Metallurgical Analysis of Cracked AA7075 Aluminum Alloy Component Used in Control System of a Satellite Launch Vehicle
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TECHNICAL ARTICLE—PEER-REVIEWED
Metallurgical Analysis of Cracked AA7075 Aluminum Alloy Component Used in Control System of a Satellite Launch Vehicle Sushant K. Manwatkar . J. Srinath . S. V. S. Narayana Murty . P. Ramesh Narayanan . S. C. Sharma . P. V. Venkitakrishnan
Submitted: 23 July 2016 / in revised form: 4 October 2016 Ó ASM International 2016
Abstract High-strength aluminum alloy AA7075 forgings are widely used in various control system components of liquid engines of satellite launch vehicles. In one such application, a forged component was found to crack under storage. Multiple cracks were noticed on the surface of the component. Fractographic features indicated intergranular mode of failure. Optical microscopic examination revealed dendritic coring in the material and crack propagation to be along the grain boundaries. Anodic dissolution of grain boundaries was also noticed. Based on detailed metallurgical investigations, it was concluded that the failure was due to ‘stress corrosion cracking.’
engines of satellite launch vehicles are fabricated out of this alloy. In one such critical application, a secondary injection thrust vector control valve body was found to be cracked during the routine visual inspection. The component was machined out of a cylindrical forged block and was anodized after fabrication. The component has cracked during storage. This paper brings out the details of metallurgical investigations carried out to establish the reason for cracking.
Keywords AA7075 alloy Intergranular cracking Dendritic Coring Stress corrosion cracking
The cracked surface was carefully separated by sectioning the component to analyze the topographical features of the fracture surface using a Carl Zeiss EVO-50 scanning electron microscope (SEM). The portion containing the crack was also sectioned for metallographic analysis. The metallographic specimens were mounted and polished using a series of SiC emery papers and final polishing was carried out with half-micron diamond lapping compound. The finish polished specimens were etched using Keller’s reagent (2 ml HF, 3 ml HCl, 5 ml HNO3, 190 ml H2O) to reveal the microstructural features. The metallographic specimens were observed using Olympus GX-71 inverted optical metallurgical microscope. An Oxford-INCA Energy dispersive X-ray spectrometer was used to obtain the chemical composition on the fracture surface. Electron probe micro-analyzer (EPMA) JEOL JXA-8530F was used to analyze the elemental segregation in the material. Transmission electron microscopy (TEM) analysis of precipitates was carried in 200 kV JEOL equipment. The microstructure was also observed under a Bruker atomic force microscope (AFM) in contact mode. Nanoindentation
Introduction Aluminum alloy AA7075 (Al–5.5Zn–2.5 Mg–1.5Cu) is widely used in the fabrication of aerospace structural components in view of its high strength coupled with excellent corrosion resistance. This alloy is non-weldable and is strengthened by precipitation hardening. Different types of structural component
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