Achievement of grain boundary engineering by transforming residual stress in selective laser-melted Inconel 718 superalloy

Abstract

The complicated geometric parts produced by selective laser melting (SLM) are not amenable to grain boundary engineering (GBE), which is primarily implemented via thermomechanical processing. Herein, the high internal residual stress produced by SLM processing is utilized as stored activation energy for direct GBE in a SLM-fabricated Inconel 718 superalloy. The grain boundary distribution characteristics, grain size, texture, and corresponding mechanical properties of the as-built specimens subjected to different heat treatments were investigated. As the annealing temperature increased, the recrystallized fraction increased, accompanied by an initial increase and subsequent decrease in the number of special grain boundaries. The numerous annealing twin boundaries effectively divided the grains, despite grain coarsening induced by grain boundary migration at a higher temperature. Compared to the as-deposited sample, the yield strength and ultimate tensile strength increased from 707 to 1071 MPa and from 947 MPa to 1271 MPa, respectively, and the ductility increased from 17% to 21% in the sample annealed at 1200 °C for 1 h followed by double aging treatment because this sample had the highest fraction of twin boundaries (64.89%). The “Additive Manufacturing + GBE” strategy based on utilizing the residual stress offers a feasible pathway for broadening the application scope of additive manufactured alloys.