Near-threshold fatigue crack growth in laser powder bed fusion produced alloy 718

Abstract

The role of microstructure in influencing fatigue crack growth behavior for laser powder bed fusion (LPBF) produced nickel superalloy 718 was examined. Two common post-build heat treatments were applied to produce two distinct microstructures, one which retained much of the solidification structure from additive manufacturing, while the other was mostly recrystallized. For both groups, residual stresses were assessed along the crack path by neutron diffraction. At low load ratios (R = 0.1), the non-recrystallized LPBF material had the lowest and most varied fatigue crack growth thresholds at 6–7.2 MPa m1/2. This is attributed to a reduction in crack path roughness induced crack closure and high superimposed residual stresses. The recrystallizing heat treatment coarsened and homogenized the grain structure substantially, thus increasing the threshold to 7.3–7.6 MPa m1/2 due to a substantially rougher crack path and completely relieved residual stresses. Both LPBF heat treatments showed negligible effect of build orientation and gave significantly lower fatigue thresholds compared to wrought material tested in the L-T orientation (∼11 MPa m1/2). At high load ratios, the difference in fatigue behavior between microstructures was reduced but still present and was attributed to differences in geometric shielding from crack path roughness, grain size, and degree of microstructural homogeneity.