Constitutive laws for the deformation estimation of extrusion die in the creep-fatigue regime

Tools are exposed to severe working conditions during the hot extrusion process. In
particular, dies and mandrels can be subjected to an excessive amount of deformation as a result of
the developed high cyclic loads and temperatures. In this scenario, a physical experiment
reproducing the thermo-mechanical conditions of a mandrel in a porthole die was performed with
the Gleeble machine on the AISI H11 tool steel with the aim to investigate the mechanisms that
influence the die deformation. The design of experiment consisted of 4 levels of temperature, 3
levels of stress and 3 types of load, i.e. pure creep, pure fatigue and creep-fatigue. In all the testing
conditions, a comparable pattern of the mandrel displacement-time curve was found reproducing
the 3 stages of softening typical of the strain evolution in a standard creep test but with a marked
primary phase. Thus, with the aim to identify an easy-applicable equation to estimate the die
deformation, the time hardening creep law was chosen. Coefficients of the time-hardening law were
optimized, for each testing condition, on the basis of experimental data starting from values for
similar alloys taken from the literature. Results in terms of mandrel displacement were then
compared to experimental data for the creep-fatigue condition at different stress and temperature
levels. The values found were validated against additional experimental data performed with
different specimen geometries. A good average agreement was found between experimental and
numerical results. The developed procedure was then applied to an industrial die.