profile on the
selective laser melting process of AlSi10Mg
Cite as: J. Laser Appl. 32, 022059 (2020); https://doi.org/10.2351/7.0000100
Submitted: 01 April 2020 . Accepted: 01 April 2020 . Published Online: 08 May 2020
Tim Marten Wischeropp, Hussein Tarhini, and Claus Emmelmann
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Paper published as part of the special topic on Proceedings of the International Congress of Applications of Lasers &
Electro-Optics (ICALEO<sup>®</sup> 2019)
Note: This paper is part of the Special Collection: Proceedings of the International Congress of Applications of Lasers
& Electro-Optics (ICALEO® 2019).
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Influence of laser beam profile on the selective
laser melting process of AlSi10Mg
Cite as: J. Laser Appl. 32, 022059 (2020); doi: 10.2351/7.0000100
View Online Export Citation CrossMar
k
Submitted: 1 April 2020 · Accepted: 1 April 2020 ·
Published Online: 8 May 2020
Tim Marten Wischeropp,
1
Hussein Tarhini,
2
and Claus Emmelmann
1,2
AFFILIATIONS
1
Fraunhofer Research Institution for Additive Manufacturing Technologies IAPT, Am Schleusengraben 14, 21029 Hamburg,
Germany
2
Institute of Laser and System Technologies (iLAS), Hamburg University of Technology, Am Schwarzenberg-Campus 1, 21073
Hamburg, Germany
Note: This paper is part of the Special Collection: Proceedings of the International Congress of Applications of Lasers &
Electro-Optics (ICALEO
®
2019).
ABSTRACT
Selective laser melting (SLM) offers great potential to manufacture customized and complex metallic parts. Major drawbacks that limit its
industrial application are the high cost of the process that is related to low process speeds and issues with reproducibility. One important
process parameter that has the potential to increase the reproducibility and speed of the process is the laser beam intensity profile. Since its
influence has not been sufficiently investigated, the goal of this study is to analyze the effect of the beam profile on the SLM process of
AlSi10Mg. Single tracks and density cubes are manufactured with different process parameters and two beam profiles (standard Gaussian
and Donut beam profiles) and analyzed with respect to appearance, the size of melt tracks, porosity, and the types of defect. The results
reveal several advantages of the Donut beam profile such as fewer defects and a significantly broader process window that promises a more
robust process.
Key words: additive manufacturing, selective laser melting, process optimization, beam shaping, beam profile
Published under license by Laser Institute of America.https://doi.org/10.2351/7.0000100
I. INTRODUCTION
Selective laser melting (SLM) is a layer-wise additive manufac-
turing (AM) technology capable of manufacturing fully dense and
high-quality metal parts by utilizing a focused laser beam to melt
and solidify a powder material according to the corresponding
3D-Computer Aided Design (CAD) model.
1,2
Due to the numerous
advantages of SLM compared with conventional manufacturing
technologies, its application has been rapidly increasing with
double-digit growth figures.
3
Major drawbacks limiting the applicability of the technology
today are high costs of the process that are directly related to the low
productivity of today’smachinesandissueswithreproducibility.
1,4–6
Current trends to increase the productivity are based on the optimiza-
tion of process parameters, the development of multilaser machines,
and automation with the goal of reducing the machine downtime.
1,7
Efforts to increase the reproducibility mainly focus on process
optimization and standardization. One major influencing parameter
that has not been investigated sufficiently is the laser beam profile.
The standard Gaussian laser beam profile used in machines today
leads to an imbalanced energy input. The high intensity at the center
provokes vaporization and spatter and limits the energy input. It is
reasonable to assume that a higher energy input and, therefore,
higher process speeds are feasible with different laser beam intensity
profiles such as Top Hat or Donut beam profiles. The lower intensity
of these profiles potentially reduces the vaporization and, therefore,
increases the maximum energy input. Additionally, the more even
energy distribution could lead to a more robust process.
Research on other laser material processes such as laser
welding and laser metal deposition demonstrates the potential of
beam shaping for process optimization.
8–10
But until now only
limited research has been carried out on this topic for SLM.
Roehling et al.
11
numerically investigated the potential of micro-
structural control by beam modulation. Zhirnov et al.
12
and
Journal of
Laser Applications ARTICLE scitation.org/journal/jla
J. Laser Appl. 32, 022059 (2020); doi: 10.2351/7.0000100 32, 022059-1
Published under license by Laser Institute of America
Metel et al.
13
investigated the effect of the beam profile on the
shape and microstructure of single tracks for different process
parameters for CoCrMo. Loh et al.
14
investigated the melt width
and depth for different process parameters for aluminum alloy
A6061 on volumetric specimens for Gaussian and uniform beam
profiles, revealing wider melt tracks and similar melt depth for the
uniform laser beam profile. This agrees with numerical investiga-
tions done by Wischeropp et al. in previous work.
15
Cloots et al.
16
compared the microstructure and crack formation of IN38LC
samples for Gaussian and Donut beam profiles demonstrating,
among other things, that less hot cracking occurs for the latter.
Although previous work has investigated the effect of beam
profiles, the laser spot diameter was not fixed. This limits the valid-
ity of the results, since the difference in the results can either come
from the beam profile or the beam size. Therefore, experiments are
necessary that keep the beam size constant and only vary the beam
profile to understand its influence.
II. EXPERIMENTAL PROCEDURE
In this study, the influence of two laser beam profiles (a stan-
dard Gaussian beam profile and a Donut beam profile) on the for-
mation of single tracks and density cubes is investigated for
different process parameters. Single-track experiments were per-
formed to get a basic understanding of the influence of the beam
profile on the melt pool size and shape and density cubes to under-
stand the influence on the multilayer SLM process.
A. Material and equipment
The experiments were done with material AlSi10Mg, the most
commonly used aluminum alloy in SLM.
17
The powder used had a
particle size distribution of 20–63 μm. The experiments were con-
ducted on an AconityLAB from Aconity3D with a Coherent
FL1000R 1 kW fiber laser and a customized optical bench that
allows an integration of beam-shaping optics in the optical path
(see Fig. 1).
19
The two different beam profiles that have been used are
shown in Fig. 2. To generate the Donut beam profile, a diffractive
optical element [M-Shaper from Holo/Or (Ref. 20)] was used. The
M-shaper is installed in the optical path of the laser beam between
the collimator and the focusing optics as shown in Fig. 1. The
M-shaper increases the size of the focused laser beam from 85 to
140 μm (1/e² beam width). To increase the diameter of the
Gaussian beam to an equivalent size, the laser beam was defocused.
For measurement and calibration of the laser power and the
laser beam shape and position along the zaxis, the 1000W-BB-34
thermal sensor, an SP928 camera, and a BeamWatch AM from
Ophir were used. For polishing the specimen, a Struers
Tegramin-30, and for the microscopic pictures for density analysis
an Olympus GX-51 light microscope were used.
B. Design of experiments
1. Single-track experiments
The effect of process parameters and beam profile on single-
track melt geometry was studied by remelting a bare aluminum
plate (AlSi10Mg). The use of powder was avoided for these experi-
ments due to the difficulties in applying single layers of powder
with a predefined thickness. Therefore, remelting of the solid plate
ensures reproducible results. It is reasonable to assume that a
powder layer will affect the melt pool size and shape to some
degree, but the authors are confident that the general behavior of
the melt pool will be similar with and without a powder layer. The
process parameters for the single-track experiments are shown in
Table I. Five tracks with a length of 22 mm were manufactured for
each parameter set to account for statistical variations of the
process. Afterward, the single tracks were analyzed by using micro-
scopic pictures (top view) and microsections (cross-sectional view).
Analyzing the top view reveals process instabilities such as balling
FIG. 1. Setup of optical bench used in the experiments (based on Ref. 18).
FIG. 2. Measured Gaussian and Donut beam profiles used for the experiments.
The maximum intensity of the Gaussian beam profile is 42% higher.
Journal of
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J. Laser Appl. 32, 022059 (2020); doi: 10.2351/7.0000100 32, 022059-2
Published under license by Laser Institute of America
and protrusions and the cross section enables the measurement of
width, depth, and shape of the melt pool.
2. Density cube experiments
Density cubes were manufactured additively with different
process parameters as shown in Table II.
One specimen as shown in Fig. 3 was manufactured for each
combination of process parameters. Afterward, the cubes were
removed from the build platform and embedded and polished to
investigate the porosity by light microscopy on one vertical micro-
section per cube. A magnification of ×50 was used, resulting in a
measured area of ∼5 × 6.6 mm
2
(1200 × 1600 px²). The density
measurement by microsections has the disadvantage that only one
layer is analyzed that might not be representative of the 3D cube.
As Spierings et al.
21
demonstrated the coefficient of variation (cv)
for high density cubes (>98%) is very low (cv ∼0.1%) for the poros-
ity analysis by microsections and comparable to the sensitivity of
Archimedean measurements, which is why no additional density
measurement was performed.
III. RESULTS AND DISCUSSION
In the following, the experimental results of the single-track
and density cube experiments are presented and discussed.
A. Results of the single-track experiments
As a first step, microscopic pictures from the top were taken
and analyzed for each single track. Depending on the appearance,
four different types of single tracks were identified as described in
the following and shown in Fig. 4.
•Type I—Homogeneous melt track: The melt track topology is
homogenous and forms a continuous track. This can be defined
as the desired shape for the single tracks.
•Type II—Marginal melting: The melt track topology is character-
ized by small and discontinuous melt tracks. This type of melt
track is the result of an energy input that is too low (e.g., low
laser power and high scanning speeds).
•Type III—Protrusion and depression: The melt track topology is
characterized by protrusions in the center and depression zones
at the side of the melt tracks. This type of track is a result of long
melt pools that come from an increased backward material flow
based on the Marangoni effect and vaporization pressure.
22
Due
to the Plateau–Rayleigh instability,
1,23
the melt pool forms a pro-
trusion that solidifies before it can break into single melt balls
(see type IV).
•Type IV—Balling: The melt track topology is characterized by
single melt “balls”along the melt track.
1,6
Due to the Plateau–
Rayleigh instability, single melt balls are formed for long melt
pool lifetimes and elongated melt pools.
6,23
TABLE I. Process parameters of a full-factorial experimental design for single
tracks.
Factor Level
Laser power (W) 200, 300, 400, 500, 600, 700, 800
Scanning speed (mm/s) 750, 1000, 1250, 1500, 1750, 2000
TABLE II. Process parameters of a full-factorial experimental design for density
cubes.
Factor Level
Laser power (W) 300, 400, 500, 600, 700, 800
Scanning speed (mm/s) 1000, 1500, 2000
Hatch distance (mm) 0.1, 0.15, 0.2
Layer thickness (mm) 0.05
Temp. build-platf. (°C) 200
FIG. 3. Overview of density cubes and their spacing on the build platform.
FIG. 4. Overview of different types of single tracks.
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J. Laser Appl. 32, 022059 (2020); doi: 10.2351/7.0000100 32, 022059-3
Published under license by Laser Institute of America
Figure 5 shows the results of the single-track top view anal-
ysis for the Gaussian and Donut beam profiles. The results
reveal several important facts: First, the region of the type I
melt tracks (homogenous melt track) is larger for the Donut
beam profile and exceeds the limits of the process parameters
tested. It can be expected that melt tracks of type I can be pro-
duced with higher scanning speeds and laser powers than the
ones tested. Second, homogeneous melt tracks were produced
with scanning speeds of up to 2000 mm/s for the Donut beam
profile, while the maximum speed that produced a continuous
melt track for the Gaussian beam profile was 1750 mm/s at
500 W. This could be an indication that higher scan speeds (and
hence productivity) are feasible with a Donut beam profile.
Third, the region where protrusion and depression are present
is significantly larger for the Gaussian beam profile, compared
with the Donut beam profile. Last but not least, no balling was
detected for the Donut beam profile, while it was present for
the Gaussian beam profile at high laser powers (600–800 W)
and high scanning speeds (1250–1750 mm/s). This can be
explained by more significant vaporization and higher tempera-
ture gradients that come along with the Gaussian beam profile.
Both lead to more elongated melt pools due to an increased
radial melt flow as described by Zhang et al.
22
This results in
the formation of protrusions and balling based on the Plateau–
Rayleigh instability.
6,22,24
Additionally, microsections of the single tracks were prepared
to analyze the width and depth of the melt tracks depending on the
process parameters and to identify possible defects such as porosity
or cracks within the single tracks. For a laser power of 200–400 W,
the cross sections of the melt tracks were barely visible. Therefore,
a reliable measurement of their width and depth was not possible
and they are excluded from the following analysis.
Table III and Figs. 6 and 7compare the appearance and the
width and depth of the single tracks for the Gaussian and Donut
beam profiles. One can see that the single tracks for the Donut
beam profile are wider and shallower than those for the Gaussian
beam profile and that defects such as cracks and protrusions are
present more for the Gaussian beam profile. On average, the width
of the single track for the Donut beam profile is 33% higher and
the depth is 36% lower. Additionally, it can be seen that the varia-
tion in melt pool width at high laser powers (700–800 W) is higher
for the Donut beam profile, but the variation in depth is lower. The
Donut profile also shows a slight linear decrease in depth with
scanning speed, whereas the Gaussian profile shows a significantly
stronger decline. This is also shown in Fig. 8, where the ratio
of melt pool width to depth is shown for different linear energy
densities (LEDs) as defined in Eq. (1),
1
LED ¼PL/vS, (1)
where P
L
is the laser power and v
S
is the scanning speed of the
laser beam.
For the Gaussian beam profile, the ratio varies between 1 and
4 and shows a decrease with an increase in LEDs. A width-to-depth
ratio smaller than 3 indicates that key hole welding is present,
25,26
which is an undesired effect in SLM since it provokes instabilities
in the process.
27,28
For the Donut beam profile, the width-to-depth
ratio varies between 3.5 and 6.2, and the fitting curve is almost
constant with an increase in LEDs. This indicates that even for
high LEDs the process remains in the heat conduction region for
the Donut beam profile (no key hole effect), since width-to-depth
ratios between 3 and 7 are reported for heat conduction welding in
the literature.
26
Overall, several advantages can be seen for the Donut beam
profile: The process window that produces stable single tracks is
larger, and stable single tracks can be produced at higher scan
speeds. In addition, fewer defects (cracks and protrusions) are
observed and the key hole effect is avoided.
B. Results of the density cube experiments
Two major advantages are expected from the optimized beam
profile: on the one hand a more robust process (in terms of a wider
process window) and on the other hand an increase in productivity.
To validate this assumption, microsections of the manufactured
cubes are prepared and the density and the type of defects analyzed
by light microscopy.
FIG. 5. Overview of the top view analysis of single tracks for the Gaussian and
Donut beam profiles.
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J. Laser Appl. 32, 022059 (2020); doi: 10.2351/7.0000100 32, 022059-4
Published under license by Laser Institute of America
1. Analysis of the effect of beam profile on process
window (stability of process)
Figures 9 and 10 show the density of the cubes for different
LEDs [Eq. (1)] and volumetric energy densities (VEDs) as defined
by Eq. (2),
1
VED ¼PL/(vShd), (2)
where P
L
is the laser power, v
S
is the scanning speed, his the hatch
distance, and dis the layer thickness.
From the results, it can be seen that the density of the cubes
manufactured by the Donut beam profile is higher for most process
parameters and that the amount of cubes with densities >99.5% is
significantly higher compared with the ones manufactured by the
Gaussian beam profile (36 compared with 26). This is especially
true for high LEDs and VEDs. Above an LED of 0.6 J/mm and a
VED of 100 J/mm³, none of the cubes manufactured with a
Gaussian beam profile has a density higher than 99.5%, while all
cubes manufactured with a Donut profile have a density above this
value. For low LEDs and VEDs, the difference between the results
for the two beam profiles is not as significant. It is interesting to
note that all cubes manufactured with a Donut beam profile and
laser powers above 500 W have a density above 99.5%, independent
of the scanning speed (between 1000 and 2000 mm/s) and hatch
distance (100–200 μm). This indicates a very broad process
window, while such a broad process window cannot be observed
for the Gaussian beam profile.
Table IV shows the microsections of the density cubes for dif-
ferent laser powers and scanning speeds for a hatch distance of
150 μm. It can be seen that on average the porosity is lower for the
Donut beam profile and that at low scanning speeds and high laser
powers (high LEDs and VEDs), gas pores can be identified in con-
siderable amount for the Gaussian beam profile, while they are
almost not present for the Donut beam profile. Gas pores are
directly related to the key hole effect and the results are, therefore,
in good agreement with the single-track experiments.
29
For low
laser powers and high scanning speeds (low LEDs and VEDs), a
lack of fusion porosity can be identified for both beam profiles.
Figure 11 visualizes the process windows for the Gaussian and
Donut beam profiles for the three different hatch distances. One
TABLE III. Comparison of single tracks for the Gaussian and Donut beam profiles for selected process parameters.
Journal of
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J. Laser Appl. 32, 022059 (2020); doi: 10.2351/7.0000100 32, 022059-5
Published under license by Laser Institute of America
can see that the process window with densities >99.5% is smaller
for the Gaussian beam profile than that for the Donut beam
profile. While the results are similar for low laser powers
(300–400 W), the density at laser powers between 500 and 800 W is
higher for the Donut beam profile. For the Gaussian beam profile,
the density gets lower with an increase in laser powers above
500 W for 100 and 150 μm hatch distance and above 600 W for
200 μm hatch distance due to an increase in gas porosity that is
directly linked to increase vaporization and the key hole effect as
can be seen in Table IV.
27,30
As stated before, all cubes manufac-
tured with a laser power above 500 W have a density >99.5% and
the highest densities of >99.75% are achieved at laser powers of
600–800 W for the Donut beam profile. A decrease in density with
an increase in laser power is not observed for the parameters
tested, since gas porosity is almost not present as a result of
reduced vaporization.
Overall, it can be stated that the process window that produces
high densities (>99.5%) is larger for the Donut beam profile than
FIG. 6. Melt pool width for the Gaussian and Donut beam profiles for different process parameters.
FIG. 7. Melt pool depth for the Gaussian and Donut beam profiles for different process parameters.
Journal of
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J. Laser Appl. 32, 022059 (2020); doi: 10.2351/7.0000100 32, 022059-6
Published under license by Laser Institute of America
that for the Gaussian beam profile. A larger process window is
more stable against variations of input parameters such as laser
power and scanning speed that can occur during the manufacturing
of parts, especially at the beginning and end of scan tracks.
Therefore, a more robust process seems to be feasible with the
Donut beam profile due to the broader process window. To validate
this, more experiments are necessary, especially to test the long-
term stability of the process and the reproducibility of the results.
2. Analysis of the effect of beam profile on the
maximum build rate of the process
Figure 12 shows the density of the cubes manufactured for
different build rates bas defined by Eq. (3),
1
b¼vShd, (3)
where v
S
is the scanning speed, his the hatch distance, and dis the
layer thickness.
From the results, one can see that for both beam profiles,
densities of >99.5% are achieved at the highest build rate of
20 mm³/s. Therefore, it is not clear if the Donut beam profile
FIG. 8. Melt pool width-to-depth ratio for different LEDs for the Gaussian (G)
and Donut (D) beam profiles with an exponential fitting curve.
FIG. 9. Density of cubes manufactured by using the Gaussian beam profile for
different LEDs and VEDs.
FIG. 10. Density of cubes manufactured by using the Donut beam profile for
different LEDs and VEDs.
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J. Laser Appl. 32, 022059 (2020); doi: 10.2351/7.0000100 32, 022059-7
Published under license by Laser Institute of America
TABLE IV. Comparison of microsections for different scanning speeds and laser powers for a hatch distance of 150 μm (G, Gaussian beam profile; D, Donut beam profile).
Journal of
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J. Laser Appl. 32, 022059 (2020); doi: 10.2351/7.0000100 32, 022059-8
Published under license by Laser Institute of America
also offers benefits in terms of productivity of the process for
AlSi10Mg. For the current experiments, a maximum build rate
of 20 mm³/s was used (a scanning speed of 2000 mm/s and a
hatch distance of 200 μm), and additional experiments with
higher build rates are necessary to investigate the maximum
feasible build rate for both profiles. Due to the broader process
window (see Fig. 11) and results from the single-track
experiments, the authors are confident that the Donut beam
FIG. 11. Process window for the Gaussian and Donut beam profiles for different hatch distances.
FIG. 12. Comparison of the density of cubes produced by the Gaussian and Donut beam profiles for different build rates.
Journal of
Laser Applications ARTICLE scitation.org/journal/jla
J. Laser Appl. 32, 022059 (2020); doi: 10.2351/7.0000100 32, 022059-9
Published under license by Laser Institute of America
profile will show benefits for higher scan speeds and hatch
distances.
C. Comparison of single-track and density cube
experiments
From the results of the single-track and density cube experi-
ments, it can be seen that general characteristics that have been
identified in the single-track experiments are also found in the
density cube experiments. Two examples for this are that the
process window that produces good results (type I melt tracks for
single tracks; >99.5% density for the density cube experiments) is
wider for the Donut beam profile and that the key hole effect/gas
porosity is not observed for the Donut beam profile in both experi-
ments. But the results also reveal that a direct correlation between
the quality of the single tracks and the density of the cubic speci-
mens is not possible. The maximum speed for a type I single track
is 1750 mm/s, while different combinations of laser powers and
scanning speeds of 2000 mm/s result in cubic specimens with den-
sities >99.5%. Nevertheless, the single-track experiments provide a
good indication about the region of a stable SLM process.
In addition, it is interesting to note that the comparable high
variation in melt pool width for the single tracks manufactured
with the Donut beam profile, especially for 700 and 800 W, does
not seem to have a negative effect on the density of the additively
manufactured cubes. This is probably due to the fact that melt pool
width, even under consideration of the variation, is still larger than
the maximum hatch distances of 200 μm tested (except for a laser
power of 500 W and scanning speeds of 1750 and 2000 mm/s).
IV. CONCLUSION
In this study, the effect of the laser beam profile (the Gaussian
and Donut beam profiles) on the SLM process has been studied for
the aluminum alloy AlSi10Mg. The goal was to investigate if alter-
native beam profiles such as the Donut beam profile offer benefits
to the SLM process in terms of stability and productivity of the
process. Therefore, single tracks and density cubes were manufac-
tured for different process parameters. For the single tracks, the
shape and size of the single tracks were analyzed based on their top
view and microsections. For the density cubes, the porosity and
types of defects were analyzed on microsections with respect to dif-
ferent key figures (LEDs, VEDs, and build rate). The most impor-
tant findings can be summarized as follows:
•The process window that produces stable single tracks and cubes
with densities of >99.5% is larger for the Donut beam profile
than that for the Gaussian beam profile.
•The single tracks of the Donut beam profile are wider than that
of the Gaussian beam profile and the effect of the process param-
eters on the melt pool depth of the Donut beam profile is not as
significant as that of the Gaussian beam profile.
•Fewer defects (cracks, protrusions, and porosity) are present in
the single tracks and the density cubes of the Donut beam profile
for almost all process parameters tested.
•The key hole effect was avoided and gas porosity almost not
present in the single tracks and the density cubes manufactured
with the Donut beam profile.
•Both beam profiles produced highly dense cubes (>99.5%) at
maximum productivity tested. Additional experiments are neces-
sary with higher scanning speeds and hatch distances to validate
that the productivity can be increased by a Donut beam profile.
ACKNOWLEDGMENTS
The presented results are based on work that was done for the
Bionic Aircraft project. This project has received funding from the
European Union’s Horizon 2020 Research and Innovation Program
under Grant Agreement No. 690689.
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