Multi-cycles deformation modeling of hot forming tools under creep-fatigue regime

Hot forming processes (extrusion, die ca sting and forging) allow the production of a wide variety of products, both in terms of worked material and achievable shapes. However, critical working conditions are involved for the tools subjected to severe thermo- mechanical loads, thus requiring an accurate design. Among the different classifications proposed in literature for hot forming die failures, one focus on separating manufacturing and in service failures. The latter category is additionally split, as proposed by the authors, in static failure, damage and deformation/deflec tion failures. Static failure appears after e reduced number of extruded billets as a consequence of an overload or of a poor initial die design. Damage and deflection failures are ind eed induced by the synergic detrimental action of creep and fatigue phenomenon. In disc riminating the relative dominant role of creep and fatigue in leading to the final die discard, the level of temperature and of the applied load as well as of the dwell-time are d ecisive. In extrusion, the latter is the time in which a constant load acts on the die and represents the time required to extruded each single billet function of both ram speed and billet length. Fatigue and creep can be seen as limit cases with zero and infinite dwell-time. A novel model is proposed for the prediction of the deformation undergone by hot forming t ools in the creep-fatigue regime after multiple cycles. The model is presented as applied to extrusion dies and is based on a modified version of a simple creep law already implemented in all the FE codes. The model is validated against small scale dies used in physical experiments and then against industrial extrusion dies.