Prediction of microstructure evolution during high temperature blade forging of a Ni−Fe based superalloy, Alloy 718

Abstract

The mechanical properties of the Ni-Fe-based Alloy 718 depend very much on grain size, as well as the strengthening phases, γ' and γ''. The grain structure of the superalloy components is mainly controlled during thermo-mechanical processes by the dynamic, meta-dynamic recrystallization and grain growth. In this investigation, the evolution of the grain structure in the process of two-step blade forging was experimentally and numerically dealt with. The evolution of the grain structure in Alloy 718 during blade forging was predicted using a 2-D FE simulator with implemented constitutive models on dynamic recrystallization and grain growth. The comparison of the simulated microstructure with the actual grain structure of the forged parts validated the prediction of the grain structure evolution. The effect of dynamic recrystallization on the evolution of grain structure is highlighted in this article.

Characteristics of VIM/VAR-Processed Alloy 718 Ingot and the Evolution of Microstructure During Cogging

“…The constitutive model for grain structure covers dynamic recrystallization, meta-dynamic recrystallization, static recrystallization, and grain growth. [5,6] The decoupled FE simulation, which considers accumulated state parameters during deformation, is previously shown to be appropriate for the prediction of microstructure of Alloy 718 [7]. The process parameters were carefully selected to represent the actual cogging process.…”

Effect of duplex grain size distributions on long term stress-rupture life at 600 °C/450 MPa of Nimonic 80A

“…Microstructure evolution during recrystallization of superalloy 718 shows that the recrystallized grain size as well as the fraction of new grains increases with the increasing deformation temperature [10]. The appearance of recrystallized necklace structure is due to the low temperature and limited strain caused by die chilling and friction during the high temperature blade forging of superalloy 718 [11]. The abnormal grain growth and expansion of necklace grain structure are observed to depend on the grain boundary structure [12,13].…”

Dynamic recrystallisation model in precipitation-hardened superalloys as a tool for the joint design of alloys and forming processes

“…This may be achievable if a model is available, capable of predicting DRX features as a function of alloy compositionand hence phase constitution -as well as of processing conditions (temperature, strain rate, strain). Most models, however, provide a description of microstructural evolution using fitted parameters and are not predictive regarding the influence of composition [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. For instance, some authors [8][9][10][11][12][13][14] have used parameterised equations, mostly relying on or deriving from the Zener-Hollomon parameter, to describe the high temperature flow stress of different superalloys, the dependence of grain size on temperature, strain rate, and/or the evolution of the recrystallised fraction with strain.…”

“…activation energies, pre-exponential factors, exponents of stress and/or strain rate) were fitted to experiment and no explicit dependence on composition or phase constitution was proposed. Many authors [15][16][17][18][19] used similar parameterised and fitted equations to describe flow stress and/or grain size during dynamic recrystallisation, and models were then implemented into finite element analyses to simulate the evolution of the grain structure in various superalloys parts during forging, as a function of thermomechanical history. Other models, instead of using global scalar variables describing the material behaviour as a continuous medium, follow a mean field approach focusing on the characteristics (e.g.…”

“…Calculating the evolution of individual grains can also be made without a mean field approach, by simulating a large and representative set of grains by a cellular automaton [22][23][24]. Nevertheless, all the abovementioned approaches [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] share the common feature that model parameters have to be fitted to each specific alloy, so that predictions cannot be extended to different alloy compositions unless model parameters have been fitted again. This differs notably from our present approach and objectives.…”