Inconel 718 (IN718) superalloy is widely used in aerospace engines and other high-temperature components. Improving its fatigue properties is crucial for ensuring the long-term stability of the components in service. Selective laser melting (SLM) technology is one of the best choices for the manufacturing of complex aerospace components owing to its high molding rate, high design freedom, and short production cycle time. However, the rapid melting and solidification of IN718 powder during SLM leads to the precipitation of a brittle Laves phase instead of a strengthening phase, inside the component. The characteristics of layer-by-layer scanning of SLM lead to the existence of high residual stress inside SLM-fabricated parts. Therefore, heat treatment is essential. This study investigates the influence of different heat treatment processes on the microstructure, phase distribution, and fatigue properties of SLM-fabricated IN718 alloy.

In this study, an IN718 alloy powder was prepared using a gas atomization method with particle size distributions between 15 and 55 μm. The IN718 specimens were prepared by EOSINT M280 from EOS, Germany and then cut along the plane of the substrate via wire cutting and removed. Subsequently, the IN718 specimens were subjected to three different heat treatments, as shown in Fig. 2. The three heat treatments is: 1) solution + double aging (SA); 2) homogenization + double aging (HA); 3) homogenization + solution + double aging (HSA). After heat treatment, the specimens were processed into fatigue specimens and subjected to a constant stress-controlled low-cycle fatigue test, at room temperature. Finally, after sample preparation and polishing, scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) photographic analyses were performed.

The distribution of precipitated phases differs significantly after different heat treatments (Fig. 5). After SA treatment, micron-level γ″ and γ′ strengthening phases and a small number of distributed δ phases exist in the matrix, whereas a large number of δ phases distribute at the grain boundaries. After HA treatment, nano-sized γ″ and γ′ strengthening phases exist in the matrix, and a small number of δ phases distribute at the grain boundaries. After HSA treatment, nano-sized γ″ and γ′ strengthening phases exist in the matrix, and δ phases exist at the grain boundaries. Differences in the distribution of the resolved phases leads to differences in fatigue performance (Fig. 6). Among them, the fatigue performance of the HSA treated IN718 specimen is the best, and the fatigue performance reaches 98.6% of the fatigue performance of the forged part. Subsequently, the specimens were prepared by wire cutting and the fracture morphologies were observed, and the fatigue morphologies of the specimens with different heat treatments were basically same (Fig. 7), that is, there are multiple fatigue source areas, obvious fatigue glow lines, and fatigue transient fracture areas with dimples and secondary cracks. The EBSD results (Fig. 8) show that the stress concentration is mainly in the δ-phase and grain boundary regions. Via analysis, it is found that dislocations slide freely inside the grain. When dislocations slide to the punctate γ′ phase, dislocations bypass the γ′ phase based on the Orowan strengthening mechanism and hinder the subsequent dislocation sliding. When dislocations slide to the flat elliptical γ″ phase, the γ″ phase hinders the dislocation movement and then γ″ phase will be cut with the accumulation of dislocations. Because the δ phase and matrix γ phase are non-conglomerative, dislocations accumulate around the δ phase. When dislocations slide to the grain boundary, the grain boundary can hinder the dislocation sliding and crack expansion. At the same time, the δ phase at the grain boundary can nail the grain boundary and delay the expansion of fatigue crack, thus improving the fatigue performance.

In this study, the microstructure and phase distribution of the IN718 alloy fabricated using SLM are regulated via heat treatment, to further analyze the effect of heat treatment on the low-cycle fatigue properties, at room temperature. The experimental results indicate that the alloy exhibited precipitation of the internal δ phase, as well as γ″ and γ′ strengthening phases precipitate inside the alloy following heat treatment, as opposed to the as-built conditions. The presence of these phases contributed to the alleviation of internal stresses within the alloy and led to a significant improvement in its fatigue performance. Based on the Orowan strengthening mechanism, the diffusely distributed γ″ and γ′ strengthening phases within the grain prevent the dislocations from sliding within the grain. The δ phase precipitated at the grain boundary can enhance the strength of the grain boundary, thus retarding microcrack extension in the matrix and increasing the fatigue cycles. Therefore, after HSA treatment, the IN718 specimen has optimized fatigue performance. The improvement of the microstructure and mechanical properties via heat treatment processes presented in this study provides a reference for the application of the SLM-fabricated IN718 components.