Achieving balanced mechanical properties in laser powder bed fusion processed Inconel 718 superalloy through a simplified heat treatment process

Abstract

Laser additively manufactured (LAM) Ni-based superalloys commonly exhibit low strength and high residual stress in the as-built state, requiring post-heat treatment to improve mechanical properties. We propose a modified heat treatment (MHT) process that only involves a single-step aging at 650°C for 4 h to achieve high strength, high ductility, and low residual stress simultaneously in a laser powder bed fusion (LPBF)-processed Inconel 718 (IN718) alloy. The MHT treated alloy exhibits comparable tensile strength (1368 MPa) to the conventional solution plus two-step aging (SA) treated alloy (1398 MPa), while the tensile elongation (∼21.7% for MHT treated alloy and 13.4% for SA treated alloy) is 60% higher and the residual stress (∼195 MPa) is 20% lower than the SA treated alloy. The balanced high performance of the MHT IN718 alloy was mainly attributed to the precipitation of abundant γ" phase with a size of ∼5 nm, while the original nano-sized Laves precipitates and dislocation cells were mostly retained. The finer size and higher fraction of γ" of the MHT sample mainly result from the dislocation structure and compositional variations in the as-built IN718, which promotes precipitation during aging. The retention of Laves phase, and cellular dislocation network in the MHT alloy also contributes to work hardening during tension and suspends the occurrence of necking. This study unveils a unique strengthening and toughening mechanism in the Ni-based superalloy produced by LAM with the presence of abundant Laves precipitates and provides a simple, low energy-consumption and cost-effective heat treatment route for achieving desirable mechanical properties.