Insights revealed by a large particle accelerator lighted a way forward – ScienceDaily


For aircraft, cargo ships, nuclear power plants, and other critical technologies, strength and durability are essential. This is why many of them contain a remarkably strong and corrosion-resistant alloy called 17-4 precipitation hardening (PH) stainless steel. Now, for the first time ever, 17-4 PH steels can be statically 3D printed while retaining their preferred properties.

A team of researchers from the National Institute of Standards and Technology (NIST), University of Wisconsin-Madison, and Argonne National Laboratory 17-4 have identified certain steel fittings that, when printed, match properties of the traditionally manufactured version. The researchers’ strategy described in the journal additive manufacturing, It is based on high-speed data about the printing process that they obtained using high-energy X-rays from a particle accelerator.

The new findings could help producers of 17-4 PH parts use 3D printing to reduce costs and increase manufacturing flexibility. The approach used for examining the materials in this study may also lead to a table setting for a better understanding of how other types of materials can be printed and to predict their properties and performance.

Despite its advantages over conventional manufacturing, 3D printing of some materials can lead to very inconsistent results for certain applications. Printing metal is particularly complex, in part because of how quickly temperatures change during the process.

“When you think about additive manufacturing of metals, we’re basically welding millions of tiny, powdery particles into one piece with a high-energy source like a laser, melting them into a liquid and cooling them into a solid,” said the NIST physicist. Fan Chang, co-author of the study. “But the cooling rate is high, sometimes above a million degrees Celsius per second, and this highly non-equilibrium condition creates a set of unusual measurement challenges.”

Because the material heats up and cools so quickly, the crystal arrangement or crystal structure of the atoms inside the material changes rapidly and is difficult to determine, Zhang said. Without understanding what happens to the crystal structure of steel as it is printed, researchers have struggled for years to 3D print 17-4 PH, in which the crystal structure must be well-suited – a type called martensite – for the material to display its highly desirable properties.

The authors of the new study aim to shed light on what happens during rapid temperature changes and find a way to push the internal structure toward martensite.

Just as a high-speed camera was needed to see a hummingbird’s wings, the researchers needed special equipment to monitor the rapid shifts in the structure that occur in milliseconds. They found the right tool for this work in synchrotron X-ray diffraction, or XRD.

“In XRD, X-rays interact with a material and will form a fingerprint-like signal corresponding to the specific crystal structure of the material,” said Lianyi Chen, professor of mechanical engineering at UW-Madison and co-author of the study.

At the Advanced Photon Source (APS), a 1,100-meter particle accelerator at Argonne National Laboratory, the authors smash high-energy X-rays into steel samples during printing.

The authors determined how the crystal structure changed over the course of printing, and revealed how certain factors they controlled — such as the composition of the mineral powder — affected the process throughout.

While iron is the primary component of 17-4 PH steel, the composition of the alloy can contain various amounts of up to ten different chemical elements. Now equipped with a clear picture of the structural dynamics during printing as a guide, the authors were able to fine-tune the composition of the steels to find a range of compositions including iron, nickel, copper, niobium, and chromium that did just that. trick.

“Control of the composition is really the key to 3D printing the alloy. By controlling the composition, we can control how it solidifies. We have also shown that, over a wide range of cooling rates, say, between 1,000 and 10 million degrees Celsius per second, our formulations Consistently lead to a PH steel entirely of 17-4 martensitic,” Zhang said.

As a bonus, some formulations have led to the formation of force-inducing nanoparticles which, in the traditional way, require the steel to be cooled and then reheated. In other words, 3D printing can allow manufacturers to skip a step that requires special equipment, additional time and production cost.

Mechanical testing showed that the 3D-printed steel, with its martensitic structure and strength-catalyzed nanoparticles, matches the strength of steel produced through conventional means.

The new study can make spatter beyond 17-4 PH steels as well. Not only can the XRD-based approach be used to improve other alloys for 3D printing, but the information it reveals could be useful for building and testing computer models aimed at predicting the quality of printed parts.

“Our 17-4 is reliable and repeatable, which lowers the barrier to commercial use,” Chen said. “If they follow this formula, manufacturers should be able to print 17-4 structures as good as conventionally manufactured parts.”



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