Behaviour and Modelling of AA6xxx Aluminium Alloys Under a Wide Range of Temperatures and Strain Rates

Abstract

This thesis addresses the behaviour and the modelling of the thermo-mechanical response of Al-Mg-Si (aluminium) alloys in tension subject to a wide range of temperatures and strain rates. The work is organised into a synopsis providing information about the objectives of the study, the necessary theoretical background and a summary of the work performed. The synopsis is followed by three articles published or submitted for publication in scientific journals. An appendix with complementary results concludes the thesis. The first paper presents a split Hopkinson tension bar (SHTB) system that was used to investigate the mechanical response of aluminium alloys under dynamic conditions. This test rig was modified to perform a thermo-mechanical study of Al-Mg-Si alloys. The test specimens were rapidly heated to a desired temperature through the use of an induction apparatus connected to a pyrometer that measured the temperature. A highspeed digital camera was coupled to the test rig to record the specimen and to measure the local deformations. The local logarithmic strain, the strain rate and the Cauchy stress were then determined up to large strain levels. The validity of this new technique was assessed using thermal and thermo-mechanical simulations in LS-DYNA. The second paper focuses on the thermo-mechanical response of three Al-Mg-Si aluminium alloys that were investigated at different nominal strain rates from 0.01 s-1 up to 750 s-1 and temperatures ranging from 20°C to 350°C. The test rig presented in the first paper was applied to determine the dynamic response of the materials considered. Lower nominal strain rates were achieved using a universal tensile test machine coupled with the same induction heater, pyrometer and a digital camera. The aluminium alloys investigated exhibited a coupling between temperature and strain rate sensitivity (SRS) subject to hot conditions. A slightly positive SRS was observed at room temperature, while the SRS was shown to increase markedly at higher temperatures. The third paper is concerned with modelling the thermo-mechanical response of the AA6082 alloy that was investigated in the second paper. A new physically based model was developed to account for the high temperature sensitivity of both yielding and work hardening. The constitutive model was fitted with test data obtained in the previous paper. It was shown to be in good agreement with the alloy response over the entire range of temperature and strain rate up to large deformations. Simulations of the tests were performed with the calibrated model, which was implemented in the non-linear finite element code LS-DYNA. This paper also evaluates the use of the local strain measurement technique developed in the first paper to characterise the mechanical response of isotropic metallic materials up to large deformations with good precision.