Comparative Study of Creep and Stress Relaxation Behaviour during Ageing of 7050 Aluminum Alloy

This work focuses on quantitatively investigating the relationship between creep and stress relaxation of 7050 aluminum alloy at ageing temperatures. The creep strain rates, creep exponent, n, and activation energy were extracted from both the creep and stress relaxation curves to explore the creep mechanisms. Results showed that higher applied stress promotes the creep and stress relaxation extents. The calculated creep strain rates from creep and stress relaxation curves located in similar ranges and share similar n values (n = 3.7 for creep and n = 3.5 for stress relaxation), indicating that both tests are dominated by the dislocation slip mechanism. Additionally, the n value for creep tests is a constant throughout the tests, indicating that the dominant creep mechanism is unchanged. However, the activation energy changes from 123 kJ/mol to 139 kJ/mol, possibly due to evolved dislocations. Such a new finding could provide a supporting mechanism for building constitutive equations depending on the evolved A value. Furthermore, compared to the single-stage creep ageing, no primary creep was observed in T74 multi-stage creep ageing while similar amount of total creep strains was achieved. This suggests an alternative loading sequence for creep age forming of 7050 aluminum alloy, i.e., loading at the second stage to avoid prolonged exposure to high temperatures, extend the tool life, and achieve similar amounts of inelastic deformations. Based on all the above studies, creep and stress relaxation have a certain relationship. This paper provides a theoretical basis for future research on creep ageing forming of 7050 aluminum alloy.