High-performance Ni-based superalloy 718 fabricated via arc plasma directed energy deposition: effect of post-deposition heat treatments on microstructure and mechanical properties

Abstract

Ni-based superalloy 718 fabricated via arc plasma direct energy deposition (IN718 AP-DED) exhibit a limited response to heat treatment due to its coarse primary microstructure and interdendritic segregation, which may prevent its use in high-integrity applications. Thus, dedicated heat treatments for IN718 AP-DED must possess a homogenization temperature as high as possible to significantly dissolve the eutectics and increase the γ′′-former elements in solid-solution. The present work proposed heat treatments for IN718 AP-DED (homogenization – 1050, 1100, 1142, and 1185 °C / 2 h – followed by aging – 718 °C / 8 h, cooling at 56 °C / h, and 621 °C / 8 h). The as-built IN718 AP-DED showed the typical coarse and oriented (cube texture) microstructure with eutectics (Laves and MC-typical carbides) in the interdendritic region, which were significantly dissolved during the homogenization, promoting a high-volume fraction of hardening phase (γ′′ and γ′) and outstanding quasi-static mechanical properties after the aging step. The present work showed that IN718 AP-DED mechanical properties can be optimized through dedicated heat treatments, meeting the ductility and yield strength requirements (room temperature) of AMS 5662. Furthermore, the heat treatments did not alter the grain morphology and texture aspect, inducing a lower Young’s modulus compared to the non-oriented material.