MEDICAL
The cold spraying technique
operates at a temperature far below
that of titanium’s melting point,
instead utilising kinetic energy as the
primary energy source. As such, the
metal powder can be deposited at
supersonic speeds to form solid-state
bonding and create porous structures,
resulting in improved mechanical
properties and biocompatibility with
bone-enveloping preosteoblast cells.
Parts fabricated via powder bed
fusion often struggle with large
internal residual stresses, whereby the
cyclic heating and cooling caused by
the scanning laser can generate shear
stresses within a 3D printed
component. In extreme scenarios,
these residual stresses can be very
destructive, warping, distorting, and
cracking the part.
Cold spraying is designed to avoid
heat-related problems altogether,
instead accelerating powder particles
to supersonic impact velocities of
around 600m/s out of a de Laval
nozzle. The idea is to give the jetted
Titanium makes the
difference
Motor nodes are one of the hardest
e-bike parts to manufacture.
When GSD Global turned to
Sandvik’s experts in metal
powder and additive
manufacturing to 3D
print their motor nodes in
titanium, they found they
could achieve a lighter, more
durable and much more energy
efficient solution.
GSD Global is an engineering
and design consultancy with longstanding
experience in creating
premium electronic bicycles, or
e-bikes. Heading the organization
is Zach Krapfl, an electric vehicle
engineer based in Paonia, Colorado,
in the United States. Krapfl
is dedicated to global energy
conservation and reducing fossil
fuel consumption — and combines
bicycles, light electric vehicles and
renewable energy technologies as a
catalyst for sustainable transport.
26 » JANUARY 2021 » WWW.MADEIN.IE
GSD Global works
with various bicycle
OEMs (original equipment
manufacturers), with the majority
of their design work focusing on
e-bikes. For almost a decade, they’ve
been partnering with Bosch e-bike
systems to testify that, up until
recently, e-bike uptake has been
slow. Part of the explanation is
thought to be that titanium parts
such as the motor node that holds
the electric motor onto the bike
frame are very difficult to machine
using traditional CNC processes —
and costly at that.
When GSD Global turned to
Sandvik to investigate the possibility
of 3D printing their titanium
components, they
found that by developing
the design of the motor nodes and
adapting them to be additively
manufactured, they could reduce
their costs by more than 50 per cent.
Using powder bed fusion laser
technology, Sandvik 3D printed the
motor nodes using its Osprey®
Ti6AI4V powder. Typically,
these grades are used in the
medical, aerospace, automotive
and engineering industries for
applications that require significant
weight saving while maintaining
high strength and performance. The
motor nodes then underwent heat
treatment and sandblasting during
post processing.
material just enough kinetic energy to
fuse with the substrate below. In the
case of Ti64, the cold spraying occurs
at around 800 – 900°C, far below the
material’s melting point of 1626°C. At
this operating temperature, the metal
is soft enough for plastic deformation
to take place, but not so soft that the
titanium loses its structural integrity
once sprayed on the substrate.
To test the technique with titanium
alloy, the team conducted a number of
basic deposition studies with the
material. They found that they could
spatially control the porosity of the
deposits with modifications to the
powder impact velocity, achieving a
maximum porosity of 30%.
It was also found that cold spraying
the material could result in
mechanical properties superior to
those of other laser-based 3D printing
techniques. Specifically, the
researchers determined an apparent
compressive yield strength of 535MPa,
which is reportedly a whole 42%
higher than high-temperature DED.
The team also states that this
compressive strength could be
improved further with post-printing
heat treatments.
Finally, owing to the porous nature
of the deposits, the cold sprayed Ti64
was proven to be biocompatible with
MC3T3-E1 SC4 murine preosteoblast
cells. This, of course, cements its
viability for use in bone implants, a
very common application of the
material both in and out of the
additive manufacturing sphere.
Ultimately, the researchers were able
to demonstrate that an alternative
single-step solid-state 3D printing
process was capable of producing
biocompatible metal parts with higher
strengths than its more conventional
high-temperature counterparts,
boding well for the future of cold
spraying.
Custom cranial implants and
bespoke medical prosthesis are not for
the distant future — the technology
needed to develop and manufacture
them already exists. MADE
/WWW.MADEIN.IE