Differential Heat Treatment Response of Cast plus Homogenized and Forged Billets of Aluminum Alloy AA7075

Abstract

High-strength aluminum alloy AA7075 is extensively used in aerospace systems in overaged temper condition due to its high strength and stress corrosion cracking resistance. When this alloy experiences insufficient deformation prior to heat treatment, it may exhibit change in yield strength. Considering this aspect, a thorough heat treatment study has been carried out on cast plus homogenized (ACH) condition billets as well as on forged (ACHF) billets. The response of the alloy in these two initial conditions to various heat treatment tempers T6, T73 as well as RRA (Retrogression and reaging) on mechanical properties and microstructure has been studied. Peak yield strength is observed in T6 temper in periphery of ACH condition (531-539 MPa) and RRA temper of ACHF (530-539 MPa), whereas lowest yield strength is observed in T73 temper of ACHF (427-433 MPa). The aging response with respect to yield strength is found to be better for ‘ACH’ samples in T6 and T73 conditions, whereas it is found to be better in ‘ACHF’ samples subjected to RRA condition. Microstructure showed the presence of fine GP zones and η′ (50-75 nm) along with stable η precipitates in T6 temper condition of ACH (periphery), whereas coarser precipitates are seen in ‘ACH’ samples taken from the core. In T73 condition, coarser precipitates (~75 nm) are found with narrow precipitate free zones (<20 nm). In RRA condition, finer precipitates of η′ (~10 nm) are observed in ACHF samples as compared to ACH samples (~50 nm). It is found that, the alloy responds to all temper heat treatments in ‘ACH’ condition as well and results in acceptable yield strength. Fractography revealed ductile features in tensile tested samples in all the temper conditions.