A Comparison of Lattice Structures in Metal Additive manufacturing.
Hanzl, Pavel ; Zetkova, Ivana ; Dana, Milan 等
A Comparison of Lattice Structures in Metal Additive manufacturing.
1. Introduction
In recent years, certain branches of the mechanical engineering
industry have been replacing solid materials with light porous
materials. Metal Additive Manufacturing (MAM) opens new possibilities in
the production of complicated components like lattice structures. Since
the manufacturing is less constrained by the limits of traditional
techniques, printed components can be smaller and more complicated. This
is one of the reasons why the interest in additive manufacturing
continues to grow. [2]
However, MAM has limits as well. Complex components with overhangs
usually require support to hold the critical areas during the process.
These support structures are built together with the component, and help
prevent distortion from thermal stresses and anchor the overhang to the
building platform. The support structures are removed after the
production is complete. [1]
In some cases, the removal of the support structures can be very
difficult, especially where the topology is very complex. This means
that the choice of suitable lattice structures for MAM is narrow.
Lattice structures with irremovable support structures would be
counterproductive. A change to the orientation of the lattice structure
can reduce the need for support structures. For these reasons, lattice
structures based on self-supporting unit cells are in high demand and
are used in additive manufacturing. [3]
Cellular lattice structures find wide range of usage due to their
unique properties. They are used in applications where high strength
accompanied by a relatively low mass is required. Porous structures also
have other advantages such as thermal insulation properties and
suppressed vibration. [7] [11]
The interest in cellular lattice structures continues to grow, and
a number of scientific articles have already been published dealing with
this area of research and development. The authors in [9] and [10] focus
on FEM analyses of different kinds of cellular lattices and the results
were verified by practical experiments.
This article deals with a comparison of structures which are
commonly used in additive manufacturing due to their self-supporting
properties. The structures are evaluated by maximum capacity load, and
the load directions (x, z) are taken into consideration.
2. Lattice structures with self-supporting cell units [2]
The experiment includes seven topologies with self-supporting unit
cells, see Figure 1. All of the samples have 5 mm unit size and volume
fraction is set at 13.5%. Cubic units such as Body Centred Cubic (BCC)
and Face Centred Cubic (FCC) form the basis for the other variations
such as PFCC and BCCz, where struts are added to gain stiffness along
the z axis. F2BCC is created by combining BCC and FCC units.
Since the struts of BCC enclose a 45[degrees] angle with the main
axes, BCC topology respects the rule of MAM and is self-supported along
the main axes. The other topologies meanwhile are self-supported only in
one z direction due to the added struts. While these lattices build
along x or y direction, the MAM could fail on the horizontal struts. A
typical representative of a fully self-supporting topology is a single
gyroid, which does not require supports in a minimum range of cell size
3-8 mm. [7] [8]
The lattice structure marked as "Rhombic" does not
respect the general rules of MAM and is therefore not self- supported.
However, it is known from theory that AM is able to bridge an overhang
of short distance without support structures. [4] Therefore its topology
could also be built using this technology with the following parameters
set.
3. Production of Lattice Structure
The samples were made using an EOS M290. The construction material
was EOS MaragingSteel MS1 and the layer thickness was set to 40 pm.
Default process parameters were used from EOS. The set consisted of 11
sample types. Samples with a rotated porous core of about 90[degrees]
were included. Some samples without a rotated core had a less suitable
orientation for additive manufacturing of lattice structure topologies
due to the existence of horizontal struts without support structures.
All of the samples had rigid plates on the contact surfaces with the
test machine. To ensure reliable production, the rigid plates were built
vertically and therefore the axis z was oriented horizontally during AM,
whereas the axis x was vertical. Some samples are shown in Figure 2.
None of the samples exhibits any defects. Only the horizontal
struts in the samples with load direction z show an impaired quality of
the lower surfaces and their diameters are slightly increased. This is
attributed to overheating of these areas due to the absence of a support
structure, resulting in the partial welding of powder particles to the
bottom.
4. Experiment
The porous samples were subjected to compression tests on the
Zwick/Roell Z 250 testing device. The results of the load capacity for
uniaxial loading along direction z and x are shown in Figure 3. Values
of the load capacities have been determined for a 0.2% permanent
deformation.
The lattice structure with the pFcc unit achieved the highest
rigidity in both load directions, but also exhibits the highest
anisotropy along the investigated direction. Deformation curves which
were obtained from the tests differed from each other significantly. The
load along direction z increased until the yield point, then came a slow
load reduction. The sample with rotated PFCC topology (load direction x)
showed a different course of load. After attaining load peak, the load
force fell sharply due to the collapse of one cell row. The peak of
maximum local load was contained five times in the deformation curve
since the sample had five rows of units. All the other topologies have
deformation curves similar to the first one. A closer look at the
deformation curves of the PFCC topology is shown in Figure 4.
5. Conclusion
This article deals with a comparison of self-supported lattice
structures. The structures are evaluated by maximum capacity load, and
the load directions (x, z) are taken into consideration. SLM is
evidently very useful in the production of complex lattice structures. A
set of porous samples with different topologies were all manufactured
without defects. Since volume fraction and unit size were set as
constant, the strut diameters were not constant and ranged from 0.64 mm
to 0.96 mm in the different 3D models of the samples.
In general, the horizontal straight struts can distort
self-supporting properties of cell unit because these struts do not
respect the minimum building angle (40[degrees]) without supporting
elements, and production may become risky. It was proved on the sample
set that additive manufacturing could bridge a distance of 4.3 mm in the
case of melting of horizontal struts with a minimum impact on the
surface quality. The situation may be different where struts are
slightly inclined from the horizontal position due to the absence of
constrains on both ends of the struts during metal additive
manufacturing. A consequence of this may be a deviation of the
unconstrained end above the level of the melted layer under the
influence of internal tensions. A collision of the recoater and strut
end could follow. In this respect, the Gyroid is the most suitable for
MAM, because its manufacturability does not depend on its orientation.
[6]
The BCC, Rhombic and Gyroid structures have consistent mechanical
properties along three orthogonal axes (x, y, z).[5] Therefore, the
samples with these topologies were tested only in one z direction. These
types of structures, including F2BCC can be used in applications where
homogeneity of rigidity is required and could be useful in applications
with combined stresses.
A higher load capacity was reached by BCCz and FCC topologies.
Adding struts to BCCz gave it a higher load capacity in both directions
than BCC. The PFCC topology was the stiffest with the least aligned
results. This may suggest that its mechanical properties will degrade
faster compared to other topologies, when the load is combined as
pressure-torsion. However, some components are stressed along one
direction and there the PFCC could be beneficial.
This conclusion corresponds with the results in article [3]. The
authors focus on FEM analyses of different kinds of self-supported
cellular lattices. It was confirmed that a double gyroid is particularly
suitable for applications with multiaxial loading, since it provides
identical rigidity of the porous sample in the x and z load axis. This
also applies to a single gyroid, although achieving less rigidity.
Geometrically simpler cellular lattices (such as BCCz, FCC, PFCC) have
significantly increased stiffness in one of the main direction.
Additional mechanical tests, such as bending strength or cyclic
fatigue, will be added in the following research activities.
DOI: 10.2507/28th.daaam.proceedings.067
6. Acknowledgments
The present contribution has been prepared under project LO1502
'Development of the Regional Technological Institute' under
the auspices of the National Sustainability Programme I of the Ministry
of Education of the Czech Republic aimed at supporting research,
experimental development and innovation.
7. References
[1] Hussein, A., Hao, L., Yan, C., Everson, R., Young, P. (2013)
Advanced lattice support structures for metal additive manufacturing,
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213 (2013) 1019-1026
[2] Contuzzi, N., Campanelli, S., Casavola, G., Ludovico, A. D.
(2010) Effect of heat treatment on selective laser melted steel parts,
Proceedings of the 21th DAAAM International Symposium, Published by
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Austria
[3] Aremu, A. O. all et. A Comparative finite element study of
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[4] Thomas, D. (2009) The Development of Design of Design Rules for
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[5] Challis, V. J. all et. (2014) High specific strength and
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[6] Yan, C., Hao, L., Hussein, A., Young, P., Raymont, D. (2014)
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Materials and Design 55 (2014) 533-541
[7] Hao, L., Raymont, D., Yan, C., Hussein, A., Young, P. (2011)
Design and Additive Manufacturing of Cellular Lattice Structures,
College of Engineering, Mathematics and Physical Sciences, University of
Exeter, Exeter EX4 4QF, Devon, United Kingdom
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[11] Marcian, P., Rehorek, Z. Florian, Z., Dlouhy, I, (2011)
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Caption: Fig. 1. Compared unit cells topologies
Caption: Fig. 2. Samples of lattice structures
Caption: Fig. 3. Load capacity of the different topologies
Caption: Fig. 4. Deformation curves of lattice with PFCC unit (a)
direction z (b) direction x
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