Ames Laboratory’s Metal Powder Manufacturing Technology Opens Up New Material Possibilities for 3D Printing
Metal powders, of the sort commonly used for additive manufacturing, look pretty similar at first glance, but their differences are many – not just between differing metals and alloys, but between different batches of the same material, depending on how those batches were produced. Before a metal material can be used to produce additively manufactured parts, it has to go through a lot of processing itself before it becomes that fine powder we see being swept and sintered into solid components.
Those methods of processing vary as much as printing methods do, and certain powder production techniques can make a dramatic difference in the quality of the material and thus the finished part. At the Ames Laboratory in Iowa, a process called gas atomization is creating powders that are smoother and better-performing than other metal manufacturing powders. The tiny powder grains, while identical to those of traditionally manufactured powders, are smoother and more spherical than the rough, randomly sized particles that make up most metal powders. As a result, the particles produced through gas atomization flow smoothly without requiring any additional pulsing or agitation during the manufacturing process, saving energy and cost in the long run.
The Ames Laboratory, owned by the US Department of Energy and operated by Iowa State University, holds at least 16 patents for the gas atomization process. Their titanium atomization patents are exclusively licensed by Praxair, which acquired the lab’s spin-off company, IPAT, and is now working to bring the technology to market.
In gas atomization, metal is melted in an induction furnace and held in a crucible with an opening at the bottom. When that opening is unstoppered, the metal flows through a pour tube and into a unique atomization nozzle that tightly focuses several round-hole gas jets on the molten metal. The individual jets of gas, which can be argon, nitrogen or helium, meld together to create a supersonic curtain of sorts that passes across the flowing metal and forces it to couple with the kinetic energy of the supersonic gas, resulting in a controlled spray of droplets.
“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 Iver Anderson, senior metallurgist at Ames Laboratory. “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.”
Those droplets then solidify as they fall through the spray chamber and are cooled by additional gas halos, creating powder particles that are separated from the combined gas flow and drop into two powder collecting cans that are connected to the end of the spray chamber. The cleaned inert gas from the process then exits through two types of filters and is expelled from the lab.
It may be a bit hard to picture, but the results clearly show the benefits of the process. The powder grains are not only smooth, spherical and uniformly sized, but they’re customizable, have minimal internal porosity, and pack together well in bulk, reducing dead air space and improving part quality. Ames Laboratory has used the technology to produce powders of iron, aluminum, nickel, copper, tin, magnesium, and, of course, titanium, as well as several other metals and alloys.
“The titanium industry is extremely interested in powder metallurgy and final-shape consolidation methods,” said post-doctoral researcher Emma White. “Titanium is expensive and the large amount of waste titanium 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 created by gas atomization have been used to create stronger alnico (aluminum, nickel, cobalt and iron) magnets, as well as an experimental power transmission cable made from an aluminum and calcium composite. Not only does the technology open up opportunities for the development of new materials, but it also allows for new exploration of old materials that have been written off as too problematic in the past.
“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,” said Anderson. “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.”
Ames Laboratory is looking to scale up its powder production capabilities, moving beyond the research capacity and ideally being able to produce up to 200 pounds of powder in one production run. With larger batches of powder, not only will the lab be able to embark on larger, collaborative research projects with outside institutions, but it will be in a position to make major headway into materials advances that could be beneficial to the additive manufacturing industry.
“The ability to make impossible shapes out of incredible alloys is my mission in life,” said Anderson. “I want to work on ways to get this done.”
Discuss in the Ames Laboratory forum at 3DPB.com.[Source/Images: Ames Laboratory]
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