Effect of the Zener-Hollomon parameter on the formability of AA6082-T6 formed at elevated temperature

Material formability is one of the main research topics in metal forming, but it is usually regarded as a secondary concern in processes carried out at high temperature. Though usually entailing higher material ductility, the high temperature used in hot forging can lead to the phenomenon of hot shortness, with fall of material formability due to the microstructural changes during the deformation. Hot metal formability modelling can imply three main issues: (i) the lack of models expressly developed for high temperatures, (ii) the exponential increase of the experimental effort which is needed for the model calibration, since different thermo-mechanical conditions need to be tested, according to the process window parameters, and consequently (iii) the more complex model calibration, especially if inverse analysis techniques are used. In this work the formability of the AA6082-T6 aluminum alloy is investigated by means of tensile tests carried out at different temperatures and strain rates. The hot shortness onset is identified as well as the negative strain rate influence on the material maximum strain at fracture. A linear dependency of the material formability on the Zener-Hollomon parameter is established for temperatures below the hot shortness point, allowing a significant simplification of the experiments needed for the calibration. A new formability model is proposed to overcome the limits of the linear correlation thanks to the introduction of a critical value of the Zener-Hollomon parameter. The Oyane-Sato damage criterion is then extended to hot conditions using the afore mentioned Zener-Hollomon dependency. Finally, the approach is validated on the reference industrial case: the cross wedge rolling of AA6082-T6 round bars carried out at elevated temperature showing axial cracking due to Mannesmann effect.