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Inqui r y I s sue
1
| 2016
Inqui r y I s sue
1
| 2016
7
LIKE SANDTHROUGH AN HOURGLASS
Iver Anderson and Emma White, metallurgists at Ames
Laboratory, like to show off samples of metal powders
encapsulated in custom-made hourglasses to visitors. Dull
gray, the powders are barely remarkable in and of themselves,
let alone in comparison to each other.
Until the hourglasses are flipped and observers can compare
how the powders flow through the narrow necks of glass. The
powder created by traditional manufacturing methods doesn’t
flow, exactly. It starts and trickles and stops. It needs shaking
and manipulating to get through. The other powder, produced
at the Laboratory’s high-pressure gas atomization facility,
pulses smoothly through the hourglass of its own accord.
It’s all because of the smooth
spherical particles produced by Ames
Laboratory’s gas atomization method,
an improvement over traditionally
manufactured powders.
“You can see they’re chunky,
randomly sized, with rough edges,”
said White of the traditionally-made
powder particles, comparing scanning
electron microscope images of the
two. “They don’t flow past each
other, and that’s going to require a pulsing mechanism or an
agitator in the manufacturing process. That’s going to cost
the manufacturer more in energy to
run their production line.”
It’s only one of the many benefits of
powders created by the gas atomization
process, which has garnered the
Laboratory at least 16 patents over
the last two decades. It also helped
create spin-off company Iowa Powder
Atomization Technologies, which
was recently acquired by Praxair, and
exclusively licenses Ames Laboratory’s
titanium atomization patents to introduce titanium powder
to an eager marketplace.
SPLITTING LIQUID INTO DROPLETS
Gas atomization is a powder production method that
uses high-pressure gas flow to distintegrate molten metal
into particles. In the Metals Development building at Ames
Laboratory, Anderson, a senior metallurgist, and White, a
post-doctoral researcher, are able to produce experimental
quantities of powder with the Laboratory’s experimental
apparatus, about half of a liter volume per production run.
Another, larger gas-atomizer at Iowa State University’s
Applied Sciences Complex can produce around three liters.
The basic operation is the same in both. Metal is melted
by an induction furnace and held in a crucible with a
stoppered opening in the bottom. When the stopper is lifted,
the metal flows through a specially designed pour tube into
an atomization nozzle (also unique to Ames Laboratory)
that focuses a number of round-hole gas jets on the molten
metal in a tight pattern. The individual jets of gas—argon,
nitrogen, or helium depending on the run—knit together
to form a supersonic “curtain” that flows directly across the
liquid metal flow and forces the melt to couple directly with
the high kinetic energy of the supersonic gas, creating a
controlled droplet spray.
“This energetic coupling happens because the gas curtain
creates a suction that pulls the melt into the atomization
zone and simultaneously forces an upward directed gas
counter-flow to form that splits the liquid as though there
was an umbrella stuck underneath it and makes it flow
sideways, across to the outer edge of that round nozzle,”
said Anderson, who is also an Iowa State University adjunct
professor of materials science and engineering. “So it gets
presented to the gas as a thin film that is forced by the gas to
turn in the gas flow direction so it can shear past the surface
of that film, and strip off waves of liquid that break at their
crest to form droplets.
“It’s the same phenomenon you can see on the surface of a
pond hit with a gust of wind. You see small ripples and a spray
of water come off that gust.”
Once the droplets form, they solidify rapidly as they fall
through the spray chamber and are cooled by additional gas
halos. The resulting powder particles are separated from the
combined gas flow and settle into two powder collector cans
that are connected to the end of the spray chamber. The
cleaned inert process gas exits through two types of final filter
devices and is exhausted from the lab.
ADVANTAGES
Ames Laboratory’s gas atomization method produces
powders that are customizable, consistently sized and
smoothly spherical. The advantages of a perfectly formed
powder aremultiple. Besides the advantage of smooth powder
flow already mentioned, the individual round particles have
little internal porosity and pack together optimally in bulk.
Both qualities reduce dead air space and improve the quality
of parts produced using these powders.
Using gas atomization, Ames Laboratory has produced
powders of iron, aluminum, nickel, copper, tin, magnesium
and various other metals and alloys, in addition to titanium,
one of its key research accomplishments.
“The titanium industry is extremely interested in powder
metallurgy and final-shape consolidation methods,” said White.
“Titanium is expensive and the large amount of titanium
waste produced during machining cast parts into final shapes
significantly increases their costs. They see advances in powder
metallurgy as an effective cost control strategy by making parts
into near-final shapes and minimizing waste titanium.”
The powders produced by this method have also been
used in the production of stronger alnico (aluminum, nickel,
cobalt, and iron) permanent magnets, and in the production
of an experimental power transmission cable fabricated out
of an aluminum and calcium composite.
And the possibilities of these metal powders don’t just
look to the future, but may also redeem materials from the
past that had been abandoned by researchers and industry as
impossible to work with.
“You can create an alloy with fantastic properties, but if you
can’t make something useful out of it, it will never get off the
lab bench. This method enables us to revisit materials that
have been around a long time, give them a second chance,
and find new potential applications for them,” said Anderson.
IMPOSSIBLE SHAPES OUT OF INCREDIBLE ALLOYS
Ames Laboratory is seeking to expand its powder
production capabilities beyond research capacity, with the
goal of being able to produce up to 200 pounds of powder in
one production run.
At that scale, new opportunities for research are possible,
explained Anderson and White. Large batches provide
sufficient samples amounts for shared research projects
among multiple national laboratories and industry partners.
With new 3D printing and additive manufacturing
capabilities expanding rapidly, Ames Laboratory will be able
to position itself as a provider of custom metal powders for
these research areas, continuing to fine-tune the abilities of
the gas atomization process.
All of this is a natural progression of the research goals
Anderson has worked toward for decades.
“The ability to make impossible shapes out of incredible
alloys is my mission in life. I want to work on ways to get this
done,” Anderson said.
B Y L A U R A M I L L S A P S
Perfect Powder:
AMES LABORATORY’S PROCESSES PERFECT METAL POWDERS FOR MANUFACTURING
High-pressure gas atomized metal powders at various levels of magnification showing the perfection of the spherical end product.
Iver Anderson
Emma White
“You can create an alloy with fantastic properties, but if you can’t make something useful out of it, it
will never get off the lab bench.This method enables us to revisit materials that have been around a long
time, give them a second chance, and find new potential applications for them.”