Influences of residual stresses and initial distortion on springback prediction of 7B04-T651 aluminium plates in creep-age forming

Abstract

Abstract. Springback behaviour of surface-machined 7B04-T651 aluminium plates of 3 to 8 mm thicknesses, creep-age formed under a single curvature bending radius of 1200 mm, has been experimentally investigated. The surface residual stress levels of the plates have been measured and the typical residual stress distribution and values in 7000-series aluminium alloys have been reviewed and analysed. Finite element models have been developed to simulate creep-age forming (CAF) processes and predict the amount of springback in the physical CAF test. Machining-induced residual stresses and distortion have been considered in the finite element models. This paper presents a first attempt to study the effect of machining-induced residual stresses on the errors of springback prediction in CAF of 7000-series aluminium alloy panel components. It has been found that the effect of machininginduced residual stresses on the accuracy of springback prediction is related to the initial loading stress levels in CAF. In addition, when the initial distortion conforms to the loading curvature, an opposing effect on the material's deformation is observed. Creep-ageing material constant related to effective creep strain rate.1 Creep-ageing material constant related to rate of age hardening.2 Creep-ageing material constant related to the depletion of solute into precipitate.3 Creep-ageing material constant related to rate of precipitate growth.4 Creep-ageing material constant controlling the effect of dislocation density (or creep) on precipitate nucleation and growth.5 Creep-ageing material constant related to rate of dislocation density.Creep-ageing material constant related to rate of dislocation hardening.Bending radius of forming tool.̅ , ̅ Normalised precipitate size and its rate term respectively. ̅ 0 Initial value for normalised precipitate size.Radius of revolvement of workheads of the flexible form tool.Creep-ageing time.1 , 2 , 3 , 4 Time of loading, heating, cooling and unloading respectively. Artificial ageing time.Solution heat treatment time., , Translational degrees of freedom in Cartesian coordinates., , Rotational degrees of freedom in Cartesian coordinates.Poisson's ratio., , Cartesian coordinates., positions of the lower and upper splines respectively.

Material modelling and its application to creep-age forming of aluminium alloy 7B04

“…Based on the FE-simulated results which show a 0.4% difference between the two plates, it can be concluded that the springback sensitivity to a 0.1 mm difference in plate thickness is close to negligible under the current CAF condition. Therefore, the 4.2% difference between the experimental springback values of the two plates is likely to be due to experimental uncertainties, which may include their difference in residual stress level and initial plate distortion prior to CAF submission [6]. Nonetheless, the measured and FE-simulated results both show a decrease in springback as the plate thickness increases from 8.1 to 8.2 mm.…”

“…Flexible forming surfaces were on both sides of the plate with an additional elastomeric sheet layer between the plate and lower flexible surface. Further details about the method of CAF tool design and test setup can be found in [5] and [6] respectively. The main steps of the forming test include:…”

“…As shown in Figure 5, only workheads of the forming pins were modelled. Detailed descriptions of the FE model and modelling procedure are available in [6], whilst the influences of residual stresses and initial distortion of plates on springback prediction are not considered in this study.…”

An investigation of creep age forming of AA7B04 stiffened plates: Experiment and FE modelling

“…Although the difference between FE and experimental results tends to increase with increasing die radius and decreasing stiffeners height, the maximum difference is only 6.7% for all tested conditions, which is good for industrial applications. One possible reason for the increasing difference may come from the residual stress present in the machined stiffened plates before CAF, as overestimate of springback by FE simulation has been reported when residual stress effect is ignored [28]. It has also been shown that the residual stress effect becomes less apparent when plates were loaded with a higher stress [29].…”

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

“…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. An alternative approach based on a -projection method for modelling the creep-ageing behaviour, especially for 2024-T3 Al alloys, were also introduced by Lin et al (2013).…”