Solid-State Transformation of an Additive Manufactured Inconel 625 Alloy at 700 °C.

Fan Zhang,Lyle E Levine,Jan Ilavsky,Mark R Stoudt,Andrew J Allen,Greta Lindwall

doi:10.3390/app11188643

Abstract

Inconel 625, a nickel-based superalloy, has drawn much attention in the emerging field of additive manufacturing (AM) because of its excellent weldability and resistance to hot cracking. The extreme processing condition of AM often introduces enormous residual stress (hundreds of MPa to GPa) in the as-fabricated parts, which requires stress-relief heat treatment to remove or reduce the internal stresses. Typical residual stress heat treatment for AM Inconel 625, conducted at 800 °C or 870 °C, introduces a substantial precipitation of the δ phase, a deleterious intermetallic phase. In this work, we used synchrotron-based in situ scattering and diffraction methods and ex situ electron microscopy to investigate the solid-state transformation of an AM Inconel 625 at 700 °C. Our results show that while the δ phase still precipitates from the matrix at this temperature, its precipitation rate and size at a given time are both smaller when compared with their counterparts during typical heat treatment temperatures of 800 °C and 870 °C. A comparison with thermodynamic modeling predictions elucidates these experimental findings. Our work provides the rigorous microstructural kinetics data required to explore the feasibility of a promising lower-temperature stress-relief heat treatment for AM Inconel 625. The combined methodology is readily extendable to investigate the solid-state transformation of other AM alloys.

Highlights

Inconel 625 (IN625) is a nickel-based solid-solution superalloy with a Ni-Cr matrix strengthened by Nb/Mo solutes [1]
To understand the microstructural responses of additive manufacturing (AM) IN625, in this study, we investigate the solid-state transformation kinetics of an AM IN625 alloy at 700 ◦C primarily using synchrotron-based in situ scattering and diffraction methods
To compare to the experimentally observed precipitation events, we calculated the precipitation kinetics using the TC-PRISMA module [27,28,29]. This module is based on the Langer–Schwartz theory [30] and Kampmann–Wagner numerical methods [31,32] and calculates the nucleation, growth, and coarsening of precipitates in a multicomponent and multiphase system by integrating thermodynamic and diffusion information provided by CALPHAD descriptions

Summary

Introduction

Inconel 625 (IN625) is a nickel-based solid-solution superalloy with a Ni-Cr matrix strengthened by Nb/Mo solutes [1]. In contrast to most studies of the effect of heat treatment on nickel-based superalloys, where experimental evidence is mainly gathered from microscopy and from in-house X-ray diffraction data, synchrotron measurements probe a fixed and significantly larger sample volume through in situ experiments that allow the annealing kinetics to be unambiguously determined. The kinetics results from the same sample volume are elucidated with thermodynamic predictions by CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) methods

Material Fabrication and Sample Preparation

In Situ Synchrotron Small Angle X-ray Scattering and X-ray Diffraction

Thermodynamic Calculations

Results and Discussion