Metallurgical investigation of cracked Al–5.5Zn–2.5Mg–1.5Cu aluminium alloy valve

• Cracking occurred on radial plane corresponding to short transverse plane of forged block used to realise the component.
• The conductivity measurement of cracked component was less than desirable vale for T7352 temper condition.
• Micro structural evidence of crack propagation along grain boundaries and anodic dissolution at crack tip, resulting in dried mud features on fracture surface were observed.
• Detailed metallurgical investigation indicated that failure was caused by stress corrosion cracking.

The high strength aluminium alloy Al–5.5Zn–2.5Mg–1.5Cu (AA7075) is being widely used in realisation of aerospace components. A component ‘fill and vent valve’ used in liquid propulsion system was fabricated from AA 7075 forgings in T7352 temper condition, and subsequently undergone various functional tests, four years back. Recently, during dye penetrant test after proof pressure test at 525 bar, a valve indicated presence of a crack. Detailed metallurgical investigation indicated that failure was caused by stress corrosion cracking.