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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
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