Abstract
The flow behavior and the microstructural evolution of aluminum alloy 7055 in two heat treatment conditions (homogenized vs. pre-rolled, solution treated, stretched and naturally aged (T3)) were investigated for a height reduction of 60% with deformation temperatures ranging from 370 °C to 450 °C and strain rates ranging from 0.01 s−1 to 10 s−1. Flow stress decline ratio maps as a function of deformation temperature and strain rate were produced along with processing maps at a strain of 0.8 to reveal optimum hot-working conditions for deformation at strain rates of 0.01 s−1 to 0.1 s−1. The results showed that the stress drop ratio during deformation is higher for the homogenized condition than for the pre-rolled, T3 condition. A higher degree of recrystallization after deformation was observed in the pre-rolled, T3 condition due to finer second phase particles, smaller grain size, and more numerous sub-grains. The mechanism for deformation softening is discussed in the context of grain boundary characteristics.
Highlights
Al-Zn-Mg-Cu alloys have been a primary choice of structural materials for aircraft due to their low weight, high specific strength, excellent fracture toughness and good stress corrosion cracking resistance [1]
The level of the flow stress consistently increases with decreasing deformation temperature
Through the misorientation profile analysis performed on line 1 and line 2 in Figure 6b,f of homogenized and pre-rolled (T3) samples after deformation, the point-to-origin misorientation in the grains of the homogenized and deformed alloy was over 15◦, indicating a good development of the progressive sub-grain rotation, which is a sign of continuous dynamic recrystallization (CDRX) [30,31]
Summary
Al-Zn-Mg-Cu alloys have been a primary choice of structural materials for aircraft due to their low weight, high specific strength, excellent fracture toughness and good stress corrosion cracking resistance [1]. Most of the thermal-mechanical simulation work on alloy 7055 has been carried out on homogenized cast samples, whilst in practical production, the alloy can be rolled to thick plates, solution treated, stretched, and stored or shipped in the naturally aged condition (i.e., T3) prior to further rolling. This kind of treatment raises questions about whether there are any differences in flow stress behavior and recrystallization mechanisms for alloy 7055 for the rolling of homogenized material compared to the rolling of the pre-rolled (T3) condition, in the event that such a final forming step is required (or desirable). The findings are discussed in relation to the sub-structure, recovery, and recrystallization mechanisms
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