Microstructure Evolution of EBM Fabricated Ti-6Al-4V

Abstract

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.