6xxx series aluminium alloys are widely applied in transportation industry due to their good formability, corrosion resistance and relatively low cost. Their moderate yield strength, however, limits their further application. A novel hybrid thermomechanical processing route, consisting of Pre-Ageing, Cold Rolling, and Re-Ageing (PA-CR-RA) that was claimed to have achieved a combination of a much higher yield strength, with acceptable ductility in sheet material [1], has been studied using AA6013. It was found that PA before CR to low to medium reductions could substantially increase the strength without losing much ductility. However, to reach an unprecedented high yield strength (450 MPa) while keeping moderate ductility (6\%), a high rolling strain (~72\%) on the naturally aged sheet is critical, followed by either long term annealing at 140 $^{\circ}$C or short term at 160 $^{\circ}$C.

The strengthening contributions from precipitates and strains in the different stages of the process were characterised using TEM and XRD whose profile was analysed by Convolutional Multiple Whole Profile (CMWP) fitting method, and EBSD, to quantify both the precipitate densities and deformation structures, which was estimated by conventional models. It was found that the strengthening contribution of the PA precipitates decreased with increasing CR strain, due to non-linear summation, as well as the repeated shearing events during CR. During CR, the PA precipitate fragments acted as strong obstacles to dynamic recovery, resulting in a higher dislocation density with retarded cellular structures after CR, and consequently higher strain hardening. During RA, two reactions, recovery and ageing, competed with each other, with recovery dominating initially while age hardening occurred at a later stage. PA strongly reduced precipitation in the RA step, resulting in a reduced potential for further age hardening. The precipitates mainly nucleated on dislocations, exerting a strong pinning effect and therefore retarding recovery while providing further age hardening. It is shown that the optimum tensile properties were obtained when the lost strengthening contribution from recovery could be compensated by age hardening, without substantial decrease in work hardenability to achieve an acceptable ductility.

[1] Z. Wang, H. Li, F. Miao, B. Fang, R. Song, and Z. Zheng, “Improving the strength and ductility of Al-Mg-Si-Cu alloys by a novel thermo-mechanical treatment,” Mater. Sci. Eng. A, vol. 607, pp. 313–317, 2014, doi: 10.1016/j.msea.2014.04.009.