Static recovery of an AlMg4.5Mn aluminium alloy during multi-pass hot-rolling

In multi-pass hot-rolling of aluminium alloys stored energy is created by the plastic deformation in the form of dislocations and sub-grains. The energy dissipates due to sub-grain growth and the annihilation or rearrangement of dislocations. Both mechanisms lead to a reduction of dislocation density and hence a decrease in yield stress, i.e. to recovery and softening. Reliable quantitative results on these aspects, experimental as well as theoretical, are of high technological importance. Literature provides many results on the hardening and softening behaviour of aluminium alloys during plastic deformation. Few quantitative investigations, however, deal with the static softening after hot deformation. In this contribution we share recent quantitative results on the static softening behaviour of an AlMg4.5Mn aluminium alloy and its implications for the industrial hot-rolling process. We show the static softening behaviour derived from stress relaxation tests on a Baehr DIL805 dilatometer. The results agree well with a microstructure model, which is based on thermally activated dislocation climb. The activation energy for static recovery, however, is found to be outside the range predicted by theory. The relaxation results together with additional thermomechanical characterization are used to calibrate a material model based on the Kocks-Mecking equation, which has been implemented in a commercial implicit Finite-Element software. Numerical results from a three-dimensional Finite-Element model of the industrial multi-pass hot-rolling process show how static recovery affects the distribution of dislocation density across the rolling ingot between passes.