Recrystallisation and γ′ precipitation during industrial open die forging of the nickel based superalloy AD730

Coyne-Grell, Angus

Thesis

Recrystallisation and γ′ precipitation during industrial open die forging of the nickel based superalloy AD730

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Creator

Coyne-Grell, Angus

Rights statement

Strathclyde Thesis Copyright

Awarding institution

University of Strathclyde

Date of award

2024

Thesis identifier

T17000

Person Identifier (Local)

201992327

Qualification Level

Doctoral (Postgraduate)

Qualification Name

Doctor of Philosophy (PhD)

Department, School or Faculty

Department of Design, Manufacturing and Engineering Management

Abstract

Ni-based superalloys are used in very challenging operating conditions in jet turbine engines. In order to withstand these challenging operating conditions, and be used in safety critical parts such as turbine discs, the microstructure of such alloys must be well controlled. Much of this control is achieved during open die forging, where as-cast ingots are converted into billets. During this process the coarse as-cast structure is massively refined through recrystallisation. To design an effective forging route, and produce the desired, refined, uniform microstructure, the microstructural evolutions happening during the open die forging must be investigated; that is the aim of this thesis. The Ni-based superalloy AD730, produced by Aubert&Duval is studied. The microstructure from different stages of the industrial forging process is analysed, and stages of this process are simulated on the lab scale to allow microstructural mechanisms to be investigated. The precipitate distribution which forms during first cooling through the solvus temperature is characterised, including with 3D observations. It is observed that this distribution is very inhomogeneous, comprising a mixture of continuous and discontinuous precipitates and some regions with no precipitation. The potential role of chemical segregation in producing the inhomogeneous precipitate distribution is investigated. It is found that the regions without precipitates have lower concentrations of the γ′ forming elements. A key finding is that cooling through the γ′ solvus temperature has a significant effect on grain size and morphology, with grain size decreasing and grain boundaries becoming more serrated. The interaction of this inhomogeneous distribution with recrystallisation during compression is studied, and it is seen that regions with different precipitation show different mechanisms and kinetics of recrystallisation. A key finding is that regions with discontinuous precipitation are harder to recrystallise than those with continuous precipitation. Local crystallographic texture also influences the kinetics of recrystallisation during compression, grains with a ⟨110⟩ axis close to the macroscopic compression direction being harder to recrystallise than grains without. The role of annealing twin formation in assisting the nucleation and growth of recrystallised grains is discussed, and examples of its effect are shown. The effect of forging parameters (cooling rate after forging, number of forging strokes used) is also investigated. A detailed study is performed on the interaction between recrystallisation and precipitates during the final sub-solvus forging stage of the industrial forging process. It is observed that precipitates can interact in at least four different ways with recrystallisation. Suggestions are made for future work, and for ways to exploit the observations made in an industrial forging process.

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