Microstructure Evolution of EBM Fabricated Ti-6Al-4V
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Författare
Typ
Examensarbete för masterexamen
Program
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Electron beam melting (EBM) is an emerging additive manufacturing technology in the
recent years, providing new opportunities for fast and sustainable production on
industrial scale for key components in fields like the aerospace sector. Ti-6Al-4V offers
outstanding mechanical properties, corrosion resistance and high temperature stability
as well as low density. Hence, the EBM fabrication of Ti-6Al-4V components is of great
interest for the aerospace industry. EBM is a complex thermal process and it is of
tremendous significance to understand its effect on the microstructure and porosity of Ti-
6Al-4V since they define the mechanical properties of the produced component.
This master thesis was executed in cooperation with GKN Aerospace, a company
developing and supplying key components to the aerospace industry. The objective of this
project was to understand how two different layer thicknesses in the printing and postprocessing
including hot isostatic pressing (HIP) and heat treatments (HT) affect the
microstructure and porosity of EBM fabricated Ti-6Al-4V components. Furthermore, the
effect of powder recycling on the powder’s microstructure was investigated.
In this work optical and electron microscopy as well as energy dispersive X-ray
spectroscopy (EDS) were used to identify the phases and morphologies in the
microstructure of the components and the powders. Image analysis techniques were
applied for quantitative analysis of the microstructure in terms of lamellae thicknesses
and porosity. To support the results Vickers hardness measurements of the components
were also conducted.
It has been found that all components featured an elongated prior b-grain structure
parallel to the build direction with a-phase at the grain boundaries. The as-built sample
with lower layer thicknesses possesses a slightly finer microstructure and postprocessing
makes it three times coarser. Finer microstructures resulted in higher
hardnesses for every component due to the Hall-Petch effect. The microstructural
features of the as-built samples at different layer thicknesses are similar but differ from
the ones of the post-processed component. Besides the fully lamellar structure consisting
of Widmannstetten a- and b-phase, martensite has been observed in the as-built state. A
few additional features are detected in all specimen such as large feature-less regions
consisting of a-phase. In all components the entire a-phase contains nano-sized particles
which could be Ti3Al precipitates. No correlation between porosity and layer thickness
has been clarified. Post-processing reduces the porosity significantly, however, it does not
close all pores. Furthermore, it has been found that the virgin Ti-6Al-4V powder has a fully
martensitic structure whereas all the recycled ones exhibit a microstructure similar to
that of the as-built samples, which almost does not change after ten times recycling.
Beskrivning
Ämne/nyckelord
Additive manufacturing, electron beam melting, titanium alloy, Ti-6Al-4V, microstructure, porosity, hardness, hot isostatic pressing, powder recycling