Researchers discover how to 3D print one of the

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image: A microscopic image of 3D printed 17-4 stainless steel. The colors in the left version of the image represent the different orientations of the crystals in the alloy.
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Credit: NIST

For airliners, freighters, nuclear power plants and other critical technologies, strength and durability are essential. That’s why many contain a remarkably strong, corrosion-resistant alloy called 17-4 precipitation hardening (PH) stainless steel. Now, for the very first time, 17-4 PH steel can be consistently 3D printed while maintaining its favorable characteristics.

A team of researchers from the National Institute of Standards and Technology (NIST), the University of Wisconsin-Madison, and Argonne National Laboratory have identified particular 17-4 steel compositions that, when printed, match the properties of the conventionally manufactured version. The researchers’ strategy, described in the review Additive manufacturing, is based on high-speed data on 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 to examine the material in this study may also set the stage for a better understanding of how to print other types of materials and predict their properties and performance.

Despite its advantages over conventional manufacturing, 3D printing some materials can produce results that are too inconsistent for some applications. Metal printing is particularly complex, in part due to the speed at which temperatures change during the process.

“When you think of additive metal manufacturing, we’re basically welding millions of tiny powder particles into one piece with a high-power source like a laser, melting them in a liquid, and cooling them in one go. solid,” said a NIST physicist. Fan Zhang, co-author of the study. “But the cooling rate is high, sometimes exceeding a million degrees Celsius per second, and this extreme non-equilibrium condition creates an extraordinary set of measurement challenges.”

Because the material heats up and cools down so quickly, the arrangement, or crystal structure, of the atoms in the material moves quickly and is difficult to pinpoint, Zhang said. Without understanding what happens to the crystal structure of steel when it’s printed, researchers have struggled for years to 3D print 17-4 PH, in which the crystal structure has to be just – a type called martensite – so that the material exhibits its highly sought-after properties.

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

Just as a high-speed camera is needed to see a hummingbird’s wing beat, the researchers needed special equipment to observe the rapid changes in structure that occur within milliseconds. They’ve found the right tool for the job in synchrotron X-ray diffraction, or XRD.

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

At Advanced photon source (APS), a 1,100-meter-long particle accelerator housed at Argonne National Laboratory, the authors shot high-energy X-rays at steel samples during printing.

The authors mapped how the crystal structure changed during a print, revealing how certain factors over which they had control – such as the composition of the powdered metal – influenced the process throughout.

Although iron is the main component of 17-4 PH steel, the composition of the alloy can contain varying amounts of up to a dozen different chemical elements. The authors, now equipped with a clear picture of structural dynamics when printed as a guide, were able to narrow down the steel composition to find a set of compositions including only iron, nickel, copper, niobium and chrome that did the trick.

“Composition control is really key with 3D printing alloys. By controlling the composition, we are able to 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 compositions consistently result in fully martensitic 17-4 PH steel,” Zhang said.

As a bonus, some compositions resulted in the formation of resistance-inducing nanoparticles which, with the traditional method, require cooling and then reheating the steel. In other words, 3D printing could allow manufacturers to skip a step that requires special equipment, time and additional production costs.

Mechanical testing showed that the 3D-printed steel, with its martensite structure and strength-inducing nanoparticles, matched the strength of steel produced by conventional means.

The new study could also cause a stir beyond 17-4 PH steel. Not only could the XRD-based approach be used to optimize other alloys for 3D printing, but the information it reveals could be useful for building and testing computer models to predict the quality of printed parts.

“Our 17-4 is reliable and repeatable, which lowers the barrier for commercial use. If they follow this composition, makers should be able to print 17-4 structures that are just as good as conventionally made parts,” Chen said.


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