Development of alternative thermomechanical processing techniques to improve forging outcomes of cast peritectic TiAl alloys

Peters, Sean

Thesis

Development of alternative thermomechanical processing techniques to improve forging outcomes of cast peritectic TiAl alloys

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Creator

Peters, Sean

Rights statement

Strathclyde Thesis Copyright

Awarding institution

University of Strathclyde

Date of award

2024

Thesis identifier

T16906

Person Identifier (Local)

201988884

Qualification Level

Doctoral (Postgraduate)

Qualification Name

Doctor of Philosophy (PhD)

Department, School or Faculty

Abstract

The aim of this research was to investigate and develop alternative thermomechanical processing techniques to improve the forging outcomes, at industrially viable temperatures, of two cast, peritectic solidifying, consolidated titanium aluminide alloys; 45XD (Ti-45Al-2Nb-2Mn-1B at.%) and 4822 (Ti-48Al-2Nb-2Cr at.%). This study differed from existing work on TiAl alloy composition-processing-microstructure relationships with an emphasis on controlling the initial microstructure to optimise primary processing (i.e., ingot breakdown) efficiency and produce a microstructure that improves the outcome of secondary processing (i.e., isothermal closed die forging, or hot rolling), ideally approaching a strain rate sensitivity of ≥0.3. For these alloys, consolidation by hot isostatic pressing (HIP) followed by primary compression alone does not remove the casting segregation or the anisotropic behaving lamellar content that hinders secondary processing; alternative processes are required. For 45XD, this study found that integrating HIP and homogenisation stages into one step, using HIP equipment, proved beneficial to the forging outcome in comparison to the traditional two step approach. Achieving an elementally homogeneous and refined fully lamellar microstructure enabled high levels of globularisation, and dynamic recrystallised material from 50 % primary compression at 1100 °C and 0.001 s-1 . This aided secondary compression, returning the highest strain rate sensitivity of 0.32. For 4822, this study investigated the impact of cyclic heat treatment (CHT) and cooling rate on HIP and homogenised material, as well as its subsequent effect on forging outcomes. For the first time, industrially relevant induction heating equipment was applied to conduct the five cycles to the single α phase temperatures (1370 °C) necessary for lamellar grain refinement. The compression results showed that CHT, irrespective of cooling, proved successful with uniform 50 % primary compression at 1100 °C and 0.001 s-1 , compared to the shearing instability of HH material. This led to secondary compression of CHT material returning material high in dynamic recrystallised content and free of lamellar morphologies, with a strain rate sensitivity of 0.25.

Advisor / supervisor