Hot bulk damage modelling of precipitation hardened AA6082 aluminium alloy

Among the bulk metal forming processes hot forging is often the only option if large reductions of the forging load are required or if the material formability needs to be significantly increased or even if specific thermally-induced microstructural changes are needed to take place during the deformation process. Moreover it still retains at some extent the positive features of the cold forging processes: high production rates, complex final shapes attainable and little to no material waste. Hot forging is therefore used for the production of large parts, with complex shapes and especially when using materials characterized by low formability and high toughness at room temperature or if particular microstructural characteristics are required. It is worth to be underlined that these are often the features of the innovative metal alloys that have been increasingly being used in the last decade and namely: Mg alloys, Al alloys, Ti alloys and superalloys. Finite Element metal forming numerical simulation has become an increasingly important process optimization tool, due to the growing computational power available at reduced costs, which spread it in the industrial world. Its use allows reducing process design time and prototyping costs as well as long and expensive plant downtimes for process variable tuning. For all these reasons hot forging has become a strategic process and its accurate numerical simulation is encountering great industrial interest. One of its main targets is the determination of the maximum strain that the material can undergo during the deformation process, since it is strictly related with both the final shape and the surface quality of the manufactured part. In this sense hot formability modelling provides a meaningful example of a topic which is both of great scientific and industrial interest. From the scientific point of view ductile damage modelling has been developed at first for cold processes, for which crack formation is usually a major issue. Last decade research efforts focused on the development of more advanced fracture criteria, keeping into account a complete characterization of the stress state dependency of formability. However the complex analytical formulation of these criteria and their expensive experimental calibration kept them de facto away from practical industrial use. On the other hand hot formability modelling was traditionally addressed by direct application of the conventional cold fracture criteria, under the implicit assumption of isothermal conditions. This approach has evident limits since it does not account for temperature influence on material formability and it does not even provide any physical insight on the different fracture mechanisms that can develop as temperature changes. In recent years some research efforts have been performed on a deeper investigation upon this point: experimental formability campaigns on different metal alloys were carried out in order to assess the temperature and strain rate influence, while some analytical models were proposed in order to describe the damage evolution at high temperature and in particular the onset of the so-called “hot shortness”. However these models are still quite simple and can describe only a limited variety of formability trends, losing in accuracy once complex microstructural phenomena take place. Moreover their validation has been done with simple laboratory tests and not with real processes industrial trials in which non-uniform thermo-mechanical conditions are present and the material can evolve through fracture mode-changing regimes. The objective of this work is the elaboration of a novel approach to hot ductile fracture modelling, capable to represent accurately the formability evolution of a metal alloy as a function of both the main thermo-mechanical variables and its physical and microstructural characteristics, yet providing a tool simple enough to be of industrial utility. To this aim the hot cross wedge rolling of a precipitation hardened aluminium alloy was taken as industrial reference case, since it is an innovative and non-standard forging process which entails variable and non-homogeneous thermo-mechanical conditions. The study case is of remarkable interest since it has a narrow process temperature window, being limited at the top by Mannesmann-type axial cracking onset and at the bottom by unwanted grain coarsening. Moreover the metal alloy used, the AA6082-T6, has the microstructural features, namely the intermetallic precipitates, that make it a good example of a metal alloy of wide industrial use, that during forming can undergo complex microstructural changes. A hot tensile test campaign was performed on a wide range of thermo-mechanical conditions and the results highlighted an unexpected negative strain rate influence on formability. Fractographic and micro-chemical analysis were then performed in order to assess the microstructural reasons of this behaviour and finally two approaches to the material formability modelling were proposed and calibrated extending to hot conditions the classic Oyane-Sato fracture criterion. The first one consists on the empirical calibration of the criterion by means of a bi-linear interpolation of the experimental data, while the second one entails a physically-based analytical formulation of the material fracture locus, which has also the advantage of being of easier calibration. These models were then validated on the cross wedge rolling process simulation by comparison with the industrial trials results and the outcomes were critically assessed