Effect of post-deposition heat treatments on the microstructure and hardness of Wire + Arc Additively Manufactured Inconel 718

Wire + Arc Additive Manufacturing (WAAM) is a high deposition rate manufacturing technique which uses an electric arc and a wire feedstock to build up near net-shape components in successive layers. In this study, the effect of various heat treatments on the microstructure of WAAM Inconel (IN) 718 nickel-base superalloy was investigated. Specimens extracted from a WAAM wall, deposited using IN718 wire and a plasma arc, were subject to different heat treatments. The microstructure of as-deposited WAAM IN718 was found to contain detrimental phases showing evidence of elemental segregation during deposition. These particles were found to remain in the microstructure after a standard homogenisation, solution and aging heat treatment. Micrographs of the modified homogenisation treated material show no trace of these micro-segregated particles, indicating that they were completely dissolved into the matrix. Vickers hardness testing results suggest that the aging treatment is responsible for majority of the strengthening effects, likely due to the precipitation of γ’ and γ’’ hardening particles. A significant improvement in the microstructure and hardness was observed in the modified heat-treated material.