Effect of microstructure induced anisotropy on fatigue behaviour of functionally graded Inconel 718 fabricated by additive manufacturing

In this paper, the effect of microstructural anisotropy on the fatigue crack growth behaviour of the functionally graded Inconel 718 fabricated through laser powder bed fusion (L-PBF) is investigated. Different manufacturing parameters, including low and high laser powers, were used to produce a variety of non-graded (NG) and functionally graded (G) specimens in two build directions, vertical and horizontal. In addition, a group of heat treated wrought samples was tested as a reference. It was observed that the different manufacturing parameters result in various grain size, crystallographic textures, precipitates and Laves phases, porosity, and un-melted particles. Three-point bending fatigue tests were conducted to measure the threshold stress intensity factor (ΔKth) and fatigue crack growth rate (FCGR),da/dN. Only the lower laser power L-BPF Inconel material was found to have comparable to the wrought heat treated material fatigue crack growth behaviour. Furthermore, a new approach of automatically controlling ΔK as a function of the crack length was employed for graded specimens to investigate the crack growth rate as a function of local microstructure. The FCGR value of the vertical L-PBF samples, in which the crack direction was perpendicular to the build direction, remained constant. In contrast, the da/dN value of the horizontal samples with the crack direction parallel to the build direction increased constantly with the increase of the crack length. This behaviour is in good agreement with the hardness profile of the graded materials. Melt pool boundaries, graded interface boundaries, and grain orientations close to 〈001〉 were found to deflect the crack path. Additionally, it was found that L-PBF material is more affected (at a low stress ratio of R = 0.1) by the roughness-induced crack closure than the wrought counterparts. This study has successfully demonstrated the feasibility of using an additive manufacturing process to fabricate functionally graded materials featuring tailorable fatigue response of the local microstructures.