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

Student thesis: Doctoral Thesis

Abstract

The aim of this research was to investigate and develop alternative thermomechanical processingtechniques 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.%) and4822 (Ti-48Al-2Nb-2Cr at.%).This study differed from existing work on TiAl alloy composition-processing-microstructurerelationships 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 ofsecondary processing (i.e., isothermal closed die forging, or hot rolling), ideally approaching a strainrate sensitivity of ≥0.3. For these alloys, consolidation by hot isostatic pressing (HIP) followed byprimary compression alone does not remove the casting segregation or the anisotropic behavinglamellar content that hinders secondary processing; alternative processes are required.For 45XD, this study found that integrating HIP and homogenisation stages into one step, using HIPequipment, proved beneficial to the forging outcome in comparison to the traditional two stepapproach. Achieving an elementally homogeneous and refined fully lamellar microstructure enabledhigh levels of globularisation, and dynamic recrystallised material from 50 % primary compression at1100 °C and 0.001 s-1. This aided secondary compression, returning the highest strain rate sensitivityof 0.32.For 4822, this study investigated the impact of cyclic heat treatment (CHT) and cooling rate on HIPand 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 singleα phase temperatures (1370 °C) necessary for lamellar grain refinement. The compression resultsshowed that CHT, irrespective of cooling, proved successful with uniform 50 % primary compressionat 1100 °C and 0.001 s-1, compared to the shearing instability of HH material. This led to secondarycompression of CHT material returning material high in dynamic recrystallised content and free oflamellar morphologies, with a strain rate sensitivity of 0.25.
Date of Award24 Apr 2024
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorJorn Mehnen (Supervisor) & Dorothy Evans (Supervisor)

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