Mechanical characteristic and stress-strain modelling of friction stir welded 6061-T6 aluminium alloy butt joints

This paper investigates the mechanical properties and softening extent of friction stir welded (FSW) 6061-T6 aluminium alloy butt joints through experimental studies and theoretical analyses. A total of 138 monotonic tensile coupon tests and 46 Vickers hardness tests, cut from 5-, 8-, 12, 16-mm-thick welded plates using various welding speed, tool rotation speed and tool configurations, were systematically performed. The surface morphology after welding, failure modes, stress-strain curves and hardness distribution laws were evaluated. An assessment of the applicability of existing constitutive model and proposed model to FSW aluminium alloys were presented, with predictive equations for the key input parameters further derived. The results show that the strength reduction and softening extent induced by FSW under the optimal welding parameters were superior to those of 6061-T6 aluminium alloy plates joined by fusion welding process in accordance with Chinese and European standards, indicating the effectiveness and applicability in aluminium alloy component forming and structural connections using FSW technology. The stress-strain curves of FSW butt joints exhibited higher strain hardening effect but lower ductility. The developed three-stage Ramberg-Osgood model, using seven known key input parameters, was shown to provide a very high accurate and consistent predictions in full-range stress-strain curves in comparison with existing constitutive models. Moreover, the proposed constitutive model, using only three common available parameters, could still achieve a great balance between accuracy and practicality.