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Zou, Zhiyi (2020) Refinement of the grain structure of selective laser melted Ti-6Al-4V via induction heat treatment. PhD thesis, University of Nottingham.

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Abstract

Selective laser melting (SLM) has been shown to be an attractive manufacturing route for the production of α/β titanium alloys, and in particular Ti-6Al-4V. Post-process heat treatments are widely used to tune the microstructure and improve the mechanical properties of the as-built parts. However, conventional heat treatments cannot transform the coarse elongated prior-β structure in a satisfying way, which limits the use of Ti-6Al-4V produced by SLM. The principal aim of the present study is to investigate how the initial microstructure formed in SLM can be exploited to achieve β-grain refinement via induction heat treatment. The microstructure and tensile properties following such heat treatments are discussed. As well as the microstructure evolution and recrystallisation phenomena responsible for the refinement.

The effect of induction heat treatment on the tensile properties and microstructure of the SLM specimens were initially investigated. It was found that, after heat treated to a temperature around β transus via induction heating (short time β annealing), a typical microstructure consisted of equiaxed or quasi-equiaxed prior-β grains can be observed. In comparison to the microstructure obtained after conventional heat treatment, a refinement of the morphology of the prior-β grains can be observed. It was also found that the width of the α lamellae is smaller than what was observed in specimens subjected to conventional heat treatment. It was found that this microstructure can provide tensile properties equivalent to or better than what was obtained after conventional heat treatment, and also might provide a potential to achieve isotropic behaviour and improve fatigue resistance.



The microstructure evolution leads to the observed β grain refinement was then investigated. It was found that a mixed morphology of prior-β grains appears when the temperature approaches β transus. Such mixed morphology consists of both newly formed equiaxed prior-β grains and original columnar prior-β grains. It was found that the equiaxed prior-β grains grow from the recrystallised high angle boundary (HAB) β, which are nucleated during the induction heat treatment. It was found that, since the HAB has higher mobility, the equiaxed HAB β grain grows faster compared to the original β grains. Therefore, the original columnar morphology is gradually replaced, and the microstructure evolves toward the observed refined morphology.

The nature of the observed recrystallisation phenomena was then investigated. It was found that the recrystallised β is related to individual adjacent α/α’ laths via Burger Orientation Relationship (BOR), and can be nucleated either in the interior of the individual α phase, at the α/β boundary or at the prior-β grain boundary. Therefore, the recrystallisation process is identified as epitaxial recrystallisation. It can be assumed that the driving force for the observed epitaxial recrystallisation origins from the stored energy of deformation that is typically expressed by the high densities of lattice defects found after SLM in the core and at the boundary of the martensitic as-built microstructure. The induction heat treatment would exacerbate such epitaxial recrystallisation and result in significant refinement of the structure.

This study achieves for the first time the refinement of the β grain structure of additively manufactured Ti alloys via simple heat treatment. Further, this study investigates for the first time the microstructure evolution during the β annealing of Ti-6Al-4V produced by SLM, especially in its early stage. Lastly, this study demonstrates for the first time how epitaxial recrystallisation nucleate and progress in additively manufactured Ti alloys.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Hague, Richard Simonelli, Marco Katrib, Juliano Dimitrakis, Georgios
Keywords:	Three-dimensional printing; Titanium alloys; Titanium alloys, Heat treatment; Induction heating
Subjects:	T Technology > TN Mining engineering. Metallurgy T Technology > TS Manufactures
Faculties/Schools:	UK Campuses > Faculty of Engineering
Item ID:	61283
Depositing User:	Zou, Zhiyi
Date Deposited:	29 Sep 2023 13:27
Last Modified:	29 Sep 2023 13:27
URI:	https://eprints.nottingham.ac.uk/id/eprint/61283

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