The AISI H11 creep-fatigue behaviour: an innovative experimental design

During the hot-extrusion manufacturing process, a number of damage and deformation mechanisms act simultaneously to produce cumulative damage to the tools, thus causing the increasing deviations from the original geometry or the final breaking [1-3]. Due to the severe cyclic thermo-mechanical loads, mandrel, i.e. the part of the hollow die that defines the internal shape of the profile, is the most critical component in the extrusion of an hollow profile. Indeed, the high pressure generated during the process creates severe friction conditions that results in longitudinal tensile stress and significant bending stresses can arise in the bridges of the mandrel during extrusion. In addition to the mechanical cyclic load, the total loading/unloading time for the whole batch and the temperature that the die is exposed to are great enough to necessitate the consideration on the creep behaviour of the die material, particularly for hollow dies. Hence, the combination of dynamic, heavy loading and high temperature determines a hostile working condition for the mandrel that is normally designed on the basis of static loading at elevated temperatures on hot-work tool steels that are tempered to reach an adequate balance of hot hardness and toughness. Premature failure may occur after a certain number of loading and unloading cycles as a result of creep-fatigue interaction. The new technologies developed for aluminium extrusion aim to minimize the tool system-material flow interference and optimize the mechanical performance of the die that is related both to design and tool steel. Aim of the present work is to illustrate the steps followed to design an innovative experimental test purposely developed to investigate the deforming mechanisms of the AISI H11 tool steel in the creep-fatigue regime. The specimens replicate the geometry and the loading scheme of a mandrel on a smaller scale and are manufactured following the same working scheme. In such a way the test is able to account for realistic stress and strain distributions and superficial roughness of a real mandrel as well as to investigate different material and heat treatments.