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One of the first things you learn in Mechanical
Engineering is how to design your inventions
in a way that is possible to manufacture and
assemble. This is a skill that takes time
to learn, primarily by working with machinists
that look at your design and laugh at the
imcompetitance of this young college kid.
From placing fastener holes in inaccessible
locations to placing a sharp inner corners
that no milling machine can achieve.
So much of our engineering capabilities are
dictated by what we can manufacture and every
time a new method of manufacturing is invented,
it ushers in new technologies once deemed
impossible. Just as the simple cylinder boring
machine facilitated the industrial revolution,
3D printing may now be opening doors to new
designs.
Complicated hollow structures are now possible.
Allowing designers to integrate cooling ducts
directly into parts, an incredible useful
tool for high temperature parts like turbine
blades and rocket nozzles. We can perform
something called topology optimization, where
we use finite element analysis, a type of
stress simulation, to tell us exactly where
material is needed. Allowing us to generate
the perfect structure for our application,
similar to how our hollow bones are formed,
and thus allowing us to save on weight. Helping
our lightweight vehicles to gain even more
performance.
Often parts are machined down from giant blocks
of raw material to their final form. In aviation
we measure this waste with the buy to fly
ratio. Which divides the weight of the final
part by the weight of the raw material it
was manufactured from. Imagine taking a material
like a titanium alloy which can cost upwards
of 30 dollars per kilo, and then throwing
away 90% of it in the manufacturing process.
Needless to say this is a massive source of
increased costs, that 3D printing could help
reduce.
All of these benefits can come together to
unshackle engineers to form the perfectly
shaped objects and perhaps one of the most
interesting applications of this is this incredible
3D printed aerospike rocket engine, that has
incorporated liquid cooling channels directly
into the rocket nozzles interior and shaped
everything optimally to provide a highly efficient
rocket nozzle that can operate efficiently
at many different altitudes.[1]
But, even with all these amazing applications.
We rarely see 3D printed parts outside of
prototyping applications like this. It’s
clear that 3D printing can be take human manufacturing
to it’s next evolution, but what’s holding
it back?
As usual the first problem is cost. If we
plot the price of a 3D printed part as a function
of the number of parts created. It would look
something like this. [2] It’s price will
be dominated by initial machine cost, and
that line will only marginally trend downwards
as we print more parts, due to the insane
time it takes to print a single part, after
all we are essentially welding hundreds of
kilometres of metal powder together.
To scale up our manufacturing we need to buy
additional machines, which will not lower
our cost. This turns our traditional economies
of scale on its head.
Take an injection molded part. Early on the
cost will be dominated by the cost of creating
an expensive mold needed to form the part,
but once that is finalised this machine can
churn out piece after piece in rapid succession,
we mostly just have to wait for the plastic
to cool down before we can eject it from the
mould and restart the process.
This results in a graph that looks something
like this where our cost per part rapidly
decreases as we build more, soon becoming
dominated by the material costs. This means
that it only makes economic sense to use 3D
printing for parts that fall behind this break
even point. Which is why it is used so frequently
for rapid prototyping. If we can reduce the
raw material cost, with better supply, and
decrease 3D printer machine costs we can lower
this line and open up more parts to being
replaced by 3D printing. That is gradually
happening as the cost of these machines lowers,
in large part due to patents expiring in the
last 5 years. However it’s not just cost
preventing 3D printed parts from entering
the market.
This month I spoke with Professor Roger Reed,
the founder of OXMET, a company taking on
the challenge of developing metal alloys and
printing techniques to improve the material
properties of these additive manufactured
parts. To get a better idea of the material
science that prevents 3D printed parts from
being approved for even specialized small
batch applications.
We have thousands of years of experience mostly
through trial and error of learning how manufacturing
techniques affect the material properties
of the metals we use. From learning how to
tailor carbon content during iron ore smelting,
to learning hour each hammer blow can affect
the crystalline structure of the metal. In
particular, we have learned how the exact
way we heat and cool a metal effects it’s
material properties, as a result of it’s
internal crystalline structure.
But additive manufacturing throws away much
of the techniques we have developed. Forcing
us to build much of our understanding up from
scratch and develop completely novel techniques
for studying and optimizing our material properties.
One of the key areas of research in this regard
is improving 3D printed metal fatigue life.[3]
Fatigue life is a measure of how many cycles
of stress a part can sustain before breaking.
Because materials CAN fracture even below
their ultimate strength, if you cycle them
at a lower stress for extended periods. This
affects every metal and is the reason continually
maintenance is always needed for machinery.
We can visualise a materials fatigue strength
by plotting on a S-N curve, which places the
magnitude of the alternating stress on the
Y-axis and the number of cycles it survived
on the y-axis. For traditional machined titanium
it looks something like this, [3][4] whereas
for 3D printed parts it looks more like this.
Put simply, 3D printed parts fail much sooner.
Stopping many of the parts from being approved
for the applications they are best suited
for, like aviation.
So why does this happen? First we need to
understand what causes fatigue fractures.
The primary cause of these fractures is crack
growth, [5] where small voids and imperfections
within the part can force stress to divert
and pile up in sharp corners and thus exceed
the metals strength locally and cause the
crack to grow. The more imperfections present,
the more likely your fatigue life is going
to suffer.
And 3D printed material tend to have a lot
of imperfections. We got a clearer look at
why this happens when researchers used high-speed
synchrotron X-ray imaging to get this phenomenal
footage of the laser melting process, which
revealed many of the phenomenon resulting
in imperfections. [6]
Here we have a powder bed of iron-nickel alloy
called Invar 36, which has been turned into
a powder by blasting a stream of molten metal
with a high pressure gas. This process is
called atomization.
As the laser moves across the powder bed it
melts it, essentially forming a weld line.
You can see that this layer tended to dig
into the powder bed. Creating a track that
varies in height. These sort of imperfection
means the final product needs a surface machining
to create a quality part.
Although it’s important to note that this
study was specifically studying something
called an overhang condition where the part
has no structure below it and has to build
on the loose bed of powder instead.
As the laser marches on, the powder in front
of it gets blown away, meaning the laser no
longer has metal powder to melt in that region
and instead forms a new beads of molten metal
ahead of the original track, which eventually
coalesce with the original. Finally we can
see some worrisome behaviour as the laser
reaches the end of this track as the molten
metal begins to cool, we see pores begin to
form in the upper surface of the track. The
exact kind of imperfections that could allow
crack growth to occur in the future.
This study also varied several factors like
laser speed and laser power to study their
effects on the melt tracks properties. Here
they increased the speed of the laser to a
point that the metal particles did not have
enough time to heat up and coalesce.
In another experiment they investigated the
interaction of two melt tracks. Here we can
see more pores forming as a result of overhangs
trapping gas, and yet more pores form in the
same manner as before as we reach the end
of our track.
Clearly, this process is much more complicated
than just melting some metal powder together
and in the end the final products that come
directly out of a 3D printer are far from
finished and need a significant amount of
post processing.
For example, we can help close these pores
by using a method called hot isostatic pressing,
where we apply heat and a very high isostatic
pressure, which just means the pressure is
the same in all directions. [7] This maintains
the overall part shape, but compresses and
heats the part up enough to close those pores
to improve our fatigue strength, but not enough
to compete with traditionally machined parts.
This of course pushes this cost bar higher,
making 3D printing again less attractive for
applications outside of rapid prototyping,
and we have yet more material property issues
to address.
We explored the science of forging with my
friend Alex Steele in a previous video. We
learned how the internal crystal grain structures
is one of the most influential factors in
determining a materials final material properties.
We can control the materials hardness and
ductility by simple heating and cooling it
in a particular way.
Typically when a piece of molten metal is
cooled, crystals grow at random from individual
nucleation points and form crystal grains.
The size and structure of these grains dictates
so many of the metals final material properties
and we have learned over thousands of years
of metal forging how to get the best out of
our materials. Once again, additive manufacturing
throws much of this knowledge out the window.
Leaving us to start from scratch.
We have learned that 3D printed materials
tend to form these columnar grains that rise
up in the direction of the print and that
the grains tend to follow the direction of
the laser. [8] Forming directional grain structures
that can almost be thought of like the grains
in a piece of wood.
This means that how the laser moves has a
massive effect on the material properties
of the material and thus we can use this to
our advantage by tailoring our laser scan
strategy. [9]
One of the most common laser scan strategies
is the islands scan strategy where a pattern
like this is formed created 5 mm islands of
laser track paths oriented perpendicularly
to each other, these islands are formed in
a random sequence. This scan strategy developed
by Concept Laser was created to alleviate
residual stresses that form as a result of
uneven heating a cooling within the metal,
which can decrease the parts overall strength.
Just another factor designers have to consider
and often requires the part to be placed in
an oven after printing to help alleviate residual
stresses
However, one study found that this scan strategy
has some unique effects on the grain structure.
Creating those aforementioned vertical grain
structures with fine grain boundaries between
each island, and these fine grain boundaries
had a high density of cracks, which again
can grow a cause fatigue failure. [8] There
are of-course alternative laser scan strategies
like this helical one. [10] Other researchers
are attempting to use thermal and other specialised
cameras inside the build chamber to observe
the phenomenon like pore formation and inform
the laser exactly how to operate with machine
learning to maximise material properties.
While I don’t see this manufacturing technique
ever being used for low cost high volume parts
where other manufacturing techniques are much
better suited, if we can improve the fatigue
life of these metals we could start seeing
them appear in more applications. Like that
incredible 3D printed aerospike engine we
saw earlier.
This is a VERY new area of research, that
could use more eyes. Just as I learned how
to design to get the best out of carbon fibre
composites and moulded plastics over the course
of my university life and industry experience.
We are now seeing young engineers beginning
their education with this form of design in
mind, allowing them to create designs that
were once deemed impossible.
I believe there is going to be a fascinating
meeting of material science and machine learning
in this space to customise laser scanning
patterns for particular parts and allow the
machines to spot and fix defects as they happen,
and I would imagine the overlap of material
scientists and machine learning coders is
a small pool of people at the moment. So perhaps
this could be a career path for you, and you
could start working towards it right now by
taking this course on Machine Learning on
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