The Co-Ni superalloy reportedly overcomes the key issue of cracking, which can plague parts fabricated via high-temperature powder bed fusion technologies such as SLM and EBM. The scientists believe their material holds “tremendous promise” for the advancement of industrial 3D printing in high-stress applications, including critical aerospace engine components and chemical-contacting nuclear components.
The problem of additive manufacturing compatibility
Speaking of the motivation behind the work, Tresa Pollock, associate dean of the College of Engineering at UC Santa Barbara, states that many of the advanced alloys in use today with conventional manufacturing methods are simply not compatible with additive manufacturing. High-performance alloys designed to withstand extreme heat and chemicals are often faced with the issue of cracking, predominantly due to excessive residual stresses caused by the cyclic heating and cooling present in the metal 3D printing process.
Pollock adds, “They can crack in their liquid state, when an object is still being printed, or in the solid state, after the material is taken out and given some thermal treatments. This has prevented people from employing alloys that we use currently in applications such as aircraft engines to print new designs that could, for example, drastically increase performance or energy efficiency.”
Intrigued by the challenge of developing additive-compatible high-performance alloys, Pollock and her team commenced the project using a $3M Vannevar Bush Faculty Fellowship award from the Department of Defense.
Nickel-based superalloys for aerospace
The final formulation ended up containing approximately equal parts cobalt and nickel, as well as smaller amounts of other elements such as aluminum and chromium. While most alloys begin to mechanically deteriorate at around 50% of their melting temperatures, the newly developed nickel-based superalloy was shown to maintain its integrity at 90% of its melting temperature, displaying tensile strengths of around 1.1GPa and an elongation at break greater than 13%.
Similar nickel-based superalloys, such as Inconel, are often characterized by their excellent mechanical properties, corrosion resistances, and creep resistances, with service temperatures typically in the 500°C+ range. As such, the material class has historically had its uses in the manufacturing of single-crystal turbine blades and vanes in the aerospace industry, where high-stress, high-temperature environments are commonplace.
Having tested the material with both laser-based SLM and EBM, the researchers discovered the crack-free printing capabilities of their new superalloy. Pollock explains, “The high percentage of cobalt allowed us to design features into the liquid and solid states of the alloy that make it compatible with a wide range of printing conditions.”
While advances in materials certainly play a role in the potential of 3D printing, there are other methods of producing defect-free components for industrial applications. For example, a team of Chinese and U.S-based researchers recently discovered a ‘speed limit’ at which part defects are less likely to occur during powder bed fusion. Through the use of X-ray imaging, the scientists precisely mapped scanning speeds to melt pool spatter levels, determining a ‘safe speed’ at which surface defects can be minimized.
Elsewhere, at Argonne National Laboratory, researchers developed an innovative machine learning-based approach to defect detection in 3D printed parts. Using real-time temperature data, together with predictive algorithms, the scientists were able to make correlative links between thermal history and the formation of subsurface defects during powder bed fusion. The method could allow potential users to avoid porosities right at the print parameter selection stage.
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Featured image shows Tresa Pollock, a professor of materials and associate dean of the College of Engineering at UC Santa Barbara. Photo via UC Santa Barbara.