A physics-based approach to modelling the surface integrity of machined nickel-based superalloys

Calderwood, Daniel (2024). A physics-based approach to modelling the surface integrity of machined nickel-based superalloys. University of Birmingham. Ph.D.

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Abstract

This thesis presents a novel method for predicting flow stress behaviour of nickel-based superalloys through the use of a newly developed microstructure informed constitutive model. Implementing the constitutive model within a finite element scheme has allowed simulations of the deformation and stress evolution, as well as microstructural changes induced by the machining process. Predictions of microstructural changes include grain refinement, γ′ distortion, and dissolution of γ′ precipitates. The constitutive model has predicted the experimentally observed ‘white layer’, showing grain refinement and γ′ distortion. These predictions will allow optimisation of the machining process to reduce the white layer, and aid in development of new alloys.

The newly developed constitutive framework predicts flow stress behaviour of precipitate strengthened alloys, taking into account the effects of a multi-modal precipitate distribution. The model includes descriptions of deformation micro-mechanisms by dislocation climb/glide and dislocations shearing precipitates, as well as a mean-field approach to modelling precipitation kinetics. The model was implemented within finite element simulations to predict flow stress behaviour during compression tests, and microstructural changes during machining, of nickel-based superalloy, RR1000.

Validation of the constitutive model was performed by comparison of model predictions to experimental data obtained from compression tests of RR1000. The model showed good agreement on yield stress data, and described the flow stress characteristics of the material.

The constitutive model was implemented within finite element simulations of the machining process, predicting the formation of an elastic shear band within the workpiece which acted as a nucleation site for microstructural changes. The predictions described how the chip formation process was a result of tensile stresses within the shear band and showed that accumulation of damage and opening of voids in the centre of the band was where the chip first started to form. The model described how reaction forces were a consequence of the chip formation process, and how the force can be related to γ′ shearing to give a real-time indication of the machinability of a material.

Additional simulations were performed to determine the impact of different cutting speeds, workpiece temperatures, and various γ′ dispersions, on machinability. By varying the initial secondary γ′ precipitate dispersion, the model demonstrated how a decrease in the interparticle spacing provided more dislocation pinning obstacles, which provided greater resistance to deformation.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):