Creep and Fatigue Damage in Hot Work Tools Steels during Copper and Aluminium Extrusion

During hot extrusion, tools experience cyclic thermo-mechanical loads that can lead to materials degradation and failure. For a process optimization and study of the occurring damage mechanisms, the finite element method (FEM) is an appropriate means. Local inelastic strains result from the interaction of the applied temperature and stress loading and can be computed by suitable inelastic constitutive equations. Stress amplitudes and dwell times during extrusion result in creepfatigue damage. A lifetime consumption model sums increments of a damage variable over time and defines materials failure as the accumulation of the resulting damage variable to a critical value. The predominant failure mechanism, i.e. creep or fatigue, can be found by the investigation of the damage rate over several cycles. A comparison of both a creep dominated (copper extrusion) and a fatigue controlled (aluminium extrusion) lifetime consumption in an extrusion die is shown with the hot work tool steel Böhler W300 ISOBLOC in comparison with W400 VMR (both ~ EN 1.2343).