Effect of heat treatments on strengthening mechanisms in electron beam melted superalloy Inconel 718

Abstract

Additive manufacturing of Inconel 718 (IN718) has gained significant interest due to its application in various industries such as aerospace and nuclear industry. In the present study, IN718 was fabricated by additive manufacturing using Electron beam melting (EBM). The superalloy was subjected to hot isostatic pressing (HIP) at 1120 °C for 4 h at a pressure of 100 MPa. It was then subjected to two different aging treatments. The standard heat treatment involved a solution treatment (954 °C/1 h), followed by a two step aging (740 °C/8 h + 635 °C/8 h) cycle (HIP + STA). In the second heat treatment, solutionizing (954 °C/1 h), as well as aging (740 °C/4 h + 635 °C/1 h) was carried out inside the hot isostatic press (combined HIP cycle). This leads to shortening of the heat treatment time as well as allows the heat treatment to be done in a single cycle. The investigation of microstructure and mechanical properties of EBM IN718 was carried out for as-built, combined HIP cycle and HIP + STA conditions. All the samples show strong <100> texture along the build direction. The as-built sample shows 0.47 ± 0.3 % defects. No defects were observed in heat treated samples. The volume fraction (%) of the γ″ (Ni3Nb) precipitates in as-built, combined HIP cycle and HIP + STA sample was ∼12%, ∼15% and ∼13% respectively. The contribution by various strengthening mechanisms to the overall yield strength was calculated by considering solid solution, grain boundary, dislocation and precipitation strengthening. The primary contribution to the overall yield strength was by precipitation strengthening. For as-built, combined HIP cycle and HIP + STA samples, the estimated precipitation strengthening contribution was ∼723 MPa, ∼701 MPa and ∼551 MPa, respectively. The difference may be attributed to the volume fraction, aspect ratio and size of the precipitates present. The as-built sample shows a lower experimental yield strength than its theoretical yield strength. This may be attributed to the defects present in the as-built sample. The combined HIP cycle is recommended as it leads to best mechanical properties. Reasons for this are discussed.