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Electron beam welding (EBW) studies have been investigated on Ti-6Al-4V (Ti64) alloy. Post-welding solution treatment and aging were carried out to study the effects of heat treatment on the microstructure of welded samples. Results showed that the martensitic 𝛼′ phase coarsened, the size of heat affected zone (HAZ) changed and the grains in the base materials (BM) had grown after the post-weld heat treatments. The tensile behaviour of electron beam welded Ti64 has been investigated using in-situ tensile testing monitored by optical microscopy. The deformation and failure were directly revealed during the in-situ tensile process. The results showed that the EBW Ti64 samples have different failure locations after receiving different post-weld heat treatments. After the post-weld aging treatment, the tensile strength and the elongation-to-failure were both decreased. The tensile strength and elongation were further reduced after a post-weld solid solution and aging treatment. The relationship between the post-weld heat treatments, microstructural evolution and mechanical properties of EBW Ti64 were investigated. Thermodynamic databases were used to predict mechanical properties-including the yield strengths-of the titanium alloy for different grain sizes, representing different post-weld heat treatment operations, and these were embedded into a finite element modelling framework to simulate the tensile testing specimens to understand the mechanical fields experienced such as stresses and strains, just prior to failure.