title: Microstructure characterizing and mechanical properties of Selective Laser Melted Ti-6AL-4V alloys
Creators: name: Do, Dang Khoa
Publisher: name: University of Glasgow
Year: 2021
Subjects: subject: Additive manufacturing, titanium, Selective Laser Melting, Ti-6Al-4V.

description: Selective laser melting (SLM) is an additive manufacturing technique that can produce complex functional metal parts of high relative density and near-net shape. Due to the complete melting of metal powders in a layer-by-layer approach during the SLM process, heterogeneity in the as-produced microstructure is a big hindrance in the application of SLM parts. The formation progress of such heterogeneous microstructure in SLM built Ti-6Al-4V and how it behaves under different working conditions are not yet fully understood. This study thus aims to better understand the evolution of microstructure within Ti-6Al-4V's as-fabricated and heat-treated conditions for the microstructural regulation. Furthermore, this research also paves the way for identifying microstructure behaviours under high strain rate conditions that are important for the successful implementation of SLM-built Ti-6Al-4V in many industries. Firstly, the effect of SLM processing parameters on the part quality and macrostructure are investigated to obtain the optimised processing window based on the optimal relative density and surface roughness achieved. Secondly, the resulting microstructure of Ti-6Al-4V parts is examined and analysed quantitatively using microstructural characterisation techniques and numerical simulations to promote the interpretation of the SLM forming process. Thirdly, the mechanical testing of SLM Ti-6Al-4V parts manufactured by a wide range of laser energy densities is conducted under quasi-static (e.g. tensile tests and Vickers microhardness tests) and high strain rate (e.g. Split-Hopkinson pressure bar tests) conditions. The dynamic behaviours of SLM Ti-6Al-4V is benchmarked against that of Ti-6Al-4V produced by traditional methods to investigate the microstructure features and dynamic recrystallization in the SLM-built material. Lastly, the influence of the heat treatment on the SLM Ti-6Al-4V microstructure is studied with a view to improving mechanical properties. Annealing strategies are designed to decompose and spheroidise the alpha’ martensitic microstructure into a bimodal to balance mechanical strength and ductility. A Phase Field model is developed to validate the experimental observation and facilitate the findings in this section. The conclusions are drawn that the heterogeneity in the microstructure and mechanical properties is indeed established during the fabrication process. The phase transformation during the cyclic thermal loading process of SLM is also shown. Differences in thermo-plasticity and auto-tempering influenced by laser energy densities lead to heterogeneity in the microstructure behaviours and mechanical properties. Additionally, the kinetic pathways of alpha’ decomposition is revealed by the Phase Field model, which facilitates understanding of microstructure evolution in isothermal heat treatment of SLM Ti-6Al-4V. The key contributions in this study further enrich the scientific knowledge regarding microstructure evolution occurring during SLM process and under different loading conditions. The findings of this thesis are a valuable reference for both microstructure regulation and employing the SLM components for industrial application.