Influence of build orientation on high temperature fatigue crack propagation mechanisms in Inconel 718 fabricated by laser powder bed fusion

Abstract

Additively manufactured (AM) Inconel 718 (IN718) was characterised in terms of microstructure, effect of post heat treatment and hence dependence of fatigue crack growth on build orientation, even after heat treatment. The AM specimens were manufactured in two orientations so that cracks would propagate either parallel (horizontal samples) or perpendicular (vertical samples) to the build direction. In addition, cast and wrought IN718 specimens were studied to benchmark AM against established processing techniques. The AM as-built microstructure exhibited a columnar, dendritic structure parallel to the build direction. No differences were found between specimen orientations in terms of Laves phase, and MC type carbides precipitating in inter-dendritic regions. There were however differences in average grain size, with vertical specimens having a coarser grain structure. Post processing heat treatments recrystallised the structure eliminating the columnar grain morphology, although some columnar-type grain shape was retained. In addition, a high percentage of twin grain boundaries were formed during the heat treatment process for both orientations, and vertical specimens again had a larger average grain size than horizontal. Laves phase were dissolved, and precipitation of common IN718 strengthening precipitates (δ-phase, ϒ’, and ϒ’’) occurred, with no difference between sample orientations. The cast and wrought material used for comparison had a finer, equiaxed microstructure with a lower area fraction of twin grain boundaries compared to heat treated L-PBF. Long crack fatigue tests were conducted on single edge notched samples at 350 and 650 ⁰C in air at a load ratio 0.1, under 1-X-1-1 trapezoidal loading waveforms with dwell times, X, of 1s and 90s at maximum load. At 350 ⁰C no apparent difference in crack growth rates was found, with transgranular fracture modes found at all ΔK levels for all tested specimens. Intergranular fracture modes became evident when testing at higher vi temperatures, with the vertical samples showing greater crack growth resistance at both frequencies. Horizontal specimens had on average a higher degree of intergranular failure. Their smaller grain size, and lower area fraction of twins resulted in a higher number of grains susceptible to high-temperature embrittlement. With longer dwell times, vertical samples again showed greater crack growth resistance, with crack arrest occurring at low ΔK values due to observed crack deflection reducing the effective stress intensity at the crack tip. For all testing at 650 ⁰C cast and wrought specimens displayed higher fatigue crack growth rates, and significant differences in grain boundary failure mechanisms. The equiaxed grain structure in cast and wrought specimens was not as effective deflecting cracks, plus the smaller grain size and lower area fraction of twins resulted in a much larger number of grains susceptible to high-temperature embrittlement. The extent of oxidation related damage formed ahead of the crack tip during dwell-fatigue was further studied by performing tests alternating between high (5 Hz) and low frequency (1-90-1-1) regimes at 650 ⁰C. Crack arrest occurred again for vertical specimens and subjected to 90 s dwells with no apparent oxidation embrittlement formed ahead of the crack tip. In contrast, horizontal and cast and wrought specimens both exhibited the formation of a large oxidation related damaged zone ahead of the crack tip during dwell fatigue promoted by dynamic embrittlement. This was linked to a large surge in crack propagation rates when switching to higher frequency (5 Hz) as the crack propagated through embrittled grain boundaries. In order to further study the effect of dwell time on damage ahead of the crack tip and understand the large differences between sample orientations, pre-cracked specimens were held under a sustained load at 650 ⁰C for 16 hours. The resultant crack tip and oxides formed were studied using scanning electron microscopy, electron backscatter diffraction and energy dispersive spectroscopy. No differences were found in the composition of oxides. However, the location of the oxide layer with respect to the crack tip was different for horizontal (ahead of the crack), and vertical (filling the crack) specimens. This resulted in a more conventional grain boundary failure process by dynamic embrittlement for horizontal specimens, comparable to that in the literature for cast and wrought IN718. The microstructure of vertical specimens, however, promoted a very tortuous crack path compromising oxygen delivery to the crack tip. In addition, the reduction in effective ΔK associated with crack deflection will have lowered the local stresses at the crack tip. Thus, at low ΔK levels the combination of limited available oxygen and low local stress at the crack tip was not sufficient to promote diffusion of oxygen ahead of the crack tip, with oxidation forming inside the crack and stopping fatigue crack propagation.

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Identifiers

ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

ORCID for Andrew Hamilton: orcid.org/0000-0003-4627-849X

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