Cracking of AFNOR 7020 Aluminum Alloy Adaptor Used for Aerospace Applications: A Metallurgical Investigation
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Aluminum alloys are frequently preferred materials for aerospace applications due to their high-specific density, high-specific stiffness, and ease of fabrication. In one such application, adaptors used in the torroidal shaped water tank of liquid propulsion system, were made of an AFNOR 7020 (Al–4.5Zn–1.5Mg) aluminum alloy extrusion. These pressurization adaptors, in T6 temper condition, were initially shrink fitted to the openings provided in water tank main body and later circumferentially welded, manual TIG, to configure the tank. Four such adaptors were welded at different locations of a water tank. During one of the qualification tests, cracks were noticed near to the weld fusion line of one of the adaptor. Detailed metallographic investigation on the cracked adaptor revealed that cracking was due to combined effect of locked-in stresses in the material and anodic dissolution of solute rich phases present at elongated grain boundaries of the HAZ: a typical case of Stress Corrosion Cracking (SCC).
KeywordsAluminum alloy AFNOR 7020 Locked-in stresses Stress corrosion cracking Anodic dissolution Contractional strain
The authors are thankful to Shri M. C. Mittal, Former Group Director, MMG and Dr. P. P. Sinha, Deputy Director (MME), VSSC for their guidance and encouragement during the work. They are indebted to Dr. K. Radakrishnanan, Director, VSSC for permission to publish this work.
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