Microscope images of the 3D-printed 17-4 stainless steel. The colors in the image on the left represent the differing orientations of crystals within the alloy. Image: NIST.
Microscope images of the 3D-printed 17-4 stainless steel. The colors in the image on the left represent the differing orientations of crystals within the alloy. Image: NIST.

For airliners, cargo ships, nuclear power plants and other critical technologies, strength and durability are essential. This is why many utilize a remarkably strong and corrosion-resistant meta alloy called 17-4 precipitation hardening (PH) stainless steel. Now, for the first time ever, researchers have developed a way to 3D-print 17-4 PH steel while retaining its favorable characteristics.

The researchers from the US National Institute of Standards and Technology (NIST), the University of Wisconsin (UW)-Madison and Argonne National Laboratory identified particular 17-4 steel compositions that, when printed, match the properties of the conventionally manufactured version. The researchers’ strategy, reported in a paper in Additive Manufacturing, is based on high-speed data about the printing process obtained using high-energy X-rays from a particle accelerator.

These new findings could help producers of 17-4 PH parts use 3D printing to cut costs and increase their manufacturing flexibility. The approach that the researchers used to examine the alloy may also lead to a better understanding of how to print other types of materials, and predict their properties and performance.

Despite its advantages over conventional manufacturing, the 3D-printing of some materials can produce results that are too inconsistent for certain applications. Printing metal is particularly tricky, in part because of how quickly temperatures shift during the process.

“When you think about additive manufacturing of metals, we are essentially welding millions of tiny, powdered particles into one piece with a high-powered source such as a laser, melting them into a liquid and cooling them into a solid,” said NIST physicist Fan Zhang, a co-author of the paper. “But the cooling rate is high, sometimes higher than one million degrees Celsius per second, and this extreme nonequilibrium condition creates a set of extraordinary measurement challenges.”

Because the material heats and cools so quickly, the arrangement, or crystal structure, of the atoms within the material shifts rapidly and is difficult to pin down. Without understanding what is happening 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 just right – a type called martensite – for the material to exhibit its highly sought-after properties.

The researchers aimed to shed light on what happens during the fast temperature changes and to find a way to drive the internal structure toward martensite. But just as a high-speed camera is needed to see a hummingbird’s flapping wings, the researchers needed special equipment to observe the rapid shifts in structure, which occur in milliseconds. They found the right tool for the job in synchrotron X-ray diffraction (XRD).

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

At the Advanced Photon Source (APS), an 1100m-long particle accelerator housed at Argonne National Lab, the researchers smashed high-energy X-rays into steel samples during printing, and then mapped out how the crystal structure changed over the course of a print. This revealed how certain factors they had control over — such as the composition of the powdered metal — influenced the process throughout.

While iron is the primary component of 17-4 PH steel, the precise composition of the alloy can vary, with differing amounts of up to a dozen chemical elements, including nickel, copper, niobium and chromium. Now equipped with a clear picture of the structural dynamics during printing as a guide, the researchers were able to fine-tune the makeup of the steel to find the optimum set of compositions.

“Composition control is truly the key to 3D-printing alloys,” said Zhang. “By controlling the composition, we are able to control how it solidifies. We also showed that, over a wide range of cooling rates, say between 1000 and 10 million degrees Celsius per second, our compositions consistently result in fully martensitic 17-4 PH steel.”

As a bonus, some compositions resulted in the formation of strength-inducing nanoparticles, which can only be produced in the traditional method by cooling and then reheating the steel. In other words, 3D printing could 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 martensite structure and strength-inducing nanoparticles, matched the strength of steel produced through conventional means.

The new study could make a splash beyond 17-4 PH steel as well. 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 meant to predict the quality of printed parts.

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

This story is adapted from material from NIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.