Cracking of AFNOR 7020 Aluminum Alloy Adaptor Used for Aerospace Applications: A Metallurgical Investigation
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CASE HISTORY—PEER-REVIEWED
Cracking of AFNOR 7020 Aluminum Alloy Adaptor Used for Aerospace Applications: A Metallurgical Investigation Abhay K. Jha Æ G. Naga Sirisha Æ P. Ramesh Narayanan Æ K. Sreekumar
Submitted: 2 December 2008 / in revised form: 14 July 2009 / Published online: 29 July 2009 Ó ASM International 2009
Abstract 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).
In this case, the alloy was used in the fabrication of a torroidal shaped water tank that provides water for initial run of turbo pump and for cooling various components in the liquid propulsion system. Pressurization adaptors are shrink fitted to the openings followed by circumferentially manual TIG welding to main body (AFNOR 7020, 1.8 mm thick sheet) in order to configure the water tank assembly (Fig. 1). These adaptors are made of AFNOR 7020-T6. During qualification tests for the water tank, one of the adaptors cracked near the weld fusion line. A systematic metallurgical investigation was carried out to understand the mechanism of failure of this adaptor. This article brings out the details of their investigation.
Keywords Aluminum alloy AFNOR 7020 Locked-in stresses Stress corrosion cracking Anodic dissolution Contractional strain
A cut piece from the cracked adaptor was analyzed for chemical constituents and results confirmed the material to be AFNOR 7020 (Table 1).
Experimental Chemical Analysis
Macroscopic Observations Introduction The medium strength aluminum alloy AFNOR 7020 (Al–4.5Zn–1.5Mg) is extensively used in the fabrication of liquid boosters and liquid upper stages of launch vehicles.
A. K. Jha (&) G. N. Sirisha P. R. Narayanan K. Sreekumar Material Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Indian Space Research Organisation, Trivandrum 695022, India e-mail: [email protected]
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The crack morphology, viewed under optical microscope with stereographic facility, indicated discontinuous cracks with sharp crack tip. Cracks were wider in middle and narrowing down at both the ends (Fig. 2). The crack path was
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