Microstructure characterisation of a friction stir welded hemi-cylinder structure using Ti-6Al-4V castings

Friction stir welding (FSW) was applied to manufacture a spacecraft propellant tank from cast Ti-6Al-4V with the aim to reduce costs and lead time compared to conventional manufacturing routes. These tanks are usually produced via forging and a combination of tungsten inert gas (TIG) and electron beam (EB) welding. FSW is a solid-state process and produces joints of high quality and fewer internal defects, given the use of appropriate welding parameters, and is expected to become an attractive alternative for future spacecraft tank manufacturing. The study presented in this paper investigates the FSW region of a cast cylinder and hemisphere shell, joined by a single curved track friction stir weld of 1.3 m length. The weld parameters, optimised in a separate development study, resulted in a stable and consistent weld around the circumference of the tank with good mechanical properties, no porosity and few defects in the microstructure. Particular emphasis was placed on the variation of the microstructure from the base material, cast Ti-6Al-4V, to the refined stir zone. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were employed to visualise these regions. Furthermore, periodic bands of tungsten contamination were detected in the stir zone.