Crack growth in laser powder bed fusion fabricated alloy 718 at 650 °C under static and cyclic loading

Abstract

The role of microstructure in influencing 650 °C crack growth behavior for laser powder bed fusion (LPBF) fabricated nickel alloy 718 was examined by applying two post-build heat treatments and comparing to wrought material. The first heat treatment (solution and ageing) retained the elongated grain structure along the build direction. The second used hot isostatic pressing (HIP) prior to the solution and aging treatment to mostly recrystallize the microstructure. At high cyclic frequency (30 Hz), crack growth was mixed transgranular and intergranualr and differences in the crack growth rates among samples were primary caused by grain size differences and corresponding transgranular crack path roughness. Under static loading or low frequency (0.1 Hz) cyclic loading, intergranular crack growth dominated. Without HIP, the LPBF material had highly anisotropic behavior with a high threshold for crack extension when the crack plane tried to cut across the elongated grain structure. After HIP, the LPBF fabricated material displayed excellent resistance to intergranular crack extension at both 0.1 Hz and constant applied load due to a large fraction of Σ3 special boundaries which are highly resistant to intergranular oxidation. The results suggest LPBF with HIP treatment can give a grain boundary engineered 718 microstructure for elevated temperature applications.