Effect of δ phase on high temperature mechanical performances of Inconel 718 fabricated with SLM process

The paper studies comparatively on the microstructures and mechanical performances at elevated temperature of SLM-fabricated Inconel 718 alloy in as-build, SHT 980, SHT 1080, SHT 980 + 1080 heat treated conditions. The morphology and distribution of delta phase in the gamma matrix are regulated by pre-designed heat treatment schemes, their effects on tensile properties and stress rupture properties at elevated temperature are discussed. The results show that the specimens of SLM-fabricated Inconel 718 alloy in SHT 980 + 1080 heat treated conditions possess the optimum mechanical performances at elevated temperature, in which the morphology and distribution of delta phase in the gamma matrix are the key factors in determining their high temperature performances, The excessive delta phases within grains and along grain boundaries can easily lead to dislocation piling up in the process of tensile test at elevated temperature, causing local stress concentration and microcrack generation in the matrix, further initiating premature failure of parts. However, lack of delta phase will reduce the high temperature strength of grain boundaries and then influence the high temperature mechanical properties. As a result, the suitable heat treatment is used to regulate the precipitation of delta phase and the moderate amount of delta phases precipitate along grain boundaries, further improving high temperature mechanical properties of IN718 alloy.