Étude de l'endommagement par fatigue thermique des moules de fonderie sous pression d'aluminium : effet de l'interaction avec l'aluminisation et l'oxydation

Abstract : Hot work tool steels damage results of a combination of Thermal Fatigue (TF) and various environmental mechanisms, especially in the case of Aluminium Die Casting. This complex damage mechanism has been observed in real life conditions as well as in laboratory tests. Beyond 500 °C, a duplex oxide layer grows on the surface of H11-13 steels exposed to ambient air. The inner layer is rich in chromium whereas the outer layer is poor. Otherwise, during contact with molten aluminium alloy, the iron from the steel interacts with aluminium atoms and forms a multiphase intermetallic layer at their interface. An experimental approach based on the decoupling of loading was designed to study damage mechanisms individually and under complex conditions. In a first step, a immersion test is carried out to examine the interaction between X38CrMoV5 tool steel and a molten AlSi9Cu3 alloy under static and isothermal conditions. The growth of the intermetallic layer is governed by diffusion and is largely dependent on a dissolution phenomenon which accelerates beyond 650 °C. In the second step, a new induction thermal fatigue rig used in air and/or inert atmospheres with reduced PO2, has been set up to study the combined effects of thermal fatigue and oxidation. Disk shaped axi-symmetric test specimens made from pre-aluminised or virgin X38CrMoV5 steel were used. Thermal fatigue experiments are performed with different thermal cycle at the edge, where Tmin = 100 °C and Tmax = 550 to 650 °C with heating rates of 420 °C. S-1. At 550 °C, damage in the case of virgin steel is due to cyclic scaling of the oxide layer and no macro-cracking is observed until 400,000 cycles. Beyond 550 °C, when the superficial layer (oxide or intermetallic) is compact and has strong adhesion to the base metal, it first undergoes regular and parallel micro-cracking. From these "un-iaxial heat checking", several macroscopic cracks propagate in the steel. However, in the absence of a compact outer layer, macro-cracks appear simultaneously as soon as the steel reaches a critical cyclic softening. At macroscopic scale, the macro-cracks propagate in mode I perpendicularly to the ortho-radial axis. The depth of these macro-cracks depends on their number. A finite element thermo-mechanical analysis was performed. The material models used in this analyses were based on "thermo elastic-plastic" (EP) and "thermo-elasto-visco plastic" (EVP) behaviour. It is shown that the behaviour model selection is not significant if the stress life approach of prediction is used. However, the EVP model is found to be better adapted if other parameters (such as the maximum stress at crack initiation) is considered, because it takes into account time-related effects (e. G. Loading rate and microstructure evolution). The concept of linear fracture mechanics was used as the crack propagation criterion. The Stress Intensity Factor (SIF) was calculated using the weight function of Bueckner and was validated through measurement of residual crack opening distance at ambient temperature.
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Mehdi Salem. Étude de l'endommagement par fatigue thermique des moules de fonderie sous pression d'aluminium : effet de l'interaction avec l'aluminisation et l'oxydation. Mécanique des matériaux [physics.class-ph]. Université de Paul Sabatier de Toulouse, 2009. Français. ⟨NNT : 2009TOU30321⟩. ⟨tel-01696352⟩

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