Precipitation kinetics in warm forming of AW-7020 alloy

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2013-01, Vol.561, p.362-370
• Main Authors: Kumar, M., Poletti, C., Degischer, H.P.
• Format: Article
• Language: English
• Subjects: Annealing; Applied sciences; AW-7020; Cold working, work hardening; annealing, quenching, tempering, recovery, and recrystallization; textures; Cross-disciplinary physics: materials science; rheology; Differential scanning calorimetry; Exact sciences and technology; Heat treatment; Materials science; Metals. Metallurgy; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Precipitation; Production techniques; Retrogression; Solid solution, precipitation, and dispersion hardening; aging; Treatment of materials and its effects on microstructure and properties; Warm forming
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2012.10.031

The warm formability of the precipitation hardening AW-7020 (AlZn 4.5Mg1) alloy is investigated by testing extruded tubes. The precipitation kinetics of different conditions before and after warm deformation is studied by differential scanning calorimetry and transmission electron microcopy. The precipitation conditions are correlated with the results of hardness tests at room temperature and of tensile tests at temperatures between 200 and 350°C at different strain rates. The yield strength decreases with increasing test temperature approaching that of samples in the annealed condition, while the strain at fracture increases. The overall influence of the strain rate on ductility is dominated by the corresponding time required for deformation. The formability of the starting condition T1 as well as the corresponding strain hardening exponent is particularly promising for high strain rates at 250°C, where the metastable precipitates of the T1 condition are dissolved. The short exposure of about 30s at 250°C re-establishes the potential for precipitation strengthening by natural ageing after the warm deformation and a following paint baking heat treatment maintains the hardness level.