Effect of Post-Processing Treatments on the Microstructure and Creep Behavior of Functionally Graded Inconel 718 Manufactured by Selective Laser Melting

Additive manufacturing (AM) or Rapid prototyping, more commonly known as 3-D printing, has been termed as one of the most disruptive technologies in the recent years as it offers a paradigm shift in the engineering design and manufacturing due to its ability to build parts with complex geometries, free usage of tool and functional part optimization. However, the process is characterized by complex thermal cycles with fast solidification rates that leads to residual stresses, process induced defects and an inhomogeneous microstructure. Therefore, there is a need to fully understand and optimize the process parameters during AM manufacturing as well as developing favorable post processing treatments in order to completely explore the advantages offered by AM and continuous research is being undertaken in that direction. The current study aims to understand the high temperature mechanical behavior of functionally graded Inconel 718 produced by Selective Laser Melting (SLM). Two different laser sources of 950 W and 250 W power were used to build cylindrical specimens having a columnar coarse-grained core and a fine equiaxed-grained shell. Additionally, cylindrical specimens were manufactured with only 250 W and 950 W laser source and the comparison was made with the developed functionally graded material (FGM). The post processing variants included combinations of solution treatment, double ageing and hot isostatic pressing (HIP). The dominant mechanism at the stress and temperature range used in this study (800-1000 MPa/630 °C) was identified to be dislocation power-law creep and the effects of size and volume fraction of γʺ, the principal strengthening phase, on the creep rate were explained. It was observed that the material built with 250 W laser source and a post process treatment comprising of solution treatment (980 °C/1h) and double ageing treatment at (720°C/8h/620°C/8h) resulted in the lowest creep rate and was found to be comparable to the reference wrought material. Furthermore, the effect of orientation relative to the build direction after the post processing treatment was found to be insignificant on the creep rate and was further confirmed by the residual stress analysis in the as-processed and post-processed condition. The creep strength of FGM was found to lie in between the material built only with 250 W and 950 W laser source, confirming the expected composite-like behavior of the produced FGMs.Mechanical Engineerin