Fatigue Behaviour of functionally graded Inconel 718 manufactured by selective laser melting

Functionally graded materials (FGMs) represent a new class of materials that consist of graded compositional and microstructural patterns allowing to tailor properties. Additive manufacturing (AM) or more commonly known as 3D-printing offers a paradigm shift in FGMs and engineering design due to its ability to produce components with complex geometries and functional part optimization. This study aims in investigate the concept of microstructural grading enabled via 3D-printing process and its feasibility to tailor site-specific fatigue behaviour. Non-graded and graded specimens of Inconel 718 featuring a range of microstructural gradients were produced by selective laser melting (SLM). In the current work the grain size, grain orientation and precipitates, resulting in distinctly different properties, were used to develop microstructural gradients. Post-process heat treatment in terms of hot isostatic pressing (HIP) and aging was also investigated. Microstructural and elastic properties as well as in-depth fatigue crack growth behaviour were characterised and compared with conventionally manufactured wrought Inconel 718. Fatigue crack growth in FGMs under cyclic loading was investigated via novel experiments and FE approach. Results from homogeneous specimens were used for estimating spatial property distribution and crack-extension effect in the graded specimens. Hence, for the graded material a constant ΔK procedure was designed to investigate the change in crack growth rate as a function of different gradients. Additionally, Digital Image Correlation (DIC) and Potential Drop (PD) were conducted to validate the constant ΔK tests. It was found that fatigue behaviour of the individual microstructures was primarily affected by the grain size, grain orientation and heat treatment. As opposed to the general trend, fine grained material (range of 10-70 µm) had a better fatigue crack propagation resistance than coarse grained (100 – 500 µm), which was attributed to the high compressive residual stresses developed during the 3D-printing. With respect to the wrought material, the 3D-printed material with fine grains showed higher ΔKth and lower da/dn values. The crack growth rate in the graded material at a constant ΔK was found to change from one microstructure to the other following the same trend as the individual microstructures. The effect of grain orientation was found to significantly affect the crack growth rate, with grains elongated parallel to crack growth showing the larger and more continuous crack growth gradient. Additionally, in the heat treated (HIP+H/T) graded material, the difference in properties between the two gradients was almost entirely diminished, indicating the necessity to optimize heat treatment regimens more suitable for developed herein FGMs. The current study not only delivered fatigue parameters and FE model for various graded and non-graded 3D-printed Inconel 718, but also successfully demonstrated the feasibility of using 3D-printing process to design microstructures and alter the fatigue behaviour in FGMs components