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[en] Highlights: • Dissimilar EB welding of Ti55-TA15 is investigated for the first time. • The Effect of beam offsetting on microstructure and mechanical properties of welded joint is studied. • The higher reinforcement of the weld is found in the welded joints with beam offset towards Ti55. • Tensile test and microhardness results are in accordance with chemical composition and microstructure observations. • The porosities are found in high beam offset welded joints in both directions. Electron beam welding was used to join dissimilar Ti55 to TA15 titanium alloys. The aim of this study was to investigate the effect of the beam offset on microstructure and mechanical properties of the welded joints. Microstructural characterization was investigated by optical microscopy, scanning electron microscopy and X-ray diffraction. Tensile test was conducted at room temperature and 550 °C. Results indicated that the fusion zone consisted of martensite α′ and acicular α. Heat affected zone (HAZ) in both Ti55 and TA15 sides consisted of two regions; Low temperature HAZ (LT-HAZ) was composed of primary α, secondary α and prior β and high temperature HAZ (HT-HAZ) was composed of martensite α′ and acicular α. The hardness of fusion zone was lower than HAZ of both Ti55 and TA15 alloy. Energy-dispersive spectroscopy was applied to display microstructure evolution inside the fusion zone. Tensile results showed that offsetting the beam position towards Ti55 alloy led to higher values of joint strength. Beam offsetting towards TA15 alloy lowered the hardness and strength of welded joints. Increasing the beam offset to the values > 0.3 mm towards Ti55 or TA15 resulted in premature fracture due to increasing the possibility of porosity formation inside the weld metal.