Dec 12, 2018 | By Cameron
A researcher at Penn State has demonstrated that 3D printed superalloys don’t exhibit an undesirable trait found in conventionally processed superalloys. Superalloys are alloys that are capable of withstanding extreme stresses, high temperatures, and highly-oxidizing environments. They’re incredibly useful in aircraft engines, gas and steam turbines, and heat exchangers for nuclear reactor systems. Superalloys are serious workhorses.
One of the most popular superalloys is the nickel-based Inconel. Inconel, like all alloys, is susceptible to dynamic strain aging (DSA), or serrations in the stress-strain curve caused by high strain rates experienced at high temperatures. Many adverse mechanical properties are associated with DSA, including a decrease in low-cycle fatigue life and loss of ductility. Basically, the superalloys lose some of their super strength when they’re put under heavy strain in 600°+ environments.
The mechanisms that cause DSA susceptibility are not fully understood, but it’s believed the issue is related to “the interaction between solute atmospheres that build up around dislocations as they move through the matrix and secondary phases within the matrix.” Don’t feel bad if you don’t understand that as this level of metallurgy is literally rocket science. The important thing is that Allison Beese, an assistant professor of Materials Sciences and Engineering at Penn State, understands it as she directed the study.
“We saw the characteristic serrated stress curves in conventionally processed Inconel 625 at elevated temperatures, where the flow stress oscillates up and down as the material is deformed up and down,” Beese said. “That is not an ideal behavior for materials to have as it could result in early breakage and unpredictable behavior.”
For the study, cylindrical specimens were extracted from the 3D printed Inconel 625 sample and the conventionally processed Inconel 625 plate and then subjected to uniaxial compression tests at room temperature, 600 °C, and 700 °C. The 3D printed sample was produced using laser-based directed energy deposition (DED), where a laser melts a stream of pre-alloyed metallic powder as it’s delivered to the melt pool via a nozzle; the conventional plate came from Special Metals Corporation. In situ neutron diffraction characterization was measured with the VULCAN instrument.
“We used a unique experimental setup to interrogate the mechanics at the grain level,” Beese said. “We wanted to understand how that contributes to the difference in macroscopic behavior that we see between these two forms of Inconel 625 that had the same elemental composition, but were manufactured in different ways. We were able to develop a mesoscopic understanding of DSA’s origins, which was previously missing.”
The results proved enlightening not only in regards to better understanding the underlying mechanisms that lead to DSA but also as it relates to the microscopic differences between additively manufactured and conventionally fabricated materials. Due to more finely dispersed particles and a more ideal crystal texture, the 3D printed Inconel showed no signs of DSA at temperatures as high as 700° while the regular Inconel exhibited DSA at 600°. Inconel and other superalloys are especially difficult and costly to machine due to their unique hardness, so reducing the machining amount with 3D printing greatly increases the efficiency of fabricating components with superalloys.
Posted in 3D Printing Application
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