Researchers from the National Institute of Standards and Technology (NIST), the University of Wisconsin-Madison and Argonne National Laboratory have recently published a paper in Additive Manufacturing on techniques to additively manufacture 17-4 PH stainless steel to meet user requirements.
To additively manufacture in 17-4 PH stainless steel, the crystal structure within must be martensite to exhibit the metal’s sought-after properties. Because the Additive Manufacturing process means materials heat and cool quickly, however, the arrangement of the material’s atoms can be de difficult to pin down.
In ‘Phase transformation dynamics guided alloy development for additive manufacturing’, researchers used a strategy evolved from high-speed data – gathered from X-rays from a particle accelerator – about the Additive Manufacturing process to repeatedly additively manufacture 17-4 PH with the necessary martensite structure.
At the Advanced Photon Source (APS), an 1,100-metre-long particle accelerator housed at Argonne National Lab, the authors smashed high-energy X-rays into steel samples during Additive Manufacturing. They then mapped out how the crystal structure over the course of the manufacturing process, revealing how certain factors (such as the composition of the powder) influenced the process.
While iron is the primary component of 17-4 PH steel, the composition of the alloy can contain differing amounts of up to a dozen different chemical elements. With a clear picture of the structural dynamics during printing as a guide, the authors were able to fine-tune the makeup of the steel to find a set of compositions including just iron, nickel, copper, niobium and chromium that resulted in the necessary martensite structures.
“Composition control is truly the key to 3D-printing alloys. By controlling the composition, we are able to control how it solidifies,” stated Fan Zhang, NIST physicist and co-author of the study. “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.”
As well as the necessary structure, some compositions resulted in the formation of strength-inducing nanoparticles that, in traditional manufacturing, require the steel to be cooled and reheated. This would allow manufacturers to skip a step that requires special equipment, additional time and production cost by utilising Additive Manufacturing.
Mechanical testing showed that the additively manufactured steel, with its martensite structure and strength-inducing nanoparticles, matched the strength of steel produced through conventional means. But this study could be relevant beyond 17-4 PH steel, as well; not only could the approach be used to optimise other alloys for Additive Manufacturing, but the information could be useful for building and testing computer models meant to predict the quality of additively manufactured parts.
“Our 17-4 is reliable and reproducible, which lowers the barrier for commercial use,” added Lianyi Chen, a professor of mechanical engineering at UW-Madison and study co-author. “If they follow this composition, manufacturers should be able to print out 17-4 structures that are just as good as conventionally manufactured parts.”
The paper is available here.