Cracking of Al–4.5Zn–1.5Mg aluminium alloy propellant tank – A metallurgical investigation

The medium strength aluminum alloy AFNOR 7020 (Al–4.5Zn–1.5Mg) is extensively used in the fabrication of common bulk head propellant tank of liquid propulsion engine. The rings used in fabrication of the tank were of different sizes and were processed almost 4 years back in T651 condition. The propellant tank developed a leak at 2.7 bar(a) during proof pressure charging to 5 bar. The cracked portion of the ring was removed from the failed tank and subjected to detailed metallurgical investigations to understand the cause of failure. This paper brings out the details of investigations carried out and thereafter conclusion arrived on.