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 (ΔK_th) 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.

16Cr-2Ni-Mn-Mo-xN (wt%) stainless steel powders were synthesized by mechanical alloying from elemental metal and chromium nitride (Cr2N) powders. Compaction of resulting powders was carried out using spark plasma sintering technology. The X-Ray diffraction and Scanning Electron Microscopy analysis showed that chemical homogeneity improves with increasing alloying time and decreasing nitrides powder content. Analysis of sintering samples showed the formation of a and ?-phase solid solution with nitrogen-containing samples containing undissolved nitrogen inclusions. The results of high-temperature tensile tests showed an increase of tensile stress for the sample containing 50% Cr2N. Furthermore, the microhardness was increased with an increase in the amount of Cr2N.