Deformation Prediction of Porthole Dies After Multiple Extrusion Cycles

Extrusion dies work under the combination of cyclic loads and high temperatures. Accounting also for the billet-die contact time during each single extrusion, the die deflecting mechanism is placed in the creep-fatigue interaction. In order to analyze the die deformation, a physical experiment reproducing the thermo-mechanical conditions of an extrusion die was performed on a Gleeble simulator. The specimens were made of AISI H11. The design of the experiment consisted of four levels of temperature, three levels of stress, and three types of load (creep, fatigue, and creep-fatigue with a dwell-time). It was found that creep is the mechanism that greatly affects die deformation, and that the time-displacement histories of the specimens showed a marked primary softening phase for each tested condition. Thus, a modified version of the time-hardening creep law was chosen to estimate the die deformation. Coefficients of the law were optimized on the basis of experimental data, starting from values found in the literature. Novel equations were formulated to express the dependency of the coefficients on the stress-temperature state, as well as on the dwell-time after multiple cycles. The presented procedure was validated against additional experimental data, performed with different specimen geometries and on an industrial extrusion die. A good average agreement was found between experimental and numerical results.