Non-isothermal annealing of AA7075 aluminum alloy – structural and mechanical effects

Abstract

Calorimetric and dilatometric tests were performed on AA7075 aluminum alloy annealed at constant heating rate of 15°C/min in temperature range 20-470°C and discussed in relation to the hardness test and structure observation results. The samples were machined from furnace cooled material (FC), furnace cooled and deformed (FCD) material, solution treated (ST) material and solution treated and deformed (STD) material. It was found that the nucleation and growth of transition η’ and stable η (MgZn2) phases caused remarkable reduction of thermal expansion coefficient t , whereas both the dissolution of η phase and formation of GP zones were accompanied by an increase of t value. While αi phase started to dissolve at 250°C, a widespread endothermic effect was observed on DSC curves. Dissolution of η particles at high annealing temperatures was accompanied by the solid solution hardening of the alloy. Mentioned hardening process was overlapped by expected material softening that was ascribed to recovery and recrystallization processes. Because of superposition the solution hardening and recrystallization softening, recrystallization temperature could not be precisely defined on the basis of simple hardness measurements.