Microstructural evolution and corrosion behaviors of Inconel 718 alloy produced by selective laser melting following different heat treatments

Abstract

The microstructures and corrosion resistances of Inconel 718 alloy prepared by selective laser melting (SLM), SLM following various heat treatments, and conventional rolling are studied. Results show that only Nb element is enriched in interdendritic regions while Fe element is abundant in dendritic trunks for the as-built Inconel 718 alloy. After solution annealing treatment, incomplete recrystallization is observed and distortion energy is released. Increasing the solution annealing temperature from 980 °C to 1020 °C (ST1∼ST3), the morphologies of δ phases turn from needle-like into short platelet shape, which reduces the anodic current density and improves the corrosion resistance compared to other heat-treated samples in 3.5 wt% NaCl solution. Corrosion morphology observation shows that obvious cracking of surface passive film occurs for the SLM, solution annealing plus double aging (SA) and rolled samples, while corrosion pits and micro-cracks appear at the δ phase boundaries of solution-annealed (ST1∼ST3) samples. The surface passive film is smooth for the rolled sample. The corrosion resistance of samples obtained by different processes follows in the order of rolled > ST3 > ST2 > ST1 > SA > SLM. The galvanic coupling effect causes the formation of corrosion pits or cracks between γ matrix and second phases. The high interface energy and lattice misfit may provide driving forces for the preferential dissolution of γ matrix rather than second phases. The inferior corrosion resistance of the as-built Inconel 718 alloy can be significantly improved through solution annealing treatment at 1020 °C.