Fatigue Crack Propagation in Additively Manufactured and Functionally Graded Inconel 718

Abstract

Additive Manufacturing (AM), commonly known as 3D printing, exemplifies the recently emerging processing methodologies that aim to substitute the conventional routes, such as to produce parts with complex geometries and eliminate expensive tooling. AM also allows high degree of freedom and rapid prototyping for functional part optimization. This has led to a renewed interest in the Functionally Graded Materials (FGMs). FGMs are a class of novel materials designed to have graded compositions or microstructures with tailored properties. Selective Laser Melting (SLM) is one of the most widely used AM method showing great potential to produce parts made from Inconel 718, a Nickel-based superalloy.This study aims to investigate the microstructural gradients in Inconel 718 produced with SLM by manipulating the thermal fields acting during the production and their subsequent effect on fatigue behavior. Two different laser powers, 950 W and 250 W were used to develop coarse grained and fine grained microstructures respectively. Ungraded and graded specimens were produced to study the fatigue crack growth in individual as well as graded microstructures under cycling loading. The effect of standard post-process heat treatments on the microstructure and fatigue properties of as-printed (AP) AM Inconel 718 was also studied. The two heat treatments under discussion here are homogenisation + solution + aging (HT) and hot isostatic pressing + HT (HIPHT). Direct Current Potential Drop (DCPD) method was used to measure the fatigue crack growth rates in standard tests to identify the fatigue properties of ungraded microstructures. A new approach of using a constant ΔK procedure was employed for graded specimens to investigate the crack growth rate as a function of the crack interaction with local microstructure.The coarse columnar grains with preferred <001> texture were found elongated along the building direction (BD) and their axis of elongation in the specimens changed as a function of BD. Fine grains were found equiaxed and randomly oriented. HT had no significant effect on this observed trend, while HIPHT entirely altered the printed microstructure. The grain sizes, orientation as well as heat treatments were found to be affecting the fatigue behavior of individual microstructures. Fine grained microstructures showed a slower fatigue crack propagation (by ≈70% in AP, ≈40% in HT and ≈45% in HIPHT) than coarse grained. Fatigue cracks propagated slower in coarse grains oriented perpendicular to the crack path in AP (≈80%) while they were slower when oriented parallel to the crack path in HT (≈75%) and HIPHT (≈9%). The interfaces produced in AP and HT graded specimens were seen to introduce barriers for crack propagation and reduce the local crack growth rate. The same was not observed in HIPHT due to diminished gradients, resulting from grain coarsening.Thus, this study has successfully demonstrated the feasibility of using AM to fabricate future FGMs featuring altered fatigue response of the local microstructures.