Influence of Thermal Post-Processing on the Microstructure and Mechanical Behavior of the Additively Manufactured Inconel 718 Superalloy using the Laser Powder Bed Fusion Process

In an attempt to improve the performance of the additively manufactured (AM) Inconel 718 (IN718) superalloy, a typical material widely used for turbine engine components in the aerospace and energy industries, the current work studies the effect of thermal post-processing on the microstructure and mechanical behavior of the AM IN718. Additive manufacturing and, in particular, the laser powder bed fusion (LPBF) of IN718 offers several advantages over the conventionally manufactured IN718 (cast and wrought). However, the existence of some inherited manufacturing defects in the as-printed parts presents an obstacle to produce components with specifications that meet the design requirements. Thus, post-heat treatment of LPBF printed IN718 is an essential and integral part of the industrial operations to mitigate these drawbacks. For this purpose, in the present study, a heat treatment time window, including a wide time range of homogenization (at 1080°C; 1 to 7h) and solution (at 980°C; 15 to 60 min) treatments, is established to study the effects of the treatments time on the microstructure and mechanical properties at room temperature (RT) and at 650°C of the LPBF printed IN718 parts. The results demonstrate that the 1h homogenization treatment is not enough to significantly change the as-printed grain structure, the strong crystallographic texture and to annihilate the primary dislocation tangles. However, a completely recrystallized IN718 material with non-distinct texture and stress relived grains are obtained after 4h. A further increase in the homogenization time to 7h results in grain growth as well as greater and coarser MC carbides. Therefore, the increase in the homogenization time from 1 up to 7h results in a progressive decrease in the mechanical properties at RT and at 650°C. For the solution treatment, the treatment time does not cause a noticeable change in the grain structure and material texture but significantly affects the precipitation amount of δ-phase. The role of the solution time in the improvement of the mechanical properties at 650°C is crucial due to the increase in the grain boundary strength through the pinning effect of δ-phase. Based on the results obtained at different treatment time, a multi-objective optimization is employed to tune homogenization and solution time and achieve the optimum heat treatments that can fulfill the required mechanical properties and material texture. The results show that, after the conditions which include 2.5 and 4h homogenization treatment at 1080°C followed by 1h solution at 980°C and standard aging treatments (2.5H/1S and 4H/1S), a significant improvement in the mechanical properties at RT and 650°C is observed, compared with the wrought IN718. Furthermore, after the 4H/1S condition, a good balance between the strength and ductility is obtained at RT. To assess the thermal stability of the obtained optimum heat treatments during the in-service conditions, the as-printed, 2.5H/1S and 4H/1S conditions are subjected to thermal cycling similar to what is encountered in the aircraft turbine engines for long periods up to 3000h. The results reveal that the 4H/1S condition possesses higher thermal stability over the in-service exposure than the as-printed and 2.5H/1S conditions, as a relatively lower strength loss of 3.3% is counted for the 4H/1S condition after 3000h thermal exposure, while for the as printed and 2.5H/1S conditions, strength loss of 7.4 and 5.3%, respectively, are counted. Furthermore, the 4H/1S treatment results in delaying the deterioration of material strength for longer thermal exposure time (after 2000h), whereas the 2.5H/1S treatment results in deterioration of the material strength after only 1000h thermal exposure due to the retarded phase transformation of the metastable γ″ into more stable δ-phase within grains interior in the former treatment