SAMPE Journal - November/December 2021 - 50

FEATURE / ADDITIVE MANUFACTURED LATTICE
An example of the optimisation algorithm
progress is shown in Figure 3, which plots the part
mass, maximum stress and eigenfrequency results
versus step number.
An example showing how the topology
easily within the available
optimised part
fits
geometric envelope is shown in Figure 4.
EXPERIMENTATION
Development Process
With the end application, a spacecraft antenna
mounting in mind, a detailed qualification of all
stages of the manufacturing process and a part
design validation was necessary. In a subsequent
serial production the inspection and proof-load
steps may possibly be relaxed, at this stage an
exhaustive characterization was performed to build
confidence in the capabilities of the AM process to
manufacture space quality lattice type parts.
A relatively complex hybrid design combining
lattice with variable cell properties and a primary
load-path reinforcing framework was predicted
from the design optimisation work to fulfill the
technical objectives, the final design pushed the
manufacturing capabilities to the limit to achieve
a minimum mass.
Process Equipment
All manufacturing was performed in an SLM
Solutions Group AG type SLM 280 type AM
machine at NETME in Brno. The equipment was
capable of meeting all of the hybrid lattice design
expectations, with truss diameters as small as
0.4 mm diameter shown to be feasible with the
dimensional accuracy and porosity level needed
for the strength performance of the part.
Materials
The choice of materials for AM laser sintering
process is more limited than conventional
manufacturing. An adequate range of aluminium,
titanium, steel and copper types exits, with
several new materials in development or recently
available. For
the
technical
objective
of
mass, Aluminium was preferred, and AlSi10Mg
aluminium in pre-alloyed powder form was used.
AlSi10Mg is a typical casting alloy with good casting
properties and is used conventionally for cast
parts with thin walls and complex geometry. The
alloy combination silicon/magnesium results in a
significant increase in the strength and hardness. It
features good dynamic properties and is therefore
used for parts subject to high loads. High strength
and low strength allow light a low mass, and good
thermal conduction is beneficial to the spacecraft
application.
Figure 4. An intermediate step in the design iteration, the lattice grid result
is drawn inside of the allowed geometric envelope for the part and interface
positions.
low
Additive Manufacturing (AM) techniques
for metallic materials are characterized by
extremely fast heating and cooling cycles, and as a
consequence, metallic parts processed by AM are
sensitive to the build-up of internal stresses. They
also tend to exhibit much finer microstructures
when compared to their counterparts processed
by more traditional processing routes such as
casting, this microstructural refinement may cause
a significant increase in strength and hardness.
Age hardening due to the precipitation of Mg2Si
is possible due to the addition of a small amount
of Mg. Typical heat treatments include (i) stress
relief at 250°C for 2 hours, or (ii) solutionised at
e.g. 510°C for 6 hours and aged at e.g. 170°C for 4
50 | SAMPE JOURNAL | NOVEMBER/DECEMBER 2021
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Figure 3. Design topology optimisation run, mass, vibration eigenmode
frequency and peak stress results indicated versus iteration step.
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