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Researchers discover cobalt-nickel superalloy for AM applications

Adam StrömbergssonJanuary 5, 2021

1 minute read

SEM micrographs of metal powder of SB-CoNi-10 used for a EBM and b SLM printing trials. Simple bar geometries have been printed for uniaxial tensile testing c, d in addition to complex geometries such as prototype turbine blades with e internal cooling channels or f thin, over-hanging platforms. IPF maps acquired through EBSD show the grain structure of the as-printed CoNi-base superalloy along the build direction manufactured through g EBM and h SLM. The scale bars for a, b and g, h are 500 μm. The scale bars for c–f are 2 cm.

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Researchers at the University of Santa Barbara and the Oak Ridge National Laboratory have discovered a new defect-resistant cobalt-nickle superalloy. Their results were published in the journal Nature.

Stronger materials in the metal AM space promise greater applicability across a range of industries. Aerospace is noted up by these researchers, but high-strength and high-heat alloys can also be deployed in diverse military applications. More reliable alloys are, moreover, useful when marketing AM as an industrial solution alongside subtractive manufacturing options.

The new alloy is designed to address a common flaw in the metal AM process whereby cracks are created when heated metal is repeatedly warmed and cooled to create an object. Welding alloys are typically used to offset these stressors, but problems, like strain-age and ductility-dip cracking, occur when welded metals are treated with heat, or when heat treatments cause small cracks to form in a semi-cooled metal.

The researchers selected a nickel-based alloy because it performs well in high-stress, high-temperature environments, like aircraft engines. Stronger nickel-based alloys are, however, more difficult to weld and thus not suitable for AM application. The difficulty welding higher strength alloys comes from the various trace metals in the alloy that cool at different rates, thus creating greater chances that the structure becomes compromised.

Stress–strain curves for quasi-static tensile tests at room temperature on the a EBM and f SLM materials in the as-printed and HIP + SHT + aged conditions compared to EBM CM 24733 and SLM IN738LC57. SEM fractography of the b–e EBM samples and the g–j SLM samples in the b, c, g, h as-printed and d, e, i, j HIP + SHT + aged conditions reveal features indicative of ductile fracture in all specimens. The higher magnification images are taken near the center of each fracture surface. The scale bars for b, d are 1 mm. The scale bars for g, i are 2 mm. The scale bars for c, e, h, j are 5 μm.

The researchers tackled this problem by introducing cobalt to create an alloy that possess the same characteristics as a nickel superalloy, but that cools using cobalt particles, thus reducing the change of cracking. The researchers claim that: “Our approach demonstrates that the CoNi-base superalloy compositional space provides opportunities for the development of superalloys that can leverage the potential of AM.”

This claim bears out on their evidence. Trials showed the SLM and electron-beam melting custom-made superalloy powders produced high-strength objects. No post-processing heat treatments were required to strengthen the parts; they withstood tests right out of the build bed. The alloy also demonstrated a more uniform grain structure, which again helps increase strength.