Failure analysis of a Ti–6Al–4V gas bottle

Ti–6Al–4V alloy has proven its suitability as a major structural material for upper stages of launch vehicles owing to excellent specific strength coupled with excellent fabrication characteristics. High pressure gas bottles being fabricated using this alloy are in use for storing gaseous helium for the pressurization of liquid propellant in the satellite launch vehicles. One such bottle with Ti–6Al–4V liner, fabricated by electron beam welding of two forged and machined hemispheres (designated as male and female domes) and wound with Kevlar fibre around it, failed prematurely during the auto-frettage cycle of pressure testing.Detailed metallurgical analyses were carried out on this cracked liner. Microstructure of the male dome consisted of colonies of alpha phase and coarse laths of alpha phase along the prior beta grain boundaries whereas that of the female dome contained equiaxed grains of primary alpha in the matrix of transformed beta. Microstructure of the electron beam (EB) weldment region showed martensitic structure. The difference in microstructures between the two domes is attributed to higher forging temperature for male dome, which caused male dome to yield at a smaller strain. Cracks initiated at the location where there was a change in the wall thickness of the dome and propagated along the alpha phase present on the prior beta grain boundaries and also along the alpha lath boundaries. This paper highlights the investigations carried out on this cracked liner and remedial measures suggested to avoid future recurrence.