A comparison between different approaches in the evaluation of extrusion die life: static strength, fatigue and creep

The demand of high quality extruded products involves the requirement of an increasing efficiency of the production systems leading to severe operational conditions for the die. Among different classifications proposed in literature for 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/deflection failures. Static failure appears after e reduced number of extruded billets as a consequence of an overload or poor initial die design. Damage and deflection failures are indeed induced by the synergic detrimental action of creep and fatigue phenomenon. All the failure mechanisms are die design and in-service conditions dependent. In particular, in discriminating the dominant role of creep and fatigue in leading to final die discard, a considerable role is played by the dwell time. This is the time of constant load acting 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 extrusion dies in the creep-fatigue regime after multiple cycles (i.e. multiple extruded billets) based on a modified version of a simple creep law already implemented in all the FE codes. The starting point is a physical experiment reproducing the thermo-mechanical conditions of a die. The model allows to account also for the dwell time. The model has been validated against small scale dies. A test was also performed on an industrial hollow die used in a controlled experiment.