Martin, who has studied metallurgy, knew the problem was at the atomic level. Metals are crystals and their atoms line up in repeating patterns that make up grains. There are sometimes defects at the boundaries where grains meet up and those defects can cause cracks.
There was also the issue of powdered metal. The process of 3D-printing metal requires that the metal begins as a powder, which is laid down in very thin layers, each one heated with a laser in order to melt it. The molten metal must cool quickly before the next layer of powder is added and laser-heated. The intense heating and quick cooling causes metal grains to solidify in odd shapes, which can lead to cracks.
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Because the grains of powder are tiny and measured in microns, Martin and his team thought they could introduce nanosized particles to beef up the metal’s strength. They used a computer program to sort through and analyze more than 4,500 different alloy and nanoparticle combinations to see which ones had atomic structures that would fit together best. The idea was that the grains of metal would glom onto the tiny nanoparticles, sort of like a water vapor droplet that nucleates around a particle of dust to create a drop of rain.
They found that a nanoparticle made of hydrogen-stabilized zirconium would work best with two different kinds of aluminum alloys. During the very first lab experiment, they laid down a thin powdery layer of metal microparticles coated with a very fine layer of nanoparticles before running the laser over it. Layer after layer, the printer created the object until it was finished.
When it was completed, the scientists cut the object in half, then polished it. There were no cracks.
“I was surprised that it worked the first time,” said Martin. “We followed physics, so I wasn’t too shocked that it worked, but it’s easy to write something down on a piece of paper. It’s much harder to follow through.”
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It worked, said Martin, because as soon as the liquid metal reached the point where it should solidify into grains, they immediately nucleated around the nanoparticles. That produced uniform cooling and created a fine grain structure that prevented cracking.
Martin and his team think the technology could be adapted to other alloys used for high-temperature applications, such as turbine blades for jets and even hypersonic vehicles.
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