Optimization of the heat treatment of additively manufactured Ni-base superalloy IN718

Benedikt Diepold,Mathias Göken,Steffen Neumeier,Nora Vorlaufer,Thomas Gartner

doi:10.1007/s12613-020-1991-6

Abstract

Additive manufacturing (AM) of Ni-base superalloy components can lead to a significant reduction of weight in aerospace applications. AM of IN718 by selective laser melting results in a very fine dendritic microstructure with a high dislocation density due to the fast solidification process. The complex phase composition of this alloy, with three different types of precipitates and high residual stresses, necessitates adjustment of the conventional heat treatment for AM parts. To find an optimized heat treatment, the microstructures and mechanical properties of differently solution heat-treated samples were investigated by transmission and scanning electron microscopy, including electron backs-catter diffraction, and compression tests. After a solution heat treatment (SHT), the Nb-rich Laves phase dissolves and the dislocation density is reduced, which eliminates the dendritic substructure. SHT at 930 or 954°C leads to the precipitation of the δ-phase, which reduces the volume fraction of the strengthening γ′- and γ″-phases formed during the subsequent two stage aging treatment. With a higher SHT temperature of 1000°C, where no δ-phase is precipitated, higher γ′ and γ″ volume fractions are achieved, which results in the optimum strength of all of the solution heat treated conditions.

Highlights

Components with reduced weight and integrated functional features for aerospace applications can be produced by additive manufacturing (AM)
The cell boundaries appear bright in the back scattered electron contrast mode (BSE) contrast due to Nb enrichment and the formation of Nb-rich Laves phases [25]
Subsequent heat treatments are very important for IN718 to dissolve Laves phases, homogenize the Nb content, reduce internal stresses, and form strengthening γ′- and γ′′precipitates

Summary

Introduction

Components with reduced weight and integrated functional features for aerospace applications can be produced by additive manufacturing (AM). Examples include the cabin bracket for the turbine of Airbus A350 XWB [1] or designs of additively manufactured liquid propellant injectors for future rocket engines by the Ariane Group [2]. Such components are already in use, the correlation between processing, subsequent heat treatments and the resulting microstructures and mechanical properties of additively manufactured alloys, such as the Ni-base superalloy IN718, is still not well understood. The fast solidification on the order of 10‒2 s combined with high thermal gradients of around 107 K/m leads to residual stresses [5,6], which may result in unexpected deformation after separation of the part from the base plate [7]. High local residual stresses can cause hot cracking, primarily at the grain

Results

Discussion

Conclusion