Researchers discover how to 3D print one of the strongest stainless steels

Side-by-side micrographs show elongated grains within the 3D-printed stainless steel.

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



For aircraft, cargo ships, nuclear power plants, and other critical technologies, strength and durability are essential. This is why many 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 steel can be consistently 3D printed while retaining 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 compositions of 17-4 steel that, when printed, match the properties of the conventionally 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 PH 17-4 part producers use 3D printing to reduce costs and increase their manufacturing flexibility. The approach used to examine the material in this study may also lay the groundwork 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 of some materials can produce results that are too inconsistent for certain applications. Metal printing is particularly complex, in part due to how quickly temperatures change during the process.

“When you think of additive manufacturing of metals, we’re essentially welding millions of tiny powder particles together with a high-power source like a laser, melting them into a liquid, and cooling them into a solid,” said the 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 out-of-equilibrium condition creates a number of extraordinary measurement challenges.”

3D printing with laser and metallic powder

A laser powder bed fusion type 3D printer in action. Laser powder bed fusion adds successive layers of metal powder and then uses a laser to fuse each layer in place on the part being created.

Because the material heats up and cools down so quickly, the arrangement, or crystal structure, of atoms within the material changes rapidly and is difficult to pin down, Zhang said. Without understanding what happens to the crystal structure of steel when 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 exhibits its highly sought-after properties.

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

Just as it takes a high-speed camera to see a hummingbird flapping its wings, the researchers needed special equipment to observe rapid changes in structure that occur in milliseconds. They found the right tool for the job in synchrotron X-ray diffraction, or XRD.

“In XRD, X-rays interact with a material and 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 study.

At the Advanced Photon Source (APS), a powerful light source at the Department of Energy’s Argonne National Laboratory, the authors smashed high-energy X-rays into steel samples during printing.

The authors mapped out how the crystal structure changed over the course of a print, revealing how factors they had control over, such as the composition of the metal powder, influenced the entire process.

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

“Composition control is truly the key to 3D printing alloys. By controlling the composition, we can 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,” said Zhang.

As an added benefit, some compositions resulted in the formation of strength-inducing nanoparticles that, with the traditional method, require the steel to be cooled and then reheated. In other words, 3D printing could allow manufacturers to skip a step that requires special equipment, additional time and cost to produce.

Mechanical tests 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 have an impact beyond 17-4 PH steel. The XRD-based approach could not only be used to optimize 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 reproducible, lowering the barrier to commercial use. By following this composition, manufacturers should be able to print 17-4 structures that are just as good as conventionally manufactured parts,” Chen said.

Paper: Q. Guo, M. Qu, CA Chuang, L. Xiong, A. Nabaaa, ZA Young, Y. Ren, P. Kenesei, F. Zhang, and L. Chen. Phase transformation dynamics guided the development of alloys for additive manufacturing. Additive manufacturing. Published online August 2, 2022. DOI: 10.1016/j.addma.2022.103068

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