Improved creep behaviour for a high strength Al-Li alloy in creep age forming: Experimental studies and constitutive modelling. (December 2022)

• Record Type: Journal Article
• Authors: Wang, Xi; Rong, Qi; Shi, Zhusheng; Lin, Jianguo
• Is Part Of: International journal of plasticity. Volume 159(2022)
• Publication Date: 2022-12
• Subjects: Creep age forming -- Al-Li alloy -- Threshold stress -- Pre-stretching -- Constitutive modelling; Plasticity -- Periodicals; Plasticité -- Périodiques; Plasticity; Periodicals; 620.11233
• DOI: 10.1016/j.ijplas.2022.103447
• Languages: English
• ISSNs: 0749-6419

Abstract

Highlights: Creep behaviour of Al-Li alloy in creep-ageing is improved via applying high stress or with pre-stretching. The breakdown of linear stress-strain loading relation leads to nonlinear increase of creep strain with stress. Enhancing effect of pre-stretching on creep strain decreases with increasing applied stress. A unified mechanism-based constitutive model is established for the improved creep behaviour. The model is applicable to both CA and PCA, agreeing well with experimental results. Abstract: The creep deformation of 3 rd generation 2xxx Al-Li alloys in creep age forming (CAF) is unsatisfactorily low due to its high strength and required low ageing temperature, especially for the commonly used T8 temper. Promoting creep deformation in such process will significantly reduce springback in CAF, making easier the springback compensation in tool design. In this study, two possible process routes of inducing increased creep deformation were explored, one by employing high applied stress to introduce small plastic strain in loading stage; and the other by conducting pre-deformation prior to creep-ageing. The creep behaviour with and without pre-deformation was investigated through creep-ageing tests under stresses ranging from 300 to 430 MPa at the ageing temperature of 143°C. Due to the decrease of threshold stress with introduced plastic strain in the loading stage and/or pre-stretching, a faster increase of creep strain with applied stress was observed for both methods when the applied stress surpassed 400 MPa. The strain promotion of the former was higher than the latter when above 415 MPa. A mechanism-based constitutive model was proposed, with additional work hardening equations to describe the relationship between threshold stress and dislocation density induced by plastic strain in the loading stage and pre-stretching, to model the microstructural evolution and reflect the nonlinear increase of creep strain with stress in both methods. The adequacy of this proposed unified model was demonstrated with good agreement with experimental data utilising both methods. The adaptability of the model in multiaxial case was verified using four-point bending CAF tests with the same Al-Li alloy.