Improving Structural Integrity of Direct Laser-Deposited Ni–Co–Cr–Al–Y Superalloys by Alloy Modification

Abstract

Ni and Co based MCrAlY (M = Ni and/or Co), dual phase β and γ/γ'; β = NiAl, γ = Ni (Co, Cr), and γ' = Ni3Al) superalloys exhibit several beneficial high-temperature (>1000°C) (2012°F) strength and corrosion resistance properties. Our study investigates the feasibility of fabricating a commercially available NiCoCrAlY alloy (tradename “Amdry 386”) using laser directed energy deposition (laser-DED). A significant challenge for laser fabrication of bulk structures with Amdry 386 (A386) stems from the presence of relatively higher amounts of brittle β phase than ductile γ/γ' phases. While prior studies report laser fabrication of these materials in coating and cladding configurations, bulk structures have not been realized. Fabrication of A386 bulk structures using laser-DED was unsuccessful due to hot cracking and delamination during and after deposition. Laser-DED fabricated A386 alloy exhibited a metastable, rapidly solidified β-rich microstructure typical to laser processes. To alleviate these challenges, this study systematically evaluates the effects of modifying the A386 alloy by adding 10, 20 and 30% by weight of a Ni-20%Cr (NiCr) alloy (mainly γ/γ'). The A386 + NiCr mixtures were processed using laser-DED, and small coupons from each composition were evaluated using SEM, XRD, and microhardness to characterize the as-processed microstructure. Thermodynamic modeling was performed to determine the phase evolution as a function of the alloy composition. Our results show that the alloy chemistry can be successfully modified to improve the structural integrity of laser processed structures.