Metal Additive Manufacturing eliminates dynamic strain ageing in conventional superalloys
January 28, 2019
Allison Beese, Assistant Professor of Materials Sciences and Engineering at The Pennsylvania State University, during experimentation with Inconel 635 at Oak Ridge National Laboratories (Courtesy ORNL)
According to a US-based team of materials scientists, the undesirable Dynamic Strain Ageing (DSA) trait found in conventionally processed superalloys, does not exist in a metal additively manufactured nickel-based superalloy. DSA occurs in metals at high temperatures subjected to stress. If present in conventionally processed materials, the strength of the material fluctuates with applied deformation, resulting in serrated stress-strain curves.
The team, led by Allison Beese, Assistant Professor of Materials Sciences and Engineering at The Pennsylvania State University, State College, Pennsylvania, USA, tested additively manufactured Inconel 625 versus traditionally processed Inconel 625 using neutron diffraction characterisation with mechanical testing at Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA.
Data gathered at the microscopic level gave a picture of the grain-level origins of the serrated stress curve, and resulted in a new understanding of the microstructure mechanisms that drive this phenomenon. It is thought that this research, published in Nature Communications, could pave the way for designing materials without Dynamic Strain Ageing. The researchers believe that this could lead to new manufacturing techniques that allow for alloys with tailored properties.
“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 explained. “That is not an ideal behaviour for materials to have as it could result in early breakage and unpredictable behaviour.”
According to the researchers, the conventional alloy had a random crystal structure but that the additively manufactured version had a better crystal texture and more finely dispersed particles. “We used a unique experimental setup to interrogate the mechanics at the grain level,” Beese continued. “We wanted to understand how that contributes to the difference in macroscopic behaviour 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 team attributed the lack of DSA in the additively manufactured material to a combination of finer particles distributed within the grains of this material and better crystal texture in the material, resulting in directionally-dependent properties, similar to wood, in which the material has differences in strength across versus with the grain.
Beese commented that additional research could allow the additively manufactured material to be further tuned for desired performance during initial processing, or with the use of heat treatments prior to fabrication to adjust the particles and grain structures. Additively manufacturing superalloys to near-net shape also is useful because superalloys, due to their strength, are difficult to machine. AM reduces the machining requirements, along with the amount of wasted material.
Further, she stated that the research could help improve longstanding models used to design and understand metals that undergo DSA during deformation, and also provide targets for the design of new metallic materials, particularly those fabricated by metal Additive Manufacturing.
