A unified constitutive model for asymmetric tension and compression creep-ageing behaviour of naturally aged Al-Cu-Li alloy

Abstract

A set of unified constitutive equations is presented that predict the asymmetric tension and compression creep behaviour and recently observed double primary creep of pre - stretched/naturally aged aluminium - cooper - lithium alloy AA2050 - T34. The evolution of the primary micro - and macro - variables related to the precipitation hardening and creep deformation of the alloy during creep age forming (CAF) are analysed and modelled. E quations for the yield strength evolution of the alloy, including an initial reversi on and subsequent strengthening, ar e proposed based on a theory of concurrent dissolution, re - nucleation and growth of precipitates during artificial ageing . We present new observations of so - called double primary creep during the CAF process . This phenomenon is then predicted by introducing effects of interact ing microstructures , including evolving precipitates, diffusing solutes and dislocations , into the sinh - law creep model. In addition, concepts of threshold creep stress ??? ??? ??? and a microstructu re - dependant creep variable H , which behave differently under different external stress directions, are proposed and incorporat ed into the creep model . This enables predict ion of the asymmetric tension and compression creep - ageing behaviour of the alloy. Q uanti tative transmission electron microscopy (TEM) and related small - angle X - ray scattering (SAXS) analysis ha ve been carried out for selected creep - aged samples to assist the development and calibration of the constitutive model. A good agreement has been achieved between the experimental results and the model. The model has the potential to be applied to creep age form ing of other heat - treatable aluminium alloys

An accelerated springback compensation method for creep age forming

“…In addition, when the total applied strain is lower than a particular level (0.165% for AA6082-T6 at 160°C), springback ratio nearly equals to 1, which indicates that nearly all deformation is recovered elastically. This is due to a threshold stress that exists for creep of material, below which no creep occurs [31]. Hence, there is a no creep zone in Fig.…”

A novel constitutive model for multi-step stress relaxation ageing of a pre-strained 7xxx series alloy

“…Based on the description of the microstructure evolution above, a model has been developed, which enables the determination of the yield strength, , and the stress relaxation at different temperatures. The model is built using an internal state variable approach (Esmaeili et al, 2003;Li et al, 2017;Myhr and Grong, 2000), which relates the micro-internal variables to the material response of interest (i.e. yield strength, , creep strain, , etc.).…”

“…A different form, Eqn. (4), is then adopted to enable a similar radius evolution with r starting from 0, as suggested in (Li et al, 2017),…”

“…Further improvements were made to describe the precipitation hardening behaviour of 7055 in creep-ageing by Zhan et al (2011), where the evolution of normalised precipitate radius was used to describe both the precipitation hardening and the solute softening effects on the creep behaviour. Later, extensive work, including improving the creep-age models, such as accounting the precipitate aspect ratio , considering the precipitate dissolution (Li et al, 2017) were performed. Studies have also been made to apply creep-ageing models to study different materials, such as AA6111 (Wang et al, 2004), AA2025 (Gu and Ngan, 2014), and study different conditions, such as T4, T651 (Lam et al, 2015), etc.…”

Influence of artificial aging on the electrochemical properties of the aluminium AA 6063 alloy

“…The precipitation hardening process requires that the second component in the aluminium alloy, is sufficiently soluble to allow extensive dissolution at an elevated temperature (solubilisation treatment temperature) and that the solubility is reduced at lower temperatures like Si and Fe in our case. 2,24,25 The aim of the presented study was to determine the aging effect on corrosion stability in differently artificially aged aluminium alloy AA6063.…”