Investigation on the influence of heat treatment on Inconel 718 fabricated by selective laser melting: Microstructure and high temperature tensile property

Abstract

Abstract The microstructure of nickel-based superalloy Inconel 718 prepared by selective laser melting (SLM) has effect on its mechanical performance. In this work, microstructure evolution and tensile properties were studied in as-SLMed state and after various heat treatments, as well as the role of defects on its fracture mechanism at high temperature. The results indicated that the as-SLMed microstructure consisted of dendritic and cellular structure, while small γ′′ phases and acicular δ phases formed after heat treatments. Two types of defects, ‘X-shaped crack’ and ‘central crack’ were formed in the as-SLMed specimens during in-situ high temperature tensile process, which suggested that the melt pool boundaries and the center of melt pools were cracks initiation places because of high density of defects and segregation. Room temperature (RT) tensile strengths of the as-SLMed specimens exceeded the cast standard but were inferior to the wrought standard. Apparently, the standard heat treatment (SHT) and direct aging treatment (AG) specimens showed highest tensile strengths (1430 MPa at RT and 1189 MPa at 650 °C). RT tensile strengths of the SHT specimens were increased by 43.7 % when compared with those of the as-SLMed ones, even 12 % higher than those of wrought standard as a result of precipitation strengthening effect. However, elongation after heat treatments decreased significantly due to dislocation pinning effect from strengthening phases. Furthermore, the strengths and elongations in horizontal and vertical directions showed obvious anisotropy because of strong preferential texture in specimens formed during SLM process. This paper looks at microstructural evolution, mechanical properties evaluation of Inconel 718 fabricated by SLM under various heat treatments, the results showed that the tensile properties can be tuned via proper treatment to suit their application requirements.