Microstructure and Mechanical Properties of Cryorolled Aluminum Alloy AA2219 in Different Thermomechanical Processing Co
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INTRODUCTION
PRECIPITATION-HARDENABLE aluminum alloys of the 2XXX and 7XXX series find extensive application in the aerospace industry as a result of their good combination of strength, toughness, and stress corrosion cracking resistance.[1–3] Among the 2XXX series alloys, Al-Cu (AA2219) alloy is used for the fabrication of propellant tanks of cryogenic stages due to its good weldability coupled with excellent properties at liquid hydrogen [fuel, boiling point 20 K ( 253 C)] and liquid oxygen [oxidizer, boiling point 90 K ( 183 C)] temperatures.[4–6] During the fabrication of
ADITYA SARKAR, Masters Student, is with the Department of Materials Science and Engineering, Indian Institute of Technology, Gandhinagar, 382 424, India, and also Graduate Student with the Indian Institute of Technology, Bombay, Mumbai, 400 076, India. K. SARAVANAN and NIRAJ NAYAN, Engineers, S.V.S. NARAYANA MURTY, Deputy Head, P. RAMESH NARAYANAN, Head of Material Characterisation Division, and P.V. VENKITAKRISHNAN, Deputy Director, are with the Materials and Mechanical Entity, Vikram Sarabhai Space Center, Trivandrum, 695 022, India. Contact e-mail: [email protected] J. MUKHOPADHYAY, Professor, is with the Department of Materials Science and Engineering, Indian Institute of Technology. Manuscript submitted June 18, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
propellant tanks, plates/sheets are welded together along with stiffener rings/end domes. During fusion welding of AA2219, a partially melted zone forms adjacent to the weld pool, which contains copper-rich cellular regions along the grain boundaries.[7,8] These eutectic films are weaker compared with the base weld material, and during tensile testing, the specimen fails along these films. With the advent of friction stir welding (FSW), the problems associated with the partially melted zone in fusion welding of AA2219 were overcome. The design of structural members is based on strength, and any improvement in the mechanical properties, particularly the yield strength of AA2219, will help in the reduction of structural mass. Furthermore, due to the improved fabrication through FSW, the overall mass benefit will accrue to the structure under consideration. Various mechanisms responsible for improvement in strength of aluminum alloys are dislocation strengthening, grain size refinement, solid solution strengthening, and precipitation hardening.[9] There are continuous efforts to improve the strength of aluminum alloys by controlling the microstructure, by changing composition, and by introduction of newer tempers. In case of AA2219, precipitation hardening is the principal strengthening mechanism as the bulk of its strength comes from the metastable h’ precipitates that form during aging.[10]
For the contributions of dislocation strengthening and grain size refinement to be significant in AA2219, it is imperative to suppress dynamic recovery (DRV) to increase the dislocation density. The increased number of dislocations will not only lead to dislocation strengthening adding to th
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