Experimental study and modelling of the role of solutes, precipitates and temperature on the work-hardening of AA6xxx aluminium alloys

Abstract

Attempts have been registered scientifically mostly for strained hardened Al alloys towards improvement of formability at warm temperatures. The varieties of possible precipitate populations add complexity in determining the mechanisms of work-hardening behaviour of the precipitation hardened alloys. Efforts were made, in the current work, to investigate the work-hardening response for EN AW-6016 and EN AW-6061 sheets of different temper (T4 and T6) at room and warm temperatures (150 and 250 °C) and at two different strain rates (0.1 and 0.01 s⁻¹) through tensile experiments. Among these variants, the influence of temperature has been detected the most on yield strength and work-hardening rate. Lowering the efficiency of storing dislocations as well as activating dynamic recovery, both are responsible for the reduction in work hardening rate proportionally with temperature. The work-hardening model originally developed by Prof. E. Nes (Nes model) has been chosen to simulate the experimental stress-strain response of both alloys at room and warm temperature as the phenomenology of the model promised a better constitutive description of work hardening. Because Nes model was primarily designed for strained hardened Al alloys, the bottleneck, to apply this model for precipitation hardened alloys, was removed by adding contribution from the precipitates in terms of mean slip length of dislocations in the current version. The validity of the model was examined for 130 °C, 150 °C and 180 °C and at a strain rate of 0.1s⁻¹, once satisfied fitting was achieved for 6016 and 6061 alloys at RT and 250 °C at a strain rate of 0.01s⁻¹. The decrease of dynamic recovery during the change of temper state (T4 to T6) during RT deformation can be explained as a competition between solute depletion and precipitate growth. With minute quantitative differences, the influence of strain rate and temperature was reproduced well by the model.