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J. Laser Appl. 27, S17006 (2015); https://doi.org/10.2351/1.4898835 27, S17006
© 2014 Laser Institute of America.
Mechanical response of TiAl6V4 lattice
structures manufactured by selective
laser melting in quasistatic and dynamic
compression tests
Cite as: J. Laser Appl. 27, S17006 (2015); https://doi.org/10.2351/1.4898835
Submitted: 10 July 2013 • Accepted: 10 October 2014 • Published Online: 09 December 2014
S. Merkt, C. Hinke, J. Bültmann, et al.
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Mechanical response of TiAl6V4 lattice structures manufactured by selective
laser melting in quasistatic and dynamic compression tests
S. Merkt
Chair for Laser Technology LLT, RWTH Aachen University, Aachen 52074, Germany
C. Hinke
Fraunhofer-Institute for Laser Technology ILT, Aachen 52074, Germany
J. B€
ultmann
Department of Ferrous Metallurgy, RWTH Aachen University, Aachen 52072, Germany
M. Brandt
School of Aerospace Mechanical and Manufacturing Engineering, RMIT University, Melbourne 3001, Australia
Y. M. Xie
Centre for Innovative Structures and Materials (CISM), RMIT University, Melbourne 3001, Australia
(Received 10 July 2013; accepted for publication 10 October 2014; published 9 December 2014)
This paper focusses on the investigation of the mechanical properties of lattice structures
manufactured by selective laser melting using contour-hatch scan strategy. The motivation for this
research is the systematic investigation of the elastic and plastic deformation of TiAl6V4 at different
strain rates. To investigate the influence of the strain rate on the mechanical response (e.g., energy
absorption) of TiAl6V4 structures, compression tests on TiAl6V4-lattice structures with different
strain rates are carried out to determine the mechanical response from the resulting stress-strain
curves. Results are compared to the mechanical response of stainless steel lattice structures (316L). It
is shown that heat-treated TiAl6V4 specimens have a larger breaking strain and a lower drop of stress
after failure initiation. Main finding is that TiAl6V4 lattice structures show brittle behavior and low
energy absorption capabilities compared to the ductile behaving 316L lattice structures. For larger
strain rates, ultimate tensile strength of TiAl6V4 structures is more than 20% higher compared to
lower strain rates due to cold work hardening. V
C2014 Laser Institute of America.
[http://dx.doi.org/10.2351/1.4898835]
Key words: selective laser melting, lattice structure, compression test, ultimate tensile strength,
brittle fractures, stress-strain curve, specific energy absorption
I. INTRODUCTION AND MOTIVATION
Global trends like mass customization and resource effi-
ciency lead to a rising demand of individual products highly
adapted to the needs of the customer.
1
To fulfill these customer
needs, flexible manufacturing technologies, such as Additive
Manufacturing, are required that allow the almost unlimited
adaption of the products functionalities to these specific
demands.
2
Layer by layer the most complex products can be
manufactured with no need for tools or molds. In a preprocess-
ing step, the 3D-computer-aided-design (CAD) model gets
sliced to get layer wise information for computing the scan
tracks of the laser beam. In a first manufacturing step powder
material with spherical particles, typically in a range of
25–50 lm, are deposited on a substrate plate as a thin layer of
30–50 lm. According to the computed scan tracks, the laser
beam melts the powder which is solidified after melting. The
substrate plate is lowered by one layer, a rubber or brush is
depositing powder onto the last layer and the powder is melted
again to represent the parts geometry (see Fig. 1). These steps
are repeated until almost 100% dense parts with serial-identical
properties are manufactured with the selective laser melting
(SLM) process directly from the 3D-CAD model.
2–4
Another
big advantage of this technology is that diverse parts can be
manufactured in one build job. Materials, such as tool steel,
stainless steel, aluminum, copper, ceramic, and titanium, can be
used as base material for the SLM process.
2
Former research on Additive Manufacturing mainly
focused on the qualification of new materials and the process
development by applying more efficient scan strategies and
higher laser powers >1 kW (High Speed Additive
Manufacturing).
5,6
These investigations lead to a significant
increase of process efficiency by more than a factor of 10 allow-
ing larger lot sizes being economically manufactured by SLM.
6
Little research so far is carried out on the new design opportuni-
ties given by SLM, e.g., for ultra-light-weight applications in
the aerospace industry. Lattice structures with load-adapted me-
chanical properties are very promising to raise light-weight
design to a new level. In sum the SLM process enables a single
component to combine the benefits of high geometrical freedom
and functional integration with series-identical mechanical
properties. For conventional manufacturing technologies such
as die casting, the piece cost highly depends on the lot size of
the product. For increasing lot sizes, the piece costs are decreas-
ing due to economies-of-scale. The fixed costs for Additive
Manufacturing are lower than for conventional manufacturing
enabling the economic production of parts in the regime of
small lot sizes (Individualization for free). Innovative business
1938-1387/2015/27(S1)/S17006/6/$28.00 V
C2014 Laser Institute of AmericaS17006-1
JOURNAL OF LASER APPLICATIONS LASER ADDITIVE MANUFACTURING FEBRUARY 2015
models such as customer co-creation can be implemented using
this big advantage of Additive Manufacturing Technologies
(see Fig. 2left). The more complex a product is, the piece cost
for manufacturing increases. For Additive Manufacturing, this
relation is not applicable. The nearly unlimited geometric free-
dom that is offered through SLM makes the piece cost almost
independent from product complexity. In some cases, manufac-
turing costs can even decrease due to lower build-up volumes
of optimized products with high geometric complexity.
Topology optimization is one design approach to save weight
while functionally adapting the product design to predefined
load cases.
7
These different relations between piece cost and
product complexity offer a unique capability for Additive
Manufacturing to manufacture innovative products perfectly
adapted to the specific technological requirements through the
integration of lattice structures (see Fig. 2right).
II. STATE-OF-THE-ART
Lattice structures manufactured by SLM have been
investigated by researchers to understand the behavior of
these structures under various loadings.
8–13
Mainly cubic-
type lattice structures (see Fig. 3) and scaffold structures
have been investigated regarding their potential for applica-
tions as lightweight structures, energy absorbers, medical
implants, buoyancy devices, vibration control, and heat
exchangers.
14,15
The most comprehensive theoretical consid-
eration about the behavior of cellular material can be found
in Ref. 16.
Rehme
15
recommends the implementation of a pointlike
scan strategy rather than using a contour-hatch scan strategy.
AMATLAB tool determines the intersection points of the lat-
tice structures with each layer which will be scanned using
infinite small scan tracks. The size of the strut diameter is
controlled by the exposure time of the laser beam. Strut
diameters <150 lm with high geometric precision are possi-
ble with pointlike scan strategy.
15
Nevertheless, the surface
roughness of struts manufactured with contour-hatch scan
strategy is better than for struts manufactured with pointlike
scan strategy. Most researchers use stainless steel 316L or
TiAl6V4 as base material for their investigations.
17
Shen
et al.
18
manufactured struts in different angles (0–45to the
FIG. 1. Schematic representation of the SLM process.
FIG. 2. Technology potential of SLM process.
S17006-2 J. Laser Appl., Vol. 27, No. S1, February 2015 Merkt et al.
substrate plate) with an exposure time of 500 ls and a laser
power of 140 W and carried out tensile tests. The mechanical
properties of the strut do not vary significantly over the range
of build angles.
18
Compressions tests of BCC- and BCC,Z-
type structures were carried out and show a range of elastic
modulus of 5–58 MPa for laser powers from 100 to 160 W at
an exposure time of 500 ls. Investigations of Shen et al.
18
show that struts manufactured at 45have a slightly lower
(5%) diameter than vertically manufactured struts. This can
lead to a decrease of elasticity modulus and yield stresses of
about 10%. G€
umr€
uk and Mines
9
considered the material
overlapping effects in theoretical and numerical approaches
in the vicinity of strut connection points to give reasonable
predictions of the mechanical response of lattice structures
under compressive load. Furthermore, the stress-strain
curves of an individual defected (high surface roughness)
micro strut manufactured using SLM process are determined
and the elasticity modulus for the micro strut is found as
97 GPa, which is 60% lower than for the bulk material of
stainless steel 316L. Smith et al.
19
have shown that the qua-
sistatic response of BCC- and BCC-Z structures can be accu-
rately described using finite element modeling with both 3D
continuum and beam element types. Compression tests on
cubic lattice structures made from stainless steel 316L have
shown that microlattice structures can absorb significant
energy in compression via the formation of plastic hinges at
the junction points within the structure.
10
Gorny et al.
20
investigated the compressive behavior of TiAl6V4 lattice
structures (type BCC) under quasistatic compression. The
SLM specimens show brittle failure mechanism. A post
SLM heat-treatment is boosting the energy absorption capa-
bilities by increasing the ductility of the structures. Little
research so far was done on dynamic compression tests on
lattice structures manufactured by SLM. Smith et al.
21
have
investigated the quasistatic and blast response of both BCC-
and BCC-Z type lattice structures. Their investigations show
that the behavior of lattice structures is predictable and that
the collapse mechanisms in blast response.
III. EXPERIMENTAL DETAILS
Cubic lattice structures (type F2CCZ) are built using a
SLM250HL machine (SLM Solutions GmbH). This type of
unit cell is used due to investigations of O. Rehme,
15
who
compared different cell types in his study and came to the
conclusion that a f2ccz unit cell type is performing best in
compression tests. Lattice blocks with 8 88 unit cells
(see Fig. 4) with a strut diameter of approx. 350 lmanda
cell width of 2 mm are manufactured from TiAl6V4 powder
(average particle size 40 lm). A laser power of 100 W and a
scan speed of 450 mm/s are applied to build lattice blocks
with material densities of almost 100%. Specimens are
attached to the substrate plate by additional vertical struts
to guarantee an easy removal of the specimens from the
base plate. Wire-cutting (electric discharge machining) is used
to detach the specimens from the substrate plate without dam-
aging any microlattice and to get a smooth surface for clamp-
ing the specimens during compression test. Relative density of
the specimens is calculated by measuring the weight and outer
dimensions of the lattice blocks. The mean value for the rela-
tive density is approx. 12.4% for all specimens. As it is known
that the microstructure of the microlattice has a huge impact
on the mechanical response
20
half of the specimens are heat-
treated. The phase fraction of a-titanium to b-titanium, the
grain size and internal stresses can be influenced by heat treat-
ment.
20
Specimens are heat-treated in a vacuum furnace at
950 C for 2 h and then cooled down until room temperature.
Compression tests on a minimum of three specimens each are
performed with three different strain rates: quasistatic,
FIG. 3. Different cubic unit cell types.
9
FIG. 4. Lattice blocks manufactured by SLM process.
J. Laser Appl., Vol. 27, No. S1, February 2015 Merkt et al. S17006-3
16 mm/s and 5 m/s. The compression tests are performed using
a High Rate Instron Test System (VHS8800) with a low rate
method. The loading capacity of the machine is 65 kN. The
load history is measured by a Kistler load cell 9071A. The dis-
placement history is measured by an inherent linear variable
differential transformer with data filter by setting cut off fre-
quency of 1000 Hz. The deformation process is also recorded
by a camera within an interval of 30 s. The specimens are
placed on the bottom compression plate which is positioned at
a considerable distance from the top plate at the beginning of
each compression test. The surface friction effect is an im-
portant factor for most compression tests. Nevertheless, no
special surface treatment to reduce friction is done prior the
tests. However, it should be noted that the friction effect on
the uniaxial plateau stress is not so significant for crushable
cellular materials, because the Poisson’s ratio during the
compression in plateau region of a specimen is close to
zero.
For the dynamic tests (16 mm/s and 5 m/s), a High Rate
Instron Test System provides a feedback mechanism by its
FastTrack
TM
VHS8800 controller to change the drive profile
of the hydraulic system according to the force history from
experimental data. Thus a relatively constant loading rate
can be achieved during later experiments. The initial drive
profile is a constant value. It can be increased according to
the force history obtained by the load cell to obtain the new
drive profile. The consistency of the strain rate during com-
pression of the specimens can be improved further by iterat-
ing the procedure several times.
IV. RESULTS AND DISCUSSION
To make the deformation behavior of the TiAl6V4 lat-
tice structures visible photographs are taken at different
strain levels (see Fig. 5). During the first 10% strain, it can
be observed that the specimens expand laterally building a
radius along the edge of the lattice blocks. This behavior is
related to the linear-elastic region and deformation is still
reversible. Most of the energy is absorbed in the joints of
the connecting struts. The plastic region of the specimens
starts at around 4% strain with large deformations of the
top and bottom row of the lattice unit cells. The lattice rows
in the middle of the structure remain almost undeformed.
Further compression leads to higher plastic deformation
resulting in a brittle fracture across the structure. From 20%
strain on, brittle fractures of single struts are evident and
more and more struts lose their connection to the structure.
Bit by bit (30–50% strain) almost every horizontal row of
the lattice structure collapse until the horizontal rows ag-
gregate with each other forming a random conglomerate of
almost loose material (70% strain). A stress-strain curve is
recorded during the compression tests. In Fig. 6, one can
find the results for the quasistatic compression test of
TiAl6V4 lattice structures as SLM processed and as heat-
treated. A stress-strain curve of a 316L structure is added to
the diagram to compare the characteristics of lattice struc-
tures made from different materials. Both the TiAl6V4 and
316L structures have the same relative density of approx.
FIG. 5. Deformation with brittle failures of heat-treated specimens.
FIG. 6. Quasistatic stress-strain curves (TiAl6V4 vs 316L), same relative density of approx. 12%.
S17006-4 J. Laser Appl., Vol. 27, No. S1, February 2015 Merkt et al.
12% to allow a comparison of the compressive behavior.
There are some significant differences in the stress-strain
curve between the as SLM processed and heat-treated
specimens. The elastic moduli (stiffness), which can be
determined in the elastic region (incline of stress-strain
curve) of both structures, are almost identical. The ultimate
tensile strength (UTS) of the heat-treated specimens is
about 47 MPa and thereby lower than the UTS of the speci-
mens as SLM processed. Compared to the SLM-lattice
structures, forged solid TiAl6V4 specimens have an UTS of
approx. 900 MPa and a breaking elongation of approx.
10%.
22
Noticeable is the sharp drop to zero-stress at around
5% of strain for specimens as SLM processed. This drop is
caused due to the brittle fracture across the structure which
was already mentioned in the deformation analysis (see
Fig. 5). The breaking elongation of the heat-treated speci-
mens is approx. 6% and thereby a little higher than for the
other specimens. The drop of stress is not so sharp for the
heat-treated specimens, but still brittle fractures occur
resulting in low stresses after the peak stress. At further
compression strains (20%–50%) both types of structures
face an increase in stress with up and downs caused by
more brittle fractures of the cell rows and struts.
Densification starts for strains higher than 50% resulting in
an increase of stress due to deformations larger than the
cell width within the structure. If one compares the com-
pressive behavior of the TiAl6V4 with the 316L specimens,
a completely different failure mechanism can be observed.
The elastic moduli of both structures are almost identical,
which can be explained by the fact that the type of unit cell
mainly appoints for the stiffness of the structure rather than
the mechanical properties of the base material. The yield
strength (R
p
) of the 316L structures is approx. 2.5 times
lower than for the TiAl6V4 structures. No sharp drop of
stress can be observed in the stress-strain diagram of 316L
structures and thereby no brittle fractures occur during
compression. The structure form plastic hinges in the joint
of the struts and the ductility of the material prevent the
structure to break during further compression. Plastic fail-
ure mechanism leads to a high energy absorption capability
of the structure compared to TiAl6V4 structures. The
TiAl6V4 structure can absorb 12.14 J/mm
3
(see Fig. 7). The
316 L structure can absorb more than 15 J/mm
3
, although
the 316L base material has a lower specific strength than
TiAl6V4. The mechanical response of the 316L structures
is more predictable than for the TiAl6V4 structures due to a
steadily increasing plateau stress without any scatter of
stress.
The dynamic response of the lattice specimens is impor-
tant for applications such as crash boxes. High energy
absorption capability at high impact velocities is the main
objective for these types of applications. To validate the suit-
ability of the lattice structures as energy absorbers at high
impact velocities, compression tests at 16 mm/s and 5 m/s
are carried out. During the impact tests a stress-strain curve
is recorded to compare the mechanical response of the struc-
tures for different impact velocities (see Fig. 8). An increase
of the stiffness of the structure (elastic modulus) can be
observed for strain rates of 18 mm/s and 5 m/s. Moreover,
the UTS increases for higher strain rates by approx. 20% due
to cold work hardening. Breaking strain tends to decrease for
higher strain rates by a very low amount. Noticeable is an
increase of plateau stress at 10%–60% strain for increasing
strain rates. As a consequence, the structure can absorb the
most energy at 5 m/s strain rate (see Fig. 7)
FIG. 7. Comparison of the Specific energy absorption for different
specimens.
FIG. 8. Stress-strain curve of quasistatic and dynamic compression tests (TiAl6V4).
J. Laser Appl., Vol. 27, No. S1, February 2015 Merkt et al. S17006-5
V. CONCLUSION AND OUTLOOK
Lattice structures offer a great opportunity for light-
weight applications and as energy absorbers. Lattice struc-
tures manufactured from TiAl6V4 powder by SLM show
brittle failure mechanism during compression test. Even a
sharp drop to zero-stress can be observed due to brittle frac-
tures across the lattice structure and struts losing their con-
nection to the structure. A postprocess heat-treatment can
improve the mechanical response of the lattice structures and
prevent the stress to drop to zero-level. Still mainly brittle
failure is present in the heat-treated structure.
For higher strain rates, UTS and plateau stress increase
by approx. 20% due to strain hardening effects. 316L lattice
structures show a more reliable failure mechanism than
TiAl6V4 structures. A high amount of energy can be
absorbed by plastic deformation (plastic hinges) of the struc-
ture throughout the whole compression test. Although 316L
is not a typical light-weight material, the authors suggest—if
applicable—to use stainless steel rather than TiAl6V4 as
base material for lattice structures manufactured by SLM. In
future research more unit cell types in different relative den-
sities and different load cases will be investigated to charac-
terize these 316L structures for the use in functional
products.
ACKNOWLEDGMENT
The authors would like to thank the German Research
Foundation DFG for the support of the depicted research
within the Cluster of Excellence "Integrative Production
Technology for High-Wage Countries."
1
G. Schuh, F. Klocke, C. Brecher, and R. Schmitt, Excellence in
Production, 1st ed. (Apprimus-Verlag, Aachen, 2007).
2
T. Wohlers, “Wohlers Report 2013,” Wohlers Associates, 2013.
3
W. Meiners, “CDirektes selektives lasersintern einkomponentiger metalli-
scher werkstoffe,” Dissertation, RWTH, Aachen, 1999.
4
C. Over, “Generative fertigung von bauteilen aus werkzeugstahl
X38CrMoV5-1 und Titan TiAL6V4 mit selective laser melting,”
Dissertation, RWTH, Aachen, 2003.
5
H. Schleifenbaum, W. Meiners, and K. Wissenbach, “Towards rapid
manufacturing for series production: An ongoing process report on increasing
the build rate of selective laser melting (SLM),” International Conference on
Rapid Prototyping & Rapid Tooling & Rapid Manufacturing, Berlin,
Germany, 2008.
6
S. Bremen, W. Meiners, and A. Diatlov, “Selective laser melting. A manu-
facturing technology for the future?,” Laser Tech. J. 9(2), 33–38 (2012).
7
X. Huang and Y. M. Xie, Evolutionary Topology Optimization of
Continuum Structures: Methods and Applications (John Wiley & Sons,
Chichester, England, 2010), 235 pp.
8
M. Santorinaios, W. Brooks, C. J. Sutcliffe, and R. A. W. Mines, “Crush
behavior of open cellular lattice structures manufactured using selective
laser melting,” High Performance Structures and Materials III (Wessex
Institute of Technology, UK, 2004), pp. 481–489.
9
R. G€
umr€
uk and R. A. W. Mines, “Compressive behavior of stainless steel
micro-lattice structures,” Int. J. Mech. Sci. 68, 125–139 (2013).
10
K. Ushijima, W. Cantwell, R. Mines, S. Tsopanos, and M. Smith, “An
investigation into the compressive properties of stainless steel micro-
lattice structures,” J. Sandwich Struct. Mater.13, 303–329 (2011).
11
C. Yan, L. Hao, A. Hussein, and D. Raymont, “Evaluations of cellular lat-
tice structures manufactured using selective laser melting,” Int. J. Mach.
Tools and Manuf. 62, 32–38 (2012).
12
D. M. Xiao, Y. Q. Yang, X. B. Su, M. H. Zhang, and D. Wang, “Study on
cellular structures as the core architectures of parts based on SLM,” Adv.
Mater. Res. 538/541, 1904–1907 (2012).
13
K. Ushijima, W. J. Cantwell, and D. H. Chen, “Prediction of the mechani-
cal properties of micro-lattice structures subjected to multi-axial loading,”
Int. J. Mech. Sci. 68, 47–55 (2013).
14
O. Rehme and C. Emmelmann, “Cellular design for laser freeform fab-
rication,” Fourth International WLT-Conference on Laser in manufactur-
ing, Munich, June, 2007.
15
O. Rehme, “Cellular design for laser freeform fabrication,” Ph.D. Thesis,
Laser Zentrum Nord, Hamburg, 2009.
16
L. J. Gibson and M. F. Ashby, Cellular Solids, Structure and Properties
(Cambridge University Press, UK, 1999).
17
L. L€
ober, D. Klemm, U. K€
uhn, and J. Eckert, “Rapid manufacturing of cel-
lular structures of steel or titaniumalumide,” Mater. Sci. Forum 690,
103–106 (2011).
18
Y. Shen, W. J. Cantwell, R. A. W. Mines, and K. Ushijima, “The proper-
ties of lattice structures manufactured using selective laser melting,” Adv.
Mater. Res. 445, 386–391 (2012).
19
M. Smith, Z. Guan, and W. J. Cantwell, “Finite element modeling of the
compressive response of lattice structures manufactured using the selective
laser melting technique,” Int. J. Mech. Sci. 67, 28–41 (2013).
20
B. Gorny, T. Niendorf, J. Lackmann, M. Thoene, T. Troester, and H. J.
Maier, “In situ characterization of the deformation and failure behavior of
non-stochastic porous structures,” Mater. Sci. Eng. A 528, 7962–7967
(2011).
21
M. Smith, W. Cantwell, Z. Guan, S. Tsopanos, M. Theobald, G. Nurick,
and G. Langdon, “The quasi-static and blast response of steel lattice
structures,” J. Sandwich Struct. Mater. 13, 479–501 (2011).
22
A. G. Zapp, www.zapp.com, 09.07.2013. [Online] Available: www.zapp.com/
fileadmin/./TiAl6V4_e_02.12.pdf.
S17006-6 J. Laser Appl., Vol. 27, No. S1, February 2015 Merkt et al.
