For the first time, stainless steel can be 3D-printed while maintaining its characteristics

Turns out synchrotron X-ray diffraction gets the job done.
Nergis Firtina
Close-up photo of a 3D printer

RodrigoEM/iStock 

Researchers from the National Institute of Standards and Technology (NIST), the University of Wisconsin-Madison, and Argonne National Laboratory have churned out particular 17-4 steel compositions. When printed, they match the properties of the conventionally manufactured version.

The results of the research were published in Additive Manufacturing's November issue. They used high-energy X-rays from a particle accelerator to acquire the data.

Strength and endurance are crucial for nuclear power plants, cargo ships, airplanes, and other key technologies, says NIST. For this reason, many are made of the extraordinarily durable alloy 17-4 precipitation hardening (PH) stainless steel. For the very first time, 17-4 PH steel can now be reliably 3D printed while maintaining its beneficial properties.

The latest research might make 3D printing more cost-effective and flexible for manufacturers of 17-4 PH items. The method utilized to investigate the substance in this study may also lay the groundwork for a better comprehension of how to print various substances and forecast their qualities and performance.

“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 study co-author.

“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.”

What happens during the fast temperature changes?

Researchers began exploring what they could do to understand what happens during rapid temperature change and orient the interior structure towards the martensite.

To examine rapid structural changes that take place in milliseconds, the researchers needed specialized tools. They discovered synchrotron X-ray diffraction, or XRD, to be the ideal technique for it.

“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 study co-author.

Graphical abstract from the research.

The authors were able to fine-tune the composition of the steel to find a set of compositions consisting just of iron, nickel, copper, niobium, and chromium that worked because they now had a good understanding of the structural dynamics during printing as a reference.

“Composition control is truly the key to 3D-printing alloys. 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 1,000 and 10 million degrees Celsius per second, our compositions consistently result in fully martensitic 17-4 PH steel,” Zhang said.

The recent work might be influential beyond 17-4 PH steel as well. The information obtained from the XRD-based method might be used to develop and test computer models intended to forecast the quality of printed items in addition to optimizing other alloys for 3D printing.

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

Abstract:

Fusion-based additive manufacturing technologies enable the fabrication of geometrically and compositionally complex parts unachievable by conventional manufacturing methods. However, the non-uniform and far-from-equilibrium heating/cooling conditions pose a significant challenge to consistently obtaining desirable phases in the as-printed parts. Here we report a martensite stainless steel development guided by phase transformation dynamics revealed by in-situ high-speed, high-energy, high-resolution X-ray diffraction. This developed stainless steel consistently forms desired fully martensitic structure across a wide range of cooling rates (102–107 ℃/s), which enables direct printing of parts with a fully martensitic structure. The as-printed material exhibits a yield strength of 1157 ± 23 MPa, comparable to its wrought counterpart after precipitation-hardening heat treatment. The as-printed property is attributed to the fully martensitic structure and the fine precipitates formed during the intrinsic heat treatment in additive manufacturing. The phase transformation dynamics guided alloy development strategy demonstrated here opens the path for developing reliable, high-performance alloys specific for additive manufacturing.

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