Experimental and numerical investigation of stress-relaxation age forming of AA6082

Abstract

This thesis presents a comprehensive experimental and modelling investigation of stress-relaxation ageing of AA6082. Based on this investigation, successful manufacture of a typical industrial component with large and complex curvatures and with high dimensional accuracy has been achieved through the stress-relaxation age forming (SRAF) process. The 12 hr stress-relaxation ageing behaviour in both the elastic and plastic regions of AA6082-T6 at 160 °C has been experimentally investigated for the first time, including stress-relaxation ageing tests, tensile tests, and microstructural observations. It has been observed that the total stress relaxed after a 12 hr test increases significantly with increasing initial strain levels, while the initial strain levels contribute little effect to the yield strength evolution during stress-relaxation ageing. Microstructural observation results and the stress exponent method have been used to analyse the stress-relaxation mechanisms of the material, and a decreasing threshold stress with increasing initial strain is proposed for stress-relaxation ageing in the plastic region. A new unified constitutive model has been proposed and calibrated to accurately predict both the stress-relaxation behaviour and the yield strength evolutions of AA6082-T6 during the SRAF process in both the elastic and plastic regions. FE simulations of the SRAF process have been carried out using the FE solver ABAQUS, in which the developed material model has been implemented through the user-defined CREEP subroutine, and their effectiveness has been validated by SRAF tests of a singly-curved plate. A springback compensation strategy has been developed for tool shape compensation and its effectiveness proved by SRAF tests of a doubly-curved plate. Finally, an industrial component with large and complex curvatures, which was selected from an in-service high-speed train, has been successfully manufactured through the SRAF process with the integrated numerical technologies developed in this thesis, including material and FE simulation models, and a springback compensation strategy. The successful production of the industrial component has proven the feasibility of the developed technologies, which can be used as an effective tool to guide the future industrial applications of the SRAF process with AA6082 alloy.