materials

Article
On the Anisotropic Mechanical Properties of
Selective Laser-Melted Stainless Steel
Leonhard Hitzler 1, * ID , Johann Hirsch 2 , Burkhard Heine 2 , Markus Merkel 2 , Wayne Hall 1
and Andreas Öchsner 1
1 Griffith School of Engineering, Griffith University, Gold Coast Campus, Southport 4222, Australia;
W.Hall@griffith.edu.au (W.H.); A.Oechsner@griffith.edu.au (A.Ö.)
2 Faculty of Mechanical Engineering and Material Science, Aalen University of Applied Sciences,
73430 Aalen, Germany; Hirsch-Johann@web.de (J.H.); Burkhard.Heine@hs-aalen.de (B.H.);
Markus.Merkel@hs-aalen.de (M.M.)
* Correspondence: Leonhard.Hitzler@Griffithuni.edu.au; Tel.: +61-452-339-044

Received: 18 August 2017; Accepted: 24 September 2017; Published: 26 September 2017

Abstract: The thorough description of the peculiarities of additively manufactured (AM) structures
represents a current challenge for aspiring freeform fabrication methods, such as selective laser melting
(SLM). These methods have an immense advantage in the fast fabrication (no special tooling or
moulds required) of components, geometrical flexibility in their design, and efficiency when only small
quantities are required. However, designs demand precise knowledge of the material properties, which
in the case of additively manufactured structures are anisotropic and, under certain circumstances,
inhomogeneous in nature. Furthermore, these characteristics are highly dependent on the fabrication
settings. In this study, the anisotropic tensile properties of selective laser-melted stainless steel
(1.4404, 316L) are investigated: the Young’s modulus ranged from 148 to 227 GPa, the ultimate tensile
strength from 512 to 699 MPa, and the breaking elongation ranged, respectively, from 12% to 43%.
The results were compared to related studies in order to classify the influence of the fabrication settings.
Furthermore, the influence of the chosen raw material was addressed by comparing deviations on the
directional dependencies reasoned from differing microstructural developments during manufacture.
Stainless steel was found to possess its maximum strength at a 45◦ layer versus loading offset, which is
precisely where AlSi10Mg was previously reported to be at its weakest.

Keywords: tensile strength; hardness; microstructure; grain morphology; epitaxial grain growth;
scan strategy; directional dependencies

1. Introduction
Additive manufacturing (AM) methods, such as the selective laser melting (SLM), represent
powerful freeform fabrication techniques which can fabricate direct deployable components without
the necessity of special tooling, and are highly efficient when only small quantities are required [1–3].
Since full melting of the raw metal powder enables the generation of fully dense parts within a
single production step, with mechanical properties exceeding the specifications of the conventional
material, the fabrication of highly specialized components (like tools, moulds, ultra-lightweight
components or medical implants) using AM is increasing [4–7]. One of the major challenges to date
is the characterization and prediction of the properties of additively manufactured structures and
their linkage with the selected fabrication settings [8]. The approach most utilized for describing the
manufacturing process is through the energy input of the laser beam per unit volume, commonly
referred to as the energy density [9].

laser power (W)

Enery density WS = m


layer thickness (m) × hatch distance (m) × scan speed s

Materials 2017, 10, 1136; doi:10.3390/ma10101136 www.mdpi.com/journal/materials

Materials 2017, 10, 1136 2 of 19

Unfortunately, it appears that this convenient approach through characterization with a single
number is not able to sufficiently express the entire complexity of powder-bed based AM processes,
like SLM [10–12]. Thus, at this stage, a proper description of the manufacturing process still requires
the listing of the individual irradiation parameters. Hu et al. [13] highlighted the importance of the
scan speed and the layer thickness. In addition to the pure irradiation, information about the raw
metal powder, mainly its size and distribution, is also of great importance and should not be neglected.
Spierings et al. [14] pointed out the necessity of having both small and large powder particles: fine
particles are easily molten and favour a good relative density and surface quality; whereas larger
particles benefit ductility. On this note, the mechanical properties, such as hardness and tensile strength,
greatly correspond to the relative density, which is without doubt the most utilized characteristic for
evaluating the quality of fabricated components [10]. To illustrate its importance, the aeronautical
industry introduced a minimum relative density of 99% as a standardized quality requirement [15].
The properties of AM-fabricated components are known to be anisotropic, for reasons to do
with their layer-wise generation; and, in addition, inhomogeneous, with the latter being related to
location-dependent alterations caused by prolonged dwell-times at elevated temperatures. It has been
shown that inhomogeneities, which are caused through age-hardening (e.g., in aluminium–silicon
alloys or steel that is age-hardened) can be overcome using a post-heat treatment [16,17]. Differing
statements and conclusions about the inherent anisotropy were reported in former studies on stainless
steel. For example, outcomes about the directional dependencies of the tensile strength differ widely.
For the polar angle (the inclination to the layers), the findings range from the predominantly accepted
formulation (the highest tensile strength and highest breaking elongation are found in a parallel layer
to the loading direction scenario; and the lowest results, with an almost linear tendency, are found
when the loading direction is in an in-built direction, i.e., perpendicular to the layers) to the following
particular findings [4,18]: Sehrt and Witt [19] reported the opposite case, with the highest breaking
elongation being obtained in the perpendicular loading scenario; Rehme and Emmelmann [20] stated
that the lowest results for both the ultimate tensile strength and the breaking elongation occurred
under a 75◦ angle to the layers, whereas the maximum values for both were examined under a 15◦
angle; Tolosa, et al. [21] found an increase in strength by increasing the inclination angle from 30◦
to 45◦ ; whereas Guan et al. [22], on the other hand, reported a minimum strength occurring at a 45◦
inclination. In a similar way, differing findings were also reported for the in-plane dependencies.
In general, the effects occurring in-plane are less pronounced; Sehrt and Witt [19] even described this
influence as being negligible. However, there are various findings and it can be concluded that the
in-plane dependencies are highly influenced by the chosen scan strategy, and thus, there is a need to
consider them dependent on the individual settings [1]. Niendorf et al. [23] stated that, in addition
to the irradiation settings, the dimensions of the structure also affected the mechanical strength and
altered the microstructure. This finding was confirmed by Niendorf et al. [24], reporting noteworthy
fluctuations of the mechanical properties and a strongly textured microstructure when high-energy
laser systems (1000 W) are utilized. Rashid et al. [17] documented that the scan strategy impacted
the martensite/austenite ratio in 1.4542, and even caused minor changes in the dimension of the
component. Similar findings were made by Mahmoudi et al. [25] who, in addition, highlighted that
additively manufactured components show, in general, better performance in compressive loading.
Another aspect to consider is the influence of the surface roughness on the material strength, with
machined components yielding better results due to the removal of cracks and defects on, or close
to, the surface [19,26]. Apart from conventional subtractive procedures, multiple contactless surface
modification feasibility studies have been undertaken which all share the benefit of offering a similar
geometric flexibility to AM. Alrbaey et al. [27] and Schanz et al. [28,29] investigated the surface
laser-polishing of AM components and reported successful roughness reductions in the range from
80% to 92%. In addition to surface quality enhancement, the laser surface modification can also be
utilized to adjust surface hardening, as shown by Martínez et al. [30]. AlMangour and Yang [31]
showed that severe shot peening is another promising process that can greatly improve the mechanical
Materials 2017, 10, 1136 3 of 19

properties of the surface. However, pure strength aside, AM provides the unique opportunity to
directly incorporate additional features, such as textured surfaces to lower the friction, or bio-activated
(open-porous) surface structures to enhance bone-ingrowth [32,33].
Besides its well-known geometric freedom and greatly enhanced yield strength, AM offers
additional opportunities to further enhance material strength [34]. In recent studies the performance
gain of 1.4404 through particle reinforcement was extensively investigated. Reinforcement through TiC
particles was shown to be capable of almost doubling the hardness with respect to the un-reinforced
material [35,36]; and reinforcement via TiB2 was capable of almost tripling hardness [37]. In addition
to the remarkable gain in surface hardness, refined microstructural characteristics and a reduced wear
rate were reported.
In this study, the anisotropic material properties of stainless steel were examined with destructive
material tests, since the characterisation of the anisotropic material properties via non-destructive
procedures was found to be inadequate [38]. Moreover, the findings of these material tests were
compared with the results reported in literature in order to acquire a comprehensive overview of
the inherent directional dependencies and their variation among various machinery and irradiation
settings. Special consideration is given to the scan strategy settings and microstructural development
throughout the process.

2. Methodology

2.1. Manufacturing Conditions

In this study, a SLM 280HL machine (SLM Solutions GmbH, Lübeck, Germany) equipped with a
400 W Yb-fibre-laser was utilized to manufacture the specimens. It features an available build space
of 280 × 280 × 320 mm3 and includes a preheating system. The selected fabrication parameters are
summarized in Table 1 and a brief explanation is provided in Figure 1. Low-carbon stainless steel
type EN 1.4404, US 316L (also known as X2CrNiMo17-12-2) was chosen as the raw material and was
supplied by SLM Solutions with the following properties: a mean particle diameter of 35.5 µm and an
apparent powder density of 3.85 g/cm3 [39].
The tensile specimens were designed in accordance with the German standard DIN
50125:2009-07 [40] as flat specimens type E 5 × 10 × 40 and fabricated in seven distinct orientations
(Table 2, Figure 2), subsequently referred to as configurations (a) to (g). The azimuth angle (Θ) describes
the inclination of the sample to the x-axis and the polar angle (Φ) describes the inclination in relation
to the xy-plane. The samples were fabricated with an oversize of 0.4 mm in width and thickness and
milled to their final shape. On a side note, various techniques and alternatives for the preparation
of tensile samples, to the chosen milling process, were investigated by Krahmer et al. [41]. Detailed
studies on the effects of the positioning and inclination on the as-built surface roughness can be found
elsewhere [39,42].

Table 1. Parameter sets utilized for irradiation in SLM for the processing of 1.4404; see description of
the parameters and the setup in Figure 1.

Parameter Scan Speed Laser Power Hatch Distance Rotation Angle Energy Density
Set (mm/s) (W) (mm) Increment (◦ ) (J/mm3 )
Contour 400 100 0.09 - 92.6
Core 800 200 0.12 33 69.4
Final layer 400 300 0.1 - 250.0
Support 875 200 - - -
Layer thickness of 30 µm
Mounting plate temperature of 200 ◦ C
Common
Nitrogen is employed as the inert gas
Contour is irradiated first, followed by the core, utilising the line scanning strategy with a scan
vector length of 10 mm
Limitation window of 90◦ , respectively ±45◦ to the y-axis
Materials 2017, 10, 1136 4 of 19
Materials 2017, 10, 1136 4 of 19
Materials 2017, 10, 1136 4 of 19

TableTable 2. Summary
2. Summary of positioningdetails
of positioning detailsforfor all
all considered
considered configurations,
configurations,grouping of individual
grouping of individual
Table 2. Summary
manufacturing ofand
jobs positioning detailstime
corresponding for all
perconsidered
job. configurations, grouping of individual
manufacturing jobs and corresponding time per job.
manufacturing jobs and corresponding time per job.
Config. Polar Angle Φ; αXY (°) Azimuth Angle Θ; αX (°) Total Runtime (h)
Config.
Config. (a) Polar
PolarAngle
Angle0Φ; Φ;ααXY (◦ ) Azimuth
XY(°) Angle
Azimuth 0 Θ; αΘ;
Angle αX (◦Total
X (°) ) Runtime
Total (h) (h)
Runtime
39.5
(a)
(a) (b) 0 0
0 090 0 39.5 39.5
(b)(b)(c) 015
0 900 90
(c)(c)
(d) 1515
45 00 0
(d)(d)(e) 4545
75 0 0 0 86.5
(e)(e)(f) 7575
90 0 0 0 86.5 86.5
(f)(f)(g) 9090
90 090 0
(g)(g) 9090 90 90
Slight deviations from 0° to 90° angles were introduced for the azimuth angle of the in-plane oriented
Slight deviations
Slight deviations
configurations 0◦ toand
from((a)
from 90◦to(b))
0° angles
90°to were
angles introduced
were
improve for thefor
introduced
the recoating azimuth angle
the azimuth
process of the in-plane
angle
by ensuring ofthat itsoriented
the in-plane configurations
bladeoriented
does not
((a) and (b)) to improve
configurations the recoating
((a)entire
and edge
(b)) to process by
the recoating process by ensuring that its blade doesedge
ensuring that its blade does not abruptly hit the entire notat once.
abruptly hit the at improve
once.
abruptly hit the entire edge at once.

Figure 1. Schematic depiction of the SLM process; (a) the representation of the geometry via single
Figure 1. Schematic depiction of the SLM process; (a) the representation of the geometry via single
Figure 1. Schematic
scan tracks depiction
and layers; (b) theofbuild
the SLM process; (a)
environment; the representation
adapted from [8]. of the geometry via single
scan tracks and layers; (b) the build environment; adapted from [8].
scan tracks and layers; (b) the build environment; adapted from [8].

Figure 2. Tensile samples on the substrate plate, overview of the positioning and arrangement.
Figure 2. Tensile samples on the substrate plate, overview of the positioning and arrangement.
Figure 2. Tensile samples on the substrate plate, overview of the positioning and arrangement.

2.2. Material Testing

2.2.1. Composition and Density

The material composition was determined with optical emission spectrometry (Q4 TASMAN,
Bruker Corp., Billerica, MA, USA) on the machined samples. Based on the determined composition and
the corresponding theoretical density, the relative density was obtained via the Archimedes method.
2.2. Material Testing

2.2.1. Composition and Density

The material composition was determined with optical emission spectrometry (Q4 TASMAN,
Bruker 2017,
Materials Corp.,
10, Billerica,
1136 MA, USA) on the machined samples. Based on the determined composition 5 of 19
and the corresponding theoretical density, the relative density was obtained via the Archimedes method.

2.2.2. Hardness
2.2.2. Hardness
Surface
Surface hardness
hardness tests
tests were
were systematically
systematically conducted
conducted on on the
the clamping areas of
clamping areas of the
the as-built
as-built and
and
machined samples. Four indentations were evaluated on each sample. The hardness measurements
machined samples. Four indentations were evaluated on each sample. The hardness measurements
were
were undertaken
undertaken with
with aa Reicherter
Reicherter KF
KF hardness
hardness tester
tester (Reicherter Georg GmbH
(Reicherter Georg GmbH Co Co Kg,
Kg, Esslingen,
Esslingen,
Germany)
Germany) in accordance to the DIN EN ISO 6507-2:2016 standard [43]. The testing force was set
in accordance to the DIN EN ISO 6507-2:2016 standard [43]. The testing force was set to
to
294.1 N and the hardness was obtained in HV30.
294.1 N and the hardness was obtained in HV30.

2.2.3. Tensile Testing

2.2.3. Tensile Testing
For
For the
the destructive
destructive material
material tests,
tests, aa tensile
tensile testing
testing machine
machine(Zwick/Roell,
(Zwick/Roell, Ulm,Ulm, Germany)
Germany) with with
an
an in-built extensometer was utilized. The maximum load for this machine and the employed load
in-built extensometer was utilized. The maximum load for this machine and the employed load
cell
cell was
was 100
100 kN,
kN, and
and thethe initial
initial distance
distance of of the
the extensometer
extensometer was was set
set to
to 50
50 mm. The testing
mm. The testing procedure
procedure
was
was carried
carried out
out in accordance with
in accordance with the
the German
German standard
standard DIN
DIN ENEN ISO
ISO 6892-1:2016
6892-1:2016 [44]
[44] with
with aa constant
constant
crosshead speed of
crosshead speed of 55 mm/min,
mm/min,and anda atotal
totalofof4242 samples
samples (6 (6 samples
samples perper configuration)
configuration) were were tested.
tested. In
In addition, 3 out of 6 samples per configuration were equipped with an additional
addition, 3 out of 6 samples per configuration were equipped with an additional strain gauge (Type strain gauge (Type
FCB-2-17-1L, Tokyo Sokki
FCB-2-17-1L, Tokyo Sokki Kenkyujo
Kenkyujo Co.,Co., Ltd., Tokyo, Japan),
Ltd., Tokyo, Japan), comprising
comprising twotwo individual measurement
individual measurement
2
grids
grids in
in aa perpendicular
perpendicular arrangement
arrangement to to each
each other
other (each
(each with
with aasize
sizeof 1.5×× 2.5
of1.5 2.5 mm
mm2). ). The
The tensile
tensile
setup is depicted in detail in Figure
setup is depicted in detail in Figure 3. 3.

Figure 3. Tensile testing setup with extensometer and strain gauge.

Figure 3. Tensile testing setup with extensometer and strain gauge.

2.2.4. Microstructure
2.2.4. Microstructure
For investigations of the microstructure, segments were taken from the tensile samples, which
For investigations of the microstructure, segments were taken from the tensile samples, which
were embedded in a hot mounting resin. Various grinding and mechanical polishing steps were
were embedded in a hot mounting resin. Various grinding and mechanical polishing steps were
performed to expose the metallurgical structure. The visibility of the scan track pattern and the
performed to expose the metallurgical structure. The visibility of the scan track pattern and the
inherent grain structure was enhanced through a subsequent etching process. High-resolution images
inherent grain structure was enhanced through a subsequent etching process. High-resolution images
of the etched micro-sections were taken with an optical light microscope (Carl Zeiss Microscopy
of the etched micro-sections were taken with an optical light microscope (Carl Zeiss Microscopy GmbH,
GmbH, Jena, Germany).
Jena, Germany).

3. Results and Discussion

3.1. Density and Composition

The chemical composition of numerous samples of both batches was examined, and the averaged
compositions are shown in Table 3. In short, the deviations among the two batches are negligible and
Materials 2017, 10, 1136 6 of 19

are within the specifications of 1.4404. With the Archimedes method, a consistent relative density
greater than 99% (machined condition) was determined.

Table 3. Chemical composition of the specimens, all values in weight-percent.

Config. Fe C Si Mn P S Cr Ni Mo N
(a)–(b) Bal 0.031 0.564 1.044 <0.005 0.007 16.837 11.691 2.371 -
(c)–(g) Bal 0.0235 0.585 1.051 <0.005 <0.005 16.994 11.257 2.390 -
DIN EN 10088-3 Bal <0.03 <1 <2 <0.045 <0.03 16.5–18.5 10–13 2–2.5 <0.1

3.2. Hardness
The hardness results were consistent in all directions, and the obtained results ranged from
223 to 234 HV for the machined condition and, respectively, from 235 to 245 HV in the as-built
condition (Table 4), which is in perfect agreement with the documented values in the literature
(Table 5). Throughout all the configurations, the surface hardness in the as-built condition exhibited an
increased hardness of, on average, about 11 HV higher.
Unlike the previous investigation on AlSi10Mg, the hardness measurements did not
exhibit noteworthy deviation along the specimens, indicating a homogeneous structure of the
1.4404 samples [16]. The reason for the inhomogeneities encountered in the AlSi10Mg samples
was related to the alloy being age-hardened. Precipitation hardening occurred within the process,
triggered by the dwell times at elevated temperatures in the build chamber. In the case of stainless
steel, the difference between the temperature present in the build chamber compared to its melting
temperature was greater, by far, than was the case for AlSi10Mg. Thus, for 1.4404 it can be concluded
that the risks of inducing microstructural inhomogeneities due to dwell times are negligible at this
point. Yadroitsev et al. [45] have shown that preheating temperatures above 500 ◦ C result in the
formation of satellites adjoined to the scan tracks and, in addition, the generation of unwanted
splatters, that negatively affect the surrounding powder-bed. Given this upper limit for suitable
preheating temperatures, coupled with the finding of Krakhmalev et al. [46] that the microstructural
characteristics of 1.4404 stays stable up to 900 ◦ C before significant grain-coarsening occurred, it can
be concluded that inhomogeneities are unlikely to occur in 1.4404. It should, however, be noted
that the correlated stress-relieving effects at higher temperatures lowered the hardness results [47].
Furthermore, one exception should be emphasised: in cases where the solidification morphology,
i.e., columnar or equiaxed growth morphology, is crucial, the temperature field and, accordingly,
the preheating temperature, is key for successful fabrication [48]. This is the case, for example, when
single crystal structures are manufactured. However, for steel types capable of being age-hardened
(such as the 1.4542 (17-4PH) stainless steel) the risk of inhomogeneous altering of the properties within
the process cannot be ignored; AlMangour and Yang [49] pointed out that for additively manufactured
1.4542, an age-hardening with superimposed grain-coarsening occurred at 480 ◦ C.
One noteworthy and consistent relationship stood out in this study: comparing the configurations
with identical polar angles, the configurations with a 90◦ azimuth angle revealed higher core hardness
results (Figure 4). This finding will be addressed at a later point, together with the findings of the
tensile test and micro-sections.
Materials 2017, 10, 1136 7 of 19
Materials 2017, 10, 1136 7 of 19

Figure
Figure 4. 4. Orientationdependency
Orientation dependencyofofthe
thesurface
surfacehardness
hardnessin
inboth
boththe
theas-build
as-build and
and machined
machined condition.
condition.

Table 4. Surface hardness results per configuration obtained on the machined samples.
Table 4. Surface hardness results per configuration obtained on the machined samples.
Vickers Hardness Standard Deviation Core Vickers Hardness Standard Deviation
Config. Vickers Hardness Standard(HV30)
Deviation Vickers Hardness
Config. Core (HV30) Contour (HV30) Standard Deviation
Contour (HV30)
(a) Core226.7
(HV30) Core (HV30)
6.1 Contour (HV30)
242.8 Contour (HV30)
18.4
(b)
(a) 234.4
226.7 6.18.0 245.0
242.8 18.4 12.2
(b)
(c) 234.4
229.9 8.05.6 245.0
243.3 12.2 26.9
(c)
(d) 229.9
227.8 5.67.2 243.3
235.5 26.9 11.0
(d)
(e) 227.8
230.4 7.26.2 235.5
240.6 11.0 14.3
(e)
(f) 230.4
223.6 6.27.9 240.6
236.6 14.3 13.7
(f) 223.6 7.9 236.6 13.7
(g) 229.2 8.3 235.2 9.3
(g) 229.2 8.3 235.2 9.3

Table 5. Hardness results for 1.4404, comparison between literature and supplier specification;
Table 5. Hardness results for 1.4404, comparison between literature and supplier specification; relative
relative densities (≥99%).
densities (≥99%).
Reference Vickers Hardness Machine Max. Laser Power [W]
This work
Reference 223–245
VickersHV30
Hardness SLMMachine
280HL Max. Laser400
Power [W]
Cherry, et
This workal. [10] 220–225 HV
223–245 HV30 Renishaw AM250
SLM 280HL 200
400
Cherry, et al. [10] 220–225
215–255 HVHV Renishaw AM250 200
Tolosa, et al. [21] 215–255 HV SLM 250 Realizer -
Tolosa, et al. [21] mean of 235 HV SLM 250 Realizer -
mean of 235 HV
Kruth, et al.
Kruth, et al.[50]
[50] 220–250 HV0.1
220–250 HV0.1 - - --
Montani,
Montani, etetal.
al.[51]
[51] 245245
HVHV Prototype, notnot
Prototype, further
furtherspecified
specified 1000
1000
Sheet metal,
Sheet typical
metal, valuevalue
typical [52–54] ~220 HV(212–217
~220 HV HB) - -
- -
[52–54] (212–217 HB)
3.3. Tensile Strength
3.3. Tensile Strength
The samples throughout all configurations revealed arbitrary occurrences of failure along the
The samples
gauge length, throughout
thus confirming theall configurations
presence revealed
of a holistic arbitrarystructure.
homogeneous occurrences of failure
Hence, along the
the deviations
gauge length, thus confirming the presence of a holistic homogeneous structure. Hence,
subsequently discussed originated solely in the inherent directional dependencies, yet differentiations the deviations
subsequently
were necessary fordiscussed
both theoriginated
polar and solely
azimuthin the inherent
angles. directional
The averaged dependencies,
results yettest,
of the tensile differentiations
as well as
their according standard deviations, are presented in Table 6. These are drawn from six samples as
were necessary for both the polar and azimuth angles. The averaged results of the tensile test, well
each,
as their
except for according standard
Poisson’s ratio, which deviations,
is based onarethree
presented in Table each.
measurements 6. These
Theare drawn
latter will from six samples
be investigated
ineach,
more except
detail infor Poisson’spublication,
a separate ratio, which is based
since on three measurements
the encountered each. The
findings are scattered latter will
throughout thebe
investigated in more detail in a separate publication, since the encountered findings
entire possible range of Poisson’s ratio (i.e., between 0 and 0.5 [55]), with one exception even being are scattered
throughout
outside the entire
this range, which possible range
necessitates of Poisson’s
consideration of ratio (i.e.,applicable
theories between to 0 porous
and 0.5and [55]), with one
composite
exception even being outside this range, which necessitates
material that can exhibit Poisson’s ratios greater than 0.5 [56]. consideration of theories applicable to
porous and composite material that can exhibit Poisson’s ratios greater than 0.5 [56].
Materials 2017, 10, 1136 8 of 19

Table 6. Averaged results for the tensile properties of 1.4404.

Young’s Modulus E Yield Strength Ultimate Tensile Elongation at Poisson’s Ratio

Config. (GPa) Rp0.2 (MPa) Strength Rm (MPa) Failure At (%) ν (-)
Average STDEV Average STDEV Average STDEV Average STDEV Average STDEV
(a) 151.01 25.56 516.51 7.16 634.43 7.39 33.24 0.57 0.444 0.031
(b) 207.57 24.22 539.47 3.29 643.67 3.25 42.74 0.82 0.155 0.014
(c) 147.87 23.59 501.32 7.70 624.65 4.36 34.09 1.12 0.479 0.058
(d) 227.35 25.12 589.89 11.86 698.98 23.65 32.56 10.17 0.203 0.024
(e) 151.43 18.80 485.65 11.93 571.23 18.63 22.84 7.27 0.558 0.020
(f) 137.78 14.25 438.60 9.69 511.99 17.95 11.76 5.38 0.453 0.005
(g) 137.83 16.25 457.21 17.29 530.22 8.09 17.46 4.42 0.170 0.085

As with the hardness evaluation, the samples with an azimuth angle of 90◦ stood out. Considering
the in-plane oriented configurations (i.e., (a) and (b)), the Young’s modulus differed by more than
30%, whereas the deviations in yield strength and UTS were marginal. Interestingly, this considerable
dependency of Young’s modulus on the azimuth angle was only present for the samples with a polar
angle of zero degrees. This finding contradicts Sehrt and Witt [19], who reported that the in-plane
orientation can be neglected. However, this simplification was qualified; Sehrt [1] added that the
in-plane tendencies correspond with the irradiation strategy and especially the rotation angle of
subsequent layers. He reported that the negligible case corresponds with a 67◦ increment between
layers, which was the only case in which Young’s modulus was independent of the azimuth angle.
On a side note, a further possibility for promoting isotropic material behaviour through a second
irradiation of each layer, with a 90◦ rotation increment between the two irradiations, was reported by
AlMangour et al. [57].
The breaking elongation was considerably higher for both cases with Θ = 90◦ , which increased by
28.5% (config. (a) to (b)) and 48.5% (config. (f) to (g)), respectively. These findings coincided with those
of Meier and Haberland [18], who also reported fluctuations of the breaking elongation with a varying
azimuth angle.
Nonetheless, the results of this investigation are in perfect agreement with the polar angle being
the major directional dependency, influencing all tensile characteristics. The polar angle dependencies
of each single characteristic are depicted in detail in Figures 5–8 and compared with the results of
related studies. Interestingly, the maxima for the Young’s modulus and the tensile strength were
evident for Φ = 45◦ . On a side note, given this superimposition with the azimuth angle dependency,
it can be anticipated that in the case at hand the combination of Φ = 45◦ and Θ = 90◦ , which was not
investigated in the present study, would yield the highest results with the utilized manufacturing
settings. Fluctuations in the strength (yield and ultimate tensile strength) and, especially, in the
breaking elongation by alterations of the polar angle were frequently reported. However, alterations in
the linear elastic behaviour have been investigated in far less detail and the few existing studies are not
in agreement. In addition to the depicted results (Figure 5), Rehme and Emmelmann [20] stated that
there is no evidence of dependency of the Young’s modulus on the polar angle. Based on the results of
this work, however, there were remarkable deviations. The nature of these deviations has not been
clarified holistically yet, and will be addressed in a future work.
The specific progress of the tendencies in the tensile strength values also differ noticeably between
independent studies (Figures 6 and 7). Considering the general formulation about the occurrence
of the highest strength and breaking elongation in the parallel layer to loading orientation, it can be
concluded that this is indeed correct in many instances, but only partially correct when considering
the big picture, and far from being applicable as a general rule. The tensile strength, whether yield
or ultimate strength, showed a general tendency to be higher for the parallel loading to layer case.
However, the progression in between the extrema (i.e., parallel and perpendicular) did not follow
a general rule. It differed greatly by chosen manufacturing settings and setup across these various
studies, and it is also highly volatile in relation to individual material characteristics. For a conventional
bulk base material, there is a direct correlation between the ultimate tensile strength and the surface
Materials 2017, 10, 1136 9 of 19
Materials 2017, 10, 1136 9 of 19

18265:2014-2
hardness, and[58]. When comparing
concluding the fluctuating
one characteristic from theresultsother is forwell-known,
the ultimatee.g., tensile strength standard
the German (Table 6),
ranging
for from 512
conversion to 699 MPa,
of hardness with
values to the constant
ultimate hardness
tensile strength results (Table
for steel DIN4),EN it can
ISO be concluded[58].
18265:2014-2 that
this conversion is not constructive for selective laser-melted samples.
When comparing the fluctuating results for the ultimate tensile strength (Table 6), ranging from 512 to A few similar findings were
reported
699 MPa, within the theliterature, which indicate
constant hardness results that
(Table the4),correlation of the tensile
it can be concluded strength
that this and surface
conversion is not
hardness is for
constructive notselective
given inlaser-melted
AM: Mertens et al. A
samples. [59]
fewshowed that through
similar findings were an optimized
reported in thepreheating
literature,
temperature,
which indicatethe thatultimate tensile strength
the correlation of the tensile of 1.2344
strength couldandbesurface
drastically
hardnessincreased; this increase,
is not given in AM:
Mertens et al. [59] showed that through an optimized preheating temperature, the ultimatethat
however, was coupled with a decrease in hardness. Abd-Elghany and Bourell [34] argued this
tensile
incongruity
strength is reasoned
of 1.2344 could be to drastically
be due to the differingthis
increased; thermal
increase,environments,
however, was i.e.,coupled
close towith the free-surface
a decrease
and core.
in hardness. Abd-Elghany and Bourell [34] argued that this incongruity is reasoned to be due to the
Proceeding
differing thermal to the breaking i.e.,
environments, elongation,
close to the thisfree-surface
appeared toand be by far the characteristic most volatile
core.
to alterations in orientation (Figure 8). In this instance, no
Proceeding to the breaking elongation, this appeared to be by far the characteristicclear tendency was evident,most the range
volatileof
reported results scattered greatly, and the progression behaviour appeared
to alterations in orientation (Figure 8). In this instance, no clear tendency was evident, the range of random at first glance,
leading to
reported the question
results scattered of how thisand
greatly, can the
be the case. Clearly,
progression to answer
behaviour this question
appeared random holistically, more
at first glance,
work needs to be done. For now, the major causes of these deviations
leading to the question of how this can be the case. Clearly, to answer this question holistically, more are anticipated to be inherent
residual
work needs stresses
to be anddone. incomplete
For now, the fusionmajor between
causesscan tracks
of these and layers.
deviations areBoth result in
anticipated toabe weakening
inherent
of the material in a predominant direction but, depending on where
residual stresses and incomplete fusion between scan tracks and layers. Both result in a weakening of the defect occurs, the weakening
varies
the in its in
material predominant
a predominant direction.
direction Furthermore,
but, depending theseon effects
whereare thegreatly
defect influenced
occurs, the by the laser
weakening
powerinutilized
varies and the direction.
its predominant ability to Furthermore,
control the thermal environment,
these effects are greatlye.g., according
influenced to the
by the laserrange
power of
available preheating temperatures, which alter microstructural
utilized and the ability to control the thermal environment, e.g., according to the range of availabledevelopment. On a side note,
Wang et al.temperatures,
preheating [60] and AlMangour et al.microstructural
which alter [61] reported that defects and pores
development. in the
On a side as-built
note, Wang state
et al.can be
[60]
overcome,
and AlMangour to a etgreat extent,
al. [61] by applying
reported that defects a subsequent
and pores inhot the isostatic
as-built statepressing
can be(HIP) treatment.
overcome, to a
Leuders et al. [62] have found that HIP increases the ductility of 1.4404,
great extent, by applying a subsequent hot isostatic pressing (HIP) treatment. Leuders et al. [62] have but due to the reduction in
strength and its already good properties in the as-built state a post-heat
found that HIP increases the ductility of 1.4404, but due to the reduction in strength and its already treatment is not required in
most properties
good cases. However, in thedue to thestate
as-built inherent residualtreatment
a post-heat stresses, which
is not can go far in
required beyond
most the yield
cases. strength
However,
of the equivalent wrought material, heat treatments prior to removal
due to the inherent residual stresses, which can go far beyond the yield strength of the equivalent from the substrate plate need to
be carried
wrought out to avoid
material, deformations
heat treatments priorand tocorrelated
removal from accuracy issues [63].
the substrate plateOnneed
a side to note, the residual
be carried out to
stresses in SLM have a predominant direction and are generally higher
avoid deformations and correlated accuracy issues [63]. On a side note, the residual stresses in SLM in the direction of the scan
track [64].
have a predominant direction and are generally higher in the direction of the scan track [64].

Figure 5. Orientation
Figure5. Orientation dependency
dependency of
ofYoung’s
Young’smodulus;
modulus;comparison
comparisonwith
withreported
reportedresults
results[4,65].
[4,65].
Materials 2017, 10, 1136 10 of 19
Materials 2017, 10,
Materials 10, 1136 10 of 19
19
Materials 2017,
2017, 10, 1136
1136 10
10 of
of 19

Figure
Figure 6. Orientation
6.6. Orientation dependency
dependency of
of the
the yield strength;
strength; comparison with reported results [4,14,18,20,21].
Figure
Figure 6.Orientation
Orientationdependency
dependencyof
ofthe
the yield strength; comparison
yield strength; comparisonwith
comparison withreported
with reportedresults
reported results[4,14,18,20,21].
results [4,14,18,20,21].
[4,14,18,20,21].

Figure 7. Orientation
Figure Orientation dependency
dependency of
of the ultimate
ultimate tensile
tensile stress;
stress; comparison
comparison with
with reported results
results
Figure 7. Orientation
7. Orientation
7. dependency of the
dependency of the
the ultimate
ultimate tensile
tensile stress;stress;
comparison with reported
comparison with reported
reported results
[4,14,18–20,66].
[4,14,18–20,66].
results [4,14,18–20,66].
[4,14,18–20,66].

Figure 8. Orientation
Figure Orientation dependency
dependency of
of the elongation
elongation at
at fracture; comparison
comparison with
with reported results
results
Figure 8.
Figure 8.
8. Orientation
Orientationdependency of the
dependency the elongation
of the at fracture;
elongationfracture; comparison
at fracture; with reported
comparison reported results
with reported
[4,14,18–20,66].
[4,14,18–20,66].
[4,14,18–20,66].
results [4,14,18–20,66].

3.4. Comparison AM and Bulk Base Material

Additively manufactured material generally exhibits higher strength, coupled with reduced
ductility (Figure 9). Yield and ultimate tensile strength were exceeded by far, yet the 40% minimum
breaking elongation of bulk base 1.4404 is a criterion that was only achieved by one out of seven
configurations. One other aspect that should be emphasised is the greatly reduced difference between
Materials 2017, 10, 1136 11 of 19

3.4. Comparison AM and Bulk Base Material

Additively manufactured material generally exhibits higher strength, coupled with reduced
ductility (Figure
Materials 2017, 9). Yield and ultimate tensile strength were exceeded by far, yet the 40% minimum
10, 1136 11 of 19
breaking elongation of bulk base 1.4404 is a criterion that was only achieved by one out of seven
configurations. One other aspect that should be emphasised is the greatly reduced difference between
the yield
the yield point
point and
and the
the ultimate
ultimate tensile
tensile strength
strength caused
caused by by AMAM [8].
[8]. In
In Table
Table 7,7, several
several achieved
achieved ratios
ratios
for 1.4404 are compared with its bulk base material. It should be noted that a similar
for 1.4404 are compared with its bulk base material. It should be noted that a similar finding was also finding was also
reported for
reported for 1.4307
1.4307 (US
(US 304L)
304L) [34].
[34]. For
For safety
safety reasons,
reasons, itit is
is beneficial
beneficial ifif the
the difference
difference between
between these
these
measures is large. Thus, this peculiarity should be noted for components designed
measures is large. Thus, this peculiarity should be noted for components designed for fabrication for fabrication
using AM.
using AM. It It should
should be be pointed
pointed outout that
that this
this behaviour
behaviour is is not
not particularly
particularly negative,
negative, but but simply
simply
something that
something that needs
needs toto be
be considered.
considered. Among
Among materials
materialswith with high
high strength,
strength, this
this isis very
very common,
common,
and AM also introduces this behaviour in materials that do not show this trait
and AM also introduces this behaviour in materials that do not show this trait when fabricated with when fabricated with
conventional procedures.
conventional procedures.

Figure 9. Comparison of additively manufactured and bulk base 1.4404 [67].

Figure 9. Comparison of additively manufactured and bulk base 1.4404 [67].
Table 7. Ratio between the yield point and the ultimate tensile strength for 1.4404.
Table 7. Ratio between the yield point and the ultimate tensile strength for 1.4404.
Reference Configurations Range Re/Rm (-) Averaged * Ratio Re/Rm (-)
this work
Reference 7
Configurations 0.8026–0.8623
Range Re /Rm (-) Averaged0.8383
* Ratio Re /Rm (-)
Meier and Haberland [18] 5 0.8621–0.9261 0.8889
this work 7 0.8026–0.8623 0.8383
Merkt
Meier and [4] [18]
Haberland 5 3 0.7819–0.7877
0.8621–0.9261 0.7844
0.8889
Rehme and Emmelmann
Merkt [4] [20] 3 150 0.8400–0.8877
0.7819–0.7877 ** 0.8597
0.7844**
Rehme and Emmelmann [20] 150 1 0.8400–0.8877
0.8177 ** 0.8597
- **
Riemer, et al. [47] 1 0.8177
Riemer, et al. [47] heat treated 2 h, 650 °C 0.7445 --
heat treated 2 h, 650 ◦ C 0.7445 -
Spierings,
Spierings, etetal.
al.[14]
[14] 3 3 0.7941–0.8648
0.7941–0.8648 0.8380
0.8380
Tolosa, etal.
Tolosa, et al.[21]
[21] 15 15 0.9163–0.9967
0.9163–0.9967 0.9565
0.9565
bulk
bulk base 1.4404[67]
base 1.4404 [67] - - 0.4
0.4 --
** Investigated
Investigated configurations
configurations are equally,
are valued valued sample
equally,
sizessample sizes
neglected; Θ = 0◦ and**Φ only
neglected;
** only = 0◦ –90
Θ◦ considered.
= 0° and
Φ = 0°–90° considered.
3.5. Comprehensive Analysis of the Directional Dependencies and Their Origin
3.5. Comprehensive Analysis of the Directional Dependencies and Their Origin
Next, the inherent directional dependencies are investigated further and, to emphasise that
theirNext, the inherent
formation directional
is greatly dependencies
influenced by the chosenare investigated
material, a further and, to emphasise
direct comparison is madethat theira
with
formation is greatly
previous study on aninfluenced by the chosen
aluminium-based material,
die-cast a direct comparison
alloy (AlSi10Mg) [16]. Whenis made with a the
comparing previous
polar
study on an aluminium-based
angle dependencies of 1.4404 withdie-cast alloy (AlSi10Mg)
the dependencies [16]. When
of AlSi10Mg, a clear comparing
influence of the polar angle
the material can
dependencies
be shown. AlSi10Mgof 1.4404exhibited
with thethedependencies of AlSi10Mg,
lowest tensile a clear
strength under ◦
Φ influence of the
= 45 , which material can
is precisely wherebe
shown.
1.4404 hadAlSi10Mg
its peakexhibited
strength.the
Thelowest tensilefor
explanation strength underisΦbased
this finding = 45°, on
which is precisely where
microstructural 1.4404
development
had
duringits AM
peakfabrication.
strength. The microstructure
explanation for of this finding
AlSi10Mg is based
revealed ontraces
clear microstructural development
of every single scan track,
which inherits an inhomogeneous nature. The Al grain boundaries are surrounded with Si particle
segregations, which mainly take place in the connection between scan tracks and layers, namely hatch
overlaps and wetting areas (see Figure 1a) [68,69]. These segregations on the grain boundaries have a
Materials 2017, 10, 1136 12 of 19

stabilization effect and prevent secondary grain-coarsening or growth. Furthermore, the Si-rich areas
along the boundaries are comparably brittle, thus representing a weak spot for fracture. Given that
the line scanning approach is most commonly coupled with a rotation angle in between subsequent
layers, these brittle predetermined breaking points are far more emphasised between layers (in the
z-direction), than is the case in either the x- or y-directions. Applying a tensile loading under 45◦ to
the layers results in the maximum shear stress acting parallel to the layers, thus shearing the layers
apart along these embrittled areas, which are rich in Si-segregations.
Considering the microstructure of 1.4404, there are no obstacles for ongoing grain growth. Thus,
subsequent heat inputs, due to the neighbouring scan track or subsequent layer being generated, alter
the grains of the already solidified material [47]. This behaviour is commonly referred to as epitaxial
grain growth, describing the tendency of needle-like grain growth towards the heat source [24,70].
Due to this tendency, the grains of 1.4404 grew through the individual layers (in the direction of the
heat source), causing an interlocking of the individual layers (Figure 10). This interlock occurs in
all directions, i.e., through layers and also through neighbouring scan tracks. Since the scan track
direction is altered between layers, this interlocking mechanism can be seen best in the build-up
direction. An exemplified depiction of these underlying mechanisms is depicted in Figure 11 and
a direct comparison of the obtained polar angle dependencies is provided as well, which clearly
points out the resulting opposing, material-dependent, progressions. Further examples of the material
dependency on the development of directional anisotropies in AM are found in the literature. In the
work of Sehrt [1], it was seen that the NiCr alloy (Hastelloy X) developed a more emphasised polar
angle dependency than the stainless steel GP1. The results of Spierings et al. [71] on an AlMgSc alloy
(Scalmalloy) suggested that, for this material, the polar angle dependency can be neglected entirely,
since the reported deviations are below 3.4%. Given this, there cannot be a true generalized statement
on the inherent anisotropic character of AM-fabricated components. At the least, differentiations on
the underlying material have to be made.
Returning to the micro-sections (Figure 10), in these can also be found the answer for the
deviations regarding the azimuth angle that were encountered. A closer look at the shape of the
single scan tracks yielded the following. The cross-section of configuration (a) samples mainly revealed
oblong scan tracks, thus indicating that the majority of the scan tracks were fabricated around a
perpendicular orientation to the longitudinal axis. For configuration (b), the other case was evident:
single bead cross-sections can be seen clearly, indicating that the single scan tracks were predominantly
fabricated parallel to the specimen’s longitudinal axis. These findings yield the following conclusion:
in configuration (a), the loading occurred predominantly perpendicular to the scan tracks; whereas in
the case of configuration (b), the loading occurred predominantly parallel to the scan tracks, with the
latter yielding the higher strength.
These differing stacking patterns are based on the applied irradiation pattern; more precisely,
on the limitation window, which limits the admissible range of irradiation directions. Its main purpose
is to exclude all scan directions that result in a laser irradiation movement towards the inert gas stream.
The reason for this limitation is to prevent any interaction of emerging weld splashes and smoke with
the ongoing irradiation [72]. These particles get transported out of the build chamber via the inert
gas flow; hence, a laser movement in the direction of the inert gas stream is likely to cause unwanted
interactions with these particles and can cause defects in the fabrication process due to a lack of highly
focused power. However, on the other hand this precaution limits the range of possible rotation
increments of subsequent layers and causes the occurrence of predominant directions. In this study,
the allowed range of possible track vectors was 90◦ , which limits the possible track vector range to
±45◦ , with the track vectors always pointing in the negative y-direction (opposed to the inert gas
stream), resulting in the lower increment border at 135◦ and the upper border at 225◦ (Figure 12),
respectively. With the rotation increment set to 33◦ (Table 1), the sequence of track vectors and scan
track vector angles, as outlined in Table 8, arises.
Materials 2017, 10, 1136 13 of 19

respectively. With the rotation increment set to 33° (Table 1), the sequence of track vectors and13scan
Materials 2017, 10, 1136 of 19
track vector angles, as outlined in Table 8, arises.

Figure 10.
Figure 10. Microstructure
Microstructure of of 1.4404,
1.4404, taken
taken from
from the
the cross-section
cross-section of
of the
the tensile
tensile samples
samples of
of (i,ii)
(i,ii)
configuration (a); (iii,iv) configuration (b); and (v,vi) configuration (d). Left-hand column:
configuration (a); (iii,iv) configuration (b); and (v,vi) configuration (d). Left-hand column: lower lower
resolution; right-hand
resolution; right-hand column:
column: higher
higher resolution.
resolution.
Materials 2017, 10, 1136 14 of 19
Materials 2017, 10, 1136 14 of 19

Materials 2017, 10, 1136 14 of 19

(a) (b)
(a) (b)

(c)
(c)

Figure
Figure11.11.
Comparison
Comparison of theof
microstructural characteristics
the microstructural of (a) 1.4404
characteristics ofstainless steelstainless
(a) 1.4404 and (b) AlSi10Mg,
steel and
as well
(b) as their
AlSi10Mg, (c) strength dependency
as well asoftheir on the loading
(c) strength dependency versus
on the layer
loadingorientation [16].
Figure 11. Comparison the microstructural characteristics of (a) 1.4404versus layer
stainless steelorientation [16].
and (b) AlSi10Mg,
as well as their (c) strength dependency on the loading versus layer orientation [16].

Figure 12. (a) Irradiation strategy and (b) limitation window; adapted from [8,72].

Figure 12.
Figure 12. (a)
(a) Irradiation
Irradiation strategy
strategy and
and (b)
(b) limitation
limitation window;
window; adapted
adapted from
from [8,72].
[8,72].
Materials 2017, 10, 1136 15 of 19

Table 8. Exemplary calculation of the direction of subsequent irradiation tracks.

Layer Track Vector Angle Scan Vector Angle

(track vector angle)
1 (bottom increment limitation border) = 135◦
± 90◦ = 45◦ ; 225◦
(track vector angle)
2 (previous track vector angle) + (rotation angle increment) = 135◦ + 33◦ = 168◦
± 90◦ = 78◦ ; 258◦
(track vector angle)
3 (previous track vector angle) + (rotation angle increment) = 168◦ + 33◦ = 201◦
± 90◦ = 111◦ ; 291◦
would be outside the limitation window!, thus:
(previous track vector angle) + (rotation angle increment)—(top Increment (track vector angle)
4
limitation border) + (bottom Increment limitation border) = 201◦ + 33◦ − 225◦ + ± 90◦ = 54◦ ; 234◦
135◦ = 144◦
(track vector angle)
5 (previous track vector angle) + (rotation angle increment) = 144◦ + 33◦ = 177◦
± 90◦ = 87◦ ; 267◦

The findings show that in SLM the properties of the generated part are greatly affected by the
applied scanning pattern, and a thorough prediction of the properties of additively manufactured
components prior to fabrication is, consequently, a challenge. Since the SLM process represents
a micro-welding process, in a well-controlled build chamber, the general correlation between the
solidification of a scan track and its microstructure is similar to the weld microstructures. Therefore,
the microstructural characteristics of a single scan track follow the rules of a rapid solidification starting
from the melt pool boundary, i.e., the contact between the melt and the solid material [73]. It should
be noted that the energy input, although the laser source operates during the core irradiation at a
constant speed and irradiation intensity, is not constant. Trapp et al. [74] investigated intensively the
fluctuations and alterations of the absorptivity in the powder-bed, and showed that the real absorption
differs greatly from the known values from powder layer and liquid metal estimates. Hence, it can
be concluded that, although the physics of welding applies, the changes and interactions between
powder state, liquid state, and consolidated metal state render a thorough prediction more complicated,
and that more work needs to be done for a full understanding. On a side note, for powder-bed based
AM techniques the single scan tracks are two-dimensional only. When considering nozzle-fed AM
technologies, the scan strategy becomes a three-dimensional construct, introducing further complexity
in terms of the prediction of the properties [75].

4. Conclusions
In this study, the peculiarities of additively manufactured material were addressed using the
example of stainless steel. It was shown that homogeneous structures can be fabricated, and preheating
temperatures of up to 200 ◦ C do not cause location-dependent alteration of the microstructure.
The scan strategy was found to influence the material characteristics significantly and even simple
precautions, such as limiting the irradiation pathways to avoid possible interactions between emerging
particles with the laser beam, promote inherent directional dependencies. In addition, the general
rule of higher strength occurring in a parallel layer to the loading direction, in comparison with
the perpendicular layer to loading scenario, was proven accurate. However, the progression of the
mechanical characteristics by altering the inclination between the loading and the layers differed,
and was shown to be highly material-dependent. Stainless steel exhibited its peak strength and
maximum Young’s modulus under a 45◦ offset between the layer and loading direction, whereas
the aluminium–silicon alloy AlSi10Mg revealed the lowest strength in this instance. In regard to the
breaking elongation, the tested specimen showed a noteworthy drop in ductility past an inclination
offset of 45◦ . Considering the disparate tendencies found in related studies, it can be concluded
that the orientation dependency of the ductility in AM is, to date, not fully understood and further
in-depth investigations need to be undertaken. Moreover, future work is aimed at the modification and
extension of classical welding theory to enable the prediction of additively manufactured components
prior to fabrication.
Materials 2017, 10, 1136 16 of 19

Acknowledgments: Sincere appreciation to Michael Sedlmajer, Rene Klink, Tim Schubert, Wilfried Salzwedel,
Markus Hubbel, and the IMFAA research institute, for their helpful support throughout the implementation and
evaluation of the experiments.
Author Contributions: Corresponding author Leonhard Hitzler: Conception of the study, Interpretation of the
data, conduction of the literature overview, preparation of the manuscript; Johann Hirsch: Conduction of the
experimental work, interpretation of the data, conduction of the literature overview, preparation of the manuscript;
Burkhard Heine: Interpretation of the data, especially the evaluation of the microstructural characteristics and
their impact on the anisotropic material properties; Markus Merkel (Supervisor of J. Hirsch): Revision of the
experimental work and revision of the manuscript; Wayne Hall (Supervisor of L. Hitzler): Revision of the
manuscript; Andreas Öchsner (Supervisor of L. Hitzler): Conception of the study and revision of the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.

References
1. Sehrt, J.T. Möglichkeiten und Grenzen bei der Generativen Herstellung Metallischer Bauteile Durch das
Strahlschmelzverfahren. Ph.D. Thesis, University Duisburg-Essen, Duisburg, Germany, 2010.
2. Campanelli, S.L.; Contuzzi, N.; Angelastro, A.; Ludovico, A.D. Capabilities and performances of the selective
laser melting process. In New Trends in Technologies: Devices, Computer, Communication and Industrial Systems;
Er, M.J., Ed.; Sciyo: Roosbeek, Belgium, 2010.
3. Hitzler, L.; Merkel, M.; Freytag, P. Design of a subframe to integrate an electric drivetrain in existing vehicles.
Mat.-Wiss. Werkstofftech. 2015, 46, 454–461. [CrossRef]
4. Merkt, S.J. Qualifizierung von Generativ Gefertigten Gitterstrukturen für maßgeschneiderte
Bauteilfunktionen. Ph.D. Thesis, RWTH Aachen, Aachen, Germany, 2015.
5. Müller-Lohmeier, K. Stahl- und Aluminiumteile: Praktische Erfahrungen mit generativem Prototyping.
In 3. Swiss Rapid Forum; Festo AG & Co. KG: St. Gallen, Switzerland, 2005.
6. Milovanovic, J.; Stojkovic, M.; Trajanovic, M. Rapid tooling of tyre tread ring mould using direct metal laser
sintering. J. Sci. Ind. Res. 2009, 68, 1038–1042.
7. Buchbinder, D.; Meiners, W.; Brandl, E.; Palm, F.; Müller-Lohmeier, K.; Wolter, M.; Over, C.; Moll, W.; Weber, J.;
Skrynecki, N.; et al. Abschlussbericht—Generative Fertigung von Aluminiumbauteilen für die Serienproduktion,
01rio639a-d, BMBF; Fraunhofer ILT: Aachen, Germany, 2010.
8. Hitzler, L.; Merkel, M.; Hall, W.; Öchsner, A. A review of metal fabricated with powder-bed based additive
manufacturing techniques: Process, nomenclature, materials, achievable properties, and its utilization in the
medical sector. Adv. Eng. Mater. 2017. under review.
9. Prashanth, K.G.; Scudino, S.; Maity, T.; Das, J.; Eckert, J. Is the energy density a reliable parameter for
materials synthesis by selective laser melting? Mater. Res. Lett. 2017, 1–5. [CrossRef]
10. Cherry, J.A.; Davies, H.M.; Mehmood, S.; Lavery, N.P.; Brown, S.G.R.; Sienz, J. Investigation into the effect of
process parameters on microstructural and physical properties of 316l stainless steel parts by selective laser
melting. Int. J. Adv. Manuf. Technol. 2015, 76, 869–879. [CrossRef]
11. Scipioni Bertoli, U.; Wolfer, A.J.; Matthews, M.J.; Delplanque, J.-P.R.; Schoenung, J.M. On the limitations of
volumetric energy density as a design parameter for selective laser melting. Mater. Des. 2017, 113, 331–340.
[CrossRef]
12. Kleszczynski, S.; zur Jacobsmühlen, J.; Sehrt, J.; Witt, G. Mechanical properties of laser beam melting
components depending on various process errors. In Digital Product and Process Development Systems;
Kovács, G.L., Kochan, D., Eds.; Springer: Berlin/Heidelberg, Germany, 2013.
13. Hu, Z.; Zhu, H.; Zhang, H.; Zeng, X. Experimental investigation on selective laser melting of 17-4PH stainless
steel. Opt. Lasers Technol. 2017, 87, 17–25. [CrossRef]
14. Spierings, A.B.; Herres, N.; Levy, G. Influence of the particle size distribution on surface quality and
mechanical properties in AM steel parts. Rapid Prototyp. J. 2011, 17, 195–202. [CrossRef]
15. Vilaro, T.; Colin, C.; Bartout, J.D. As-fabricated and heat-treated microstructures of the Ti-6Al-4V alloy
processed by selective laser melting. Metall. Mater. Trans. A 2011, 42A, 3190–3199. [CrossRef]
16. Hitzler, L.; Janousch, C.; Schanz, J.; Merkel, M.; Heine, B.; Mack, F.; Hall, W.; Öchsner, A. Direction and
location dependency of selective laser melted AlSi10Mg specimens. J. Mater. Process. Technol. 2017, 243,
48–61. [CrossRef]
Materials 2017, 10, 1136 17 of 19

17. Rashid, R.; Masood, S.H.; Ruan, D.; Palanisamy, S.; Rahman Rashid, R.A.; Brandt, M. Effect of scan strategy
on density and metallurgical properties of 17-4PH parts printed by selective laser melting (SLM). J. Mater.
Process. Technol. 2017, 249, 502–511. [CrossRef]
18. Meier, H.; Haberland, C. Experimental studies on selective laser melting of metallic parts. Mat.-Wiss.
Werkstofftech. 2008, 39, 665–670. [CrossRef]
19. Sehrt, J.; Witt, G. Auswirkung des anisotropen Gefüges strahlgeschmolzener Bauteile auf mechanische
Eigenschaftswerte. In RTejournal; Forum für Rapid Technologie: Duisburg, Germany, 2009; Volume 6.
20. Rehme, O.; Emmelmann, C. Rapid Manufacturing of Lattice Structures with Selective Laser Melting; Laser-Based
Micropackaging: San Jose, CA, USA, 2006; p. 61070K.
21. Tolosa, I.; Garciandia, F.; Zubiri, F.; Zapirain, F.; Esnaola, A. Study of mechanical properties of AISI 316
stainless steel processed by “selective laser melting”, following different manufacturing strategies. Int. J.
Adv. Manuf. Technol. 2010, 51, 639–647. [CrossRef]
22. Guan, K.; Wang, Z.; Gao, M.; Li, X.; Zeng, X. Effects of processing parameters on tensile properties of selective
laser melted 304 stainless steel. Mater. Des. 2013, 50, 581–586. [CrossRef]
23. Niendorf, T.; Brenne, F.; Schaper, M. Lattice structures manufactured by SLM: On the effect of geometrical
dimensions on microstructure evolution during processing. Metall. Mater. Trans. B 2014, 45, 1181–1185.
[CrossRef]
24. Niendorf, T.; Leuders, S.; Riemer, A.; Richard, H.A.; Tröster, T.; Schwarze, D. Highly anisotropic steel
processed by selective laser melting. Metall. Mater. Trans. B 2013, 44, 794–796. [CrossRef]
25. Mahmoudi, M.; Elwany, A.; Yadollahi, A.; Thompson, S.M.; Bian, L.; Shamsaei, N. Mechanical properties
and microstructural characterization of selective laser melted 17-4 PH stainless steel. Rapid Prototyp. J. 2017,
23, 280–294. [CrossRef]
26. Zhang, D. Entwicklung des Selective Laser Melting (SLM) für Aluminumwerkstoffe. Ph.D. Thesis, RWTH
Aachen, Aachen, Germany, 2004.
27. Alrbaey, K.; Wimpenny, D.; Tosi, R.; Manning, W.; Moroz, A. On optimization of surface roughness of
selective laser melted stainless steel parts: A statistical study. J. Mater. Eng. Perform. 2014, 23, 2139–2148.
[CrossRef]
28. Schanz, J.; Hofele, M.; Hitzler, L.; Merkel, M.; Riegel, H. Laser polishing of additive manufactured AlSi10Mg
parts with an oscillating laser beam. In Machining, Joining and Modifications of Advanced Materials; Springer:
Singapore, 2016; pp. 159–169.
29. Schanz, J.; Hofele, M.; Ruck, S.; Schubert, T.; Hitzler, L.; Schneider, G.; Merkel, M.; Riegel, H. Metallurgical
investigations of laser remelted additively manufactured AlSi10Mg parts. Mat.-Wiss. Werkstofftech. 2017, 48,
463–476. [CrossRef]
30. Martínez, S.; Lamikiz, A.; Ukar, E.; Calleja, A.; Arrizubieta, J.A.; Lopez de Lacalle, L.N. Analysis of the
regimes in the scanner-based laser hardening process. Opt. Lasers Eng. 2017, 90, 72–80. [CrossRef]
31. AlMangour, B.; Yang, J.-M. Improving the surface quality and mechanical properties by shot-peening of 17-4
stainless steel fabricated by additive manufacturing. Mater. Des. 2016, 110, 914–924. [CrossRef]
32. Yadroitsev, I.; Bertrand, P.; Antonenkova, G.; Grigoriev, S.; Smurov, I. Use of track/layer morphology to
develop functional parts by selective laser melting. J. Laser Appl. 2013, 25, 052003. [CrossRef]
33. Figliuzzi, M.; Mangano, F.; Mangano, C. A novel root analogue dental implant using CT scan and CAD/CAM:
Selective laser melting technology. Int. J. Oral. Maxillofac. Surg. 2012, 41, 858–862. [CrossRef] [PubMed]
34. Abd-Elghany, K.; Bourell, D.L. Property evaluation of 304L stainless steel fabricated by selective laser melting.
Rapid Prototyp. J. 2012, 18, 420–428. [CrossRef]
35. AlMangour, B.; Grzesiak, D.; Jenn, M. Selective laser melting of TiC reinforced 316L stainless steel matrix
nanocomposites: Influence of starting tic particle size and volume content. Mater. Des. 2016, 104, 141–151.
[CrossRef]
36. AlMangour, B.; Grzesiak, D.; Yang, J.-M. In-situ formation of novel TiC-particle-reinforced 316L stainless
steel bulk-form composites by selective laser melting. J. Alloys Compd. 2017, 706, 409–418. [CrossRef]
37. AlMangour, B.; Grzesiak, D.; Yang, J.-M. Rapid fabrication of bulk-form TiB2/316L stainless steel
nanocomposites with novel reinforcement architecture and improved performance by selective laser melting.
J. Alloys Compd. 2016, 680, 480–493. [CrossRef]
Materials 2017, 10, 1136 18 of 19

38. Hitzler, L.; Janousch, C.; Schanz, J.; Merkel, M.; Mack, F.; Öchsner, A. Non-destructive evaluation of AlSi10Mg
prismatic samples generated by selective laser melting: Influence of manufacturing conditions. Mat.-Wiss.
Werkstofftech. 2016, 47, 564–581. [CrossRef]
39. Hitzler, L.; Hirsch, J.; Merkel, M.; Hall, W.; Öchsner, A. Position dependent surface quality in selective laser
melting. Mat.-Wiss. Werkstofftech. 2017, 48, 327–334. [CrossRef]
40. Prüfung Metallischer Werkstoffe—Zugproben; DIN 50125; Deutsches Institut für Normung e.V.: Berlin,
Germany, 2009.
41. Krahmer, D.M.; Polvorosa, R.; López de Lacalle, L.N.; Alonso-Pinillos, U.; Abate, G.; Riu, F. Alternatives for
specimen manufacturing in tensile testing of steel plates. Exp. Tech. 2016, 40, 1555–1565. [CrossRef]
42. Hitzler, L.; Hirsch, J.; Merkel, M.; Öchsner, A. Thermal environment and inclination angle dependencies on
the surface quality of selective laser melted 316L steel. Defect Diffus. Forum 2017, 372, 202–207. [CrossRef]
43. Metallic Materials—Vickers Hardness Test—Part 2: Verification and Calibration of Testing Machines; DIN EN ISO
6507-2; Deutsches Institut für Normung e.V.: Berlin, Germany, 2016.
44. Metallic Materials—Tensile Testing—Part 1: Method of Test at Room Temperature; DIN EN ISO 6892-1; Deutsches
Institut für Normung e.V.: Berlin, Germany, 2016.
45. Yadroitsev, I.; Krakhmalev, P.; Yadroitsava, I.; Johansson, S.; Smurov, I. Energy input effect on morphology
and microstructure of selective laser melting single track from metallic powder. J. Mater. Process. Technol.
2013, 213, 606–613. [CrossRef]
46. Krakhmalev, P.; Yadroitsava, I.; Fredriksson, G.; Yadroitsev, I. Microstructural and thermal stability of
selective laser melted 316L stainless steel single tracks. S. Afr. J. Ind. Eng. 2017, 28. [CrossRef]
47. Riemer, A.; Leuders, S.; Thöne, M.; Richard, H.A.; Tröster, T.; Niendorf, T. On the fatigue crack growth
behavior in 316L stainless steel manufactured by selective laser melting. Eng. Fract. Mech. 2014, 120, 15–25.
[CrossRef]
48. Gäumann, M.; Bezençon, C.; Canalis, P.; Kurz, W. Single-crystal laser deposition of superalloys:
Processing-microstructure maps. Acta Mater. 2001, 49, 1051–1062. [CrossRef]
49. AlMangour, B.; Yang, J.-M. Understanding the deformation behavior of 17-4 precipitate hardenable stainless
steel produced by direct metal laser sintering using micropillar compression and tem. Int. J. Adv. Manuf.
Technol. 2016, 90, 119–126. [CrossRef]
50. Kruth, J.P.; Mercelis, P.; Van Vaerenbergh, J.; Froyen, L.; Rombouts, M. Binding mechanisms in selective laser
sintering and selective laser melting. Rapid Prototyp. J. 2005, 11, 26–36. [CrossRef]
51. Montani, M.; Demir, A.G.; Mostaed, E.; Vedani, M.; Previtali, B. Processability of pure Zn and pure Fe by
SLM for biodegradable metallic implant manufacturing. Rapid Prototyp. J. 2017, 23, 514–523. [CrossRef]
52. Woite, M. Edelstahl 1.4404. Available online: http://www.woite-edelstahl.info/14404de.html (accessed on
14 June 2017).
53. AZO Materials. Stainless Steel—Grade 316L—Properties, Fabrication and Applications (UNS S31603).
Available online: http://www.azom.com/article.aspx?ArticleID=2382# (accessed on 14 June 2017).
54. Atlas Steels. Grade Data Sheet, 316 316L 316H. Available online: http://www.atlassteels.com.au/
documents/Atlas_Grade_datasheet_316_rev_Jan_2011.pdf (accessed on 14 June 2017).
55. Hibbeler, R.C. Mechanics of Materials, 8th ed.; Pearson Prentice Hall: Upper Saddle River, NJ, USA, 2011;
ISBN1 10: 0136022308. ISBN2 13: 9780136022305.
56. Daggett, S.S. A Simplified Approach to Determining “Effective” Elastic Mechanical Properties of
Fiber-Polymer Composite Systems. Ph.D. Thesis, Wichita State University, Wichita, KS, USA, 1997.
57. AlMangour, B.; Grzesiak, D.; Yang, J.-M. Scanning strategies for texture and anisotropy tailoring during
selective laser melting of TiC/316L stainless steel nanocomposites. J. Alloys Compd. 2017, 728, 424–435.
[CrossRef]
58. Metallic Materials—Conversion of Hardness Values; DIN EN ISO 18265:2014-02; Deutsches Institut für Normung
e.V.: Berlin, Germany, 2014.
59. Mertens, R.; Vrancken, B.; Holmstock, N.; Kinds, Y.; Kruth, J.P.; Van Humbeeck, J. Influence of powder bed
preheating on microstructure and mechanical properties of H13 tool steel SLM parts. Phys. Procedia 2016, 83,
882–890. [CrossRef]
60. Wang, Z.; Shi, Y.; Li, R.; Wei, Q.; Liu, J. Manufacturing AISI316L components via selective laser melting
coupled with hot isostatic pressing. In Advanced Material Science and Technology, Pts 1 and 2; Tan, Y., Ju, D.Y.,
Eds.; Trans Tech Publications Ltd.: Zurich, Switzerland, 2011; Volume 675–677, pp. 853–856.
Materials 2017, 10, 1136 19 of 19

61. AlMangour, B.; Grzesiak, D.; Yang, J.-M. Selective laser melting of TiB2/316L stainless steel composites:
The roles of powder preparation and hot isostatic pressing post-treatment. Powder Technol. 2017, 309, 37–48.
[CrossRef]
62. Leuders, S.; Lieneke, T.; Lammers, S.; Tröster, T.; Niendorf, T. On the fatigue properties of metals
manufactured by selective laser melting—The role of ductility. J. Mater. Res. 2014, 29, 1911–1919. [CrossRef]
63. Yadroitsava, I.; Yadroitsev, I. Residual stress in metal specimens produced by direct metal laser sintering.
In Proceedings of the SFF Symposium, Austin, TX, USA, 10–12 August 2015.
64. Yadroitsava, I.; Yadroitsev, I. Evaluation of Residual Stress in Selective Laser Melting of 316L Steel. In
Proceedings of the 1st International Conference on Progress in Additive Manufacturing (Pro-AM 2014),
Nanyang Executive Centre NTU, Singapore, 26–28 May 2014; pp. 278–283. [CrossRef]
65. Yadroitsev, I.; Yadroitsava, I.; Smurov, I. Strategy of Fabrication of Complex Shape Parts Based on the Stability of
Single Laser Melted Track; Pfleging, W., Lu, Y., Washio, K., Eds.; SPIE: San Francisco, CA, USA, 2011; p. 79210C.
66. Wen, S.; Li, S.; Wei, Q.; Yan, C.; Zhang, S.; Shi, Y. Effect of molten pool boundaries on the mechanical
properties of selective laser melting parts. J. Mater. Process. Technol. 2014, 214, 2660–2667. [CrossRef]
67. ThyssenKrupp. Nichtrostender Austenitischer Stahl TK 1.4404. Available online: http://www.thyssenkrupp.
at/files/rohre/Werkstoffdatenblaetter/1.4404.pdf (accessed on 21 August 2017).
68. Aboulkhair, N.T.; Tuck, C.; Ashcroft, I.; Maskery, I.; Everitt, N.M. On the precipitation hardening of selective
laser melted AlSi10Mg. Metall. Mater. Trans. A 2015, 46, 3337–3341. [CrossRef]
69. Tang, M.; Pistorius, P.C. Anisotropic mechanical behavior of AlSi10Mg parts produced by selective laser
melting. JOM 2017, 69, 516–522. [CrossRef]
70. Niendorf, T.; Leuders, S.; Riemer, A.; Brenne, F.; Tröster, T.; Richard, H.A.; Schwarze, D. Functionally graded
alloys obtained by additive manufacturing. Adv. Eng. Mater. 2014, 16, 857–861. [CrossRef]
71. Spierings, A.B.; Dawson, K.; Voegtlin, M.; Palm, F.; Uggowitzer, P.J. Microstructure and mechanical properties
of as-processed scandium-modified aluminium using selective laser melting. CIRP Ann. Manuf. Technol.
2016, 65, 213–216. [CrossRef]
72. Hirsch, J. Anisotropes Materialverhalten von Edelstahl 316L beim selektiven Laserstrahlschmelzen. Master’s
Thesis, Aalen University, Aalen, Germany, 2017.
73. David, S.A.; Vitek, J.M. Correlation between solidification parameters and weld microstructures. Int. Mater.
Rev. 1989, 34, 213–245. [CrossRef]
74. Trapp, J.; Rubenchik, A.M.; Guss, G.; Matthews, M.J. In situ absorptivity measurements of metallic powders
during laser powder-bed fusion additive manufacturing. Appl. Mater. Today 2017, 9, 341–349. [CrossRef]
75. Calleja, A.; Tabernero, I.; Fernández, A.; Celaya, A.; Lamikiz, A.; López de Lacalle, L.N. Improvement
of strategies and parameters for multi-axis laser cladding operations. Opt. Lasers Eng. 2014, 56, 113–120.
[CrossRef]

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