INCONEL 718 Single and Multipass Modelling of Hot Forging

Abstract

A better understanding of the competition between several mechanisms (dynamic recovery, dynamic recrystallization and plasticity hardening) is crucial for aircraft engine manufacturers. The aim of this paper is to improve the microstructure and therefore the mechanical properties of a nickel based superalloy used for rotating forged pieces. A nickel superalloy microstructure is the result of several successive hot forging processes: multipass processes, with intermediate dwell time and quenching. In this paper, an original three dimensional approach able to simulate these processes is proposed. The specific role of the different steps of the processes is analysed. In this approach, several forging thermo-mechanical parameters are taken into account: the working temperature, the strain rate, the final strain, the interpass time, etc. At high forging temperature, the studied INCONEL 718 presents an austenitic matrix γ (face centred cubic) assumed to be in a single phase.This approach proposes a sequential coupling of two models, one devoted to deformation and the other to recrystallization. Such a coupling enables the estimation of the effect of deformation and of different recrystallization types on mechanical behaviour and on micro-structural evolution. The approach is performed at the grain scale and takes into account the whole thermo-mechanical cycle with a focus on the dynamic behaviour.The first polycrystalline model is based on the plasticity mechanisms at the grains scale. The framework corresponds to finite transformations (large lattice rotations and small elastic strains). The model is implemented in ABAQUS and CAST3M finite element codes. The second model is based on the recrystallization theory and uses a 3D cellular automaton. It describes dynamic recrystallization phenomena such as nucleation-growth and static or postdynamic recrystallization. Such recrystallization mechanisms were observed during interpass time or during the successive heatings depending on the thermomechanical paths used in multipass forging. Dislocation densities are the internal variables common to the two models. The simulations are performed on a 3D Representative Elementary Volume (aggregate) obtained from Electron Back Scattering mapping. Numerical results are compared to experimental microstructures.

Tailored Time–Temperature Transformation Diagram for IN718 Alloy Obtained via Powder Bed Fusion Additive Manufacturing: Phase Behavior and Precipitation Dynamic

“…Thermo-Calc ® software (https://thermocalc.com (accessed on 13 November 2023), Stockholm, Sweden) version 2019.1, which uses the CALPHAD method, was utilized to predict the phase equilibria in multi-component systems. The Thermo-Calc Nickel-based superalloys databases (TCNI8, MOBNI4) were used to calculate phase fractions, Scheil-Gulliver solidification, and diagrams in temperatures ranging from 300 to 1400 • C. The simulation parameters for AM-manufactured goods, such as dislocation density alloys or the wetting angle, were adopted from the literature [14,15]. Experimental data on morphology and microstructure used in the simulations were obtained from our previous investigation on IN718 [13].…”

“…The coherence between these phases limits the EBSD technique's ability to determine their distribution and crystal orientation accurately [28]. A comparison with SEM morphological shapes from previous studies was carried out to confirm the presence of Laves and NbC phases [14]. For identification, the lattice parameters in Å were used: Ni3Nb (a = 3.62, b = 3.62, c = 7.41), Laves (a EBSD analysis (Figure 7) was conducted to identify Ni 3 Nb, Laves, and NbC carbides within the microstructure.…”

Aspects of High Strain Rate Industrial Forging of Inconel 718

“…The velocity of tool impact on the workpiece can reach up to 1200 mm/s in a screw press and it can go much higher, up to 9000 mm/s, in the case of hammers [4]. This, when coupled with a complicated geometry forged in single or multiple blows, results in complex thermal and strain histories [5]. It is not realistically possible to predict the resulting highly inhomogeneous deformation and complex microstructures using models derived from macro-level data.…”

Mean-field modelling of the intermetallic precipitate phases during heat treatment and additive manufacture of Inconel 718

“…The mean-field precipitation model has been developed to assist in the design of heat treatments. Typical heat treatments for this alloy involve a solid-solution treatment for 1 hour at a temperature varying between 930 and 980C • C, followed by a fast quench rate and a two step aging process [39]. The aging process lasts 20 hours, with the first 8 hours at 720 • C followed by a furnace cool to 620 • C. The results show that this is possible when performing the SST at a temperature below the γ * solvus (∼940 • C) with a fast quench rate.…”