Metallurgical analysis of cracking in weldment of propellant tank

Abstract Medium strength Al–Zn–Mg aluminium alloy is extensively used in propellant tanks of satellite launch vehicles. The cylindrical tanks are fabricated by TIG welding of sheets and rings into larger diameter pressure vessels. End domes are fabricated by welding petals, and nozzle openings are provided by shrink fit nozzles, welded to the petals. A crack, partly on the overlap region was noticed on one such nozzle weld. A detailed analysis revealed that the crack initiated from the root of pits, propagated under the high longitudinal restraint at the weld stop area, through solute enriched dendrites at the weld fusion line, resulting in an intergranular mode of failure. This is typical of stress corrosion cracking (SCC).