Evolution of globularisation in Ti-6Al-4V alloy during secondary cogging operations under industrial scale conditions

Understanding the relationship between process parameters and microstructural evolution during thermo-mechanical processing of Titanium alloys is essential to ensure that the desired mechanical properties are achieved in the final component and to reduce production costs. This thesis particularly focuses on the evolution of globularisation in Ti-6Al-4V alloy during secondary cogging operations, with an attempt to model the phenomenon based on the dominant mechanisms.A methodology has been developed to evaluate the effect of processing parameters such as forging temperature, strain and strain path on the evolution of microstructure during open die forging operations. It replicates industrial conditions of cogging process using large cubic samples and a 500T hydraulic press.The methodology was benchmarked against the literature and validated using uniaxial forging trials. The main mechanisms of globularisation were found to be the creation of α/α boundaries separating laths into smaller grains, termination migration and edge spheroidisation. The kinetics of globularisation increases with strain and annealing duration, and is unaffected by the interruption of the deformation and intermediate annealing.During multiaxial forging with orthogonal compressions, a range of behaviours were observed with some α laths that retained their hard orientation with regard to the compression direction and resisted deformation. On the other hand, laths with favourable orientations further deformed during additional forging steps, resulting in heterogeneous deformation and a lower level of globularisation than those that underwent uniaxial compression. For the range of conditions tested in this work, the process route and the deformation increment had limited effect on the kinetics of globularisation during cogging; whereas the forging temperature affected the main deformation mechanisms with the lower temperature (viz.900°C) producing more heterogeneous deformation and lower kinetics of globularisation.A novel empirical model was developed to predict globularisation during cogging operations based on the classic Avrami equation. It includes modifications to the existing models by taking into consideration the effects of strain path and inter-step annealing for the prediction of the kinetics of globularisation. This newly developed empirical model has been successfully implemented into a finite element framework, using DEFORM software package, as a user subroutine that can help industries investigate new processing routes to optimise the existing manufacturing routes and to explore new methodologies.

Name dc:type.qualificationname

engd

Level dc:type.qualificationlevel

doctoral-pg

Grantor dc:publisher.institution

University of Strathclyde

Year dc:date.issued

2022

Author dc:creator

• Fabris, Mathieu

Advisors dc:contributor.advisor

• Rahimi, Salaheddin
• Rosochowski, Andrzej

DOI

10.48730/a3ej-vv24

Identifier

T16223

Author Identifier

201478976

OAI identifier oai:identifier

oai:strathclyde:w3763734f